The Raman Spectra of Some Volatile Fluorides Citation Gross, Siegfried Theodore (1935) The Raman Spectra of Some Volatile Fluorides. Master's thesis, California Institute of Technology. doi:10.7907/wdvh-m211. https://resolver.caltech.edu/CaltechTHESIS:10292024-153525943 Abstract No abstract. Item Type: Thesis (Master's thesis) 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 1935 Record Number: CaltechTHESIS:10292024-153525943 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:10292024-153525943 DOI: 10.7907/wdvh-m211 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16820 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 29 Oct 2024 22:04 Last Modified: 29 Oct 2024 22:06 Thesis Files PDF - Final Version See Usage Policy. 5MB Repository Staff Only: item control page

The Raman Spectre of some Volatile Fluortrles- by Siegfried T. Gross • In partial fulti 1 lment. of the req,11rements for tbe degree of Master of Science. Callfornta Institute of Tec bnology, Fssaaena, Californ le.. 1936 Ac know led ~ement. These i nvestigations were undertaken at tbe suggestion and with the active cooperation of T;r. Don M. Yoet. The autbor wishes to express his ~ratltude for the lnv al uvb 1e ass is tanoe rece ivecl. He wishes to acknowled ge also bis indebtedness to Carsten Steffens, and Dr. E, . B. Wilson Jr., v,bo developed the mathematical theory necessary for e part of tbe work; and to Professor Dic kinson, wh o furnished invaluable su~g estions concerning tbe apparatus and tec hn i.que nece ssa~y. Much of t h e material ln t hls t he sis ts identical witb t ba t in t he article, 1 Tbe Ha rnan Spectra and MoJ e culer constants of the Hexafluoriaee of Su 1 fur~,- Selenium and Tellurium,' Yost, Steffens, am'! Gross, Journa J of Chem. Phys., 2, 311-16 ( 1934). Tbe Ra man Spectra o f ~ Volatile Fluoric'les. Electron diffraction experiments have been made upon many of tbe volatile fluorides, but no previous spectroscopic measurements bad been made to determine the fu,ndamentel molecular frequencies of tbese compounds, although such data is nec e ssary for further information concernin g bona strengths and thermodynamic quantities. As one step in determining such c'lat6 , the Raman spectra of some of these substances were determined, in botb Jiquia a nd gaseous states. In tbe experi ments on t be liquias, the material was transferre d to a Pyrex Y, lass Raman tube, 1 cm. tn a1ameter, equip pe <'l at one e nf! with a plane ~lass window, and at the other v-iitL a ri gb t cone of black nicke l-oxide Pyrex g lass. The use of t he nickel-oxide g lass was adopter' afte r effor·te bad been mod e to f i nd a liquid suitable for cool ing which wou Jd not re move tbe optics 1 black ord 1no ri ly used on Raman tubes. Alcohol, benzene., gasoline a na other liquids of low fr e ezing point were tried, but in every caee, t h e liquid would either become cloudy, or tb e paint would flake off. The Harna n tub e was supporte n vertically in a De war flask whic h had been silvered ab out one-half of its circumference. Close above the Raman tube a totally reflecting prism was mounted, wbicb served to direct the scatte red radia tion to t h e coll e cting lens. The exciting radiations were the 4046 ana 4358 j 1 ines of the mercury spectrum, obtained from a mercury arc i n Pyrex g lasE, e nclosed in a wate r ,1 acket. -1- The apparatus designed to obtain the Haman spectra of the gaseous material was constructed entirely of quortz. Within the spect:rograpL \HlS placed a <'lisb of mercury, tbe vapor serving to absorb o lar ~e frsction of t he 2537 i radiation user to excite the spectra. The Raman tube, shown in F'i g . 1 was 2.5 cm. in diameter, anc1 30 c m. lon&r, , equipped at one ena witb a 1.3 cm. window, end at tbe other, wltb a right cone, similar to that ordinarily used 1n obtaining the Raman spectra of 11.quirls, coated with optical blE, ck paint. Tbe tube was fi l lec tbrou gh a brass connection equipped wl th a stopcock anc'! pressure gauge, anr! connecter to the Raman tube by means of a three inch long wax sea 1. The pressure was obtained by liquifying or soli<'lifying tbe ges tn a smell heavy waller Pyrex glass tube witb liquid air, an<'! then ellowtng it to evaporate into the Raman tube, until the required pressure, about 75 lbs. por sq. in., was obtained. I Fig. 1. (¼ natural size) The Raman lines were me asured by a linear extrapolation from adjacent lines of an overlapping iron spectra. The accuracy estimated for the spectra of the liqui<ls is about ± 3 cm-1, anrl for tbe ~ases about ± 0.1 cm-1. -2- Hexafluoric'les 2.£ sulfur., selenium~ tellurium. The hexafluorides were preparea by treating the elementary substances c untained in a copper tube., with gaseous fluorine. With the exception of tellurium hexafluoride., which was condensed in a trap immersed in a carbon dioxide cooling mixture, tbe compounds were all collected over water. The resulting material via s t ben in each case purified by means of a series of fractional distillations under vacuo with the use of liquid air. A compound which was not studied., but wbtcb may be of cheinica 1 interest was obtained as an -tmpur-1 ty w:t. tb the sulfur he xaf 1uoriae. It v,as, at 1 ow temp- era tures, a white solid, wh ich cban:;,;ecl in appearance sligh tly as it a r proach e d its meltin~ point. It melted toe colorless liquid, with a boiling poi n t s ome wb Ht less tban -20° c. The vapor pressure was at -20° a pproximately 22 cm. of mercury. No Te 2 F6 (?) * was d e tected in tbe tel luri am hexafluoride reaction product. It was usually necessary to pass the ~ases through a tube containing phosphorous pentoxide tube to re move the la s t traces of water before mak ing t be spe ctra. The gases were store d l n evacuate~ six liter flasks., equtppef with gas inlets. In the experiments on the liquids, alcoh ol (cooled with carbon dioxide) was used for the sulfur hexafluoride, while ice-water served wi t h the selenium and tellurium hexafluorides. In each case the Hquiflec'l material was present 1mfler a vapor pressure whicb varied fr um five atmos ph ere s, f er SF6 , to * Yost an~ Clausen, J. Am. Cbem. Soc., 55, 885-91 (1933). - -3 -. nine atmospheres for the SeF6 • The sulfur cornpound was usea -so 0 c. while the selenium end tellurium compounrls were r un at o0 c. at The strongest Hama n line appeared after an exposure of t wenty mi nutes, but to obta i n all of the line s t wo or three h ours were necessary. In e acb cs se t h ree line s a r re ar-ea, re- sult i n g fr om a s ing le exc i t i n g frequency, and under no conditions could more lines be obts ln ea from any of the comp ounds. F'or a given length of exp osure, the Raman J 1nes from the te11urlum be xa fluoride were Urn most i ntense anrl t h ose from the sulfur compound t he least. Lengths of e x posures for t he gases va ried from 30 minutes to 30 hours. The tellurium bexafluorlce yield e d three Raman lin t: - s, t he one of greatest Av strong anr! s b arp wi tb only a trace of diffusene ss on th e lon g wave-leng th side; the line of least .6. v was 19 cm- 1 broad, ana lmiforml y intense over mo s t of its width, a ppearin g also as anti-Stokes radiation; while the weake st line had a well-definer1 center anc! faint di f fuse win gs. Tbe other two gases yielded on l y one strong sharp li ne each, correspondin g to the line of greatest .6.v in the TeF6• For e given length of e x p osure, the mo f t intense line s were ob ta i ned from tellurium bexafluoriae, t he least from th e sulfur be xaflllor1r. e. The lines ob ta i ned f rom the gas e s were much s barper t ba :, thO f'e obtainea from tbe liquids. The results of tbese expe riments are presente<l ln Table r. The numbers in parenthesis a a .1oinin g the s :.r mbols v are multi p licities, those adjoin i n g t be freq uency va lue s are approxima te relative in tensities. -4-. Table I.Substance SF6 SeF6 v 1 (1) cm-1 cm-1 v2 (2) v3 (3) Liquid 776( 10) 642(2} 522(2) Gas 772.4 Liquid 710(10) 708.0 697( 10) 701.2( 10) 662(2) 405(2) 672(2) 674.4(1) 316(4) 313*( 4) State Gas Liquid TeF6 Gas cm-1 * The width 1s about 19 cm-1. Nearly tbe same va 1 uo s for v1 were obtained for the gases as for the liquids, a result usually founa with nonpolar substances. Theoretical. i,.'ormulae for tlle fundamental mo~es of vibration of the regular octeherlra 1 molecule have been derived by Redlich, Kurz and Rosenfeld as a function of cen- tral force constants, but ignoring the forces present 1n tbe equilibrium position of the atoms somewhat oversimp lified the problem. Carsten Steffens using a procedure usually applied to non-holonomic systems, derived frequency formulae am~ obtained equations ( 1-6), the multiplicity of the frequencies being given by the figure in parenthesis. k 01 , k 12 , and k 24 are force constants for the reactions between the centr8l and a fluorine at om, bet~een two adjacent fluorine atoms , and between two opposite fluorine atoms respectively. m0 enc m1 r e present the mass of tbe centre. 1 an<'! of any per- iphera 1 particle, respectively. -b, -b, enc -j, denote forces to correspon~ln~ equilibrium dlstenceaj where the forces are tbose a t eq:..i.ilibrium due to reaction between the Redlioh, Kurz and hosenfeld , Zeits.f.pbysik.Cbemie B19,231 (1932) centre l and a peripheral particle, between two ar jacent per- lpberal particles, and between two opposite peripheral particles, respectively. 4rr2vl2(1) ( l ). 4~2v22(2) = (ko1~k12•2k24-t3h)/m1, ( 2). = (2k 12 -2h)/m 1 , ( 3). = (kur•h-2j)/m 1 , ( 4). 4ir2v 2 (3} 3 2 4-rr2r'4 (3) 4ir2 v 2 = (ko1~4k12~2k24)/m1, 5,6 1 2m0 m1 (3 ) = tA±{ A2 -t4m0 { rn 0 t6m1 )( -k 0 1ku:?+3k 0 1h+2k0 1j+4( 2b-tj) ~ (k12+h))]I}, (5). (m0 +2m1)k 0 1-t3m0 k12-(m 0 -tl6m1 )h-2(m 0 14m1)j, A: v12 = ..;22,. (3/2)V32• (6). Another set of frequency formulae for the XY6 molecule was derived by Dr. E. a. Wilson, Jr. using a restricted type of valence force potential function involving only three constants, k 0 1, the X-Y force constant, p the interaction constant for the effect of extension of the X-Y bond, and k~ the constant for the bending of tbe valence bonr!s. The formulae are the following: = (ko1-t4p)/m1, ( 7). 4~2v 2 2(2) • (k 01 -2p)/m1, (8). 4~2 v3 2 (3) = 4k~/m 1 , ( 9). 4it2y4 2 (3) • 2k~/m1, ( 10). 4,r2y/Z(l} 4rr2 v2 (3) = 4 2 ~ ; ; t.lm1/m0 )~±( B -48 ~• B • 24 =!~kul :!] [ :~J2 1t 6 k 0 1k4) *}, ( 11) • ♦ 6 ~• 4:!]2kq +2 ~• 4 =~1 (k J •kf), 0 ( 12). Of these six frequencies, \Ip v2 and v3 are permitted by the Hemen selection rules as determined by Dr. E. B. Wilson, Jr., wbo applied Ploozek's* theory of scettering to oeduce the selection end polarization rules for tne octa- hedral type molecule. with ~ • 6/7. y2 and v3 will be depolarized equally The modes of vibration are shown in Fig. 2 and are in agreement with those described by Rerllich, Kurz and Rosenfeld. Pairwise combinations give y 5 and })6. 1'"1g. 2. Fundamental modes of vibration of regular octahedral model of XY6 • Tbe assignment of the strongest line from each substance to the completely symmetrical mode of vibration, v1 ( 1) is in agreement with the tbeory end is e lmost certainly correct. In the CE\ Se of the gsses, tbe strongest line is sharp wl tll only a trace of diffuseness on the long wavelength side. Thts incioates a Q branch and is to be oxpecter. from the rnooe of vibT'ation. The weakest line from TeF6 was assign- ---- * Cf. Debye, The Structllre of Molecules, p. 86, Bleckie and Son, Lonaon, 1932. -7-. ed to V2, end has e well defined center with Jess intense, diffuse r,ings on eitber side wbicb were, however, rendered somewhat indistinct by tbe continuous background. The broad line of least .6.\J is moderately intenee end is assigned to v3 • The broaaness is dot1btless due to unresolved rotational structure, and tbe uniform intensity over its wtdth indicates tbe presence of P, Q, enc'! R branches. These consic'lerations, together with the relative magnitudes of the frequencies to be expected from the modes of vibration involved, would seem to confirm the assignments given to tbe 1 ines. To render them even more certain the cons tent-free relation between the allowed frequencies was employed. From this re lat ion 1 t is evident that Y is the greatest of the three 1 frequencies. It was found that the best agreement was obtained when the sme l lest frequency v.ras assigned to values of ~ 2 2 ~ (3/2)v3 2]t v3 • The are from 10 to 17 percent greet- er than those observed for '1 1 , the best agreement being obtained wt th TeF5 • Al though polarization experiments hod been considered, tbey were d1scontinuea when it was learned that tbe depoJ e.rizetion factor• and v3 • e• wi 11 be the same for v2 Both sets of frequency formulae were used to estlmate the magnitudes of the forbidden frequencies. These values are only rough approximations. The de.ta ere insufficient to determine a 11 of the cons tan ts in equations ( 1-5).. From the rel:E, ticn discovered by Professor R. M. Badger * it was p~ssible to determine 1 at * R.M.Badger, Jour. Chem. Phys. 2, 128-31 (1934). -e--. least approximately, the values of k 0 1, an0, with less certainty, k,1 2 • k 24 1s small and is disregarded. h may be calculated from 2 alone, and elementary lnver•se square v law considerations show that j = h/~ri. k 01 and k 12 were used to c~lculate \/1 in order to test the approximate oorrectnes.s of the constants. The agreement with exper- iment ie not exact but is satisfactory. The results of the cal cul etlons are presented in 'fable II. The negative values for h and j indicate the existence of repulsive forces between fluorine atoms. Table II. Estimated force constants and forbidc'!en frequencies. Equstione (1-5) (central forces). Substance k0 1 k12 Y1 V1 -b Estimated foT"bid• xio-5 xio-5 ca.le .. obs. x10•5 den f ,requenoies. v, vs 1'6 1510 960 eso 5.9 0.50 .8 44 772 1.0 280 620 s.a o.36 ao7 10a 0.55 230 s7o 5.9 0.26 790 101 0.2a 190 420 Thev's are expressed in om- 1 ; tbe constants dynes/em .. In Table III are results of similar ea lculations f-r•om using equations (7-11). In tbis case the three oonsttints v,ere feterminea directly from tbe experimental data. Table III. Substance SFe, ee,~6 TeF5 Estimated .t'orbiddan frequencies . Equations (7-11) ( va }enee bonds) . -1 v6 cm- l p k k V4Cm Y5 cm·l x18l5 xio--5 xio-5 5.30 o.344 0«765 370 540 970 5.15 5.23 0.123 o.460 0.086 0.274 286 222 430 310 650 610 After the above work was completed, Eucken, from infrared spectra, and specif'ic heats observed tbe values given -9-. in Table IV for sulfur hexafluoride. Table IV. Observed and calculated forbidden Frequencies for SF6 -. Euken 's values-. 1 v4 cm1 v5 cm-1 ~6 C m. 363 617 965 oa 1cul a te d '" centre.] forces '. valence bonds. 280 370 620 540 1510 970 It is evident that the theory postulating valence bonds gives better results in predicting the unknown frequencies than that po~tulating central forces-. Since tbe spectra is readily obtained, it is probable thet the bonds are not of the extreme ionic type. Thermodynamic Const.ants of Sulfur Hexafluoride. ---------- - --- -----'fbe virtual entropy of sulfur hexafluoride may be expressed as si9 a = Sa • Sf, wbere Sr is the entropy aasoo- ia ted wi tt; the firbidden freqllencies and S 8 is the remaining entropy. The value of 8 8 is 64 •.6 cal./deg., and was calculated from the aata in Table I, wltb the help of the statistical equation Sv29ao • . fhv e•bv/kt. R !Ft i-e-hv71d. St was calculated from Euken's values, and has a value of 4.7 cel./deg•. s~ 98 then becomes 69.3 oal./aeg. The ent- ropy of fluorine is 48 cal/deg., and that of rhombic sulfur is 7.6 cal./aeg. ~t 25° ana l atmosphere. Tbe beat of formation of sulfur boxafllloride at constant pressure and 25° is 262,000 cal.* When these vs lues ere combtned in the we 11 known equation AF • Ali - TAS, a provisional value for * Yost encl Clausen, J. Am. Chem. Soc., 55, 885-91 (1933). -10-. 0 .b.F29e is obtained of -2.3 7 ,000 ca 1. The error is due princ- 1pa lly to that in .c.H, namely,± 3000 calories. Tetrafluorioes of carbon and sl 11con. --------- ---• The carbon tetrafluoride was prepared by the interaction of gaseous fluorine ana sug~1r cllarcoa l, the procuct being col leoted over water. It was purified by fractional distillation in vacuo, and stored in previously evacuated six liter flasks at a pressure of one atmosphere. In addition to the ~F6 and C3Fa commonly present as irnpuri ties, a col orless liquid with a boiling range or from .450 c. to +3o 0 c. was obtained• wbioh seemed to consist wholly of a mixture of carbon fluorides. A Dumas' molecular Yieight determination upon the higher boiling portion of the mixture gave an approximate value of 244. The Rarn~n spectra of the purified gaseous carbon tetrafluoride was photographe.d, but due to the presence of a continuous background, only one definite line could be obtained, regardless of the length of <:.exposure. This line, v'i,. aue to tbe symmetrical vibration of the mol- ecule, bad e wave number of 907. 7 cm- 1 • Photographs were not obtained in the liquid state. The silicon tetre.fluori<'le, prepared in the usual manner f~om silica, calcium fluoride, and sulphuric acid, was purified by firs.t passing tbe gaseous product through a carbon a ioxide cooled tra.p., to remove rnoret of the HF, H20, etc., and tben through a tube containing sodium fluoride to remove BF3 (from t be pyrex gloss used in tb3 generating a r parstus)., and the laet traces of th€ HF. It was finally fractionated by complete the purification. In the quartz -11- Raman tube used for gases, an absorption spectrum was obtained, with several band-beads in tbe neigbborbooa of 2800 R. Attempts to obtain the Raman lines in the liquid state yielded but one line, with a wave number of 799.0 cm -1 • The dif f1culty in attempting to obtain the Raman lines of this substance seems to be similar to tba,t experienced with some of tbe other silicon compounds. Boron trifluoride. --- The boron trifluoride was prepared from bo:ric anhydride, st1lphuric acid, anc calcium fluoride, followed by fractionation to purify tbe compound. Attempts to obtain the Raman spectra in the quartz apparatus for gases, led to the decompos1t1 on of the material, although when the pbotog:raphio plate tn the spectrograph was developed it yielded one line, having a wave number of 862 om-1 .. This was not confirmed by taking other photographs because of the deoompositi on. The spectra of the liquid was not 1nvest1geted. Oxygen d1fluor1de. The compound was prepared in the manner described by Ruff .from gaseous fluorine and di lute sodium bydro.x1de solution. The oxygen fluoride is given off as a gas, mixed wl tb an almost equal amount of oxygen. To obtain the Raman spectra 1 t was not necessary to purify the mater la 1, since tbe spectrum due to oxygen is quite well known. The gases ylelaea from tbe interaction of tbe fluorine wtth the sodium hydroxide a ol utlon were collected in a trap immersea in liq- utd air, and finally transferred to a Pyrex Raman tube with -12- .. aid of super-cooled liquid air. The Raman tube in each oese contained about 15 to 20 cc~ of the liquifled gas mixture~ In the first effort to obtain the spectra, tbe material was illuminated with no filter interposed between the Pyrex mercury arc and the Raman tube~· Soon after tbe beginning of the exposure, a photocbemioal change was obs..:. erved inside the tube• a deposit of yellov1 material form.;. ing on the glsss-.;.-eventually becoming a red solid, possibly the OzF2 described by Ruff~ To prev~nt this photocbemica l change from oocuring., the green line of tbe mercury arc was usea for excitation~ Tb1s was eccompl!sbed by making use of a filter of cupricchlorine in alcohol. Due to the weak intensity of this line a long exposul"e was c'lesirable~ Witb a five hour expos~ ure no Haman lines could be detected~ All spectra were made at liquid air te •·n peratures, the Raman tube immersed in liquid air containen ln a Pyrex dewar; silvere d about one half of 1 ts circumference, and at a pressure of one atmosphere. Due to the use of the super-cooled liquid air; necessary to transfer the material from the collecting trap to the Raman tube, it is possible that a small amount of liquid nitrogen might have been present in the material~ -13-- §~mary. These experiments were undertaken for the purpose or securing data concerning tbe molecular forces and frequencies of the molecules, and to obtain thermodynamic date, notably the virtual entropies. From the entropy, with the aid or specific heat date the Free Energy can be determinen, and it becomes poEsible to predict the direction in Which e reaction wi 11 go, provided that such a reaction may occur. The Raman frequencies determined are: State. yl cm- 1 Substancev2 cm-1 776 (10) 772.4 710 ( 10) 708.0 697 (10) 701.2(10) 646 (2) v3 cm-1 522 (2) 662 (2) 405 (2) 672 (2) 674.4(1) 316 ( 4) 313~~(4) SF6 Liquid. SeF6 Liquid. TeF 6 Liquid. CF4 Gas. SiF4 Liquid. BF3 Gae. (Line obtained during decomp.) Gas. Gas. Gas. 907.7 799.0 862 cm· 1 *The width is about 19 cm- 1 . No Raman lines were obtained with 0F2. An absorption spectrum was obtained with S1F4 in the ultra violet with several band beads visible near 2800 i. Oxygen fluoride, OF2, exposed at 11qu1d air temperatures to an unfiltered Pyrex mercury arc undergoes a photochemica 1 reaction of some type as yet undetermined. The entropy and free energy of sulfur bexafluorid e were ea 1 cu lated, and the vs lues are: S~98 (SF6) • 69.3 cals./deg • .6F~9 a ( SF6 ) = -237 ,ooo * 3000 ca ls. -14-.