13.15 : IR スペクトルのピーク分割: 対称振動と非対称振動

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.