Ligand Exchange Reactions of Sodium Cation Complexes Examined Using Guided Ion Beam Mass Spectrometry: Relative and Absolute Dissociation Free Energies and Entropies

Abstract

Guided ion beam mass spectrometry is used to study the ligand exchange reactions of Na+L1 with L2, where L1, L2 = H2O, C6H6, CH3OH, CH3OCH3, NH3, and C2H5OH, as a function of kinetic energy. For the endothermic ligand exchange reactions, reaction endothermicities are obtained by analyzing the kinetic energy dependence of the cross sections using our empirical threshold modeling equation. The thresholds are found to be systematically higher than values previously determined using competitive CID experiments by 0.07−0.2 eV. An analysis of the endothermic cross sections using a bimolecular, polyatomic phase theory model and a competitive, bimolecular RRKM model demonstrates that the systematic deviations result from a competitive shift between the thermoneutral reactions back to the reactants and the endothermic reactions to the ligand exchange products. For all reactions, thermal rate constants, k(298), are determined by modeling the cross sections in the low-energy region and integrating the model over a M...