Aromatic hydrocarbons ultraviolet spectrum

The ultraviolet spectra of these compounds are similar to those of trans stilbene or of 2- and 4-stilbazole. The effect on the ultraviolet spectrum of various substituents have been found to parallel in many respects the efiects produced by the corresponding group in derivatives of aromatic hydrocarbons (142). 

Azulene. The absorption spectrum of azulene, a nonbenzenoid aromatic hydrocarbon with odd-membered rings, can be considered as two distinct spectra, the visible absorption due to the 1Lb band (0-0 band near 700 nm) and the ultraviolet absorption of the 1L0 band.29 This latter band is very similar to the long wavelength bands of benzene and naphthalene CLb) and shows the same 130 cm-1 blue shift when adsorbed on silica gel from cyclohexane.7 As in the case of benzene and naphthalene, this blue shift is due to the fact that the red shift, relative to the vapor spectra, is smaller (305 cm"1) for the adsorbed molecule than in cyclohexane solution (435 cm"1). Thus it would appear that the red shifts of the 1La band are solely due to dispersive forces interacting with the aromatic molecule, in agreement with Weigang s prediction,29 and dipole-dipole interaction is negligible. 

The ultraviolet spectra of the polynuclear aromatic hydrocarbons possess charaeteristie shapes and fine structure. In the study of spectra of substituted polynuclear aromatic derivatives, it is common practice to compare them with the spectrum of the unsubstituted hydrocarbon. The nature of the chromophore can be identified on the basis of similarity of peak shapes and fine structure. This technique involves the use of model compounds. Section 7.15 will discuss it further. 

It should not be assumed that all simple mixtures can be analyzed by either of the above techniques. Consider the aromatic hydrocarbon pyrene in the presence of an equal amount of anthracene. The ultraviolet absorption (and fluorescence excitation) spectrum of anthracene completely overlaps that of pyrene. It can be... 

Hgure 7 Influence of the modulation amplitude a [8] on the shape of a second-order derivative spectrum [19]. The example was determined by means of an aqueous sample containing approx. Imgl of each benzene, toluene, p-xylene. (Reprinted with permission from Vogt F, Tacke M, Jakusch M, and Mizaikoff B (2000) An ultraviolet spectroscopic method for monitoring aromatic hydrocarbons dissolved in water. Analytica Chimica Acta 422 187-198 Elsevier cf. Erratum (2001) Analytica Chimica Acta 431 167.)... 

If the solvent is to be used for ultraviolet spectroscopy, it is necessary to remove all the aromatic compounds. This may be done by shaking the hydrocarbon with a mi.xture of concentrated sulfuric and nitric acids, which will nitrate the aromatic compounds. The hydrocarbon is separated, washed with water, distilled, and passed through a column of activated alumina which will remove any residual unsaturated or nonhydrocarbon materials. The spectrum of the solvent is monitored as it passes from the column, and when significant absorption at 210 m/i is observed, the alumina is replaced. 

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