title: Observation of Evidence for the π*-σ* Hyperconjugation in the S1 State of o-, m-, and p-Fluorotoluenes by Double-Resonance Infrared Spectroscopy.
abstract: 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.
authors.affiliation: Department Chemistry, Graduate School of Science, Tohoku University , Sendai 980-8578, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Chiba
authors.affiliation: Department of Chemical Biology and Applied Chemistry, College of Engineering, Nihon University , Koriyama 963-8642, Japan.
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Okuyama
authors.affiliation: Department Chemistry, Graduate School of Science, Tohoku University , Sendai 980-8578, Japan.
authors.forename: Asuka
authors.initials: A
authors.lastname: Fujii