Torsion-Vibration Interactions in S₀ and S₁ Phenylsilane

We report the results of a study of the interaction between torsion and the low frequency out-of-plane silyl wag vibration in the ground, S₀, and excited, S₁, electronic states of phenylsilane. These studies follow the observation of interactions between methyl torsion and the out-of-plane methyl wagging vibration in toluene, several fluoro-substituted toluenes and N-methylpyrrole. The interaction leads to various spectroscopic constants becoming divorced from their usual physical meaning. It also provides a mechanism for the redistribution of vibrational energy within the electronic state. Substitution of silyl for methyl changes the electronic interactions with the phenyl ring while retaining analogous nuclear motions. Thus, comparison of phenylsilane with toluene provides insight into the role of electronic interactions on torsion-vibration interactions. It is found that there is strong mixing between silyl torsion and the out-of-plane silyl wag vibration in both the S₀ and S₁ states of phenylsilane. Interestingly, the coupling constants for phenylsilane are similar in magnitude to those for toluene. This adds to the mounting evidence that when an XH₃ group is attached to a planar frame, the interaction of torsion with the XH₃ out-of-plane wagging vibration is generic and hence this type of interaction is likely to be widespread. The spectra show that the staggered conformation is the minimum in both electronic states.