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 q uasi-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 ma in-ring which varies from 4 to 7 atoms. The molecules were isolated in low- temperature matrices and investigated by IR spectroscopy. The strength of t he OH . . .O=C interaction, depending on the size of the main ring, was spe ctroscopically revealed by the position of the infrared bands due to the st retching and torsional vibrations of the OH group. All the intermediate cas es from very weak hydrogen-bond-like interaction to medium strength hydroge n bonding were observed, indicating that the strict boundary between the ca ses "hydrogen bond'' and "no hydrogen bond'' does not exist. This experimen tal conclusion is supported by theoretical calculations, performed at HF/6- 31++G**, MP2/6-31+G* and DFT(B3LYP)/6-31++G** levels.