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

A conformational potential energy contour diagram for perfluorodimetho xymethane [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 coor dinates between -180-degrees and 180-degrees. Additional RHF/6-31G-//R HF/6-31G geometries arid energies were determined fora number of equi librium and transition state stationary points. Many of the conformers exhibited a helical structure, which has been attributed to dipolar r epulsions which destabilize parallel C-F bond dipoles. Several rotamer s were found to contain two nonequivalent, but closely related molecul ar 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 f ound that the gauche-gauche rotamer is greatly stabilized relative to other conformations by the anomeric effect. In contrast, the most stab le conformations for PFDMOM correspond to either or both of the torsio nal angles having values in the region near 180-degrees. In addition, the barriers to internal rotation in PFDMOM are significantly lower th an in DMOM. The basis for the differences in the energy diagrams of th e two ethers and its implications on the structure of their polymeric forms [(-CF2O-)n and (-CH2O-)n] are discussed.