Tautomerism: Methods and Theories

Content: 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 1  1.2 Causes of Reversal in Tautomeric Form: Aromatic Resonance 3  1.3 Causes of Reversal in Tautomeric Form: Lone-Pair and Dipolar Repulsion 4  1.4 Causes of Reversal in Tautomeric Form: Selective Stabilization Through "Far" Intramolecular Hydrogen Bonding 5  1.5 Changes in Tautomeric Form Brought About by Electronegative Substituents 7  1.6 The Influence of Solvent on Tautomeric Form 8  1.7 Tautomeric Equilibrium: Historical Overview of an Analytical Problem 9  1.8 Short Historical Overview of Tautomerization Dynamics 13  1.9 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 25  2.2 Quantitative Analysis of Tautomeric Equilibria 26  2.2.1 Classical Spectrophotometric Analysis, Limitations, and Early Attempts to Find a Solution 26  2.2.2 Quantitative Analysis by Using Bands Decomposition 29  2.2.3 Change in the Environment Affects the Equilibrium: Physical Meaning and Mathematical Expression 33  2.3 Analysis of Real Tautomeric Systems 37  2.3.1 Keto-Enol Tautomerism in 4-(phenyldiazenyl)naphthalen-1-ol: Solvent Effect 37  2.3.2 Keto-Enol Tautomerism in 1-((phenylimino)methyl)naphthalen-2-ol: Verification of the Approach 40  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 41  2.3.4 Ammonium-Azonium Tautomerism in 4-((4-aminophenyl)diazenyl)-N,N-dimethylaniline: Effect of Protonation and Solvent 43  2.4 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 49  3.2 Photoinduced Proton/Hydrogen Atom Transfer 50  3.2.1 Direct Intramolecular Proton Transfer Reactions 50  3.2.2 Solvent-Mediated NH Tautomerism 52  3.3 Fluorescence Techniques for Studying Tautomerism 52  3.3.1 Steady-State Fluorescence Methods 52  3.3.2 Time-Resolved Fluorescence Approaches 54  3.3.3 Advanced Techniques in Fluorescence Spectroscopy 54  3.3.3.1 Fluorescence Anisotropy 54  3.3.3.2 Fluorescence Microscopy and Fluorescence Correlation Spectroscopy 55  3.4 Tautomerism in Bifunctional NH/N Azaaromatics 56  3.4.1 Intramolecular NH/N Tautomerization 56  3.4.2 Intermolecular NH/N Tautomerization in Hydrogen-Bonded Dimers 59  3.4.3 Tautomerization in Solute-Solvent Hydrogen-Bonded Complexes 60  3.5 Ab initio and DFT Computational Methods 67  3.5.1 Reaction Mechanisms and Cooperativity in Proton Migrations 67  3.5.1.1 Concerted versus Stepwise Mechanism 67  3.5.2 Reaction Path Calculations and Energy Barriers for Proton Transfer 69  3.5.3 Challenges for Molecular Dynamics and QM/MM Simulations 71  3.6 NH Tautomerism as a Tool in Biophysics 72  3.7 Concluding Remarks 74  Acknowledgment 74  References 75  4 Femtosecond Pump-Probe Spectroscopy of Photoinduced Tautomerism 79 Stefan Lochbrunner  4.1 Introduction 79  4.2 Ultrafast Pump-Probe Spectroscopy 81  4.2.1 Time-Resolved Absorption Measurements 82  4.2.2 Fluorescence Upconversion 84  4.2.3 Ionization Techniques 84  4.2.4 Time-Resolved Infrared Spectroscopy 84  4.3 Dynamics from Pump-Probe Spectroscopy 85  4.3.1 Ultrafast Transient Absorption Signatures of ESIPT 85  4.3.2 Data Analysis 87  4.3.3 Ballistic Wavepacket Motion 88  4.3.4 Coherently Excited Vibrations in Product Modes 90  4.3.5 Ultrafast IR Studies 92  4.3.6 Other Tautomeric Reactions 92  4.4 Reaction Mechanism 93  4.5 Reaction-Path-Specific Wavepacket Dynamics in Double ESIPT 96  4.6 Internal Conversion 97  4.7 Summary and Conclusions 99  Acknowledgments 100  References 100  5 NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State 103 Erich Kleinpeter  5.1 Introduction 103  5.2 Methodologies of NMR Spectroscopy to Study Tautomerism 104  5.3 Types of Tautomerism Studied by NMR Spectroscopy 109  5.3.1 Ring-Chain Tautomerism 109  5.3.2 Tetrazole-Azide Tautomerism 111  5.3.3 Transannular Tautomerism 111  5.3.4 Keto-Enol Tautomerism 112  5.3.5 Imine-Amine Tautomerism 116  5.3.6 Lactam-Lactim Tautomerism 124  5.3.7 Annular Tautomerism of Five- or Six-Membered Heterocyclic Compounds 126  5.3.8 Nitroso (N-Oxide)-Oxime Tautomerism 128  5.3.9 Tautomeric Structures in Nucleosides, Nucleotides, and Proteins 129  5.3.10 Tautomerism in Porphyrins 131  5.3.11 Carbohydrate Tautomerism 132  5.3.12 Azo-Hydrazone Tautomerism 133  5.3.13 Tautomerism of Phosphorus Compounds 134  5.3.14 Miscellaneous Tautomerisms 136  5.4 Conclusions and Outlook 137  Acknowledgments 138  References 138  6 Isotope Effects on Chemical Shifts as a Tool in the Study of Tautomeric Equilibria 145 Poul Erik Hansen  6.1 Introduction 145  6.2 Experimental Requirements 148  6.2.1 One-Tube Experiments 148  6.2.2 Exchange of Isotopes 149  6.2.3 Concentric Tubes 149  6.2.4 Couplings 149  6.2.5 Primary Isotope Effects 149  6.2.6 Temperature 150  6.2.7 Variation of Solvent 150  6.2.8 Isotope Labeling 151  6.3 Isotope Effects on Chemical Shifts 151  6.3.1 Intrinsic Isotope Effects 151  6.3.1.1 Intrinsic Deuterium Isotope Effects on 13C CS 153  6.3.1.2 Intrinsic Deuterium Isotope Effects on 15N Chemical Shifts 154  6.3.1.3 Deuterium Isotope Effects on 17O Chemical Shifts 154  6.3.1.4 Deuterium Isotope Effects on 19F CS 154  6.3.1.5 18O Isotope Effects on 13C Chemical Shifts 155  6.4 Secondary Equilibrium Isotope Effects on CS 156  6.4.1 Isotopic Perturbation of Equilibrium 160  6.5 Primary Isotope Effects 161  6.6 Solid State 164  6.7 Theoretical Calculations 165  6.8 Examples 166  6.8.1 beta-Thioxoketones 166  6.8.2 Multiple Equilibria 169  6.9 Overview 172  References 173  7 Tautomer-Selective Spectroscopy of Nucleobases, Isolated in the Gas Phase 177 Mattanjah S. de Vries  7.1 Introduction 177  7.2 Techniques 177  7.3 Guanine 179  7.4 Adenine 187  7.5 Cytosine 187  7.6 Uracil and Thymine 188  7.7 Base Pairs 189  7.8 Outlook 191  Acknowledgments 192  References 192  8 Direct Evidence of Solid-State Tautomerism by Diffraction Methods: Isomers, Equilibria, and Kinetics 197 Pan!ce Naumov and Subash Chandra Sahoo  8.1 Application of X-Ray Diffraction to Study Tautomerism 197  8.2 Examples of X-Ray Diffraction Analysis of Proton Transfer 199  8.2.1 Tautomerism, Proton Transfer, and Resonance-Assisted Hydrogen Bonding 199  8.2.2 Examples of Thermally Induced Tautomerism 203  8.2.3 Photoinduced Tautomeric Processes 208  8.3 Other Diffraction Methods Used to Study Proton Transfer Reactions 211  References 211  9 Dynamics of Ground- and Excited-State Intramolecular Proton Transfer Reactions 213 Gert van der Zwan  9.1 Introduction 213  9.2 Transition State Theory 216  9.3 Two Examples of Tautomerization 218  9.4 The Role of the Solvent 221  9.5 Solvent Friction and Solvent Dynamics 224  9.6 The Solvent Coordinate: Basics 226  9.7 Polarization Fluctuations 229  9.8 The Solvent Coordinate: An Application 231  9.9 Electronic Rearrangement 233  9.10 The Rug that Ties the (Classical) Room Together 234  9.11 Quantum and Classical 236  9.12 Quantum Decay 239  9.13 Coupling Quantum and Classical Motion: A Simple Example 242  9.14 Nonlinear Optics 246  9.15 Femtochemistry 247  9.16 Concluding Remarks 249  References 250  10 Force Field Treatment of Proton and Hydrogen Transfer in Molecular Systems 253 Jing Huang and Markus Meuwly  10.1 Introduction 253  10.2 Computational Approaches to Proton Transfer 254  10.3 Proton Transfer Reactions with MMPT 256  10.4 Applications of MMPT 259  10.4.1 Infrared Spectroscopy 259  10.4.2 Classical and Quantum Proton Transfer in the Gas Phase 261  10.4.3 Condensed-Phase Proton Transfer 263  10.4.4 MMPT for NMR Properties 264  10.5 Discussion and Outlook 267  Acknowledgments 268  References 268  11 The Scope and Limitations of LSER in the Study of Tautomer Ratio 277 Peter J. Taylor  11.1 Introduction 277  11.2 The Taft-Kamlet LSER Methodology 277  11.2.1 The pi * Scale 278  11.2.2 The beta Scale 279  11.2.3 The alpha Scale 281  11.2.4 The beta Value for Water 283  11.2.5 pi * for the Gas Phase 286  11.3 LSER Case Histories in the Field of Tautomerism 287  11.3.1 Enol Formation from beta-Diketones and Related Compounds 288  11.3.2 Tautomerism in Schiff Bases and Related Azo Compounds 292  11.3.3 Three-Way Tautomerism in the Pyrazolone 25 295  11.4 Overview 298  Appendix 11.A: Earlier Approaches 301  References 302  12 The "Basicity Method" for Estimating Tautomer Ratio: A Radical Re-appraisal 305 Peter J. Taylor  12.1 Introduction 305  12.2 Experimental Protocol 307  12.3 The Derivation of Correction Factors 308  12.3.1 Amidines and Related Compounds 308  12.3.2 Conformational Effects on Amidine Correction Factors 309  12.3.3 Lactams: Simultaneous Determination of (NMe) and (OMe) 310  12.3.4 Vinylogous Lactams: Simultaneous Estimation of (NMe) and (OMe) 312  12.3.5 (NMe) in Acylamidines and Imides 313  12.3.6 (NMe) for Compounds with Contiguous Nitrogen Atoms 315  12.3.7 (NMe) for Thiolactams and Their Vinylogues 317  12.3.8 An Attempt at (SMe) for Thiolactams and Their Vinylogues 319  12.3.9 Correction Factors: Summary and Speculations 320  12.4 Regularities Revealed by Correction Factors 320  12.4.1 Benzofusion to Give Benzenoid Structures in Six-Membered Ring Oxoheterocycles 321  12.4.2 Benzofusion to Give Quinonoid Structures in a Variety of Compounds 324  12.5 Complicating Factors in the Use of the "Basicity Method" 326  12.5.1 Complications Caused by Steric and Stereoelectronic Factors 326  12.5.2 Complications Caused by Hydrogen Bonding 327  12.5.3 Complications Caused by Protonation at the "Wrong Site" 328  12.6 Tautomeric Problems to Which the "Basicity Method" Is Inapplicable 330  12.7 Overview 332  References 333  13 Quantum Chemical Calculation of Tautomeric Equilibria 337 Walter M.F. Fabian  13.1 Introduction 337  13.2 Computational Procedures 338  13.2.1 Wave-Function-Based Methods 338  13.2.1.1 Independent Particle Methods 339  13.2.1.2 Correlated Treatments 340  13.2.2 Density Functional Procedures 342  13.2.2.1 Overview of Density Functionals 343  13.2.2.2 Validation of Density Functionals 346  13.2.3 Choice of Basis Set 347  13.2.4 Calculation of Spectroscopic Properties 351  13.3 Solvent Effects 353  13.3.1 Continuum Solvation Models 353  13.3.2 Explicit Solvent Models 355  13.4 Applications of Quantum Chemical Methods to Tautomeric Equilibria 357  13.4.1 The SAMPL2 Challenge of Predicting Tautomer Ratios 357  13.4.2 Lactam-Lactim Tautomerism of 2-Hydroxypyridine 360  13.4.3 Annular Tautomerism in Tetrazole 363  13.5 Concluding Remarks 363  References 364  Index 369