Electron binding energies of SO2 at the surface of a water cluster

The electronic properties of SO2 at the surface of a water cluster were investigated by employing a combination of Born-Oppenheimer molecular dynamics and electron propagator theory (EPT). In our work, we utilized a revised version of the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, which incorporates empirical corrections for dispersion interactions in line with a recent study of the air-water interface conducted by Ohto et al. [J. Phys. Chem. Lett. 10(17), 4914-4919 (2019)]. Polarization effects induce a significant broadening of the electron binding energy distribution, as predicted by EPT. This broadening can result in a substantial increase in electron affinity, impacting the chemical reactivity of SO2 at the air-water interface, a topic of significant and recent research interest. We discuss the relationship between electron binding energies (EBEs) and the specific connections of SO2 to water. The results indicate that configurations involving an OS⋯H bond tend to yield higher electron affinities compared to complex formation through S⋯OW bonds. Surprisingly, SO2 molecules not bound to water molecules according to a specific criterion may also exhibit higher electron affinities. This feature can be explained by the role played by the polarization field from water molecules. Our best estimate for the HOMO-LUMO (H-L) gap of SO2 at the surface of a water cluster is 11.6 eV. Very similar H-L gaps are predicted for isolated and micro-solvated SO2. Fukui functions for the gas phase, and the micro-solvated SO2-H2O complex supports the view that the LUMO is predominantly localized on the SO2 moiety.