N—H Bending Vibrations Amide II Band

All primary amides show a sharp absorption band in dilute solution (amide II band) resulting from NH2 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, 1620-1590 cm-1, and normally is separated from the amide I band. Multiple bands may appear in the spectra of concentrated solutions, arising from the free and associated states. The nature of the R group 

Secondary acyclic amides in the solid state display an amide II band in the region of 1570-1515 cm-1. In dilute solution, the band occurs in the 1550-1510 cm-1 region. This band results from interaction between the N—H bending and the C—N stretching of the C—N—H group. A second, weaker band near 1250 cm-1 also results from interaction between the N—H bending and C—N stretching. 

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