Absorption band, chromonics

The pyrano[3,2-c]xanthen-7-one exhibits the two absorption bands associated with a heteroatom at a peri position of the benzopyran nucleus. Pyrano[2,3-a]xanthen-12-one 44, in which the fusion of the chromone ring is reversed, shows an intense single band blue-shifted some 14 nm relative to 2,2-diphenyl-2//-naphtho[l,2-6]pyran [Xmax 403, 482 nm (PhMe)]. Both isomers are readily degraded <01HCA117>. It is noteworthy that 44 could not be... 

The absorption data of many chromone-2-carboxylic acids and esters have been recorded [24, 27,28, 32,36,40, 55,63,77,88, 123,133], but the majority of these were determined in one solvent, usually methanol or ethanol. A recent spectral study [ 134] of several simple chromones, including chromone-2-carboxylic acid, its ethyl ester, the corresponding carboxamide and nitrile, in cyclohexane, methanol and water led to assignment of some of the absorption bands. As the polarity of the solvent decreased, fine structure appeared in some of the bands, as shown in Figure 2.7. Table 2.2 lists the absorption maxima and absorptivities of a number of compounds related to chromone-2-carboxylic acid. One notable feature is a shift to longer wavelength of the 311 nm band of the parent ester when an amino or hydroxyl... 

Table 2.3 some infrared absorption bands OF chromone-2-carboxylic aqd (KBr disc) [28]... 

Chromones differ from coumarins (vc=o 1710 cm ) by the position of the C=0 absorption band in the IR (vc=o 1660 cm ). Chromones, especially flavones, are characterized by two UV absorption bands in the region of 240-285 and 300-400 nm. Chromones and flavones show analogies to 4H-pyran-4-one in their reactivity, that is, they behave as masked 1,3-dicarbonyl systems. 

The spectra of chromone and many of its derivatives consist of four bands centred around 205, 225, 240 and 300 nm. The major absorption peaks at 240 and 300 nm are separated by a minimum at ca. 270 nm. In contrast to coumarins, chromones do not show a second minimum. 

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