Ultraviolet spectroscopy absorption bands

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

The most powerful method for stmcture elucidation of steroid compounds during the classical period of steroid chemistry (- 1940 1950s) was ir-spectroscopy. As with the ultraviolet spectra, data collected on the infrared spectra of steroids are available in several books, spectmm atiases, and review articles (265,266). Unlike ultraviolet spectroscopy, even the least substituted steroid derivatives are relatively rich in characteristic absorption bands in infrared spectroscopy (264). 

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. 

In ultraviolet spectroscopy the absorption bands appeared as maxima but in infrared spectroscopy the wave length is plotted against transmittance and hence the peaks appear as dips. In ultraviolet the absorbance is plotted against wave length while in the infrared transmittance is plotted wave length. 

We have found a direct calorimetric determination of both K and AH° to be capable of producing much more reliable thermodynamic data than infrared or ultraviolet-visible spectroscopy. Furthermore, things that react extensively have enthalpies but may not have convenient changes in absorption bands. The procedure for the simultaneous determination of K and AH° from calorimetric measurements has been described (20—22). 

A great deal of work has been done on the photodissociation dynamics of the halogen containing compounds. This is partially due to the production of halogen atoms and radicals that can be studied by TOF and partially due to the fact there are intense absorption bands in the ultraviolet and near ultraviolet region. These reasons, coupled with the fact that the spectroscopy of some precursor molecules is fairly well understood, have resulted in many workers looking at these compounds. Finally, some of these compounds have been studied as possible candidates for various types of lasers. 

For oxirane rings an IR absorption around 890 cm-1 is characteristic. This is also observed in the case of K-region epoxides and can be used for diagnostic purposes, but it is not sensitive enough to provide detailed structural information. The oxepins ordinarily do not show this band. Ultraviolet spectroscopy has been invaluable in studying the dynamic equilibrium between the arene oxides and oxepins. The solvent variation of UV spectra has also been exploited very effectively.8... 

The spectra (absorption or emission) of atoms are much sharper than those of molecules, because every electronic energy level in a molecule has a rich complement of vibronic levels and rotational sublevels (Fig. 3.15). In the late nineteenth century these smaller features could not be resolved in visible-ultraviolet spectroscopy, so, in ignorance of all the quantum effects explained decades later, the sharper spectra of atoms were called "line spectra," while the broadened spectra of molecules were called "band spectra." Cooling the molecules to 77 K or 4.2 K does resolve some of the vibronic substructure, even in visible-ultraviolet absorption spectroscopy. 

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. 

McGrath and Morrow " studied the reactions of both 0( D) and 0( P) with cyanogen at room temperature by flash photolysis. The 0( D) was produced by the photolysis of ozone. The reaction was monitored by absorption spectroscopy. At first, they attributed the previously unidentified ultraviolet absorption bands at 3250-3330 A to the fulminate radical (CNO), concluding that the initial step of the reaction was... 

U nlike ir spectroscopy, ultraviolet spectroscopy lends itself to precise quantitative analysis of substances. The intensity of an absorption band is usually given by the molar extinction coefficient e, which, according to the Beer-Lambert Law, is equal to the absorbance A, divided by the product of the molar concentration c, and the path length /, in centimeters. 

Using ultraviolet/visible (UV/Vis) absorption spectroscopy, it is possible to measure the protein concentration using Beer s Law A = e c, where A is the measured absorbance of a solution, e is the absorptivity of the protein, is the pathlength of the cell used to determine the absorbance, and c is the protein concentration. Proteins typically exhibit two strong, broad absorption bands in the UV/Vis part of the spectrum. The first and most intense band is centered at 214 nm and arises from absorption of light by the peptide backbone. The second absorption band is typically found at 280nm. This band arises from absorbance from the aromatic side chains of Trp, Tyr, and Phe. Disulfide bonds may exhibit weak absorption in this range as well. 

Ultraviolet spectrophotometry is considered a valuable tool as an aid for confirming the identification of pesticide residues. A correlation between the UV spectrum and the structure of several pesticides is discussed. Knowledge of such correlation may provide clues about the general type of chromophore present and may help the analyst to design analytical procedures. The transparency of many groups in the near UV imposes a limitation on interpretations of the absorption bands in this region. However, when taken in conjunction with the information obtained by IR, NMR, and mass spectroscopy, UV spectra may lead to structural proposals of value to the pesticide analyst. A discussion of the methods that have been utilized for the analysis of pesticides on the submicrogram level is also presented. 

Flavonoids are a well-defined group of compounds with established physical and chemical characteristics. This especially counts for their absorption of ultraviolet (UV) radiation, which makes their UV spectra very characteristic and UV spectroscopy a method of choice for their characterization. Two main absorption bands are observed 1) band 1 (300-380 nm) due to absorption of ring B and 2) band 11 (240-280 nm) due to absorption of ring A. The position of these bands gives information about the kind of the flavonoid and its substimtion pattern thus UV spectroscopy is used as a main method for the identification and the quantification of flavonoids for decades. 

---