Raman spectral evidence of methyl rotation in liquid toluene.
Phys Chem Chem Phys
; 10(7): 1003-8, 2008 Feb 21.
Article
em En
| MEDLINE
| ID: mdl-18259640
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 CH3 group in the liquid phase rotates almost freely, similarly as in the gaseous phase, and that the molecule consequently exhibits effectively C(2v) 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.
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Base de dados:
MEDLINE
Assunto principal:
Teoria Quântica
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Análise Espectral Raman
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Tolueno
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Simulação por Computador
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Modelos Químicos
Idioma:
En
Ano de publicação:
2008
Tipo de documento:
Article