Ultraviolet molar absorptivities

Byrne, R.H., Young, R.W. and Miller, W.L. (1981) Lead chloride complexation using ultraviolet molar absorptivity characteristics./. Sol. Chem. 10, 243-251. 

The kinetic determination of any concentration as a function of time yields k, as in Eqs. (3-61) and (3-63). This is true no matter whether one follows [A] or [P],. The latter point, although correct, can sometimes seem illogical. Suppose one measures the buildup of P2 (say), by monitoring an infrared peak, an ultraviolet band, or an NMR signal. Assume that neither A nor any product other than P2 contributes to the signal. Surely then, will it not be k2 that is obtained from the kinetic analysis The answer is no. Consider the result from Eq. (3-63), which gives the concentration of the one product in terms of its absorbance (per unit optical path) and molar absorptivity (62) ... 

Generally the first thing to be done in preparation for the photochemical study of a compound is to determine the visible and ultraviolet absorption spectrum of the compound. Besides furnishing information concerning the nature of the excited state potentially involved in the photochemistry (see Section 1.4), the absorption spectrum furnishes information of a more applied nature as to the wavelength range in which the material absorbs and its molar absorptivity e. From this information it is possible to decide what type of light source to use for the irradiation, what solvents can be used to... 

A second factor (which could potentially affect ultraviolet initiators as well) is the attenuation of light through the sample. Depending on the thickness of the sample, the molar absorptivity of the initiator (e), and the concentration of the initiator ([A]), the differences between conversion at the surface and in the bulk of the sample can be appreciably different. These differences are the result of an exponential decay in the light intensity as a function of depth in the sample. 

Ultraviolet spectroscopy does not lend itself to leucovorin analysis for two reasons. First, because commercial samples are frequently contaminated with uv-absorbing impurities, a reliable molar absorptivity has not been determined for leucovorin. Recently, in this laboratory a value of 3.09 x 10 M-1 cm-1 was derived from thorough analysis of a relatively pure sample. This value is in reasonable agreement with that of Zakrzewski and Sanson.21... 

Aromatic compounds have very high molar absorptivities that usually lie in the vacuum ultraviolet region and are not useful for routine analysis. Modest absorption peaks are found between 200 and 300 nm. Substituted benxene compounds show dramatic effects from electron-withdrawing substituents. These substituents are known as auxo-chromes since they do not absorb electromagnetic radiation but they have a significant effect on the main chromophore. For example, phenol and aniline have molar absorptivities that are six times the molar absorptivity of benzene or toluene at similar wavelengths. 

It is often difficult to quantitate one particular amino acid in the presence of others because of chemical similarities. Interference from substances other than amino acids is also a problem in many reputedly specific methods. Ultraviolet spectroscopy is of little value in the detection of aromatic amino acids because they have similar absorbance maxima and considerably different molar absorption coefficients. 

For ultraviolet and visible spectroscopic detectors, a standard solution of a compound whose molar absorption constant is known must be prepared, and placed in the flow cell. The absorbance obtained is then compared with the value measured by a standard spectrophotometer. 

The molar absorptivities for the two betaines and the three sulfobetaines in aqueous solution are listed in Table I. Before being used for surface tension measurements, aqueous solution of surfactants were further purified by repeated passage (12) through minicolumns (SEP-PAK Cjs Cartridge, Waters Assoc., Milford Mass.) of octadecylsilanized silica gel. The concentration of surfactant in the effluent from these columns was determined by ultraviolet absorbance, using the molar absorptivities listed in Table I. 

The useful spectral range is usually found between 700 and 200 nm. (The region below 200 nm is the vacuum ultraviolet, which requires special instrumentation and, for this reason, is less important.) The spectrum is the result of measuring the absorption of light versus wavelength. The position of the absorption maximum (wavelength = peak position), is important, as is related to the amount of radiation absorbed (molar absorptivity, e). 

The ultraviolet absorption spectra of benzoic in 96 % ethanol (concentration = 8.3 ppm) and in 0.01 N HCl (concentration = 8.2 ppm) were recorded on a Shimadzu UV-265 spectrophotometer, and are shown in Figure 4. The spectra were not found to be greatly affected by the nature of the solvent used. An absorbance maximum of 227 nm was noted for the 96 % ethanolic solution, while a maximum at 229 nm was found in the 0.01 N HCl solvent. The absorbencies within these two solutions were found to be 0.815 and 0.768, respectively, so that the respective molar absorptivities are computed to be 12000 and 11400 liter/cm mole. 

The ultraviolet spectrum of brinzolamide in reagent alcohol exhibited a maximum at 252 nm, with a molar absorptivity of 9,400 liters/mole. The spectrum is shown in Figure 5. 

The ultraviolet absorption spectrum of diloxanide furoate was recorded on a Shimadzu model 1601 PC LJV/VIS spectrophotometer, and is shown in Figure 3. In aqueous solution, the spectrum exhibited a single absorption maximum located at 260 nm. For this band, the A o/ .ic , value was 700, and the molar absorptivity equal to 22970. 

The ultraviolet absorption spectrum of isoxsuprine HCl dissolved in 0.1 N HCl is shown in Figure 3, and was recorded using a Shimadzu UV-VIS model 1601 PC spectrophotometer. The transition characteristic of a hydroxy-substituted phenyl group was found to split into two maxima, with the first being noted at 274.7 nm (Al% 1cm = 66, and molar absorptivity 2230 L/mole cm), and the other at 268.9 nm (Al% 1 cm = 67, and molar absorptivity 2265 L/mole cm). The far more intense singlet->singlet transitions of the phenyl systems are evident in the rapidly rising absorption noted below 240 nm. 

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... 

Scott and coworkers52 have investigated some of the parameters of the dehydration reaction in concentrated sulfuric acid solution, in order to use it as an analytical method. The concentration of the sulfuric acid was found to be the most important variable in the reaction it influences the position and the intensity of the ultraviolet-absorption maximum, the stabilities of the reaction products, and, for certain pentoses and hexuronic acids, the course of the reaction. In general, an increase in the concentration of the sulfuric acid causes a shift of the 2-furaldehyde absorption to higher wavelength, and an increase in the molar absorptivity. 

Peptides that contain 2-aminoacrylic acid and 2-aminocrotonic acid groups absorb in the ultraviolet region, the former at a wavelength maximum of 240 nm, and the latter with a more generalized absorption. At 240 nm, the molar absorptivity is the same (4,200) for the two... 

Ultraviolet absorption spectra of tryptophan (Trp), tyrosine (Tyr), and phenylalanine (Phe) at pH 6. The molar absorptivity is reflected in the extinction coefficient, with the concentration of the absorbing species expressed in moles per liter. (Source From D. B. Wetlaufer, Adv. Protein Chem. 17 303-390, 1962.)... 

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

The ultraviolet spectrum of benazepril hydrochloride (obtained at a concentration of 0.1 mg/mL in water), which is shown in Figure 3, was recorded using a Shimadzu model 1601 PC ultraviolet-visible spectrometer. The drug exhibited only one maximum at 238.5 nm, A[l%, 1 cm] = 198.7, and molar absorptivity = 9160. 

Molar Absorptivity and AValues from the Ultraviolet Absorption Spectrum of Dipyridamole in Different Media... 

Solution absorption spectra of Bk(III) and Bk(IV) are shown in Figs. 2 and 3, respectively. The spectrum of Bk(III) is characterized by sharp absorption bands of low molar absorptivity attributed to Laporte-for-bidden f-f transitions and by intense absorption bands in the ultraviolet region, which are attributed to f-d transitions (96). The spectrum of Bk(IV) is dominated by a strong absorption band at 250-290 nm, the peak position of which is strongly dependent on the degree of complex-ation of Bk(IV) by the solvent medium. This band is attributed to a charge-transfer mechanism (96). 

The term c (in 1 mol- cm-1) is known as the molar absorptivity. The intensity of an absorption band in the ultraviolet spectrum is usually expressed as molar absorptivity at the maximum, emax or log emax. However, the molecular... 

Yellow anhydrous disodium hexachloroplatinate(IV) is quite hygroscopic and readily gains six water molecules of hydration when stored at 25° and a relative humidity of 50% or greater. Orange disodium hexachloroplatinate(IV) hexahydrate, Na2-PtCl6-6H20, is reconverted to the anhydrous compound by heating at 110° for one hour. The ultraviolet spectrum of hexa-chloroplatinate(IV) is so distinctive that it serves as a valuable criterion for the sole presence of this ion in solution as well as for the purity of disodium hexachloroplatinate(IV). In 1 F hydrochloric acid medium the spectrum exhibits a maximum at 262 nm. and a minimum at 232 nm. with molar absorptivities of 24,500 and 3080 l./mole-cm., respectively 4 the molar absorptivity is 5090 l./mole-cm. at 300 nm. 

The a,ft unsaturated carbonyls exhibit an ultraviolet absorbance with an intense maximum in the range 220 nm and a weak one at 280 nm, depending on the polarity of their solvent (hexane 215 nm, methanol 221 nm, water 224 nm) (Fig. 5.5). This characteristic has been extensively exploited for the characterization of 4-hydroxynonenal produced during the peroxidation of hepatic tissues. Table 5.1 lists the molar absorption co-efficients for free hydroxyalkenals (average e= 10000-14000 M- cm-1 (Esterbauer and Weger, 1967). 

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