A Computational and Experimental View of Hydrogen Bonding in Glycerol Water Clusters.

Abstract

Polyol-water clusters provide a template to probe ionization and solvation processes of paramount interest in atmospheric and interstellar chemistry. We generate glycerol water clusters in a continuous supersonic jet expansion and interrogate the neutral species with synchrotron-based tunable vacuum ultraviolet photoionization mass spectrometry. A series of glycerol fragments (m/z 44, 61, 62, and 74) clustered with water are observed. A judicious combination of backing pressure, nozzle temperature, and water vapor pressure allows for tuning the mol % of glycerol. The recorded appearance energies of the water cluster series m/z 62 and 74 are similar to that observed in pure glycerol, while the m/z 61 series shows a dependence on cluster composition. Furthermore, this series also tracks the water concentration of the beam. Theoretical calculations on neutral and ionized clusters visualize the hydrogen bond network in these water clusters and provide an assessment of the number of glycerol-glycerol, glycerol-water, and water-water hydrogen bonds in the cluster, as well as their interaction energies. This method of bond counting and interaction energy assessment explains the changes in the mass spectrum as a function of mol % and offers a glimpse of the disruption of the hydrogen bond network in glycerol-water clusters. The calculations also reveal interesting barrierless chemical processes in photoionized glycerol water clusters that are either activated or do not occur without the presence of water. Examples include spontaneous intramolecular proton transfer within glycerol to form a distonic ion, nonactivated breaking of a C-C bond, and spontaneous proton transfer from glycerol to water. These results appear relevant to radiation-induced chemical processing of alcohol-water ices in the interstellar medium.