Ultraviolet-visible spectroscopy molar absorptivity

Ultraviolet-Visible Spectroscopy Ultraviolet-visible (UV-VIS) molecular absorption spectrophotometry (often called light absorption spectrophotometry or just UV-visible spectrophotometry) is a technique based on measuring the absorption of near-UV or visible radiation (180-770 nm) by molecules in solution.35,36 Reference standard characterization by UV-VIS spectophotometry includes determining the absorption spectra and the molar extinction coefficient. These two spectral characterizations are used as identifiers of reference standards. 

Abscicic acid, 1027 Absolute configuration, 267—271, 292 Absorption of electromagnetic radiation, 489 in infrared spectroscopy, 518 in nuclear magnetic resonance spectroscopy, 490—493 in ultraviolet-visible spectroscopy, 524—525 Absorptivity. See Molar absorptivity Acetaldehyde, 655 bond angles, 657 enolization of, 706... 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

In connection with visible-ultraviolet spectroscopy, know the meaning of nanometer, electronic transition, Beer s law, molar absorptivity or extinction coefficient. 

The ultraviolet-visible spectra of most compounds are of limited value for qualitative analysis and have been largely superseded by the more definitive infrared and mass spectroscopies. Qualitative analytical use of ultraviolet-visible spectra has largely involved describing compounds in terms of the positions and molar absorptivities of their absorption maxima, occasionally including their absorption minima. Indeed, some organic compounds are still characterized in terms of the number of peaks in the UV-visible spectrum and their absorbance ratios. This is usually the case in phytochemistry and photodiode array chromatography and when the analyst has a limited range of compounds to work with whose spectra are known to differ. In the pharmacopeias, however, absorbance ratios have found use in identity tests, and are referred to as Q-values in the U.S. Pharmacopia (USP). 

Extensions fo Complicated Equilibria Equation 14-10 can he modified to lit many other cases. It can be extended to account for the formation of polynuclear complexes and for multiple equilibria. Ultraviolet and visible absorption spectroscopy is not. however, particularly well suited for multiple equilibria because of its lack of specificity and because each additional equilibrium adds two unknowns, a formation constant term and a molar absorptivity term. 

A The Magnitude of Molar Absorptivitics 367 14B Absorbing Species 367 14C Qualitative Applications of Ultraviolet-Visible Absorption Spectroscopy 372... 

Ultraviolet and visible spectroscopy gives a way of estimating the extent of conjugation in a molecule. Peaks in electronic spectra are usually broad and are reported as (nm). Their relative intensities are given by the molar absorptivity (extinction coefficient) e. 

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