Hyperconjugation in the s1 state of substituted  toluene probed by infrared spectroscopy

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https://hdl.handle.net/2142/91119

Title

Hyperconjugation in the s1 state of substituted toluene probed by infrared spectroscopy

Author(s)

Chiba, Takashi

Contributor(s)

Issue Date

2016-06-21

Keyword(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.
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.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

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http://hdl.handle.net/2142/91119

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