|Laser-initiated shuttling of a water molecule between distinct hydrogen bonding sites on a peptide, trans-formanilide (TFA), has recently been reported . We present here results of a computational study of the kinetics of water molecule shuttling, with particular focus on the influence of energy transfer between TFA and the attached water molecule. Since stimulated emission pumping is used in the experimental study to initiate the reaction, excitation is initially localized in the TFA molecule. The ergodicity threshold in TFA is calculated using ab initio values for the anharmonic constants to be about 1500 cm-1, exceeding the experimentally established barrier for water shuttling of between about 750 cm-1 and 1000 cm-1, indicating that collisions may play an important role in energy scrambling involved in reaction. Coupling between TFA and water also enables energy flow in the complex below 1500 cm-1. We compute the time for energy transfer from TFA into the intermolecular modes of the complex to be typically between 20 and 100 ps, times that significantly influence water shuttling kinetics, yielding a rate that is several orders of magnitude below the RRKM theory estimate.|
1. J. R. Clarkson, E. Baquero, V. A. Shubert, E. M. Myshakin, K. D. Jordan, T. S. Zwier, Science 307, 1443 (2005).
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