Modern Fourier Transform Infrared Spectroscopy (Comprehensive Analytical Chemistry)

356 Pages·2001·14.64 MB·English

by  A.A. Christy

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