Experimental and Theoretical Investigations of Energy Transfer and Hydrogen-Bond Breaking in Small Water and HCl Clusters

Conspectus Water is one of the most pervasive molecules on earth and other planetary bodies; it is the molecule that is searched for as the presumptive precursor to extraterrestrial life. It is also the paradigm substance illustrating ubiquitous hydrogen bonding (H-bonding) in the gas phase, liquids, crystals, and amorphous solids. Moreover, H-bonding with other molecules and between different molecules is of the utmost importance in chemistry and biology. It is no wonder, then, that for nearly a century theoreticians and experimentalists have tried to understand all aspects of H-bonding and its influence on reactivity. It is somewhat surprising, therefore, that several fundamental aspects of H-bonding that are particularly important for benchmarking theoretical models have remained unexplored experimentally. For example, even the binding strength between two gas-phase water molecules has never been determined with sufficient accuracy for comparison with high-level electronic structure calculations. Likewise, the effect of cooperativity (nonadditivity) in small H-bonded networks is not known with sufficient accuracy. An even greater challenge for both theory and experiment is the description of the dissociation dynamics of H-bonded small clusters upon acquiring vibrational excitation. This is because of the long lifetimes of many clusters, which requires running classical trajectories for many nanoseconds to achieve dissociation. In this Account, we describe recent progress and ongoing research that demonstrates how the combined and complementary efforts of theory and experiment are enlisted to determine bond dissociation energies (D 0) of small dimers and cyclic trimers of water and HCl with unprecedented accuracy, describe dissociation dynamics, and assess the effects of cooperativity. The experimental techniques rely on IR excitation of H-bonded X–H stretch vibrations, measuring velocity distributions of fragments in specific rovibrational states, and determining product state distributions at the pair-correlation level. The theoretical methods are based on high-level ab initio potential energy surfaces used in quantum and classical dynamical calculations. We achieve excellent agreement on D 0 between theory and experiments for all of the clusters that we have compared, as well as for cooperativity in ring trimers of water and HCl. We also show that both the long-range and the repulsive parts of the potential must be involved in bond breaking. We explai