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

Changes in atmospheric humidity affect the number of\nwater molecules\nsurrounding <i>o</i>-nitrophenol (ONP), creating an anisotropic\nchemical environment. It, in turn, influences the photodynamic behaviors\nof ONP, differing from those observed in the gas phase and in solution.\nRecently, we explored the excited-state decay and the generation of\nthe hydroxyl (OH) radical before proton transfer of ONP in the microsolvated\nenvironment using the MS-CASPT2//CASSCF approach. As is well known,\nONP is capable of converting to its aci-nitro isomer (aciONP) via\nan excited-state intramolecular proton transfer (ESIPT) process. In\nthe present work, the photoinduced dynamics of aciONP, which can lead\nto an OH radical and nitrous acid (HONO), was studied using the same\ncomputational model. Our calculations demonstrated that increasing\nthe number of water molecules affects the molecular geometries, particularly\nthe key bond lengths and dihedral angles of the HONO group, while\nalso reducing the relative energies of minima and intersections. Moreover,\nwe identified two distinct types of minimum structures: one that retains\nthe intramolecular hydrogen bond and the other that breaks the hydrogen\nbond with the H atom flipping outward. The latter structure, compared\nwith the former, has a different electronic-state character and facilitates\nintersystem crossing processes. Subsequently, two major excited-state\ndecay paths were proposed: (PATH I) ESIPT → S<sub>1</sub> →\nS<sub>1</sub>S<sub>0</sub> → S<sub>0</sub>; (PATH II) ESIPT\n→ S<sub>1</sub> → S<sub>1</sub>-2 → S<sub>1</sub>T<sub>1</sub> → T<sub>1</sub> → S<sub>0</sub>T<sub>1</sub> → S<sub>0</sub>. Furthermore, the T<sub>1</sub> state\nhas a relatively long lifetime, allowing for the formation of the\nOH radical and HONO, and the corresponding energy barriers decrease\nas the number of water molecules increases. These theoretical findings\nprovide valuable insights into the photodynamics of aciONP in the\nmicrosolvated atmospheric environment.