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Abstract The effect of the molecular structure on the properties of C-H·s O bonds was investigated in a set of small cyclic carbonyl compounds, using vibrational spectroscopy and ab initio calculations. Two main effects were studied: the size of the ring and the inclusion of oxygen atoms in the ring. The analysis of the band profile of the carbonyl stretching mode reveals the presence of single dimerization equilibria in the series cyclobutanone-cyclopentanone-cyclohexanone. The smallest cyclobutanone was found to have the lowest dimerization energy, with an experimental ?H value of 3.7± 0.6 kJ mol-1. The systems with oxygen atoms in the ring, ?-butyrolactone and ethylene carbonate were found to present more complex equilibria. Ab initio calculations suggest that this complexity is due to the presence of additional dimer structures and higher oligomers. [ABSTRACT FROM AUTHOR]