Infrared and Raman spectra and normal coordinate calculations for methylisothiocyanate

The infrared spectra (3200-50 cm−1) of gaseous and solid CH3NCS and CD3NCS and the Raman spectra (3200-10 cm−1) of the liquids and solids have been recorded. The spectra have been interpreted on the basis of a "pseudo-symmetric top" with C3v symmetry. An assignment of the fundamental vibrations in both molecules, based on their infrared band contours, depolarization values and group frequencies, is given and discussed. Particularly interesting is the low-frequency region where band maxima were observed at 152 and 80 cm−1 for CH3NCS and 139 and 71 cm−1 for CD3NCS in the infrared spectra of the gases. A normal coordinate analysis has also been carried out based on C3v symmetry. Considerable mixing was found between the CαN stretch and NCS symmetric stretch in both isotopic species. The other normal modes in CH3NCS are reasonably pure but, for the CD3NCS molecule, considerable mixing was found between the CD3 stretches and NCS antisymmetric stretch. The proposed vibrational assignment and the results of the normal coordinate calculations are discussed and compared with the results obtained for similar molecules.