Mechanism of NH2+CO2 formation in OH+HNCO reaction: Rate constant evaluation via ab initio calculations and statistical theory

Portions of the [CH2NO2] potential energy surface related to the OH+HNCO reaction were calculated by means of ab initio molecular orbital theory at the QCISD(T)/6-311++G(d,p) level based on UMP2/6-31G(d,p) optimized geometries. Of all possible three channels considered, the hydrogen abstraction turns out to be the dominant reaction channel. The addition to C atom requires activation energy slightly larger than that of the abstraction but smaller than that of the N addition, in contrast to the H+HNCO reaction. The structural and energetic parameters for the channels thus characterized were further utilized for the calculation of rate constants in the framework of a quantum statistical theory (QRRK). The contributions of the individual reaction channel towards the total rate constant have been examined. Although the OH+HNCO→NH2+CO2 reaction is more exothermic than the hydrogen abstraction OH+HNCO→H2+NCO, it is confirmed that rate constant for CO2 loss is much lower than that of H2O-elimination. The standard heat of formation of the adduct HNC(OH)O is estimated to be ΔHf298=−41.1±3 kcal/mol.