Vibrational overtone spectroscopy of pyridine and related compounds Original Research Article

The vibrational overtone spectra of gaseous pyridine, 3-fluoropyridine, and 2,6-difluoropyridine were recorded using photoacoustic absorption spectroscopy. The pyridine, 3-fluoropyridine, and 2,6-difluoropyridine overtone spectra exhibit two local mode progressions. The appearance of these two progressions in the difluorinated compound results from a splitting of the C(3,5)-H and C(4)-H stretches which overlapped in pyridine. In 3-fluoropyridine, ab initio CH bond lengths were used to assign the lower energy progression to the C(2,6)-H stretches and the higher energy progression to the C(4,5)-H stretches. Alternatively these CH stretch absorptions may be mixed with other vibrational modes and cannot be assigned to the separate CH oscillators at the (2,6), (3,5) and 4 positions. Comparisons are made to the spectra of benzene and pyrrole which exhibit only one overtone peak at each quantum level.