Solvation and reorganization energies in polarizable molecular and continuum solvents

The solvation free energy difference, Delta G, and reorganization energy, lambda, of the electronic transition between the ground and first excited state of formaldehyde are investigated as a function of the solvent electronic polarizability in aqueous solution. Solvent shifts are difficult to measure experimentally for formaldehyde due to oligomer formation shifts for acetone, which have been measured experimentally, are used instead for comparison with computational results. Predictions of the Poisson-Boltzmann equation of dielectric continuum theory with molecular shaped cavities and charges on atomic sites calculated from ab initio quantum chemistry are compared with direct molecular dynamics simulations using the fluctuating charge model of polarizable water. The explicit molecule simulations agree with the acetone experimental results, but the continuum dielectric calculations do not agree with explicit solvent or with experiment when the default model cavity is used for both the ground and excited state molecule. Several different algorithms are used to define the size of the molecular cavity in the ground and excited states, but we are unable to find a single set of atomic radii that describe adequately all the data. Quantitative calculations from a continuum model might therefore require charge-dependent solute cavity radii. (C) 1997 American Institute of Physics.