Benzenoid absorption

In the majority of cases, the absorbance at 250-280 nm was measured in solutions containing 1 X 10 4 M of the benzaldehyde studied. In solutions containing higher concentrations of DMSO, these benzenoid absorption bands were overlapped by a cut-off caused by solvent absorption. In those cases, the absorbance corresponding to n — 7r transition of the carbonyl group was measured. Because of the lower extinction coefficient of this absorption band, measurements were then carried out in 5 X 10 4M benzaldehyde solutions. 

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. 

Ultra-violet and visible spectrophotometry can be effectively used for the control of purification and specification of purity of compounds. If a compound is transparent in the near ultra-violet and the visible regions, the purification is continued until the absorbancy is reduced to a minimum (e < 1). Traces of impurities present in pure transparent organic compounds can be readily detected and estimated, provided the impurities themselves have fairly intense, absorption bands. Before a liquid is used as a spectroscopic solvent, it should be tested for spectrophotometric purity. For example, commercial absolute alcohol usually contains benzene as impurity. The absence of benzene in the Alcohol should be confirmed spectrophoto-metrically by using sufficiently large cells (4 or 10 cm cells), before using the alcohol as a solvent. The presence of carbon disulphide in carbon tetrachloride may be detected by the presence of the disulphide absorption tend at 318 mytt. The detection of the characteristic benzenoid absorption in the spectra of many organic compounds (e.g. diethyl ether, cyclohexene) showed that the bands attributed to these compounds earlier were only due to the contamination by benzene1. 

Annelation increases the complexity of the spectra just as it does in the carbocyclic series, and the spectra are not unlike those of the aromatic carbocycle obtained by formally replacing the heteroatom by two aromatic carbon atoms (—CH=CH—). Although quantitatively less marked, the same trend for the longest wavelength band to undergo a bathochromic shift in the heteroatom sequence O < NH < S < Se < Te is discernible in the spectra of the benzo[Z>] heterocycles (Table 17). As might perhaps have been anticipated, the effect of the fusion of a second benzenoid ring on to these heterocycles is to reduce further the differences in their spectroscopic properties (cf. Table 18). The absorption of the benzo[c]... 

Similar results were obtained when TCDD in methanol was exposed to natural sunlight in sealed borosilicate glass tubes or beakers (Figure 3). After about 36 hours exposure, a yellow non-volatile gum was obtained as the sole product by evaporation of the solvent. It showed no UV absorption and did not seem to retain the benzenoid chromophore ... 

At this point the oxidation stage of quinonediimine has been fully reached its (very unstable) salts have scarcely any colour. The production of colour only takes place when quinonoid and benzenoid systems are present together. The molecular union of the two substances at different stages of oxidation produces the intense absorption which is a prerequisite for the formation of a dye (Willstatter and Piccard). This union need not take place in the proportion 1 1, which obtains in the present case. The relations between quinhydrone and quinone-quinol are quite similar (p. 314). 

Polydimethylsiloxanes containing phenyl groups exhibit absorptions in the UV which are similar to those observed for other phenyl containing compounds(8), They include a principal band in the far UV near 215 nanometer (nm), designated L (9) and a weak intensity band multiplet which is centered near 260 nm, designated 1Ljj(9)-The latter is often used to recognize and occasionally to quantitate benzenoid content. 

The long-wavelength absorption bands exhibited by solutions of methiodides of aza analogues of benzenoid hydrocarbons have been attributed to the presence of charge-transfer complexes.01 There is a correlation between the excitation energies of the bands and the calculated electron affinity of the cations, in agreement with the delocalized rather than the localized model of the excited state in the charge-transfer complex.91... 

The addition of ethanol appears to have an appreciable influence on the absorptivity of substituted benzaldehydes. Generally the molar absorptivity decreases by about 40% when the ethanol content of the solution is increased from 1 to 90 vol %. Moreover, there is a slight shift of both the benzenoid and the... 

Chromene is variously reported to exhibit maxima at 264 and 307 nm in ethanol (63CPB1042), 266.5 and 314 nm in hexane (64T1185), and 263 nm in ethanol (62JA813). Both absorptions may be benzenoid (B) bands or the peak near 260 nm may be a conjugation (K) band both are associated with v-tir transitions. 

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 stability of betaines of type 246 and 248 is thus mainly determined by the environment of the deprotonated hydroxyl group, and their further transformations may be sterically retarded (cf. Section III,E,2). At the same time, on treatment of salt 249 with triethylamine in anhydrous solvents, betain 250 was detected by electronic absorption spectra as a shortlived compound. Its instability is obviously explained by the disturbance of aromaticity in the annelated benzenoid ring, resulting in an ortho-quinonoid fragment (87RRC417). 

C60H36 isomer depends from the synthetic route. The synthesis under dry conditions under high H2 pressure favours the D3d isomer (Meletov et at. 2001 Bini et al. 1998) while the wet synthesis with Zn/HCl it was initially thought to favour the S6 isomer. However, as discussed in the section of the electronic absorption spectra, considerations based on the absorption peak of both C60H36 and C60D36 and on their molar extinction coefficient have lead to assign the Th symmetry to these molecules. Th isomer is the sole isomer characterized by isolated double bonds and is free from benzenoid rings. 

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