Drastic changes of the methyl internal rotation potential energy functions upon the electronic excitation have been reported for o- and m-fluorotolunes [Okuyama, K., Mikami, N., and Ito, M. J. Phys. Chem. 1985, 89, 5617-5625], and their physical origin has been attributed to the π^*-σ^* hyperconjugation. To observe direct evidence of the π^*-σ^* hyperconjugation, double-resonance infrared spectroscopy was carried out in the CH stretching vibrational region in both the S₀ and S₁ states of jet-cooled o-, m-, and p-fluorotoluenes. In the spectra of both o- and m-fluorotoluenes, some of the methyl CH bands were red-shifted upon the electronic excitation while the residual CH bands stayed in the same frequency region. The normal-mode analysis demonstrated that the shift behavior correlates to the relative conformation between the methyl CH bond and the phenyl ring plane. This conformation-dependent methyl CH bond weakening clearly supports the presence of the π^*-σ^* hyperconjugation in o- and m-fluorotoluenes. The similar red-shift of the methyl CH bands upon the electronic excitation was seen also in p-fluorotoluene though the magnitude of the shift was much smaller. The mechanism of its internal rotation potential energy behavior, however, can be different from those of the o- and m-isomers.

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