Liquid phase overtone spectral investigations of 2,6-dimethylaniline and 2,4-dimethylaniline-evidence for steric nature of the ortho effect and the consequent base weakening.

The near infrared vibrational overtone absorption spectrum of liquid phase 2,6-dimethylaniline and 2,4-dimethylaniline are reported in the region deltanu = 2, 3 and 4. The aryl CH, methyl CH and NH local mode mechanical frequency values obtained from fitting the overtones are analysed and compared. The observation supports the conclusions drawn from earlier base strength studies on methylation of aniline and structural studies of toluidines by ab initio calculations. The introduction of the ortho methyl group into aniline weakens its base strength due to steric strain.