Reactions of mass-selected cluster ions in a thermal bath gas

Technological developments of fast-flow tubes that led to major advances in the study of cluster ion reactions are reviewed, including the coupling of high-pressure cluster ion sources to flowing-afterglow and selected-ion flow tube (SIFT) instruments. Several areas of cluster ion chemistry that have been studied recently in our laboratory, using a SIFT instrument with a supersonic expansion ion source, are reviewed. Firstly the thermal destruction of cluster ions is highlighted by a discussion of the competition between electron detachment and thermal dissociation in hydrated electron clusters (H2O)(n)(-). Rates and activation energies for the thermal destruction (dissociation plus detachment) of these clusters are discussed. The reactivity of hydrated electron clusters with several neutral electron scavengers is also reviewed. Secondly cluster ion chemistry related to trace neutral detection of atmospheric species using chemical ionization mass spectrometry is discussed. Recent rate measurements needed for chemical ionization detection of SO2 and H2SO4 using CO3-(H2O)(n) ions are reviewed. Thirdly the effects of solvation on nucleophilic displacement (S(N)2) reactions are highlighted by the reactions of CH3Br with OH-(H2O)(n), Cl-(H2O)(n) and F-(H2O)(n). Our measurements show that rates of S(N)2 reactions decrease with increasing hydration, and the reactions preferentially lead to unhydrated products, corroborating previous experimental and theoretical studies. The cluster studies also demonstrate that, in the absence of a fast S(N)2 reaction channel, other mechanisms such as association and ligand switching can become important. In the reaction of Cl-(H2O)(n) with CH3Br, a reaction where ligand switching plays an important role, the interesting thermal dissociation of Cl-(CH3Br) product ions is discussed. Finally, the use of cluster reactivity studies to resolve issues about the detailed nature of solvation, that is whether the core ion is internally or externally solvated, is discussed. Size-dependent rate constants for the reactions of X-(H2O)(n) (X = F, Cl, Br or I) with Cl-2, and the reactions of OH-(H2O)(n) with CO2 and with HBr were used to determine the closing of initial solvation shells.