π*-σ* hyperconjugation mechanism on the rotational barrier of the methyl group (I): Substituted toluenes in the ground, excited, and anionic states

In the ground, excited and anionic states, the internal rotations of the methyl group in substituted toluenes such as fluorotoluene, toluidine, cresol and tolunitrile are theoretically investigated. The rotational barriers in the second excited state and second anionic states were analyzed. Orbital pictures are given for the barrier variations by excitation and electron attachment. A conjugation mechanism is introduced to interpret barrier variations.