Conformers, Vibrational Spectra and Infrared-induced rotamerization of dichloroacetic acid in argon and krypton matrices

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-0) conformations of CCI,HCO,H 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-' and interpreted. The skewlscis form (H-C-C-0 dihedral angle equal to ca. 144") converts to the most stable syn/scis conformer (H-C-C-0: 0") upon irradiation in the v(0-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* a6 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.