Ultraviolet spectroscopy spectra

Figure 8.6 The He I ultraviolet photoelectron spectrum of argon. (Reproduced from Turner, D. W., Baker, C., Baker, A. D. and Brundle, C. R., Molecular Photoelectron Spectroscopy, p. 41, John Wiley, London, 1970)...

These structural problems are also insoluble by physical methods alone. The infrared spectrum often gives an unambiguous decision about the structure in the solid state the characteristic bands of the carbonyl or the hydroxyl group decided whether the compound in question is a carbinolamine or an amino-aldehyde. However, tautomeric equilibria occur only in solution or in the liquid or gaseous states. Neither infrared nor ultraviolet spectroscopy are sufficiently sensitive to investigate equilibria in which the concentration of one of the isomers is very small but is still not negligible with respect to the chemical reaction. 

Many of the properties oj -hydroxypyridines are typical of phenols. It was long assumed that they existed exclusively in the hydroxy form, and early physical measurements seemed to confirm this. For example, the ultraviolet spectrum of a methanolic solution of 3-hydroxypyridine is very similar to that of the 3-methoxy analog, and the value of the dipole moment of 3-hydroxypyridine obtained in dioxane indicates little, if any, zwitterion formation. However, it has now become clear that the hydroxy form is greatly predominant only in solvents of low dielectric constant. Comparison of the pK values of 3-hydroxypyridine with those of the alternative methylated forms indicated that the two tautomeric forms are of comparable stability in aqueous solution (Table II), and this was confirmed using ultraviolet spectroscopy. The ratios calculated from the ultraviolet spectral data are in good agreement with those de-... 

The most convenient and sensitive method for detecting the presence of an isoindole is by means of ultraviolet spectroscopy. The isoindole chromophore shows a characteristic series of bands in the range 220-390 m/x, with the band at longest wavelength usually well separated from the remainder of the spectrum and of high intensity... 

Aromatic rings are detectable by ultraviolet spectroscopy because they contain a conjugated rr electron system. In general, aromatic compounds show a series of bands, with a fairly intense absorption near 205 nm and a less intense absorption in the 255 to 275 nm range. The presence of these bands in the ultraviolet spectrum of a molecule is a sure indication of an aromatic ring. 

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since turns to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the turns isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. 

The identification of chemical substances by examination of their spectra (singular, spectrum). Both infrared and ultraviolet spectroscopy are used in the study of rubber problems such as identifying the type of polymer or the nature of a contaminant. 

The effects of d-d transitions can be studied using spectroscopy. If the absorbed energy is in the visible part of the electromagnetic spectrum, giving a coloured compound, visible spectroscopy is used. If the absorbed energy is in the ultraviolet part of the electromagnetic spectrum, the compound will be colourless and ultraviolet spectroscopy is used. 

Molecular absorption spectroscopy deals with measurement of the ultraviolet-visible spectrum of electromagnetic radiation transmitted or reflected by a sample as a function of the wavelength. Ordinarily, the intensity of the energy transmitted is compared to that transmitted by some other system that serves as a standard. 

In spite of many studies the complete analysis of the ultraviolet absorption spectrum has not been successful [Herzberg (16), p. 511]. The spectroscopy and photochemistry of S02 may be conveniently discussed for four... 

After the reaction product at the Rf value of 0.40 was eluted with water from the chromatogram, its spectrum in neutral solvent and its ionization Ac curve were recorded. The neutral spectrum and the ionization Ac curve of bis-4-hydroxy-3,5-dimethoxyphenylmethane in 47.5% ethanol were also recorded the wavelengths of the ultraviolet absorption maxima of the two compounds—eluted product and reference compound—are given in Table II. The spectra of the compounds possess maxima at nearly identical wavelengths. Thus, paper chromatography and ultraviolet spectroscopy indicate positively that bis-4-hydroxy-3,5-dimethoxyphenylmethane is a product of the reaction of syringyl alcohol with alkali in aqueous solution. 

Reusch, W. (1999). Virtual Textbook of Organic Chemistry Visible and Ultraviolet Spectroscopy. http //www.cem.msu.edU/ reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm uvl. Accessed April 22, 2009. 

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

Infrared spectroscopy is an important technique for studying acidity. Acidic OH groups can be studied directly. Probe molecules such as pyridine may be used to study both Bronsted and Lewis acidity since two forms of adsorbed probes are easily distinguished by their infrared spectra. Quantitative infrared spectroscopy may be performed by measuring the spectrum of acidic OH or probes adsorbed on thin, self-supporting wafers of the acidic solid. Other spectroscopic methods which may provide information in specific cases include Fourier Transform Raman spectroscopy, electron spin resonance spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. 

The hydroxyl radical, OH, occupies an extremely important position in spectroscopy, in free radical laboratory chemistry, and in atmospheric, cometary and interstellar chemistry. Its ultraviolet electronic spectrum has been described in many papers published over the past seventy years. It was the first short lived gaseous free radical to be studied by microwave spectroscopy, described in a classic paper by Dousmanis, Sanders and Townes [121] in 1955. The details of this work are presented in chapter 10. It was the first free radical to be studied by microwave magnetic resonance, in pioneering work by Radford [141] the microwave and far-infrared laser magnetic resonance studies are... 

Ultraviolet Spectroscopy The ultraviolet spectra of aromatic compounds are quite different from those of nonaromatic polyenes. For example, benzene has three absorptions in the ultraviolet region an intense band at Amax = 184 nm (e = 68,000), a moderate band at Amax = 204 nm (e = 8800), and a characteristic low-intensity band of multiple absorptions centered around 254 nm (e = 200 to 300). In the UV spectrum of benzene in Figure 16-19, the absorption at 184 nm does not appear because wavelengths shorter than 200 nm are not accessible by standard UV-visible spectrometers. 

Second derivative ultraviolet spectroscopy is a simple yet powerful technique for highlighting the fine structure of spectral curves. It involves calculating the second derivative of a spectrum with respect to wavelength and plotting the derivative rather than the spectrum itself. Second derivative absorbance spectra of lignin model compounds, milled wood lignins and lignosulfonates have been published (Lin 1982). 

CO has been detected in the interstellar absorption spectrum of f Ophiuchi and has thus become the second interstellar molecule to be detected by rocket ultraviolet spectroscopy. Smith and Stecher (1971) have detected eight transitions in the fourth positive system of 12C160 and four of 13C160, which yielded a 12C/13C ratio of 105. It seems likely that interstellar molecular detections in the vacuum ultraviolet will follow, especially of polyatomic molecules like H20. 

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