Some Molecular Data from Raman Spectra, Electron Diffraction and Low Temperature Colorimetry Citation Russell, Horace, Jr. (1941) Some Molecular Data from Raman Spectra, Electron Diffraction and Low Temperature Colorimetry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/cv71-j597. https://resolver.caltech.edu/CaltechTHESIS:02282025-000015219 Abstract No abstract. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: (Chemistry) Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Yost, Don M. Thesis Committee: Unknown, Unknown Defense Date: 1 January 1941 Record Number: CaltechTHESIS:02282025-000015219 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:02282025-000015219 DOI: 10.7907/cv71-j597 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 17033 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 28 Feb 2025 19:05 Last Modified: 28 Feb 2025 19:23 Thesis Files PDF - Final Version See Usage Policy. 20MB Repository Staff Only: item control page

SOME MOLECULAR DATA F'ROM _RAMAN SFEC-TRA, ELECTRON DIFFRACTION AND LOvi TEMPERATURE CALORIMETRY Thesis by Horace Russell, Jr. In Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 1941 ACKl{OWLEDGEMENTS Dr. David P. Stevenson directed and aided in the electron diffraction experiments. Dr. Darrell Osborne is jointly responsible for the measurements on methyl mercaptan, except for the final heat of vaporization. these measurements Dr. Thor Ru.bin assisted. In The calibration of the resistance thermometers and the construction of the heat of vaporizat ion apparatus was done in cooperation with Professor Don M. Yost and Dr. Rubin. Measurements on arsenic trifluoride were made jointly with Mr. Robert Rundle. Dr. C.S.Lu made the graphs and figures for this thesis. To all of these I am greatly indebted. Especially do I wish to thank Professor Yost, under whom I have worked for the last three years. can come up to his direction and example. I hope my work CONTENTS Page Propositions Summary I. The Preparation and Raman Spectrum of Sulfuryl Fluoride Preparation 2 Raman Spectrum 5 Frequency Assignments for Sulfuryl Fluoride and Sulfuryl Chloride 9 II. Electron Diffraction Studies on Sulfuryl Fluoride and Thiophosphoryl Fluoride 11 III. Low Temperature Calorimetry Description of Apparatus 16 The Temperature Scale 18 Construction of a Heat of Vaporization Apparatus 26 Apparatus f or Vapor Pressure Measurements 28 Operation of the Heat of Vaporization Apparatws 29 IV. Calorimetric Data on l•iethyl Mer captan Preparation of Material 35 Vapor Pressures 36 Heat Capacities 37 Heat of Transition 37 Heat of Fusion 42 Heat of Vaporization 42 Entropy from Calorimetric Data 44 Discussion of Transition 46 Spectroscopic Entropy 47 Comparison of Calculated and Observed Entropies 50 CONTENTS V. Page Calorimetric Data on Arsenic Trifluoride Preparation of Material 51 Vapor Pressures 53 Heat of Vaporization 55 Heat Oapaci ties 5'7 Heat of Fusion 59 Entropy from Calorimetric Data 59 Comparison of Experimental and Calculated Entropies 61 INTRODUCT ION This thesis presents the results of molecular structure investigations carried out during the last three years by means of Raman Sl)ectra, . electron diffraction, and low temperature calorimetry. The general aim of the work was the gaining of experience in several methods of structure determination as well as the accumulation and interpretation of data. Thus work was done in several fields, although the principle field of research was low temperature calorimetry. In this f'ield valuable experimental training was gained in the calibration of a set of resistance thermometers and in the construction of an apparatus for the accurate measurement of heats of vaporization. Three of the compounds treated are inorganic fluorine compounds; the data gained extend and in some cases verify previous knowledge of this typ.e of compound-- much of which originated in these laboratories. The methyl mercaptan is an interesting case in which restricted internal rotation occurs; it is the third of such cases examined in this laboratory. The work is presented in essentially independent sections and in the chronological order of its performance. are given at the close of each section. -1- References PROPOSITIONS 1. Corrections I'rom the real gas to the ideal gas may be made in entropy calculations by use of the experimentally derived quantities p, T, dp/ dT, and L).li. v The accuracy is equal to that ob- tained using the Berthelot equation of state and it is not necessary to know critical constants for the gas. 2. A reinvestigation of the low temperature calorL~etric work on methyl alcohol should be made before too much importance is attached to the discrepency between the barrier to internal rotation as calculated by Dennison and Kohler from calorimetric data by Crawford. 1. 2. 3. 1 and as calculated 2 Dennison and Kohler, Phys. Rev. June 1940. Crawford, J. Chem. Phys . .§, 744 (1940). The quantity actually measured in ordinary calorimetric work is the saturated heat capacity rather than the heat capacity at constant pressure as commonl.y assumed. Appreciable error may be made in liquid heat capacities near room temperature but the entropy is very little affected. 4. With the data now available, rea.sona.ble correlations can be made of the frequencies of simple inorganic molecules containing S-O and S-X bonds such as SOF , SO3 ClH, etc. 2 Further polarization data are desirable. 5. An interesting program of research would be the experimental study of low temperature transitions in the solid using an automatically regulated adiab~tic calorimeter. A design is propoaed for the regulator. 6. In measuring heats of vaporization as described in this thesis the exact temperature of the vaporizing surface does not have to be known. PHOPOS I TI ONB ( cont I d) 7. The sophomore course in analytical chemistry for physics majors could be more closely adapted to the nee ds and interests of the students. Two ways to do this are through outside re- ports on advanced topics and through special experiments. a. Diffraction of slow electrons in a gas should be studied more thoroughly. 9. The sulfur point should be eliminated in the calibration of platinum resistance thermometers for low temperatur.e work. is desirable to find a good fixed point at about -120°0. for the calibration. If such a one cannot be found the mercury boiling point could be substituted for the sulf'ur point. It SUMMARY Results of investigations by Raman spectra, electron diffraction, and low temperature calorimetry are given with calorimetry as the main field of research. The results are presented in five sections. I. Sulfuryl fluoride was prepared and tested for purity. Although no evidence of impurity was found, a melting point of 1;5.9° K. was found instead of the previously reported 152° K. The Raman spectrum of both the gas and liquid were examined. Nine lines were found in the liquid, ;88 (6), 456 (0), 544 (8), 845 (10), 885 (6b), 1109 (lb), 1262 (10), 1J87 ( 0 ), and 1492 (5) cm-l (intensities in parenthesis). Two lines were found in the 1 gas, 859 (10), and 1279 (8) cm- . Assignments to normal vi- brations of the frequencies of sulfuryl fluoride and chloride are made. The 1;87 cm-1 frequency is assigned as a combination; the others as funda.~entals. ..:1 A frequency of about 350 cm- is suggested as the missing frequency. II. Electron diffraction technique was used to investigate the structures of sulfuryl fluoride and thiophosphoryl fluoride. The interatomic distances and bond angles are: for sulfuryl and L F-S-F = 100±10°; and for thiophosphoryl fluoride, P-F = 1.51±.02 A, P-S c 1.85~.02 A., L F-P-F = 99 -5±2°, and L F-P-S = 118±2°. III: A brief description of the adiabatic calorimeter and of the temperature scale of the laboratory is given. A detailed discussion of the calibration of three platinum resistance thermometers, CT 2, 4, and 7, for the international temperature scale range, and against the laboratory standard CT 26 in the SUMMARY (con 1 t) range from 14° to 90° K. Details of the construction of an improved heat of vaporization calorimeter and of the associated manometer are given next. Special features of the calorimeter are both internal and external heaters, two difference thermocouples to measure gradients on the calorimeter, a special absolute thermocouple to measure the temperature of the emerging gas, and close control over the temperature of the filling tube . The method of operation and of calculation are given. IV. Calorimetric data on methyl mercaptan are presented including heat capacities from 15° to 279° K. A transition is found at 1;7.6t.15° with a heat of transition of 50.5;1,.5 cal/mole. The melting point is 150.162°±.02° K. with a heat of fusion of 14ll .4 cal/mole. The heat of vapori 2.ation at the boiling point (279.11° K. ) is 5872±4 cal/mole. The entropy from calorimetric data is 60.16±.1 e. u. at 1 atm. and 279.l1° K. Comparison of this value with that calculated from spectroscopic and structual data shows a 2.02 e. u. contribution from the internal rotation in the molecule. A calculation using Pitzer 1 s tables gives a barrier of 1¥&0 cal . V. Heat capacities and other data on arsenic trifluoride are given for the ~ange from 14° to 292.50° K. Vapor pressures are also given. The melting point is 267.205°.02° K. with a heat of fusion of 2486 cal. cal/mole at 292.50° K. The heat of vaporization is 8566 8 The entropy from the data is 72.12:t:.1 e.u. at 141.6 mm. pressure and 292.50° K. The spectroscopic entropy is calculated as 72.1; at the same pr essure and temperature. The entropy at i atm. and 298.1° K. is 69.08 e. u. I Preparations and Raman Spectrum of Sulfuryl Fluoride. PREP.A.BATIOH OF SULFURYL FLUORIDE. The method of Trautz and Ehrman was used. Three separate operations are necessaryi (1) The distillation of fluosulfonic acid from a mixture of fuming sulfuric acid and calcium fluoride, ( 2) the formation of the barium salt, and (3) the decomposition of the salt at 500° c. to get sulfuryl fluoride. The reactions are: Ca.Fl.+ 2S03+ H.:i,SO.._ • Ca so't+ 2 S03FH 2 S03FH + BaCla,_ • (F S0.3 )~ Ba+ 2HC1 (FSOJ )~ Ba • S0.2.F~ + BaSO't Iron reaction vessels were used except for the finaJ. trap. Chemicals were of C.P. grade thoroughly dried (BaCl ~ at 200°)except for the acid which was technical fuming 60% sulfuric acid. A number of runs were necessary to get the right technique and the yield in the last run was only about 30% based on the supposed so3 content of the fuming acid. This yield could be improved. It is essential that the fuming acid be of full strength (60% at least) and that the loss of so3 be small on the addition of the CA,F~ . In the decomposition of Ba(FS~)~ care IIIllst be taken not to heat the container too hot as S04F~ reacts with red hot iron to give SOa,_. A Monel container would probably be better. The crude product was collected in a glass trap in liquid air. At -78° it was a yellow liquid smelling strongly of sulfur dioxide. This product was washed through water, then through silver nitrate solution to remove hydrogen chloride, and then through slightly acid saturated potassium permanganate solution to remove sulfur dioxide. Some hydrogen chloride passed through the silver nitrate solution (which was partly used up by precipitating silver sulfite) and wa,s oxidized by the permanganate. The trace of chlorine was removed by washing the gas with a solution of sodium thiosulfate. Tests showed that thiosulfate removes chlorine quite completely with no formation of sulfur dioxide. After a second washing with permanganate, the gas was dried with phosphorus pentoxide. observed on the l?.,._OS' surface. A faint pink color was About forty cubic centimeters of colorless liquid (at -78°) were received. The gas was colorless, odorless, and neutral to moist litmus paper. The material was frozen and pumped on, then melted and the process repeated. Finally it was distilled from a dry ice trap in an all glass system into the Ra.man tube, leaving about two cubic centimeters as a last fraction. A small flask fitted with a ground glass joint and stopcock, was filled with gas to 200 mm. pressure for the electron diffraction pictures. Later some material. was lost when the dry ice bath level in the storage Dewar dropped, and another set of two runs was made. Raman plates and diffraction pictures were made with both preparations. After most of the Ra.man plates were taken, the melting point was determined with a calibrated thermocouple. An approximate vaJ.ue was obtained of 135.2°K. although the result the result was not very satisfactory due to drift. The gas was washed again through per- manganate solution and dried -- this time wi t h magnesium perchlorate. It was distilled four times from a dry ice trap, leaving about one cc. the first three times as a last fraction. The last distillation was directly into the adiabatic calorimeter of Drs. Garner and Osborne. The melting point was determined to be 135.9°:1: .20 K. with a calculated liquid soluble, solid insoluble impurity of .5 mole per cent. After the determination-, the liquid was dis- tilled into a Raman tube,rejecting three cc. as a first fraction and 10 cc. as a last fraction. A good thirteen hour plate then obtained was one of three measured. The melting point is in serious disagreement with the results of Trautz and Ehrman (151. 7±..5° K.) and Moissa.n. (152° K.) obtained with pentane thermometers. found at that temperature. No sign of a transition was Since our temperature scale is almost surely correct (See later section on calibration of resistance thermometers) and the adiabatic method well tested, the question of purity and identity must be examined. The gas density was determined by weighing a 280 cc. flask, first evacuated and then filled with sulfuryl fluoride. The results are compared with those of Trautz : This Thesis 24.7° D741.7 mm. 16.6° Trautz D Observed Cale. as perfect gas = 4.05 :t:.Ol g/l 4.08 g/l = 4.23 g/1 4.27 g/l 75t1-.6 mm. About a gram of gas was absorbed in a basic solution, acidified, and oxidized, part with hot nitric acid and part with hot acid permanganate. Q,uaJ.itative tests sh.01Ved large amounts of sulfate and fluoride, but no trace of chloride or phosphate. A quantitative test for SOa._ showed less than .1% to be present • .No Raman lines of any known, possible impurity are found. All plates have the same relative intensity for different lines as nearly as can oe judged. The sulfuryl fluoride is thus believed to be about 99.5% pure, aJ.thougb. a quantative analysis is projected to be sure. The Ba.man Spectrum of Sulfuryl Fluoride. Fifteen Raman plates were ta.ken of the liquid with exposures of from three to thirteen hours. Three plates were ta.ken using a sodium nitrite solution filter to prevent any ambiguity in assig,:i.ment of the exciting line . About twenty cu.bic centimeters of the liquid were contained in a standard Pyrex Raman tube mounted in a dry ice-alcohol bath. The Dewar vessel holding the bath was unsilvered on one side and through this sid_e the tube was illuminated by a water cooled Pyrex mercury arc. comparison spectrum. An iron arc was used as a The spectograph was a glass prism instrument with a dispersion of about 601 per millimeter. The plates were measured on a comparator and a plot of deviation from linearity determined from the comparison iron spectrum used to fix positions of the lines. A five hour and a thirteen hour plate were used for the final measurements. Microphotometer traces aided in identifying the lines and in the intensity estimations. listed in Table I. The lines found are TABLE I Raman Frequencies of Liquid Sulfuryl Fluoride. Frequency Intensity No. of Exciting Lines Scattered fr om ( cm. - 1) 6 4 g 3 0 l 10 5 885 6b 3 1109 lb l 1262 10 5 0 l 5 2 Measurements are ± l cm:- 1 for the sharp lines and ± 3 cm :- 1 for t he broad lines. -6- The Ra.man spectrum of the gas was photographed using the mercury resonance line from a quartz water-cooled arc and a quartz . prism spectrograph. Mercury vapor inside the spectograph case ab- sorbed much of the exciting line. The gas was contained in a quartz tube under ninety pounds gauge pressure. Four plates were taken with exposures of from ten to twenty houra. Longer expos- ures had excessive backgroun.d. Only the two strongest lines could be found aa sho~n in Table II. TABLE II Raman Frequencies in Sulfuryl Fluoride Gas. Frequency Intensity ( cm.-1) 859 10 1279 8 Measurements are± l cm.-l Discussion of Raman Spectrum. The structure of th• molecule is known from electron diffraction experiments (see next section of this thesis) to have sym.rnetry c v. 2 A normal coord ina.te treatment haa never been made., but the vibrations, resolved into those of symmetry coordinates, • b y ~poner. " 2 are given The characteristics and selection rules t·or the vibrations · are easily computed from te.bJ.es given by 3 Kohlrauach. The vibrations are pictured in Table III, . together with the selection rules. Assignments for aulfuryl chloride and sulfuryl fluoride are given in Table IV. Sulfuryl chloride is included as it ha.a not been previously done and as the polarization -7- data and comparisons are necessar y for the assignments. These assignments are made on the basis of empirical rules and analogy and are not to be considered as final, although they h) are probably correct. Most doubtful are the ~ ...L and the frequencies for sulfuryl fluoride. collected by Yost 5 ~ Much use was made of the data on the Raman spectra of volatile fluorides. The empirical rule that :M - F • 1.7 - 2.0 is seen to hold quite M - Cl well. 01 The ~ II frequency for sulfuryl fluoride is suggested from it and from ratios of what are presumably similar frequencies in thionyl chloride and fluoride. Polarization data for sulfuryl fluoride would aid considerably in the assignments. For example, the 1387 frequency, assigned as a combination, should be polarized. Since none of the vibrations are degenerate, calculations of thermodynamic functions would not be affected by interchange of assignments. REFERENCES • l. Trautz and Ehrman, J our. fur prakt. Chem. 142, .· 79 ( 1935) • 2. Sponer, 3. Kohlrauach, , 11 Der Smekal-Raman-Effekt 11 , Erganzungsband, p. 37-52 ( 1938) . • 4. Cabannes and Rousset, Ann. phys. 19, 229 (1933). 5. Yost, Proc. of the Indian Academy of Science, , VIII, 333 (1938). 11 Molekulspektren 11 , .Vol. II,, P• 87 (1936). - 8- TABLE IV Vibration Assignments Sulfuryl Fluoride Sulfuryl Chloride* 845 (10) 4o8 (10) P 1262 (10) 1190 (1) p 885 (6b) 390 (5) D v('L)..L 1492 (5) 1415 ( 0) D ~(l)II D5~ 214 (5) p v;{l) II ¼(Z-\ \\ v(l)...L 1 s{,.,\\ 544 (8) 556-63 (5) P 't/11_ ?456 (0) 280 (2) D ~(1'-L 388(6) 365 (1) D ~ v.; I I/ 1109 (lb) &t'Z.-11 Estimated intensity in(). --t- 1387 ( 0) P means polarized; D depolarized. *Measurements by Caban.nes and Rousset, Ref. 4. ? 576 (0) D TAELE III Vibrations of a Molecule of the S02.F~ Type - Symmetry Ca.v 0 ... "'"¥ 0 0 --~1)---te)-- z. 0 Vibration Movement of Atoms Cl)j ~(1)11 V:~11 ~~ ~ ~ yl'1__ C¼ ~ vtz-1 A p M1..l s-o II stretching ~ Jul_ ~ ~ o~l ,L' ~ 0 s-x II stretching B D My ' s-x ..L stretching B D Mxj s-o .l.. stretching A p M z @J; ? l s- x II bending I ¾ A p Mz.l s-o II bending I B D My l s- x .l bending I r ft, B D t Mxl s-o l. bending I ~ i M2I .f> i ~ ct /' ,. ~ ~~ / "'° b .f9J s:'n Description / p ~ 6 ~ / A ~ ~:' II ,,, , ~ ~ t- Type Section (Placzek) Rule I A D ia ltorsion bending fa, P means polarized; D, depolarized; Mx etc. indicates direction of electric moment produced; ia means inactive in infrared. -10- II Electron Diffr action Studies On Sulfury Fluoride and Thiophosphoryl Fluoride. [Reprinted from the Journal of the American Chemical Society, 61, 3264 (I 939) . ) [CONTRIBUTION FROM THE GATES AND CRE LLIN LABORATORIES OF CHEMISTRY, CALIFORNIA I NSTITUT E OF TECHNOLOGY, No. 736] The Molecular Structures of Su uryl Fluoride and Thiophosphoryl Fluoride BY D. P. STEVENSON 1 AND HORACE RUSSELL, JR. The relations between bond length a nd bond character in molec es co taining bonds of the type sometimes called se • olar are not well understood. Among the compounds of phosphorus and sulfur there are a great number of such molecules, some of which have been investigated recently. 2 •3 •4 In this paper we give the results of an electron diffraction investigation of the structures of two such molecules containing fluorine atoms as well as semipolar double bonds. Experimental.-The electron diffraction apparatus used in this investigation has been described by Brockway. 5 The camera distance was 10.84 cm., and the wave length of the electrons, determined in the usual fashion, 5 was 0.0611 A. Fifty cc. (S. T. P .) of pure thiophosphoryl fluoride was given to us by Professor H. S. Booth and Miss M. C. Cassidy' of Western Reserve University, who have described the preparation and purification of the substance. 6 The sample was transferred to a bulb of about 150 cc. capacity which could be attached to the apparatus by means of a ground joint. e preparation and purity of the sulfuryl fluoused will be described in a separate paper on Raman spectrum of this compound. 7 The ure of the gas in the sample bulb was approximately 200 mm. Interpretation.-Both the radial distribution method 8 and the correlation method 9 were used in interpreting the electron diffraction photographs. The modified radial distribution formula '°' sinl Snl D(l) = L, Cn - Cn = f(In, Sn) n (1) Sn has been discussed by Schomaker. 10 (1) National Research Fellow. (2) L. 0. Brockway and J. Y. Beach, THIS JOURNAL , 60, 1836 (1938). (3) K. J. Palmer, ibid., 80, 2360 (1938). (4) J. Y. Beach and D. P. Stevenson, J. Chem. Phys., 6, 75 (1 938) . (5) L. 0. Brockway, R,g. Mod. Phys., 8, 231 (1936). (6) H. S. Booth and M. C. Cassidy, Boston , A. C. S . meeting , September, 1939. (7) Horace Russell , Jr., and D . M. Yost , to be published . (8) L. Pauling and L. 0. Brockway, THIS J OURN AL, 57, 2684 (193 5) . (9) L. Pauling and L. 0. Brockway, J. Chem . Phys ., 2,867 (1934). (10) V. Schomaker, B'.lltimore , A. C. S. meeting, April , 19~9. -n- It has been remarked in the literature 9• 11 that the coefficients zizi in the simplified theoretic:al scattering formula ordinarily used (2) are not completely satisfactory when the atoms i and j are members of different rows of the periodic system, and that the more exact expression l(s) = LL(Zi -;- /i)(Z; - J;) si~;~i;s (3) should be used. Since the corresponding coefficients in (2) and (3) differ by as much as 30% when 4 ~ s ~ 10 for sulfuryl fluoride and thiophosphoryl fluoride, it was thought that it might be necessary to use (3) rather than (2) in the application of the correlation method to the interpretation of the photographs of these molecules. However, curves calculated according to (2) and (3) were found to be indistinguishable qualitatively and to have differences in the position of their maxima and minima of about 1% "thin the range of s indicated above. Since f s 12 the coefficients of (2) and (3) are the same within 2%, it is thus satisfactory to use (2) ther than (3) in interpreting the photographs of sulfuryl fluoride and thiophosphoryl fluoride when s > 12. When s < 12 the photographs were compared with curves calculated according to (3). Inasmuch as the two molecules under consideration are rather rigid, it was thought unnecessary to include a temperature factor 5 in the coefficients of the scattering formulas. Sulfuryl Fluoride.-The photographs of this molecule show seven measurable maxima, six measurable minima, and two shelf-like features. The first shelf, designated a, is a shoulder on the inner side of the first maximum. The second shelf, feature b, is on the outside of the third maximum. The intensities of the maxima decrease uniformly from the first through the seventh. The only irregularity in the decreasing depth of the tp.inima is that the fifth minimum is shallower than either (11 ) M . H . Pirenne , J . Chem . Phys., 1, 144 (1939). • Dec., 1939 STRUCTUR.d F SULFUkYL FLUORIDE AN 3265 THIOPHOSPHORYL FLUORIDE the fourth or sixth· minimum. The visually estimated relative intensities and the measured values of s(so = 4; sin~) of the maxima and minima are given"in columns 3 and 5 of Table I. A TABLE I Max. Min . Io Co 2 6 -7 8 5 -8 8 -6 6 -3 5 -6 4 -2 2 so s• s•f so sb sb/so 3.43 5.81 5 . 54 (0,954)C 5.57 (0,959)C 2 7.22 7.24 1.003 7.22 1.000 8 . 80 8.92 1.014 8 . 92 1.014 11.21 3 12.26 12.09 0.986 12.03 0.981 13.92 13.89 0.998 13 . 72 0 . 986 4 15 . 41 15 . 48 15 . 42 1.005 1.001 16.77 16.9¼ 1.010 16.91 1.008 5 17 . 82 18 . 12 18 . 00 1.017 1.010 19.10 19.10 1.000 19 . 01 0.995 6 20.51 20.49 0.999 20 . 50 1.000 22 . 09 21.89 .991 21.83 0.988 7 23 . 25 23 . 22 23.21 . 999 . 998 25 . 02 24 . 55 ( ,981)C 24.49 ( ,979)C Average of 11 features 1.002 = 0 . 007 0 . 998 = 0.008 5 10 -9 8 4 -6 6 -4 4 -2 3 -3 2 -1 a 1 2 b 3 4 5 6 7 • From curve for model with S - F / S - O = l.56/1.43. fl - O/S - 0 = 2.36 / 1.43. b From curve for model with S - F / S - 0 = l.56 / 1.43, F - O/ S - 0 = 2.38/1.43. • Omitted from average. I - e radial distribution function (1), calculated with the coefficients C0 of column 4, Table I, is sho n in Fig. 1, curve B. The first maximum at .48 A. is an unresolved average of the sulfur-r oxygen and the sulfur-fluorine distances. If one assumes the sulfur-oxygen distance to be 1.43 A., the distance found in sulfuryl chloride, 4 one may estimate the sulfur-fluorine distance to be 1.52 A. Since the term involving the fluorine-oxygen distance is four times as important in the scattering formula as the terms involving the fluorine-fluorine and oxygen- oxygen distances, the second peak at 2.37 A. probably corresponds closely to the fluorine- oxygen distance. If one assumes, as has been found to be the case for sulfuryl chloride, 4 that the sulfuryl fluoride molecule is a bisphenoid with symmetry C2v, there are three parameters which determine the form 'of the intensity curves. These may be conveniently taken as P1, the ratio of the sulfurfluorine to sulfur-oxygen distance, p2, the ratio of the fluorine- oxygen to sulfur- oxygen distance, and (3, the fluorine-sulfur- fluorine angle. The radial distribution function indicates that the value of p 1 is in the vicinity of l:52/1.43 and the valu~ of P2 is near 2.37 /1.43. Eighteen curves were calculated, 2 for models with the parameters p 1 and f>2 in the ranges l.48/1.43 ( P1 ( 1.60/1.43 and 2.32/1.43 ( P2 ( 2.44/1.43. - 12- B II 1 2 3 l, A. Fig. 1.-Radial distribution function, D(l) vs . l: Curve A, thiophoophoryl :fluoride; Curve B, sulfuryl fluoride. The vertical lines represent the internuclear distances found . The parameter (3 was assumed ·to be 100° for all thes~ cui,-ves. This is roughly the value found for the chlorine-sulfur-chlorine angle in sulfuryl and thionyl chlorides, and the qualitative and quantitative character of the theoretical curves is quite insensitive to variations in the value of this parameter. Some of these curves are shown in Fig. 2. Of these curves only the two with p1 = l.56/1.43 and P2 = 2.36/ 1.43 or 2.38/1.43 are. in agreement with the appearance of the photographs. Curves were then calculated for 1nodels with these values of p 1 and p2 with (3 = ·go and 110°. The curves with (3 = 90° are unsatisfactory because feature b disappears while for (3 = 110° the fifth maximum is too low. This is illustrated by curves E1 and & of Fig. 2. Quantitative comparison of the photographs with the curves for p 1 = l.56/ 1.43, (3 = 100° and P2 = 2.36/ 1.43 and 2.38/ 1.43 is made in Table I, columns 7 and 9. The agreement is quite satisfactory. The final results for the structure of sulfuryl fluoride are S-F = 1.56 = 0.02 A. S-O = 1.43 = 0.02 A. F-O = 2.37 = 0.02 A. LF-$--0 = 105 = 2° LF-S-F = 100 = 8° L O-S-O = 130 = 10° Thiophosphoryl Fluoride.-The photographs of thiophosphoryl fluoride were exceptionally clear, showing nine rings. The first and third maxima appear weaker than the second maximum, and the fifth maximum is weaker than the 3266 D. P. STEVENSON Vol. 61 ND HORACE RUSSELL, JR. TABLE II .~T:t ,,. 2 2.95 5 -9 -5 3.62 2 2 5.06 10 6 4.84 (0. 957)b, -2 -2 5.84 6.28 3 (1.075) 3 7.01 3 3 6.89 (0. 983) -9 7.67 -8 4 7.71 1.005 4 9.17 8 9.09 0.991 9 -5 -7 10.15 5 10.34 1.019 5 4 11.46 11.35 6 0.990 -6 6 -9 12.36 12.42 1.005 6 6 10 13.89 13.73 0.988 7 -4 -7 15.57 15.70 1.008 7 4 7 17.67 17.65 0.999 -3 8 -5 19 .36 19 . 13 0.988 8 3 5 21. 73 22.00 1.013 -1 -2 23.25 9 23.45 1.009 9 1 2 25.19 24,.90 0.988 Average of 12 features 1.001 ± 0.010 . P - F 1.52 • From model with - - = and LF-P-F = 99°. P - S 1.85 b Values in parentheses omitted from average because of unreliability of measurements. Max. Min. Jo Cn So 1 s ....... Fig. 2.-Theoretical scattering curves for sulfuryl fluoride: Curve A, p1 = l.52/1.43; P2 = 2 ..32/1.43; and {3= 100° Curve. 2.36/1.43 100° 2.40/1.43 100° 2.32/1.43 . 100° , Pl = l.56/1.43 2.36/1.43 100° 2.38/1 . 43 100° 2.40/1.43 100° 2.32/1.48 100° , Pl = 1. 60/1. 43 Curv.t I, 2.36/1.43 100° Curve J, 2.40/1.43 100° 2.36/1.43 110° Curve E1, P1 = l.56/1.43 Curve E2, 2.36/1.43 90° fourth and sixth maximit--' The fourth minimum is deeper than the thitd minimum and the sixth minimum is slightly deeper than the fifth minimum. The visually estimated relative intensities are given in Table II along with the values of Sobs.• T1i radial distribution function calealated according to (1), with the coefficients C0 , ti*n in column 4 of Table II, has four maxdha (see curve A, Fig. 1) at 1.50, 1.85, 2.33, and 2.90 A. These correspond to the four distances, phosphorus-fluorine, phosphorus-sulfur, fluorine-fluorine and fluorine-sulfur. The fluorine-phosphorus-fluorine and fluorine-phosphorus-sulfur ·an s computed from these distances are 100 arul:I 118°, respectively. I£ this molecul has tbe symmetcy structural parameters determine the -l3- s -+-. Fig. 3.-Theoretical scattering curves for thiopho~phoryl fluoride. i•, Curve A, / .;, 1. 48/1. 85 a "" 96° Curve B, r•., l '99° Curve C, • 102° Curve D, p = 1.52/1.85 96° Curve ', 99° Curve F, 102 ° Curve G, p = 1.54/1.85 96° 99° Curve H, .IAJ20 Dec., 1939 STRUCTURES.OF .r-r. ;, 1.52 ± 0.02A. PF, . OPFs. 1.52 ± o.m OPF,CI I.Al ± 0 .02 OPFCh 1.50 ± 0 .03 OPCI, PCb )C, , .• SPFfbc 1.51 ± 0 .02 3 SPct, 0 This'd'tslance was assumed. 3267 IJLFURYL FLUORIDE AND TlilOPHOSPHORYL FLUORIDE TABLE !IJ P - CJ P-Y Reference 12 and 8 2 . 01 ± 2.02 ± 2.02 ± 2.00 ± 0 .02 A. 0.02 0 .02 0 .02 ' 2.01 ± 0 .02 107 ± 2° 1.56 0.02 A. 1 .55 :1: o.O'J ·, .. 11)6 ± 3 ° 1.54 ± o.oa , 106 ± 3 ° (1.58}°;., . ,106±). 0 l0l t:_ 2° 1.85 ± (l.02 99.5 2° 107 ± 3° 1.94 ± 0 . 03 ~ 2 2 2 12 and 8 This research 4 ~, apptOpriate.models and the form of t4eth~oretical inte,i$ty curves. These parameters <;O!J· ve:ttiently taken as the ratio p of the phosphorh fluorine to the phosphorus-sulfur distance, and a the fluorine- phosphorus-fluorine angle. Curves were calculated for eighteen models, in which a was varied from .90-108° and p was varied from 1.54/1.85 to ).46/1.85. A selection of these curves is shown in Fig. 3. All models with 96° ?: a ?: 102° could be eliminated becau the appearance of the third maximum on the rves is not that of the photographs and they show an asymmetry on either the outer side of the sixth maximum or the inn side of the seventh maximum while on the photographs these maxima appear quite symmetrical. The model with a = 99° and p = 1.54/ 1.85 is unsatisfactory because its fourth minimum is not deeper than its third and the sixth minimum is not deeper than the fifth. The curves for models with p = 1.50/ 1.85 and a = 99° show a shelf on the inner side of the ninth maximum and are unsatisfactory for this reason. The curve calculated for the model p = 1.52/ 1.85 and a = 99° is in good qualitative agreement with the photographs. The quantitative comparison of this model with the photographs is shown in Table II. The final results for the structure of thiophosphoryl fluoride are P- F = 1.51 :1: 0.02 A., P-S = 1.85 :1: 0.02 A., L F-P- F = 99.5 ± 2°, and L F- P-S = 118 = 2°. Discu·ssion.-Table III presents a summary of the structures found for molecules of the type XaPY. The distances predicted by the covalent radius table 11 are P-F (single bond) = 1.74 A., P-Cl (single bond) = 2.09 A., P-O (double bond) = 1.57 A., and P-S (double bond) = 1.95 A. The structure of thiophosphoryl fluoride presents two interesting differences from the structures (12) L. 0 . Brockway and F . T. Wall, Tms JOURNAL, 56, 2373 (1934) . (13) L. Pauling and L . O. Brockway, ibid., 119, 1223 (1937) . of the other . molecules in tilt group that have been studied, a decrease of 0.09 A. in the phosphorus-sulfur distance and a small fluorinephosphorus-fluorine angle. An explanation for the first of these differences may be found from a consideration of the various resonating valence bond structures at may be written for molecules of this type. X X I X-P Y +x 1 I X III X II X x+ I II -x+p x-p Y I X IV I P-Y- x+ II x-p_y- Y X I x+ V VI 11 X I - xP==Y+ I X VII xVIII The much greater shortening observed for the phosphorus-fluorine distance, 0.22 A., than for the phosphorus-chlorine distance, 0.08 A., has been explained by the assumption of a larger contribution of structures IV- VIII to the resultant structure of molecules containing fluorine, made possible by the larger electronegativity 14 of fluorine. Since the electronegativity of oxygen is so much larger than the electronegativity of phosphorus and sulfur, structures VII and VIII are unimportant in th molecules XaPO. Thus we interpret the extra shortening of the phosphorus-sulfur bond in thiophosphoryl fluoride as due to considerable triple bond character made possible by the large electronegativity of fluorine and the small difference in the electronegativity between phosphorus and sulfur. Although several explanations for the small fluorine-phosphorus-fluorine angle in thiophos(14) L. 0. Brockway, J . Pllys. Clt,m., U, 185 (1937). 3268 D. P. STEVENSON AND HORACE RUSSELL, JR. phoryl fluoride present themselves, a satisfactory one cannot be advanced until more data have been acquired. The sulfur-fluorine distance in sulfuryl fluoride, 1.56 A., is 0.12 A. less than the single bond distance predicted by the covalent radius table. 13 This shortening is not as great as is observed in the fluorine derivatives of phosphorus. A similar situation obtains for the corresponding chlorine compounds of sulfur and phosphorus. It is of interest to note that, although there is much more shortening in the sulfur-halogen bond in sulfuryl fluoride than in sulfuryl chloride, the sulfur-oxygen distance in sulfuryl fluoride, 1.43 A., is just that found for this distance in sulfur dioxide, 15 sulfur trioxide, 3 and sulfuryl chloride. 3 In conclusion it should be remarked that the (15) V. Schomaker and D. P. Stevenson, to be published. -15- Vol. 61 large uncertainty in the fluorine-sulfur-fluorine angle, ± 8°, introduces an uncertainty of less than 0.1 E . U. into the calculation of "P.ii rotational entropy from the structure of the1~olecule. Summary The structures of sulfuryl fluoride and thiophosphoryl fluoride have been investigated by the electron diffraction technique. The interatomic distances and bond angles are: for sulfuryl fluoride, S-F = 1.56 :1: 0.02 A., S-0 = 1.43 :1: 0.02 A., L F-S-0 = 105 :1: 2°, and L F-S-F = 100 ± 10°; and for thiophosphoryl fluoride, P-F = 1.51 :1: 0.02 A., P-S = 1.85 :1: 0.02 A., L F-P-F = 99.5 ± 2°, and L F-P-S = 118 ± 2°. The relationship between the structures found and the resonating valence bond structures is briefly discussed. PASADENA, CALIF. RECEIVED SEPTEMBER 25, 1939 , III Low Temperature CaJ.orimetry Introduction to the Calorimetric Work. Description of Apparatus. During the last five years calorimetric apparatus has been constructed in this laboratory for the accurate determination of vapor pressures, ,heat capacities, and heats of transition, fusion, and vaporization. The first apparatus built was a non-adiabatic 1 calorimeter for the use to 60° K. described by Dr. Osborne and 2 Dr. Garner in their theses in 1938. The calorimeter was used for preIn 1938-39 Professor liminary measurements on trichlorofluoro-methane. Yost, Dr. Osborne and Dr. Garner constructed an adiabatic calorimeter for use from 15° K. to 300° K. This apparatus is described by in his thesis and in a paper soon to be published. 4 Dr. Doescher 3 The calorimeter was extremely satisfactory with advantages of a simple, reproducible temperature scale, freedom from heat leak corrections, and low liquid hydrogen consumption. The apparatus was used also for vapor pressures and heats of vaporization. acetylene, With it measurements were made on dimethyl 5 dimethyl sulfide 6 and trichlorofluoro-methane, 3, 7 and methyl mer9aptan 8 (listed in chronological order). In July, 194o, at the close of the work on methyl mercaptan, failure to close a stopcock after vapor pressure measurements allowed mercury to flow into the calorimeter. It was consequently dismantled in the fall of 1940 and the calorimeter taken out. The apparatus was then reassembled for the thermometer calibration described in the next section of this thesis. Also a new and faster vacuum system was built using a three stage oil diffusion pump of R.C.A. design backed by a Cenco Megavac pump. About the same time work was started on the design and construction of the heat of vaporization apparatus described in a later section. When the thermometer calibrations were completed, and after the construction of the heat of vaporization apparatus, the replated calorimeter was put in place with certain changes in the apparatus. A heater winding of a single layer of No. 30 constantin wire was put on the filling tube with No. 36 copper leads soldered on at 12 cm. intervals as both current leads and difference thermocouples. The lowest junction is 9 'l!Ull. above the top of the radiation shield and may be read absolutely. The heater leads from the shield to the calorimeter were made five cm. long instead of the six cni. previously. On tests it was discovered that the filling tube was getting very cold and that heat leaked along it out the calorimeter. The thermal dam provided by a bak:elite varnish joint at the shield was not good enough. The filling tube was then soldered to the top of the radiation shield and a foil radiation shield placed between the hydrogen can and the loops in the filling tube. The performance was much improved and the drifts were negligible. An interesting result of the improved vacuum system is that with the can constant at 15° K. almost no current is required . t o maintain the radiation shield at a fixed temperature anywhere in the range 15°-900 K. Both radiation and gas conduction losses are negligible. The heat capacity of the empty adiabatic calorimeter was run in April, 1941, and measurements on arsenic trifluoride wer e made immediately afterward. The Temperature Scale. The first temperature scale of the laboratory was a copper-constantin thermocouple calibrated indirectly against the Berkeley temperature scale. This was replaced in January, 1939, by a scale based on a platinum resistance thermometer of laboratory designation CT 2'6: • which has been used in all measurements in the adiabatic calorimeter through the arsenic trifluoride ones. This thermometer was constructed according to the designs of Meyers 9 and the specifications of the Bureau of Standards. was .762 cm. in diameter and eight cm. long. The platinum case It contained two separate coils, one of chemically pu.re annealed platinum wire (.0036 in. in diameter) and the other of No. 4o bare constantin wire (129-n-). Two platinum leads were gold soldered to the constantin coil, and 1 where they emerge through the glass head of the thermometer case, each was soft soldered to two copper wires which served as current and potential leads, respectively , to the heater. Each end ofthe platinum thermometer coil was welded to two platinum lead wires, the branch points in this case being inside the thermometer case. The thermometer was calibrated for use with the Callender-Van Dusen formula : R-Ro t - t t t 100 100 100 - - - + i° ( _ -l) ( _ ) + ;3 ( <1. Ro 1)( t 3 100 ) The Ro was measured in an ice bath; then t( was determined by comparison in a steam bath with a Leeds and Northrup resistance thermometer which had been calibrated three months previously by the Bureau of Standards. The ~ was determined at the mercury freezing point taken as -38.870° c. lO The thermometer was then mounted in the adiabatic calorimeter and the oxygen fixed point used to determine /3 . With this calibration, it was believed the temperature scale did not differ more than .02° from the international temperature scale. lO Below 90° K. thermometer CT 26 was calibrated 4 by means of a helium gas thermometer ( corrected to the thermodynamic scale) according t o the procedure described by Hoge and Brickwedde. 11 The ice point is taken as 273.16° K. with a maximum deviation of .02° Forty-four points were measured and an average deviation of .007° • Accordingly it is believed that in the range of 14°K. thermometer CT 26 measures temperatures that probably do not differ by more than.02° from the thermodynamic scale. The complete calibration of CT 26 was carried out between December, 1938 and June, 1939, by Professor Yost, Dr. Osborne and Dr. Garner. In June, 1940, through the courtesy of Dr. Huffman of the biology department we secured a set of three resistance thermometers made by Dr. Robert Ruehrwein in the same way as CT 26. given laboratory designations CT 2, CT 4, and CT 7. These were A.n all platinum thermometer, CT 22, had been sent to the Bureau of Standards in the fall of 1939 for calibration from 14° K. to 44o 0 a. and it was hoped that all thermometers could be calibrated against it. However, the thermometer had not come back, so other calibrations were made. After an ice point measurement, constants and in the Callender-- Van Dusen equation were determined by comparison with the same Bureau of Standard's certified resistance thermometer that was used for the determination of cf... for CT 26. Comparisons were made in baths of boiling water and boiling 0-toluidine (200° C.) As a check the melting po int of mercury was determined. for the three new thermometers. After those determinations five thermometers, CT 26, CT 2, CT 4, CT 7, and H 3, a thermometer belonging to Dr. Huffman and of the same type as the others, were mounted with stopcock grease in closely fitting holes in a cylindrical copper block, 3.8 cm. by 11 cm. with OT 26 in the center. The block was equipped with a heater and difference thermocouple and hung by fish line from the top of the radiation shield in the adiabatic calorimeter. Comparisons were made at the oxygen point ( -182.97° C.) and the constant,4 fixed for CT 2, CT 4, CT 7 on the basis of the oxygen point of CT 26. A few comparisons were made in the international temperature scale range; then a complete set of comparisons, at intervals of a few degrees, were made from 14° K. to 90° K. In these as for all other calibrations discussed, the switch board coil in Room 13, Gates, was taken as standard, R = 100.000 C ( 1+9xl0-o (t-25) ). A standard current of one milliampere was used. Measurements were made with an autocalibrated White potentiometer. Results of the calibrations are given in Tables I to III. Below 90° K. smoothed tables for the thermometers CT 2 and CT 7 were prepared. A table for CT 4 has not been made although data is available. With the smoothed tables CT 2 and CT 7 give the same temperatures from 14° to 90° K. as CT 26 within at least .01°. The greatest uncertainty in the CT 2,4,7 calibrations is in the values of th ~'s. to There was a spread in values obtained of .015 unit corresponding .0390 at -117° C. The cause of the spread is unknown as a high degree of constancy of temperature was achieved during the nations. determ i- The deviations at the mercury point and at -117° c. 1 indicate the 8 s are somn1hat low. An error in S would have its maximum effect (below o° C.) at about -117° C. However the observed deviations are only slightly more than the uncertainty in the international scale, 14,15 correct to ±: .007 unit. so it is believed the ~ 1 s are probably Thermometer CT 4 has a much higher b than allowed by the international temperature scale and shows marked deviations. A plot was made of its deviations from the average of the other three thermometers ( Figure l ) and from it a smoothed table constructed.. -21- Table I Constants of the Thermometers for the callender-Van Dusen Formula. Thermometer R0 (Int. ohms) X lOl. CT 26 CT 2 CT 7, CT 22 CT 4 26 .2820 45.7725 45.1234 22. 63930 46.0269 .39094 .391757 ,391751 . 391349 .390622 1.509 1.499 1.494 1.5118 1.551 .1264 .1211 .1233 .12143 .1336 38.870° b -38.871 ° -38.872° Freezing Point Hg.a -38.888° a The Fl? of Hg is -38,864° on the international scale. b Calibration point 12 13 , Table II Temp. oc. 23.s -117.9 -180.8 2 .002° .010 .001 7 .001° .022 .001 4 .000° .062 .oo4 Table III o... Values of 6 T -- t+I.e r.,.. . - b t Co..(- V,D. Temp. 0 c. 2 7 26 4 - .044° - .0450 - . t>4o0 - .062 a . As determined by the corrected helium gas thermometer. b.t calculated from the Callender-- Van Dusen equation. -22- Figure l \ ·. - :: --- --l----f1-----1f-'--+-'-::-_,+__:_:._1--:..J-~.:.. / : >-rl k.·:-tl--:-c_+_--_ " - -:+-t--+-'-----1 ----J..'...'.....'.+ i ! • . T. l ; .. ; . . • • • • • • ~+ . fl: . ·-, - : •:. I . I \ .. \. _· :.:i-------:t:--:-:--:-t-:-:-:±,--:--l-:------+,-----ii---.:.+,--------1.:..:..:.+:..:.:.J----l : •: . '.\--\ -r--:-t,--:--t .. ----:+,--:--t-----+-------'-j!.(),.........:+:--:..+.........:-1--:...1--:.:..:..:.-1-------l---~.:.j T-7'":·--t--:+-t-:..+--1--'-'--'--l-----l--~ -- · :r • • : · j1:, . •: ..: ,:_ ,.· ··•1. 1fil ·: • ; •- --1 .. :+ ;::_ I - - - -~- __ - ----!----·_i.;c.. : -- . -·:\ ::.: .: I -:-±--:--\-'-:-+:.:....\.........:+.:.:+.........:..J--:--:.:..:.j..........::.j..:....:..:..j-• : • : I: . _T _ ---1r :---t:-:-:--i·-+:-:-!-' :/' · : _: •,_:_1---.---:t-:-:-t:--,-,--t-:--+--'-1 ---,-,--F+..:.:i:..:..!..:.+.:-c-:+~.........:.;.:.:.: -- - • : : l - . ... .. .. ,i .. ! ! ~--- -- :-: ::'rf-1 -+-+-'--'----F+.:..:..:.: t--+:._+-----+ ··+.:.:- -~~: :~l-:-cl l -I -23- - - : I I . • The all platinum thermometer CT 22 has now been returned from the Bureau of Standards and its constants are listed in Table I also. No comparisons have been made between it and the other thermo- meters. Recently thermometer H 3 has been compared by Dr. Huffman , in his laboratory, with H 25, a thermometer just calibrated by the Eureau of Standards for the range 14° K. to 44o° C. Our oxygen point was closely confirmed. R/Ro of H 3 T by H 25 T by CT 26 . 246796 90.664° K• 90.662° K. Preliminary ~easurements in the range 36° - 90° K. indicate that the scales differ by not more than .025° at any point and in general are much closer. On January l, 1941, the switchboard coil used for all measurements to that date was replaced by a Leeds and Northrop standard 100 ohm resistor No. 288075, certified by them to be within .01% of its nominal value. ohms. For convenience it is taken as 100.000 ( l + ( t-25)8xl0 Accurate comparisons with two other Leeds and Northrop standard coils and the results of the ice point determination on CT 22 show the resistance differs from its nominal value by about .001%. Ice points for the thermometers were redetermined and the tables of resistance versus temperature adjusted accordingly. of ice point data is given in Table IV. -24- A summary -6 ) Tabl e IV Ro for ResistBJ~ce Thermometers in Int. Ohms. Thermometer CT 26 January, 1939 26 .2820 OT 2 46 .0269a 45.1234a 22.63930b December, 1940 January 6 . 1941 CT 22 22.6J35a 45.'7'725a August., 1940 CT 7 OT 4 26.2829° 45.7734° ( 46 .0278) d 45.1240° a. With switchboard coil Room 13, _Gates. b. Determined by the Bureau of Standards. c. With Leeds and Northrup coil No. 288075. d • . Value assumed. The temperature scale of this laboratory is seen to consist of two pa.rte, one being the thermodynamic scale (14° - 90° K.) and the otner being the international scale (above 90° K.). It is known that the international acale may differ by as much as .os 0 from the thermodynamic scale at some temperatures., out even tne sign of the 5 deviations is not agreed upon141 l • Data are therefore reported in terms of the international scale above 90@ K. and in terms of the thermodynamic scale below 90° K. -25- 22.6396c of Construction of a Heat AVaporization Apparatus . The adiabatic calorimeter is undoubtedly best suited for measurements of heat capacities and heats of transition and fusion. It was decided, however, that for the determination of vapor pressures and heats of vaporization the Nernst-Giauque lb type of calorimeter offers definite advantages. In particular the massive copper block with its hi gh heat capacity is more convenient to operate than a light radiation shield. By building a special calorimeter certain improvements of design for the particular measurement desired can be made. These are: 1) Heater windings on the filling tube with thermocouple junctions at frequent intervals; 2) Vertical vanes within the calorimeter so that the surface area does not change abruptly; 3) A spray cap to prevent loss of u.nvaporized liquid and to insure heating of the gas to a known temperature as it leaves the calorimeter; 4) A calorimeter fairly small in capacity; 5) Internal and external heate r s to check heat loss corrections and to see if the method of heating influences the result; 6) An absolute thermocouple at the top of the calorimeter to measure the temperature of the emerging gas. Accordingly such a calorimeter was built. Figu.re two shows the general plan of construction. The calorimeter is of gold plated copper with a wall thickness of .5 mm. and is provided with three separate heaters. The first, usually called I mm. l O T JIJcm.----+ ~ - - - 3 t n c h<! s ---->l Figure 2 the inside heater, is of constantin (147.J. o.. ) wound inside the resistance thermometer case; '.t he second, or bottom heater, is wound on the lower half of the calorimeter and the third is second. A single layer of No. just above the 26 bare copper wire is wound over these outer heaters and, after being impregnated with ba.kelite varnish and baked, was filed flat and smooth in a lathe. The junctions of two copper-constantin difference thermocouples are soldered to the smoothed surface, one at the center of each outside heater. The other junctions are joined and thermally anchored to the heavy ( 2 Kg.) silver plated copper radiation shield. An absolute thermocouple is soldered to the one mm. ID Monel filling tube at the point where it joins the calorimeter. With this arrangement of heaters and thermocouples, it is possible to vaporize the liquid in the calorimeter by means of internal or external heating and at the same time observe the temperature grad.iants along the calorimeter. With the absolute thermocouple the temperature of the emerging vapor is measured. The free volume of the calorimeter is 82 cc. It is provided with sixteen vertical copper vanes which are soldered to the reentrant tube holding resistance thermometer CT 4. Both the inside and outside of the calorimeter, including the vanes, are heavily gold plated. The Monel filling tube is covered by a single layer of No. 30 constantin wire to which is soldered at 25 cm. intervals No. 36 copper leads. The tube is soldered to the top of the block and below the block is a centimeter long winding on the tube of No. 30 constantin. This last winding is used only during the filling of the calorimeter. The filling tube can thus be heated in sections at will and the temperature determined at a number of points. -27- Energy input and temperatures are measured as for the adiabatic calorimeter. Current leads from the block to the calorimeter are six inches of No. 36 copper wire. Correction for heat generated in the leads is made, assuming half goes to the caloThe maximum correction is .cfo for the rimeter and half to the block. bottom heater and .15% for the inside heater. Other leads are from one foot to one and a half feet long. Vapor pressure meaS1.1rements are made by means of a manometer shown in Figµre 3. The desired amount of mercury is raised through the stopcock into the manometer by atmospheric pressure or by compressed air and the stopcock closed. The pressure to be meaS1.1red.is applied in the middle arm with gold leaf in the line to protect the calorimeter from mercury. The double low pressure arms of the manometer are connected to a mercury diffusion pump and are used in the leveling of the Hensen cathetometer used for the readings. MeaS1.1rements are made in terms of a standard Gartner steel meter bar ruled in millimeters. Fractions of a millimeter are read with the fine adjustment screw of the cathetometer (100 divisions to the millimeter). is .05 mm. or better. Precision of reading A check of the total length of the meter bar scale against the scale of the cathetometer which had been calibrated against a standard decimeter from the Bureau of Standards showed it t o be correct to .02 mm. The manometer and scale are enclosed in a wooden box with plate glass windows. The case temperature is measured with two calibrated mercury thermometers. Corrections are made to o0 c. tog: 980.655 from g = 979.57 for Pasadena, and for the meniscus height. Figure) 1. Mercury thermometers To Pum; 1 - - 2. Capillary tube to the calorimeter ~~ )· Steel meter bar 4. Pyrex tubing, 15 rrun. ID 5. Glas s Wool damper 6. Inlet for compressed air 7. Iron reserv oir f or mercury ·o ~ J J+ ~ ,,, -~ - -+- I "-- -- . .... ~-----' I -~ ~ -+ -! ..,, "> T -~ <>- ' 'I 'I' I I I I I I I ~- l I " 7 I I I I I I '- i!h -:: ---4-,--- I ' I ' I I - 6 7f /n l I -2Sa.- Operation of the Heat of Vaporization Apparatus. About 70 cc. of the compound to be measured is weighed either in a bulb provided with a stopcock or in a bulb provided with a thin tube to be broken with a magnetic hammer. This last type of container is wei@led first, then the compound distilled in through a ground joint, the joint sealed off and the container and joint weighed again. method of handling liquids has proved quite satisfactory. This A great advantage is that if delays make storage necessary, the compound is sealed in glass, avoiding slow attack: on or solution of the stopcock grease and resultant leakage. The material is next distilled into the previously evacuated calorimeter. At the end of the distillation the filling tube is heated above room temperature to clear it of all liquid. After the calorimeter is filled, it is cooled to about thirty degrees below the melting point and the percentage of impurity determined by heat capacity and melting point measurements. If the compound is satisfactorily pure, liquid heat capacity measurements are made over the range where the vaporization is to tBke place. About four points are necessary to fix the slope of the curve well. Heat capacity runs are made either adiabatically or by measuring the drift bfore and after the run. Tests have shown that the bottom and inside heaters give the same results. Calculation is more difficult with the bottom heater as two difference thermocouples must be read and a correction applied for their di fference. The inside heater is thus generally used for heat capacities. If vapor pressures are unknown, they are next determined, as the data is needed for heat of vaporization runs. At least two readings are made at each point with material being taken out betweBn readings. This must be collected and weighed if the runs are made at the beginning. If possible, therefore, the runs are left until last. After the preliminary work the heat of vaporization run is made. The procedure is similar to that described by Doescher 3 but differs in many details. The calorimeter is heated to the desired temperature maintaining the block and filling tube hotter than the calorimeter. Current sufficient to give the desired rate of evaporation during the run is started through an exercising resistance. Good rates are .2 moles per hour at a pressure of 760 mm. and .os moles per hour at l4o mm. put in place. A vaporization bulb is evacuatecl, weighed and Other methods of collecting the gas could be used, but the present method of allowing the vapor to bubble through a few millimeters of mercury into a trap cooled in liquid air is quite satisfactory. Constant pressure is provided by a twenty liter buffer jar of dry nitrogen connected to the trap. Trap and bu.bbler are a single unit equipped with two ground glass joints. With the bulb in place the buffer pressure is adjusted to the correct value. The block is adjusted until it is hotter than the calorimeter by .2° to .s 0 and the difference maintained constant during a drift reading lasting about fifteen minutes. Du.ring the drift the absolute thermocouple is calibrated against the resistance thermometer if necessary. Near room temperature, thermocouple tables already available are found accurate. After a good linear drift is obtained, the energy input is started. About the same time the stopcock to the bu.lb is opened and al~ernate readings begun on the two difference thermocouples at half minute intervals. After the initial rapid change in temperature of the calorimeter, readings are made less frequently. During the heating readings are made of the electrical energy going in and of the absolute thermocouple at 100 to 300 second intervals. The approximate gas flow is noted. The temperature of the filling tube is checked occasionally and kept several degTees above that of the calorimeter. A selector switch automatically turns off t he tube heater current when thennocouple readings are made. At the end of the heating period, frequent readings are again taken of the difference thermocouples. The absolute thermocouple is read until the flow of gas has stopped. The stopcock on the vaporization bulb is closed when the flow of gas bas slowed or stopped-- usually five to fifteen minutes after the heating has been stopped. After equilibrium is established a final set of drifts is taken. The vaporiza- tion bulb is carefully evacuated, the material melted and the bulb again evacuated. It is then weighed. Calculation of Results. The following resultant changes are assumed in going from the equilibrium state at temperature T1 to the equilibrium T~ . M-t C+Mt is heated under constant pressure from T1 to T :l. . -m,+ b:\1. is heated under constant pressure to Ts , there vaporized. and the vapor heated under constant pressure to T1' . th3 is heated from T1 to T5 there vaporized and the vapor heated under constant pressure to T;l.. . Ls the moles of material collected in the vaporization bulb. is increase in moles of material in the line from the calorimeter to the stopcock on the vaporization bu.lb. is the increase in moles of gas in the calorimeter due to the removal of m and m and the pressure change. M is the moles of liquid finally present in the calorimeter. is the moles of gas present in the calorimeter at T 1 . C is the calorimeter. is temperature of vaporizing surface. is temperature of emerging gas. This listing of the changes is not entirely precise. M+- M~ + m1 + m. 2_:t ~ are really heated from T, to Ts under the vapor pressure of the material rather than under constant pressure. However, the difference between the saturated heat capacities and the constant pressure heat capacities is very small. This, added to the fact that t he heat used in this way is only a few per cent. of the total, makes the error less than .01%, The chief question is whether the vaporization occurs under conditions approximating reversible ones. This can only be tested experimentally by varying the conditions of vaporization and is one of the chief reasons for the use of two different heaters in a set of runs. The agreement obtained in runs using different heaters, different amounts of material and different heating rates show the assumption of reversible- conditions to be a good one. In making the numerical calculations no single formula is used, only the basic changes listed above. The quantities needed for the calculation and the methods of their determination are listed below: direct measurement with the resistance thermometer. Energy Added from the known electrical power input plus the energy received. from the shield. This energy is the average area under the readings of the two difference thermocouples times the average of the fore and after drift constants. determined from the buffer pressure plus the mercury in the bubbler plus pressure drop due to gas flow in the filling tube. determined by graphical averaging of the absolute thermocouple readings weighted by the rate of flow. by weighing va.-porization bulb . m,_, M,- by measuring volume and temperature of the line and knowing the va;por pressure. by knowing the volume of the calorimeter and the amount and density of the material present. M by weighing material before it is put in the calorimeter. Heat Capacities determined. experimentally for the calorimeter and liquid. Gas heat capacities are either calcu.lated, found in the literature or guessed. They need be known only to 10ft, or so . From these quantities we find the amount of heat needed to vaporize m1+ ~ + m3 moles of material at temperature T s . Agreement of runs made with different temperature gradients along the calorimeter show the heat leak corrections are ma.de correctly. From consideration of possible errors and from the experimental results obt ained, we conclude that this is the most accurate method and apparatus available for determination of heats of vaporization. favorable conditions an error of less than .06% is made. -33- Under References 1. D. W. Osborne, Ph.D. Thesis, California Institute of Technology, , ( 1938) • 2. C. s. Garner, Ph.D. Thesis, California Institute of Technology, ( 1938) • 3. R. Doescher, Ph.D. Thesis, California Institute of Technology, (1940). (See also the next reference). 4. Yost, Garner, Osborne, Rubin and Russell, to be published in J.A.c.s. 4. Yost, Garner and Osborne, J. Chem. Phys.~, 131 (1940). 6. Osborne, Doescher and Yost, J. Chem. Phys • .§., 506 (1940). 7. Osborne, Doescher and Yost, _to be published in J.A.C.S. 8. This thesis, Part IV. 9. Meyers, J. Research Bureau of Standards, _Jt, 807 (1932). 10. Burgess, J. Research Bureau of Standards, 1, 635 (1928) • 11. Hoge and Brickwedde, J. Research Bureau of Standards, 22, 351 ( 1939). 12. Heuse and Otto, Ann. d. Physik (5) 2,, 486 (1931). 13. Wilhelm, Bulletin Bureau of Standards 13, 655 (1916) (a correction of .009° must be applied to the published result to include effect of the term) • 14. Reuse and Otto,. Ann. der Physik (5) 14, 181 ( 1932). 15. Keeaon and De.mmers, Phyaica _g, 1051, 1080 (1935). 16. Giauque and Egan, J. Chem. Phys. ,£, 45 ( 1937) • IV Calorimetric Data on Methyl Mercaptan. Calorimetric Data on Methyl Mercaptan. An accurate calorimetric value of the entropy of methyl mercaptan was desired so that a comparison could be made with the value calculated from spectroscopic and structural data. It was e:xpected, and found, tbat evidence would be gained for the existance of a potential hindering rotation about the carbon-sulfur bond. All calorimetric measurements, except the final heat of vaporization ones, were made in the adiabatic calorimeter using thermometer 26. The heat of vaporization was determined in the heat of vaporization apparatus using thermometer OT 4. The ice point is taken as 273.16° K. Resistance is in international ohms and energy is expressed in the defined calorie ( l caJ.orie = 4.1833 int. joules). Vapor pressures are given in international millimeters of mercury. A description of the apparatus and methods of measurement is given in a previous section of this thesis. Correction of heat capacities for vaporization into the small gas space and for premelting were made when necessary. Heat capacities are accurate to 1% or better below 30° K. and to .1% above that temperature, except in the range between the transition point and melting point ( 137.6° - 150.2° K}where the error may be several per cent. In making corrections, the liquid densities of Ellis and l Reid • were used to get the specific volume: V : l .11754 ( 1 + .0016239t) tin° O. Preparation of Material. For the purification of methyl mercaptan there was constructed a one and a half meter, helices-packed, vacuum jacketed column. The condenser, set just above the vacuum jacket, was an insulated metal can seven inches across and ten inches deep equipped with a stirrer. This condenser was filled with crushed ice. the temperature at 0° C. However, the stirrer did not hold It was found best to set the stirrer at a constant speed and merely add ice at the top to maintain a constant ice level. Under these conditions pressure fluctuations were reduced but a slow change was observed. The average distillation temperature was 3.5° c. Two bottles of one hundred grams each of methyl mercaptan, obtained from the Eastman Kodak Company, were purified. The bottles were distilled separately; the procedure being first to pour the liquid into a glass trap equipped with stopcocks lu brica.ted with l\piezon L grease as were all other stopcocks used. This was done under a hood. methyl mercaptan sme1ls like spoiled cabbage. In fairly high concentrations In low concentrations t , he odor becomes more like that from a distant sktmk and is quite persistant. With proper handling, however, the material is quite easy to work with. After the transfer, the glass trap was sealed to the pot of the still through a two foot P~ Os- tube, the mercaptan frozen and the air pumped out. Frozen in liquid air the mercaptan was pink; melted it was yellow. Upon distilling it into the pot, most of the yellow remained in a high boiling liquid. The first two sections of the Pi o., tube turned turned yellow; the mercaptan appeared to react with phosphoric acid slightly, but not with P~O_, . The distillation was made at a reflux ratio of 20:1. The first and main fractions were not pink when frozen in liquid air but the last one was. It was also observed tbat on breaking open one of the last fractions while frozen that a pink color quickly appeared in the upper layers of solid. The nature of the pink material is not known. -35.. The middle fractions from both bottles were combined (115g) and distilled again at a reflux ratio of 30:1. About ninety, - four grams ( 1.9462 moles )of purified material were received in a weighed bulb equipped wi t h a fragile tip to be broken by a magnetic hammer. The bulb was then sealed off. Purity of Material and Melting Point. The mercaptan was distilled into the adiabatic calorimeter and a premelting curve run on it. The results showed .052 mole per cent. impurity. Per cent.melted Equilibrium Temperature Calculated Temp. 150.002° K. 150.108 150.129 150.1a8 150.1 3 10.3 31.4 50.5 69.6 88.8 150.002° K. 150.109 150.129 150.138 150.143 Melting point is 150.162 ±. .02° K. The calculated temperatures come from the freezing point lowering equation. ~ T - To - R T0 ~ X %nielted x: mole per cent. impurity. After these measurements the filling tube was cleared of vapor by admitting suddenly about an atmosphere of helium. After the calorimeter was coQled to 90° K. all but 8 mm. of the helium was pumped out. Vapor Pressure Measurements. The vapor pressure of methyl mercaptan was measured after the heat capacity runs were finished with the results given in Ta.ole I. TABLE I T ( oK.) Observed Pressure ( int. mm. Hg ) Calculated Pressure (mm.) Obs, Cale. P (mm.) 221.880° 249.288 263.686 273.1~9 279.137 41.22 201.64 399.43 399.47 760.74 41.20 201.48 399.43 599.63 760.73 .02 .16 .00 -.16 .Ol Calculated pressures are from the equation log ,o P mm= 18.27439 - 176~• 05 ~ 3.70248 log ,0 T The calculated boiling point is 279.11° K. Heat Capacities. Heat capacities were run from 15° K. to the boiling point, 279,11 0 K. The first points in a series of ru.~s beginning at 90° K. were low due to supercooling of a few per cent. of the high temperature form of the solid. Care was then taken to hold the calorimeter just below the transition temperature for some time before cooling it farther. Results were then quite reproducible. Two other series of points were taken over the low temperature range (below 137° K.). The complete results, exc~pt for the low points first mentioned, are given in Table II. a smooth curve. Table III shows the heat capacities as taken from Figu.re 1 shows the shape of the curve. The Transition Point. At about 137° K. a transition was found which went extremely slowly. Effects were complicated also by appreciable premelting in this region. Four series of runs were made in an effort to get equilibrium and to fix, if possible, the temperature of the transition. In investigations of such transitions as these an adiabatic calorimeter is especially desir able as the time of waiting for equilibrium -37- 24 24- - ---,._, 20 /& / / ·:,,r, 1 ~ $ I _ _ ,.__,_,_ - ~- "~ - - - J ,r. ~~ 0 ~ /' r;,_ ,2 /< I J / l ~ lv.,;__ // / . 8 4- I / I I 8 I ' 4 I 0 0 0 80 '-r /20 160 200 T , ol< He~t Capacities of Methyl Mercaptan -37a- .320 TABLE II Molal Heat Capacities of Methyl Mercaptan . Run No. 55 72 56 73 57 74 58 59 75 60 76 61 77 62 78 63 ii80 65 81 66 82 67 83 68 69 84 35 70 36 71 37 38 39 40 41 11 42 12 13 14 15 16 T ~T o K. ~l:"!'8X. 14.97 15.82 16.47 17 .48 17 .98 19.35 19.49 21.17 21.50 23.26 23.94 25.63 26.57 1.461 1.490 1.522 1.823 1.515 1.912 1.499 1.855 2.388 2.327 2.502 2.419 2.747 2.655 2.861 2.950 3.409 2.827 3.495 3.627 3,813 3.655 4.191 3.688 4.751 4.107 4.572 5.165 5.155 5.076 5.134 5.061 5.139 5.175 5.206 5.272 5.336 5.4o5 5.4o2 5.151 5.376 5,594 5.4o8 5.248 28.17 29.37 30.97 32.51 33.86 35.96 37.09 39.61 40.73 43.61 44.4o 48.08 48.30 52.64 53.04 56.72 57 .46 61.86 62.53 67.00 72.15 77.34 82.58 87 .89 88.84 93.26 94.12 99.38 104.87 110.37 115.70 °K. Co caf.Jdeg, / o. 773 0.885 0.983 1.126 1.226 1.453 1.476 l. 773 l.835 2.168 2.300 2.641 2.831 3.155 3.396 3.719 4.022 4.285 4.690 4.902 5.352 5.555 6.038 6.170 6.763 6.798 7.435 7.486 7.948 8.038 8.548 8.625 9.106 9.600 10.094 10.510 10.92a 10.99 11.332 11.384 11. 738 12.099 12.466 12 . 811 mol. TABLE II . Cont. Molal Heat Capacities of Methyl Mercaptan. Run No. T o K. 25 17 26 18 27 19 45 28 49 a~ 30. 52 31 53 85 33 86 34 87 88 89 90 91 92 93 94 95 96 97 98 117.20 120. 87 122.29 125.90 128.06 130.81 133.03 133. 21 133.22 137.6 14o.42 14o.6o 141.09 143.~5 145.88 146.57 150.16 154.16 155.62 160.50 162. 07 166.80 174.47 184.11 194.31 204.51 214.63 224.68 234.63 244.48 254.23 263.89 271.06 279.11 A T °K J4,p!1ex. cP cal. /deg. 5.169 5.098 5.009 4.968 4,873 4.84o 4. 711 5.425 5.344 12.890 13.153 13.231 13.457 13.614 13.795 13.949 13.972 13.963 Transition 3.163 .165 .938 2.902 4.64o 3.34o a 1487 14.97 15.66 16.62 16.98 17.47 not corr. for premelting 21.27 21.27 21.17 21.27 21.27 21.17 21.12 21.12 21.07 20.97 20.87 21.07 20.97 20.87 20.80 20.75 20.71 20.70 20.73 20.81 20.88 21.00 21.13 M.p. 6.350 6.457 6.317 6.449 6.293 9.033 10.248 10.235 10.164 10. 083 10.003 9.903 9-798 9. 709 9. 599 4.752 B.p. ao.so 20. 75 20. 71 20. 70 20. 74 20.82 20.89 21.02 21.15 A few extra pts. are not included in this table. - 39- mol. TABLE III MolaJ. Heat Capacities of Methyl Mercaptan. T Op o K. 15 20 25 30 35 4o 45 50 60 70 80 90 100 110 120 130 137 .6 140 150 150.16 160 170 180 190 200 210 220 230 240 250 260 270 280 cal .deg:- 1 mole-l . o. 777 1.565 2.515 3.523 4.508 5.425 6.265 7.053 8.333 9.411 10.300 11.081 11. 785 12.43s 13.oso 13.74o transition 14.54 16.58 m.p. 21.18 21.03 20.91 20.83 20. 77 20.73 20.70 20.72 20.76 20,85 20.97 21.13 21.30 -40- Corrected f or } premelting. is limited only by the patience and endurance of the operators. In this case as long as eight hours were spent at a single point without assurance of equilibrium. E:xmnples of typical observations are given. 89% Transformed. 18% Transformed Time after Heating ( hrs:min.) 137,591 .561 .547 .528 .519 .512 .509 .506 : 31 :41 1:06 1: 38 2:08 2:38 3:08 3:38 4: 38 5: 26 .503 .499 .497: Time after Heating (hrs;min.) T o K. :05 138.017 137,908 .864 .756 .734 .716 .707 .696 .692 .688 .685 :10 : 30 1:00 1:30 2:30 :J.4 a5:00 : 27 5:30 6:00 On the basis of these and other points the transition temperature was taken as 137.6 ± .15 ° K. The heat of transition is given in Table IV. TABLE IV Run No. Temp. Interval Heat Input Cal/mole Cal/mole Incl .premelting Cal/mole 29 135.925 - 138.618 91.04 38.38 52.64 46 135.382 - 139.021 104.19 51.91 52.28 50 135.887 - 138.833 94.57 42.04 52,53 Avergge 52.5 Premelting at the upper temperature made this determination somewhat difficult. -41- Heat of Fusion. The heat of fusion determination is given in Table V, assuming the melting point as 150.162° K. TAJ3.LE V Temperature Interval A HF cal/mole 148.196 - 152.390 1411.2 148 .. 238 - 15;,. 519 1411.6 Average 1411 .4 Heat of Vaporization. The heat of vaporization was determined as described previously in this thesis. Tests showed that no appr.e ciable amount of mercaptan was absorbed in the stopcock grease. Rubber bands were used to hold the stopcocks in dur i ng the weighing of the liquid mercaptan. Two runs were made using the adiabatic calorimeter, but disagreed badly. When the new heat of vaporization apparatus was completed, five runs were made with excellent precision. in the first set was given no weight in the average. are _given in Table VI. The low run Results TABLE VI. Heat of Vaporization of Methyl Mercaptan. Run No. Heater Moles Vaporized Time required 1a Inside .17335 3000 5815b 2a Inside .19343 3600 5871 l Inside .17313 3600 5876 2 Inside .174o1 3600 5868 3 Bottom outside .13943 4200 5869 4 Inside .11630 3000 5874 5 Bottom outside .11075 3000 5876 Average 5872 :r.. 4 6 Hv at 279.11° a usingitdiatbatic calorimeter b given no weight in avergge. The heat of vaporization as calculated from the vapor pressure and Berthelot 1 s equation of state i s 5866 cal/mole including a Berthelot correction of 161 calories. Critical constants used were Tc~ 1'-70° K ~ and Pc : 71.4 (;Utm s. The Entropy of Methyl Mercaptan. Below 15° K. a Debye function was used for the extrapolation of the heat capacities. The fit with experimental points is shown belowt _, I-• T Cp C~ I. tea .,., e oK. observed 14.97 • 773 • 773 15.82 .885 .889 16.47 .983 .981 17 .48 1.126 1.128 17 .98 1.226 1.203 19.35 1.453 1.412 19.49 1.476 1.433 21.17 1. 773 1.691 21.50 1.835 1. 741 3 R X D (122.8 ° ) T The correction of the gas to the ideal gas state was made using the thermodynamic equation: and Berthelot 1 s equation of state. The criticalconstants were taken as Tc = 470° K~ pc = 71.4 atm. 2 . 5 ideal - 5actual = 27 RTc P/32 T Pc The correction is The entropy calculations are summarized in Table VII. -44- = .11 cal.deg - 1 mole7' TABLE VII. Entropy of Methyl Mercaptan. 0-15°K. Debye function O • 122.s 0 .331 15° -137.6° K, graphical i4.679 52.5/137.6 .350 Transition 137.6-150.16° K. graphical 1.339 Fusion 1411.4/150.16 9.399 150.16° -279.11° 12.968 graphical Vaporization 5872/279.11 21.036 Entropy of actual gas at b9i ling point 60.047 Correction for gas imperfection Entropy of ideal gas at 760 mm. and 279.110 °K.. .112 60.16 cal deg - 1 mole-\ Discussion of Transition. Data on a number of low temperature transitions in the solid phase have recently been obtained. 3 As yet no very general or satisfactory theory has been proposed. 3 It is of considerable interest to obtain information on a large number of such transitions to aid in the development of a better theory. The transition in methyl mercaptan appears to be isothermal at 139.6° K., although it progresses very slowly. over a range of a few tenths degree. 50.5±.5 cal/mole. At most it extends The heat of transition is It is possible to supercool to 90°K. a few per cent., at least, of the high temperature form. No deliberate attempts were made to supercool the high temperature form. No further experimental information is available concerning the transition. A transition has been found also in methyl alcohol. It has been investigated calorimetrically by Parks 6 and Kelley 4 , and with dielectric constant measurements by Smyth and Mc Neight 5. The calorimetric experiments found the transition to occur over a range of about 15° with a heat of transition of 154 cal/mole, Temperatures of 161.1° tures, 6 and 157.4° 4 were picked as transition tempera- The large range found was probably due to the non-adiabatic method used, Dielectric constant measurements show a sharp transition at 159.9° with rising temperature.5 falling temperature. Hysteresis was observed with No transitions have been found in higher alcohols by either calorimetric or dielectric constant studies. Smyth and Mc Neight believe that those results exclude the possibility that -46- the transition merely involves the ro t ation of the hydroxyl group. They postulate a rotation of the whole molecule about an axis along the C-0 bond. The methyl mercaptan transition is believed to be quite similar to that in the methyl alcohol and no better explanation can be offered at present than that of Smyth and Mc Neight. Spectroscopic Entropy. The spectroscopic entropy was calculated using the following constants: l cal.s 4.1833 Int. joules -l R = 1.98708 cal.deg. k = 1.3805 x 16 -\b N = 6.023 ~ 10 he -I erg deg -I molecule -I !l.3 1 h - 6.624 X 10- J- R mole erg sec. : 1.4384 cm.de g. The translational and vibrational contributions to the entropy are calculated by the standard formulae given below. The vibrational degree of freedom associated with the rotation of t he methyl group is omitted, however, and its contribution calculated separately. Strans :: 3/2 R ln M+. 5/2 R ln 1r - R ln p atm. R ln ,t5/ 2 ( 2-rr k) N 5/2 - 2. 314 3/ 2R =- 2.Jl:4 cal.deg.~I -\ mole h3 Svi b. = ln Q, -t T ~ l a ~ Q, = 2n e - ( n+ ½ ) hcw/kT The vibrational contributions are given in tables by Gordon and Barnes 7 for vard.ous values of he w/ k T. The calculation of the rotational contribution is more difficult. It is customary to calculate moments of inertia for the molecule as though it were rigid. The contributions to the entropy of the overall rotation is calculated by the usual formula using the symmetry number of the rigid molecule (CJ • l for CM~ SH). 5 rot = 1 / 2 R ln i ly Iz..+ 3/2 R ln T - R ln6 -5.385 where l's are in At.wt. units xi , The contribution of the internal rotation is then evaluated separately. In the cas e of free rotation methods of calculation have been given by Eidinoff and Aston ij and by Kassel 9 . In the case of hindered rotation a special treatment is necessary. One has been given by • t.ions b etween th e P 1• t zer lO who neglecte d angular momentum 1nterac rotating top and the framework. Re gives tables of functions which are quite accurate for high potential barriers, but which may be in error for low barriers. Crawf ord 11 has presented a treatment for hindered rotation which includes angu.lar momentum coupling effects and is suitable for molecules where the top axis does not pass through -48- the center of gTavity of the molecule. If the top is not too large relative to the rest of the molecule the treatment is quite satisfactory. In his method a parameter.!l. is used which depends on the orientation of the top relative to the principle axes of the molecule and on the relative mass of the top to that of the framework. Foril. : 1 his treatment is the same as Pitzer 1 s. For ..0. small, approxi- mations made in Crawford's method become serious and further investigation of them seems necessary. Both treatments use a cosine form of hindering potential. The molecular weight of methyl mercaptan was taken as 48.102. The vibrational assignments of Thompson and Skerrett 12 were used. The frequencies and their degeneracies are 3000 (2), 2870, 1335, 1475 1430, 704, 1060. 957. 2597 and 863. Distances taken were 0 S-C • l.83 A as determined by electron diffraction, S-H ~ 1.35 R, '- C - s- H = 100° and a tetrahedral methyl gToup with C-H • 1.09 1( . The parameter .0. : .34921 and a further investigation of Crawford's method seems desirable but has not been completed. calculations is given in Table VII I. A sum.mary of T.AELE VIII Entropy of Methyl Mercaptan at 279.11° K. an~ l atmosphere. Translational 37. 207 e. u. Rotational (omitting internal rotation) 20.333 Vibrational _.5.9_g_ Total omitting internal rotation 58.137 e. u. Experimental entropy . 60.16 Contribution of hindered rotation as experimentally determined 2.02 e. u. Contribution from free rotation 2.60 e. u. Contribution (~f barrier potential of l ltGO calories is taken (Pitzer' s Tables.) 2.02 e.u. The height of t he barrier would be raised somewhat by using Crawford 1 s treatment. We can thus say·that the data presented adds to the experimental evidence for hindered rotation and that a . lower limit to the barrier is about 1 480 calories. References 1. Ellis and Reid, J.A.C.S. 2, 1674 (1932) 2, Berthelot and Brun, J. chim.phys. 21. 143 (1924) 3, Eu.c~en, Zeit fur Elektrochem., ~' 126) 1939) 4. K~ Kelley, J.A.C.S. 51., 779 (1929) 5, Smyth and Mc N6 ight, J.A.C.S. 5§_, 1597 (1936) 6. Parks, J.A.C.S . .!±7., 338 (1925) 7, Gordon and Barnes, J. Chem.Phys._J:._297 (1933) S. Eidinoff and Aston, J.Chem.Phys._j_, 379 (1935) 9, Kassel, J. Chem. Phys. ~. 276 (1936) 10. Pitzer, J. Chem, Phys. 5_, 469 (1937) 11. Crawford, J.Chem. Phys. 8 , 274, (1940) 12. Thompson and Skerrett, Trans. Faraday Soc. j§_, 812, (194o) V Calorimetric Data on Arsenic Trifluoride. ' Calorimetric Data on Arsenic Trifluoride. Arsenic trifluoride was selected, for two reason s , as being an interesting compound to investigate. One was that it is a relatively simple molecule for which an accurate value of the entropy could presumably be calculated from electron diffraction and Raman spectrwn data. The comparison with the calorimetric value would check: to a certain extent both the frequency assignments and the experimental method. In such a molecule the entropy would be expected to approach zero at absolute zero. The other reason was that the solid bad been observed to emit light on cooling to liquid air temperatures. The investigation would reveal_ whether any thermal effects were associated with the phenomenon. Preparation of Material. 1 the directions of Ruff Arsenic trifluoride was made according to from C.P. sulfuric acid and finely mixed reagent grade arsenious oxide and calcium fluoride. was carried out in an all glass system. The preparation About fifty grams of dried sodium fluoride in the receiver served to remove hydrogen fluoride. After the collection of the crude product, the receiver, which was equipped with a fragile tip, was sealed off. It was allowed to stand for two or three days to allow the arsenic trifluoride time to react with possible water. fluoride formed. The sodium fluoride was to remove the hydrogen This procedure was quite successful and at no time have we had evidence of pressure being built up in a sealed bulb of arsenic trifluoride, Dry arsenic trifluoride does not attack glass. The material was next distilled in an all glass sy~tem without stopcocks, through a two foot sodium fluoride tube into the pot of a five foot,g'hass helices packed column. -51- The column was insulated and wound with heater wire. In order to get a variable reflux ratio without the use of a stopcock, the take-off tube was sloped up and equipped with a condenser, then sloped down and supplied with anotber condenser. Through the first condenser, water from a constant temperature bath (about 50°) was circulated by a small centrifugal pump. second cold water was passed. Through the The reflux ratio was varied either by changing the speed of the pump or the temperature of the bath. This system worked reasonably well, although it had to be watched carefully. Distillations were made at a reflux ratio of 10:l. A ground glass joint aJ.lowed receivers to be changed without admitting moist air. An inner tube joined by a ring seal above the joint prevented any liquid from coming into contact with the grease (on the outer third of the joint only). Apiezon M grease was early found to be appreciably attacked, but Apiezon L was only very slowly attacked and was used throughout. Distillations were made at atmospheric pressure with a series of drying tubes in the open line. During the course of the distillations the boiling point did not change more than .01°. There was no evidence of a more volatile impurity in the sodium fluoride treated material, but the high boiling portion left in the still had red-brown particles of solid in it. The purified arsenic trifluoride was a colorless, very mobile liquid which froze ~o a white crystalline solid in a dry ice bath. On either freezing down to, or warming up from, liquid air temperatures loud, cracking noises are made and flashes of blue-green light emitted. Under proper conditions light is emitted also above -7so C. but not in general. On trying to put the first purified sample obtained into the calorimeter, the trap broke while the solid was warming up. -52- Solid arsenic trifluoride has apparently a high coefficient of expansion and extreme care is necessary to prevent breakage. Even with care the trap broke - five in the course of the pre paration. The recovered arsenic trifluoride plus more cru.de material was a gain distilled, this time cutting a first fraction of 30 cc., a ~~ond of 70 cc. and a third of 60 cc., leaving 4o cc. in the pot as a last fraction. The distillation proceded as before. The Vapor Pressure of Arsenic Trifluo r ide. The weighed third fraction was distilled into the evacuated heat of vaporization apparatus without incident. A melting point curve gave a liquid soluble, solid insoluble impurity of .04 mole per cent. made . Vapor pressure measurements were then Consiierable trouble was found in getting good reproducibility. Later a slight leak was fpund in the line, but as material was pumped out at each point, no serious error is thought to have been made. Results are in Table I. -53- TABLE I Vapor Pressure of Arsenic Trifluoride. T p obs. (Int.mm.Hg) p calc. 265.019 2s.90 29.00 .10 26s.235 36.15 35.85 -.30 272.058 45.68 45.67 -.01 275.271 55.22 55.53 .31 280.182 74.20 73.88 -.32 288.055 112. 70 113.10 .4o 291.816 136.96 136.90 -.06 291.879 137 .46 137 .30 -.16 oK. (mm) p calc.- p obs. (mm) Calculated values are from the equation: log to Pmm, 61.37969 - 4149.784 - 18.26403 log , 0 T. T Calorimeter Data. Calorimeter work, except for the heat of vaporization, was done in the rebuilt adiabatic calorimeter using thermometer CT 26 measured with Leeds and Northrop coil# 288075. The heat of vaporization was measured in the special apparatus for that purpose. In all results T0 = 273,16° and energy is expressed in the defined calorie (1 calorie= 4.1833 int. joules). Heat capacities are accurate to 1% or better below 30° K. and to .1% above that temperature, except for the liquid heat capacities which may be in error about .2j,. Corrections were made when necessary for vaporization into the small gas space and for premelting. In these corrections,the density equation of Thorpe 2 was used. d 'tt : 2.oc6 59 - . 003839 t + .00000 435t (o0 to 60° c.) Some evidence indicates these densities a re low 1 2 ' ,but no appreciable error in the entropy would result. Heat of Vaporization. Four runs were made on the heat of vapor1zation. Results are in Table II. TABLE II Heat of Vaporization of Arsenic Trifluoride. Run No. Heater used. Mole vaporized Time sec. 6 Hv at 292.50° K. cal/mole l Inside .037722 3600 8531 2 Bottom .042432 3300 8566 3 Bottom .058841 2500 8529 4 Inside .048593 3000 8565 The runs fall into two sets. The explanation is almost surely as follows: Runs l and 2 were done one day and Runs 3 and 4 the next day. Only one container was used each day with the second sample being collected directly after the first. Due to the marked propensity of arsenic trifluoride to break traps, the collected material W--ds not pumped on, melted, frozen and again pumped on, but only pumped on as it had collected. About a third of the nitrogen in the bulb was apparently trapped in the solid and weighed each time. The first sample weights were then high each day while the second were approximately correct as there was then nitrogen weighed in each weighing. The heat of vaporiza- tion is: 6 Hv at 292. 50° = 8566 : 8 cal/mole of the arsenic trifluoride. Heat Capacities. Fractions two and three of the arsenic trifluoride were combined and distilled into the adiabatic calorimeter (l.7975 moles) The filling tube was cleared of vapor with helium. A small amount of helium was left in to promote equilibrium of the solid. corrections were made. Suitable A melting point curve showed about .62 mole per cent. impurity. Per cent. melted Equilibrium Temperature oK. 17.5 267.143 Calculated Temp. oK. 267.141 35.9 267.173 267.174 60.3 267.190 267.187 86.5 267.192 267.192 Melting Point = 267.205° .:t: .02 0 K. The calculated temperature came from the freezing point lowering equation using a mole per cent. impurity of .0195. The second fraction was thus purer than the third. Heat capacities are given in Table III. transit i on was observed. No sign of a The only irregularity was that in the region 100° _ 1200 K. equilibrium was reached more slowly than at either higheror lower temperatures. The origin of the light observed is probably due static discharges caused by strain in the crystals on warming or coolung. A similar effect is observed for bydrogen cya.nide. 6 -57- 32 32 _,- f - < - - 28 24 ·-·-·· .,,,../ ,/ _,,,/ 20 ,.,, // ~t 'i~ _.,, / 0 ll. ~o ,,.,,,,,, 0 i 'i> ,r,, -,, _./ 16 ~ /,/ ~!. ii _/ ~. Q V l(._).tl.. // 1-:Z. /' ,2 / ~ I -~----- -- 8 ·- · - - · 8 I I - 4 ./ , 0 0 0 40 go ,,o z40 T, 01< Heat Capacities of Arsenic Trifluoride TABLE III Heat Capacities of Arsenic Trifluoride Run No. T oK. 1 2 3 4 5 6 7 8 9 10 13.588 14.677 15.868 17 .250 19.166 21.504 23.992 26.893 30.329 33.873 11 37.220 12 40.673 44.305 48.273 15 52.678 16 57.433 17 63.072 18 69.868 77.120 19 78.186 23 20 84.4o7 24 87.074 21 92.365 25 95.685 22 101.289 26 104.139 112.163 27 28 120.456 29 129.6s3 138.460 30 31 147.190 32 156.354 165 .6 79 33 34 175.036 185.032 35 36 194.454 201.673 37 211.116 38 220.869 39 4o 230.631 41 239.601 42 247 .802 Melting Point260.205° 44 276.203 280.091 45 46 2s3.95e 47 287.810 ~a T C (approx.) caf../mole 1~03 1.15 1.23 1.53 2.30 2.38 2.60 3.20 3.67 3.42 3.28 3.63 3.64 4.30 4.52 5.00 6.28 7.31 7.22 9.26 7.34 8.52 8.58 8. 70 9.27 8.21 7.84 8.75 8.95 s.60 8.86 9.47 9.18 9.54 9.25 9.60 9.28 9.60 9.90 9.62 8.32 , .09 1.258 1.480 1.712 2.044 2.513 3.084 3.703 4.388 5.143 5.870 6.488 7.056 7.653 8.207 8.799 9.320 9.920 10.610 11.287 11.35i 11.89 12.541 12.788 13.183 13.433 13.953 14.525 15.122 15.685 16.209 16.749 17. 282 17.800 18.342 18.856 19.172 19.744 20.254 20.736 21.172 21.569 3.90 3.88 3.86 3.85 29.96 30.02 ,0.07 30.07 -58- 12.118 The Heat of Fusion. Due to lack of time only one run was made on the heat of fusion. The Jess accurate value obtained during ~he impurity determination in the heat of vaporization apparatus agreed within .3% but is given no weight. TABLE IV Heat of Fusion of Arsenic Trifluoride. Temperature Interval 6 HF cal/mole 251.847° - 274.255° 2486.4 The Entropy of Arsenic Trifluoride. The correction to the ideal gas is made by use of the relation . the elapeyron equation, d p d T and the Berthelot equation of state pv = R T [ l t 9PT c 128pcT From these equations we may derive -gJ_ 192 R Tc p T pc r 1.. < 1· _- o'l'c ) TC :: 2R t ~ t T We might neglect the l compared to 6 T;-, /T ?... which introduces an error of from 4 to 7 %. A mean correction of 5 %.is taken but may be made more accurately if desired. term. -59- We may neglect also the 2PVe/T We then get 3 A S = Sideal - S actual • 27 R Tc P 32 T :pc -[1 P .6Hv T~ dp 1.05 dT - The quantities of p, L1Hv, T, and dp/dT are all experimentally determined quantities. dp/dT. The least accurate term is probably If it is known to .4 %the ~S would be given accurately to .016 e. u., assuming the Berthelot equation holds for the gas. Entropy ca.'l;culations are su:!!marized in Table V. -60- TABLE V The Entropy of A_rsenic Trifluoride o0 - 13 . 338° K. Debye function e ~- 91.1°' ;454 e.~. 13.338° - 267.205&graphical 30.272 Fu.sion 2484.6 / 267 .21 9.305 267.21° - 292.50° graphical 2. 713 Vaporization 8566/292.50 29.285 Real gas at 141.6 mm. and 292.50° K. 72.029 Correction for gas imperfections .09 72.12 e. 1.4.. Entropy of ideal gas at 141.6 mm. and 292.50° K. Entropy Compa~isons The entropy of the molecule was calculated by standard statistical mechanical methods using the formular given in the methyl mercaptan sect ion. For the calculations of moments of inertia the electron diffraction data of Pauling and Brockway 4 were used. 0 The As - F distance is l. 72 A and the F - As - F angle 100°. The angle is somewhat uncertain, but small changes in the parameters make bu.t slight changes in the product of the moments of inertia and consequently in the entropy. Effect of Parameter Changes on the Entropy of Arsenic Trifluoride. 0 0 As-F 1. 72 A 1-73 i 173 A F-As-F 100° 100° 105B S29s.1° 69.083 e u. -61- 69.118 e.u. 69.1 7) e.u. The frequency assignments of Wilson and Howard 5 are used. The frequencies and their degeneraces are 707, 341, 644 {2), and 274 (2) cm- 1 . The frequencies were obtained from the Raman spectrum of the liquid and may be somewhat different from the frequencies in the gas. Results of the entropy calculation and comparisons with the experimental value are presented in Table VI. In view of the uncertainties in the structural and spectroscopic values, the perfect agreement is fortuitous. Comparison of Calculated and Observed Entropies for ~rsenic Trifluoride. TOK, 292. 50 ( 141 . 6nun) Actual Gas Ideal Gas Spectroscopic 72. 03 :±. .I 72.12 72.13 298 .10 ( I ~t~.) 69.os -62- References. 1. Ruff, "Die Chemie des Fluors, 11 p. 27 01920) 2. International Critical Tables, Vol . 3, p.23 (1929) 3. Malone and Ferguson, J. Chem.Phys.~, 95 (1934) 4. Brockway and Pauling, J.A.C.S. 51, 2684 (1935) 5. Howard and Wilson, J. Chem. Phy s. 2 , 630 (1934) 6. Giauque and Ruehrwein, J.A.C.S. 61. 2626 (1939)