Observation of Evidence for the π*−σ* Hyperconjugation in the S₁ State of <i>o-</i>, <i>m-</i>, and <i>p-</i>Fluorotoluenes by Double-Resonance Infrared Spectroscopy

Drastic changes of the methyl internal rotation potential energy functions upon the electronic excitation have been reported for <i>o</i>- and <i>m</i>-fluorotolunes [Okuyama, K.; Mikami, N.; 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 <i>o-</i>, <i>m-</i>, and <i>p</i>-fluorotoluenes. In the spectra of both <i>o-</i> and <i>m-</i>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 <i>o</i>- and <i>m</i>-fluorotoluenes. The similar red-shift of the methyl CH bands upon the electronic excitation was seen also in <i>p-</i>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 <i>o-</i> and <i>m-</i>isomers