Kinetics and Mechanism of the NH (X³Σ^-) + SO (X³Σ^-) Reaction: A Theoretical Approach

The reaction mechanism, product branching ratios, and relevant rate constants for the reaction of imidogen (NH) with sulfur monoxide (SO) over singlet and triplet potential energy surfaces are theoretically investigated. Various quantum chemical methods at the single-reference methods (PBE, M06-2X, MP2, GBS-QB3, G3MP2B3, and CCSD(T)) and the multi-reference methods of CASPT2 are carried out to examine the characteristics of the title reaction's potential energy surface. Eighteen chemically activated intermediates and more than 35 different reaction channels are predicted over the singlet surface, while less species and channels are distinguished over the triplet surface. The entrance channels for both surfaces appeared to be barrier-less association reactions to form pre-reaction energized intermediates of singlet or triplet HNSO or HNOS. OH and NS radicals are indicated as the major products for the title reaction on both surfaces in agreement with the reported experimental observations. The RRKM-steady state approximation method is used to calculate the rate constants and branching ratios of the main products. The obtained overall rate constant is in agreement with the available reported experimental data over the wide range of temperature from 300 to 3000 K. By considering single-reference calculations, the singlet and triplet total rate constants were found to be k(T) = 5.04 × 10¹⁰ and 2.47 × 10¹² T^-0.83 exp(-1.56 kJ mol^-1/T), respectively. Also, the total rate constant for the consumption of reactants by inclusion of multi-reference calculations was found to be in the range of 3.86 × 10¹⁰ to 4.18 × 10¹⁰, depending on the level of calculations. In addition, our results revealed that the total rate constant for the NH + SO reaction is pressure-independent in the range of 0.1-2000 Torr.