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

Molecular geometries and energies for stationary point conformations o f perfluoro(isopropyl methyl ether) (PFIPME), (CF3)(2)CFOCF3, were cal culated using ab initio molecular orbital theory. Geometries were opti mized with the HF/6-31G basis set; single point energies were also ca lculated at the MP2/6-31G level. CO bond lengths and COC and OCC angl es vary in a manner consistent with severe steric strain in conformati ons where the terminal perfluoromethoxy group either eclipses one perf luoromethyl group or is gauche to both perfluoromethyl groups. The tor sional potential surface of PFIPME is markedly more complex than that reported for the analogous non-fluorinated ether, exhibiting three min ima and three saddle points in the range, 0 degrees < Phi < 180 degree s. The global minimum occurs at Phi = 0 degrees where the perfluoromet hoxy group directly eclipses the fluorine bonded to the tertiary carbo n. A comparison of the rotational potential with those reported for tw o linear perfluoroethers reveals that internal rotation about the cent ral CO bond in PFIPME is very restricted relative to the linear molecu les and provides a logical explanation of the high viscosity activatio n energies experimentally observed in branched perfluoropolyalkyl ethe r lubricants.