Hydrogen Bond Breaking Dynamics of the Water Trimer in the Translational and Librational Band Region of Liquid Water Frank N. Keutsch, Ray S. Fellers, ‡ Mac G. Brown, § Mark R. Viant, | Poul B. Petersen, and Richard J. Saykally* Contribution from the Department of Chemistry, UniVersity of California, Berkeley, California 94720 ReceiVed October 16, 2000 Abstract: The effect of exciting each of the three classes of intermolecular vibrations on the hydrogen bond lifetime (τ H ) of the isolated water trimer is investigated by far-infrared laser spectroscopy. Single excitation of a librational vibration decreases τ H by 3 orders of magnitude to τ H ) 1-6 ps, comparable to the time scale of a number of important bulk water dynamical relaxation processes. In contrast, excitation of translational or torsional vibrations has no significant effect (τ H ) 1-2 ns). Although such a dependence of τ H on intermolecular motions has also been proposed for liquid water via computer simulations, these are the first experiments that provide a detailed molecular picture of the respective motions without extensive interpretation. The hydrogen bond network and its fluctuations determine the unique properties of liquid water. Many different experi- ments have addressed these dynamics in terms of the underlying intermolecular motions, but they either were insensitive to the microscopic details or required extensive interpretation. 1-8 Far- infrared vibration-rotation-tunneling (VRT) spectroscopy has proven to be a powerful experimental method for unraveling the molecular details of the structures and hydrogen bond rearrangement dynamics of small water clusters. 9-13 However, none of the previous water cluster studies either sampled motions directly corresponding to those existing in the bulk or employed excitation energies relevant to ambient bulk processes. In this first detailed spectroscopic study of the translational and librational vibrations of a water cluster, we realize both ends and exploit the water trimer as a model for the local hydrogen bond breaking dynamics of bulk water. There are two strong intermolecular vibrational bands of liquid water, the “translational band”, centered at 180 cm -1 in H 2 O, and the prominent “librational band”, extending from ca. 300 to 1000 cm -1 in H 2 O. The internal motions giving rise to the “translational band” are essentially hydrogen bond stretching vibrations, while hindered rotational motions of water molecules give rise to the second absorption feature. The results of numerous experiments, e.g., neutron diffraction, dielectric relaxation measurements, and photon-echo spectroscopy, in conjunction with interpretation by theoretical models, have suggested that both the “translational” and the “librational” motions are directly involved in a number of dynamical processes, like the prototropic mobility and the solvation and relaxation dynamics of the liquid. 3,4,6,7,14-18 In recent femtosecond pump-probe experiments, Bakker et al. have studied the dependence of reorentiational (librational) motion on the hydrogen bond stretching dynamics. 19 They propose that excitation of translational motions facilitates the librations that are, in turn, responsible for hydrogen bond breaking through weakening of the hydrogen bond. 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