It's my understanding that the infrared absorption spectrum of primary amides beyond having a broad peak also has 2 sharp peaks representing the symmetric and antisymmetric stretch. My question is why isn't there a third peak representing a bending motion where the two hydrogen molecules bend towards each other?
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1$\begingroup$ Bending vibrations tend to have quite different frequencies, much lower compared to stretching vibrations. They tend to fall in regions of the spectrum which are more difficult to interpret. This page gives the primary amide N-H bend as 1570-1515 cm-1. So, the bend is indeed there, but the stretch is diagnostically much more useful. $\endgroup$– orthocresol ♦Jan 25, 2017 at 18:29
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