J. Phys. Chem. 1991, 95, 518-520
SI8
orbital to an ion-pair excited state SrZ+(NH3),,(NH3),- whose stability increases with cluster size. In summary, several small metal-ammonia clusters have been studied by using pseudopotentials suitable for bulk-phase simulations. As the number of ammonia molecules increases, the valence (excess) electron of the metal is driven into a Rydberg-like state. The nitrogen atoms of the ammonia ligands coordinate so strongly about the metal ion that the electron is squeezed out. Thus, the "solvent" ammonia molecules compete with the electron for the chemically stable cation, the reverse of the usual description of such a process. In fact, ammonia competes so successfully that it takes only from four to six ammonia molecules to destabilize the valence electron of Sr+which is bound by 1 1 eV! This picture, which gives a semiquantitative explanation of the experimental
photodissociation spectra, suggests that stepwise solvation involves a slow leakage of charge density from around the metal atom into the bulk rather than a competition between a localized metal atom state and a charge-separated excited state that eventually cross. In the future, we plan to study larger clusters and the bulk in order to see whether a true charge-separated ground state can be stabilized in a large cluster. Further experimental measurements and quantum calculations on metalammonia clusters would be welcome.
Acknowledgment. We thank Professor Farrar for helpful comments. This research was supported by the National Science Foundation under Grants CHE-872248 1, CHE-88 15130, and DMR88-19885.
Solld-State Magnetlc Resonance Spectroscopy of Fullatenes R. Tycko,* R. C. Haddon, G. Dabbagh, S. H. Glarum, D. C. Douglass, and A. M. Mujsce AT& T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 (Received: November 2, 1990)
We report solid-state 13CNMR measurementson powder samples of C, and of a mixture of Cso and Cs. The NMR results show that, at 296 K, C, molecules rotate rapidly and isotropically in the solid state, while C70 molecules rotate somewhat more anisotropically. These results are consistent with the proposed spherical geometry of C, and prolate spheroidal geometry of C70. The rotational correlation time of C,, molecules in the solid state becomes greater than 50 p s at about 100 K.
Introduction Interest in the structures and properties of fullerenes' has received new impetus from the recent discovery that the molecules C, and C70can be prepared in large quantities by comparatively simple procedures.2 The ready availability of solid samples of C,, and C70 now permits their characterization by a variety of physical methods. In this paper, we report the results of solid-state l3C nuclear magnetic resonance (NMR) measurements on powder samples of C, and of a mixture of C, and C7@Our NMR results indicate that Cm rotates rapidly and nearly isotropically in the solid state at 296 K and that C70 also rotates at 296 K, although somewhat anisotropically. The rotation of Cmmolecules becomes slow on the time scale of our measurements at about 100 K. Materials and Methods Powder samples containing a mixture of Cm and C70 were prepared as described previously.*J The 13CN M R spectrum of the dissolved powder in C6D6 shows the line at 143.2 ppm characteristic of C,,. Solid-state magic angle spinning (MAS) I3CNMR spectra (see below) show five additional lines from C70 and indicate a Ca:C70 ratio of 3.9 f 0.6:l. Comparison of the integrated "C MAS N M R signal from the fullerene powder with the 13C N M R signal from a known weight of isopropyl alcohol indicates that the entire mass of the powder is accounted for in the 13CMAS spectrum, within the accuracy of the measurement (estimated to be 10%). Mass spectrometry shows no species other than Cm and C70 in the mass range 700-900 amu. Samples of pure Cm were prepared by column chromatography on neutral alumina, using hexane as the solvent. (1) Kroto, H.
W.;Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R.
E. Nature 1985, 318, 162.
(2) Kratschmer, W.; Lamb, L. D.;Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (3) Haufler, R. E.; Conceicao,J.; Chibante, L. P. F.; et al. J. Phys. Chem. 1990, 94, 8634.
Solid-state I3C N M R spectra a t 100.48 MHz were obtained on a Chemagnetics CMX spectrometer, using home-built static and MAS probes. Approximately 70 mg of powder iiere used for the N M R measurements. Frequency scales in the figures are in ppm with respect to T M S or in kilohertz with respect to the carrier frequency. Solid-state electron spin resonance (ESR) spectra at 9.25 GHz were also measured, using microwave power levels
