Computational Studies of the Isomerization Pathways of Isocyanic Acid HNCO

Paths of isomerizations of isocyanic acid HNCO into various chain isomers via ring intermediates have been investigated by ab initio MO calculations with the MRD-CI procedure employing the 6-31G** basis sets. Geometry optimizations were accomplished at the RHF, UHF, or CASSCF level of theory. It is found that the isomerization from HNCO to cyanic acid HOCN is energetically the most favorable and that it should proceed via successive 1,2-hydrogen migrations rather than by a single 1,3-hydrogen migration. The overall barrier height is calculated to be 423 kJ mol−1, which is ca. 60 kJ mol−1 higher than the critical energy for the main decomposition channel, HNCO→NH(3Σ)+CO. Implications of the results to the kinetics of the thermal decompostion of HNCO at high temperatures are discussed.