N—H Bending Vibrations

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. 

Liquid samples of primary and secondary amines display medium to strong broad absorption in the 909-666 cm-1 region of the spectrum arising from N—H wagging. The position of this band depends on the degree of hydrogen bonding. 

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Finally, in this region are the N H bending vibrations (1640-1550 cm ), which will have corresponding stretching vibrations between 3600 and 3200 cm, These absorptions are often difficult to distinguish from C—C vibrations. The N-H vibration of the secondary amide paracetamol is probably responsible for the absorption at 1564 cm (Figure 3.4), while that of aniline (Figure 3.7) may be partly responsible (in addition to the CC stretch) for the peak at 1620 cm. ... 

Bands resulting from the primary and secondary N—H bending vibrations appear near 1650 cm-1 and 1550 cm-1 respectively in the solid phase, and the large difference in these amide II bands enables primary and secondary amides to be distinguished. The 1550 cm -1 band is not a simple N—H bending mode, but is believed to result from coupling of this deformation with a C—N stretching vibration. 

N—H Bending Vibrations (Amide II Band) All primary amides show a sharp absorption band in dilute solution (amide II band) resulting from N—H, bending at a somewhat lower frequency than the C=0 band. This band has an intensity of one-half to one-third of the C=0 absorption band. In mulls and pellets the band occurs near 1655-1620 cm-1 and is usually under the envelope of the amide I band. In dilute solutions, the band appears at lower frequency,... 

Isotope effects are vibrational in origin206-210. It has been claimed213 that the isotope shift must involve the in-plane C—N—H bending vibrations and the associated C—C—XH bond-angle distortions. This distortion can be regarded as a perturbation on the hybridization which, in conjugated systems, is likely to spread over the whole... 

Region 2 (2000-1500 cmr1) this is known as the functional group region and includes the stretching frequencies for C=C, C=0, C=N, N=0 and N—H bending vibrations. 

Examine region 2 (2000-1500cm 1). Here you will find C=0 stretch, usually the most intense band in the spectrum C=C and C=N stretches, less intense and sharper N=0 stretch (from N02) intense and sharp and with a twin band in region 3 N-H bending vibrations - do not confuse with C=0. 

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