CONFORMERS, VIBRATIONAL-SPECTRA AND INFRARED-INDUCED ROTAMERIZATION OF DICHLOROACETIC ACID IN ARGON AND KRYPTON MATRICES

The results of a combined study of dichloroacetic acid monomer underta ken by matrix-isolated low-temperature infrared spectroscopy in both a rgon and krypton matrices and ab initio SCFMO calculations are present ed. Two s-cis (C-O) conformations of CCl2HCO2H differing by internal r otation about the C-C bond and one s-trans form were found in the matr ices. Their spectra are reported within the range 4000-400 cm-1 and in terpreted. The skew/s-cis form (H-C-C=O dihedral angle equal to ca. 14 4-degrees) converts to the most stable syn/s-cis conformer (H-C-C=O: 0 -degrees) upon irradiation in the v(O-H) region. The temperature depen dences (annealing) of the isolated matrix vibrational spectra of the s tudied molecule before and after irradiation were found to be differen t, and a model is proposed to explain the experimental observations. I n addition, 3-21G and 6-31G ab initio- MO calculations were carried o ut and the structures and energies of the relevant conformations are r eported. The theoretical results agree with the experimental data and provide a good insight into the intramolecular interactions which dete rmine the relative stability of the various conformers. Finally, a nor mal-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.