Near IR overtone spectral investigations of liquid phase imidazole and the blue-shifting hydrogen bonds.

The near infrared vibrational overtone absorption spectrum of imidazole dissolved in carbon tetrachloride is examined in the region Delta v = 2, 3 and 4. Instead of the standard hydrogen bonding, unusual blue-shifted hydrogen bonds are observed in NH positions. This observation is in agreement with the recent conclusions drawn from theoretical studies of the coupling character between the Imidazole-Imidazolium complex, which is strongly hydrogen bonded. The NH local mode mechanical frequency values and anharmonicity values obtained from fitting the overtones are analysed. The anharmonicity of blue-shifted hydrogen bonded NH stretching bands is greater compared to those of the free NH stretching bands. It is contrary to the decrease in anharmonicity due to the standard red shifting of NH bonds.