Benzenoid bands

Figure 22-3 Ultraviolet absorption spectrum of benzene (in cyclohexane) showing the benzenoid" band...

Effects of Structure on Electronic Absorption Corresponding to the Benzenoid Band... 

The most characteristic part of the spectrum is the band centered at 254 nm, called the benzenoid band. About three to six small, sharp peaks (called fine structure) usually appear in this band. Their molar absorptivities are weak, usually 200 to 300. These benzenoid absorptions correspond to additional forbidden transitions. 

Simple benzene derivatives show most of the characteristics of benzene, including the moderate band in the 210-nm region and the benzenoid band in the 260-nm region. Alkyl and halogen substituents increase the values of Amax by about 5 nm, as shown by the examples in Table 16-2. An additional conjugated double bond can increase the value of Amax by about 30 nm, as shown by the UV spectrum of styrene in Figure 16-19. 

The presence of aromatic units from the X 200 nm and X 260 nm (benzenoid) bands. 

The spectrum of decamethylbiphenyl is quite similar to that of hexamethyl-benzene in double concentration (Figure 8.3). The replacement of the oriho-methyl groups by bulkier groups, like the t-butyl group, results in a further decrease in intensity and increase in the fine structure of the benzenoid band at 260 m/i. In addition, the primary band also undergoes a blue-shift. The effect of increasing the size of the substituents in 2,2 -disubstituted biphenyls... 

Finally, in this vein, while the benzenoid bands in benzene are centered around Xmax 255 nm (e = 230), they are found near 380 (e = 7900) for colorless anthracene (CwHio) (Figure 6.32) near 480 (e = 11,000) for yellow napthacene (tetracene, CigHn) (Figure 6.32) near X ax 580 (e = 12,600) for blue pentacene (C22H14) (Figure 6.32) and between 605 and 699 nm (e = 300) for azulene (Figure 6.31). 

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