Conjugation bands

It is quite evident that the conjugated systems might fail to display the expected conjugated bands due to the following two reasons, namely ... 

In the spectra of isoflavones (64) and flavanones (25), in which rings A and B are not conjugated, band I frequently appears only as a shoulder on band II. Not surprisingly, introduction of oxygen substituents in ring B has little effect on the spectra, whereas substitution in ring A shifts band II to the red. 

Dehydro- -Cyclocitrylideneacetic Acid (49). In order to determine the influence of a supplementary double bond in the cycle of 3-ionone on the proposed mechanism, we were interested in the irradiation of compounds of the dehydro- 3-ionone series. Dehydro- 8-cyclocitrylidene-acetic acid (49), prepared from- -cyclocitiyhdeneacetic acid according to the method of Henbest used by him to obtain dehydro- 3-ionone) has been irradiated in hexane. The conjugation bands 1610-1630 cm. and CH=CH trans absorption at 960 cm. characteristic of 49, disappeared and were replaced by a methenic band at 895, cm. the carbonyl vibration shifted from 1695 to 1705 cm. The UV absorption at 320 mitt disappeared while maxima at 200, 235, and 270 my appeared. 

The ultraviolet-visible spectra of both PDA—H and PDA- apparently have a common band (3, = 295 nm (15 700) 296 (16 700)), the so-called amine tt-conjugation band (p. 411), involving the nitrogen lone electron pair (Ballester and Riera, 1967). Two additional bands are found in PDA-(470 nm (2300) 665 (425)), which are inherent to the radical character, much like the two observed in PTM radicals (380, 500-560 nm) (p. 419) (Olivella, 1973). Because of these features, it is concluded that each of the two nitrogen nonbonding orbitals interacts strongly with both pentachlorophenyl Jt-systems, and therefore they are nearly equivalent. Consequently, efforts to identify the semioccupied orbital (Walter, 1966), might be pointless, at least in PDA-. 

Polychlorinated benzenes show overlapping primary bands and secondary bands around 220-250 and 300 nm respectively. It is well known that in conjugated biphenyls, styrenes and stilbenes, a medium-intense to intense band appears (the so-called K or conjugation band), located between the primary bands and the secondary band. In biphenyls, it is an intense band at 250-270 nm (Fig. 21) in styrenes, in the range 260-280 nm (Fig. 22) and in stilbenes, between 270 and 300 nm (Fig. 23). 

Fig. 25 Perchloroindene, an almost planar, sterically distorted 7t-system. Its conjugation band is not significantly inhibited, but is exceptionally bathochromically shifted.

Soret band of the macrocycle, a ti to tt transition, is excited instead. It has been found that the vinyl groups do not participate in the conjugated system [4]. This is based on the fact that the vinyl C=C stretch does not... 

Band 5, 6-25y. (1601 cm. i). Aromatic (phenyl) ring absorption. The weak shoulder at 6-33p. (1580 cm. i) may be noted. When the aromatic ring is conjugated, as in the present example, the aromatic (phenyl) band is often split into a doublet and is usually more pronounced (Table III). 

The former exhibits absorption tjrpical of an isolated keto group, whereas the latter shows a high intensity -band associated with the conjugated system HO—C=C—C=0. The proportions of the two forms under various conditions are readily determined from the ultraviolet spectra. The ultraviolet spectra in various solvents are shown in Fig. A, 7, 2. Since the absorption of the keto form is negligible, the percentage of enol present is 100(em/e ), where e is the observed extinction at 245 mp. and that of the pure enol. It was shown that in alcoholic solution is 1900 and the percentage of enol is 12. Thus e is ca. 16000, and use of this value permits the approximate evaluation of the enol content in different solvents. The results are collected in Table XII. 

Table 1. Optical Band Gaps For Conjugated Polymers...

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

Since IR spectra are essentially due to vibrational transitions, many substituents with single bonds or isolated double bonds give rise to characteristic absorption bands within a limited frequency range in contrast, the absorption due to conjugated multiple bonds is usually not characteristic and cannot be ascribed to any particular grouping. Thus IR spectra afford reference data for identification of pyrimidines, for the identification of certain attached groups and as an aid in studying qualitatively the tautomerism (if any) of pyrimidinones, pyrimidinethiones and pyrimidinamines in the solid state or in non-protic solvents (see Section 2.13.1.8). 

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