Examination of the Mechanism of the Yield of N₂O from Nitroxyl (HNO) in the Solution Phase by Theoretical Calculations

The dimerization of HNO and subsequent yield of N₂O in aqueous solution are studied based on the theoretical calculations and kinetic simulations. The initial dimerization reactions were computed at various levels of theory, and large divergence was observed in the predictions of the gas-phase free energies. The T₁ diagnostics at CCSD(T)/aug-cc-pVTZ suggests multireference characteristics of the HNO dimers and the transition states. The solution-phase free energies were obtained using the wB97XD method and the SMD solvation model. The pK_a values of the (HNO)₂ tautomers and their first protonated and deprotonated products were estimated using the cluster-continuum approach. The theoretical results confirmed the original conclusion that the favored cis-pathway is comprised of several rapid proton transfer steps leading to either cis-HONNOH or cis-HONNŌ before decomposition. Several new water-catalyzed and H₃O⁺/water catalyzed reactions are presented to explain the fast kinetics observed in the experiments. To validate the proposed mechanism, kinetic simulations with the consideration of diffusion-limited kinetics were implemented on several related systems, based on which the previously reported global rate constant was explained as the kinetics of the initial dimerization step and the global kinetics in very dilute HNO solutions.