PNNL

Abstract

The previously formulated Quantum Mechanical (QM) / Molecular Mechanical (MM) model applicable to micro-solvated solute excited state, the QM/MM-pol-vib/CAV model, has been combined with a Monte Carlo averaging scheme to derive the averaged properties of the solvated solutes. The methodology was applied to the electronic 1(p*n) excitation of formaldehyde in water. We first performed Monte Carlo MM/MM calculations to generate the water configurations. Then, we chose 400 configurations for the QM/MM excitation energy calculations. Finally, we carried out Complete Active Space Self-Consistent Field (CASSCF) calculations to derive the average excitation energy. Several different sizes of water clusters with 23, 54 and 108 water molecules were used. The first solvent shell of the clusters was found to be well structured. We also calculated the shift of the vertical excitation energies and of the dipole moments resulting from micro-solvation. The calculated blue shift of the vertical excitation energies using a non-polarizable MM potential was in the range 2610 ~ 2690 cm-1, and using a polarizable MM potential, was in the range 2540 ~ 2660 cm-1. Thus the treatment that considered polarization improved the results, although the improvement was not significant. The cluster size dependence has been found to be small which indicates that the outer water molecules have little influence to the solute-solvent interaction. The dipole moments of the ground and excited states showed a significant increase arising from micro-solvation. The ground state dipole moment showed larger solvent shifts than the excited state dipole moment. This leads to a decrease in the strength of the hydrogen bond between the oxygen atom of formaldehyde and hydrogen atoms of water after excitation. We analyzed the structures of the solvent configurations that produced both high and low blue shifts. The first solvent shell is proven to play a principal role in the solvent effect.