Series Editor's Preface This book on Modern Fourier Transform Infrared Spectroscopy is a useful addition to the Comprehensive Analytical Chemistry series. The work contains different chapters that cover both fundamental and applied aspects of infrared spectroscopy. Particular attention is given to fundamentals of vibrational spectroscopy and to the recent develop- ments of hyphenated chromatographic techniques. In addition, the major portion of the applications described in this book deal with polymeric and biological materials. Chemometric interpretation and data analysis are also described in detail in the last chapter of the book, indicating their relevance in infrared spectroscopy. The book can be used as an academic text and as reference book either for those with more expertise or for those starting with this technique. Overall, the book covers an important technique increasingly used in analytical chemistry. Finally I would like to thank the authors of the book for their time and efforts in preparing their contributions. Without their engagement this reference work on infrared spectroscopy would certainly not have been possible. D. Barcel6 xvii Acknowledgements Writing a book needs a lot of reading, careful planning and writing. The process is time consuming and requires the assistance of people on whom we can rely. During the process of writing this book many have helped us with physical work, ideas and support. It is not possible to thank everyone who has contributed to the book. However, there are some who have contributed to elevate the quality of the book and we are grateful for their efforts. In this respect, we would like to thank Professor Rolf Manne, Department of Chemistry, Agder University College for critically reviewing Chapter 4. We would like to thank Dr. Hideki Kandori (Kyoto University) for critically reviewing parts of Chapter 8 and Ms. Seiko Hino for polishing the English in Chapters 3, 5, 7, and parts of Chapters 8 and 9. In addition one of the authors (VGG) would like to thank Sheila E. Rodman of Polaroid Corporation for the collection of some of the materials used in Chapters 6, 7 and 8. Finally, we would like to thank our families and friends who have given us moral support and helped us through some difficult times during the writing of the book. xviii Authors' Preface Infrared spectroscopy has a history of more than a century: the charact- eristic absorptions of functional groups in the infrared region were known even in the 19th century; the first infrared atlas was published in 1905, twenty years before the birth of quantum mechanics. However, even though infrared spectroscopy is a relatively old technique, it has always been a popular technique for chemical analysis. Developments in computer technology, sensitive detectors and accessories for new sampling methodologies in the infrared region have made infrared spectroscopy one of the most powerful and widespread spectroscopic tools of the 20th and 21st centuries. The applications of infrared spectroscopy, and of Fourier transform infrared spectroscopy (FT-IR) in particular, are ever expanding, due to its versatile nature. The enormous number of articles and research papers published every year that deal with infrared spectroscopy and its applications is clear evidence to this. The book "Modern Fourier Transform Infrared Spectroscopy" has been written to reflect the popularity of infrared spectroscopy in sev- eral different fields of science. The chapters are designed to give the reader not only the understanding of the basics of infrared spectroscopy but also ideas on how to apply the technique in these different fields. The book is suitable for students at graduate level as well as experienced researchers in academia and industry. The first three chapters deal with the fundamentals of vibrational spectroscopy. Since spectroscopy is the study of the interaction of electromagnetic radiation with matter, the first two chapters deal with the characteristics, properties and absorption of electromagnetic radiation. Chapter 3 provides the basis for vibrations in molecules from both classical and quantum mechanical points of view. The absorption of infrared radiation by a vibration in a molecule depends on the symmetry of the molecule and the symmetry of the vibrations. As this aspect is not usually treated in textbooks on infrared spectroscopy, Chapter 4 deals with the symmetry aspects of molecules and illustrates how the reader can determine the vibrations that are infrared active. Chapter 6 is an overview of the instrumentation used to perform the majority of Fourier transformed infrared spectroscopic experiments xix today. The chapter starts with an overview of the history of FT-IR spectroscopy from the construction of the first interferometers in 1880 to the present day and continues with a description of the components of an interferometer and the various scanning techniques (continuous- scan and step-scan). Chapter 7 first describes sampling techniques used in transmission and reflection spectroscopy and then a variety of the so-called hyphenated techniques that combine the use of FT-IR spectroscopy with another analytical technique. Thermogravimetric analysis (TGA/FT-IR), liquid chromatography (LC/FT-IR), gas chroma- tography (GC/FT-IR) and supercritical fluid chromatography (SFC/FT- IR) are the combinations discussed in this book. Chapter 8 depicts certain applications of FT-IR spectroscopic tech- niques to basic and industrial research. Specifically, a large portion of the chapter deals with the characterization of polymers and polymeric surfaces, whereas the remaining part describes applications to organic thin films and biological molecules. One subcategory treated in detail is the determination of molecular orientation in polymers via static and dynamic FT-IR experiments. Another subcategory is the applications that involve optically active materials and conducting polymers. In addition, very significant developments have recently taken place in the area of infrared microspectroscopy and especially in infrared imag- ing with the introduction of focal plane array detection. Part of this chapter is dedicated to an explanation of the experimental procedures associated with these imaging experiments along with selected examples from the recent literature. Finally, Chapter 9 deals with some modern analytical methods in infrared spectroscopy. Again, the methods described here are not very common in books on infrared spectroscopy. The first part of the chapter deals with chemometric techniques that can be applied to semi-quanti- tative and quantitative analysis of infrared spectroscopic profiles. The text is designed to give the theoretical basis of these techniques and in particular how they can be applied to infrared spectroscopic data profiles. In this chapter, the subject of two-dimensional correlation spectroscopy (2D-IR) is also discussed. The principles of the technique along with selected examples of the applications of the 2D-IR treat- ment are presented. Alfred A. Christy Yukihiro Ozaki Vasilis G. Gregoriou April 2001 xx Chapter 1 Electromagnetic radiation and the electromagnetic spectrum We have come across people talking about microwave, UV radiation, radio waves, x-rays, radar, cosmic rays and so on. We understand the dangers of UV radiations from the sun and the use of microwave in heating food. What do we associate with all these different terms? We understand without learning the physics of these different radiations that they are associated with different energies. For example, white light is a form of energy and it comprises a mixture or spectrum of seven different colours, which are all visible to the human eye. All these colours of which white light is composed have different energies in the descending order: violet, indigo, blue, green, yellow, orange, red. A photographic plate is readily affected by violet light, unlike red light which has almost no effect. From the above discussion, it is clear that the spectrum of different radiations falls into a larger scale of spectrum where white light is a very small part. The larger scale containing the spectrum of different radiations is called electromagnetics pectrum. The physical properties of radiations cannot be explained by a single theory. Some properties such as propagation of radiation through a medium, diffraction and reflection are better explained by a theory called wave theory and properties like momentum are better explained by particle theory. The propagation of radiation through space involves electric and magnetic components of the radiation and hence the term "electromagnetic". Spectroscopy generally deals with the interaction of electromag- netic radiation and matter. In order to understand this interaction, we must understand the characteristics of the electromagnetic radiation and the matter involved in the interaction. 1 1.1 WAVE NATURE OF ELECTROMAGNETIC RADIATION-WAVE CHARACTERISTICS AND WAVE PARAMETERS According to electromagnetic theory, electromagnetic radiation is a form of energy that is composed of oscillating electric and magnetic fields acting in planes that are perpendicular to each other and to the direction of propagation (Fig. 1.1). The oscillating electric field is simple harmonic and propagates as waves with a velocity (c) 3x108 m s- in vacuum. The propagation velocity varies with the refractive index of the medium. In a two-dimensional representation, the variation of the electric field strength of an electromagnetic radiation with propagation time in space can be paralleled to the variation of the y co-ordinate of the trace of a particle moving around a circle of radius A with a constant angular velocity o radians per second (Fig. 1.2). Let us assume that OX and OY represent the positive directions of the x and y axes and O represents the origin of these axes. Let us also assume that at time zero the particle passes through x and then consider the position of the particle after t seconds. The angle traversed by the particle in t seconds is ot radians. The y co-ordinate of the position of the particle can be given by equation y =A sin ot (1.1) Electric field streng~ Direction of propagation Fig. 1.1. Oscillating electric and magnetic fields of electromagnetic radiation. 2 ct=r/2 s-1 act=r wt= 3r12 Fig. 1.2. Trace of a particle moving around a circle of radius A with an angular velocity rads-1. At time zero (i.e. ot = 0) the particle is at x and this function is minimum with a value 0. At time 7d/20 (i.e ot = c/2) the particle is at y and the function is maximum with a value A, the amplitude of the function. At time nl/c (i.e cot = g), the particle is at Z and the function has another minimum. Similarly the function will have another maximum and another minimum at times 3/2co (cot = 37/2) and 2/co (ot = 2), respectively. The particle takes 2/o seconds (remember one circle is 2i7 radians) to complete the journey through the circular path once (one cycle). This is called the period of the motion and denoted by . The number of cycles the particle traces through in one second is o/2t {1/ (2t/o)} and is called the frequency (v) of the motion. The frequency is abbreviated by the symbol Hz. The angular velocity of the motion can then be written in terms of the frequency as co = 2v (1.2) A two-dimensional plot of the variation of the y co-ordinate of the position of the particle with time is shown in Fig 1.3. Equation (1.1) can be written to include the frequency of the motion and time as y =A sin 2vt (1.3) 3 A O.5 A A >1 0-- -0.5A -A Time in seconds Fig. 1.3. A two-dimensional plot of the variation of the y co-ordinate moving in a circle as shown in Fig. 1.2. 0.SA 0 -0.5A -A Propagation distance in metres Fig. 1.4. The propagation of electromagnetic radiation. The propagation of the electromagnetic radiation in space is 3x108 m s l. Figure 1.3 can be redrawn (Fig. 1.4) in a similar manner to represent the distance of propagation of electromagnetic radiation along the x axis. When the x axis represents the distance, we can define some other parameters characteristic to electromagnetic radiation. In the 4 preceding discussion, we learnt that the y co-ordinate of the particle has zero values at times 0, n/co and 27/co. That is at distances 0, rc/lo and 2nc/o, the propagating electric field of the electromagnetic radiation will have zero field strength. But the distance between the extreme points, that is, the distance travelled during a complete cycle of oscilla- tion is called wavelength (k). However, because of the symmetry of the sine wave, the wavelength can be defined as the distance between two similar points in the wave. The wave has a frequency v and therefore the velocity of propagation can be written as kv = c (in metres per second) (1.4) This implies that the wavelength has dimension m (metre). The inverse of the wavelength, when the wavelength is expressed in centi- metres is called wavenumber and is denoted by v. The dimension for wavenumber is cm-1. v = 1/k cm-1 (1.5) The wavenumber can also be thought of as the number of waves in 1 cm length. The above two equations combined give Eq. (1.6). v = Vc (c in centimetres per second) (1.6) The propagation distance s of an electromagnetic radiation in t seconds is given by s = kvt = ct (1.7) Equation (1.7) can be combined with Eq. (1.1) to give a relationship describing the variation of the field strength of an electromagnetic radiation with the distance of propagation and its wavelength as follows y =A sin (2nrs/k) (1.8) The flux of energy of an electromagnetic radiation along the direction of propagation is equally divided between electric and magnetic fields. Electromagnetic radiations are produced by accelerating electric charges and electric charge accelerations are produced when 5