Excited-State Decay and Photolysis of <i>O</i>-Nitrophenol after Proton Transfer. II: A Theoretical Investigation in the Microsolvated Atmospheric Environment

Changes in atmospheric humidity affect the number of water molecules surrounding <i>o</i>-nitrophenol (ONP), creating an anisotropic chemical environment. It, in turn, influences the photodynamic behaviors of ONP, differing from those observed in the gas phase and in solution. Recently, we explored the excited-state decay and the generation of the hydroxyl (OH) radical before proton transfer of ONP in the microsolvated environment using the MS-CASPT2//CASSCF approach. As is well known, ONP is capable of converting to its aci-nitro isomer (aciONP) via an excited-state intramolecular proton transfer (ESIPT) process. In the present work, the photoinduced dynamics of aciONP, which can lead to an OH radical and nitrous acid (HONO), was studied using the same computational model. Our calculations demonstrated that increasing the number of water molecules affects the molecular geometries, particularly the key bond lengths and dihedral angles of the HONO group, while also reducing the relative energies of minima and intersections. Moreover, we identified two distinct types of minimum structures: one that retains the intramolecular hydrogen bond and the other that breaks the hydrogen bond with the H atom flipping outward. The latter structure, compared with the former, has a different electronic-state character and facilitates intersystem crossing processes. Subsequently, two major excited-state decay paths were proposed: (PATH I) ESIPT → S₁ → S₁S₀ → S₀; (PATH II) ESIPT → S₁ → S₁-2 → S₁T₁ → T₁ → S₀T₁ → S₀. Furthermore, the T₁ state has a relatively long lifetime, allowing for the formation of the OH radical and HONO, and the corresponding energy barriers decrease as the number of water molecules increases. These theoretical findings provide valuable insights into the photodynamics of aciONP in the microsolvated atmospheric environment.