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Issue 7, 2008
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Raman spectral evidence of methyl rotation in liquid toluene

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In order to rationalize subtle details in the liquid phase toluene Raman backscattering spectra, an analysis was performed based on a quantum-mechanical Hamiltonian operator comprising rotation of the methyl group and the angular dependence of vibrational frequencies and polarizability derivatives. The separation of the methyl torsion from the other vibrational motions appears to be necessary in order to explain relative intensity ratios of several bands and an anomalous broadening of spectral intensity observed at 1440 cm−1. These results suggest that the CH3group in the liquid phase rotates almost freely, similarly as in the gaseous phase, and that the molecule consequently exhibits effectively C2v point group symmetry. A classical description and an adiabatic separation of the methyl rotation from other molecular motion previously used in peptide models is not applicable to toluene because of a strong coupling with other vibrational motions. Density functional computations, particularly the BPW91 functional, provide reasonable estimates of harmonic frequencies and spectral intensities, as well as qualitatively correct fourth-order anharmonic corrections to the vibrational potential.

Graphical abstract: Raman spectral evidence of methyl rotation in liquid toluene

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The article was received on 12 Sep 2007, accepted on 16 Nov 2007 and first published on 10 Dec 2007

Article type: Paper
DOI: 10.1039/B713965A
Phys. Chem. Chem. Phys., 2008,10, 1003-1008

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    Raman spectral evidence of methyl rotation in liquid toluene

    J. Kapitán, L. Hecht and P. Bouř, Phys. Chem. Chem. Phys., 2008, 10, 1003
    DOI: 10.1039/B713965A

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