Ailan Cheng

C60O: a molecular dynamics study of rotation in the solid phase solid phase

Constant-pressure molecular dynamics simulations have been performed for solid C60O using a pair-wise additive atom–atom potential. The simulation results suggest that the presence of oxygen atoms on C60 framework does not change the lattice structure significantly. The onset of the orientational freezing is predicted to occur in a similar temperature range as pure C60. The molecular dipoles prefer to point along crystal (100) directions so that the oxygen atom can occupy one of the vacant octahedral sites. Since the cavity associated with the octahedral site is large, solid C60O has the same structure as C60(i.e. f.c.c.) with essentially the same room-temperature lattice parameter. At room temperature, the C60O molecules are found to be rotating anisotropically. This prediction could be tested by appropriate NMR measurements.

Orientational ordering in the solid fullerene oxide: C60O

Constant-pressure molecular dynamics simulations have been employed to investigate the orientational ordering in the solid phases of fullerene oxide, C60O. A pairwise additive atom–atom intermolecular potential model developed for solid C60 is modified slightly to reflect the functionalized character of the C60O molecule. The simulation results indicate that at low temperature the carbon cages are frozen into a Patext-decoration:overline3-like structure, as in the pure C60 solid. Most oxygen atoms point randomly to one of the neighbouring octahedral interstitial sites (i. e.〈100〉 direction) but about 20 % point to the smaller tetrahedral sites (〈111〉 direction). Above the transition temperature, estimated to be around 210 K compared with the measured value of 278 ± 2 K, C60O molecules rotate about the centre-of-mass–oxygen axis. The bridging oxygen atoms tend to wobble in their interstitial sites rather freely but they cannot move from one pocket to another. However, at very high temperature (ca. 800 K), the oxygen atoms are able to hop between different interstitial sites on the molecular dynamics timescale.

Intermolecular interactions and the nature of orientational ordering in the solid fullerenes C60 and C70

We have proposed an intermolecular potential for C60 molecules that not only reproduces the correct low-temperature structure, but also correlates a wide range of experimental properties, including the molecular reorientational time in the room-temperature rotator phase, the volume change at the orientational ordering transition, and the librational frequencies in the low-temperature phase. The low- pressure phases in solid C70 have been explored using constant-pressure molecular dynamics and an intermolecular potential derived from one that gives an excellent account of the properties of solid C60. The molecular dynamics calculations predict three low-pressure phases: a high-temperature rotator phase, a partly ordered phase with trigonal symmetry, and an ordered monoclinic phase. The calculations on C70 were carried out on a cluster of IBM RS/6000s, operating in parallel.

Intermolecular Interactions and the Nature of Orientational Ordering in the Solid Fullerenes CFormula and CFormula [and Discussion]

We have proposed an intermolecular potential for C60 molecules that not only reproduces the correct low-temperature structure, but also correlates a wide range of experimental properties, including the molecular reorientational time in the room-temperature rotator phase, the volume change at the orientational ordering transition, and the librational frequencies in the low-temperature phase. The low- pressure phases in solid C70 have been explored using constant-pressure molecular dynamics and an intermolecular potential derived from one that gives an excellent account of the properties of solid C60. The molecular dynamics calculations predict three low-pressure phases: a high-temperature rotator phase, a partly ordered phase with trigonal symmetry, and an ordered monoclinic phase. The calculations on C70 were carried out on a cluster of IBM RS/6000s, operating in parallel.