Solvent effect on the UV/Vis absorption spectra in aqueous solution: The nonequilibrium polarization with an explicit representation of the solvent environment

An explicit solvation model was presented in this work to estimate the solvent effect on the UV/Vis absorption spectra in aqueous solution. Different from others’ concern, we concentrate on the establishment of proper electrostatic solvation energy formula for nonequilibrium polarization in explicit solvent model with adopting the constrained equilibrium approach, which was originally proposed by Leontovich and firstly introduced by us to cope with the nonequilibrium solvation problem. The formulation of this model was deduced entirely based on the explicit representation of the solvent surrounding with discrete dipoles and polarizability for each solvent molecule. The external field, applied to construct the virtual constrained equilibrium state, can be expressed by the physical quantities before and after the Franck–Condon transition. The newly established solvation energy formulation for nonequilibrium was implemented under the popular QM/MM strategy with modifying the nonequilibrium module in the averaged solvent electrostatic potential/molecular dynamics program to carry out numerical calculations. The new codes were further utilized to study the solvatochromic shift of the n → π∗ transition in formaldehyde in aqueous solution. Different contributions to the total solvent shift were analyzed and the main component comes from the electrostatic interaction between the solute and solvent and the solvent polarization also has a significant influence. The solute molecule properties and the microscopic solvent structure were also discussed to uncover more details.