On extinction coefficient

Carotenoids can be converted into mixtures of geometrical isomers under appropriate conditions, the most common being iodine catalyzed photoisomerization. This produces an equilibrium mixture of isomers, in general the all-trans isomers predominates. These isomers in an isomeric mixture cannot be measured separately by simple spectrophotometric determination. The usual method of subsequent measurement would be chromatographic separation, diode-array detection, and spectral analysis. In the absence of any definitive data on extinction coefficients for cfv-isomcrs, they are quantified against the all-trans isomer. Modem procedures involve the direct synthesis of c/.v-carotcnoids. 

Spectrophotometric assessment of chlorophyll content is based on the strong electronic absorption spectra of these pigments. Arnon (1949) developed an early method measuring 80% acetone/20% water plant extracts based on the electronic absorption spectra of chlorophylls a and b. Absorbance of the extract was measured at different wavelengths, and simultaneous equations were constructed based on extinction coefficients for each derivative s unique electronic absorption maxima. Over the... 

The homologous azirine (143) with a one-atom bridge gave quite different results.70 Photolysis led to the 3,5-fused bicyclic dihydropyrrole (144). The isomeric azirine (145) also led to (144), although the initial products included dihydropyrrole (146) which apparently converted to (144) as photolysis continued. Azirines (143) and (145) were shown to not interconvert and die postulated two discrete azomethine ylides were trapped with methyl trifluoroacetate. Formation of dihydropyrrole (144) was explained based on a two-step cycloaddition process involving a common diradical intermediate. The observation of (146) from photolysis of (145) but not (143) can be explained based on extinction coefficient differences. Azirine (145) has a high extinction coefficient as does (146). The initial product (146) can then be optically pumped to (144) with a low extinction coefficient. Azirine (143) also has a low extinction coefficient and any (146) that formed from it would be optically pumped to (144) before observa-... 

Absorption spectra of polysilanes anisotropic absorption for helical conformation, 549 bathochromic shifts, 549 effect of conformation on extinction coefficient, 547, 548/ low-energy feature for al -trans conformations, 547, 548/, 549 Acetylenes... 

Goel, K., and Chang, W. S. C., Effect of asymmetry on extinction coefficient of crossing channel LiNbOj waveguide switches, SPIE J. Integrated Opt. Circuit Eng. V, 835, 118 (1987). 

Absolute concentrations are, however, often difficult to establish due to the lack of information on extinction coefficients and due to different adsorption probabilities under the prevailing conditions. Instructive examples for research along these lines are the observation of Cu-Zn interactions in working methanol synthesis catalysts [28] and of coexistence of metal and ionic Ru sites in alumina-supported Ru catalysts for partial oxidation of methane [29]. 

Resonance stabilization is also forwarded as a rationale for the relatively high stability of monomeric aromatic sulfide radical cations. Extensive studies have been conducted with respect to the one-electron oxidation of diphenyl sulfide and thioanisol [130-132]. The absorption spectra of the radical cations typically show several bands based on transitions within the aromatic 7i-system but also involving the non-bonding electrons at sulfur (i ax 360 nm and 740 nm for (C5H5)2S, and 310 nm, 530 nm and possibly 740 nm for QHs-S -CH ). In their elaborate study on a variety of thioanisol derivatives, lolele cf a/. [ 131 ] provided information on extinction coefficients, being typically on the order of 1 x lO and 6 X 10 M" cm" for the UV and visible band, respectively. Oxidations of the parent sulfides by OH and several one-electron oxidants (TF, SO/", Br2 ", etc.) occur with rate constants on the order of some 10 M s . Dimerization to the corresponding (>S S<) could not be achieved in most cases, most likely due to solubility limitations. 

Effect of Solvent on Extinction Coefficients op Peotonated Bases OF THE Ho Type Relative to the Hb Type ... 

Another interesting physical feature relates to the cliromophoric character of fullerenes. Based on the symmetry prohibitions, solutions of [60]fullerene absorb predominantly in the UV region, with distinct maxima at 220, 260 and 330 nm. In contrast to extinction coefficients on the order of 10 cm at these wavelengths, the visible region shows only relatively weak transitions (X at 536 nm s =710 cm ) [142]. 

The intensity of a spectral absorption band at a given wave length is expressed in terms of absorption or extinction coefficients, dehned on the basis of the Beer-Lambert law. The latter states that the fraction of incident light absorbed is proportional to the number of molecules in the light path, i.e., to the concentration (c) and the path length (1). The law may be expressed mathematically as ... 

Optical Properties. The index of refraction and extinction coefficient of vacuum-deposited aluminum films have been reported (8,9) as have the total reflectance at various wavelengths and emissivity at various temperatures (10). Emissivity increases significantly as the thickness of the oxide film on aluminum increases and can be 70—80% for oxide films of 100 nm. 

Methyl Red (4-dimethylaminoazobenzene-2 -carboxylic acid) [493-52-7] M 269.3, m 181-182 , Cl 13020, pK j 2.30, pK2 4.82. The acid is extracted with boiling toluene using a Soxhlel apparatus. The crystals which separated on slow cooling to room temperature are filtered off, washed with a little toluene and recrysld from glacial acetic acid, benzene or toluene followed by pyridine/waler. Alternatively, dissolved in aq 5% NaHC03 soln, and ppted from hot soln by dropwise addition of aq HCl. Repealed until the extinction coefficients did not increase. 

The extinction coefficient is dependent on the presence of gases and molecules that scatter and absorb light in the atmosphere. The extinction coefficient may be considered as the sum of the air and pollutant scattering and absorption interactions, as shown in Eq. (10-4) ... 

The Rayleigh scattering extinction coefficient for particle-free air is 0.012 km for "green" light (y = 0.05 /rm) at sea level (4). This permits a visual range of —320 km. The particle-free, or Rayleigh scattering, case represents the best visibility possible with the current atmosphere on earth. 

The second comment concerns the possibility of avoiding fixed derivatives, using instead the effect of the temperature on the spectrum. Assuming that the extinction coefficients are independent of the temperature, then the changes in the spectrum should correspond to changes in Kt (In Kj- = -4///RT -I- 45/R). Therefore, in principle it is possible to determine the equilibrium enthalpy without the use of model compounds [95ACA(314)225]. 

When 1, 3, 3-triethoxypropene was hydrolyzed with IN sulfuric acid, a solution of malonaldehyde whose optical density was perfectly stable at 350 m/x for at least one week was obtained. If the solution was made alkaline, the optical density at the same wavelength increased by a small value and then remained virtually constant for at least one week (56). It was also observed that in these solutions the extinction coefficient at 350 m/x was very low (observed 8.3, 61.5 and 69, for solutions of pH 0.4, 7.15 and 9.4 respectively) compared with previously reported values which varied from 200 ( 40) to 1000 ( 48). On the other hand, the absorption of solutions having a pH of 3 to 5, increased considerably with time (at pH 4.75, the extinction coefficient of malonaldehyde at 350 m/x was initially about 40 after four weeks a value of about 930 was recorded and the optical density of the solution was still increasing). This increase in absorption was accompanied by a marked decrease in the malonaldehyde content of the solution, as measured by the thiobarbituric acid method. As a corollary, it was found that aqueous solutions of malonaldehyde, prepared by autocatalyzed hydrolysis (33) of the same acetal and which had a pH of about 3.5, showed, at the completion of the hydrolysis, considerably higher extinction coefficient values at 350 m/x than did those malonaldehyde solutions which were prepared by hydrolysis with IN acid and subsequently adjusted to pH 4. It appears, therefore, that at pH values at which most of the periodate oxidations are carried out, malonaldehyde is unstable and undergoes a chemical reaction, the nature of which is not, as yet, known. 

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

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