MOLECULAR-DYNAMICS SIMULATION OF TIME-RESOLVED FLUORESCENCE AND NONEQUILIBRIUM SOLVATION OF FORMALDEHYDE IN WATER

The results of molecular dynamics simulations of nonequilibrium solvation and time-resolved fluorescence from the first excited singlet state of formaldehyde in water are reported. The laser excitation is simulated by instantaneously switching the solute charges and force constants from ground- to excited-state values during the course of a molecular dynamics simulation of the ground-state species in water. Eighty nonequilibrium trajectories were completed, starting from different configurations obtained from the ground-state simulation. The fluorescence shift and solvation relax nonexponentially on a very fast time scale. The major portion of the relaxation occurs within 100 fs; additional relaxation is observed up to 1 ps. Time-dependent radial and angular distribution functions for solute-solvent interactions have been calculated. The relaxation is dominated by changes in the structure of the first solvation shell. Translational motion of the solvent provides the major mechanism for the relaxation. The relationship of the present computer experiments to theories of nonequilibrium solvation is discussed. © 1990 American Chemical Society.