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pi --> n fluorescence transition in formaldehyde in aqueous solution: a combined quantum chemical statistical mechanical study.

The solvent shift to the fluorescence transition pi --> n in formaldehyde in aqueous solution is theoretically analyzed. The solvent model has explicit representation of the solvent and uses the complete active space state interaction (CASSI) method to obtain a description of the wave function of the solute similar to what the complete active space self-consistent-field (CASSCF) method would give. In the description of the solute-solvent interaction the discrete set of solvent molecules perturb the solute not only through an electrostatic perturbation but also through a nonelectrostatic operator. The latter describes in a way analogous to pseudopotential theory the effect the Pauli principle has on the solute embedded in the solvent. This way the exchange repulsion between solute and solvent is accounted for which therefore can be anisotropic. The best estimate of the average shift is a blue shift of 0.003 eV, and for the current transition the nonelectrostatic perturbation broadens the distribution but has no significant effect on the average shift.