A polarizable mixed Hamiltonian model of electronic structure for solvated excited states. II. Application to the blue shift of the H2CO 1(π*←n) excitation in water

We illustrate an application of a polarizable mixed Hamiltonian model of solvation developed in the companion Paper I [J. Chem. Phys. 117, 1242 (2002)] and describe the structure of electronically excited formaldehyde in water. We used Hartree-Fock and multiconfiguration wave functions together with the tip3p, pol1, and pol2 interaction potentials combined with the Bartlett–Shavitt vibrational potential for water. We calculated the structure of H2CO (A11, A23, and A21) micro-solvated with 1 or 2 water molecules and we mimicked the aqueous environment with up to 81 waters with equilibrium solute–solvent configurations. We calculated the vertical and adiabatic excitations energies. The vertical absorption energy shows a blue shift between ∼1000 and ∼2500 cm−1 due to solvation, that is in fact already present in the micro-solvated systems and increases with the degree of solvation. The dipole moments of the ground and excited states show a marked increase with the degree of solvation. The polarizable character of the pol1 and pol2 water potentials has only a minor effect on the magnitude of the shift, even on the vertical excitation shift, with a reduction of ∼100 cm−1 in blue shift. The polarizable mixed model gives a satisfactory description of the formaldehyde–water hydrogen-bond structure and of the energetics. Those are very similar to the all-quantum chemical description when considering ground-state H2CO. For the excited states (A23 and A21) the H2CO---HOH distance in H2CO:1w is calculated to be ∼0.10 Å shorter with the polarizable mixed model than with the all-quantum chemical model, albeit the calculated hydrogen bond energies are in accord with the all-quantum chemical results and smaller than for the ground state. This finding suggests that, at least in the equilibrium solvation regime, the σ and ε Lennard-Jones parameters for the excited states of H2CO should have larger values than those used for the ground state, in accord with simple arguments based on the increased size and polarizability of the molecular excited state.