Experimental and Theoretical Studies of Silylenes, Silicenium Ions, and Organometallic Reactive Intermediates

Citation

Shin, Seung Koo (1989) Experimental and Theoretical Studies of Silylenes, Silicenium Ions, and Organometallic Reactive Intermediates. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/sdj1-2y57. https://resolver.caltech.edu/CaltechTHESIS:11122009-115104264

Abstract

Fourier transform ion cyclotron resonance spectroscopy has been used to investigate thermochemistry and relative stabilities of silylenes, silaethylene, and silicenium ions in the gas phase. Proton affinities of silylene, methylsilylene and silaethylene have been derived from studies of kinetics and thermochemistry of proton transfer from the corresponding silicenium ions to a series of n-donor bases with well-established gas-phase base strengths. Values of proton affinities combined with the known heats of formation of the corresponding silicenium ions yield heats of formation of silylene, methylsilylene, and silaethylene. Experimental results for the relative stability between methylsilylene and silaethylene are corroborated by ab initio generalized valence bond (GVB)-configuration interaction (CI) calculations which indicate that silaethylene is more stable than methylsilylene. Hydride affinities of the methyl-substituted silicenium ions have been precisely determined from examination of kinetics and equilibria of hydride-transfer reactions of methyl-substituted silanes with various hydrocarbons having well-established gas-phase hydride affinities. The result shows that the silicenium ions are significantly more stable than the corresponding carbonium ions in the gas phase with H⁻ as a reference base.

Photoelectron spectroscopy and mass spectrometry have been employed to identify the gas-phase reactive intermediate in the chiorosilane chemical vapor deposition under the heterogeneous flash vacuum pyrolytic condition. The result indicates that dichlorosilylene and hydrogen chloride are the major gas-phase products and monochlorosilylene is not an abundant gas-phase intermediate.

The ab initio theoretical methods have been used to calculate the equlibrium ge-ometries and singlet-triplet splittings of chlorine- and fluorine-substituted silylenes and methylenes. The GVB-dissociation consistent CI (DCCI) method has been developed to accurately predict singlet-triplet energy gaps within 1 kcal/mol error.

Finally, we have employed Fourier transform ion cyclotron resonance spec-troscopy combined with a line tunable CW CO₂ laser to isolate the coordinatively unsaturated organometallic intermediates and examine structures, reactivities, and spectroscopic properties of the isolated intermediates for the methyl-migratory decarbonylation reaction and ligand displacement reaction. The results show that the CF₃ group is an ideal infrared chromophore to investigate the infrared photochemistry of organometallic complexes, LₙM-CF₃, structures, and reaction mechanisms of their coordinatively unsaturated intermediates containing metal-bonded CF₃ groups. The infrared multiphoton dissociation spectra of the isolated intermediates containing metal-bonded CF₃ group are presented.

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