Strongly hydrogen-bonded molecular solids with 'highly anomalous' infrared OH absorption

The infrared spectra of solid isonicotinic acid (1), 6-hydroxy-2-pyridone (2) and 6-hydroxy-l-methyl-2-pyridone (3) have very intense, broad, plateau-like, OH absorption bands, centred near 1000 cm-1, with prominent transmission windows. After O -deuteration these bands remain very broad but become much less intense, and the centre of gravity is shifted to higher frequencies, by c . 100-250 cm-1; most of the transmission windows become much shallower, or disappear, or are replaced by ordinary bands pointing upwards. Full explanations of the deuteration effects on the transmission windows cannot be given, but it is clear that the amplitude of hydroxyl stretching must be much greater for OH than for OD. This vibration must be very highly anharmonic, and it is shown that any physically reasonable potential energy curve for it must be highly unsymmetrical. For solids (2) and (3) the potential energy hypersurface, for all stretching vibrations, may be symmetrical, with a double minimum, but no such symmetry can be present in solid isonicotinic acid.