NMR of Ordered Liquids

NMR of Ordered Liquids

Edited by

E. Elliott Burnell University of British Columbia, Canada

and

Comelis A. de Lange University of Amsterdam, The Netherlands

Springer-Science+Business Media, B.Y.

A c.I.P. Catalogue record for this book is available from the Library of Congress.

ISBN 978-90-481-6305-2 ISBN 978-94-017-0221-8 (eBook) DOI 10.1007/978-94-017-0221-8

Printed on acid-free paper

All Rights Reserved © 2003 Springer Science+Business Media Dordrecht

Originally published by Kluwer Academic Publishers in 2003.

Softcover reprint of the hardcover 1 st edition 2003 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work.

This book is dedicated to our dear wives Bonnie and

Annette who put up with the many hours and late

evenings, not to mention the early mornings, spent

discussing and arguing about the various details of

this project

Contents

Contributing Authors xiii

Preface XXI

Introduction XXv

E.E. Burnell and CA. de Lange

Part I Basics

Basics ofNMR of molecules in uniaxial anisotropic environments 5 CA. de Lange and E.E. Burnell

1 Introduction 5 2 General Hamiltonian in uniaxial anisotropic liquids 6 3 The high-field approximation 9 4 Transformation to molecule-fixed axes 11 5 Orientation parameters 12 6 Molecular symmetry 15 7 Simple examples ofNMR spectra of orientationally ordered molecules 17 8 Orientational order induced by anisotropic solvents 21 9 Orientational order induced by strong electric fields 22 10 Orientational order induced by strong magnetic fields 23 11 Internal motion 24 12 Summary 26

2 Density matrix methods in NMR 27 M. Bloom, E.E. Burnell and CA. de Lange

1 Introduction 2 Brief introduction to the density matrix 3 PuisedNMR 4 Summary

3 Coherent averaging and correlation of anisotropic spin interactions in oriented

molecules Malgorzata Marjanska, Robert H. Havlin and Dimitris Sakellariou

1 Introduction 2 Spatial reorientation techniques

Vll

27 27 30 43

45

45 45

viii

4

3 4 5 6

Two-dimensional dynamic director correlations Spin manipulation based techniques Multidimensional techniques Conclusions

NMR OF ORDERED LIQUIDS

50 51 57 63

Multiple Quantum NMR Spectroscopy in Orientationally Ordered Fluids Leslie D. Field

67

1 NMR in orientationally ordered fluids 2 Spectral simplification by MQNMR 3 Excitation and detection of multiple quantum coherence 4 Selective deuteration for spectral simplification 5 Spectral analysis and simulation 6 Structural studies using MQNMR 7 Structural studies by 1 H MQNMR 8 Heteronuclear MQNMR 9 Other applications ofMQNMR in liquid crystalline solvents

5

67 68 69 76 77 77

80 84 84

Spectral Analysis of Orientationally Ordered Molecules Raymond T Syvitski

89

1 Introduction 2 Tools of the trade 3 Some examples of putting it together 4 Summary

89 90 96

103

Part II NMR of solute atoms and molecules

6 NMR of Noble Gases Dissolved in Liquid Crystals Jukka Jokisaari

109

1 Introduction 2 NMR properties of noble gases 3 Chemical shift 4 Quadrupole coupling 5 Relaxation 6 129Xe self-diffusion 7 Conclusions

7

109 109 110 121 127 130 133

NMR of partially ordered solutes with emphasis on structure determination 137 c.L. Khetrapal and G.A. Nagana Gowda

1 Introduction 137 2 Basic principles 138 3 Spectral analysis 141 4 Scope and limitations 142 5 Practical considerations for deriving precise molecular structural information 142 6 Aids for spectral analysis 145 7 Emerging developments and possible future directions 153 8 Conclusions 158

Contents IX

8 Observation and interpretation of residual dipolar couplings in biomolecules 163 Jean-Franr;ois Trempe and Kalle Gehring

1 Introduction 163 2 Theory 164 3 Measurement of residual dipolar couplings 166 4 Interpretation and applications of dipolar couplings and CSA in structural

biology 177 5 Summary 186

9 The search for high-resolution NMR methods for membrane peptide structure Christophe Fares and James H. Davis

191

1 Introduction: NMR of orientationally ordered systems 2 Theoretical background 3 Separated local field spectroscopy 4 High-resolution 1 H MAS NMR of small membrane proteins 5 Conclusions

191 193 197

209 210

Part III Theory, models, and simulations

10 Solutes as probes of simplified models of orientational order 221 E.E. Burnell and CA. de Lange

I Introduction 221 2 Obtaining a self-consistent set of solute orientational order parameters 224 3 Factors affecting solute orientational order 226 4 Orientational order of solutes in "magic mixtures" 230 5 Comparison of experimental and calculated orientational order in "pure"

liquid crystals and "magic mixtures" 234 6 Conclusions 239

11 Molecular Models of Orientational Order Alberta Ferrarini and Giorgio J Mora

241

1 Phenomenological models for short-range interactions 2 Modeling electrostatic interactions 3 Density functional theory 4 Conclusion Appendix: Polarization induced by a charge distribution in a dielectric

12 Molecular theory of orientational order Demetri J Photinos

1 Introduction 2 Order parameters, molecular structure and interactions 3 Approximation schemes for the potential of mean torque 4 Molecular models 5 Summary.

242 246 251 256 256

259

259 260 268 273 281

x NMR OF ORDERED LIQUIDS

13 Very Flexible Solutes: Alkyl Chains and Derivatives Edward T. Samulski

1 Introduction 2 Chronology ofNMR studies of flexible solutes 3 Models of flexible molecules 4 Conclusions

14 NMR Studies of Solutes in Liquid Crystals: Small Flexible Molecules Giorgio Celebre and Marcello Longeri

1 Introduction 2 Theoretical background 3 The conformational problem 4 Selected examples 5 Conclusions

15 Simulations of Orientational Order of Solutes in Liquid Crystals James M Polson

1 Introduction 2 Orientational distribution functions and mean-field potentials 3 Conformational behaviour of flexible solutes 4 Electrostatic interactions 5 Conclusions

Part IV Dynamic aspects and relaxation

16 Spin relaxation in orientationally ordered molecules Ronald Y. Dong

1 Introduction 2 Average Hamiltonian 3 Spin relaxation theory 4 Motional models 5 Applications of spin relaxation

17 Low-frequency NMR relaxometry of spatially constrained liquid crystals F Grinberg, M Vilfan and E. Anoardo

1 Introduction 2 Field-cycling relaxometry 3 Low-field proton relaxometry of confined liquid crystals 4 The dipolar-correlation effect 5 Deuteron NMR relaxometry of confined liquid crystals

18 NMR on macroscopically oriented lyotropic systems G. Oriidd and G. Lindblom

1 Introduction 2 Orientation dependent NMR interactions 3 Lipid translational diffusion 4 Preparation of macroscopically oriented lamellar systems

285

285 289 293 303

305

305 306 307 312 322

325

325 327 333 337 343

349

349 350 354 356 360

375

375 378 380 384 391

399

399 399 404 407

Contents

5 Examples

19 Dynamic NMR in liquid crystals and liquid crystalline solutions Zeev Luz

1 2 3 4 5

Introduction Dynamic proton NMR of solutes in nematic solvents Dynamic deuterium NMR spectra Dynamic carbon-13 MAS NMR Concluding remarks

Xl

411

419

419 420 428 439 447

Contributing Authors

Esteban Anoardo (anoardo@mail.famaf.unc.edu.ar) was born in 1964 in Cordoba, Argentina. He studied Physics (grad­uate and Ph.D.) at the University of Cordoba, with specializa­tion in Field-Cycling NMR applied to liquid crystal materials. During 1999-2000 he worked in Italy with Stelar srI for the development of a new Field-Cycling reI axometer. Later he moved to the University ofUlm (Germany) for a post-doctoral (AVH) research stay with Professor Rainer Kimmich. He is currently Professor of Physics at the University of Cordoba, with research interests in NMR relaxation and the associated instrumentation.

Myer Bloom (bloom@physics.ubc.ca) was born in 1928 in Montreal, Canada. He received B.Sc. and M.Sc. degrees in Physics from McGill University in 1949 and 1950 and com­pleted his Ph.D. in Physics at the University of Illinois with Charlie Slichter in 1954. He is currently Emeritus Professor of Physics at the University of British Columbia, with research interests in the physics of biological systems. The main fo­cus of his current research involves the role of the dominant poly-unsaturated lipids in animal brains in relation to the en­largement and evolution of the human brain that probably took place more than (approximately) 100,000 years ago.

E. Elliott Burnell (elliott.burnell@ubc.ca) was born in 1943 in St. John's, Newfoundland. He received B.Sc. and M.Sc. de­grees in Chemistry from Memorial University of Newfound­land in 1965 and 1967. He did his Ph.D. in Theoretical Chem­istry in Bristol (UK) with Prof. A.D. Buckingham in 1970. He is currently Professor of Chemistry at the University of British Columbia, with research interests in NMR of orientationally ordered molecules and in intermolecular forces.

xiii

xiv NMR OF ORDERED LIQUIDS

Giorgio Celebre (giorgio.celebre@unical.it) was born in 1958 in Reggio Calabria, Italy. He graduated in Chemistry from the University of Calabria in 1984. He is currently Associate Professor of Physical Chemistry at the University of Calabria (Rende), with research interests in intermolecular forces and in conformational analysis, studied by Liquid Crystal NMR.

Jim Davis (jhd@physics.uoguelph.ca) was born in 1946 in Alexandria, MN, USA. He obtained Bachelors degrees in both Physics and Mathematics from Moorhead State University in 1969 and his Ph.D. from the University of Manitoba in Win­nipeg in 1975. He then worked as an NRC post-doctoral fellow with Prof. Myer Bloom at the University of British Columbia before joining the faculty at the Department of Physics, Uni­versity of Guelph in 1980. He has been using NMR for over 30 years.

Cornelis A. de Lange (delange@science.uva.nl) was born in 1943 in Zaandam, The Netherlands. He graduated in experi­mental physics from the University of Amsterdam in 1966 and did his Ph.D. in Theoretical Chemistry in Bristol (UK) with Prof. A.D. Buckingham in 1969. He is currently Professor of Laser Spectroscopy at the University of Amsterdam, with re­search interests in atmospheric chemistry and NMR of oriented molecules.

Ronald Y. Dong (Dong@BrandonU.ca) was born in 1942 in Shanghai, China. He graduated from Engineering Science at the University of Toronto in 1966, and obtained his Ph.D. in Experimental Physics at the University of British Columbia with Prof. Myer Bloom in 1969. He is currently a professor of Physics at Brandon University and an adjunct Physics profes­sor at the University of Manitoba, Canada. Research interests include solid-state NMR, statistical theory, molecular dynam­ics, liquid crystals and biomolecules.

Contributing Authors

I

~., IJI'

xv

Christophe Fares (cfares@uoguelph.ca) was born in 1972 in Montreal, Canada. He obtained a Bachelors of Science in Biochemistry from McGill University in 1994 and is expected to fi nish his Ph.D. in Biophysics at the University of Guelph under the supervision of Prof. lH. Davis and Prof. F.l Sharom in 2003. He is interested in pursuing research towards the application and development of solid state NMR methods on biological systems.

Alberta Ferrarini (albertaf@chfi .unipd.it) received the degree in Chemistry (1983) and the Ph.D. in Chemical Sciences (1989) from the University of Pad ova (Italy), under the supervision of Pier Luigi Nordio. She spent research periods in the groups of Gerd Kothe (University of Stuttgart, Germany) and Jack H. Freed (Cornell University, USA). In 1990 she became a re­search associate in the Physical Chemistry Department of the University of Padova, where since 2000 she has been an asso­ciate professor. Her research interests include the molecular interpretation of equilibrium and dynamic properties ofliquids and complex fuids (liquid crystals, membranes, colloids).

Leslie D. Field (L. Field@chem.usyd.edu.au) was born in 1953 in New South Wales, Australia. He graduated with fi rst class honours in Chemistry in 1975 and undertook his Ph.D. in Or­ganic Chemistry at the University of Sydney working with Pro­fessor Sev Sternhell. He spent postdoctoral periods studying with Professor George Olah at the University of Southern Cali­fornia (USA) and with Professor Jack Baldwin at the University of Oxford (UK) before returning to Australia to take up a post at the University of Sydney. He was Head of the School of Chem­istry from 1996-2000 and he is currently Professor of Organic Chemistry with research interests in organometallic chemistry, catalysis and chemical applications ofNMR spectroscopy.

xvi NMR OF ORDERED LIQUIDS

Kalle Gehring (kalle.gehring@mcgill.ca) was born in 1958 and raised in Ann Arbor, Michigan. He has degrees from Brown University, the University of Michigan and the Uni­versity of California (Berkeley). Following postdoctoral stud­ies at Lawrence Berkeley Laboratory, the Ecole Polytechnique and the French CNRS, he took up a faculty position in Bio­chemistry at McGill University in 1994. His current research interests are structural genomics and residual dipolar couplings in NMR structure determination.

G.A. Nagana Gowda (gangcbmr@hotmail.com) was born in 1961 in Musandihal, India. He graduated from the Univer­sity of Mysore in 1985 and completed his Ph.D. with Prof. C. L. Khetrapal in 1999 from the Bangalore University. He worked at the Indian Institute of Science till 2001 and presently he is working as Assistant Professor at the Center of Biomedical Magnetic Resonance, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, India. His Research Interests are in the area of Biomedical and Chemical Magnetic Resonance and NMR of Oriented Systems.

Farida A. Grinberg (grinberg@mnr.mpi-stuttgart.mpg.de) was born in 1957 in Kazan, Russia. She graduated in experi­mental physics from the University of Kazan in 1979 and did her Ph.D. in NMR of polymers in 1987 in the same University. In 2002 she did her habilitation (second dissertation) work in slow molecular dynamics with Prof. R. Kimmich in the Univer­sity ofUlm (Germany). She is currently a research scientist at the Max Planck Institute for Metals Research, Stuttgart, and a Privatdozentin at the University ofUlm. Her research interests include NMR of anisotropic liquids, diffusion studies, NMR imaging, nanostructured materials, and computer simulations.

Robert H. Havlin (bob@rabi.cchem.berkeley.edu) was born in 1975 in Belleville, Illinois, USA. He graduated from the Uni­versity of Illinois at Urbana-Champaign in 1997 with a B.S. in Chemistry where he did research with Prof. Eric Oldfi eld. Then he fi nished his Ph.D. in Physical Chemistry with Prof. Alexan­der Pines at the University of California, Berkeley where his research focused on switched angle spinning of liquid crystals and solid-state NMR dipolar recoupling methods.

Contributing Authors XVll

Jukka Jokisaari Gukka.jokisaari@oulu.fi ) was born in Kemi, Northern Finland. He graduated in experimental physics from the University of Oulu in 1968 and got his Ph.D. at the same university in 1974. He worked as a post-doctoral fellow with Professor Peter Diehl, University of Basel, Switzerland, dur­ing several periods between the late 70's and early 90's. He is currently Professor of Physics (atomic and molecular spec­troscopy) at the University of Oulu, with research interests in NMR of noble gases in liquid crystals, determination of nuclear shielding, spin-spin coupling and quadrupole coupling tensors, applying NMR of solute molecules in liquid crystals, and de­termination of pore sizes in micro- and mesoporous materials applying 129Xe NMR.

C.L. Khetrapal (clkhetrapal@hotmail.com) was born in 1937 in Sahival in Undivided India. He graduated from the Uni­versity of Allahabad in 1959 and completed his Ph.D. with Prof. S.S. Dharmatti in 1965 from the Bombay University. He worked at the Tata Institute of Fundamental Research, Bom­bay as Reader, at the Raman Research Institute, Bangalore as Associate Professor, at the Indian Institute of Science as Profes­sor, at the University of Allahabad as the Vice-Chancellor and currently he is Distinguished Professor at Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, India. His Research Interests are in the area of Biomedical and Chemical Magnetic Resonance and NMR of Oriented Systems.

Goran Lindblom (goran.lindblom@chem.umu.se) was born in 1942 in Kalmar, Sweden. He graduated as Master in Chemi­cal Engineering from the Lund Institute of Technology in 1969 and did his Ph.D. in Physical Chemistry, Lund University in 1974. He has been Professor in Physical Chemistry since 1981 at Umea University with research interests in Biophysi­cal Chemistry, in particular of biological membranes and solid state NMR spectroscopy.

Marcello Longeri (m.longeri@unical.it) was born in 1947 in Piombino, Italy. He graduated in Chemistry from the Uni­versity of Pis a in 1972. He is currently Professor of Physical Chemistry at the University of Calabria (Rende), with research interests in NMR of Liquid Crystals.

xviii NMR OF ORDERED LIQUIDS

Demonstrating physical properties of thin-layer lyotropic liquid crystals (a soap fi 1m catenoid contained between two rings).

Zeev Luz (zeev.luz@weizmann.ac.il) was born in 1932 in Munich, Germany and emigrated to Isreal (then Palestine) in 1934. He graduated with an M.Sc. in Physical Chemistry from the Hebrew University in Jerusalem in 1957, and obtained his Ph.D. from the Weizmann Institute of Science in 1961. He joined the scientifi c staff of the Weizmann Institute in 1964, and since 1997 has been Professor Emeritus in that Institute. Research interests include applications of magnetic resonance spectroscopy to study structure and dynamics in condensed phases.

Malgorzata Marjanska (gosia@cmrr.umn.edu) was born in 1974 in Monki, Poland. She graduated in Chemistry from Loyola University of Chicago and did her Ph.D. in Physi­cal Chemistry in the University of California, Berkeley with Prof. Alexander Pines in 2002. She is currently a post-doctoral fellow in the Center for Magnetic Resonance Research at the University of Minnesota working with Prof. Kamil Ugurbil.

Giorgio J. Moro (g.moro@chfi .unipd.it) has carried out re­search in physical and theoretical chemistry at Padova Uni­versity, Italy, since 1987 as an Associate Professor and since November 2000 as a Full Professor of Physical Chemistry. His research activities focus on the theoretical study of condensed matter properties. Specifi c fi elds of interest are the simulation of spectroscopic observables, the effects of the solvent (and in particular of its local structure) on rotational and translational dynamics, conformational dynamics of chains and polymers, and stochastic models of kinetic processes. Liquid crystals represent the privileged systems for molecular dynamics stud­ies and for the elaboration of original methods predicting order and structure on the basis of molecular organization. His work has resulted in more than 70 articles so far.

Contributing Authors xix

Greger Oradd (greger.oradd@chem.umu.se) was born in 1961 in Asele, Sweden. He graduated in physics from Umea University in 1988 and did his Ph.D. in Physical Chemistry, Umea University in 1995. He is currently a Lecturer in Bio­physical Chemistry at Umea University, with research interests in the pulsed fi eld gradient technique for self-diffusion mea­surements applied to ion conducting polymers and lipid bilayer systems of biological interest.

Demetri J. Photinos (photinos@upatras.gr) was born in Cairo in 1947. He graduated in Physics from the University of Athens in 1970 and did his Ph.D. in Physics at Kent State University with Prof. D.S. Moroi in 1973. He is currently professor and chair of the Department of Materials Science at the Univer­sity of Patras, with research interests in soft matter theory and computer simulations.

James M. Polson (jpolson@upei.ca) was born in Windsor, Ontario, Canada. He received his B.Sc. and M.Sc. degrees in Physics from the University of Guelph in 1988 and 1990, respectively. He completed his Ph.D. in Physics with Profs. El­liott Burnell and Myer Bloom at the University of British Columbia in 1996. He is currently Assistant Professor of Physics at the University of Prince Edward Island, with re­search interests in the simulation of soft condensed matter sys­tems.

Dimitris Sakellariou (dsakella@waugh.cchem.berkeley.edu) was born in 1974 in Athens, Greece. He graduated in Phys­ical Chemistry from the Ecole Normale Superieure de Lyon (France) in 1996. He did his Ph.D. in solid-state NMR in the Ecole Normale Superieure de Lyon with Prof. Lyndon Emsley in 2000. He is currently a post-doctoral fellow at the Lawrence Berkeley National Laboratory and University of California, Berkeley working with Prof. Alexander Pines. His current re­search interests focus on NMR of strongly oriented molecular systems.

xx NMR OF ORDERED LIQUIDS

Edward T. Samulski (et@unc.edu) was born in 1943 in Au­gusta, Georgia, USA. He graduated in textile chemistry from Clemson University in 1965 and did his Ph.D. in Physical Chemistry at Princeton University with Prof. A.V. Tobolsky in 1969. He is currently Cary C. Boshamer Professor of Chem­istry at the University of North Carolina, Chapel Hill, with research interest in oriented soft matter.

Raymond T. Syvitski (Ray.Syvitski@dal.ca)grewup in Thun­der Bay, Canada on the western edge of Lake Superior. He graduated with a B.Sc. in chemistry from Lakehead University and received his Ph.D. (2000) in Anisotropic Intermolecular Forces in Liquid Crystals using NMR from the University of British Columbia in Vancouver, Canada. Ray is currently a Killam Postdoctoral Fellow at Dalhousie University in Halifax and is studying the structure of membrane bound proteins by solution and solid state NMR.

Jean-Fran~ois Trempe (jean. francois.trempe@bioch.ox.ac.uk) was born in 1978 in Montreal, Quebec, Canada. He obtained a B.Sc. degree in biochemistry from McGill University in 2000. Two years later, he completed a M.Sc. degree in biochemistry with Professor Kalle B. Gehring, where he studied the application of polymer-stabilized liquid crystals in biomolecular NMR. He is currently pursuing a D.Phil. in structural biology in the University of Oxford (UK).

Marija Vilfan (mika.vilfan@ijs.si) was born in Ljubljana and received her Ph.D. in physics in 1978 from the University of Ljubljana, Slovenia. She is currently research consultant at the Jozef Stefan Institute and associate professor of physics at the University of Ljubljana. NMR research of liquid crystals has been her main professional interest for more than 20 years. She is married and has two grown up children - both physicists by profession.

Preface

Since the fi rst successful detection of Nuclear Magnetic Resonance (NMR) in 1945, the importance of the technique has increased enormously through countless applica­tions ofNMR to a huge variety of systems of physical, chemical, and biological interest. The impressive level of sophistication that NMR has achieved today is an excellent example of how the interplay between technological development and fundamental sci­ence continually rejuvenates an experimental method. The present book contains an overview of modem applications ofNMR to the most intriguing fourth state of matter which comprises anisotropic condensed fuid phases such as liquid crystals. Although the fi rst observation by Reinitzer of liquid crystals dates from 1888, the fascinating properties of these unusual, partially ordered liquids have continued to generate much scientifi c and technological interest and activity to the present day. As the contents of the present book show, the marriage between NMR and the study of anisotropic phases is generally a happy one. In this volume we hope to convey to the general reader some of the excitement that is felt by those active in the areas of both NMR and orientationally ordered liquids. We hope that the book may be a useful compendium of contributions that are of interest to experts in the fi eld, to those wishing to learn about this area of research, as well as to students who wish to obtain an overview about NMR and its applications to the study of ordered liquids. The title may seem presumptuous to some, as by no means do we even attempt to cover the entire range of ordered liquids that are known today. In fact, most of the ordered liquids discussed are liquid crystalline phases, with a large emphasis on the simplest of these, the ne­matic and smectic A uniaxial phases. In this context it should be noted that biological membranes are themselves uniaxial smectic A phases.

The philosophy behind the present book is that a full understanding of simpler systems may lead to a proper investigation of more complex ones. The applications discussed in the book span amazing ranges from various points of view. The molecular systems studied go all the way from molecular hydrogen to proteins, from the physical to the biological world. The highly detailed information obtained covers both time independent (spectroscopic) and time dependent (dynamic) facets of the realm of ori­entationally ordered molecules. The contributions included cover the entire spectrum between fundamental and technological science. Experimental results are discussed and interpreted in terms of varying approaches such as formal theory, phenomenolog­ical modeling, and computer simulations. This book does not and cannot solve all the

xxi

xxii NMR OF ORDERED LIQUIDS

outstanding questions and debates that make the present area of research such a lively one. However, we feel that the disparate range of ideas that sometimes arises from the pages of this book is a true reftction of how science works, and will help in defi ning future directions for the fi eld.

ELLIOTT BURNELL AND CORNELIS DE LANGE, FEBRUARY 2003

Introduction

E.E. Burnell and C.A. de Lange

Liquid crystals represent a fascinating state of matter that is intermediate between the liquid and the solid state. They are characterized by orientational, and sometimes positional order, while translational motion is often hindered but not prohibited. This so-called fourth state of matter has been known since 1888 [1], and nowadays comprises a staggering variety of molecules in the condensed phase that show partially ordered behaviour, ranging from relatively simple organic liquids to biological systems that can be designated as large by any standard. Liquid crystals can be broadly subdivided into thermotropics and lyotropics.

Thermotropics form homogeneous anisotropic liquid-crystalline phases over certain temperature ranges. With rod-shaped molecules we have a calimatic liquid crystal, and with disc-shaped a discotic. The ''simplest'' phase is the nematic, in which the molecules have orientational but no positional order, with the average direction of orientational order called the director. Cholesteric phases are formed from chiral ne­matics, resulting in a helical twist to the director throughout the sample. Smectic phases possess orientational plus translational order, where the molecules have a tendency to form planes. The smectic A phase has the director normal to the planes, whereas it is tilted in the smectic C phase. Depending on the arrangement of the molecules in the layers, there are many additional smectic phases.

Lyotropic liquid crystals depend on the heterogeneous mixing of at least two com­ponents, often water with an amphiphilic molecule such as a long-chain fatty acid salt or a phospholipid. The insolubility of water in the hydrocarbon, i.e. oil part, leads to many fascinating phases, including micelle (water surrounding clumps of lipids with their polar groups near the water), hexagonal (the micelles elongate and form hexago­nally packed rods), lamellar (layers of lipid separated by layers of water, thus excellent models of biological membranes), and cubic. The vesicles (closed bilayers) formed by phospholipids are excellent models of biological cells and organelles.

All the above are characterized by molecular packing in domains in which the molecules have long-range orientational order. As this is not a book on liquid crystals, the reader is referred to excellent monographs on this subject for more detail [2,3]. In this monograph the focus will be on the simple uniaxial systems.

xxiii

XXIV NMR OF ORDERED LIQUIDS

What all the liquid-crystalline phases have in common is their liquid-like behaviour and their anisotropic intermolecular potential. The study of this anisotropic potential is a topic of great interest, both from the fundamental and the technological point of view. This book is about the investigation of these fascinating systems using the technique of nuclear magnetic resonance, NMR. Since its discovery in 1945 [4, 5], the NMR method has developed into a tool that is unequalled in the study of an abundance of physical, chemical and biological systems. The state-of-the-art arsenal of sophisticated pulse methods for manipulating nuclear spins, and the development of high-fi eld spectrometers with associated sensitivity and multinuclear capacity, make NMR a very attractive tool for the study ofthe liquid phase. Since the anisotropic NMR interactions usually dominate over the isotropic ones in partially ordered systems, NMR has developed into a unique tool for studying molecules that constitute liquid-crystal phases, as well as for monitoring small, well characterized solute molecules that serve to probe the anisotropic environment of these phases. The detailed information that is then obtainable from NMR studies is not available by any other physical method. For example, the spectral parameters obtained from the NMR of solutes in liquid crystals have in the past provided a wealth of molecular information, such as accurate molecular structures, anisotropies of chemical shielding and indirect spin-spin coupling tensors, quadrupole coupling tensors, information on intramolecular motions, and values of orientational order parameters. Many examples are to be found in this book. In addition, NMR is invaluable in the investigation of dynamics in molecular systems, and the several Chapters that are devoted to this aspect cover topics such as spin relaxation measurements, motional narrowing, and the direct measurement of molecular diffusion constants.

In the last two decades much research has been carried out with the aim of eluci­dating the physical and chemical mechanisms that lie at the root of the orientational order in liquid crystalline phases. Extensive studies on molecules dissolved as probes in the anisotropic environment, the use ofliquid-crystal mixtures with surprising prop­erties, and the development of models, both theoretical and phenomenological, for the description of orientational order have all contributed to a signifi cant degree to our present level of understanding. In addition, computer simulation techniques have now developed to the point where solutes in an anisotropic environment can be treated at a satisfactory level. The combination of these experimental and theoretical approaches has proved to be instrumental in gaining a basic understanding of the factors that determine the behaviour of anisotropic liquids.

The book is divided into four parts. Part I provides a basic introduction to NMR spectroscopy and to some of the pulse techniques employed in the investigation of ordered liquids. Part II discusses applications ofNMR to studies ranging from atomic solutes dissolved in liquid crystalline solvents to the investigation of the structure of biological macromolecules - the methods developed for small solutes providing the basis for such investigations. In Part III we review the various models, theories and simulation approaches that have been used to investigate the interactions in ordered fuids at a molecular level. Part IV deals with some of the measurements that have been so successful in investigations of dynamics in ordered liquids.

INTRODUCTION xxv

With the current book we wish to achieve a number of aims. First, we believe that its publication is timely. As will become apparent from its contents, much understanding has been gained in recent years, but the discussion about fundamental issues is still heated and lively. We hope that this book in the years to come will help in focusing and guiding this scientifi c discussion in a way that benefi ts the entire community. This community is an active one, with many senior scientists playing a key role in developing novel ideas, with research students starting their careers in a highly stimulating area, and with young students being exposed to a competitive fi eld of science for the fi rst time. All these groups should benefi t from the book. With a signifi cant emphasis on introductory Chapters that outline the basic ideas necessary to understand the NMR of oriented liquids, the book provides useful material for specialized courses.

In editing a multi-authored book we, the editors, are very dependent on colleagues who are willing to dedicate much of their valuable time to writing Chapters that fi t into the overall framework of the book. With great pleasure we acknowledge the co-operation of all of them. They made our task a most enjoyable one.

References [I] Reinitzer, F. (1888), Monatsh. Chern., 9:421.

[2] de Gennes, P.G., and Prost, J. The Physics of Liquid Crystals, 2nd edition. Oxford University Press, Oxford, 1993.

[3] Chandrasekhar, S. Cambridge Monographs in Physics: Liquid Crystals. Cambridge University Press, Cambridge, 1977.

[4] Purcell, E.M., Torrey, H.C., and Pound, R.V. (1946), Phys. Rev., 69:37.

[5] Bloch, E, Hansen, w.w., and Packard, M. (1946), Phys. Rev., 70:474.