Origin of threefold symmetric torsional potential of methyl group in 4-methylstyrene

To understand the effect of the para position vinyl group substitution in toluene on methyl torsion, we investigated 4-methylstyrene, a benchmark molecule with an extended π conjugation. The assignment for a 33 cm-1 band in the excitation spectrum to the 3 a2 torsional transition, in addition to the assignments suggested previously for the other bands in the excitation spectrum, leads to the model potentials for the ground as well as excited states with V3″ =19.6 cm-1, V6″ =-16.4 cm-1 and V3′ =25.6 cm-1, V6′ =-30.1 cm-1, respectively. These potentials reveal that both in ground and excited states, the methyl group conformations are staggered with a 60°phase shift between them. MP2 ab initio calculations support the ground state conformations determined from experiments, whereas Hartree-Fock calculations fail to do so. The origin of the modified ground state potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis shows that the local delocalization (bond-antibond hyperconjugation) interactions of the methyl group with the parent molecule is sixfold symmetric. The threefold symmetric potential, on the other hand, stems from the interaction of the vinyl group and the adjacent ring π bond. The threefold symmetric structural energy arising predominantly from the π electron contribution is the barrier forming term that overwhelms the antibarrier contribution of the delocalization energy. The observed 60°phase shift of the excited state potential is attributed to the π* - σ* hyperconjugation between out of plane hydrogens of the methyl group and the benzene ring.