Contents. Preface XV List of Contributors XIX

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1 VII Preface XV List of Contributors XIX 1 Tautomerism: Introduction, History, and Recent Developments in Experimental and Theoretical Methods 1 Peter J. Taylor, Gert van der Zwan, and Liudmil Antonov 1.1 The Definition and Scope of Tautomerism: Principles and Practicalities Causes of Reversal in Tautomeric Form: Aromatic Resonance Causes of Reversal in Tautomeric Form: Lone-Pair and Dipolar Repulsion Causes of Reversal in Tautomeric Form: Selective Stabilization Through Far Intramolecular Hydrogen Bonding Changes in Tautomeric Form Brought About by Electronegative Substituents The Influence of Solvent on Tautomeric Form Tautomeric Equilibrium: Historical Overview of an Analytical Problem Short Historical Overview of Tautomerization Dynamics Conclusions and Outlook 19 References 20 2 Absorption UV vis Spectroscopy and Chemometrics: From Qualitative Conclusions to Quantitative Analysis 25 Liudmil Antonov 2.1 Introduction Quantitative Analysis of Tautomeric Equilibria Classical Spectrophotometric Analysis, Limitations, and Early Attempts to Find a Solution Quantitative Analysis by Using Bands Decomposition Change in the Environment Affects the Equilibrium: Physical Meaning and Mathematical Expression 33

2 VIII 2.3 Analysis of Real Tautomeric Systems Keto Enol Tautomerism in 4-(phenyldiazenyl)naphthalen-1-ol: Solvent Effect Keto Enol Tautomerism in 1-((phenylimino)methyl)naphthalen-2-ol: Verification of the Approach 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 Ammonium Azonium Tautomerism in 4-((4-aminophenyl)diazenyl)-N,N-dimethylaniline: Effect of Protonation and Solvent Concluding Remarks 46 References 46 3 Studies of Photoinduced NH Tautomerism by Stationary and Time-Resolved Fluorescence Techniques 49 Alexander Kyrychenko, Jerzy Herbich, and Jacek Waluk 3.1 Introduction Photoinduced Proton/Hydrogen Atom Transfer Direct Intramolecular Proton Transfer Reactions Solvent-Mediated NH Tautomerism Fluorescence Techniques for Studying Tautomerism Steady-State Fluorescence Methods Time-Resolved Fluorescence Approaches Advanced Techniques in Fluorescence Spectroscopy Fluorescence Anisotropy Fluorescence Microscopy and Fluorescence Correlation Spectroscopy Tautomerism in Bifunctional NH/N Azaaromatics Intramolecular NH/N Tautomerization Intermolecular NH/N Tautomerization in Hydrogen-Bonded Dimers Tautomerization in Solute Solvent Hydrogen-Bonded Complexes Ab initio and DFT Computational Methods Reaction Mechanisms and Cooperativity in Proton Migrations Concerted versus Stepwise Mechanism Reaction Path Calculations and Energy Barriers for Proton Transfer Challenges for Molecular Dynamics and QM/MM Simulations NH Tautomerism as a Tool in Biophysics Concluding Remarks 74 Acknowledgment 74 References 75

3 IX 4 Femtosecond Pump Probe Spectroscopy of Photoinduced Tautomerism 79 Stefan Lochbrunner 4.1 Introduction Ultrafast Pump Probe Spectroscopy Time-Resolved Absorption Measurements Fluorescence Upconversion Ionization Techniques Time-Resolved Infrared Spectroscopy Dynamics from Pump Probe Spectroscopy Ultrafast Transient Absorption Signatures of ESIPT Data Analysis Ballistic Wavepacket Motion Coherently Excited Vibrations in Product Modes Ultrafast IR Studies Other Tautomeric Reactions Reaction Mechanism Reaction-Path-Specific Wavepacket Dynamics in Double ESIPT Internal Conversion Summary and Conclusions 99 Acknowledgments 100 References NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State 103 Erich Kleinpeter 5.1 Introduction Methodologies of NMR Spectroscopy to Study Tautomerism Types of Tautomerism Studied by NMR Spectroscopy Ring Chain Tautomerism Tetrazole Azide Tautomerism Transannular Tautomerism Keto Enol Tautomerism Imine Amine Tautomerism Lactam Lactim Tautomerism Annular Tautomerism of Five- or Six-Membered Heterocyclic Compounds Nitroso (N-Oxide) Oxime Tautomerism Tautomeric Structures in Nucleosides, Nucleotides, and Proteins Tautomerism in Porphyrins Carbohydrate Tautomerism Azo Hydrazone Tautomerism Tautomerism of Phosphorus Compounds Miscellaneous Tautomerisms Conclusions and Outlook 137

4 X Acknowledgments 138 References Isotope Effects on Chemical Shifts as a Tool in the Study of Tautomeric Equilibria 145 Poul Erik Hansen 6.1 Introduction Experimental Requirements One-Tube Experiments Exchange of Isotopes Concentric Tubes Couplings Primary Isotope Effects Temperature Variation of Solvent Isotope Labeling Isotope Effects on Chemical Shifts Intrinsic Isotope Effects Intrinsic Deuterium Isotope Effects on 13 CCS Intrinsic Deuterium Isotope Effects on 15 N Chemical Shifts Deuterium Isotope Effects on 17 O Chemical Shifts Deuterium Isotope Effects on 19 FCS O Isotope Effects on 13 C Chemical Shifts Secondary Equilibrium Isotope Effects on CS Isotopic Perturbation of Equilibrium Primary Isotope Effects Solid State Theoretical Calculations Examples β-thioxoketones Multiple Equilibria Overview 172 References Tautomer-Selective Spectroscopy of Nucleobases, Isolated in the Gas Phase 177 Mattanjah S. de Vries 7.1 Introduction Techniques Guanine Adenine Cytosine Uracil and Thymine Base Pairs Outlook 191

5 XI Acknowledgments 192 References Direct Evidence of Solid-State Tautomerism by Diffraction Methods: Isomers, Equilibria, and Kinetics 197 Panče Naumov and Subash Chandra Sahoo 8.1 Application of X-Ray Diffraction to Study Tautomerism Examples of X-Ray Diffraction Analysis of Proton Transfer Tautomerism, Proton Transfer, and Resonance-Assisted Hydrogen Bonding Examples of Thermally Induced Tautomerism Photoinduced Tautomeric Processes Other Diffraction Methods Used to Study Proton Transfer Reactions 211 References Dynamics of Ground- and Excited-State Intramolecular Proton Transfer Reactions 213 Gert van der Zwan 9.1 Introduction Transition State Theory Two Examples of Tautomerization The Role of the Solvent Solvent Friction and Solvent Dynamics The Solvent Coordinate: Basics Polarization Fluctuations The Solvent Coordinate: An Application Electronic Rearrangement The Rug that Ties the (Classical) Room Together Quantum and Classical Quantum Decay Coupling Quantum and Classical Motion: A Simple Example Nonlinear Optics Femtochemistry Concluding Remarks 249 References Force Field Treatment of Proton and Hydrogen Transfer in Molecular Systems 253 Jing Huang and Markus Meuwly 10.1 Introduction Computational Approaches to Proton Transfer Proton Transfer Reactions with MMPT Applications of MMPT Infrared Spectroscopy 259

6 XII Classical and Quantum Proton Transfer in the Gas Phase Condensed-Phase Proton Transfer MMPT for NMR Properties Discussion and Outlook 267 Acknowledgments 268 References The Scope and Limitations of LSER in the Study of Tautomer Ratio 277 Peter J. Taylor 11.1 Introduction The Taft Kamlet LSER Methodology The π Scale The β Scale The α Scale The β Value for Water π for the Gas Phase LSER Case Histories in the Field of Tautomerism Enol Formation from β-diketones and Related Compounds Tautomerism in Schiff Bases and Related Azo Compounds Three-Way Tautomerism in the Pyrazolone Overview 298 Appendix 11.A: Earlier Approaches 301 References The Basicity Method for Estimating Tautomer Ratio: A Radical Re-appraisal 305 Peter J. Taylor 12.1 Introduction Experimental Protocol The Derivation of Correction Factors Amidines and Related Compounds Conformational Effects on Amidine Correction Factors Lactams: Simultaneous Determination of (NMe) and (OMe) Vinylogous Lactams: Simultaneous Estimation of (NMe) and (OMe) (NMe) in Acylamidines and Imides (NMe) for Compounds with Contiguous Nitrogen Atoms (NMe) for Thiolactams and Their Vinylogues An Attempt at (SMe) for Thiolactams and Their Vinylogues Correction Factors: Summary and Speculations Regularities Revealed by Correction Factors Benzofusion to Give Benzenoid Structures in Six-Membered Ring Oxoheterocycles Benzofusion to Give Quinonoid Structures in a Variety of Compounds 324

7 XIII 12.5 Complicating Factors in the Use of the Basicity Method Complications Caused by Steric and Stereoelectronic Factors Complications Caused by Hydrogen Bonding Complications Caused by Protonation at the Wrong Site Tautomeric Problems to Which the Basicity Method Is Inapplicable Overview 332 References Quantum Chemical Calculation of Tautomeric Equilibria 337 Walter M.F. Fabian 13.1 Introduction Computational Procedures Wave-Function-Based Methods Independent Particle Methods Correlated Treatments Density Functional Procedures Overview of Density Functionals Validation of Density Functionals Choice of Basis Set Calculation of Spectroscopic Properties Solvent Effects Continuum Solvation Models Explicit Solvent Models Applications of Quantum Chemical Methods to Tautomeric Equilibria The SAMPL2 Challenge of Predicting Tautomer Ratios Lactam Lactim Tautomerism of 2-Hydroxypyridine Annular Tautomerism in Tetrazole Concluding Remarks 363 References 364 Index 369

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