Abstract Structural and vibrational properties of CClH 2 COCl, CClH 2 COOH and CClH 2 COOCH 3 as well as conformational effects produced by interactions operating in condensed phases have been studied by a combination of molecular mechanics and vibrational spectroscopy. Conformer energy differences and barriers to internal rotation are determined. In particular, temperature dependent intensities in the Raman spectra of liquid CClH 2 COOCH 3 yield an enthalpy difference of approximately 0.9 kJ mol −1 between the gauche and the syn conformers. This value is larger than the corresponding result calculated by molecular mechanics for the isolated molecule (≅;0.2 kJ mol −1 ), thus revealing the importance of intermolecular interactions in stabilizing the more polar conformer ( syn form) in the liquid. Spectroscopic results show that only this conformer is present in the solid. Various trends in carbonyl stretching frequencies of CClH 2 COCl, CClH 2 COOH and CClH 2 COOCH 3 are also reported and analyzed.