Internal rotation of the methyl group in o- and m-fluorotoluenes has been investigated by the ab initio molecular orbital method. The calculated rotational barriers in the S0 and S1 states are in reasonable agreement with experimental values. Steric and electrostatic effects are not important for the changes in the rotational barriers. An electronic effect is found to be the most significant. Especially, the rotational barriers in the S1 states are determined by the stability of the LUMOs, which involves a new type of hyperconjugation between the ortho-carbon and the hydrogens of the methyl group.
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