IR Spectrum Peak Broadening: Hydrogen Bonding

The vibrational frequency of a bond is directly proportional to its bond strength. As a result, stronger bonds vibrate at higher frequencies, while weaker bonds vibrate at lower frequencies. The stretching vibration of the strong O–H bond in alcohols and phenols (very dilute solution or gas phase) appears as a sharp peak at 3600–3650 cm⁻¹.

However, the extent of hydrogen bonding influences the observed stretching frequency and band broadening. Intermolecular or intramolecular hydrogen bonding weakens the O–H bond, resulting in a range of bond strengths (3300–3400 cm⁻¹). So, the hydrogen-bonded O–H stretching vibration in concentrated solutions appears as a broad signal in IR spectroscopy. In moderately dilute solutions, the sharp signal from the free O–H stretching appears beside the broad signal from the hydrogen-bonded O–H.

When the presence of bulky groups prevents hydrogen-bonding interactions, a sharp band is seen irrespective of the alcohol or phenol's physical state. Similarly, molecules exhibiting intramolecular hydrogen bonding show a broad band independent of concentration. For instance, ortho-carbonyl substituted phenols exhibit a broad signal at 3200 cm⁻¹ at all concentrations.

In carboxylic acids, intermolecular hydrogen bonding between two molecules leads to the formation of dimers. The O–H band appears as a very broad peak (2400–3400 cm⁻¹) centered at ≈3000 cm⁻¹.