Experimental and computational studies of dynamic processes in ionic water clusters

Abstract

Clusters of water molecules mimic the transition from gas phase to bulk water. Cluster species of the desired sizes can be selected using mass spectrometric techniques and their size-dependent properties can thereby be measured. The properties of small ionic clusters are of particular relevance to atmospheric science, providing insights into nucleation phenomena. This work is dedicated to the investigation of properties of selected ionic molecular clusters and their gas phase reactions with heavy water or ammonia, with a strong emphasis on proton transfer phenomena. This has been achieved both experimentally in cluster beam experiments and by quantum chemical calculations. Both in the abundance spectra and the evaporation patterns of the investigated aqueous clusters “magic numbers” discontinuities in otherwise smoothly varying distributions were observed, and are discussed. To further examine a marked difference in the observed “magic-number” behaviour of H+(pyridine)1(H2O)n and H+(NH3)1(pyridine)1(H2O)n clusters, quantum chemical calculations have been employed. Next, relative reaction cross sections were measured for cluster ions reacting with D2O and with NH3 in the collision cell. Analysis of the results for the reaction H+(pyridine)1(H2O)n + NH3 allowed us to improve a kinetic model of the atmospheric positive ion composition. Upon reaction of a cluster with D2O a short-lived reaction intermediate is formed, which is followed by subsequent loss of D2O, HDO or H2O. The reaction channel leading to the loss of HDO requires proton mobility within the cluster, involving O–H-bond activation. The loss of HDO was not observed for protonated water clusters containing one pyridine molecule, a consequence of the immobilizing effect on the extra proton by the nitrogen base site. Similarly, the rates of protium/deuterium exchange for water clusters containing alkali metal ions are consistently extremely low. However, the experiments show enhanced proton mobility in water clusters containing two or three pyridine molecules (H+(pyridine)2–3(H2O)n), in 2,2'-bipyridine and 2,2'-ethylenebipyridine containing water clusters as well as in bisulfate water clusters (HSO4?(H2O)n). On the basis of systematic quantum chemical calculations we present consistent mechanisms for low energy water rearrangement and proton transfer along preformed "wires" of hydrogen bonds between the two distinct sites provided by these core ions in complete support of the experimental findings.