Maximum extinction coefficient

The commonest modern method for determining the degree of hydration is to measure the intensity of the broad n- carbonyl absorption band at about 280 m/x, which disappears on hydration. Early measurements (Schou, 1926, 1929 Harold and Wolf, 1929, 1931) show considerable discrepancies, but the results of later workers are in reasonable agreement. The main uncertainty lies in the value to be assigned to the maximum extinction coefficient of the unhydrated carbonyl compound, which varies between 12 and 80 for different compounds. This is commonly taken as the value measured in a non-hydroxylic solvent such as hexane or cyclohexane, but this is not strictly valid, since the intensities of n-n- transitionsvary somewhat with the solvent (see e.g. Dertooz and Nasielki, 1961) moreover, since the shape of the band and the value of e are also solvent-dependent it may make some difference whether the extinction coefficients are compared at the same wavelength, at the respective maxima, or in terms of the band area. Special difficulties arise... 

Maximum extinction coefficients will not do, even for non-rigorous discussions, since band widths differ so widely. Quantitative discussions should include corrections for instrumental broadening the procedures available are described in (59). 

A = solution absorption spectrum B = diffuse reflectance spectrum C = crystal transmission spectrum D = transmission spectrum as KC1 pellet E = absorption spectrum of melt at 400 °C maximum extinction coefficient in parentheses following the band maxima. 

Their absorption spectra were typical for macrocycles of this type (Fig. 1). They all displayed an intense split Q band and an intense single peak in the Soret region. The peripheral functionalization affected the electronic structure of the tetraaza-porphyrin ir-system and resulted in perturbations of the Q bands. Their maximum extinction coefficients were much higher than those of ordinary diarylethenes. 

An interesting consequence of this approach concerns the absorption intensities of the discrete but broadened lines. The broadened lines all appear with the same heights (maximum extinction coefficients) but differ in widths. However, when the intensities of all the lines are summed, this aspect is obscured. 

The enthalpy of formation of crystalline imidazole is 14.6 0.8 kcal mole-1 its heat of sublimation is 16.0 1.0 kcal mole-1, and the heat of formation of gaseous imidazole is 30.6 1.8 kcal mole-1.291 From these data the resonance energy of imidazole has been calculated as 14.2 kcal mole-1.291,292 From an infrared examination of the association of 4-methylimidazole in carbon tetrachloride and in 1,1,2,2-tetrachloroethane,293 the concentration dependence of the maximum extinction coefficient of the free NH valency band allowed determination of the monomer content, the mean viscosity, and the equilibrium constants K12 and Kl >. From the temperature dependence of these values the mean heat of addition (8.2 0.5 kcal mole-1) and the heats of formation of the dimers and trimers (10.2 and 8.1 kcal mole-1, respectively) were obtained. Although the mean heat of addition was found to be about 3 kcal mole-1 higher than that of Zimmermann203 this is not contradictory to the assumption that the imidazoles form chainlike associations with angled structures. 

Each compound will have particular bonding characteristics, which will generate different values of kmax- As noted earlier, bonding depends on the behaviour of electrons and electronic transitions. These electronic transitions will be observed at different wavelengths on the absorption spectrum and reveal important features of the compounds and chromophores being studied. Values of kmax and absorbance can be taken from the absorbance spectmm, and from this the maximum extinction coefficient (smax) can be determined using the Beer-Lambert law equation. Table 5.1 gives information on electronic absorption characteristics of typical chromophores in non-aromatic compounds. 

IL absorption bands are probably least distinguishable from CT bands. Since both IL and CT transitions may be strongly allowed, the associated absorption bands have maximum extinction coefficients in the same order of magnitude (10 - 10 cm ). 

The maximum extinction coefficients of CT bands are typically in the range 10 to 10, but are obviously sensitive to environmental factors. One feature of CT spectra which is not greatly affected by solvent or temperature variations is the oscillator strength,which is given approximately by... 

In classical electron transfer studies, excitation of the normal singlet states leads to electron transfer from the excited state. However, because of the dipole moment formed by the separated oxidized and reduced centers, a direct transfer of charge can occur. A similar perturbation theory approach taking into account the harmonic potential sites of the protein yields the prediction that there should be a charge-transfer band with maximum extinction coefficient at photon energy Avmax ... 

Table 4.1 Summary of the main Chromophore/Fluorophore residues in proteins and nucleic acids with absorption and fluorescence characteristics given Amax is wavelength of maximum absorbance, m is maximum extinction coefficient value, is wavelength of maximum fluorescence intensity, and tr is radiative lifetime.

In a electronic system all transitions are determined by these relations, in a high-spin i system, however, only the quintet-quintet transitions. The latter may be distinguished from the quintet-triplet transitions by their band positions, as in chromium(II) fluorides, or at least by their band intensities, as in manganese(III) fluorides. The intensity criterion must be handled with some care, however, as the maximum extinction coefficients of intercombination bands may well reach the order of magnitude of spin-allowed bands. Thus the absorption maxima at higher energies, which are of interest here, cannot be definitely localized least in MnFs 18) but neither in the other manganese(III) compounds (see Fig. 6). 

In an effort to examine the scope of the message in a bottle strategy and to develop phosphorescent pH sensors with longer excitation wavelengths, the case of 5 is examined. In contrast to sensor 3 which had a maximum extinction coefficient of 5300 M-lcm l at 283nm in methanol, 5 possesses the corresponding value of 15300 M"Icm l at 325nm. The quaternary salt 6 and other 1-bromo, 4-alkanoyl naphthalene derivatives were previously employed by Turro as phosphorescent probes of... 

Equation 4.1 can be approximated to a useful expression relating x for a molecule to the maximum extinction coefficient (A,), determined from the absorption spectrum as a function of wavelength (X) ... 

The aromatic side chains, phenylalanine, tyrosine and tryptophan all have transitions in the near-UV region (Figure 4). At neutral pH, the indole of tryptophan has two or more transitions in the 240-290 nm region with total maximum extinction coefficient e J(279 nm) -5000 moH dm cm i tyrosine has one transition with 1400 mol" ... 

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