1. Instituto de Quimica-Fisica ‘Rocasolano’, C.S.I.C., C/Serrano 119, 28006 Madrid, Spain, Dow Corning Corporation, P.O. Box 994, Mail CO1232, Midland, Michigan, 48686-0994, and Department of Chemistry, University of Reading, Whiteknights, P.O. Box 224, Reading, RG6 6AD, U.K.