The topography of the HNCO(S1) potential energy surface and its implications for photodissociation dynamics

We report the results of potential energy surface calculations for the S1(11A″) state of HNCO. The ab initio calculations are performed at the MRCI level using the CASSCF method for constructing reference wavefunctions. The most crucial degrees of freedom—the HN and the NC bond distances and the NCO bending angle—are explicitly varied. There is a tiny barrier of about 550 cm−1 towards the NH(α1Δ) + CO(X1Σ+) channel, but a large barrier of ca. 8700 cm−1 blocks the H(2S) + NCO(11A″) channel. The features of the calculated S1 potential energy surface qualitatively explains many results of recent photodissociation experiments.