Ultraviolet spectroscopy absorption spectrum

The preceding empirical measures have taken chemical reactions as model processes. Now we consider a different class of model process, namely, a transition from one energy level to another within a molecule. The various forms of spectroscopy allow us to observe these transitions thus, electronic transitions give rise to ultraviolet—visible absorption spectra and fluorescence spectra. Because of solute-solvent interactions, the electronic energy levels of a solute are influenced by the solvent in which it is dissolved therefore, the absorption and fluorescence spectra contain information about the solute-solvent interactions. A change in electronic absorption spectrum caused by a change in the solvent is called solvatochromism. 

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. 

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... 

Spectroscopy and photochemistry of CF3NO in the visible region have been extensively studied by Roellig and Houston (113,114), Bower et al. (115), Jones et al. (116), and Spears and Hoffland (117). Trifluoronitrosomethane has a weak structured absorption spectrum in the visible near 700 nm. Absorption of light in this region induced fluorescence as well as dissociation, which is similar to the H2CO photochemistry in the ultraviolet described earlier. 

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. 

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. 

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. 

The FT direct absorption spectra [28] of OCIO provide an example of the capabilities of FT spectroscopy in the visible and ultraviolet regions for the study of short-lived species. In Figure 14, part of the near-UV absorption spectrum is... 

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. 

Solvents used in ultraviolet, visible, infrared, microwave, and radiowave spectroscopy must meet the following requirements transparency and stability toward the radiation used, solubility and chemical stability of the substance to be examined, and a high and reproducible purity ( optical constancy ). Normally, intermolecular interaction with the solute should be minimal. On the other hand, important information about the solute can be obtained from the changes in the absorption spectrum arising from such interactions. 

Partition Coefficient. Partition coefficients of the TFMS herbicides in both the presence and absence of surfactant were determined between 1-octanol and pH 1.0 water (made acid by addition of HC104) by ultraviolet spectroscopy. The absorption spectrum of Tween 80 did not interfere with the spectra of the sulfonanilides (6). 

One of the problems that has received some attention concerns the nature of the electronic state of the nitrene produced on photolysis. Reiser and co-workers approached this question by comparing the photolysis of 2-azidobiphenyl (356) in a solid matrix and in solution. It was found that photolysis gave the nitrene (357) as identified by its electronic absorption spectrum, and that the reaction could be followed by ultraviolet spectroscopy, monitoring azide disappearance, nitrene appearance and disappearance, and finally, the... 

If the solvent is to be used for ultraviolet spectroscopy, it is necessary to remove all the aromatic compounds. This may be done by shaking the hydrocarbon with a mi.xture of concentrated sulfuric and nitric acids, which will nitrate the aromatic compounds. The hydrocarbon is separated, washed with water, distilled, and passed through a column of activated alumina which will remove any residual unsaturated or nonhydrocarbon materials. The spectrum of the solvent is monitored as it passes from the column, and when significant absorption at 210 m/i is observed, the alumina is replaced. 

A major difference between infrared and ultraviolet spectroscopy is in the concentrations required for assay In infrared spectroscopy as much as a 10% w/v solution of sample must be prepared. This means that the path length of the cells used in infrared must be very short, usually 0.025-0.1 mm (otherwise absorbance values would be too high). Another problem with infrared spectra is that the solvent used in the assay (usually chloroform or dichloromethane) also possesses chemical bonds that will absorb infrared radiation in some part of the spectrum, obscuring the absorption by the sample at these wavelengths. Samples are prepared in solution, in a mull or paste made with liquid paraffin (Nujol), or in a solid disc prepared by trituration with dry potassium bromide followed by compression in a hydraulic press. 

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