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Liudmil Antonov is Professor of Physical Organic and Organic Analytical Chemistry at Bulgarian Academy of Sciences. Having obtained MSc (1989) and PhD (1994) degrees from the University of Sofia, he spent one year at Tokyo Institute of Technology before taking an appointment at University of Forestry (UF) in Sofia. In 1998, he was appointed as youngest in Bulgaria Associated Professor in Analytical Chemistry and from 1999 to 2002 headed the Department of Chemistry UF. After stays at AIST (Japan, 2002, 2006) as JSPS Fellow and at Humboldt University of Berlin (2003-2004) as Alexander von Humboldt Fellow and work for European Commission as reviewer and panel member (Directorate General for Research, 2001-2005), he joined Bulgarian Academy of Sciences in 2007. He has authored more than 90 research papers, reviews and book chapters, being cited over 1500 times.

List of Contributors XV

Preface XIX

1 Tautomerism: A Historical Perspective 1
José Elguero

1.1 Thermodynamic Aspects 2

1.2 Kinetic Aspects 5

1.3 Conclusions 6

References 7

2 “Triage” for Tautomers: The Choice between Experiment and Computation 11
Peter J. Taylor and Liudmil Antonov

2.1 Introduction (Original TextWritten by Peter J. Taylor) 11

2.2 cis-Amides 12

2.3 Tautomerism in Alicyclic Lactams: Six-Membered Rings 13

2.4 Tautomerism in Alicyclic Lactams: 2-Pyrrolidinone 14

2.5 Tautomerism in Other Five-Membered Ring Lactams 16

2.6 Tautomeric Ratios Requiring Computation: Alicyclic β-Diketones 17

2.7 Tautomeric Ratios Requiring Computation: “Maleic Hydrazide” 18

2.8 Tautomer Ratios Requiring Computation: 2-Oxo Derivatives of Pyrrole, Furan, andThiazole 20

2.9 Tautomeric Ratios Requiring Computation: Compounds Containing Contiguous Carbonyl Groups 22

2.10 Tautomeric Ratios Requiring Computation: Compounds Containing Contiguous π-Donors 24

2.11 Compounds Equally Suited to Experiment or Computation: “Azapentalenes” 25

2.12 Phenomena Susceptible to Experiment or Computation: Lone Pair Effects 29

2.13 Conformational Effects on Aminoenone Stability: A Computational Approach 30

2.14 Overview (Original TextWritten by Peter J. Taylor) 32

References 32

3 Methods to Distinguish Tautomeric Cases fromStatic Ones 35
Poul Erik Hansen

3.1 Introduction 35

3.2 The Liquid State 36

3.3 UV/VIS Spectroscopy 53

3.4 Infra Red Spectroscopy 55

3.5 Tautomerism in the Excited State 56

3.6 Near-Edge X-Ray 56

3.7 Energy-Dispersive X-Ray 57

3.8 Solid State 57

3.9 Single Molecule Tautomerization 59

3.10 Gas Phase 60

3.11 Theoretical Calculations 61

References 71

4 Electron-Transfer-Induced Tautomerizations 75
Thomas Bally

4.1 Introduction 75

4.2 Methodology 76

4.3 O-Alkyl Phenyl Ketones 77

4.4 Conclusions 93

Acknowledgments 93

References 94

5 The Fault Line in Prototropic Tautomerism 95
Peter J. Taylor and Liudmil Antonov

5.1 Introduction: “N-Type” and “C-Type” Tautomerism 95

5.2 Tautomerism in Symmetrical Amidines 96

5.3 Tautomer Ratio in Asymmetric Heteroaromatic Amidines 100

5.4 Tautomer Ratio in the Imine–Enamine System: Substitution at Nitrogen 102

5.5 Tautomer Ratio in the Imine–Enamine System: Substitution at Carbon 105

5.6 The Resonance Contribution to Ketone and Amide Tautomerism 107

5.7 The Field-Resonance Balance in Vinylogous Heteroaromatic Amidines 108

5.8 Conclusions 110

References 111

6 Theoretical Consideration of In-Solution Tautomeric Equilibria in Relation to Drug Design 113
Peter I. Nagy

6.1 Introduction 113

6.2 Methodology 114

6.3 Equilibration Mechanism 119

6.4 Relation to Drug Design 123

6.5 In-solution Equilibrium Calculations 127

6.6 Concluding Remarks 142

References 143

7 Direct Observation and Control of Single-Molecule Tautomerization by Low-Temperature Scanning Tunneling Microscopy 147
Takashi Kumagai and Leonhard Grill

7.1 Brief Introduction to STM 148

7.2 Direct Observation of Single-Molecule Tautomerization Using STM 152

7.3 Concluding Remarks 172

Acknowledgments 172

References 172

8 Switching of the Nonlinear Optical Responses of Anil Derivatives: From Dilute Solutions to the Solid State 175
Frédéric Castet and Benoît Champagne

8.1 Introduction 175

8.2 Experimental and Theoretical Methods 178

8.3 Second-Order Nonlinear Optical Responses of Anils 187

8.4 Conclusions 196

Acknowledgments 197

References 197

9 Tautomerism in Oxoporphyrinogens and Pyrazinacenes 203
Jonathan P. Hill, Jan Labuta, Shinsuke Ishihara, Gary J. Richards, Yongshu Xie, Francis D’Souza, and KatsuhikoAriga

9.1 Introduction 203

9.2 Tautomerism in Oxoporphyrinogen, OxP 205

9.3 Multichromic Acidity Indicator Involving Tautomerism 211

9.4 Polytautomerism in Oxocorrologen, OxC 212

9.5 Tautomerism in Linear Reduced Fused Oligo-1,4-pyrazines (Pyrazinacenes) 219

9.6 Conclusion 225

References 226

10 Enolimine–Ketoenamine Tautomerism for Chemosensing 229
Alexander D. Dubonosov, Vladimir A. Bren, and Vladimir I.Minkin

10.1 Introduction 229

10.2 Prototropic Enolimine–Ketoenamine Tautomerism 229

10.3 Ionochromic Enolimine–Ketoenamine Tautomeric Systems for Ions Sensing 234

10.4 Concluding Remarks 247

Acknowledgments 247

References 247

11 Tautomerizable Azophenol Dyes: Cornerstones for Advanced Light-Responsive Materials 253
Jaume Garcia-Amorós and Dolores Velasco

11.1 Azobenzene-Based Light-Sensitive Materials 253

11.2 Azophenols: Tautomerizable Photochromes with Fast Switching Speeds 255

11.3 Sub-Millisecond Thermally Isomerizing Azophenols for Optically Triggered Oscillating Materials 262

11.4 Fast-Responding Artificial Muscles with Azophenol-Based Liquid Single Crystal Elastomers 266

11.5 Conclusion 268

References 269

12 Controlled Tautomerism: Is It Possible? 273
Daniela Nedeltcheva-Antonova and Liudmil Antonov

12.1 Introduction 273

12.2 Manipulation of Electronic Properties of the Substituents 275

12.3 Tautomeric Tweezers 278

12.4 Tautomeric Cavities 279

12.5 Proton Cranes 282

12.6 Rotary Switches 290

12.7 Concluding Remarks 291

Acknowledgments 291

References 291

13 Supramolecular Control over Tautomerism in Organic Solids 295
Krunoslav Užarevic, Vladimir Stilinovic, and Mirta Rubcic

13.1 Crystal Engineering and Tautomerism in Molecular Solids 297

13.2 Supramolecular Synthons 298

13.3 Solid-State Tautomerism, Proton Transfer, and Hydrogen Bonding 300

13.4 Supramolecular Stabilization of Metastable Tautomers 304

13.5 Identification of Tautomeric Properties and Connectivity Preferences 305

13.6 Synthetic Methods 306

13.7 Supramolecular Interactions in Other Tautomeric Solids 310

References 324

14 Proton Tautomerism in Systems of Increasing Complexity: Examples from Organic Molecules to Enzymes 329
Hans-Heinrich Limbach, Gleb S. Denisov, Ilya G. Shenderovich, and Peter M. Tolstoy

14.1 Introduction 329

14.2 Hydrogen Bond Geometries and Proton Transfer 330

14.3 Tautomerizations without Requiring Reorganization of the Environment 333

14.4 Tautomerizations Requiring Reorganization of the Environment 346

14.5 Conclusions 364

Acknowledgments 365

References 365

Index 373