Ab initio theoretical studies on photodissociation of HNCO upon S-1((1)A '')<- S-0((1)A(')) excitation: The role of internal conversion and intersystem crossing

Photodissociation of isocyanic acid, HNCO, was studied with high-level ab i nitio methods. Geometry optimizations of stationary points and surface cros sing seams were performed with the complete active space self-consistent-fi eld (CASSCF) method, and the energetics were re-evaluated with single-point second-order multireference perturbation theory (CASPT2). The three produc t 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 10 9.7 (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 bot h, channel [2] and [3], whose respective reverse heights are 11.3 and 1.2 k cal/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 i somerization transition states as well as numerous minima on the seam of su rface crossings (MSX's) were also found. At photoexcitation energies near c hannel [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 photoexci tation 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 excit ed HN mode on a channel [2] yield, and the band origin of the S-1<-- S-0 ex citation spectrum. (C) 1999 American Institute of Physics. [S0021-9606(99)3 0535-3].