NIR vibrational overtone spectra of toluidines-evidence for steric and electronic effects in o-toluidine

Vibrational overtone spectra of o-fluorophenol and the "anomalous" order of intramolecular hydrogen bonding strengths

The near infrared vibrational overtone absorption spectrum of liquid phase o-fluorophenol is examined in the region deltav=2, 3 and 4. The OH frequencies are compared with that of o-chlorophenol and phenol. Considering the relative electronegativities of all halogens, one might expect the order of intramolecular hydrogen bond strength for o-fluorophenol to be the greatest among all halogenophenols. It is evident that o-fluorophenol forms a weaker intramolecular hydrogen bond (an anomalous trend) contrary to that expected from relative electronegativities of halogens. The local mode mechanical frequency values and anharmonicity values obtained from fitting the overtones are analysed. Our observation is in agreement with the previous experimental as well as the recent theoretical vibrational analysis of halogenophenols using density functional theory (B3LYP). The overtone spectra of o-fluorophenol in carbon tetrachloride in different concentrations are also examined. It is noted that the OH-red shift which arises due to the intermolecular bond formation between the cis and trans conformers of o-fluorophenol (dimerization) increases with concentration.

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.

Near IR overtone spectral investigations of cyclohexanol using local mode model--evidence for variation of anharmonicity with concentration due to hydrogen bonding

The near infrared vibrational overtone absorption spectrum of liquid phase cyclohexanol in carbon tetrachloride in different concentrations are examined in the region Deltav=2, 3 and 4. The free and bonded OH local mode mechanical frequency values and anharmonicity values obtained from fitting the overtones are analysed. The observation supports the conclusions drawn from earlier experimental studies on anharmonicity variation of OH-stretching vibrations of alcohols due to intermolecular hydrogen bonding. Our observation is also in agreement with the ab initio calculations on water dimer and trimer. Mechanical anharmonicity of bonded OH-stretching bands tends to increase as a consequence of strong hydrogen bonding at higher concentrations.