Calculating absorption maxima

The structure of the open-chain form was assigned on the basis of its negative solvatochromic behaviour, which is similar to that of other meropolymethines such as the pyridinium A-phenolate betaines [108]. The correlation shown in Fig. 7-4 allows one to calculate absorption maxima of the merocyanine dye in other solvents for which x(30) values are known. 

The calculated absorption maxima are summarized in Table 2. In the case of neutral Ln, the calculated peaks are 323 nm, 338 nm and 339 nm, were nearly equal to the experimental value for Ln (335 nm). Calculated peaks of Anion-2 , which was supposed as M440 by H-NMR, are 423, 434 and 434 nm, i.e., nearly equal to 440 nm. Thus, the experimental absorption peaks of Ln and M440 were properly predicted by the first-principles calculation. However, each of the calculated absorption maxima of Anion-3 , which could be assigned to M420 by H-NMR, was at a much longer wavelength than the experimental value. 

Table 2. Calculated absorption maxima and those oscillator strengths...

Fig. 4 Comparison of the calculated blue lines) absorption spectrum of the same complexes, X=NCS left). Cl right), with the corresponding experimental spectra red lines) in water solution. Inset, detail of the low-energy spectral region. Energies in eV. The Intensity of the experimental spectrum has been rescaled so that the intensity of the experimental and calculated absorption maxima of low-energy bands match. Reprinted from [104] with kind permission of Elsevier...

Figure 2.8 Ergocalciferol (vitamin D2) and the calculated absorption maximum...

An isomer of the molecule used in Example 5.6 is shown in Example 5.7. The calculated absorption maximum is 286 nm. This spectrum is different in several ways from Example 5.6. For example, the carbon at the S position has only one alkyl substitution. 

An isomer of the molecule used in Example 5.6 is shown in Example 5.7. The calculated absorption maximum is 286 nm. This spectrum is different in several ways from Example 5.6. For example, the carbon at the 5-position has only one alkyl substitution. The double bond between the y- and 5-positions is exocyclic to a ring in this case and therefore increases the wavelength of the absorption maximum. The conjugated system is not within a ring, and therefore, contributions from ring currents are not observed. These two isomers could be distinguished from each other by their UV spectra. 

In this system the product of the first reaction possesses an absorption maximum at 222 nm and the final product has k ax = 288 nm. The initial reactant is essentially nonabsorbing at these wavelengths. Hence, spectrophotometric observation at 222 and 288 nm allowed two simultaneous equations to be written, and thus Cb and Cc were determined as functions of time. From the known quantity c°, the concentration Ca was calculated with Eq. (3-28). The rate constant A , was then found from the plot of In Ca vs. time. An estimate of rate constant k was obtained from a plot of In Cb vs. time in the late stages of the reaction, and this value was refined by curvefitting the Cb and Cc data. Figure 3-6 shows the data and final curve fits. 

Typical absorption and fluorescence spectra are shown in Figure 11.3. Since energy is lost in the activated state (S ) before fluorescence, the emission maximum always occurs at a lower wavenumber than the absorption maximum. The difference, which is termed the Stokes shift, can be calculated approximately from the absorption spectrum using the Pestemer rule [17,18]. This rule states that the Stokes shift is 2.5 times the half-bandwidth at the absorption maximum. 

When parameters of the Pariser-Parr-Pople configuration interaction molecular orbital (PPP-CI MO) method were modified so as to reproduce the Aol)s values for l,3-di(5-aryl-l,3,4-oxadiazol-2-yl)benzenes 16 and 17, the calculated HOMO and LUMO energy levels corresponded with the experimental ionization potential and electron affinity values. The relationships between the electrical properties and molecular structures for the dyes were investigated. The absorption maximum wavelengths for amorphous films were found to be nearly equal to those for solution samples <1997PCA2350>. 

It is possible to measure the absorbance of a sample of a known compound at its absorption maximum and to calculate the actual concentration of the compound in the sample using a known value for the molar absorption coefficient (often obtainable from published spectral tables). In Figure 2.2 the absorption spectrum of ADP shows an absorbance of 0.22 at 258 nm. The quoted value for the molar absorption coefficient of ADP at this wavelength is 1.54 X 104 1 mol-1 cm-1 and hence the concentration of ADP in the sample used can be calculated from the Beer-Lambert equation ... 

Experimentally, while the determination of absorption cross sections is fairly straightforward, measuring primary quantum yields is not, due to interference from rapid secondary reactions. As a result, in cases where quantum yield data are not available, calculations of maximum rates of photolysis are often carried out in which it is assumed that (A) = 1.0. It should be emphasized in such cases that this represents only a maximum rate constant for photolysis the true rate constant may be much smaller, even zero, if photophysical fates of the excited molecule such as fluorescence or quenching predominate. 

Person 1 Calculate the maximum band gap energy for which absorption of visible light is possible. 

We tacitly assumed in the preceding paragraph that the absorption maximum in the region where e is negative occurs at the Frohlich frequency. Although this is not strictly correct, the Frohlich frequency is usually approximately equal to the frequency of maximum absorption the precise position of the maximum depends on the behavior of the dielectric function and can be determined only by detailed calculations. This is analogous to the position of absorption peaks in the bulk material we usually assume that such peaks occur at the maxima of c", whereas this is only approximately correct. 

The free dye (M) has an absorption maximum at 520 nm while Ca M absorbs maximally at 480 nm. The concentration of Ca2 + can be calculated from a standard curve in which A480 is plotted as a function of [Ca2+] or preferably from a standard curve of (A520 — A480) as a function of [Ca2+] which is less curved and more sensitive (Figure 5.9). Using this method, the [Ca2+] in milk was found to be 2.53-3.4 mM and appears to be 0.8 mM higher than that determined by the other methods. 

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