Dependence of the CH3SiH3 barrier to internal rotation on vibrational coordinates: Testing of models and effect of vibrations on the observed barrier height

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Abstract
The torsional satellite spectrum associated with the J=0→1 rotational transition in several symmetric isotopes of methylisilane is analyzed to yield the internal rotation barrier derivatives ∂V3/∂S1, where S1 is an A1 vibrational symmetry coordinate. Small basis set Hartree‐Fock calculations of the same quantities give remarkable agreement with the ``experimental'' values. But we have been unable to construct any simpler model that will reproduce the derivatives corresponding to either the methyl or silyl bend or the CSi stretch. The shielded proton model‐and variants which take into account the leftover negative charge as well as non‐Coulombic forces‐have been tried without success. From the derivatives we calculate a reduction in V3 of 54 cal/mole due to distortion of the equilibrium geometry in the eclipsed configuration. The effect of zero‐point motions on the observed barrier height is examined for the first time. It is estimated that the pure harmonic terms arising from just the A1 vibrations contribute about 155 cal/mole, and it is quite possible that the total correction could be 3 or 4 times larger.