Group frequencies benzene ring

We shall also find good group frequencies for rings, such as those in benzene compounds. In most instances the vibration involves a CH group, so that again the small mass of hydrogen is the prime factor which makes these vibrations retain fairly constant spectral positions. ... 

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. 

The stretching frequency of the Ge—O bond in Ge(OCgH2(CH2NMe2)3-2,4,6)2 recorded in benzene solution, has been reported to lie at 1040 cm-1143. However, this germylene is additionally stabilized by intramolecular coordination of the Ge center by the N atoms of two dimethylaminomethyl groups attached to different benzene rings in the ortho-positions and in fact can be considered as an intramolecular donor-acceptor complex143. 

In tlie aliphatic nitro derivatives, Raman frequencies characteristic for the nitro group occur at 1,383 cm and 1,555 cm These values are between those observed in compounds containing a single N—O bond (approximately 1,000 cm ) and those containing a double N=0 bond (approximately 1,640 cm ). In nitrobenzene these characteristic frequencies are lower, being 1,345 cm i and 1,523 cm respectively, which is most probably due to molecular resonance with the benzene ring and the contribution of the structure ... 

Extensive correlation tables and discussions of characteristic group frequencies can be found in specialized references. As one example, consider the band patterns of toluene, and of o-, m-, and p-xylene, which appear in the frequency range 2000 to 1650 cm" (Fig. 2). These band patterns are due to changes in the dipole moment accompanying changes in vibrational modes of the aromatic ring and are surprisingly similar to those for monosubstituted and other o-, m-, and p-disubstituted benzenes. 

Some chemical structures exhibit typical distances that occur independently of secondary features, which mainly affect the intensity distribution. In particular, aromatic systems contain at least a distance pattern of ortho-, meta-, and para-carbon atoms in the aromatic ring. A monocyclic aromatic system shows additionally a typical frequency distribution. Consequently, Cartesian RDF descriptors for benzene, toluene, and xylene isomers show a typical pattern for the three C-C distances of ortho-, meta-, and para-position (1.4, 2.4, and 2.8 A, respectively) within a benzene ring. This pattern is unique and indicates a benzene ring. Additional patterns occur for the substituted derivatives (3.8 and 4.3 A) that are also typical for phenyl systems. The increasing distance of the methyl groups in meta- and para-Xylene is indicated by a peak shift at 5.1 and 5.8 A, respectively. These types of patterns are primarily used in rule bases for the modeling of structures explained in detail in the application for structure prediction with infrared spectra. 

IR measures the frequencies of molecular vibrations which depend on the masses of atoms and the force constants (i.e. the stiffness ) of chemical bonds (see Topic C8). Spectra can be measured for pure gaseous and liquid samples, but solids are usually measured by grinding them to make a mull with a heavy hydrocarbon liquid ( nujol ) which has relatively few, and well known, IR bands. Many types of chemical bond, such as C-H and C=0, give bands with characteristic IR frequencies and can thus be identified. In the case of compound X discussed above, bands appear which are characteristic of aromatic C-H bonds (suggesting a C6H6 benzene ring) and of C=0 groups bound to... 

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