SOLVENT EFFECTS ON MOLECULAR-SPECTRA .2. SIMULATIONS OF HYDRATED CLUSTERS AND DILUTE-SOLUTIONS OF PYRIMIDINE IN ITS LOWEST (N,PI-ASTERISK) SINGLET EXCITED-STATE

Hydrogen bonding between pyrimidine in its lowest (n,pi) singlet exci ted state and water in dilute solution is investigated using NPT-ensem ble Monte Carlo simulations, and the properties of the pyrimidine-wate r complex are examined using molecular dynamics. The 1(n,pi) excitati on, known experimentally to be delocalized in the gas phase, is shown to remain delocalized when pyrimidine undergoes hydrogen bonding. Assu ming that the intermolecular interactions are electronic state indepen dent, Kollman's Lennard-Jones interactions are combined with molecular charges obtained in four different ways, generating a variety of inte rmolecular pair potentials. It is found that, in solution, both pyrimi dine-water-hydrogen bonds are considerably weakened in the excited sta te; however, on average, one hydrogen bond per pyrimidine remains. The excited state hydrogen bonding gives rise to structures in the liquid more like those found in van der Waals bonded systems than in strongl y hydrogen-bonded systems such as ground-state pyrimidine in water. A blue shift of the absorption band origin is correctly predicted and it s magnitude and Franck-Condon contribution are in reasonable agreement with experiment. For pyrimidine-water clusters, minor changes in the intermolecular potential surfaces are shown to cause major qualitative differences in the excited state cluster dynamics, mimicking the obse rved spectral properties of several different hydrogen-bonded pyrimidi ne clusters.