AB-INITIO INVESTIGATION OF CONFORMATIONAL GEOMETRIES AND THE TORSIONAL POTENTIAL SURFACE IN PERFLUORODIMENTOXYMETHANE

A conformational potential energy contour diagram for perfluorodimethoxymethane [PFDMOM] was constructed by RHF/6-31G*//RHF/6-31G*(C,O) [the latter is the 6-31G basis set with polarization functions on C and O] calculations at 60-degrees increments of the two CF2-O torsional coordinates between -180-degrees and 180-degrees. Additional RHF/6-31G-//RHF/6-31G* geometries arid energies were determined fora number of equilibrium and transition state stationary points. Many of the conformers exhibited a helical structure, which has been attributed to dipolar repulsions which destabilize parallel C-F bond dipoles. Several rotamers were found to contain two nonequivalent, but closely related molecular structures, corresponding to (a) the same or (b) different relative directions of helical twisting in the two halves of the molecule. The potential energy contour diagram of PFDMOM is dramatically different from that reported earlier for dimethoxymethane [DMOM], where it was found that the gauche-gauche rotamer is greatly stabilized relative to other conformations by the anomeric effect. In contrast, the most stable conformations for PFDMOM correspond to either or both of the torsional angles having values in the region near 180-degrees. In addition, the barriers to internal rotation in PFDMOM are significantly lower than in DMOM. The basis for the differences in the energy diagrams of the two ethers and its implications on the structure of their polymeric forms [(-CF2O-)n and (-CH2O-)n] are discussed.