Vibrational Energy Flow in the Uracil-H2O Complexes.

Abstract

In the uracil-H2O complex, the vibrational energy initially stored in the OH(v = 1) stretch efficiently transfers to the first overtone-bending mode under a near-resonant condition. The relaxation of the overtone vibration redistributes its energy to uracil and the two hydrogen bonds in the intermolecular zone, which consists of the OH bond and the bonds between nearby C, N, O, and H atoms of uracil. The uracil NH bond and the hydrogen bond it formed with the H2O molecule, N-H···O, store the major portion of the energy released by the relaxing bending mode, thus forming a localized hot band in the intermolecular zone. Energy transfer to the bonds beyond the zone is found to be not significant. The excited uracil NH is found to transfer its energy to the bending mode, thus indicating that the hydrogen bond of N-H···O is the principal energy pathway in both directions. In the presence of efficient near-resonant energy transfer pathways, the time evolution of the centers of mass distance shows the phenomenon of beats. One global and two different local minima energy structures are considered. The results of energy transfer do not differ significantly, suggesting that the two hydrogen bonds in all three structures have similar contributions to the energy transfer.