LOCATION OF MUONIUM AND HYDROGEN IN C-60 FULLERENE AND ASSOCIATED ELECTRONIC-STRUCTURE AND HYPERFINE PROPERTIES

The unrestricted Hartree-Fock (UHF) procedure is used to investigate t he locations, associated electronic structures and hyperfine interacti ons for muonium and hydrogen in C-60 fullerene. Our results indicate t hat from total energy considerations, in keeping with earlier investig ations, the exohedral model has the lowest energy. However, the energi es of the endohedral model involving the muonium (hydrogen) inside the fullerene and bonded to one of the carbon atoms, and of the muon at t he center are found to be almost equal, contrary to earlier results. T he hyperfine interaction constant for the endohedral site is in good a greement with that required to explain the lower observed muon spin-ro tation (mu SR) frequency in the C-60-muonium system. The same appears to be the case for the exohedral model. However, there seems to be som e uncertainty about the theoretical result in the latter case due to s ignificant admixtures of higher spin states in the UHF wave-function. Additionally, in solid fullerene, the calculated location of the muoni um for the exohedral model is such that it could be bonded to two full erene molecules and therefore a muonium attached to a simple fullerene may not be representative of the exohedral state. This feature as wel l as the difficulty for the exohedral model of explaining the observed equality of the correlation times for relaxation effects associated w ith both mu SR and C-13 relaxation times in nuclear magnetic resonance (NMR) experiments suggests that the endohedral model for muonium cann ot at present be ruled out as a viable model in favor of the exohedral model. Possible avenues for future investigations to resolve some of the problems for both exohedral and endohedral models are discussed. R esults obtained for muonium at the center of fullerene are presented a nd compared to the features of the observed high frequency mu SR signa l, and possible improvements in theory are discussed.