MOLECULAR-GEOMETRY AND THE ROTATIONAL POTENTIAL SURFACE IN PERFLUORO(ISOPROPYL METHYL-ETHER)

Molecular geometries and energies for stationary point conformations of perfluoro(isopropyl methyl ether) (PFIPME), (CF3)(2)CFOCF3, were calculated using ab initio molecular orbital theory. Geometries were optimized with the HF/6-31G* basis set; single point energies were also calculated at the MP2/6-31G* level. CO bond lengths and COC and OCC angles vary in a manner consistent with severe steric strain in conformations where the terminal perfluoromethoxy group either eclipses one perfluoromethyl group or is gauche to both perfluoromethyl groups. The torsional potential surface of PFIPME is markedly more complex than that reported for the analogous non-fluorinated ether, exhibiting three minima and three saddle points in the range, 0 degrees < Phi < 180 degrees. The global minimum occurs at Phi = 0 degrees where the perfluoromethoxy group directly eclipses the fluorine bonded to the tertiary carbon. A comparison of the rotational potential with those reported for two linear perfluoroethers reveals that internal rotation about the central CO bond in PFIPME is very restricted relative to the linear molecules and provides a logical explanation of the high viscosity activation energies experimentally observed in branched perfluoropolyalkyl ether lubricants.