Benzene absorption bands

Haaland also measured the infrared spectra of benzene adsorbed on Pt/Al203 that had been regenerated after previous benzene/cyclohexane adsorptions (247) the surface was thought to retain structured carbonaceous deposits. In this case, the broad yCH feature was centered at ca. 3030 cm 1 (with components at 3042, 3031, 3024, and 3014 cm 1) rather than 3040 cm 1 for the species on the freshly prepared catalyst, and a weaker companion band occurred at 2947 cm-1. The benzene absorption bands at wavenumbers <1500 cm 1 were little changed in position but become more prominent in room-temperature spectra in which the 2947-cm 1 feature was weakened. Spectra measured over the range 300-650 K showed that the 2947-cm 1 feature disappeared at 435 K, whereas the vCH aromatic bands retained considerable intensity at temperatures up to 560 K. 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

From about 1970, but before the availability of suitable lasers, Parmenter and others obtained SVLF spectra, particularly of benzene, using radiation from an intense high-pressure xenon arc source (see Section 3.4.4) and passing it through a monochromator to select a narrow band ca 20 cm wide) of radiation to excite the sample within a particular absorption band. 

Ultraviolet. Benzene has a series of relatively low intensity absorption bands in the region of 230 to 270 nm. When there is a substituent on the ring with nonbonding electrons, such as an amino group, there is a pronounced increase in the intensity of these bands and a shift to longer wavelength. Aniline shows an absorption band at 230 nm (e = 8600) and a secondary band at 280 nm (e = 1430). Protonation of the amino groups reduces these effects and the spectmm resembles that of the unsubstituted benzene. 

The charge-tranter concept of Mulliken was introduced to account for a type of molecular complex formation in which a new electronic absorption band, attributable to neither of the isolated interactants, is observed. The iodine (solute)— benzene (solvent) system studied by Benesi and Hildebrand shows such behavior. Let D represent an interactant capable of functioning as an electron donor and A an interactant that can serve as an electron acceptor. The ground state of the 1 1 complex of D and A is described by the wave function i [Pg.394]

The aforementioned conditions make an analysis of the effect of substituents in thiophene on the UV spectrum more difficult than in the benzene series. In benzene there are two widely separated areas of absorption with different intensities. In thiophene there are instead two or three absorption bands due to electronic transitions which overlap and are of similar intensity. Finally, two very low-intensity bands at 313 and 318 mja have been found in thiophene. ... 

A solution of the 2-azido ester or amide (ca. 2 g) in a mixture of MeOII (95 mL) and sodium-dried THF (95 mL) was photolyzed under N2 in a Hanovia photochemical reactor (110-W medium-pressure Hg lamp with a Pyrex filter). The reaction was monitored by observing the rate of disappearance of the absorption band (Nf) at 2140 cm 1 (irradiation times of 3-5 h were generally required). When the reaction was complete the solvent was removed in vacuo and the brown residual oil chromatographed on alumina [petroleum ether (bp 60-803C)/benzene 7 3]. Further elution with benzene followed by removal of the solvent gave the product (the esters as pale yellow oils, the amides as crystalline solids), which were further purified by vacuum distillation or by recrysiallization. 

Except in simple cases, it is very difficult to predict the infrared absorption spectrum of a polyatomic molecule, because each of the modes has its characteristic absorption frequency rather than just the single frequency of a diatomic molecule. However, certain groups, such as a benzene ring or a carbonyl group, have characteristic frequencies, and their presence can often be detected in a spectrum. Thus, an infrared spectrum can be used to identify the species present in a sample by looking for the characteristic absorption bands associated with various groups. An example and its analysis is shown in Fig. 3. 

Solvent — The transition energy responsible for the main absorption band is dependent on the refractive index of the solvent, the transition energy being lower as the refractive index of the solvent increases. In other words, the values are similar in petroleum ether, hexane, and diethyl ether and much higher in benzene, toluene, and chlorinated solvents. Therefore, for comparison of the UV-Vis spectrum features, the same solvent should be used to obtain all carotenoid data. In addition, because of this solvent effect, special care should be taken when information about a chromophore is taken from a UV-Vis spectrum measured online by a PDA detector during HPLC analysis. 

In contrast to the dihalogens, there are only a few spectral studies of complex formation of halocarbon acceptors in solution. Indeed, the appearance of new absorption bands is observed in the tetrabromomethane solutions with diazabicyclooctene [49,50] and with halide anions [5]. The formation of tetrachloromethane complexes with aromatic donors has been suggested without definitive spectral characterization [51,52]. Moreover, recent spectral measurements of the intermolecular interactions of CBr4 or CHBr3 with alkyl-, amino- and methoxy-substituted benzenes and polycyclic aromatic donors reveal the appearance of new absorption bands only in the case of the strongest donors, viz. Act = 380 nm with tetramethyl-p-phenylendiamine (TMPD) and Act = 300 nm with 9,10-dimethoxy-l,4 5,8-... 

A method that is stated to be applicable to residues of benzene hexachloride (20) is based on the fact that benzene hexachloride yields essentially 1,2,4-trichlorobenzene on dehydrohalogenation with alkali. This product possesses a characteristic absorption band in the ultraviolet, which permits its quantitative determination. 

Similar vivid colorations are observed when other aromatic donors (such as methylbenzenes, naphthalenes and anthracenes) are exposed to 0s04.218 The quantitative effect of such dramatic colorations is illustrated in Fig. 13 by the systematic spectral shift in the new electronic absorption bands that parallels the decrease in the arene ionization potentials in the order benzene 9.23 eV, naphthalene 8.12 eV, anthracene 7.55 eV. The progressive bathochromic shift in the charge-transfer transitions (hvct) in Fig. 13 is in accord with the Mulliken theory for a related series of [D, A] complexes. 

Fig. 13 Charge-transfer absorption bands from dichloromethane solutions containing Os04 and various (a) benzene, (b) naphthalene, and (c) anthracene donors (as indicated) showing the progressive bathochromic shift with aromatic donor strength. Reproduced with permission from Ref. 96b.

In the cyclophane 1, although the overlap between the n-system (2p) and the bridging cr-bonds (2s2p) is most effective, these orbital energy levels match worst, the first ionization potentials being 9.25 eV for benzene and 12.1 eV for ethane. As a result, the HOMOs are the almost pure it MOs with the b2g and b3g combinations. Both the PE spectrum and theoretical calculation demonstrate the degeneracy of the two HOMO levels. The absorption bands are attributed to the 17-17 transitions associated with the HOMOs. 

Among the cyclophanes 12, 17, and 22, the absorption band of 22 appears at the longest position. However, the bathochromic shifts in these cyclophanes should be compared with the corresponding acyclic compounds l,4-bis(pentamethyldisilanyl)benzene 23, l,4-bis(penta-methyldigermanyl)benzene 24, and l,4-bis(pentaisopropyldistanna-nyl)benzene 25 (Fig. 13). 

Figure 8.32 A broad, fairly strong absorption band at 3300 cm-1 indicating an alcohol. No benzene ring absorptions or carbonyl group. It is an aliphatic alcohol. 

Figure 8.33 Strong, sharp band at 1700 cnY1 indicating a carbonyl group. Absorption bands on the high side of 3000 cnY1 and a series of weak bands between 1700 and 2000 cnY1 indicating a benzene ring. Possibly benzaldehyde, or a similar compound.

Figure 8.34 Strong, sharp absorption at 1700 cm 1 indicating a carbonyl group. No other significant patterns except the C-H pattern on the low side of 3000 cm . It is an aliphatic aldehyde or ketone. Figure 8.35 A benzene ring is indicated because of the band on the high side of 3000 cm-1 and the series of weak peaks between 1700 and 2000 cm . Aliphatic C-H bonds are also indicated (absorption bands on the low side of 3000 cm-1). Possibly ethylbenzene, or a similar structure.

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