Comparison between Experimental and Three-Dimensional Quantum Mechanical Rate Constants for the NH(D) + NO Reactions

Abstract

We describe a three-dimensional quantum mechanical study within the nonreactive infinite order sudden approximation (IOSA) for the title systems. The study was performed using a recently introduced global potential energy surface (Bradley et al. J. Chem. Phys. 1995, 102, 6696). Integral total cross sections for the two reactions, namely, NO + NH and NO + ND, were calculated as a function of kinetic energy in the range 0.05−0.50 eV. Using these cross sections, temperature-dependent rate constants were calculated. Our main findings are: (a) The energy-dependent cross sections for the two isotopic reactions are very similar; still, the cross sections for NO + NH are larger at the low-energy region while those for NO + ND are somewhat larger at the high-energy region. (b) The two cross section curves start to increase at zero kinetic energy, indicating the absence of a potential energy barrier or the existence of a mild one at most. (c) Compared with quasi-classical-trajectory cross sections the present quantum mechanical cross sections for NH + NO are half as large at the low energy region but tend to become similar at the higher one. (d) The calculated rate constants for NH + NO were compared with experiment (for a wide range of temperatures) and with one quasi-classical trajectory result (at T = 300 K). There is a very encouraging agreement at the high-temperature region (1200 ≤ T ≤ 5000 K) but large discrepancies (1 order of magnitude difference) at low temperatures. The fit with the (single) classical result was reasonably good. (e) Rate constants calculated for ND + NO were found to be very similar to the rate constants of NH + NO. A single measured value, k(T = 300 K) = (2.7 ± 0.4) 10-11 molec-1 cm3 s-1, reported here for the first time, is equal to half the corresponding value for NH + NO, namely, k(T = 300 K) = (5.5 ± 0.3) × 10-11 molec-1 cm3 s-1, and it therefore fit the QM curve slightly better.