Structural Rearrangements and Magic Numbers in Reactions between Pyridine-Containing Water Clusters and Ammonia

Molecular cluster ions H+(H2O), H+(pyridine)(H2O)(n), H+(pyridine)(2)(H2O)(n), and H+(NH3)(pyridine)(H2O)(n) (n = 16-27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H+(NH3)(pyridine)(H2O)(n), and H+(NH3)(pyridine)(2)(H2O)(n) at n = 18, 20, and 27. The reactions between H+(pyridine)(m)(H2O)(n) and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster. The pattern of the observed magic numbers suggest that H+(NH3)(pyridine)(H2O)(n) have structures consisting of a NH4+(H2O)(n) core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters.