FTIR Spectroscopy of Flavonols in Argon and Methanol/Argon Matrixes at 10 K. Reexamination of the Carbonyl Stretch Frequency of 3-Hydroxyflavone

The FTIR spectra of six argon-matrix-isolated flavonols were measured. Carbonyl stretching frequencies for the following compounds were the following: flavone, 1669 cm-1; chromone, 1676 cm-1; 5-hydroxyflavone (5HF), 1660 cm-1; 3-hydroxyflavone (3HF), 1652 cm-1; 5-methoxyflavone (5MF), 1672 cm-1; and 3-methoxyflavone (3MF), 1658 cm-1. The 3HF carbonyl stretching frequency assignment represents a correction to the literature values: (1628.6 cm-1 in an argon matrix and 1621 cm-1 in the liquid phase).1,2-5 The hydroxyl stretch modes for 5HF and 3HF were observed at 2935 and 3320 cm-1, respectively, in agreement with the literature values.2-5 While the hydroxyl stretch modes predict that the 5HF intramolecular-hydrogen bond to the carbonyl is stronger than that of 3HF, the carbonyl stretch frequency of 3HF is red-shifted more than that of 5HF. In analogy to hydroxyl-substituted flavones, methoxyl substitution at the flavone 3 position also results in a greater carbonyl red-shift than at the 5 position. Relative to the methoxyflavones, the effect of hydroxyl substitution on the carbonyl is clearly greater in 5HF than in 3HF. Density functional calculations at the B3LYP/6-31G(d) level are consistent with the experimental findings. Calculated OH···O intramolecular hydrogen bond distances and O−H···O angles were 1.70 Å and 149° for 5HF and 1.94 Å and 122° for 3HF, consistent with greater hydrogen-bonding in 5HF than in 3HF. Changes in the bond distances from 5MF to 5HF are consistent with hydrogen bonding, encouraging a resonance form resembling the excited-state proton-transfer tautomer of 5HF that weakens the carbonyl. Similarly, changes in the bond distances from 3MF to 3HF suggest that hydrogen bonding stabilizes a zwitterionic resonance form that also weakens its carbonyl. This zwitterionic resonance form of 3HF resembles the excited state proposed for its unusual fluorescence and could be responsible for the facile energy transfer to this state.