MATRIX-ISOLATION INFRARED AND AB-INITIO STUDIES ON CONFORMERS OF FLUOROACETIC AND CHLOROACETIC ACID

The infrared spectra of matrix isolated fluoroacetic acid and chloroac etic acid were studied in solid Ar, Kr, and Xe matrices. The energies and spectra of possible conformers of fluoroacetic acid and chloroacet ic acid were simulated by extensive ab initio calculations at the MP2/ 6-311G* (energies, geometries), MP2/6-31G** (barriers), and MP2/4-31G (numerical frequencies levels. The present calculations propose the existence of four distinct conformers. For fluoroacetic acid the two, lowest energy conformers, i.e., Tt and Ct, were found to be present af ter deposition of the solids These conformers differ by 180-degrees ro tation of the fluoromethyl group and for both conformers the carboxyli c hydrogen atom is in cis position with respect to the C=O bond. In ag reement with the experimental results, the ab initio calculated energy difference between Tt and Ct is ca. 1 kJ mol-1, and the barrier for T t --> Ct interconversion is ca. 1 3 kJ mol-1. For chloroacetic acid th ree conformers were identified in the matrix spectra. The lowest energ y form Tt and C(s) symmetry. The second lowest energy conformer Gt dif fers from Tt by rotation of the chloromethyl group about the C-C bond (dihedral angle Cl-C-C-O = 77-degrees). The observed data and the calc ulations predict consistently a barrier of ca. 5 kJ mol-1 for this int erconversion. For both conformers the carboxylic hydrogen atom is in c is position with respect to the C=O bond. The third observed conformer is Cc with intramolecular hydrogen bonding Cl-H. A normal coordinate analysis based on the ab initio calculated force field is presented fo r observed conformers and compared with the experimental spectra. The similarities with related molecules are discussed.