An ab initio study of the dissociation of HNCO in the S1 electronic state

Abstract

Regions of the S1 potential energy surface of HNCO relevant to N–H and C–N bond photodissociation have been investigated with ab initio calculations. Geometries of minima and transition states on S1 as well as those of the product photofragments and the HNCO ground state have been optimized with the CASSCF method, and their energies calculated with MRSDCI and CASPT2 methods. Deep planar trans and cis minima exist on the S1 surface, and are connected by transition states for isomerization. The S0→S1 electronic transition is brighter for trans configurations than for cis, and the initial excitation and dynamics are most likely to proceed through trans configurations. The N–H fission on S1 has a substantial barrier; it occurs more easily through the planar cis transition state, which is about 20 kcal/mol above the dissociation threshold, than through the trans transition state. The C–N fission on S1 can take place both via the planar trans and the planar cis transition state with a low barrier over the dissociation threshold; the reverse barrier is estimated to be a few kcal/mol.