Infrared studies of chromones—II Isotopically substituted 5-hydroxychromones

By deuteration of the OH group, it has been shown that two bands, at 1665 and 1630 cm⁻¹ (CCl₄), in the IR spectra of 5-hydroxychromones are associated with the H-bonded CO stretching vibration. Nuclear deuteration of 5-hydroxy-2-methylchromone (1) under acidic conditions gave a tri- and a hexadeutero derivative; the latter, isotopically substituted at C-3, shows a single CO band at 1649 cm⁻¹. Hydrolysis of 3-acetyl-5-hydroxy-2-methylchromone with sodium carbonate in deuterium oxide furnished 5-hydroxy-2-trideuteromethylchromone-3-d which also exhibits a single CO absorption. Partial incorporation of O¹⁸ into the CO group of 1 results in a single ν_C=O¹⁸ at 1593 cm⁻¹. It is suggested that the doublet CO absorption of 5-hydroxychromones arises from a Fermi resonance involving a low-energy vibrational mode of the vinyl proton on the nuclear C-3 position.