Knowledge Bank

The visible vibrational spectra of gaseous pyridine, 3-fluoropyridine, and 2,6-difluoropyridine have been recorded up to v-6 using intracavity dye laser photoacoustic spectroscopy. The overtone spectrum of each compound shows a progression of two peaks at each vibrational level and in each case the two peaks, taken separately, fit well to a Birge-Sponer plot. Compared to the pyridine vibrational overtones, the peak maxima for 3-fluoropyridine and 2,6-difluoropyridine are blue-shifted by about 20v and $50vcm-1$, respectively, where v is the vibrational quantum number. In earlier local mode $analyses1,2$ of the liquid phase overtone spectrum of pyridine the two peaks were assigned to the $C\alpha -H$ and $C\beta ,\gamma H$ stretches. Our results are compared to this model and assignments are suggested for the overtone spectra of the substituted pyridines.