Ab initio theoretical studies on photodissociation of HNCO upon S1(1A")←S0(1A′) excitation:: The role of internal conversion and intersystem crossing

Photodissociation of isocyanic acid, HNCO, was studied with high-level ab initio methods. Geometry optimizations of stationary points and surface crossing seams were performed with the complete active space self-consistent-field (CASSCF) method, and the energetics were re-evaluated with single-point second-order multireference perturbation theory (CASPT2). The three product channels that participate in the photodissociation process are [1] HN(X (3)Sigma(-))+CO at 86.0 (calculated 79.6) kcal/mol, [2] H+NCO(X (2)Pi) at 109.7 (108.7) kcal/mol, and [3] HN(a (1)Delta)+CO at 122.2 (120.8) kcal/mol. The four electronic states, S-0, S-1, T-1, and T-2, that interconnect these channels were studied in detail. S-1 exhibits dissociation barriers to both, channel [2] and [3], whose respective reverse heights are 11.3 and 1.2 kcal/mol, in good agreement with experiment as well as previous theoretical works. The two triplets, T-1 and T-2, show barriers of similar heights for HN bond fission, while S-0 has no barriers to either channel. Various key isomerization transition states as well as numerous minima on the seam of surface crossings (MSX's) were also found. At photoexcitation energies near channel [3] threshold, products to channel [3] are likely to be formed via S-1-->[3] (if enough energy in excitation) and S-1--> S-0-->[3]. Channel [2] can be formed via S-1--> S-0-->[2]; (HN-mode quanta)+S-1--> T-1-->[2]; S-1--> T-2-->[2]; S-1--> T-2--> T-1-->[2], and channel [1] via S-1--> S-0--> T-1-->[1], S-1--> T-1-->[1] and S-1--> T-2--> T-1-->[1]. At higher photoexcitation energies the S-1-->[3] pathway is expected to be dominant while S-1-->[2], with the higher activation energy, is expected to drop rapidly. Also addressed are such important issues as the impact of a vibrationally excited HN mode on a channel [2] yield, and the band origin of the S-1<-- S-0 excitation spectrum. (C) 1999 American Institute of Physics. [S0021-9606(99)30535-3].