SOLVATION AND REORGANIZATION ENERGIES IN POLARIZABLE MOLECULAR AND CONTINUUM SOLVENTS

The solvation free energy difference, Delta G, and reorganization ener gy, lambda, of the electronic transition between the ground and first excited state of formaldehyde are investigated as a function of the so lvent electronic polarizability in aqueous solution. Solvent shifts ar e 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. Predicti ons of the Poisson-Boltzmann equation of dielectric continuum theory w ith molecular shaped cavities and charges on atomic sites calculated f rom ab initio quantum chemistry are compared with direct molecular dyn amics simulations using the fluctuating charge model of polarizable wa ter. The explicit molecule simulations agree with the acetone experime ntal results, but the continuum dielectric calculations do not agree w ith explicit solvent or with experiment when the default model cavity is used for both the ground and excited state molecule. Several differ ent 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 ca lculations from a continuum model might therefore require charge-depen dent solute cavity radii. (C) 1997 American Institute of Physics.