Ab initio potential energy surfaces for the large-amplitude motions of quasi-symmetric top molecules: CH3NCS and SiH3NCO

The potential energy surfaces of methyl isothiocyanate, CH3NCS, and silyl isocyanate, SiH3NCO, have been investigated in quantum mechanical ab initio calculations at the SCF, MP2-MP4 and CCSD (T) levels of theory. Total energies of the two molecules were calculated as functions of coordinates of the two large-amplitude motions: the CNC/SiNC bending motion and internal rotation of the methyl/silyl group. At the most advanced level employed, CCSD(T) with the cc-pVTZ basis set, the theoretical results are in good agreement with experimental data and consistent with the quasi-symmetric top model of both molecules.