Vibrational assignments and derived potential energy distributions for tri- and difluoromethyl ketene by density functional calculations

The structures of 3,3,3- trifluoromethyl ketene and 3,3- difluoromethyl ket ene were studied by utilizing ab initio calculations with the 6-311++G** ba sis set at the (B3LYP) Density Functional level. Full optimization was perf ormed for both molecules in their ground and transition states. Energy opti mization of the systems under investigation shows that trifluoromethyl kete ne exists only in the cis conformation (fluorine atom eclipses the ketene g roup). Difluoromethyl ketene was predicted to have two stable conformations : the cis (hydrogen atom eclipses the ketene group) and the gauche (fluorin e atom eclipses the ketene group) form. The conformational stability of the molecules was found to be governed mainly by electrostatic and molecular o rbital interactions. The vibrational frequencies were computed and complete assignments were provided on the basis of normal coordinate calculations a nd comparison with similar molecules. The potential energy distributions (P ED) among symmetry coordinates were derived for the stable conformations of the two molecules.