N—H Stretching Vibrations

1 N—H Stretching Vibrations In dilute solution in nonpolar solvents, primary amides show two moderately intense NH stretching frequencies corresponding to the asymmetrical and symmetrical NH stretching vibrations. These bands occur near 3520 and 3400 cm-1, respectively. In the spectra of solid samples, these bands are observed near 3350 and 3180 cm-1 because of hydrogen bonding. 

In IR spectra of secondary amides, which exist mainly in the trans conformation, the free NH stretching vibration observed in dilute solutions occurs near 3500-3400 cm-1. In more concentrated solutions and in solid samples, the free NH band is replaced by multiple bands in the 3330-3060 cm-1 region. Multiple bands are observed since the amide group can bond to produce dimers with an s-cis conformation and polymers with an s-trans conformation. 

2 C=0 Stretching Vibrations (Amide I Band) The C=0 absorption of amides occurs at lower frequencies than normal carbonyl absorption due to the resonance effect (see Section 3.6.10.1). The position of absorption depends on the same environmental factors as the carbonyl absorption of other compounds. 

Primary amides (except acetamide, whose C=0 bond absorbs at 1694 cm-1) have a strong amide I band in the region of 1650 cm-1 when examined in the solid phase. When the amide is examined in dilute solution, the absorption is observed at a higher frequency, near 1690 cm-1. In more concentrated solutions, the C=0 frequency is observed at some intermediate value, depending on the degree of hydrogen bonding. 

Simple, open-chain, secondary amides absorb near 1640 cm-1 when examined in the solid state. In dilute solution, the frequency of the amide I band may be raised to 1680 cm-1 and even to 1700 cm-1 in the case of the anilides. In th anilide structure there is competition between thering and the C=0 for the non-bonded electron pair of the nitrogen. 

FIGURE 2.27. Benzoic anhydride. Aromatic C—H stretch, 3067,3013 cm Asymmetric and symmetric 0=0 coupled stretching, respectively 1779,1717 cm1. See Table 2.6. C—CO—O—CO—C stretch, 1046 cm 1. 

FIGURE 2.28. Acrylamide. N—H stretch, coupled, primary amide, hydrogen bonded asymmetric, 3352 cm-1 symmetric, 3198 cm-1. Overlap C=0 stretch, amide I band, 1679 cm see Table 2.6. N—H bend, amide II band, 1617 cm-1. C—N stretch, 1432 cm-1. Broad N—H out-of-plane bend 700-600 cm-.  

---

C-nmr data have been recorded and assigned for a great number of hydantoin derivatives (24). As in the case of H-nmr, useful correlations between chemical shifts and electronic parameters have been found. For example, Hammett constants of substituents in the aromatic portion of the molecule correlate weU to chemical shifts of C-5 and C-a in 5-arylmethylenehydantoins (23). Comparison between C-nmr spectra of hydantoins and those of their conjugate bases has been used for the calculation of their piC values (12,25). N-nmr spectra of hydantoins and their thio analogues have been studied (26). The N -nmr chemical shifts show a linear correlation with the frequencies of the N—H stretching vibrations in the infrared spectra. 

The broadband at 3270 cm-1 is due to the O-H stretching vibration of the hydroxyl group. Moreover, the N-H stretching vibration absorption for open-chain amides occurs near 3270 cm-1 in the niclosamide solid state. 

The IR spectra recorded by the fluorescence detection technique, where the population of a species in the ground state is monitored by laser-induced fluorescence with UV laser excitation, can be used successfully to detect dihydrogen bonds in the gas phase. It has been found, for example, that the v(O-H) and v(N-H) stretching vibrations in such IR experiments are very sensitive to dihydrogen bonding. 

Table 10 N-H Stretching vibrations of some A-amino <88SA(A)283>.

In lactams of medium ring size, the amide group is forced into the s-cis conformation. Solid lactams absorb strongly near 3200 cm-1 because of the N—H stretching vibration. This band does not shift appreciably with dilution since the s-cis form remains associated at relatively low concentrations. 

N—H Stretching Vibrations The ammonium ion displays strong, broad absorption in the 3300-3030 cm-1 region because of N—H stretching vibrations (see Fig. 3.24). There is also a combination band in the 2000-1709 cm-1 region. 

Sodium salts of amino acids show the normal N—H stretching vibrations at 3400-3200 cm-1 common to other amines. The characteristic carboxylate ion bands appear near 1600-1590 cm-1 and near 1400 cm-1. 

Abstract—The relative shifts in the frequency of the N—H stretching vibration were measured for a number of amines in various solvents, particularly those in which there is strong interaction. 

Triazine-3,5-diones (33) show two N—H stretching vibrations at 3378 and 3374 cm-1 which were assigned to H-4 and H-2 according to comparisons with the 2- and 4-methyl derivatives. Also two bands in the carbonyl region were observed with 1,2,4-triazine-3,5-diones at 1730 and 1700 cm-1. The first band is shifted to 1723-1720 cm-1 if N-2 is alkylated, while the second band is shifted to 1687-1680 cm-1 when N-4 is alkylated. IR spectroscopy can therefore be used to determine the position of substituents in 1,2,4-triazine-3,5-diones (61CCC1680). 

A subsequent examination of this material (633) did not show the expected N-H stretching vibrations in the IR spectrum. Furthermore, thin-layer chromatography of the yellow crystals revealed the presence of two components. Further studies on this compound are needed before an unambiguous characterization can be claimed. 

Amines. In dilute solution primary amines show two absorption bands, one near 3500 cm -1 and the other near 3400 cm-1, arising from the asymmetric and symmetric stretching vibrations of the two NH bonds (cf. the vibrations of a methylene group, Fig. 3.1). Secondary amines show just one band near 3300 cm-1 due to the single N—H stretching vibration, while tertiary amines do not absorb in this region. These characteristic absorptions allow one to dis-... 

In dilute solution primary amides show two sharp bands resulting from the asymmetric and symmetric N—H stretching vibrations near 3520 cm-1 and 3400 cm-1 (the normal N—H region). In solid samples these appear near 3350cm-1 and 3180cm-1. In dilute solution secondary amides show only one band near 3460-3420 cm-1 on low-resolution instruments. However, under conditions of high resolution the band can frequently be split into two components which have been assigned to the cis and trans rotational isomers. 

The more complex a-amino acids additionally show characteristic bands which aid in their identification. Thus in the spectrum of L-tryptophan (Fig. 3.37) the N—H stretching vibration, and the out-of-plane hydrogen wag deformation which appears at 742 cm-1 (four adjacent hydrogens), readily allow it to be distinguished from other a-amino acids. 

In what follows, the LNT spectra will mainly be considered and the attention will be focused on the high-frequency region where bands due to the X-H (O- H or N H) stretching vibrations are expected to appear. 

In spectrum (b), the symmetrical Si-NH2 stretching vibration (3425 cm1) exhibits a tailing, which becomes a prominent shoulder in spectrum (d). This is due to a band in the region 3400 - 3350 cm1, which can be assigned to the N-H stretching vibration of silazane species. Amine species are progressively converted to silazane species as the reaction temperature rises. 

Figure 12.10 shows the FTIR spectrum of a trichlorosilylated silica gel, ammoniated at 1023 K. This spectrum confirms the above statements. The free hydroxyl band is caused by reaction (B). The symmetric Si-NH2 band (3425 cm 1) exhibits a distinct tailing, due to the Si-NH-Si (N-H stretching) vibration (3400 - 3350 cm 1). 

Comparison of the integrated peak areas of the primary and secondary N-H stretching vibrations revealed that at reaction temperatures of 650 K, the secondary reaction (O) occurs for more than 50%. 

The weaker 0-H stretching vibrations arising primarily from polar head groups of the lipids absorb at higher wavenumbers ( 3400 cm-1) than the hydrogen bonded N-H stretching vibrations ( 3300 cm-1) associated primarily with the keratinized proteins. 

The 0-H stretching and N-H stretching bands form the broad band between 3500 and 3200 cm-. Infrared bands associated with the 0-H and N-H stretching vibrations shift towards higher wavenumbers as the hydrogen bond decreases in energy and is eventually lost. 

---