Relative Gas-Phase Bromide and Iodide Affinity of Simple Solvent Molecules Determined by FT-ICR

Abstract

The gas-phase ion/molecule reaction of CH3O- with bromo- and iodobenzene has been used to generate Br-(CH3OH) and I-(CH3OH) ions in an FT-ICR spectrometer. These ions are shown to undergo rapid solvent exchange with molecules of comparable or higher halide affinity. The equilibrium constants have been determined for these exchange reactions and the relative bromide free-energy affinity at 335 K (−ΔG°/kcal mol-1) is shown to increase in the order H2O (0.0) < CH3OH (1.07) < CH3CN (1.51) ∼ C2H5OH (1.53) < i-C3H7OH (1.88) < CH3NO2 (2.34). For I-, a similar trend is observed except that C2H5OH < CH3CN. Ab initio calculations were carried out at the MP4(SDTQ) level with geometry optimization at the MP2 level and using the generator coordinate method to add diffuse functions to the Br basis set. Changes in entropies for the gas-phase equilibrium experiments were estimated from the calculated vibrational frequencies, and the enthalpies of solvation were derived by using the reported value for Br-(CH3OH) as a reference point.