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.
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