Combination bands substituted aromatics

Weak combination and overtone bands appear in the 2000-1650 cm-1 region. The pattern of the overtone bands is not a reliable guide to the substitution pattern of the ring. Because they are weak, the overtone and combination bands are most readily observed in spectra obtained from thick samples. The spectrum of Figure 3.13 is that of a typical aromatic (benzenoid) compound. 

Industrially manufactured polystyrene, polymerized with radical initiators, exists mainly as the atactic polymer. The IR spectrum of this form of polystyrene features bands consistent of a mono-substituted aromatic compound. In addition to the normal aromatic absorptions, such as the C—H stretching between 3110 and 3000 cm , and the characteristic ring vibrations at approximately 1600 and 1500 cm (1601 and 1493 cm for polystyrene), the spectrum also contains the combination bands characteristic of monosubstitution (at 1942, 1868, 1802, and 1741 cm ), and the out-of-plane C —H ring deformations for monosubstitution (757, 699, and 541 cm ). Note that the C—H stretching reflects the alternating methylene... 

Infrared Analysis. The infrared spectrum of 5-nitrosalicylic acid (Fig. 6.26 on page 238) is characteristic of aromatic carboxylic adds (see discussion in Experiment [7]). Note that (1) the substitution of the ring is revealed by the presence of the 1,2,4-combination band pattern with peaks at 1940,1860, and 1815 cm (2) the strongest band in the spectrum below 1750 cm is assigned to the symmetric stretch of the —NO2 group found at 1339 cm , and (3) the conjugated carboxyl C=0 stretch is located at 1675 cm . ... 

Substitution on the benzene ring reduces the symmetry of the molecule, potentially allowing for additional vibrational peaks. In the case of alkyl aromatics, there is also the addition of the alkyl absorptions. Figure 4.2 compares the near-infrared spectra of benzene and toluene. The primary differences appear to be the addition of the methyl combination bands near 4300 cm (2300 nm) and the methyl first overtones near 5800 cm (1750 nm). There is also a noticeable peak at 3836 cm (2607 nm) that could be related to the sum of a C-H stretch and one of the ring-wagging vibrations involving a ring with five adjacent protons. 

Aromatic compounds Monosubstituted Q- 3100-3000 1600-1500 770-730 (s) 710-690 (s) All show weak combination and overtone bands between 2000 and 16,500 cm-1 see aromatic substitution pattern chart... 

The pattern of bands in this region shown by various substituted benzene ring compounds is illustrated in Figure 5-26. It can be seen that a distinct pattern is found for each type of aromatic substitution. For example, a monosubstituted benzene derivative shows a series of four maxima beginning at about 1880 cm" The assignments of these frequencies to combination and overtone bands have been given by Kakiuti and Whiffen [ ]. Since for most compounds these are weak bands, a thicker cell or more concentrated solution is used if this region is to be examined carefully. 

Most aromatic chloro and bromo compounds have strong absorptions at 760-395cm" (13.10-25.32pm) and 650-395cm" (15.38-25.32pm) respectively, which is due to a combination of vibrational modes. Monosubstituted benzenes, dihalogen-substituted benzenes, and compounds with electron-donor or methyl substituents in the para position of halobenzenes all exhibit the former band. 

Figure 6 is from benzyl alcohol (Ph-CH2-OH). The hydroxyl bands are dominant despite only one O-H to seven C-H vibrators this is due mainly to the effects of hydrogen bonding. The aromatic bands are all consistent with or indicative of single substitution, including four from combinations of lower-frequency modes (not labelled) between 2000 and 1700 cm F These latter are usually too weak to be of use in any other than simple molecules. 

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