Absorption band broadening

In addition to the data already discussed on acids or Lewis acids as solvents, some data are available for solvents in which the interpretation in terms of molecular complexing is less obvious. For example, the ionization of trityl chloride has been compared spectroscopically in nitromethane, nitroethane, and 2-nitropropane.198 Unfortunately the absorption band broadens as the solvent is changed, rendering a quantitative interpretation difficult. In the author s laboratory two... 

Indeed, the correlation of the electronic spectra with different metallo-porphyrin states is not unequivocal in many specific cases. The distinctions between 2 and 4 are not clearcut, as is the case with 5, 8 and 9. Further complications arise when the metalloporphyrin aggregates, which usually leads to absorption-band broadening. In many of these cases magnetic data will provide additional evidence, or certain electrochemical regularities (see V.l) can be used. 

As the radius of the sphere is increased, the n>ro band of the bulk mode appears in the spectrum. Eor example, for ZnS particles this occurs at r > 2 p,m. As the radius increases further, (i) both the surface- and bulk-mode absorption bands broaden and split due to the appearance of higher order surface modes, (ii) the band maxima shift toward lower frequencies, and (iii) the ratio of the intensities of the surface to bulk modes decreases [293]. As can be deduced from Eq. (3.35), an increase in the dielectric constant of the surrounding medium will also cause the surface modes to shift toward the red, as does an increase in the particle dimensions. The explanation is the same as for plane-parallel films (Section 3.3.1). The presence of a dispersion in particle size causes the two absorption bands to broaden, the fine sfiucture to disappear, and the a>io band to shift to lower frequencies. An analogous effect can be observed if the damping constant y of the sphere material increases. 

Table 2 Fundamental absorption band, broadening index, detection index and characteristic of densitometric band, modified contrast index, densitometric densitometric visualizing index, and linearity range of ( )-a-tocopherol on silica gel 60.

Detectors Most of the detectors used in HPLC also find use in capillary electrophoresis. Among the more common detectors are those based on the absorption of UV/Vis radiation, fluorescence, conductivity, amperometry, and mass spectrometry. Whenever possible, detection is done on-column before the solutes elute from the capillary tube and additional band broadening occurs. 

Moderately slow exchange. The state lifetime is 2t we ask how the absorption band is affected as this becomes smaller. The uncertainty principle argument given earlier is applicable here lifetime broadening will occur as the state lifetime decreases. Thus, we expect resonance absorption at (or near) frequencies Va nnd Vb but the bands will be broader than in the very slow exchange limit. Equation (4-68) is applicable in this regime. 

Thiophenes substituted with groups such as alkyl, halogens, OCH3, and SCH3 show small but characteristic differences between 2- and 3-substituted compounds. In these cases, however, it is the 2-isomer which shows the less complex spectrum. Thus, 2-substituted alkylthio-phenes and halothiophenes show a single band with greater extinction than the 3-isomers whose spectra exhibit two peaks in a broadened absorption band. These differences are also present in the spectra of 2,5- and 3,4-dihalosubstituted compounds. In 2-substituted thiophenes, the intensity of the band varies inversely as the electronega-... 

This result reveals that exciplex formation plays a principal role in the initiation of polymerization. Since the absorption band is broadened toward longer wavelengths as the result of formation of CTC between AN and aniline, a certain concentration of aniline can be chosen so that 365-nm light is absorbed only by the CTC but not by the aniline molecule. Therefore, in this case the photopolymerization may be ascribed to the CTC excitation selected. For example, a 5 x 10 mol/L aniline solution in AN could absorb light of 365 nm, while solutions in DMF or cyclohexane with the same concentration will show no absorption. Obviously, in this case the polymerization of AN is caused by CTC excitation. The rates of polymerization for different amines were found to be in the following order (Table 12) ... 

Complex formation constants could also be determined directly from UV spectrophotometric measurements. Addition of tert.-butyl hydroperoxide to a solution of nitroxide I in heptane at RT causes a shift of the characteristic absorption band of NO at 460 nm to lower wavelengths (Fig. 9). This displacement allows calculation of a complex equilibrium constant of 5 1 1/Mol. Addition of amine II to the same solution causes reverse shift of theC NO" absorption band. From this one can estimate a complex formation constant for amine II and +00H of 12 5 1/Mol (23 2 1/Mol was obtained for tert.-butyl hydroperoxide and 2,2,6,6-tetramethylpipe-ridine in ref. 64b). Further confirmation for an interaction between hindered amines and hydroperoxides is supplied by NMR measurements. Figure 10a shows part of the +00H spectrum in toluene-dg (concentration 0.2 Mol/1) with the signal for the hydroperoxy proton at 6.7 ppm. Addition of as little as 0.002 Mol/1 of tetra-methylpiperidine to the same solution results in a displacement and marked broadening of the band (Fig. 10b). 

Apart from molecular vibrations, also rotational states bear a significant influence on the appearance of vibrational spectra. Similar to electronic transitions that are influenced by the vibrational states of the molecules (e.g. fluorescence, Figure 3-f), vibrational transitions involve the rotational state of a molecule. In the gas phase the rotational states may superimpose a rotational fine structure on the (mid-)IR bands, like the multitude of narrow water vapour absorption bands. In condensed phases, intermolecular interactions blur the rotational states, resulting in band broadening and band shifting effects rather than isolated bands. 

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