Identical bonds within a polyatomic group can stretch symmetrically (in-phase) or asymmetrically (out-of-phase). Similar to hydrogen bonding, these vibrations also influence the shape of the IR peak. Generally, asymmetric stretching frequencies are higher than symmetric stretching frequencies. For example, primary amines exhibit two distinct IR peaks between 3300–3500 cm−1 corresponding to the symmetric and asymmetric N-H stretching, while secondary amines exhibit a single stretching vibration at ≈ 3300 cm−1.
Similarly, the N–H bonds in primary amides exhibit two stretching vibrations at ≈3350 cm−1 (asymmetric stretching) and ≈3180 cm−1 (symmetric stretching) in the solid state, while a single N–H stretching vibration is seen for secondary amides at 3300 cm−1.
In the case of the –NO2 (nitro) group, delocalization generates two equivalent N-O bonds that show both asymmetric and symmetric stretching absorption peaks at ≈1550 and ≈1365 cm−1, respectively.
Recall that a polyatomic group bearing a minimum of two identical atoms can stretch symmetrically or asymmetrically.
So, the two N–H bonds of RNH2 either stretch in phase as symmetric stretching, or out of phase exhibiting asymmetric stretching.
Thus, RNH2 give rise to two distinct IR peaks in the range of 3300-3500 cm−1.
The higher frequency band results from asymmetric stretching, while the lower frequency absorption corresponds to symmetric stretching.
In contrast, as R2NH bear only one N–H bond, their IR spectra are characterized by a single stretching peak at ~3300 cm−1.
Similarly, consider the stretching pattern of the –NO2 group. While the two N–O bonds appear non-equivalent, delocalization generates equivalent bonds. Thus, the –NO2 group shows both symmetric and asymmetric stretching absorption peaks.
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