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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 reportedwhile 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.N,Mikami.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 Type: Text Language: En URI: http://hdl.handle.net/2142/91119 Rights Information: Copyright 2016 by the authors Date Available in IDEALS: 2017-01-26
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