Solvation of halide ions with water and acetonitrile in the gas phase

Abstract

The gas-phase solvation reactions X/sup -/(S)/sub n-1/ + S = X/sup -/(S)/sub n/, for S = H/sub 2/O and CH/sub 3/CN and X/sup -/ = F/sup -/, Cl/sup -/, Br/sup -/, and I/sup -/, were studied by using a pulsed electron beam high-pressure mass spectrometer. For hydration reactions, the values of -..delta..G/sup 0//sub n-1,n/ are in the order F/sup -/ > Cl/sup -/ > Br/sup -/ > I/sup -/ for all n. However, the -..delta..H/sup 0//sub n-1,n/ values for Cl/sup -/ decrease more rapidly than those for Br/sup -/, and a crossover is observed between n = 3 and 4. For the solvation reactions with CH/sub 2/CN, the -..delta..H/sup 0//sub 0,1/ value for F/sup -/ was found to be much larger than those for other halide ions, indicating the participation of covalent bonding in the cluster F/sup -/ ...H/sub 3/CCN. The covalent bond formation leads to the rapid decrease in the -..delta..H/sup 0//sub n-1,n/ values and -..delta..H/sup 0//sub n-1,n/ for F/sup -/ becomes smaller than that for Cl/sup -/ at n = 7. Much more gradual decreases in -..delta..H/sup 0//sub n-1,n/ values for Cl/sup -/, Br/sup -/, and I/sup -/ are observed up to the highest values of n measured;more » i.e., the completion of the first solvation shell does not seem to occur up to n = 8. Due to crowding in the inner shell, the -..delta..S/sup 0//sub n-1,n/ values increase with n. The difference between the cumulative enthalpies and entropies (i.e., -..delta..H/sup 0//sub 0,n/ and -..delta..S/sup 0//sub 0,n/) does not approach the differential bulk solvation enthalpies and entropies, respectively.« less