Internal rotation of methyl group in cationic state of substituted toluenes has been investigated by the ab initio theory. Substituted groups studied here are F (fluorotoluene), NH2 (toluidine), OH (cresol), and CN (tolunitrile). The rotational barriers in the neutral and cationic ground states calculated by the Hartree-Fock method are in reasonable agreement with experimental values. Variations of the rotational barriers by the ionization are shown to be directly correlated with the stability of the HOMO. In the HOMO and HOMO-1, a new type of orbital interaction named π-σ hyperconjugation (HC) appears and determines their stability. The π-σ HC mechanism clarifies the differences between ortho- and meta-systems, between π-electron donating and accepting substituents in the cationic state.
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