RELATIVE GAS-PHASE BROMIDE AND IODIDE AFFINITY OF SIMPLE SOLVENT MOLECULES DETERMINED BY FT-ICR

The gas-phase ion/molecule reaction of CH3O- with bromo- and iodobenze ne has been used to generate Br-(CH3OH) and I-(CH3OH) ions in an FT-IC R spectrometer. These ions are shown to undergo rapid solvent exchange with molecules of comparable or higher halide affinity. The equilibri um constants have been determined for these exchange reactions and the relative bromide free-energy affinity at 335 K (-Delta G degrees/kcal mol(-1)) is shown to increase in the order H2O (0.0) < CH3OH (1.07) < CH3CN (1.51) similar to C2H5OH (1.53) < i-C3H7OH (1.88) < CH3NO2 (2.3 4). For I-, a similar trend is observed except that C2H5OH < CH3CN. Ab initio calculations were carried out at the MP4(SDTQ) level with geom etry optimization at the MP2 level and using the generator coordinate method to add diffuse functions to the Br basis set. Changes in entrop ies for the gas-phase equilibrium experiments were estimated from the calculated vibrational frequencies, and the enthalpies of solvation we re derived by using the reported value for Br-(CH3OH) as a reference p oint.