《材料测试技术及方法》课程教学资源（书籍文献）分子光谱之紫外可见光谱 UV-VIS Spectroscopy and Its Applications，Translated by H. Charlotte Grinter and Dr. T. L. Threlfall.pdf_大学文库

Preface UV-VIS spectroscopy is one of the oldest methods in molecular spectroscopy.The definitive formulation of the Bouguer-La 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- mln me mn of eomlee With the development of quantum chemistry,increasing atten- s paid to the en light abs tion a and the structur the lt that ent de t dis ssions of the theory of des a numbe (UV-VIS and c spectr spectroscopy)have been publishe Consequently,this extremely interesting as ect o mo r 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 spectro opy owes its importance not least to its varied in This book to show W UV-VIS 0P2 be applied an to the kine tics of chemical reac ctions,incluc ng phot theoretical section has been kept to a minimum since,as men tioned above,excellent discussions of such matters are ava lable 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 expe ime tal mples ous me ents ha e beer

Preface UV-VIS spectroscopy is one of the oldest methods in molecular spectroscopy. The definitive formulation of the Bouguer-LambertBeer 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 photoelement and photocell, to the photomultiplier and from the photographic plate to the present silicon-diode detector both of which allow simultaneous measurement of the complete spectrum. With the development of quantum chemistry, increasing attention 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 spectroscopy 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 mentioned 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 spectroscopic techniques are discussed. However, in this case the selection has been influenced by the author's own interests. In order to obtain experimental examples, numerous measurements have been

1 Introduction Optical spectroscopy is based on the Bohr-Einstein frequency relationship (1) This relationship links the discrete atomic or molecular energy states Ej 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 spectroscopy, it is appropriate to use the wavenumber v instead of frequency v. Equation (1) then takes the form: LIE = E2 -Ej = hcv where V=CIA=CV (2) Absorbed or emitted radiation of frequency v or wavenumber v can thus be assigned to specific energy differences or, applying the definition of the 'term value' (energy level), to specific energy-level differences: (3) Tj = E/h c is the term value. From this definition it follows that it has dimension m -j in the SI -system. However, it is still commonly given as 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 extreme 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 principles 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-waveH.-H. Perkampus, UV-VIS Spectroscopy and Its Applications © Springer-Verlag Berlin Heidelberg 1992

4 Principles "absorption spectrum"of a compound vary by several orders of magnitude within the absorption spectrum of a single inorganic or organic compound,the logarithmic value lg e=f()can be used instead of e=f()to plot an absorption spectrum [2]. The Bouguer-Lambert-Beer law is a limiting law for dilute solutions,i.e. the assertion that the extinction coefficient e is independent of the concen- tration of a substance at the given wavenumber y (wavelength a)applies on to dilute solutions.e is no longer constant for concentrated solutions but the of the solution i1l.at concentrations up to 102n ive index and lies 1 or 2 powers of ten below the usual photometri accu owed with precise easurements on aqueous solutions of K [Fe(CN[3]. According to Eq.(4),the applicati of the Bouguer-Lambert-Beer law of the relationship between theight int Iand However,when measuring in quartz cuvettesUVV sitic cuvettes made of special optical glass(VIS region),part of the light is los through reflection at the cuvette surfaces.In order to eliminate thi s source of error,a reference measurement is made in a cuvette with the same pathlength but not containing the substance to be measured.Since most UV-VIS spectroscopy is carried out with solutions,the standard cuvette con- tains the pure solvent,which ideally should not absorb in the spectral region under c nsideration. after the light has traversed the standard or the sample has tra versed the cuvette containing ing on the otmiona mode of n as either analog or digital form.This result is independen )or eration of the of T losses due to reflection and the influence of the solvent. This still presupposes that the two cuvettes used for the meas have the same pathlength and have been matched prior to making the mea surements.Most manufacturers keep the accuracy of the pathlengths of a matched cuvette set within a few ppm.However,the continued matching of a previously used pair of cuvettes depends entirely on the care taken by the individual user of an UV-VIS spectrophotometer.In many applications, standard cuvettes are suitable;they are available in pathlengths of 1,2,5, 102050 nd 100 m and.depe nding on the spectral region,are produced ther fron or S artz glass or special optical glass.Fur- thermo r special methods of measure ment [4]. The choice of solvent depends on an adequate solubility y of the substance to be measured.For example,n-heptane water and triflu 0 ethanol r hexafluoroisopropanol may be considered as gooc scopic so bcu th r paret o th UVVI ever,below 200 nm the pathlength must be reduced to 1 mm and in this region the spectrophotometer must be flushed with pure nitrogen in order

4 Principles The functional correlation between eji and wavenumber v is called the "absorption spectrum" of a compound. Since the extinction coefficient can vary by several orders of magnitude within the absorption spectrum of a single inorganic or organic compound, the logarithmic value 19 e = f(v) can be used instead of e = f(v) to plot an absorption spectrum [2]. The Bouguer-Lambert-Beer law is a limiting law for dilute solutions, i.e. the assertion that the extinction coefficient e is independent of the concentration of a substance at the given wavenumber v (wavelength A) applies only to dilute solutions. e is no longer constant for concentrated solutions but depends on the refractive index of the solution [1]. At concentrations up to c ~ 10 - 2 moll-I, the effect is slight and lies 1 or 2 powers of ten below the usual photometric accuracy, as Kortiim showed with precise measurements on aqueous solutions of K3 [Fe(CN)6] [3]. According to Eq. (4), the application of the Bouguer-Lambert-Beer law presupposes a measurement of the relationship between the light intensities I and 10, However, when measuring in quartz cuvettes (UV-VIS region) or cuvettes made of special optical glass (VIS region), part of the light is lost through reflection at the cuvette surfaces. In order to eliminate this source of error, a reference measurement is made in a cuvette with the same pathlength but not containing the substance to be measured. Since most UV-VIS spectroscopy is carried out with solutions, the standard cuvette contains the pure solvent, which ideally should not absorb in the spectral region under consideration. Thus, 10 is measured after the light has traversed the standard or reference cuvette and I after the light has traversed the cuvette containing the sample. Depending on the construction and mode of operation of the equipment, the relationship I1Io is shown as a value of T ji (070) or Aji in either analog or digital form. This result is independent of losses due to reflection and the influence of the solvent. This still presupposes that the two cuvettes used for the measurement have the same pathlength and have been matched prior to making the measurements. Most manufacturers keep the accuracy of the pathlengths of a matched cuvette set within a few ppm. However, the continued matching of a previously used pair of cuvettes depends entirely on the care taken by the individual user of an UV-VIS spectrophotometer. In many applications, standard cuvettes are suitable; they are available in pathlengths of 1, 2, 5, 10,20,50 and 100 mm and, depending on the spectral region, are produced either from Suprasil or Spectrosil quartz glass or special optical glass. Furthermore, there is a wide range of cuvettes for special methods of measurement [4]. The choice of solvent depends on an adequate solubility of the substance to be measured. For example, n-heptane, water and trifluoroethanol or hexafluoroisopropanol may be considered as good spectroscopic solvents because they are transparent from ca. 180 nm in the UV-VIS region. However, below 200 nm the pathlength must be reduced to 1 mm and in this region the spectrophotometer must be flushed with pure nitrogen in order