Aromatic CH wag

The =CH wag is doubly degenerate when the substituent has axial symmetry, but when a plane of symmetry is present, the in-plane and out-ofplane =CH wag bands separate into a doublet. Phenyl acetylene has these bands at 648 cm (in-plane wag) and 611 cm (out-of-plane wag). Also in common with C=CH2 wag and aromatic CH wag, the C=CH wag band has a relatively prominent broad overtone at 1375-1225 cm . ... 

The in-phase, out-of-plane aromatic CH wag vibrations give rise to strong infrared bands which are arranged according to the number of adjacent hydrogens in Table 8.2. The general usefulness of the adjacent hydrogen wag correlation is illustrated by the fact that it can be extended to include... 

The total electron densities on the ring carbons have been calculated for various polysubstituted benzenes from molecular orbital theory. It was found that various substituents tend to make additive contributions to the excess total electron densities on the ring carbons. This means that most aromatic CH wag wavenumbers can be calculated approximately as a linear combination of the wavenumber P(900 cm ) of the phenyl groups whose substituents are like those in the poly-substituted benzenes.The procedures are listed in Table 8.5 along with the observed ranges for the particular group and the standard deviations between the calculated and observed wavenumbers. 

There are two types of aromatic CH wag that do not correlate well with j/(900 cm ), namely, the 2-adjacent CH wag of para and 1,2,4 trisubstituted benzenes in thS 820 cm region, and the 5-adjacent CH wag of monosubstituted benzenes in the 750 cm" region. Most likely, these two show the same kind of sensitivity to mechanical interaction with the substituents, possibly because these modes have a small out-of-plane motion of the substituted ring carbons. Some examples of 5-adjacent CH wag wavenumbers for various phenyl groups are listed in Table 8.6. 

The =CH wag gives rise to a broad, strong IR band at 700-578 cm" In common with aromatics and CH=CH2 groups, the CH wag frequency is raised or lowered when substituents decrease or increase the total electron density on the CH carbon. 

Aliphatic and aromatic isothiocyanates (—N=C=S) give rise to a very strong band at 2150-2050 cm as a result of the out-of-phase stretching of the NCS bonds. A shoulder usually appears at 2221-2150 cm In compounds with an adjacent CH2 or CH group (R—CH2 N=C=S) the CH2 or CH wagging gives rise to a strong band at 1347-1318 cm" The in-phase NCS vibration absorbs at 945-925 cm" in aryl isothiocyanates and 700-650 cm in akyl isothiocyanates. ... 

Pl etc. stands for the monosubstituted benzene ring CH wag wavenumber near 900 cm" (from Table 8.4), where the substituent is like the substituent in position number 1 in the aromatic considered. 

AROMATIC AND HETEROAROMATIC RINGS TABLE 8.6 Approximate Phenyl 5-Adjacent CH Wag Bands in cm ... 

Aromatic chlor Bromine comp< -CHjCHjBr ne compounds unds ( -sensitive bands m- m m m para — ortho CH wagging m-s -s... 

Hydrogen bending vibrations absorb from 1600 to 700 cm Bending vibrations for CH2 and CH3 absorb from 1500 to 1350 cm and very useful out-of-plane CH wag vibrations in olefins and aromatics absorb from 1000 to 700 cm In olefins these can be used to distinguish vinyls, vinylidines, and cis and trans 1,2-disubstituted... 

Sulphones -CH2OH Aromatic disul -C=SNH.CH. )hides 1 m-s SO > wagging vib ... 

---