PNNL

Abstract

The spectroscopic features of protonated water species in dilute acid solutions have been long sought after for understanding the microscopic behavior of the proton in water with gas-phase water clusters H+(H2O)n extensively studied as bottom-up model systems. We present a new, novel protocol for the calculation of the Infrared spectra of complex systems, which combines fragment-based Coupled Cluster and anharmonic vibrational quasi-degenerate perturbation theory and demonstrate its accuracy in yielding the first complete and accurate assignment of the IR spectra of the H+(H2O)21 cluster over the entire frequency region of the experiment. The site-specific IR spectral signatures reveal two distinct structures for the internal and surface four-coordinated water molecules, which are ice-like and liquid-like, respectively. The revelation of the spectral signature of the excess proton offers deeper insight into the nature of charge accommodation in the extended hydrogen-bonding network underpinning this aqueous cluster.