N—H absorption bands

Brown and Mason converted the hydrates into alcoholates by boiling them with alcohol. The hydrate and alcoholate of 6,7-diethyl-2-hydroxypteridine showed two N—H absorption bands in the infrared, and, when either compound was heated at 120° in vacuo, the band of higher frequency was strongly reduced in intensity. These results led to a new test for covalent hydration the substance is refluxed with... 

Table 2. Shifts of the N—H absorption bands on adsorption on alumosilicagel...

The preference of the peptide group, found in N-methylacetamide, to associate selectively with water rather than with itself has been demonstrated through investigation in the near-infrared region by spectroscopic investigation following the N—H absorption band in the 1.5-micron region (8). 

The shape of an absorption band can be helpful in identifying the compound responsible for an IR spectrum. For example, both O — H and N—H bonds stretch at wavenumbers above 3100 cm but the shapes of their stretches are distinctive. Notice the difference in the shape of these absorption bands in the IR spectra of 1-hexanol (Figure 13.19), pentanoic acid (Figure 13.20), and isopentylamine (Figure 13.25). An N—H absorption band ( 3300cm ) is narrower and less intense than an O — H absorption band ( 3300 cm ), and the O—H absorption band of a carboxylic acid ( 3300-2500 cm ) is broader than the O — H absorption band of an alcohol (Sections 13.9 and 13.10). Notice that two absorption bands are detectable in Figure 13.25 for the N—H stretch because there are two N—H bonds in the compound. 

The IR spectra of compounds (41) display a N—H absorption band around 3010 cm and two carbonyl absorption bands at ca. 1710 cm and 1645 cm indicative of no hydroxyl form of the compounds <88CPB4403). 

Brunette et al investigated the changes in the frequency, bandwidth, and intensity of the bonded N-H absorption band following heat treatment in model PUs and correlated them to structural changes as evidenced by DSC [147],... 

The N—H functional group of amines is also easy to spot in the IR, with a characteristic absorption in the 3300 to 3500 cm-1 range. Although alcohols absorb in the same range, an N—H absorption is much sharper and less intense than an O-H band. 

The IR spectrum of aniline (Figure 3.7) shows a typical N-H absorption pattern for a primary amine, with stretches at 3432, 3355 and 3214 cm , while that of paracetamol (Figure 3.4) shows the single stretch observed for a secondary amide at 3325 cm and we can also see the contrast between this sharp peak and the much broader O-H at 3161 cm . The spectrum of benzocaine (Figure 3.8) also shows the three bands at 3423, 3342 and 3221 cm which characterize a primary amine. 

Hydroxypyrrolo[3,2-c]pyridine (13) exists substantially in the carbonyl form shown. In the IR spectrum there is no detectable O—H absorption bands at 3150 and 3250 cm-1 are assigned to two N—H frequencies. The carbonyl band is at 1648 cm-1 (s). Several 2-hydroxypyrrolopyridines have been examined and most show the presence of both tautomers but the equilibrium position varies considerably. Compound (14) has a strong... 

Amine salts and also amino acids are characterised by strong absorptions between 3200 and 2800 cm-1 due to the N—H stretching bands of the ions... 

Irradiation of L MRe(CO)3(L-L) (L M = (COlgMn, (COlgRe, PhaSn, MeaSn a diimine ligand (L-L) coordinated on one of the metal centers) complexes also causes Re—M bond homolysis, but the reaction pathway is different. The longest wavelen h absorption band of this type of complex was attributed to a MLCT(Re dn n (L-L)) transition. Stufkens et al. reported that the Re—M bond homolysis proceeds via the ab n (L-L) excited state produced by internal conversion from the MLCT excited state 36). 

The IR spectra of all LC fractions from both Synthoil and shale oil contain a very intense feature at 750 cm and a less intense band at 820 cm". These bands, together with a strong feature at 3495 cm", are indicative of N-H stretching vibrations, thus implying the presence of amines and/or N-heterocyclics in substantial quantities in these samples. Indeed, previous analyses (26) of Synthoil have demonstrated the presence of relatively high concentrations of indole (210 ppm) and 3-methyhndole (130 ppm). However, a careful examination of MI fluorescence spectra of indole and a number of substituted indoles shows no match with any of the major unknown bands in the MI fluorescence spectra of the various Synthoil and shale oil fractions (27). Thus, it remains to identify the unknown fluorescent constituents of these samples, and the identities of the compounds responsible for the intense N-H absorptions in the FTIR spectra remain to be established. 

The partial IR spectrum of acid Subfraction 1 shows IR absorption at 3460 cm because of the pyrrolic nitrogen N-H absorption of carba-zole-like compounds. Amide carbonyl absorption appears at 1685 cm" The partial IR spectrum of acid Subfraction 2 shows the same two IR bands and additional bands at 3585 cm and 1650 cm owing to phenols and a second amide type. The partial IR spectrum of acid Subfraction 3 shows phenol absorption at 3585 cm S pyrrolic nitrogen absorption at 3460 cm S and strong carbonyl absorption at 1695 cm and 1725 cm characteristic of carboxylic acid dimers and monomers. In addition, absorption of hydrogen-bonded carboxylic acid and phenolic hydroxyl groups can be seen in the region of 3500-2300 cm" ... 

The partial IR spectrum of base Subfraction 1 shows N-H absorption at 3460 cm S amide absorption at 1690 cm S and aromatic absorption at 1600 cm. Base Subfraction 2 shows increased amounts of aromatic absorption at 1600 cm and an additional band at 1720 cm S which is thought to be an amide carbonyl absorption. Subfraction 3 shows N-H absorption at 3460 cm S amide absorption at 1685 cm S and large amounts of aromatic absorption at 1600 cm that shows asymmetry typical of pyridine benzologs. Strong bases such as pyridine benzologs appear to be the predominant basic compound type in Subfraction 3. 

In addition to looking for characteristic N-H absorptions, there s also a simple trick for telling whether a compound is an amine. Addition of small amount of HCl produces a broad and strong ammonium band in til 2200-3000 cm" range if the sample contains an amino group. All protS nated amines show this readily observable absorption caused by the anuno nium RjN-H bond. Figure 24.9 gives an example. 

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