Overtone aliphatic amines

Primary amines show characteristic first overtone bands at 1450, 1550 and 1000 nm which represent the first and second overtones of N-H stretching bands. Tertiary amines produce no N-H overtones, but do produce C-H and 0-H stretching combination bands at 1260-1270 nm. The first overtones of the N-H stretching bands of aliphatic amines are shifted to lower wavelengths. Primary amides may be characterized by combination bands at 1930-2250 nm and an N-H stretching overtone band at 1450-1550 nm. Secondary amides produce overtone bands in the 1350-1550 nm region and combination bands in the 1990-2250 nm region. 

In the mid-infrared region, the amine NH-stretch absorption is weak relative to primary alcohols, about 1-2 1/mol-cm compared to 50-100 1/mol-cm. However, the intensity of the first overtone of aliphatic amines is of the same order of magnitude as the fundamental. For example, n-butyl amine s first overtone has an absorptivity of 0.61/mol-cm compared to 2.41/mol-cm for the fundamental. The overtones of amines and hydroxyls are of approximately the same magnitude, and it may be easier to detect an amine in the presence of alcohols in the near-infrared than the mid-infrared. 

The NH-stretching bands of aromatie amines such as aniline show a doublet in the first overtone region, but it is shifted to lower wavelengths relative to aliphatic amines. Figure 8.2 compares the near-infrared spectrum of aniline in CCI4 with that of n-butyl amine. The asymmetric vibration occurs... 

As in the case of the aliphatic amines, secondary aromatic amines show only one NH-stretching overtone. For A-butyl aniline in CCI4, for example, the overtone is at 6675 cm" (1498 nm). The effects of hydrogen bonding on the mechanical anharmonicity of the NH-stretching vibration of N-methylaniline have been reported."... 

In heterocyclic aromatic amines such as pyrrols, indoles, and carbazoles, Ihoe is a first overtone NH-stretching between 6803 cm (1470 nm) and 6897 cm" (1440 nm)." Unlike the aliphatic amines, where the intensity of the overtone is approximately the same as the intensity of the fundamental, the aromatic and heterocyclic amines have a much greater absorptivity difference between fundamental and overtone. The overtone intensities do not vary widely, but the intensities of the fundamentals vary by a factor of about 100 from aliphatic to heterocyclic. Table 8.1 lists some of the intensities, given in integrated peak area. 

FIGURE 2.29. 2-Methyl-l,5-pentanediamine. N—H stretch, hydrogen-bonded, primary amine coupled doublet asymmetric, 3368 cm-1. Symmetric, 3291 cm. (Shoulder at about 3200 cm-1, Fermi resonance band with overtone of band at 1601 cm-1. Aliphatic C—H stretch, 2928,2859 cm-1. N—H bend (scissoring) 1601 cm-1. SsCH2 (scissoring), 1470 cm-1. C—N stretch, 1069 cm-1. N—H wag (neat sample), —900-700 cm-1. 

NIR spectrophotometry in the region from 8000 to 4000 cm-1 was used to measure the kinetics of copolymerization of an aromatic bismaleimide (72) derived from an aromatic diamine (e.g. 5a), taking place at 160 to 180 °C. The following NIR spectral ranges were useful for this study primary amine first overtones (vn h) at 7000 to 6400 cm-1, double bond first overtone (vc=c-h) at 6100 cm-1, aromatic first overtones (vc-h) at 6000 to 5750 cm-1, aliphatic first overtones (vc-h) at 5750 to 5350 cm-1 and primary aromatic amine combination bands first overtones (vn h + <5nh2) at 5150 to 4800 cm-1. The process consisted mainly of a second-order Michael addition, as depicted in equation 14, and not the plausible imide opening to yield a maleic dianilide (119), as shown in equation 15. A Michael addition between maleimide moieties and secondary amine moieties present in the products (118) also takes place, however at a rate of about one fourth of that of the primary amine moieties. To improve the SNR of the measurements, usually the results of... 

There is some correlation between molecular structure and band position for certain bands, but because these are often overtone and combination bands, their positions are not as structure dependent as the fundamental bands in the mid-IR. For example, primary amines, both aliphatic and aromatic, have two absorption bands, one at about 1500 nm and the second at about 1990 nm. Secondary amines have only one band at about 1500 nm. As expected, a tertiary amine has no NH band. Amides with an -NH2 group can be distinguished from R-NH-R amides by the number and position of the N H bands. The reference by Goddu has a detailed table of NIR structure-wavelength correlations. 

The first overtone of the aliphatic NH symmedical stretch occurs in the 1500 nm region. Unlike the combination band, both primary and secondary amines absorb in this region. Interference from the first overtone of the OH stretching vibrations at 1400 nm is easily avoided with the high resolution available from NIR insuiiments. 

---