Π*–σ* Hyperconjugation mechanism on the rotational barrier of the methyl group (II): 1- and 2-methylnaphthalenes in the S₀, S₁, C₀, and A₁ states

Internal rotation of the methyl group in 1- and 2-methylnaphthalenes has been investigated by the ab initio theory. The rotational barriers in the S₀ and S₁ states calculated by the Hartree-Fock and configuration-interaction with single-excitation operator methods are in reasonable agreement with experimental values. The variations of the rotational barriers by excitation (S₀→S₁), ionization (S₀→C₀), and electron attachment (S₀→A₁) are shown to be directly connected with the stability of the HOMO and/or LUMO by the first-order treatment. In the HOMO and LUMO, a new type of orbital interaction named π*-σ* hyperconjugation appears and determines their stability. The interpretation based on the π*-σ* hyperconjugation can consistently and comprehensively explain the barrier variations.