THERMOCHEMISTRY OF THE REACTION OF SI+((2)P) WITH METHANOL - A G2 MOLECULAR-ORBITAL STUDY

Ab initio molecular orbital calculations at G1 and G2 levels of theory have been used to study the most outstanding features of the [H-4,C,O,Si]+ potential energy surface. The structures, vibrational frequencies, relative stabilities and heats of formation of the most stable species are discussed. The global minimum of this potential energy surface corresponds to the insertion of Si+ into the C-O bond, while the insertion into the O-H bond is much less favorable. This is a consequence of the stabilizing effect of the methyl substitution at the Si atom with respect to the methyl substitution at the oxygen. Consistent with this, while the energy gap between the SiOH+ and HSiO+ cations is ca. 63 kcal/mol, that between their methylated counterparts, SiOCH3+ and H3CSiO+, is 34 kcal/mol smaller, and their formation in Si+ + methanol reactions is exothermic. There are certain structural similarities between the H3COH-Si+ and H3COH-C+ systems, which are not reflected however in the corresponding relative stabilities. The Si+-methanol and Si+-ethane potential energy surfaces present several features in common. In both cases most of the intermediates are below both reactants and products and the primary intermediates may be interconverted via a cyclic local minimum.