Experimental and Theoretical Investigations of the Dissociation Energy (<i>D</i>₀) and Dynamics of the Water Trimer, (H₂O)₃

We report a joint experimental-theoretical study of the predissociation dynamics of the water trimer following excitation of the hydrogen bonded OH-stretch fundamental. The bond dissociation energy (<i>D</i>₀) for the (H₂O)₃ → H₂O + (H₂O)₂ dissociation channel is determined from fitting the speed distributions of selected rovibrational states of the water monomer fragment using velocity map imaging. The experimental value, <i>D</i>₀ = 2650 ± 150 cm^–1, is in good agreement with the previously determined theoretical value, 2726 ± 30 cm^–1, obtained using an ab initio full-dimensional potential energy surface (PES) together with Diffusion Monte Carlo calculations [Wang; Bowman. J. Chem. Phys. 2011, 135, 131101]. Comparing this value to <i>D</i>₀ of the dimer places the contribution of nonpairwise additivity to the hydrogen bonding at 450–500 cm^–1. Quasiclassical trajectory (QCT) calculations using this PES help elucidate the reaction mechanism. The trajectories show that most often one hydrogen bond breaks first, followed by breaking and re-forming of hydrogen bonds (often with different hydrogen bonds breaking) until, after many picoseconds, a water monomer is finally released. The translational energy distributions calculated by QCT for selected rotational levels of the monomer fragment agree with the experimental observations. The product translational and rotational energy distributions calculated by QCT also agree with statistical predictions. The availability of low-lying intermolecular vibrational levels in the dimer fragment is likely to facilitate energy transfer before dissociation occurs, leading to statistical-like product state distributions