Gas‐phase solvated negative ions

The association of negative ions to neutral molecules has been an active field of research in gas‐phase ion chemistry since 1970. For anions, which correspond to the conjugate base of well‐known acids in solution, these species are often referred to as gas‐phase solvated anions. The ability to form anions bound to a progressive number of neutrals has been particularly explored as a means to understand ion solvation and to bridge the behavior between isolated molecules in the gas‐phase and in the bulk. Different mass spectrometric techniques have been used to generate these ions and to determine their stability, structure, and spectroscopic characteristics. The thermodynamic quantities associated with the binding of these negative ions with protic and aprotic solvent‐type molecules have been determined over the years. For mono‐solvated anions, binding energies to a series of homologous neutrals scale with the gas‐phase acidity of the solvent molecule. Although hydrogen bonding is very important for anions with strongly localized charges, ion‐dipole interactions become almost as important for large anions like Br − and I − . Theoretical calculations and spectroscopic measurements have provided some unusual insight into the structure of these species, and the present view is that ions like Cl − and Br − do not display a shell‐like structure even when attached to a large number of solvent molecules (∼10). Considerable progress has also been made in the study of differences in the reactivity of gas‐phase “nude” anions compared with “solvated” anions. The present review outlines some of the most important developments along these different lines. © 1999 John Wiley & Sons, Inc. Mass Spec Rev 17: 409–430, 1998