Solvent effects on the n->pi transition of pyrimidine in aqueous solution

A hybrid quantum mechanical and molecular mechanical potential is used in Monte Carlo simulations to examine the solvent effects on the electronic excitation energy for the n --> pi* transition of pyrimidine in aqueous solution. In the present study, the pyrimidine molecule is described by the semi-empirical AM I model, while the solvent molecules are treated classically. Two sets of calculations are performed: the first involves the use of the pairwise three-paint charge TIP3P model for water, and the second computation employs a polarizable many-body potential for the solvent. The latter calculation takes into account the effect of solvent polarization following the solute electronic excitation, and makes a correction to the energies determined using pairwise potentials, which neglects such fast polarization effects and overestimates the solute-solvent interactions on the Franck-Condon excited states, Our simulation studies of pyrimidine in water indicate that the solvent charge redistribution following the solute excitation makes modest corrections (about -130 cm(-1)) to the energy predicted by using pairwise potentials. Specific hydrogen bonding interactions between pyrimidine and water are important for the prediction of solvatochromic shifts for pyrimidine. The computed n --> pi* blue shift is 2275 +/- 110 cm(-1), which may be compared with the experimental value (2700 cm(-1)) from isooctane to water.