N-H bond-stretching

Figure 5. N—H bond lengths in N—H O hydrogen bonding systems. The phenomenon of N—H bond stretching is only weakly represented (if at all), probably because there are only few nearly matched systems that have been characterized by neutron diffraction. Neutron diffraction data are from the CSD. From top to bottom all N—ammonium-carboxylate, ApKa 5 neutral heterocycle-...

Figure 5-7. Generation of some symmetry coordinates of HNNH (a) Symmetry coordinates corresponding to N-H bond stretches (b) Symmetry coordinates representing in-plane deformation.

We start with the six conventional curvilinear vibrational coordinates for ammonia [7,14] three (N-H) bond stretches r1 and three interbond angle (H-N-H) distortions 0i, (i = 1, 2, 3). The molecular vibrational Hamiltonian and, in particular, our analytical PES are built with reference to the molecular totally symmetrical planar configuration. A basic feature of this formalism is its full ZHh symmetrization. For this purpose we define symmetrized curvilinear vibrational coordinates in a somewhat unconventional, complex form ... 

Six vibrational modes are expected for the pseudohalide hydracids for which the normal mode description and energies together with the molecular force constants are shown in Table 6. It is worth noting here that the N-H bond stretching force constant for HN3 is lower than that for HNCO and HNCS. This indicates that the latter are thermodynamically more stable compared with HN3. 

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. 

These two vibrations are clearly visible at 3270 and 3380 cm in the IR spectrum of butylamine shown in Figure 22 la Secondary amines such as diethylamme shown m Figure 22 7i> exhibit only one peak which is due to N—H stretching at 3280 cm Ter tiary amines of course are transparent m this region because they have no N—H bonds... 

I The region from 4000 to 2500 cm"1 corresponds to absorptions caused by N-H, C-H, and O-H single-bond stretching motions. N—H and O—H bonds absorb in the 3300 to 3600 cm-1 range C-H bond-stretching occurs near 3000 cm"1. 

Primary and secondary amines can be identified by a characteristic N—H stretching absorption in the 3300 to 3500 cm"1 range of the IR spectrum. Alcohols also absorb in this range (Section 17.11), but amine absorption bands are generally sharper and less intense than hydroxyl bands. Primary amines show a pair of bands at about 3350 and 3450 cm-1, and secondary amines show a single band at 3350 cm-1. Tertiary amines have no absorption in this region because they have no N-H bonds. An IR spectrum of cyclohexylamine is shown in figure 24.7. 

The AES results indicate that the aniline coverage is more than two times greater than the maximum coverage based on van der Waals radii. The TPR results show this species is too stable to be a condensed multilayer. Hence, we conclude that aniline polymerized forming a very stable polymer layer. In addition, the absence of infrared bands corresponding to C=C stretches or ring vibrations indicated that the poly(aniline) film was formed with the phenyl rings parallel to surface. The infrared results also indicated that the poly(aniline) film had N-H bonds which were oriented perpendicular relative to the surface. 

S. Jarmelo, N. Maiti, V. Anderson, P. R. Carey, and R. Fausto, Ca H bond stretching frequency in alcohols as a probe of hydrogen bonding strength A combined vibrational spectroscopic and theoretical study of n [1 Djpropanol. J. Phys. Chem. A 109, 2069 2077 (2005). 

The infrared spectra of cimines show one or two N-H stretches in the 3500-3200 cm region. Primary cimines usually have two bands, while secondary amines usually have one band. Obviously, since there are no N-H bonds, tertiary amines have no N-H stretch. The bands are small and sharp in compcirison the corresponding alcohol peaks. 

A 3360 cm N-H amide stretch Two bands indicating restricted rotation about the N-CO bond resulting in stereoisomers... 

Piperidine has an N—H bond absorbing at 3500 cm and H—C(sp ) stretch below 3000cm. Pyridine has no N—H has H—C(sp ) stretch above 3000cm C=C and C=N stretches near 1600 and 1500cm , respectively aromatic ring vibrations near 1200 and 1050 cm and C—H deformations at 750 cm". The peak at 750 varies with substitution in the pyridine ring. 

The stretching of the N-H covalent bond in N-H—O systems, involving N-H bond lengths of 0.95-1.10 A, is not very significant (Figure 5), in comparison with the O-H—O system, 0.85-1.20 A (Figure 3) or in comparison with experimental error. The reason for the small variation in N-H bond lengths is probably that there are no or few exactly-matched structures reported in the CSD. We noted a difference... 

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