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A priori theoretical considerations lead one to expect that Rb 3 C 60 is a Mott-Hubbard insulator. This has led to the proposal that the superconductor "Rb 3 C 60 " is, in fact, Rb 32´C60 ͑´ϳ 0.06͒, and, furthermore, that the vacancies are associated with an unexpected, robust splitting in 87 Rb NMR. We demonstrate that vacancies, if present, are unrelated to observed NMR splittings. [S0031-9007(97)04810-2]

Rapid and Efficient Synthesis of Alkali Metal-C60 Compounds in Liquid Ammonia.-MxC60 superconductors (Mx: K3, Rb3, CsRb2, RbCs2, KRbCs) are prepared rapidly in quantitative yield by reaction of stoichiometric amounts of alkali metals with C60 in liquid ammonia and annealing of the product 24-48 h at 375 • C. From the 13C and 87Rb line shapes for two Rb3C60 samples, a dependency of the degree of rotational motion on the method of preparation and the subsequent thermal treatment is supposed. A correlation between the degree of rotational motion and the superconducting fraction is attibuted to the amount of disorder in the sample. 87Rb NMR indicate the presence of three different cation sites in Rb3C60 rather than two suggested from the XRD data. The third site, occupying about 15% of the Rb ions, is a subset of the tetrahedral sites in the cubic close-packed C60 lattice. -(BUFFINGER, D. R.; ZIEBARTH, R. P.; STENGER, V. A.; RECCHIA, C.; PENNINGTON, C. H.; J. Am. Chem. Soc. 115 (1993) 20, 9267-9270; Dep. Chem., Ohio State Univ., Columbus, OH 43210, USA; EN)