Benzene derivatives, absorption spectra

Although maleimides form ground-state complexes with benzene, such complexes are not responsible for the occurrence of the photoaddition. Their absorption spectra are hidden below the absorption spectrum of benzene and do not extend beyond 270 nm, yet excitation above 280 nm readily leads to photoaddition of maleimide and some of its /V-substi tutcd derivatives. The mechanism proposed by Bryce-Smith [38,46] accounts for all of the facts (Scheme 13). 

To determine the photoinduced structure, the orange crystals that were formed after irradiation of benzene solutions of 6-phenoxy-5,12-naphthacenequinone were used.46 It turned out that the elemental composition of the photoinduced form coincided with the ana-quinone composition. In addition, the interaction of 6-phenoxy-5,12-naphthacenequinone with ammonia and aniline in benzene resulted in the formation of compounds that were identified as derivatives of ana-naphthace-nequinone by the counter synthesis. The ana structure of the photoinduced form was supported by the similarity of the absorption spectrum of the ana form to the spectra of unsubstituted and 5-bromo derivatives of 6,12-naphthacenequinone46 as well as by the analysis of the IR spectra of initial and photoinduced forms of 6-phenoxy-5,12-naphthacenequinone. 4... 

The aromatic nature of the [18]porphyrin-(2.1.0.1) derivative 3.135, as well as analogs 3.136, 3.137, and 3.144 (systems we will hereafter refer to as corrphycenes), was confirmed by the observation of sustained diamagnetic ring current effects in its H NMR spectrum. The UV-vis absorption spectrum (in benzene) is also consistent with the proposed aromaticity and with an 18-7t-electron formulation. Specifically, a... 

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). 

Although the valence ji-ji excitation spectra of benzene derivatives have been extensively studied over the past 65 years both experimentally and theoretically, much less is known about that of phenol, apart from its lowest excited state. In general, absorption and fluorescence spectroscopy of a benzene ring can be used to detect its presence in a larger compound and to probe its environment. While the relative constancy of the valence jt-n excitation spectrum allows a qualitative identification of spectral bands by a correspondence with those in free benzene, detailed quantitative differences could indicate the nature of substituents, ligands or medium. Key information on substituted benzene includes the excitation energies, transition moments and their direction, and electrostatic... 

The most significant feature of the spectrum shown is the strong C=0 stretch at 1688 cm which is characteristic of aromatic carbonyl compounds (simple saturated carbonyl compounds have a C=0 stretch at about 1720 cm M The presence of a benzene ring can be confirmed by the moderate to strong bands at 1601/ 1585, 1500 and 1450 cm". These absorptions are characteristic of the C=C stretch of benzene derivatives. Further spectral evidence supporting the hypothesis of a carbonyl functionality can be seen by the two weak bands at 2750 cm" and 2820 cm . These absorptions are characteristic of the C-H stretch of aldehydes. Another important feature of this spectrum is the moderate band at 1050 cm which is the typical absorption of the C-0 stretch of ethers. We now know that compound P is an aromatic aldehyde with a possible ether linkage. 

Figure 7.15. Typical second-derivative absorption spectra of gaseous samples. A Spectrum of an automobile exhaust. B Spectrum of 14 ppm benzene. Spectra courtesy of Lear Siegler Inc., Environmental Technology Division, Englewood, Colorado.

Electronic Absorption Spectroscopy. Doping with iodine and SO3 had a significant effect on the absorption spectrum of /3-carotene. Triplet-triplet absorption spectra have been obtained for six carotenoids, e.g. canthaxanthin carotene-4,4 -dione (181)] in benzene, " and bimolecular rate constants for energy transfer from singlet oxygen to carotenoids evaluated. U.v. spectra of retinal, retinyl acetate, and axerophtene (182) in solid films have been determined. Several papers discuss the light absorption spectra of retinal derivatives as rhodopsin models. ... 

The hexane spectrum (Figure 14.15) shows that the C-H stretch at 2850-2960 cm-i and the signals at 1350-1470 cm-i correlate with the absorptions in Table 14.3. Benzene derivatives and other aromatic compormds usually show absorption for the C-H units at 3000-3100 cm" but also at 675-870 cm i (in the fingerprint region). Note the subtle shift of the C-H absorption to lower energy for the aromatic compounds. Other compounds that have a C-H absorption are those for alkenes and alkynes. The CsC-H absorption is at lower energy than the C=C-H absorption, which is lower in energy than the C-C-H absorption for alkanes. 

Aromatic hydrocarbons such as benzene and substituted benzene derivatives have a few distinguishing features in the infrared. This section concludes with the spectra of benzene (Figure 14.21A, an old-style infrared spectrum) and sec-butylbenzene (2-phenylbutane, Figure 14.21B). Benzene has C-H absorptions that are clearly evident, as well as a C=C absorption at about 1600-1630 cm. There is little to indicate that this is an aromatic hydrocarbon. Note the very weak signals at about 1750-2000 cm. These are known as C-H overtone absorptions, and they usually appear only with benzene derivatives. They are very weak and may easily be missed. 

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