Information on Ion-solvent Interactions Obtained from the Study of Isolated Complexes Containing the Ion and a Small Number of Solvent Molecules

The measurement of gas phase ion-molecule equilibria by special mass spectrometric techniques (high pressure mass spec., ICR, Flowing Afterglow) provides for the first time abundant and accurate thermochemical data for organic and inorganic ions of interest. The energies of formation so obtained used in Born cycles lead to energies of solvation of ions from gas phase to a partiular solvent. Ion-solvent molecule equilibria Ion±(SS)n_1 + S IoniSZ)n measured in the gas phase with the above techniques lead to LH°n_i n and tG°n, n These data show that the essential features of the enertes of solvatton ofthe ions in a given liquid solvent S are contained in the energetics of the ion solvent molecule clusters Ion±(S2)n where n can be as low as 4 or 5. The substituent effects on the solvation of pyridinium cations and phenoxide anions in protic solvents are examined. Substituents that stabilize the ions in the gas phase (because of charge delocalization) reduce the hydrogen bonding interactions of the solvent with the ion. This can be directly observed by measuring the G° -l n and LH° 1 n values for water clustering to the substituted ions. n n