AB-INITIO POTENTIAL-ENERGY SURFACES OF LARGE-AMPLITUDE MOTIONS FOR QUASI-SYMMETRICAL TOP MOLECULES - CH3NCO

The potential energy surface of methyl isocyanate, CH3NCO, has been in vestigated in quantum mechanical ab initio calculations at the SCF, MP 2-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-ampli tude motions: the CNC bending motion and internal rotation of the meth yl group. At the most advanced level employed, CCSD(T) with cc-pVTZ ba sis set, the barriers to linearity of the CNCO skeleton and to interna l rotation of the methyl group were determined to be 920 and 22 cm(-1) , respectively. The theoretical results are in good agreement with exp erimental data and consistent with the quasi-symmetric top model of th e molecule.