Gas Phase Investigations of the Lewis Acid Properties of Electron Deficient Compounds of Boron, Carbon and Silicon

Author: Murphy, Milton Keith

Year: 1977

Degree: Dissertation (Ph.D.)

Advisor: Beauchamp, Jesse L.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/wxyg-3z40

Abstract

Following a brief overview (Chapter I) which is intended to define in very general terms the main direction of this thesis, the results of four distinct but closely interrelated investigations into the gas phase chemistry of a variety of Lewis acids of carbon, boron and silicon are presented.

Chapter II describes photoionization mass spectrometry studies of positive ions generated from the fluoromethylsilanes (CH3)nF4-nSi (n = 1-4). Results provide accurate adiabatic ionizations of the neutrals and appearance potentials of the parent-minus-methyl fragments generated by photon impact. These data permit the derivation of a variety of thermochemical data describing silicon containing species, including estimates of heats of formation of the ionic species, fluoride affinities (heterolytic bond dissociation energies, D[R+-F-]) of the various siliconium ions (CH3)nF3-nSi+ and ionization potentials of the silyl radicals (CH3)nF3-nSi (n = 0-3).

Chapter III describes ion cyclotron resonance spectroscopy (ICR) studies of fluoride transfer reactions observed to occur between various fluorine and methyl substituted siliconium ions and carbonium ions (CH3)nF3-nM+ (M = C, Si; n = 0-3). These studies provide the first experimental comparisons of the relative stabilities of analogous carbonium and siliconium ions free from the influence of solvation and establish the gas phase ion stability order F3Si+ < CH3SiF2+ < CH3+ < CF3+ < (CH3)2SiF+ < CH3CF2+ < CH3CH2+ < (CH3)3Si+ < (CH3)2CF+ < (CH3)3C+ for F- as reference base. In conjunction with quantitative thermochemical data obtained in the photoionization studies (Chapter II), these studies yield insights into the effects of α methyl and fluorine substitution on carbon and silicon positive ion centers. Comparisons of the present results with available hydride affinity data yield insights into the influence of the nature of the reference species on the strengths of acid-base interactions.

Chapters IV and V describe trapped-anion ICR investigations of the gas phase Lewis acidities of a variety of neutral boranes R3B and R2FB (where R = CH3, C2H5, i-C3H7, and F) and silanes (CH3)nF4-nSi (n = 0-3), respectively, using F- as the reference Lewis base. Alkyl substituents are observed to decrease the Lewis acidity of both boron and silicon acceptor centers relative to fluorine substituents, but the magnitude of this effect is found to be more pronounced in the case of the silane Lewis acids. For the borane Lewis acids, increasing the size of alkyl groups α to the acceptor center results in increased stability (measured as fluoride affinities, D [R3B-F-]) of the borane-fluoride Lewis adducts. The combined results of these two investigations permit quantification of the observed gas phase ordering of Lewis acidities BF3 > SiF4 > (i-C3H7)2FB > (i-C3H7)3B > (C2H5)2FB > (C2H5)3B > CH3SiF3 > (CH3)2FB > (CH3)3B > (CH3)3SiF2 > SF4 > (CH3)3SiF for fluoride ion as the reference base. Interesting insights into the variations in stabilities of the pentacoordinate silicon anions (CH3)nF5-nSi- (n = 0-3) are provided by comparisons with available information on the isoelectronic neutral fluoromethylphosphoranes (CH3)nF5-nP (n =0-4).

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