Ultraviolet absorption aromatic compounds

Determination of purity. The ultraviolet and visible absorption is often a fairly intensive property thus e values of high intensity bands may be of the order of 10 -10 . In infrared spectra e values rarely exceed 10 . It is therefore often easy to pick out a characteristic band of a substance present in small concentration in admixture with other materials. Thus small amounts of aromatic compounds can be detected in hexane or in cyclohexane. 

Aromatic rings are detectable by ultraviolet spectroscopy because they contain a conjugated rr electron system. In general, aromatic compounds show a series of bands, with a fairly intense absorption near 205 nm and a less intense absorption in the 255 to 275 nm range. The presence of these bands in the ultraviolet spectrum of a molecule is a sure indication of an aromatic ring. 

Armstrong and Boalch [60] have examined the ultraviolet absorption of seawater, particularly in the wavelengths between 250 and 300 nm, where the absorption is considered to result from the presence of aromatic compounds. Light absorption is a particularly useful measure, if it can be made to work, since it is not too difficult to construct an in situ colorimeter which can produce continuous profiles of dissolved organic carbon with distance or depth [71]. 

Ames NP, Hartley RD, Akin DE. Distribution of aromatic compounds in coastal Bermudagrass cell walls using ultraviolet absorption scanning microdensitometry. Food Struct 1992 11 25-32. 

Aromatic compounds have very high molar absorptivities that usually lie in the vacuum ultraviolet region and are not useful for routine analysis. Modest absorption peaks are found between 200 and 300 nm. Substituted benxene compounds show dramatic effects from electron-withdrawing substituents. These substituents are known as auxo-chromes since they do not absorb electromagnetic radiation but they have a significant effect on the main chromophore. For example, phenol and aniline have molar absorptivities that are six times the molar absorptivity of benzene or toluene at similar wavelengths. 

Compared to straight-chain conjugated polyenes, aromatic compounds have relatively complex absorption spectra with several bands in the ultraviolet region. Benzene and the alkylbenzenes show two bands in which we shall be primarily interested, one near 200 nm and the other near 260 nm. The 200-nm band is of fairly high intensity and corresponds to excitation of a 77 electron... 

Aromatic compounds. These compounds exhibit characteristic absorption in the ultraviolet-visible region of the spectrum, and although they are frequently easily recognised from their other spectroscopic properties, examination of their electronic spectra can often lead to the elucidation or confirmation of some of the detailed structural features. 

The ultraviolet spectra of coals, examined as suspensions in potassium bromide, show an absorption band at 2650 A that becomes more pronounced with increasing rank of the coal. This band has been assigned to aromatic nuclei, and on the basis of data obtained from comparison between the specific extinction coefficients of coal and those of standard condensed aromatic compounds, it has been concluded that the concentration of aromatic systems in coal is lower than had previously been believed. 

The organopolysilanes are those compounds containing at least one silicon-silicon bond and one silicon-carbon linkage. This review is mainly concerned with the chemistry of aliphatic derivatives of polysilanes. Consideration of aromatic organopolysilanes is excluded from this review except as far as they are used as intermediates for synthesis and their properties correlate with the aliphatic silicon-silicon compounds, because the aromatic organopolysilanes have recently been well reviewed elsewhere (31,51, 73, 76a, 212). Physical properties of the polysilanes also are excluded from consideration except for spectral properties of ultraviolet absorption and nuclear magnetic resonance, since they are well summarized in earlier excellent reviews and texts (8, 34, 35, 51,131,132). 

The formulation of enamines of quinuclidine in a mesomeric form would break Bredt s rule. No mesomerism occurs in 2,3-benzo-quinuclidine between the nitrogen atom and the aromatic ring thus, the compound does not exhibit the characteristic ultraviolet absorption of aromatic amines.43 Dehydroquinuclidine may only be formulated as l-azabicyclo[2.2.2]oct-2-ene the overlap of the olefinic ir-orbital and the lone pair orbital on nitrogen is precluded. Its ultraviolet spectrum exhibits merely end-absorption the compound is... 

Ultraviolet absorption spectra have been used for many years in a qualitative way to indicate similarities in the bonding patterns of different compounds. The recorded UV spectra of azoloazines are characterized by a band at 250-280 nm which is ascribed to a n-n transition and which finds similarity in indole as well as other aromatic carbocyclic and heterocyclic compounds (84CHEC-1(4)497). 

Ultraviolet Spectroscopy The ultraviolet spectra of aromatic compounds are quite different from those of nonaromatic polyenes. For example, benzene has three absorptions in the ultraviolet region an intense band at Amax = 184 nm (e = 68,000), a moderate band at Amax = 204 nm (e = 8800), and a characteristic low-intensity band of multiple absorptions centered around 254 nm (e = 200 to 300). In the UV spectrum of benzene in Figure 16-19, the absorption at 184 nm does not appear because wavelengths shorter than 200 nm are not accessible by standard UV-visible spectrometers. 

In Tables 1, 2 and 3 ultraviolet absorption spectra of acyclic or cyclic, aliphatic and aromatic azo compounds are compiled. 

The absorption bands in the ultraviolet and visible part of the spectrum correspond to changes in the energy of the electrons but simultaneously in the vibrational and rotational energy of the molecule. In this way a system of bands is produced in the gaseous state. In the liquid state there is nothing of the rotational fine structure to be seen, and usually little or nothing of the vibrational structure, as a result of the interaction with the molecules of the solvent. With aromatic compounds in non-polar solvents such as hexane and carbon tetrachloride the vibrational structure is, however, still clearly visible in the ultraviolet absorption spectrum. This vibrational structure is mainly determined by the vibrations of the excited state, which therefore do not occur in the infrared and Raman spectrum of the normal molecule. 

Mycoside A.—This compound haa been obtained as a nearly colorless solid, melting at 105°, [a] -37° (CHCl,) C, 72.2 H, 11.3 OCH, 8.6 N, 0 P, 0%. The ultraviolet absorption spectrum shows maxima at 222, 274, and 278 m/i (in hexane). Mycoside A contains three different 0-methylated 6-deoxyhexoses, which have been identified as 2-0-methyl-fucose, 2-0-methylrhamnose, and 2,4-di-0-methylrhamnose. The lipid moiety of mycoside A is a di- or tri-mycocerosate of an aromatic alcohol. 

Aromatic Compounds. The correlation between the absorption spectra and structural features of aromatic compounds will be discussed because of the widespread occurrence of aromatic rings among pesticides. Although the infrared spectra form the most generally applicable method of recognition of the presence of a C-aromatic ring, valuable information may be gained by careful study of the ultraviolet spectra of certain classes of aromatic compounds. 

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. 

It should however be borne in mind that positions 9 and 10 in anthracene are not typically aromatic. They are manifested ]>y a higher reactivity than positions Ur and 0 as established by MO calculation [50]. In addition 9-nitroanthracene shows a non-planar structure with the nitro group out of plane [51] as pointed out by Cerfontain and Telder [48]. This is very similar to the position of the nitro group in o-dinitrobenzene and all derivatives of benzene with two ortho nitro groups. It is well known that the nitro groups in o-dinitrobenzene are not planar and there is no conjugation of double bonds in this compound. Tlie fact is also reflected in ultraviolet-absorption spectrum of o-dinitrobenzene which deviates from those of m- and p-dinitrobenzenes (Vol. I, p. 169, Table 20). 

Aromatic compounds also show characteristic infrared and ultraviolet absorption spectra. In the mass spectrum of aromatic substances, peaks corresponding to ions such as CgH and CgHg" are often seen. A commonly observed peak occurs at m/z 91, corresponding to the stable ion CgHgCH./, These features are all helpful in assigning aromatic character. 

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