Tautomerism -- Contents -- Preface -- List of Contributors -- 1 Tautomerism: Introduction, History, and Recent Developments in Experimental and Theoretical Methods -- 1.1 The Definition and Scope of Tautomerism: Principles and Practicalities -- 1.2 Causes of Reversal in Tautomeric Form: Aromatic Resonance -- 1.3 Causes of Reversal in Tautomeric Form: Lone-Pair and Dipolar Repulsion -- 1.4 Causes of Reversal in Tautomeric Form: Selective Stabilization Through "Far" Intramolecular Hydrogen Bonding -- 1.5 Changes in Tautomeric Form Brought About by Electronegative Substituents -- 1.6 The Influence of Solvent on Tautomeric Form -- 1.7 Tautomeric Equilibrium: Historical Overview of an Analytical Problem -- 1.8 Short Historical Overview of Tautomerization Dynamics -- 1.9 Conclusions and Outlook -- References -- 2 Absorption UV-vis Spectroscopy and Chemometrics: From Qualitative Conclusions to Quantitative Analysis -- 2.1 Introduction -- 2.2 Quantitative Analysis of Tautomeric Equilibria -- 2.2.1 Classical Spectrophotometric Analysis, Limitations, and Early Attempts to Find a Solution -- 2.2.2 Quantitative Analysis by Using Bands Decomposition -- 2.2.3 Change in the Environment Affects the Equilibrium: Physical Meaning and Mathematical Expression -- 2.3 Analysis of Real Tautomeric Systems -- 2.3.1 Keto-Enol Tautomerism in 4-(phenyldiazenyl)naphthalen-1-ol: Solvent Effect -- 2.3.2 Keto-Enol Tautomerism in 1-((phenylimino)methyl)naphthalen-2-ol: Verification of the Approach -- 2.3.3 Keto-Enol Tautomerism in 1-(phenyldiazenyl)naphthalen-2-ol and 1-((phenylimino)methyl)naphthalen-2-ol: Effects of the Temperature and the Strength of Intramolecular Hydrogen Bonding -- 2.3.4 Ammonium-Azonium Tautomerism in 4-((4-aminophenyl)diazenyl)-N,N-dimethylaniline: Effect of Protonation and Solvent -- 2.4 Concluding Remarks -- References.

3 Studies of Photoinduced NH Tautomerism by Stationary and Time-Resolved Fluorescence Techniques -- 3.1 Introduction -- 3.2 Photoinduced Proton/Hydrogen Atom Transfer -- 3.2.1 Direct Intramolecular Proton Transfer Reactions -- 3.2.2 Solvent-Mediated NH Tautomerism -- 3.3 Fluorescence Techniques for Studying Tautomerism -- 3.3.1 Steady-State Fluorescence Methods -- 3.3.2 Time-Resolved Fluorescence Approaches -- 3.3.3 Advanced Techniques in Fluorescence Spectroscopy -- 3.3.3.1 Fluorescence Anisotropy -- 3.3.3.2 Fluorescence Microscopy and Fluorescence Correlation Spectroscopy -- 3.4 Tautomerism in Bifunctional NH/N Azaaromatics -- 3.4.1 Intramolecular NH/N Tautomerization -- 3.4.2 Intermolecular NH/N Tautomerization in Hydrogen-Bonded Dimers -- 3.4.3 Tautomerization in Solute-Solvent Hydrogen-Bonded Complexes -- 3.5 Ab initio and DFT Computational Methods -- 3.5.1 Reaction Mechanisms and Cooperativity in Proton Migrations -- 3.5.1.1 Concerted versus Stepwise Mechanism -- 3.5.2 Reaction Path Calculations and Energy Barriers for Proton Transfer -- 3.5.3 Challenges for Molecular Dynamics and QM/MM Simulations -- 3.6 NH Tautomerism as a Tool in Biophysics -- 3.7 Concluding Remarks -- Acknowledgment -- References -- 4 Femtosecond Pump-Probe Spectroscopy of Photoinduced Tautomerism -- 4.1 Introduction -- 4.2 Ultrafast Pump-Probe Spectroscopy -- 4.2.1 Time-Resolved Absorption Measurements -- 4.2.2 Fluorescence Upconversion -- 4.2.3 Ionization Techniques -- 4.2.4 Time-Resolved Infrared Spectroscopy -- 4.3 Dynamics from Pump-Probe Spectroscopy -- 4.3.1 Ultrafast Transient Absorption Signatures of ESIPT -- 4.3.2 Data Analysis -- 4.3.3 Ballistic Wavepacket Motion -- 4.3.4 Coherently Excited Vibrations in Product Modes -- 4.3.5 Ultrafast IR Studies -- 4.3.6 Other Tautomeric Reactions -- 4.4 Reaction Mechanism.

4.5 Reaction-Path-Specific Wavepacket Dynamics in Double ESIPT -- 4.6 Internal Conversion -- 4.7 Summary and Conclusions -- Acknowledgments -- References -- 5 NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State -- 5.1 Introduction -- 5.2 Methodologies of NMR Spectroscopy to Study Tautomerism -- 5.3 Types of Tautomerism Studied by NMR Spectroscopy -- 5.3.1 Ring-Chain Tautomerism -- 5.3.2 Tetrazole-Azide Tautomerism -- 5.3.3 Transannular Tautomerism -- 5.3.4 Keto-Enol Tautomerism -- 5.3.5 Imine-Amine Tautomerism -- 5.3.6 Lactam-Lactim Tautomerism -- 5.3.7 Annular Tautomerism of Five- or Six-Membered Heterocyclic Compounds -- 5.3.8 Nitroso (N-Oxide)-Oxime Tautomerism -- 5.3.9 Tautomeric Structures in Nucleosides, Nucleotides, and Proteins -- 5.3.10 Tautomerism in Porphyrins -- 5.3.11 Carbohydrate Tautomerism -- 5.3.12 Azo-Hydrazone Tautomerism -- 5.3.13 Tautomerism of Phosphorus Compounds -- 5.3.14 Miscellaneous Tautomerisms -- 5.4 Conclusions and Outlook -- Acknowledgments -- References -- 6 Isotope Effects on Chemical Shifts as a Tool in the Study of Tautomeric Equilibria -- 6.1 Introduction -- 6.2 Experimental Requirements -- 6.2.1 One-Tube Experiments -- 6.2.2 Exchange of Isotopes -- 6.2.3 Concentric Tubes -- 6.2.4 Couplings -- 6.2.5 Primary Isotope Effects -- 6.2.6 Temperature -- 6.2.7 Variation of Solvent -- 6.2.8 Isotope Labeling -- 6.3 Isotope Effects on Chemical Shifts -- 6.3.1 Intrinsic Isotope Effects -- 6.3.1.1 Intrinsic Deuterium Isotope Effects on 13C CS -- 6.3.1.2 Intrinsic Deuterium Isotope Effects on 15N Chemical Shifts -- 6.3.1.3 Deuterium Isotope Effects on 17O Chemical Shifts -- 6.3.1.4 Deuterium Isotope Effects on 19F CS -- 6.3.1.5 18O Isotope Effects on 13C Chemical Shifts -- 6.4 Secondary Equilibrium Isotope Effects on CS -- 6.4.1 Isotopic Perturbation of Equilibrium -- 6.5 Primary Isotope Effects.

6.6 Solid State -- 6.7 Theoretical Calculations -- 6.8 Examples -- 6.8.1 ß-Thioxoketones -- 6.8.2 Multiple Equilibria -- 6.9 Overview -- References -- 7 Tautomer-Selective Spectroscopy of Nucleobases, Isolated in the Gas Phase -- 7.1 Introduction -- 7.2 Techniques -- 7.3 Guanine -- 7.4 Adenine -- 7.5 Cytosine -- 7.6 Uracil and Thymine -- 7.7 Base Pairs -- 7.8 Outlook -- Acknowledgments -- References -- 8 Direct Evidence of Solid-State Tautomerism by Diffraction Methods: Isomers, Equilibria, and Kinetics -- 8.1 Application of X-Ray Diffraction to Study Tautomerism -- 8.2 Examples of X-Ray Diffraction Analysis of Proton Transfer -- 8.2.1 Tautomerism, Proton Transfer, and Resonance-Assisted Hydrogen Bonding -- 8.2.2 Examples of Thermally Induced Tautomerism -- 8.2.3 Photoinduced Tautomeric Processes -- 8.3 Other Diffraction Methods Used to Study Proton Transfer Reactions -- References -- 9 Dynamics of Ground- and Excited-State Intramolecular Proton Transfer Reactions -- 9.1 Introduction -- 9.2 Transition State Theory -- 9.3 Two Examples of Tautomerization -- 9.4 The Role of the Solvent -- 9.5 Solvent Friction and Solvent Dynamics -- 9.6 The Solvent Coordinate: Basics -- 9.7 Polarization Fluctuations -- 9.8 The Solvent Coordinate: An Application -- 9.9 Electronic Rearrangement -- 9.10 The Rug that Ties the (Classical) Room Together -- 9.11 Quantum and Classical -- 9.12 Quantum Decay -- 9.13 Coupling Quantum and Classical Motion: A Simple Example -- 9.14 Nonlinear Optics -- 9.15 Femtochemistry -- 9.16 Concluding Remarks -- References -- 10 Force Field Treatment of Proton and Hydrogen Transfer in Molecular Systems -- 10.1 Introduction -- 10.2 Computational Approaches to Proton Transfer -- 10.3 Proton Transfer Reactions with MMPT -- 10.4 Applications of MMPT -- 10.4.1 Infrared Spectroscopy -- 10.4.2 Classical and Quantum Proton Transfer in the Gas Phase.

10.4.3 Condensed-Phase Proton Transfer -- 10.4.4 MMPT for NMR Properties -- 10.5 Discussion and Outlook -- Acknowledgments -- References -- 11 The Scope and Limitations of LSER in the Study of Tautomer Ratio -- 11.1 Introduction -- 11.2 The Taft-Kamlet LSER Methodology -- 11.2.1 The π* Scale -- 11.2.2 The ß Scale -- 11.2.3 The α Scale -- 11.2.4 The ß Value for Water -- 11.2.5 π* for the Gas Phase -- 11.3 LSER Case Histories in the Field of Tautomerism -- 11.3.1 Enol Formation from ß-Diketones and Related Compounds -- 11.3.2 Tautomerism in Schiff Bases and Related Azo Compounds -- 11.3.3 Three-Way Tautomerism in the Pyrazolone 25 -- 11.4 Overview -- Appendix 11.A: Earlier Approaches -- References -- 12 The "Basicity Method" for Estimating Tautomer Ratio: A Radical Re-appraisal -- 12.1 Introduction -- 12.2 Experimental Protocol -- 12.3 The Derivation of Correction Factors -- 12.3.1 Amidines and Related Compounds -- 12.3.2 Conformational Effects on Amidine Correction Factors -- 12.3.3 Lactams: Simultaneous Determination of (NMe) and (OMe) -- 12.3.4 Vinylogous Lactams: Simultaneous Estimation of (NMe) and (OMe) -- 12.3.5 (NMe) in Acylamidines and Imides -- 12.3.6 (NMe) for Compounds with Contiguous Nitrogen Atoms -- 12.3.7 (NMe) for Thiolactams and Their Vinylogues -- 12.3.8 An Attempt at (SMe) for Thiolactams and Their Vinylogues -- 12.3.9 Correction Factors: Summary and Speculations -- 12.4 Regularities Revealed by Correction Factors -- 12.4.1 Benzofusion to Give Benzenoid Structures in Six-Membered Ring Oxoheterocycles -- 12.4.2 Benzofusion to Give Quinonoid Structures in a Variety of Compounds -- 12.5 Complicating Factors in the Use of the "Basicity Method" -- 12.5.1 Complications Caused by Steric and Stereoelectronic Factors -- 12.5.2 Complications Caused by Hydrogen Bonding -- 12.5.3 Complications Caused by Protonation at the "Wrong Site".

12.6 Tautomeric Problems to Which the "Basicity Method" Is Inapplicable.