Nitrogen NMR PDF
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NITROGEN NMR NITROGEN NMR Edited by M. Witanowski Institute of Organic Chemistry Polish Academy of Sciences Warsaw, Poland and G. A. Webb Department of Chemical Physics University of Surrey Guildford, Surrey England PLENUM PRESS-LONDON AND NEW YORK-1973 Library of Congress Catalog Card Number: 72-95065 ISBN-13: 978-1-4684-8177-8 e-ISBN-13: 978-1-4684-8175-4 DOl: 10.1007/978-1-4684-8175-4 Copyright © 1973 by Plenum Publishing Company Ltd Softcover reprint of the hardcover 1st edition 1973 Plenum Publishing Company Ltd 8 Scrubs Lane Harlcsden London NWI0 6SE Telephone 01-969 4727 u.S. Edition published by Plenum Publishing Corporation 227 West 17th Street New York, New York 10011 All Rights Reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, micromming, recording or otherwise, withont written permission from the Publisher Preface To date nitrogen NMR has been discussed in research papers and review articles throughout the scientific literature. It has been our aim in preparing this book to provide a comprehen- sive account of the widely spread applications of nitrogen NMR. The relevant literature has been surveyed from the beginnings of NMR until early 1972. The steady annual growth in the number of references cited since 1965 is ample evidence of the ever increasing importance of the subject. Sufficient theoretical and experimental background is given for an understanding of the applications dealt with in later chapters. The basic principles of NMR are developed with a theoretical approach to chemical shifts and spin-spin coupling constants, particular emphasis being given to nitrogen nuclei. Following this the experimental aspects of nitrogen NMR are adequately described. Special emphasis is given to the observable effects of the nuclear quadrupole moment of the 14 N nucleus. It is appro- priate that this topic be dealt with in depth since quadrupolar interactions frequently dominate the information available from a study of the 14 N nucleus and other nuclei spin- coupled to it. The applications of nitrogen chemical shift data to organic and inorganic molecules are covered in two extensive chapters which include the effects of paramagnetism on nitrogen NMR. The continuing discussion on the choice of chemical shift scales and reference compounds is also covered. Spin-spin coupling constant values are often available from the spectra of 1 5 N nuclei and other nuclei spin-coupled to 1 5 N. This subject is amply dealt with and the correlations of coupling constants with molecular structure are discussed in some detail. v PREFACE Finally we would like to express our appreciation to all the contributors for their dedication and forbearance during the preparation of this volume. M. Witanowski G. A. Webb Warsaw, Poland. Guildford, England. July 1972 vi Contributors T. Axenrod Department of Chemistry, The City College of The City University of New York, New York, N.Y. 10031, U.S.A. H. Januszewski Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland. J. P. Kintzinger Institute of Chemistry, Louis Pastuer Uni- versity, Strasbourg, France. J. M. Lehn Institute of Chemistry, Louis Pasteur Uni- versity, Strasbourg, France. N. Logan Department of Chemistry, University of Nottingham, Nottingham, England. E. W. Randall Department of Chemistry, Queen Mary College, London, England. L. Stefaniak Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland. G. A. Webb Department of Chemical Physics, University of Surrey, Guildford, Surrey, England. M. Witanowski Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland. vii Contents Preface v Contributors Vll 1 Theoretical Background G. A. Webb and to Nitrogen NMR M. Witanowski 1 2 Experimental Aspects of Nitrogen NMR E. W. Randall 41 3 Nitrogen 14 Nuclear J. M. Lehn and Quadrupole Effects J. P. Kintzinger 79 4 Nitrogen Chemical Shifts M. Witanowski, L. Stefaniak in Organic Compounds and H. Januszewski 163 5 Correlations of Nitrogen Coupling Constants with Molecular Structure T. Axenrod 261 6 Applications of 14 N NMR Data in the Study of Inorganic Molecules N. Logan 319 Subject Index 383 IX CHAPTER 1 Theoretical Background to Nitrogen NMR G.A. Webb Department of Chemical Physics, University of Surrey, Gu£ldford, Surrey and M. Witanowski Institute of Organic Chemistry, PoHsh Academy of SC£ences, Warsaw. 1.1 BASIC PRINCIPLES OF NMR 1 1.1.1 Introduction 1 1.1.2 The Resonance Condition 3 1.1.3 Nuclear Screening 4 1.1.4 Relaxation Times and Linewidth 5 1.1.5 Spin-spin Coupling 8 1.1.6 Nuclear Spin Hamiltonian and Spectral Analysis 10 1.2 THEORY OF CHEMICAL SHIFTS 12 1.3 THEORY OF SPIN-SPIN COUPLING 31 REFERENCES 37 1.1 Basic Principles of NMR 1.1.1 Introduction There can be no doubt that nitrogen is one of the most important atoms in organic-, inorganic- and bio-chemistry. Its common molecular occurrence in a variety of valence states 2 G. A. WEBB and M. WITANOWSKI with various types of bonding and stereochemistry has caused the nitrogen atom to be widely studied by chemical and physical techniques. In natural abundance nitrogen exists in two isotopic forms, the most common is 14N, which is 99.635% abundant and 15 N which has an abundance of 0.365%. Both of these isotopes may be studied by means of nuclear magnetic resonance (NMR), additionally 14N provides nuclear quadrupole resonance (NQR) data as discussed in Chapter 3. These studies and the interpretation of the ensuing results have attracted the interests of a number of physical and theoretical chemists, for reasons which are made apparent in later chapters. Although the 1 4 N nucleus was investigated in the early days of NMR the number of papers dealing with nitrogen NMR, published between 1950 and 1964, are very few in comparison with those on other nuclei especially 1 H and 19 F. This is partly due to the low sensitivity of the 14 N and 1 5 N nuclei, 0.00101 and 0.00104 respectively, relative to that of a proton in the same applied magnetic field. Another factor is the advent of commercially available NMR spectrometers to allow the exploitation of this sensitivity difference. The low natural abundance of 1 5 N is another reason for its neglect. This can be largely overcome by means of isotopic enrich- ment, which is rather expensive in practice; 15 NH! costs about U.S. $400 per gram of contained 15 N. However, recently molecules containing 1 5 N in natural abundance have been successfully studied with both continuous wave [1] and pulse [2] methods, together with spectrum accumulation tech- niques. The experimental details of these studies are discussed in Chapter 2. Since 1964, improvements in experimental techniques and instrumentation have generated a wide and growing interest in both 1 4 Nand 1 5 N NMR spectroscopy. However, the 14 N nucleus is inherently difficult to study not only on account of its low relative sensitivity but also because it has an electric quadrupole moment (Section l.l.4). However, the difficulties experienced in 14 N NMR measurements are at least partially compensated for by the additional information, on the en- vironment of the nucleus, which may be obtained from the quadrupolar broadening of the 1 4 N resonance lines (Section 3.4). THEORETICAL BACKGROUND TO NITROGEN NMR 3 1.1.2 The Resonance Condition Both common isotopes of nitrogen have a nuclear spin denoted by the quantum number 1, 1 = 1 for 14 Nand 1 = ! for IS N. Since nuclei are charged bodies there is a magnetic moment JL collinear with the spin vector I, where the length of the spin vector is yl(1 + l)h. Assuming that the nucleus can be considered as a spmnmg charged sphere, a simple classical calculation gives JL = 'YIh (1.1 ) where 'Y is the gyromagnetic ratio of the nucleus. In the presence of an external magnetic field, Bo defined to be in the z direction, 21 + 1 levels are produced whose energIes, relative to that in zero magnetic field E, are given by (1.2) where m[ represents the allowed component of 1 m the z direction and m[ = 1,1 - 1, ..., -1 (1.3) Since the selection rule governing magnetic-dipole transitions IS tlm[ = ±1 (1.4) and the separation between adjacent energy levels, M, is given by tlE = 'YhBo (1.5) NMR occurs when transitions take place between these levels. This can occur by the absorption of photons from an oscillating external field, having the correct polarization and satisfying the frequency conditions: hv = tlE = 'YhBo ( 1.6) or more simply: 'YBo v=-- (1. 7) 211" where v is the frequency of the radiation from an external oscillating field which is absorbed by the nuclear spins. Equation (1.7) expresses the resonance condition for NMR experiments, the practical details of which are described in Chapter 2.
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