Revealing the Photochemical Pathways of Nitrate in Water through First-Principles Simulations

Nitrate anion (NO3-) is a ubiquitous species in aqueous phases in the environment, including atmospheric particles, aerosol droplets, surface waters, and snow. The photolysis of nitrate is a 'renoxification' process, which converts \nitrate solvated in water or deposited on surfaces back into NOx to the atmosphere. Nitrate photolysis under environmental conditions can follow two channels: (1) NO2 and O-; (2) nitrite and O. Despite the well-studied macroscopic kinetics of the two channels, the microscopic picture of the photolysis still needs to be explored. Furthermore, previous experiments have shown that nitrate photolysis in aqueous solutions has a low quantum yield of ~1% leading to a solvation cage effect hypothesis. A previous theoretical study has indicated that the low quantum yield may be due to the direct spin-forbidden absorption of \nitrate to its triplet state. Here, we employ first-principles molecular dynamics simulations at the level of hybrid DFT with enhanced sampling to explore the two channels in an aqueous solution to unravel the atomistic and electronic structure details of the photolysis, as well as investigate the causes of its low quantum yield under a solvation environment. The direct spin-forbidden absorption to T1 state is viable through spin-orbit coupling and is ~15 times weaker than the spin-allowed absorption to S1 state. A solvation cage complex is identified as a metastable state that requires additional thermal energy to complete the dissociation of the N-O bond at the triplet state. This metastable state allows the photo fragments to recombine or deactivate through non-radiative processes. Our simulations also qualitatively explain the temperature dependence of the two channels observed in experiments based on the rearrangement of H-bonds. This work provides a novel molecular picture illustrating the significantly low quantum yield and temperature dependence of nitrate photolysis under environmental conditions and a starting point for future studies of environmental nitrate photochemistry.