Thermodynamics and structures in solvents coordinating through nitrogen. I. Enthalpies of transfer from water to pyridine and acetonitrile for some monovalent cations and anions

The enthalpies of transfer ΔHtr�, between water and the aprotic solvents pyridine and acetonitrile have been determined for a number of 1-1 electrolytes. To calculate ΔHtr� for the single ions, the tatb assumption that ΔHtr�,(Ph4As+) = ΔHtr�BPh4-) for all solvents has been applied. The results are compared with those found earlier for the transfer between water and dimethyl sulfoxide. The values of ΔHtr�, differ widely between different ions, from exothermic by c. 30 kJ/mol to endothermic by about the same amount. Endothermic values are found for Cl- and, though much smaller, for Br-, and also for the large tetraalkylammonium ion Bu4N+. Compared to the differences in ΔHtr�, found between the various ions, the differences found for a certain ion between different solvents are small. The decrease of AH,", in the sequence Cl- > Br- > I- runs almost parallel in the three aprotic solvents, and is much less steep than in water. This reflects the influence of ligand hydrogen bonding in the protic solvent.