Reaction of Isocyanic Acid and Hydrogen Atom (H + HNCO): Theoretical Characterization

Abstract

The potential energy surface related to the H + HNCO reactions has been explored by means of an ab initio molecular orbital method at the PUMP4/6-311++G(d,p) and QCISD(TC)/6-311++G(2df,2pd)+ZPE levels. The addition of H to N is confirmed to be the dominant reaction channel, giving the H2NCO radical as the primary intermediate and H2N + CO as the fragment products. The contribution of both C- and O-additions to H2NCO formation is negligible. The hydrogen abstraction requires an activation energy larger than that of N-addition but smaller than that of C- and O-additions. Using a quantum statistical Rice−Ramsperger−Kassel (QRRK) model, the kinetics of the N-addition and H-abstraction have been analyzed. The following rate constant expressions in the temperature range 300−3300 K and at a pressure of 1 atm are suggested: for H + HNCO → H2N + CO (via H2NCO*), K = 3.59 × 104T2.49 exp(−1180/T); for H + HNCO → H2NCO, K = 1.63 × 1011T-1.90 exp(−1390/T); for H + HNCO → H2 + NCO, K = 1.76 × 105T2.41 exp(−6190/T); and for H2 + NCO → H + HNCO, K = 1.63 × 104T2.58 exp(−2720/T) in cm3 mol-1 s-1. Kinetic calculations using the exact stochastic method coupled with RRKM theory have also been performed which fully support the simplified QRRK treatment. Overall, calculated rate constants are in excellent agreement with available experimental values. Some thermochemical parameters have also been predicted (at 0 K): (H2NCO) = 3 ± 8 kJ mol-1, IEa(H2NCO) = 7.19 ± 0.2 eV, (H2NCO+) = 696 ± 8 kJ·mol-1, and PA(HNCO) = 723 ± 8 kJ mol-1.