Trapped Ion Studies of Ion-Molecule Reactions

Abstract

Ion-molecule reactions have been studied using a novel high pressure mass spectrometer employing crossed electric and magnetic fields for ion trapping. These studies have been supplemented by ones undertaken using trapped-ion ion cyclotron resonance (ICR) spectrometry. Reaction rate constants at low ion energies have been measured in many systems, and compare favourably with results of other studies. Correlations of measured rates with theoretical models have characterized certain features of ion-neutral collision processes. Comparisons between chemically similar systems have also provided information about these encounters. Internal excitation of reactant ions has been shown to induce thermodynamically disallowed reactions. Evidence suggesting the participation of vibrationally excited ions reacting at different rates than their ground state counterparts is presented. The formation and lifetimes of excited intermediates in ethylene and methanol has been investigated in detail. Relative rates of collisional stabilization of (C 5 H 9 + )* by inert gases have been determined and compared with similar processes in neutral systems. Stabilization of (C 5 H 9 + )* by ethylene itself exhibits different characteristics than those of inert gases. The equilibrium constant of a reversible ion-molecule reaction has been measured at two temperatures and used to determine the enthalpy and entropy change for the process. The rates of various thermoneutral proton and charge-transfer reactions have been measured using trapped ion-ion ejection ICR spectrometry. This has enabled a detailed characterization of the collisional phenomena in these systems. The kinetic energy distribution of CH 3 + arising from ionization of methyl halides has been determined using ICR spectrometry and photoionization spectroscopy. Inferences about the nature of the dissociative states are presented.