VRT spectroscopy and hydrogen bond-breaking dynamics in water clusters

High K_a Rotation-Tunneling (RT) Spectra of (D₂O) ₂ and (H2O)₂ were measured with backward wave oscillators between 2 and 17 cm−1. These extend the characterization of the water dimer vibrational ground state data set to energies comparable to those of intermolecular vibrations. Comparison with (D₂O) ₂ RT eigenstates, calculated on the VRT(ASP-W) potential energy surface demonstrates that further improvements of VRT(ASP-W) are required.; Three new Vibration-Rotation-Tunneling (VRT) bands of the water trimer were measured between 17 and 530 cm−1, corresponding to the three distinct types of intermolecular vibrations. The analysis of a new torsional hot band of (D₂O)₃ at 19.5 cm −1 confirmed the previous description of the (D₂O) ₃ torsional manifold and the validity of the Hamiltonian developed to treat the coupling between torsional and overall rotation. The band also showed effects of previously unobserved higher order perturbations arising from Coriolis coupling. A perpendicular band of (D₂O)₃ at 142.8 cm −1 was assigned to the degenerate, antisymmetric hydrogen bond stretch, corresponding to the first direct observation of a hydrogen bond stretching (translational) vibration for a water cluster. Three parallel bands of (H₂O)₃ were measured between 510 and 525 cm −1 and assigned to three of the four bifurcation tunneling components of the nondegenerate out of plane libration. This first observed librational band for a water cluster demonstrates that the bifurcation tunneling splitting is increased dramatically relative to the ground state. Bifurcation tunneling is of special significance, as it is the lowest energy pathway for breaking and reforming the hydrogen bonds. All types of intermolecular vibrations have thus been characterized for the water trimer, which is essential for the ongoing efforts to determine an accurate potential for liquid water.; The results presented here for the water trimer constitute the first study of mode selectivity in the hydrogen bond breaking dynamics in a water cluster. Excitation of a torsional or translational vibration is shown to have no significant effect on the hydrogen bond lifetime (τ_H = 1–2 ns). In contrast, single excitation of the librational vibration decreases τ_H by three orders of magnitude (τ_H = 1–6 ps). Although a similar dependence of τ_H on intermolecular motions has 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 fact that the bifurcation motion is highly localized makes these trimer dynamics relevant to those in the liquid.