Ab initio potential energy surfaces of large-amplitude motions for quasi-symmetric top molecules: CH3NCO

The potential energy surface of methyl isocyanate, CH3NCO, has been investigated in quantum mechanical ab initio calculations at the SCF, MP2–MP4, and CCSD(T) levels of theory. The total energy of the molecule was calculated as a function of the coordinates of the two large-amplitude motions: the CNC bending motion and internal rotation of the methyl group. At the most advanced level employed, CCSD(T) with cc-pVTZ basis set, the barriers to linearity of the CNCO skeleton and to internal rotation of the methyl group were determined to be 920 and 22 cm−1, respectively. The theoretical results are in good agreement with experimental data and consistent with the quasi-symmetric top model of the molecule.