4-Hydroxycoumarin/2-hydroxychromone tautomerism: Infrared spectra of 3-substituted-2-13c-4-hydroxycoumarins

By isotopic replacement of the carbonyl carbon with ¹³C, the C?O stretching frequency was identified as the highest frequency strongly absorbing band in the 1550–1750 cm^?1 region of the infrared spectra of several 3-substituted 4-hydroxycoumarins and 3-substituted 4-alkoxycoumarins. The compounds selected for study were either known to crystallize as the coumarin tautomeric form by x-ray diffraction studies or were congeners of such compounds. The carbonyl band varied from 1664 cm^?1 in inter- or intramolecularly hydrogen bonded derivatives to 1718 cm^?1.