Infrared Spectroscopy absorptivity, molar

Infrared spectroscopy can be used to measure the number of functional groups in a molecule, e.g. the number of —OH or —NH2 groups. It has been found that the molar absorptivity of the bands corresponding to a particular group is proportional to the number of groups that are present, i.e. each group has its own intensity which does not vary drastically from molecule to molecule. 

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... 

Near-Infrared Spectroscopy NIR spectroscopy uses the NIR region of the electromagnetic spectrum (from about 800 to 2500nm). The molar absorptivity in the NIR region is typically quite small however, NIR can typically penetrate much further into a sample than MIR radiation. So, in spite of the lack of sensitivity, NIR spectroscopy can be very useful in probing bulk material with little or no sample preparation. 

Infrared spectroscopy is one method used to identify polymers, as discussed in Section 1.9.4. The degree of branching of polymers can also be determined if the absorption bands of the branch groups can be identified. Similarly in copolymers, the relative composition can be obtained if the different types of repeat unit have distinct vibrational modes and thus absorption bands. To make this a quantitative measure of fractional content, the absorbance in each band is measured via the Beer-Lambert law A = eel, where s is the molar absorptivity, c is the concentration of a given species and / is the path length. For a copolymer with two different types of repeat unit the ratio of absorbances yields the ratio of concentrations if the molar absorptivities are known, for example having being measured previously for samples of known composition. 

Venyaminov SY, Prendergast FG. Water (HjO and DjO) molar absorptivity in the 1000 000 cm" range and quantitative infrared spectroscopy of aqueous solutions. Anal Biochem 1997 248 234-245. 

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... 

Amerov AK, Chen J, Arnold MA. Molar absorptivities of glucose, water and other biological molecules over the first overtone and combination regions of the near infrared spectrum. Applied Spectroscopy 2004, 58, 1195-1204. 

As can be seen in Fig. 5, N conversion using H-ZSM-11 zeolite seems to be correlated with the number of Bronsted sites on the external surface (deduced from measurements of methylene blue adsorption capacity) and not with the total niunber of Bronsted sites (determined by the total pyridine adsorbed on Bronsted sites and desorbed at 150°C by FT-IR spectroscopy), using the literature data on the integrated molar extinction coefficients [17], (for infrared absorption bands of pyridine adsorbed on solids acid catalyst [17], providing no dependence of the integrated coefficients on the catalyst or strength of the sites). 

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). 

Temperatnre- and concentration-dependent variations in self-association of 1-octanol have also been studied by two-dimensional (2-D) Fourier transform near-infrared correlation spectroscopy." The population of the free OH groups increases with temperature, reportedly reaching 13% at 80°C. The molar absorptivities of the first and second overtones of the monomer were found to be similar in several non-hydrogen-bonding solvents such as carbon tetrachloride, heptane, and octane. The first overtone s absorptivity was about 1.7 1/mol-cm, which agrees with Goddu s data. The absorptivity of the second overtone is about one twentieth that of the first. 

Spectroelectrochemistry encompasses a group of techniques that allow simultaneous acquisition of electrochemical and spectroscopic information in situ in an electrochemical cell. A wide range of spectroscopic techniques may be combined with electrochemistry, including electronic (UV-visible) absorption and reflectance spectroscopy, luminescence spectroscopy, infrared and Raman spectroscopies, electron spin resonance spectroscopy and ellipsometry. Molecular properties such as molar absorption coefficients, vibrational absorption frequencies and electronic or magnetic resonance frequencies, in addition to electrical parameters such as current, voltage or charge, are now being used routinely for the study of electron transfer reaction pathways and the fundamental molecular states at interfaces. In this article the principles and practice of electronic spectroelectrochemistry are introduced. 

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