UV and VIS Spectroscopy

At the dawn of the nineteenth century, Ritter announced the discovery of light beyond the violet end of the VIS spectrum. This higher-energy radiation (shorter wavelength) was called UV. 

This corresponds to energy greater than about 25,000cm or, since one wave number is equivalent to 2.86calmoU, about 71.5kcalmol . 

As transitions become more facile, the wavelengths at which absorbance occurs become longer and spectra can be taken in the VIS region. For example, in aqueous solution, both chromate (Cr04 ), which is yellow, and dichromate (Cr20y ), which is orange, have maxima (Jw) below 400 nm but, both tail into the VIS, absorbing 

This particular portion (i.e., where X ca,200nm) is often called the vacuum UV. Special experimental techniques are necessary, including evacuation of the sample chamber, because the common atmospheric gases (i.e., O2, N2, and H2O) have nonbonded electrons and thus their n transitions occur and those of the sample might be obscured. 

TABLE 2.2. An Approximate Relationship between Visible Color Absorbed and the Color Seen as a Consequence 

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Ultraviolet (UV) and Vis spectroscopy has not found appreciable application to direct measurements of soil. They have, however, found considerable use in identifying and measuring components extracted from soil (see Section 8.3). A more detailed explanation of spectroscopic methods applied to soil is given in Chapter 14. 

UV and VIS spectroscopy provide a handy and efficient tool for the generation of semi-empiriail scales of solute-solute and solute-solvent interaction parameters which are currently used for correlations. Table VII gives a survey. 

Mass selective laser excitation was applied originally for the UV and VIS spectroscopy of neutral molecules. However, the benefits of both mass selected UV spectra and UV-resonance selective mass spectra were soon recognized. 

The use of ultraviolet (UV) spectroscopy for on-line analysis is a relatively recent development. Previously, on-line analysis in the UV-visible (UV-vis) region of the electromagnetic spectrum was limited to visible light applications such as color measurement, or chemical concentration measurements made with filter photometers. Three advances of the past two decades have propelled UV spectroscopy into the realm of on-line measurement and opened up a variety of new applications for both on-line UV and visible spectroscopy. These advances are high-quality UV-grade optical fiber, sensitive and affordable array detectors, and chemometrics. 

To identify the phosphorus-containing compounds described in the previous sections and the related species containing aluminum, molybdenum, cobalt, or nickel which might be present in hydrotreating catalysts, it is convenient to use techniques such as NMR, IR, UV. and Raman spectroscopies and XRD. XRD is useful for characterizing crystalline bulk compounds, and other techniques are appropriate for well-dispersed species and amorphous phases. Typical IR, Raman, and NMR data presented in Tables VI, VII, and VIII, respectively, could be the basis for such identifications. 

The fundamental equations according to Lambert-Beer s law have been given in Sections 1.4.3 and 3.3.2. The general problem in using spectroscopy data in UV and Vis is the poor selectivity in this wavelength range. In principle, a multicomponent analysis has to be done to extract concentrations of the different components. 

The ESR spectra of PIB irradiated at low temperature consists of a broad doublet with a hyperfine coupling constant of 2 mT, attributed to either radical VIII (182) or a combination of contributions from radicals VII and VIII (183, 184). The primary radicals V and VI (Fig. 2) have not been observed by ESR spectroscopy. Despite extensive studies of the radiochemical changes in PIB by ESR, volatile product analysis, and UV and IR spectroscopy, the mechanism of degradation had been imcertain imtil the advent of high resolution NMR methods. In a comprehensive study, Bremner and co-workers (185) and Hill and... 

Circular dichroism (CD) is a chiroptical spectroscopy that measures the differential absorption of left versus right circularly polarized light. Its higher sensitivity to molecular conformation and configuration has made CD spectroscopy a more powerful tool in the structural analysis of various chiral supramolecular systems than its parent achiral absorption spectroscopies such as ultraviolet (UV), visible (vis), and infrared (IR) spectra (Figure 1). CD measurements in the UV and vis regions are the most widely... 

Raman scatter, and excitation emission fluorescence spectroscopy (EEFS). They use interaction with radiation from different regions of the electromagnetic spectrum to identify the chemical nature of molecules. For example, absorption of UV and VIS radiation causes valence electron transitions in molecules which can be used to measure species down to parts per million concentrations for fluorophores (i.e., EEFS) determination can even go down to parts per billion levels. Whereas UV, VIS, and EEFS are limited to a smaller, select group of molecules, the NIR, IR, and Raman scatter spectroscopy techniques are probing molecular vibrations present in almost any species their quantification limits are somewhat higher but can still be impressive. The reader is referred to textbooks for further details on basic principles of these spectroscopic techniques [3]. 

Some surfactants have a built-in chromophore and are hence easily characterized with respect to concentration, C, through UV/visible (vis) spectroscopy. Here, we make use of the Lambert-Beer law relating the intensity of the transmitted light, 7, to the concentration of the chromophore, as follows ... 

The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

Fortunately, azachalcone derivatives (2.4a-g, Scheme 2.4) turned out to be extremely suitable dienophiles for Lewis-add catalysed Diels-Alder reactions with cyclopentadiene (2.5). This reaction is outlined in Scheme 2.4 and a large part of this thesis will be devoted to the mechanistic details of this process. The presence of a chromophore in 2.4 allows kinetic studies as well as complexation studies by means of UV-vis spectroscopy. Furthermore, the reactivity of 2.4 is such that also the... 

In the kinetic runs always a large excess of catalyst was used. Under these conditions IQ does not influence the apparent rate of the Diels-Alder reaction. Kinetic studies by UV-vis spectroscopy require a low concentration of the dienophile( 10" M). The use of only a catalytic amount of Lewis-acid will seriously hamper complexation of the dienophile because of the very low concentrations of both reaction partners under these conditions. The contributions of and to the observed apparent rate constant have been determined by measuring k pp and Ka separately. ... 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

As diverse as these techniques are all of them are based on the absorption of energy by a molecule and all measure how a molecule responds to that absorption In describing these techniques our emphasis will be on then application to structure determination We 11 start with a brief discussion of electromagnetic radiation which is the source of the energy that a molecule absorbs m NMR IR and UV VIS spectroscopy... 

With this as background we will now discuss spectioscopic techniques mdividu ally NMR IR and UV VIS spectroscopy provide complementaiy mfoimation and all are useful Among them NMR provides the mfoimation that is most duectly related to moleculai stiuctuie and is the one we 11 examine hist... 

The structural unit associated with an electronic transition m UV VIS spectroscopy IS called a chromophore Chemists often refer to model compounds to help interpret UV VIS spectra An appropriate model is a simple compound of known structure that mcor porates the chromophore suspected of being present m the sample Because remote sub stituents do not affect Xmax of the chromophore a strong similarity between the spectrum of the model compound and that of the unknown can serve to identify the kind of rr electron system present m the sample There is a substantial body of data concerning the UV VIS spectra of a great many chromophores as well as empirical correlations of sub stituent effects on k Such data are helpful when using UV VIS spectroscopy as a tool for structure determination... 

Materials characterization techniques, ie, atomic and molecular identification and analysis, ate discussed ia articles the tides of which, for the most part, are descriptive of the analytical method. For example, both iaftared (it) and near iaftared analysis (nira) are described ia Infrared and raman SPECTROSCOPY. Nucleai magaetic resoaance (nmr) and electron spia resonance (esr) are discussed ia Magnetic spin resonance. Ultraviolet (uv) and visible (vis), absorption and emission, as well as Raman spectroscopy, circular dichroism (cd), etc are discussed ia Spectroscopy (see also Chemiluminescence Electho-analytical techniques It unoassay Mass specthot thy Microscopy Microwave technology Plasma technology and X-ray technology). 

Instrumentation. The k region was developed usiag dispersive techniques adapted as appropriate from uv—vis spectroscopy. Unfortunately, k sources and detectors tend to be kiefficient compared to those for other spectral regions. 

Ketenes absorb near 2100-2130cm . When the photolysis was carried out and the IR spectrum of the solution monitored, it was found that a band appeared at 2118 cm , grew, and then decreased as photolysis proceeded. The observation of this characteristic absorption constitutes good evidence for a ketene intermediate. As with UV-VIS spectroscopy, the amount of intermediate that can be detected depends both on the intensity of the absorption band and the presence of interfering bands. In general, IR spectroscopy requires somewhat higher concentration for detection than does UV-VIS spectroscopy. 

With this as background, we will now discuss spectroscopic techniques individually. NMR, IR, and UV-VIS spectroscopy provide complementary information, and all are useful. Among them, NMR provides the information that is most directly related to molecular- structure and is the one we ll examine first. 

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

Further structural information is available from physical methods of surface analysis such as scanning electron microscopy (SEM), X-ray photoelectron or Auger electron spectroscopy (XPS), or secondary-ion mass spectrometry (SIMS), and transmission or reflectance IR and UV/VIS spectroscopy. The application of both electroanalytical and surface spectroscopic methods has been thoroughly reviewed and appropriate methods are given in most of the references of this chapter. 

The red tetrasulfide radical anion 84 has been proposed as a constituent of sulfur-doped alkali hahdes, of alkah polysulfide solutions in DMF [84, 86], HMPA [89] and acetone [136] and as a product of the electrochemical reduction of 8s in DM80 or DMF [12]. However, in all these cases no convincing proof for the molecular composition of the species observed by either E8R, Raman, infrared or UV-Vis spectroscopy has been provided. The problem is that the red species is formed only in sulfur-rich solutions where long-chain polysulfide dianions are present also and these are of orange to red color, too (for a description of this dilemma, see [89]). Furthermore, the presence of the orange radical anion 8e (see below) cannot be excluded in such systems. 

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