High overtones of the C-H stretching vibrations in anisole and thioanisole

The high overtone C-H stretching vibrations of anisole and thioanisole have been measured in liquid phases by using optical absorption and optoacoustic spectroscopy. The local mode model satisfactorily describes the experimental data and provides anharmonicities and force constants. The data obtained by the DFT calculations allow a complete assignment of all the observed bands in the vibrational spectra of the two molecules. The comparative analysis of the overtone spectra and of the DFT results confirms the occurrence of the methyl C-H symmetric stretching at low wavenumber (2836 cm-1) in anisole as due to a strong anharmonic coupling, deriving from a Fermi resonance with the first overtone of the methyl symmetric deformation mode. This effect is typical of the methoxy group, because it does not occur in thioanisole as well as in toluene. The present investigation demonstrates the existence of a strong effect of electron conjugation of OCH3 toward the aromatic system, in comparison with the SCH3 group, as evidenced also by the study of the rotational barriers and the proton magnetic resonance data.