GAS-PHASE SOLVATED NEGATIVE IONS Keiko Takashima, 1 Jose ´ M. Riveros 2 * 1 Department of Chemistry, University of Londrina, Londrina, PR, Brazil 2 Institute of Chemistry, University of Sa ˜o Paulo, Caixa Postal 26077, Sa ˜o Paulo, Brazil, CEP 05599-970 Received 15 December 1998; revised 18 February 1999; accepted 3 March 1999 I. Introduction ........................................................................................................ 409 II. General Experimental Techniques .................................................................................... 410 III. Formation of Solvated Negative Ions at Low Pressures ................................................................. 411 IV. Binding Energies and Structure of Solvated Negative Ions A. Gas-Phase Hydration of Negative Ions............................................................................. 413 1. Monohydration of Simple Anions .............................................................................. 413 2. Multiple Hydration of Halide and OH - Ions .................................................................... 415 3. Hydration of Oxyanions ....................................................................................... 416 B. Negative Ions Associated with Other Solvent Molecules ............................................................ 417 V. Reactivity of Solvated Ions .......................................................................................... 419 A. Nucleophilic Displacement Reactions .............................................................................. 419 B. Proton Transfer Reactions ........................................................................................ 420 C. Gas-Phase Reaction of Solvated Anions with Substrates of Biological and Atmospheric Interest ........................ 421 D. Reactions with Organic Substrates ................................................................................ 421 VI. Spectroscopic Studies ............................................................................................... 422 A. Vibrational Spectroscopy ......................................................................................... 422 B. Photoelectron Spectroscopy of Solvated Anions .................................................................... 422 VII. Concluding Remarks ................................................................................................ 423 References .............................................................................................................. 423 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 iso- lated 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 character- istics. 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 bond- ing is very important for anions with strongly localized charges, ion-dipole interactions become almost as important for large an- ions like Br - and I - . Theoretical calculations and spectroscopic measurements have provided some unusual insight into the struc- ture 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 Keywords: solvated negative ions; gas-phase; thermochemistry; structure I. INTRODUCTION Solvation plays a major role in determining the reactivity and properties of chemical and biochemical systems. This role is particularly important in processes mediated by ionic species as a result of the strong electrostatic interactions Contract grant sponsors: Sa ˜o Paulo Science Foundation (FAPESP) and the Brazilian Research Council (CNPq). * Correspondence to: Prof. Jose ´ M. Riveros: phone/FAX: 55-11-818-3888; e-mail: jmrnigra@quim.iq.usp.br. Mass Spectrometry Reviews, 1998, 17, 409 – 430 © 1999 by John Wiley & Sons, Inc. CCC 0277-7037/98/060409-22