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Title:Hyperconjugation in the s1 state of substituted toluene probed by infrared spectroscopy
Author(s):Chiba, Takashi
Contributor(s):Fujii, Asuka; Okuyama, Katsuhiko
Subject(s):Spectroscopy of Large Amplitude Motions
Abstract:Internal rotation of the methyl group in substituted toluenes is one of prototypes of large amplitude motions in polyatomic molecules. The internal rotation of {\it o}-fluorotoluene is strongly hindered in the S$_{0}$ state, but that of {\it m}-fluorotoluene is almost free. For the S$_{1}$ state, however, the substantial changes of the internal rotation potentials have been reported
while the potential barrier in the {\it o}-isomer drastically decreases and the methyl group becomes almost a free rotor, the barrier in the {\it m}-isomer largely increases\footnote{K,Okuyama.
M,Ito. {\it J.Phys.Chem.} {\bf 1985}, {\it 89}, 5617.}. These surprising barrier changes have been attributed to the methyl conformation-dependent stabilization in the S$_{1}$ state by the $\pi$-$\sigma$ hyperconjugation\footnote{H,Nakai.
M,Kawai. {\it Chem.Phys.Lett} {\bf 1999}, {\it 307}, 272.}. In the present study, to test this interpretation, we observed infrared spectra of {\it o}- and {\it m}-fluorotoluenes in the S$_{0}$ and S$_{1}$ states. Both the isomers showed decrease of the methyl CH stretch frequencies upon the electronic excitation. We concluded that this frequency decrease is the evidence of the $\pi$-$\sigma$ hyperconjugation.
Issue Date:2016-06-21
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper/Presentation
Rights Information:Copyright 2016 by the authors
Date Available in IDEALS:2017-01-26

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