Experiment and theory elucidate the multichannel predissociation dynamics of the HCl trimer: breaking up is hard to do.

The breaking of hydrogen bonds in molecular systems has profound effects on liquids, e.g., water, biomolecules, e.g., DNA, etc., and so it is no exaggeration to assert the importance of these bonds to living systems. However, despite years of extensive research on hydrogen bonds, many of the details of how these bonds break and the corresponding energy redistribution processes remain poorly understood. Here we report extensive experimental and theoretical insights into the breakup of two or three hydrogen bonds in the dissociation of a paradigm system of a hydrogen-bonded network, the ring HCl trimer. Experimental state-to-state vibrational predissociation dynamics of the trimer following vibrational excitation were studied by using velocity map imaging and resonance-enhanced multiphoton ionization, providing dissociation energies and product state distributions for the trimer's breakup into three separate monomers or into dimer + monomer. Accompanying the experiments are high-level calculations using diffusion Monte Carlo and quasiclassical simulations, whose results validate the experimental ones and further elucidate energy distributions in the products. The calculations make use of a new, highly accurate potential energy surface. Simulations indicate that the dissociation mechanism requires the excitation to first relax into low-frequency motions of the trimer, resulting in the breaking of a single hydrogen bond. This allows the system to explore a critical van der Waals minimum region from which dissociation occurs readily to monomer + dimer.