Interactions of ionic molecular clusters H+(H2O)n, H+(NH3)1(H2O)n and H+(pyridine)1-3(H2O)n with heavy water in cluster beam experiments

Sammanfattning: Molecular cluster ions are fascinating subjects of study. Bridging the size gap between molecules and bulk, they often display non-trivial size dependent behavior and properties. Clusters—apart from being interesting in their own right and serving as useful model system in a number of applications—have a very real and important role in the atmosphere. Clusters acts as precursors for atmospheric particle formation, and as such, any uncertainties regarding clusters have a significant impact on the current estimates of global warming. This work investigates the properties of some ionic molecular clusters: H+(H2O)n, H+(NH3)1(H2O)n and H+(pyridine)1-3(H2O)n, as well as their reactions with gas phase heavy water, D2O. This is done in a cluster beam experiment, using a setup consisting of a quadrupole mass filter, a collision cell, and a time-of-flight mass spectrometer. The relative reaction cross section between the clusters and D2O was investigated; it was found that the pure water clusters and ammonia containing cluster have similar cross sections. Furthermore, the cross sections of the pyridine containing clusters differed from those of the pure water clusters and ammonia containing clusters. However, among the pyridine containing clusters, the reaction cross section depended only on the water content and not on the number of pyridine molecules. Previous studies in the field have concluded that the proton mobility in a cationic pure water cluster is large, leading to a H/D scrambling process during the lifetime of the reaction intermediate formed upon collision of the cluster with D2O. This is seen also in the experiments presented here. However, some of the cluster types investigated—H+(NH3)1(H2O)n and H+(pyridine)1(H2O)n —do not show this behavior. For these clusters, the high proton affinity of the basic molecule leads to the proton being locked in place by the nitrogen atom. Surprisingly, it was found that adding additional pyridine molecules to the latter cluster types negates this effect, thus H+(pyridine)2-3(H2O)n clusters exhibit H/D scrambling just like pure protonated water clusters. Quantum chemical calculations show that for H+(pyridine)2(H2O)4-6 there are stable structures where the proton is found on a water molecule. Although these structures are higher in energy compared to the most stable ones (with the proton situated on a pyridine molecule), the thermodynamical barrier for transferring the proton from a pyridine molecule to a water molecule is likely to decrease with cluster size, thus allowing hydronium containing stable cluster isomers.