Theoretical Study on the Formation of H- and O-Atoms, HONO, OH, NO, and NO₂ from the Lowest Lying Singlet and Triplet States in Ortho-Nitrophenol Photolysis

The photolysis of nitrophenols was proposed as a source of reactive radicals and NOx compounds in polluted air. The S-0 singlet ground state and T-1 first excited triplet state of nitrophenol were investigated to assess the energy dependence of the photofragmentation product distribution as a function of the reaction conditions, based on quantum chemical calculations at the G3SX//M06-2X/aug-cc-pVTZ level of theory combined with RRKM master equation calculations. On both potential energy surfaces, we find rapid isomerization with the aci-nitrophenol isomer, as well as pathways forming NO, NO2, OH, HONO, and H-, and O-atoms, extending earlier studies on the T-1 state and in agreement with available work on other nitroaromatics. We find that accessing the lowest photofragmentation channel from the S-0 ground state requires only 268 kJ/mol of activation energy, but at a pressure of 1 atm collisional energy loss dominates such that significant fragmentation only occurs at internal energies exceeding 550 kJ/mol, making this surface unimportant for atmospheric photolysis. Intersystem crossing to the T-1 triplet state leads more readily to fragmentation, with dissociation occurring at energies of approximate to 450 kJ/mol above the singlet ground state even at 1 atm. The main product is found to be OH + nitrosophenoxy, followed by formation of hydroxyphenoxy + NO and phenyloxyl + HONO. The predictions are compared against available experimental data