Computational chemistry: A useful (sometimes mandatory) tool in mass spectrometry studies

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.