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|Type:||Artigo de periódico|
|Title:||Acidities And Spectral Properties Of α-silyl And α-germyl Carboxylic Acids And Their Carboxylates|
|Abstract:||The order of acidity observed for R3MCO2H (R = CH3 and/or C6H5, M = C, Si, Ge; R = H, M = C, Ge) is Si ≥ Ge > C. We find this order is best explained in terms of the larger sizes and polarizabilities of Si and Ge relative to C which would facilitate stabilization of the negative charge in the conjugate base. CNDO/2 results on the model systems H3CCO2H, H3SiCO2H, and their anions are presented (with and without inclusion of d orbitals for Si) which predict the correct order of acidity and indicate pπ → dπ interaction is of little importance, contrary to previous suggestions. The importance of Si-O (1,3) dπ-pπ interactions is also considered. For a consistent thermodynamic explanation the relative electron affinities of CH3 · and SiH3 · (Si > C) appear to be the key factors. The electronic spectra of the title compounds indicate a symmetry (C2v) forbidden n → π* transition as the lowest energy transition. CNDO/2 predicts an allowed σ → π* which cannot be ruled out at present as incorrect, dπ-π* interactions (M = Si) do appear important in determining electronic properties for which there are significant excited-state contributions. Differences in CO stretching frequencies are explained qualitatively in terms of relative Si and C inductive effects. However, comparison of the ir (v(CO)) characteristics of the acids with those of the α-group-IV metal ketones and dihalocarbonyls (for which force constant and normal coordinate data are available) suggest kinetic effects (mass and geometric differences) and potential energy terms not involving the CO bond are more important than electronic effects for determining v(CO).|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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