(±)-1-Indanone-2-acetic and -2-propionic acids: Catemeric versus dimeric hydrogen bonding in homologous γ- and δ-keto acids

The crystal structure of (±)-2,3-dihydro-1-oxo-1H-indene-2-acetic acid (C₁₁H₁₀O₃) involves catemeric hydrogen bonds, which progress from the carboxyl H atom of one molecule to the ketone O atom of a neighbor screw-related in b [O⋯O = 2.709 (2) Å]. This yields centrosymmetrically related counterdirectional pairs of hydrogen-bonding chains following parallel helical axes. Four intermolecular C=O⋯H-C close contacts were found, involving both carbonyl groups. By contrast, the hydrogen bonding in (±)-3-(2,3-dihydro-1-oxo-1H-inden-2-yl)propionic acid (C₁₂H₁₂O₃) produces carboxyl dimers [O⋯O = 2.639 (2) Å], which also have a 2.62 Å C=O⋯H-C close contact involving the ketone carbonyl group and the peri-hydrogen on a centrosymmetrically related molecule. The difference in hydrogen-bonding behavior between the two molecules is discussed.