UV-VIS Spectroscopy and Its Applications

251 Pages·1992·9.55 MB·English

by  Professor em. Dr. Heinz-Helmut Perkampus (auth.)

SPRINGER
LABORATORY
Heinz-Helmut Perkampus
UV-VIS Spectroscopy
and Its Applications
Translated by H. Charlotte Grinter
and Dr. T. L. Threlfall
With 78 Figures and 21 Tables
Springer- Ver lag
Berlin Heidelberg New York London Paris
Tokyo Hong Kong Barcelona Budapest
Professor em. Dr. HEINZ-HELMUT PERKAMPUS
Heinrich-Heine-Universitat
Physikalische Chemie
und Elektrochemie I
UniversitatsstraBe 26.43.02
W-4000 Dusseldorf 1
ISBN-13: 978-3-642-77479-9 e-ISBN-13: 978-3-642-77477-5
DOl: 10.1007/978-3-642-77477-5
Library of Congress Cataloging-in-Publication Data. Perkampus, Heinz-Helmut. [UV-VIS
Spektroskopie und ihre Anwendungen. English] UV-VIS spectroscopy and its applications/
Heinz-Helmut Perkampus; translated by H. Charlotte Grinter and T. L. Threlfall. p. cm.
Includes bibliographical references and indexes. lSBN·U, 978-3-642-77479-9 (alk. paper): DM 168.00.
ISBN-13 978-3-642-77479-9 (alk. paper: U.S.) 1. Ultraviolet spectroscopy. I. Title.
QD96.U4P4713 1992 543'.08585-dc20 92-20077
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Preface
UV-VIS spectroscopy is one of the oldest methods in molecular
spectroscopy. The definitive formulation of the Bouguer-Lambert
Beer law in 1852 created the basis for the quantitative evaluation
of absorption measurements at an early date. This led firstly to
colorimetry, then to photometry and finally to spectrophotometry.
This evolution ran parallel with the development of detectors for
measuring light intensities, i.e. from the human eye via the photo
element and photocell, to the photomultiplier and from the photo
graphic plate to the present silicon-diode detector both of which
allow simultaneous measurement of the complete spectrum.
With the development of quantum chemistry, increasing atten
tion was paid to the correlation between light absorption and the
structure of matter with the result that in recent decades a number
of excellent discussions of the theory of electronic spectroscopy
(UV-VIS and luminescence sp,~ctroscopy) have been published.
Consequently, this extremely ivteresting aspect of molecular spec
troscopy has dominated the teaching of the subject both in my
own lectures and those of others. However, it is often overlooked
that, in addition to the theory, applications of spectroscopic
methods are of particular interest to scientists. For this reason, a
lecture series about electronic spectroscopy given in the Institute
for Physical Chemistry at the Heinrich-Heine-University in
Dusseldorf was supplemented by one about "UV-VIS spectroscopy
and its applications". This formed the basis of the present book.
UV-VIS spectroscopy owes its importance not least to its varied
applications in chemistry, physics and biochemistry. This book
aims to show how UV-VIS spectroscopy can be applied to
analytical problems, to the investigation of chemical equilibria and
to the kinetics of chemical reactions, including photokinetics. The
theoretical section has been kept to a minimum since, as men
tioned above, excellent discussions of such matters are available in
the literature. The details of the equipment are also described very
briefly because G. KortUm gave an outstanding discussion of this
subject in volume II of the series "Anleitung fUr die chemische
Laboratoriumspraxis"; and its basic details still apply today.
In addition to the applications, a number of UV-VIS spec
troscopic techniques are discussed. However, in this case the selec
tion has been influenced by the author's own interests. In order to
obtain experimental examples, numerous measurements have been
VI Preface
made which might also be set as practical work for students of ad
vanced physical chemistry.
I would like to thank my colleagues for making these measure
ments, and drawing the diagrams.
The English translation of the second edition of this volume is
due to the stimulating interest of the "Ultraviolet Spectrometry
Group", London, to whom I am very grateful.
Mrs. Charlotte Grinter undertook the translation with great in
terest and engagement, professionally supported by Dr. T. L.
Threlfall and Dr. Grinter. I would like to express my sincere thanks
to Mrs. Grinter and the colleagues mentioned above for their hard
labors.
A few additions have been made to the first edition, i.e. the brief
section "Chemometrics" was added by the English colleagues,
some figures have been changed, others are new and the cited
literature has been updated where necessary. Here also the English
colleagues proved to be very helpful for which I would like to ex
press my thanks.
Thanks are also due to Dr. Enders of the Springer-Verlag for his
interest and support of the publication of the English edition.
Dusseldorf, June 1992 HEINZ-HELMUT PERKAMPUS
Contents
1 Introduction 1
2 Principles .................................. . 3
2.1 The Bouguer-Lambert-Beer Law
and Its Practical Application ................. . 3
2.2 Primary Photophysical Processes .............. . 5
2.3 Vibrational Structure of Electronic Spectra 6
2.4 Electronic Spectra and Molecular Structure ..... . 8
References ......................................... 9
3 Photometers and Spectrophotometers .......... . 10
3.1 Photometers ................................ . 11
3.2 Spectrophotometers .......................... . 12
3.3 The Stray Light Error ....................... . 17
3.3.1 General Observations ........................ . 17
3.3.2 The Stray Light Error of Transmission
and Absorbance and Its Measurement ......... . 19
3.4 Light Sources for UV-VIS Spectroscopy ........ . 21
References ......................................... 24
4 Analytical Applications of UV-VIS Spectroscopy 26
4.1 Photometric Determination of a Single Substance 26
4.1.1 Photometric Determination of Elements
by Means of Complexing Agents .............. . 29
4.1.2 Photometric Determination of Anions
and Ammonia .............................. . 38
4.1.3 Photometric Water Analyses .................. . 43
4.1.4 Photometric Determination
of Organic Compounds ...................... . 44
4.1.5 Enzymatic Analysis and Enzyme Kinetics ...... . 49
4.2 Multicomponent Analysis .................... . 58
4.2.1 Basic Equations ............................. . 58
4.2.2 An Example of a Multicomponent Analysis .... . 65
4.3 Identification and Structure Determination ..... . 68
VIII .Contents
4.4 Chemometrics 75
References ......................................... 76
5 Recent Developments in UV-VIS Spectroscopy ... . 81
5.1 Dual-Wavelength Spectroscopy ................ . 81
5.2 Derivative Spectroscopy ...................... . 88
5.3 Reflectance Spectroscopy ..................... . 95
5.4 Photoacoustic Spectroscopy .................. . 101
5.4.1 Principles of PAS ........................... . 101
5.4.2 PAS Applications ........................... . 110
5.5 Luminescence-Excitation Spectroscopy ......... . 120
References ......................................... 128
6 Investigation of Equilibria ..................... 131
6.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6.2 Protolytic Equilibria; pK-Values ............... . 132
6.3 Complex-Forming Equilibria .................. . 142
6.3.1 H-Bond Association ......................... . 143
6.3.2 EDA Complexes ............................ . 149
6.3.3 Metal Complexes ............................ . 158
References ........................................ . 162
7 Investigation of the Kinetics
of Chemical Reactions ........................ 165
7.1 Fundamental Equations of Kinetics... . . . .. . . . .. 165
7.1.1 Introduction of Absorbance
as a Measurement Parameter .................. 165
7.1.2 Classification of Other 'JYpes of Reaction ....... 167
7.1.2.1 2nd Order Reactions .......................... 167
7.1.2.2 3rd Order Reactions .......................... 169
7.1.2.3 Pseudo 1s t Order Reactions ................... 171
7.1.2.4 Consecutive Reactions ........................ 172
7.1.2.5 Parallel Reactions ............................ 173
7.2 The Number of Linearly Independent
Partial Reactions ............................. 175
7.3 Evaluation of Kinetic Measurements ........... . 179
7.4 Examples ................................... . 183
7.5 Fast Reactions .............................. . 190
7.5.1 Flow Methods: The Stopped-Flow Technique ... . 190
7.5.2 Spectroscopic Relaxation Techniques ........... . 193
Contents IX
7.6 Photo reactions .............................. . 197
7.7 Spectrometers for Kinetic Measurements ....... . 203
7.7.1 Rapid Spectrometers ......................... . 204
7.7.2 FT-UV Spectrometers ........................ . 204
7.7.3 . Diode Array Spectrometers ................... . 205
7.8 Determination of the Spectra of Intermediates .. . 207
References ......................................... 210
8 Evaluation of UV-VIS Spectral Bands ........... 215
8.1 Oscillator Strength and Transition Moment ...... 215
8.2 Band Analysis ............................... 220
8.2.1 Gaussian and Lorentzian Functions ............. 220
8.2.2 Application of Derivative Spectra .............. 223
8.3 Vibrational Structure ........................ . 228
References ........................................ . 233
Index of llIustrated Absorption Spectra ............... . 235
Subject Index ..................................... . 237
1 Introduction
Optical spectroscopy is based on the Bohr-Einstein frequency relationship
(1)
This relationship links the discrete atomic or molecular energy states E
j
with the frequency v of the electromagnetic radiation. The proportionality
constant h is Planck's constant (6.626 x 10-34 J s or 6.626 x 10-27 erg s). In
v
spectroscopy, it is appropriate to use the wavenumber instead of frequency
v. Equation (1) then takes the form:
LIE = E2-Ej = hcv
where
V=CIA=CV (2)
v
Absorbed or emitted radiation of frequency v or wavenumber can thus
be assigned to specific energy differences or, applying the definition of the
'term value' (energy level), to specific energy-level differences:
(3)
T = E/h c is the term value. From this definition it follows that it has
j
dimension m in the SI -system. However, it is still commonly given as
-j
cm-I; thus, wavenumber v as a term difference may be given in m-I or
cm -I. Since the wavenumber is always given in cm -I in the literature, it
will also be used in this book (1 cm-I~100m-I).
For absorption spectroscopy in the ultraviolet (UV) and visible (VIS)
region, this range can be characterized by the information in Fig. 1.
Within the overall range of electromagnetic radiation which is of interest
to chemists, UV- and VIS-absorption spectroscopy occupies only a very
narrow frequency or wavenumber region. Nevertheless, this range is of ex
treme importance, since the energy differences correspond to those of the
electronic states of atoms and molecules; hence the concept of "electronic
spectroscopy". Furthermore, in the visible spectral region the interactions
between matter and electromagnetic radiation manifest themselves as color.
This led the early investigators to methods of measurement, the basic prin
ciples of which still apply today.
The limits given in Fig. 1 are not fixed limits because molecules exhibit
absorption below 200 nm ie. above 50000 cm -I. However, this spectral
region is not accessible to routine measuring techniques. The short-wave-
2 Introduction
A-
m
~ ~ ill ~ ~ ~~
I I I I I I I I I
60 57 '103cm-1 50 40 30 25 20 12.5 10
-v
I_ ---1°11-.------
vacu~m UV Nz-UV UV -------.If.o-~-VIS • liNear IR-
I I
Short-wavelength Long-wavelength
, limit limit ,
Monochromators Photomultiplier
Solvent Empirical; definition
of the VIS-region
02-absorption
Extension of Extension of
range:-+ - 57 x 103cm-1 range -+ near IR
CaF2-prisms Photocells, PbS-celis
{
N2-f1ushing Less problematical than
> 57 x 103cm-1 at short-wavelength limit
Grating spectrometer
VacuilmUV
Fig. 1. The ranges of electronic spectra and their limits
length limit is restricted by apparatus and by experimental techniques. The
long-wavelength limit depends less on considerations of apparatus because,
apart from a few exceptions, most compounds exhibit no absorption
traceable to electronic excitation in this region. There are exceptions; for ex
ample polymethine dyes, used as photographic sensitizers, and some in
organic complexes which have absorption bands that can be observed up to
2~m~5xl05m-l (5000cm-1).