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 This work is subject to copyright. All rights are reserved, whether the whole or part of the ma terial is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recita tion, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. © Springer-Verlag Berlin Heidelberg 1992 Sotlcover reprint of the hardcover 1st edition 1992 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. lYPesetting: K + V Fotosatz GmbH, Beerfelden 52/3145-5 4 3 2 1 0 - Printed on acid-free paper 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).