AN INTRODUCTION TO PRACTICAL INFRA-RED SPECTROSCOPY A. D. CROSS, Ph.D., D.Sc., F.R.I.C. Vice President, Syntex Corporation, Apartado M-10063, Mexico I, D.F. R. ALAN JONES, M.A., B.Sc., Ph.D. Lecturer, School of Chemical Sciences, University of East Ang/ia, Norwich THIRD EDITION Springer Science+Business Media, LLC Published in the U.S.A. by PLENUM PRESS a division of PLENUM PUBLISHING CORPORATION 227 West 17th Street, Ncw York, N.Y. 10011 First published by ßutterworth & Co. (Publishcrs) Ltd. First l:.äition 1960 Reprinted 1962 Second Edition 1964 Reprinted 1967 Third Edition 1969 © Springer Sciencet-Business Media New York 1969 Originally published by Butterworth & Co. (Publishers) Ltd in 1969. ISBN 978-1-4899-6274-4 ISBN 978-1-4899-6596-7 (eBook) DOI 10.1007/978-1-4899-6596-7 Suggested U.D.C. Number: 543·422·4 Library of Congress Catalog Card Number 69-20393 PREFACE FEW organic chemists engaged in molecular structural research or analytical control can now avoid calling upon infra-red absorption measurements for guidance at one time or another. Whether they have to make the measure ments themselves or receive spectral charts of their compounds recorded by technicians, it is vital that they should have sufficient understanding of what is involved ifthey are to assess the results and make the correct interpretation. Here, too, it is not only a question of knowing what can be done by this method, but also what it cannot do and where real ambiguities can arise which must be solved by using other physico-chemical techniques. A nurober of authoritative books and articles now exist about all this work, but this little manual prepared by Dr. Cross will none the less be useful, since it outlines concisely all the essentials of the theory, measurements and applications required by the majority of organic chemists. He has collected together a weil chosen set of standard reference data and set it out in a readily usable form. I am sure that the book will interest a wide circle of users and undergraduate readers. H. W. THOMPSON St. John's College, Oxford iii ACKNOWLEDGEMENTS SEVERAL major works already exist which provide comprehensive surveys of the literature, theory and practice of infra-red spectroscopy. They contain a wealth of detail and references to which all others interested in this subject must inevitably turn. I wish to acknowledge the great help derived from Dr. Bellamy's masterful book, the text of Jones and Sandorfy, and the earlier works of Randall, Fowler, Fuson and Dangl, and of Barnes, Gore, Liddeli and Van Zandt Williams. I am deeply indebted to Dr. E. S. Waight and Dr. C. J. Timmons for invaluable advice during the preparation of the manuscipt and to Dr. L. J. Bellamy, Dr. G. Eglinton, Dr. G. W. Kirby and Dr. J. K. Sutherland who subsequently read and constructively criticized the script. I record here my thanks to Professor S. Shibata for information on Japanese spectrophotometers; to the many research spectroscopists who spared time to discuss problems with me and to demoostrate their various infra-red instruments; and to all the instrument manufacturers for their generaus provision of technical and scientific information. My sincere gratitude is owing to Dr. H. W. Thompson for his Preface. My final thanks are reserved for my mentor, Professor D. H. R. Barton who, besides contributing a Foreword, offered continuous personal encouragement. A.D.C. Imperial College of Science and Technology I WISH to acknowledge the encouragement given to me by Dr. A. D. Cross and by Professor A. R. Katritzky during the preparation ofthe Third Edition. I am also indebted to my colleagues at the University of East Anglia for their helpful advice and criticism; to Professors L. Yakhontov and M. Hamana for information on Russian and Japanese spectrometers respectively; and to all instrument manufacturers for their continued assistance by their pro vision of technical information. R.A.J. University of East Anglia IV CONTENTS Page Preface by Dr. H. W. Thompson iii Acknowledgements iv Foreword by Professor D. H .•R . Barton Vll Authors' Notes on Third Edition viii PART I Introduction 1 Elementary theory 2 Uses of infra-red spectroscopy 6 Instruments: construction and operation 10 Cells and sampling techniques 20 Correct phase and sample diluent 30 Prisms and gratings 36 Quantitative analysis 38 Hydrogen bonding 43 Interpretation of a spectrum 45 PART II Introduction 63 Table of abbreviations 66 Correlation charts 67 Tables of group absorption frequencies (and wavelengths) 73 Reciprocal Tables 98 Index 101 V FOREWORD THE past two decades have witnessed the arrival ofthe infra-red spectrophoto meter as an instrument of outstanding usefulness in all branches of chemistry. Already in widespread application in research, analytical and industrial laboratories, it is being belatedly introduced into the Undergraduate's course of study-a development which will undoubtedly spread with the arrival of robust low-cost instruments. It is primarily to aid the organic chemist, un familiar with practical infra-red spectroscopy, that this book has been written, though it should serve too as a convenient summary oftechniques and data for those more familiar with the field. Over the past few years Dr. Cross has introduced many newcomers to the generat operating procedures and sampling techniques ofinfra-red absorption spectroscopy. He has shown that, · given comprehensive initial instruction, and adequate supervision subsequently, students may operate instruments very satisfactorily and with great benefit to their understanding of the subject and to the progress of their work. Such experience prompted his opinion that these practices should be adopted more generally and that an Introduction to the field outside of review articles and special publications would be helpful. The present book meets these requirements in a very satisfactory manner. I am glad to recommend it to all students of Organic Chemistry and to others who may wish for an initiation into the practical applications of infra-red spectroscopy. In addition, the numerous tables provide welcome reference data in easily available form. The modest price of this book is another feature which is highly commendable. Imperial College, London D. H. R. BARTON vii AUTHORS' NO'PES ON THIRD EDITION EIGHT years have passed since the original text was brought to completion and four since composition of the second edition. Continued progress in infra-red spectroscopy, methods and machines, militated in favour of a third edition at this time rather than another reprint. Furthermore, the growing overlap of infra-red spectroscopic evaluation of problems with other spectro scopic analyses suggested expansion of the examples to illustrate this inter dependence. The tables correlating absorption frequencies with structure have been thoroughly revised and updated, with much additional information brought in which deals with heterocyclic systems. Dr. A. D. Cross is no Ionger active in the field of infra-red spectroscopy, and authorship was expanded therefore to insure recognition of the latest developments. Dr. R. Alan Jones has been responsible for the improvements and modifications introduced over previous editions. A.D.C. R.A.J. vüi PART I INTRODUCTION THE first half of the book is devoted to the simple theory and practical aspects of infra-red spectroscopy. The emphasis, throughout, is on the qualitative rather than quantitative analytical value of infra-red, since it is with the identification of specific functional groups that the organic ehernist is mostly concerned. A relatively small section on theory is included since this is adequately dealt with elsewhere1-4• This is followed by a brief summary of the various uses of infra-red spectroscopy. The next section deals with the spectro photometer itself and includes information on machines currently available or to be available shortly. Discussion is limited to double-beam instruments since their single-beam Counterparts are less convenient for routine analysis in organic chemistry research laboratories. Cell construction and sample preparation are considered in detail, as is also the importance of correct selections of prisms or gratings and of phase and sample diluents; these choices present valuable flexibility in instrument operation. The advantages and limitations of these variables are elaborated and the calibration procedure for the ultimate spectrum is included. A separate section is devoted to problems of intensity measurements and the information to be gained from them. Hydrogen bonding is of especial interest in infra-red studies and the penultimate section deals with this pheno menon. Finally an outline is given of the procedure for interpretation of a spectrum, together with examples of the integrated use of infra-red data with that from other spectroscopic techniques for the complete elucidation of structure. Throughout, attention is drawn repeatedly to sources of error and to limitations of the instrument, materials and operator. Following the recom mendations of the Royal Society Committee, wave numbers, rather than wavelengths, are referred to in the text. The argument for general adoption of frequency with the wave nurober scale has been briefly outlined by JONES and SANDORFY5• However, since many results are still published using wavelengths, the corresponding values on this scale are given in parentheses throughout the text and in the tables of both Part I and Part II; a table of reciprocals is also provided. Several methods for presentation of intensity measurements are currently in use, but there is an urgent need for an inter nationally agreed system of units for the presentation of results. Important references are given throughout the text but comprehensive collections ofrelevant references, whose inclusion would substantially increase the size and cost of this book, are available elsewhere1-12• ELEMENTARY THEORY OF INFRA-RED SPECTROSCOPY INFRA-RED radiation promotes transitions in a molecule between rotational and vibrational energy Ievels of the ground (lowest) electronic energy state*. In a simple diatornie molecule A-B the only vibration which can occur is a periodic stretching along the A-B bond. Stretchingvibrations resemble the oscillations of two bodies connected by a spring and the same mathematical treatment, namely Hooke's law, is applicable to a first approximation. For stretching of the bond A-B, the vibrational frequency v (cm-1) is given by equation (1) . (1) where c is the velocity of light, f the force constant of the bond, and IL the reduced mass of the system, as defined by equation (2) . (2) where mA and mB are the individual masses of A and B. Stretching vibrations of individual bonds within more complex molecules may be considered similarly, though other vibrations become possible and absorption band frequencies are influenced by other factors (vide infra). Substitution13 in equation (1) of accepted numerical values of c, fand IL for the C-H bond gives a frequency of 3,040 cm-1 (3·29JL), which is in tolerable agreement with found bond frequencies of 2,975-2,950 cm-1 (3·36-3·39 JL) and 2,885-2,860 cm-1 (3·47-3·50 JL) for methyl group C-H stretching vibra tions. Such calculations are of greatest value when the atoms joined by the bond have !arge mass difference and when the remainder of the molecule exerts little influence on the bond motion under consideration. This condition is met when one atom is hydrogen and, in consequence, the A-H stretching mode frequencies are among the most thoroughly studied and valuable bands available for diagnostic purposes. By convention, band positions are quoted in units of wave number (v) which are expressed in reciprocal centimetres (cm-1), usually styled band frequencies. However, the true unit of frequency (v) is given in reciprocal seconds (sec-1). An alternative term, wavelength (.\), measured in microns (JL) is also used to indicate band position. The relation between these units is given in the expressions v = ~; ii = X (c = velocity of light) lJL = 10-' cm = 10' A; hence, 10 IL = 1,000 cm-1 and 2·50 IL = 4,000 cm-1• Measurement of absorption band intensity is considered on p. 38. A complete review has been made of the presentation of absorption spectra • This is in contrast to the energetically more powerful ultra-violet radiation, which facilitates transitions between vibrational and rotational energy levels of different electronic levels. 2