Quantum Chemical Studies of the Structures and Vibrations of Trisilylmethane and Tetrasilylmethane

Abstract

Quantum chemical studies of trisilylmethane, (SiH3)3CH, (TSM) and tetrasilylmethane, (SiH3)4C, (TTSM) have been made by the HF, MP2, and B3LYP methods. The silyl groups in each compound are twisted through 12−20° from the completely staggered configurations, yielding the point groups C 3 (TSM) and T (TTSM), respectively, in fair agreement with electron diffraction results. Structures have also been calculated for the higher energy, fully staggered configurations. These exhibit H···H distances between neighboring silyl groups, which are significantly less than those in the staggered, equilibrium structures calculated for disilylmethane (DSM). In a survey of QC-based torsional frequencies in molecules containing silyl groups, TSM, TTSM, and DSM are exceptional in exhibiting marked variations in their calculated in-phase torsional frequencies according to the computational method employed. In TSM and TTSM, MP2 values are much larger than B3LYP ones, reflecting similar differences in internal rotation barrier. This may be due to the longer bond lengths and nonbonded H···H distances yielded by the DFT method. QC-based vibration frequencies and overall intensities of SiH and CH stretching bands in TSM and TTSM are scaled using earlier experimental data from related compounds such as DSM. Agreement with the sparse infrared data currently available for condensed phases is only fair. Electrical properties of the Si−H and C−H bonds are compared with those of related silyl compounds. Infrared intensities of CH and SiH stretching bands show the expected behavior due to inductive effects on the charges on the H atoms involved.