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Vibrational contribution to the internal rotation potential of toulene and nitromethane

The vibrational contribution to the internal rotation potential of toluene and nitromethane is calculated for different force fields which were constructed using permutation-inversion group and dependent on the internal rotation angle. The assignments of Raman spectra were made using experimental data obtained for the liquid and vapour (toluene) states and supported by vibrational analysis calculation. The vibrational contribution calculated with a rho-independent force field represents the kinetic energy contribution to the internal rotation potential of the molecule in question. For toluene (C6H5CH3) and nitromethane (CH3NO2) the kinetic energy contribution is zero, and for C6D5CHD2 and CHD2NO2 it has a cos (2 rho) dependence. This sort of calculation may serve as a check of any force field used for assigning molecular vibrations by comparing calculated internal rotation potentials in excited vibrational states with the parameters of the potential found from the fit of vapour band contours.

methyl group; toluene; nitromethane; internal rotation; vapour

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