IR Spectrum Peak Broadening: Hydrogen Bonding

Recall that the vibrational frequency of a bond is proportional to the square root of the force constant, considering only bond strength effects and not reduced mass factors.

Thus, stronger bonds with higher force constants vibrate at higher frequencies upon IR absorption. Weaker bonds vibrate at lower frequencies.

For instance, intermolecular hydrogen bonding observed in alcohols and phenols weakens the existing O–H bond, thereby influencing the position of the characteristic O–H peak.

In a very dilute solution or gas phase, the absence of intermolecular H-bonds results in a sharp peak at ≈3600 cm⁻¹.

In concentrated solutions, as the molecules are involved in varying degrees of H-bonding , there results in a very broad band in the range of 3200–3550 cm⁻¹.

In moderately dilute solutions, both sharp and broad peaks are observed corresponding to the free and H-bonded O–H vibrations, respectively.

Moreso, the presence of bulky groups prevents H-bonding interactions, generating only a sharp peak irrespective of the physical state of the alcohol or phenol.