VIBRATIONAL FREQUENCIES, NORMAL-COORDINATE ANALYSES AND POTENTIAL FUNCTIONS FOR INTERNAL ROTATIONS IN 3,3,3-TRIFLUOROPROPIONYL FLUORIDE AND3,3,3-TRICHLOROPROPIONYL CHLORIDE BASED ON AB-INITIO CALCULATIONS

The conformational behavior and structure of 3,3,3-trifluoropropionyl fluoride and 3,3,3-trichloropropionyl chloride were investigated by ut ilizing ab initio calculations with 6-31G basis set at RHF and MP2 le vels. The chloride was predicted to exist only in the cis conformation with minimum chlorine-chlorine interaction. For the fluoride, the cis conformer was predicted by the two levels to be slightly the lower en ergy form than the gauche and trans conformations. Full optimization w as performed at the transition state of the fluoride and the barrier t o internal rotation between the cis and gauche conformers was calculat ed. The vibrational frequencies were computed at HF level and the zero energy corrections were included into the calculated barrier. The sym metric rotational barrier of each of the CCl3 and the CF3 rotors was c alculated. Normal coordinate calculations were carried out and potenti al energy distributions were calculated for the stable conformers of t he molecules. (C) 1998 Elsevier Science B.V. All rights reserved.