Aromatics overtone bands

D 1840-1940 cm Aromatic overtone region Quite clear fingerprint but does not reflect 2 band pattern proposed for p-disubstitution.- ... 

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. 

C-H bending vibrations Asym 1450 20 cm-1 1445 Aromatics overtones and Two bands... 

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 final spectrum, figure 9.24, is that of p-nitroaniline. Although complex it is immediately recognizable as being aromatic (C—H above 3 000 cm-1, skeletal and overtone bands between 1 400 and 2 000 cm-1 and a C—H out-of-plane bending vibrations at 835 cm-1 indicating / -substitution). Asymmetric and symmetric N=0 stretching bands are very prominent ai... 

Near-IR spectroscopy proved valuable for the analysis of pharmaceutical powders in a 1981 paper by Becconsall et al. [73]. Near-IR and UV photoacoustic spectroscopy were used for determination of propranolol (PR)/magnesium carbonate mixtures. Spectra were collected from 1300 to 2600 nm with carbon black as the reference. An aromatic C-H combination band at 2200 nm and an overtone band at 1720 nm were used to quantify PR. In this case, the UV data were nonlinear, while the NIR method provided a linear calibration. 

Many weak combination and overtone absorptions appear between 2000 and 1667 cm The relative shapes and number of these peaks can be used to tell whether an aromatic ring is mono-, di-, tri-, tetra-, penta-, or hexasubstituted. Positional isomers can also be distinguished. Since the absorptions are weak, these bands are best observed by using neat liquids or concentrated solutions. If the compound has a high-frequency carbonyl group, this absorption will overlap the weak overtone bands so that no useful information can be obtained from the analysis of the region. 

With regard to higher vibrational states, the second overtone of the aromatic C-H stretch in alkylated benzenes occurs at 8734 cm (1145 nm), whereas the alkyl C-H stretch occurs at 8389 cm" (1192 nm). Approximately 5- to 10-nm shifts were observed to lower frequency (longer wavelength) as compared with benzene s second CH overtone band. 

N-H (2v) symmetric, primary aromatic amine in CCI4 as para-NH2 grouping N-H (2v),. CONH2 N-H (2v) stretching bonded NH, disordered phase from polyamide 11 N-H (2v),. CONH2 N-H (2v), secondary amine as (R-NH-R), dimethylamine (vapor), first overtone band intensity comparisons, CCI4 solution C-H, methyne (1-hexyne) as (R-C-C=C-H)... 

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. 

Near-infrared spectrometry is a valuable tool for analyzing mixtures of aromatic amines. Primary aromatic amines are characterized by two relatively intense absorption bands near 1.97 and 1.49 pm. The band at 1.97 pm is a combination ofN—H bending and stretching modes, and the one at 1.49 pm is the first overtone of the symmetric N—H stretching vibration. Secondary amines exhibit an overtone band, but they do not absorb appreciably in the combination region. Secondary amines exhibit an overtone band, but they do... 

---