Dienones, absorption wavelength

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

Sets of empirical rules, often referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 7.11) and enones and dienones (Table 7.12) to be predicted. To the respective base values (absorption wavelength of parent compound) are added the increments for the structural features or substituent groups present. When necessary, a solvent correction is also applied (Table 7.13). 

Tables 6.3-6.5 record data developed to undertake structural analysis in systems possessing chromophores that are conjugated or otherwise interact with each other. Chromophores within a molecule interact when linked directly to each other or when they are forced into proximity owing to structural constraints. Certain combinations of functional groups comprise chromophoric systems that exhibit characteristic absorption bands. In the era when UV-VIS was one of the principal spectral methods available to the organic chemist, sets of empirical rules were developed to extract as much information as possible from the spectra. The correlations referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 6.3) and enones and dienones (Table 6.4) to be predicted. When this method is applied, wavelength increments correlated to structural features are added to the respective base values (absorption wavelength of parent compound). The data refer to spectra determined in methanol or ethanol. When other solvents are used, a numerical correction must be applied. These corrections are recorded in Table 6.5.

TABLE 6.4 Absorption Wavelength of Enones and Dienones (continued)... 

All photoreactions which have been mentioned in the previous sections, are effected by ra — ir excitation. Alternatively, -ir- - v excitation was frequently found more convenient to selectively conduct single reaction steps in the dienone and bicyclohexenone field. Conversion of the initially formed Tr,ir singlet excited state to the lower energy n,ir excited state should be very rapid in general. However, quantitative studies of the product distribution from the bicyclohexenone 33a (Chart 5) showed that the relative amounts of products 36a and 38a, both formed from the same spirocyclic precursor, dienone 34a, varied somewhat when light of wavelengths was employed that corresponded to either absorption band. Apparently, the dienone rearrangement may compete to some extent with this internal conversion. 

The wavelengths of maximum absorption of conjugated dienes and polyenes and conjugated enones and dienones are given by the Woodward and Fieser Rules, Tables 1 and 2. 

Effect of Photooxidation on the Luminescent a,/ -Unsaturated Carbonyl Groups. The longest wavelength n —w absorption bands (15,16, 17) of both the enone and dienone chromophoric impurities overlap the high energy end of the spectrum of natural sunlight known to be harmful to the commercial polyolefins (20,21) (i.e., 300-350 nm). 

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