Infrared spectra in the cesium bromide region are presented for 43 monosubstituted pyridines. Two- and 3-substituted pyridines are characterized in this region by two relatively intense bands near 600 and 400 cm−1. No strong characteristic bands are found for 4-substituted pyridines. The vapor-phase contours of the far infrared bands of 2-fluoro- and 2-chloropyridine indicate that the 600 and 400 cm−1 bands arise, respectively, from an in-plane and an out-of-plane vibration. The pyridine spectra are compared with the spectra of some substituted benzenes.
Raymond Isaac, Freeman F. Bentley, Helene Sternglanz, W. C. Coburn, C. V. Stephenson, and W. S. Wilcox, "The Far Infrared Spectra of Monosubstituted Pyridines," Appl. Spectrosc. 17, 90-97 (1963)