Structure and dynamics of fullerene-enclosed small molecules

Synthetic studies of cage-opening reactions on C60 were carried out, and a novel open-cage fullerene was synthesized by means of a hybrid method that combined two previously published syntheses. The resulting fullerene was found to incorporate water spontaneously into its cage, and its structure was confirmed with MALDI-TOF mass spectrometry and NMR spectroscopy. A simple reduction of open-cage fullerenes was discovered and mass spectrometric fragmentation studies suggested that this new compound might be pyrolyzed to create a compound with a smaller orifice. Further pyrolysis studies were undertaken with open-cage fullerenes on platinum surfaces, and mass spectrometry indicated the possibility of regenerating C60 from the open-cage fullerenes. Ammonia was shown to enter fullerene cages under elevated pressures at room temperature, with an incorporation fraction of 35-50%. The encapsulation of ammonia was spontaneous even at liquid ammonia temperatures and atmospheric pressure, yielding an incorporation fraction of less than 5%, as determined by NMR spectroscopy. Novel 14N-1H NMR decoupling methods were employed to examine the properties of ammonia, and it was shown that the large quadrupolar moment of 14N relaxes the protons very efficiently. Methane was encapsulated into an open-cage fullerene with an incorporation fraction of 30-35%. Its encapsulation was confirmed with NMR spectroscopy and MALDI-TOF mass spectrometry. Theoretical studies were undertaken to determine the translational-rotational behavior of methane inside of an open-cage fullerene, as well as its vibrational dynamics. These studies revealed that the symmetric (a1) stretch of CH 4 is blueshifted by 25 cm-1 from its gas-phase value because of unfavorable interactions of the methane protons with the fullerene cage. The theoretical results also indicated that the heat capacity of a molecule inside a fullerene shows anomalous features, caused by the strongly anharmonic translational potential induced by the fullerene cage. The equilibrium incorporation fraction obtained for methane by means of this theoretical model is in good agreement with the experimentally observed incorporation fraction.