Ligand exchange reactions of sodium cation complexes examined using guided ion beam mass spectrometry: Relative and absolute dissociation free energies and entropies

Guided ion beam mass spectrometry is used to study the ligand exchange reactions of Na ⁺L ₁ with L ₂, where L ₁, L ₂ = H ₂O, C ₆H ₆, CH ₃OH, CH ₃OCH ₃, NH ₃, and C ₂H ₅OH, 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 Maxwell-Boltzmann distribution of relative energies. From the rate constants for the forward and reverse reactions, equilibrium constants and relative free energies at 298 K for the ligand exchange processes are determined. The relative free energies are converted to absolute Na ⁺-L dissociation free energies, ΔG ₂₉₈, by minimizing the differences with a set of ΔG ₂₉₈ values obtained from equilibrium studies using FT-ICR mass spectrometry. Comparisons are made to previous experimental and theoretical absolute Na ⁺-L dissociation free energies from several sources. Using the absolute Na ⁺-L dissociation free energies from this work and absolute Na ⁺-L dissociation enthalpies measured previously in our laboratory, dissociation entropies are determined for the Na ⁺-L complexes.