1. Lawrence Livermore National Laboratory, University of California, P.O. Box 808, Livermore, California 94551, Department of Inorganic Chemistry, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, D-14195 Berlin, Germany, and R&D Center, Ethyl Corporation, P.O. Box 2189, Richmond, Virginia 23217