IR spectral and theoretical characterization of intramolecular hydrogen bonds closing five-membered rings

A series of molecules with a common structural feature of a five-membered quasi-ring closed by an intramolecular hydrogen bond OH··· O=C interaction has been studied. In the series the compounds differ by the size of the main-ring which varies from 4 to 7 atoms. The molecules were isolated in low-temperature matrices and investigated by IR spectroscopy. The strength of the OH··· O=C interaction, depending on the size of the main ring, was spectroscopically revealed by the position of the infrared bands due to the stretching and torsional vibrations of the OH group. All the intermediate cases from very weak hydrogen-bond-like interaction to medium strength hydrogen bonding were observed, indicating that the strict boundary between the cases "hydrogen bond" and "no hydrogen bond" does not exist. This experimental conclusion is supported by theoretical calculations, performed at HF/6-31++G**, MP2/6-31 + G* and DFT(B3LYP)/6-31 ++G** levels.