VIBRATIONAL ANALYSIS OF C-H STRETCHING OVERTONES IN PYRIDINE AND 2,6-LUTIDINE

Aryl and methyl C-H stretchings of pyridine and 2,6-lutidine in the liquid phase have been studied in the region of the fundamentals and of the overtones up to the fifth. Standard spectrometers were used to investigate IR and near-IR regions. Thermal lensing laser spectroscopy was used in the visible region. DELTA-n = 1, 2 spectra show a complex pattern that becomes simpler on going to higher DELTA-n values. In this case, the interpretation of the spectra can be made by using the local-mode approach. Stretching frequencies are mainly perturbed by the local environment of bonds, and then new details on molecular structures can be obtained. The observation of the double bands in high-overtone spectra of pyridine is due to differences between properties of the C-H bonds in the 2,6- and 3,4,5-positions. The lower frequency structures in 2,6-lutidine are attributed to conformationally nonequivalent C-H bonds of the methyl groups. Nonnegligible interactions between methyl C-H bonds and the nitrogen atom are responsible for a smaller anharmonicity of the in-plane C-H stretching.