Stepwise Hydration and Multibody Deprotonation with Steep Negative Temperature Dependence in the Benzene^•+−Water System

We studied the stepwise hydration and solvent-mediated deprotonation of the benzene•+ cation (Bz•+) and found several unusual features. The solvent binding energies ΔH on-1,n for the reactions Bz•+(H2O)n-1 + H2O → Bz•+(H2O)n are nearly constant at 9 ± 1 kcal mol-1 for n = 1 to 8. We observed a remarkable sudden decrease in the entropy of association accompanying the formation of Bz•+(H2O)7 and Bz•+(H2O)8, indicating strong orientational restraint in the hydration shells of these clusters consistent with the formation of cagelike structures. We observed the size-dependent deprotonation of Bz•+ in a cooperative multibody process, where n H2O molecules (n ≥ 4) can remove a proton from Bz•+ to form protonated water clusters. We measured, for the first time, the temperature dependence of such a process and found a negative temperature coefficient of a magnitude unprecedented in any chemical reaction, of the form k = AT-67± 4, or in an Arrhenius form having an activation energy of −34 ± 1 kcal mol-1. The temperature effect may be explained by Bz•+ and four H2O molecules needing to be assembled from gas-phase components to form the reactive species. Such large temperature effects may be therefore general in solvent cluster-mediated reactions

Ion Funnel Trap Interface for Orthogonal Time-of-Flight Mass Spectrometry

A combined electrodynamic ion funnel and ion trap coupled to an orthogonal acceleration (oa)-time-of-flight mass spectrometer was developed and characterized. The ion trap was incorporated through the use of added terminal electrodynamic ion funnel electrodes enabling control over the axial dc gradient in the trap section. The ion trap operates efficiently at a pressure of ∼1 Torr, and measurements indicate a maximum charge capacity of ∼3 × 107 charges. An order of magnitude increase in sensitivity was observed in the analysis of low concentration peptides mixtures with orthogonal acceleration (oa)-time-of-flight mass spectrometry (oa-TOF MS) in the trapping mode as compared to the continuous regime. A signal increase in the trapping mode was accompanied by reduction in the chemical background, due to more efficient desolvation of, for example, solvent related clusters. Controlling the ion trap ejection time was found to result in efficient removal of singly charged species and improving signal-to-noise ratio (S/N) for the multiply charged analytes