Excited-State Decay\nand Photolysis of <i>O</i>‑Nitrophenol before Proton\nTransfer. I: A Theoretical Investigation\nin the Microsolvated Atmospheric Environment

As a potential source of the hydroxyl\n(OH) radical and\nnitrous\nacid (HONO), photolysis of <i>o</i>-nitrophenol (ONP) is\nof significant interest in both experimental and theoretical studies.\nIn the atmospheric environment, the number of water molecules surrounding\nONP changes with the humidity of the air, leading to an anisotropic\nchemical environment. This may have an impact on the photodynamics\nof ONP and provide a mechanism that differs from previously reported\nones in the gas phase or in solution. Herein, the high-level MS-CASPT2//CASSCF\nmethod was performed to elucidate the excited-state decay and the\ngeneration of the OH radical for ONP before proton transfer in the\nmicrosolvated surrounding. We found that the varying number of water\nmolecules affects the ground-state structures and alters the energy\nlevels of nπ* and ππ* at the Franck–Condon\n(FC) region. Nevertheless, this is not the case for the excited-state\nminima, which exhibit very similar adiabatic excitation properties.\nIn addition, the presence of water molecules also significantly influences\nthe intersection structures since hydrogen bonds will hinder or alleviate\nthe rotation or pyramidalization of the nitro (NO<sub>2</sub>) group.\nThis will, in turn, change the excited-state relaxation mechanism\nof ONP. Finally, we speculated that the OH radical might be formed\nin the hot ground state of ONP in the microsolvated surrounding after\nexploring all possible electronic states.