N—H Stretching Frequencies

Section 22 19 The N—H stretching frequency of primary and secondary amines appears m the infrared m the 3000-3500 cm region In the NMR spectra of amines protons and carbons of the type H—C—N are more shielded than H—C—O... 

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

In their studies of the effect of solvent upon the N—H stretching frequency in pyrrole, Fuson and Josien [1] have shown the distinction between the solvent-solute interaction which is a function of dielectric constant alone [2, 3] and that which is more specific, involving N—H hydrogen bonding. The most pronounced frequency shifts are those caused by pyridine [4] (K—M N bonding) and by acetone (N—H 0 bonding). The choice of pyrrole for these studies was presumably partly governed by convenience since the N—H band in pyrrole is considerably more intense than in the more basic secondary amines. We have attempted an extension of this work in two directions ... 

Table 3, N—H stretching frequencies (cm""1) in N-methyianilines Carbon tetrachloride solutions, 0 05 M in 0 4 mm cells...

In the gas-phase IR spectrum of piperidine, two bands with Q-branch maxima at 6577 and 6499 cnF1 occur, corresponding to first overtone N-H stretching frequencies. Because stretching frequencies of equatorial substituents are usually higher than those of axial substituents, it seemed probable that the more intense 6577 cm-1 band arose from the N-H equatorial con-former 93. Support for this assignment came from a detailed study of the PQR band shapes (Fig. 7). 

A strong carbonyl absorption is evident in the spectra of all amides, although the frequency of absorption varies somewhat with the structure of the amide. Thus primary amides generally absorb near 1680 cm 1, whereas secondary and tertiary amides absorb at slightly lower frequencies. The N—H stretching frequencies of amides are closely similar to those of amines and show shifts of 100 cm-1 to 200 cm 1 to lower frequencies as the result of hydrogen bonding. Primary amides have two N—H bands of medium intensity near 3500 cm 1 and 3400 cm 1, whereas secondary amides, to a first approximation, have only one N—H band near 3440 cm 1. However, a closer look reveals that the number, position, and intensity of the N—H bands of mono-substituted amides depend on the conformation of the amide, which can be either cis or trans ... 

A straightforward example of the type of information we can obtain from the vibrational spectra may be obtained from the behavior of the N-H stretching frequencies. In Table V the 10 N-H stretching frequencies in the aR helical conformation and the C7eq conformation are given as well as the residue in which they occur. We note first of all that the vibrational frequency of the N-H is extremely sensitive to the location of the residue in the chain. 

Further insight into histidine encapsulation of nanoclusters (Au, Ag, Cu, Pt) was provided by infrared spectroscopy. IR spectra of free histidine showed a set of stretching bands for the N-H bond of the amine and strong peaks (1630 cm, 1413 cm ) attributed to an asymmetrical and symmetrical C02 stretch. As a result of nanocluster stabilization and the subsequent formation of a metal-N(His) interface, the N-H stretching frequencies broadened and the number of bands decreased relative to free histidine due to the interactions with the dielectric field at the nanocluster surface. ... 

In 1960, Bryson and Werner carried out an extensive study on N—H stretching frequencies of substituted 1- and 2-naphthylamines. These frequencies were tentatively related to the pKa. values of the naphthylamines52. 

In 1964, Lady and Whetsel focused on the problem of the first overtone N—H stretching bands of 52 ortho-substituted anilines, in cyclohexane and in CCI4, in order to evaluate the intramolecular hydrogen bond between N—H and ortho substituents54. The N—H stretching frequencies in ortho-substituted anilines have also been discussed by G. Moritz55. 

The N—H stretching frequencies have been reported for a wide variety of... 

The calculated and experimental results, shown in Table 1, closely follow the same trend. The cooperative effects are demonstrated by the N - H stretching frequencies. Relative to Id, ab initio calculation indicates that the formation of a two-center H-bond in a five-membered ring in lc produces a small blueshift of the N - H stretching frequency, indicating the weakness of this two-center H-bond which alone does not have the strength to produce the expected redshift. IR experiment showed that the N - H stretching fre-... 

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