Inversion Barriers of Methylsilole and Methylgermole Monoanions

摘要

Density functional MO calculations for the methylsilole anion of [C4H4SiMe]− and methylgermole anion of [C4H4GeMe]− at the B3LYP (full)/6-311+G* level (GAUSSIAN 94) were carried out and characterized by frequency analysis. The ground state structure for the methylsilole anion and methylgermole anion is that the methyl group is pyramidalized with highly localized structure. The difference between the calculated Cα-Cβ and Cβ-Cβ distances are 9.4 and 11.5 pm, respectively. The E-Me vector forms an angle of 67.9o and 78.2o with the C4E plane, respectively. The optimized structures of the saddle point state for the methylsilole anion and methylgermole anion have been also found as a planar with highly delocalized structure. The optimized Cα-Cβ and Cβ-Cβ distances are nearly equal for both cases. The methyl group is located in the plane of C4E ring and the angle between the E-Me vector and the C4E plane for the methylsilole anion and methylgermole anion is 2.0o and 2.3o, respectively. The energy difference between the ground state structure and the transition state structure is only 5.1 kcal mol−1 for the methylsilole anion. However, the energy difference of the methylgermole anion is 14.9 kcal mol−1, which is much higher than that for the corresponding methylsilole monoanion by 9.8 kcal mol−1. Based on MO calculations, we suggest that the head-to-tail dimer compound, 4, result from [2+2] cycloaddition of silicon-carbon double bond character in the highly delocalized transition state of 1. However, the inversion barrier for the methylgermole anion is too high to dimerize.