Conformers, vibrational spectra and infrared-induced rotamerization of dichloroacetic acid in argon and krypton matrices

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Abstract
The results of a combined study of dichloroacetic acid monomer undertaken by matrix-isolated low-temperature infrared spectroscopy in both argon and krypton matrices and ab initio SCFMO calculations are presented. Two s-cis(C—O) conformations of CCl2HCO2H differing by internal rotation about the C—C bond and one s-trans form were found in the matrices. Their spectra are reported within the range 4000–400 cm–1 and interpreted. The skew/s-cis form (H—C—CO dihedral angle equal to ca. 144°) converts to the most stable syn/s-cis conformer (H—C—CO: 0°) upon irradiation in the ν(O—H) region. The temperature dependences (annealing) of the isolated matrix vibrational spectra of the studied molecule before and after irradiation were found to be different, and a model is proposed to explain the experimental observations. In addition, 3-21G and 6-31G*ab initio- MO calculations were carried out and the structures and energies of the relevant conformations are reported. The theoretical results agree with the experimental data and provide a good insight into the intramolecular interactions which determine the relative stability of the various conformers. Finally, a normal-mode analysis based on the 6-31G* harmonic force fields is used to review previous assignments of the vibrational spectra of the various conformers of the studied molecule.

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