A THEORETICAL-EXAMINATION OF SOLVATOCHROMISM AND SOLUTE SOLVENT STRUCTURING IN SIMPLE ALKYL CARBONYL-COMPOUNDS - SIMULATIONS USING STATISTICAL MECHANICAL FREE-ENERGY PERTURBATION-METHODS

With use of the statistical mechanical free energy perturbation method (FEP), the difference in Gibbs free energies (ΔΔGs) and potential energies (ΔΔVs) of solvation for the ground state versus the n 1⟶ π⋆ excited state were calculated for the carbonyl containing chromophores formaldehyde and acetone. ΔΔV, corresponds to the difference in excitation energy (ground state to Franck-Condon state transition) for a chromophore in the vapor phase versus when solvated, i.e. the solvent shift energy. ΔΔGs corresponds to the solvated versus vapor phase energy difference (ground to solvent equilibrated excited state), i.e. the difference in free energy between the two adiabatic states. By the term “solvent equilibrated excited state” we refer to the thermally relaxed, solvent equilibrated system containing the electronically excited solute molecule, where Franck-Condon strain has been alleviated. Results were obtained for the carbonyl solutes in TIP3P water, modified-OPLS methanol, and five-center CC14model solvents. Although experimentally measured UV solvent shifts were not quantitatively reproduced in all cases, the calculated difference-energy values exhibited the proper qualitative trend in magnitudes with respect to this series of solvents. The calculated difference-energy values were found to compare particularly well with experimentally observed UV solvent shifts for the n ⟶ π⋆ electronic transitions in water and methanol. Characteristics of solute-solvent orientational structuring were examined for both ground- and excited-state solutes, especially in their relationship to solute-solvent interaction energies. Solute-solvent radial distribution functions provide interesting insight into the different characteristics of the average solvent structures around the ground- and excited-state solutes. Intermolecular energy pair distribution functions reported for the ground, the Franck-Condon, and the equilibrated-excited states illustrate a progressive loss of solute-solvent hydrogen bonding and indicate further desolvation of the solute after excitation in polar solvents. The solvent's response upon n ⟶ π⋆ solute excitation is consistent with current ideas regarding the solvation of hydrophobic moieties. An examination of the solvent-solvent interactions in the solute near-shell and bulk solvent illuminates how the change in the relative strengths of solvent-solvent versus solute-solvent interactions drives the restructuring of the near-shell solvent cage, thus contributing to the differential solvation of ground- and excited-state solutes. © 1990, American Chemical Society. All rights reserved.