Reaction between NO and NH under Tropospheric Conditions: A Quantum Chemical and Chemical Kinetic Investigation.

The gas-phase reaction of NO with NH under tropospheric conditions has been carried out employing quantum chemical calculations at the CCSD(T)/CBS//MP2/aug-cc-pVTZ level of theory. The activation barrier of the reaction was found to be 13.5 kcal mol with respect to isolated reactants. Chemical kinetic calculations were carried out under pre-equilibrium approximation using transition-state theory employing Eckart tunneling, and the rate coefficient was found to be 1.02 × 10 cm molecule s at 298 K. To check the reliability of the result, calculations have also been carried out using the canonical variational transition-state theory employing both zero- and small-curvature tunneling as well as the master equation. The results obtained from these methods were found to be consistent with those obtained from the transition-state theory. The rate coefficient was found to show positive temperature dependence, and its rate of change with temperature was found to be very similar for all three methods. Further, the pressure dependence of the rate coefficient has been checked and it was found that it shows negligible pressure dependence under tropospheric conditions. Besides, all of the electronic structure calculations have been carried out at the CCSD(T)-F12/cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory and the rate coefficients using all of the above-mentioned kinetic models have been computed using these results. The results were found to match closely with the CCSD(T)//CBS//MP2/aug-cc-pVTZ results.