Mechanism and rate constants for ammonia photochemical oxidation with O2 on the singlet and triplet potential energy surfaces

Abstract

The mechanism and thermodynamic of NH3 + O2 reaction on the singlet and triplet potential energy surfaces (PES), were carried out using the RMP2 and CCSD (T)//RMP2 theoretical approaches in connection with the 6-311++G(d, p) basis set. Three pre-reactive complexes, 1C1, 1C2, and 3C1 on the singlet and triplet PES were formed between ammonia and molecular oxygen. With variety of pre-reactive complexes, six types of products are obtained, of which two types are found to be thermodynamically stable. The mechanistic properties of all products channels are discussed. Results show that production of HONO + H2 and HN(OH)2 are the main reaction channels in thermodynamic viewpoint with the Gibbs free energy of ∆G° = −34.681 and −27.153 kcal/mol, respectively. Rate constants of the title reaction over the temperature range of (200–1000 K) show kinetic products are different from thermodynamic products.