On the structural parameters and vibrational spectra of CH3NCS, SiH3NCS and GeH3NCS

The infrared spectra (3500–400 cm−1) of CH3NCS in gas, xenon solution (−60 to −100 °C) and solid phases have been recorded. The effect of free internal rotation of the methyl group is observed on the CH3 antisymmetric stretch and deformation bands from which zeta values have been obtained. The band centers for the degenerate CH3 vibrations have also been determined from the low-temperature spectra in liquid xenon and their values are significantly different from the ones in the gas. By combining previously reported rotational constants for CH3NCS, SiH3NCS and GeH3NCS with the ab initio MP2(full)/6-311+G(d,p) predicted parameters, adjusted r0 parameters have been obtained. The structural parameters for CH3NCS (Cs) are: r(CN) = 1.441(5), r(NC) = 1.206(5), r(CS) = 1.586(5), r(CHs) = 1.090(2) and r(CHa) = 1.093(2) Å with ∠CNC of 143.9(3) and ∠NCS of 175.5(2)°; for SiH3NCS (C3v): r(SiN) = 1.704(5), r(NC) = 1.197(5), r(CS) = 1.567(5), r(SiH) = 1.476(3) Å with ∠NSiH = 108.5(3)°; for GeH3NCS (Cs): r(GeN) = 1.843(5), r(NC) = 1.209(5), r(CS) = 1.576(5), r(GeHs) = 1.519(3) and r(GeHa) = 1.517(3) Å with ∠GeNC = 149.9(5) and ∠NCS = 176.5(2)°. These parameters are significantly different from the values previously reported for all three molecules. Ab initio calculations by the Møller-Plesset perturbation method to the second order MP2(full) and density functional theory calculations by the B3LYP method utilizing a variety of basis sets have been carried out for all three molecules to predict barriers to rotation and linearity as well as force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios and centrifugal distortion constants which are compared to the experimental quantities. Some significant changes have been made for the assignments of some of the fundamentals for CH3NCS which has been clearly shown to have Cs symmetry in the ground vibrational state as well as for GeH3NCS with Cs symmetry. The results are discussed and compared to corresponding quantities of some similar molecules.