Abstract
| I. | Introduction | 196 |
| II. | Theoretical Models | 197 |
| | A. Hartree‐Fock (HF) Theory | 197 |
| | B. Configuration Interaction (CI) | 197 |
| | C. Perturbation Theory | 198 |
| | D. Coupled Cluster Theory | 198 |
| | E. Quadratic Configuration Interaction | 199 |
| | F. Composite Procedures | 199 |
| | G. Multiconfiguration Interaction Theories | 199 |
| | H. Density Functional Theory | 200 |
| | I. CASSCF‐DFT Approaches | 201 |
| | J. Wave Function Analysis | 202 |
| III. | Performance of the Model | 203 |
| | A. Geometries | 203 |
| | B. Harmonic Vibrational Frequencies and Infrared Intensities | 206 |
| | C. Energies | 206 |
| | D. Entropies | 208 |
| | E. Convergency Problems of the Moller Plesset Series | 209 |
| | F. Radicals and Other Open‐Shell Systems | 210 |
| | G. Dications | 213 |
| IV. | Bond Weakening vs. Bond Reinforcement in Ion‐Molecule Interactions | 214 |
| | A. Dissociative Proton Attachment | 219 |
| | B. Ionization Effects in Three‐Membered Rings | 221 |
| V. | Theoretical Characterization of Elusive or Non‐Conventional Structures of Relevance in Gas‐Phase Ion Chemistry Confirmed by Experiment | 223 |
| | A. Oxywater | 224 |
| | B. Ammonia Oxide | 224 |
| | C. Silicon‐Containing Molecules | 225 |
| | D. Fluoroformic Acid | 225 |
| | E. Sulfur‐ and Phosphorus‐Containing Molecules | 225 |
| | F. Ylides | 226 |
| | G. Ethylenedione | 227 |
| | H. Cumulenes and Heterocumulenes | 227 |
| | I. Carbenes | 230 |
| | J. Hydrogen‐Bridged Radical Cations | 230 |
| VI. | The Role of Non‐Classical Structures in Ion‐Molecule Interactions | 231 |
| | A. Hydrocarbon Protonated Forms | 231 |
| | B. Protonated Forms of X~4~ (X=P, As, Sb, Bi) and Other Small Clusters | 233 |
| | C. Molecular Planetary Systems | 234 |
| Acknowledgments | 235 |
| References | 235 |
In this review, we present a brief summary of the theoretical methods most frequently used in gas‐phase ion chemistry. In subsequent sections, the performance of these methods is analyzed, paying attention to the reliability of geometries, vibrational frequencies, energies, and entropies. The possible pathologies of the different methods, in the form of instabilities of the wave function or spin contamination problems, are discussed. Several examples are presented to illustrate the usefulness of ab initio or density functional theory (DFT) methods to predict the existence of elusive molecules and/or to characterize non‐conventional structures, and to rationalize the charge redistributions normally associated with ion–molecule interactions and which result in bond‐weakening or bond‐reinforcement effects. Finally, the role of non‐classical structures in ion–molecule interactions is also illustrated with different examples. © 2002 John Wiley & Sons, Inc., Mass Spec Rev 20:195–245, 2001