NMR evidence for the charge transfer from interstitial NO or O2 to molecule C60 in solid C60

Solid C₆₀ was stored in NO and O₂ under high pressure, and the molecule NO and O₂ were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Their ¹³C NMR MAS spectra composed of a primary resonance at 143.7 ppm, accompanied by several minor peaks shifted downfield respectively. The spectra at various temperatures from 210 to 300 K have been measured, and the chemical shifts of the minor peaks referenced to the primary resonance versus inverse temperature show straight lines. But, the lines do not go through origin, and their intercepts are equal to 0.13 ppm or its multiple. Due to a non-zero intercept at high-temperature limit, we proposed that there is a charge transfer from NO and from O₂ to C₆₀. The quantity of the charge transferred has been calculated, that is about 0.065 q_e^- (q_e^- for the elementary charge on an electron).     

Crystal field and molecular structure in solid C60

The crystal field in the orientationally disordered phase of C₉₀-fullerite is derived from an intermolecular potential model, which takes into account the geometric difference between double bonds and single bonds. The molecules are modelled as rigid bodies, atoms and single bonds are treated as single interaction centers, while double bonds are described by a distribution of interaction centers along the bond. The crystal field is expanded in terms of cubic rotator functions. The calculated expansion coefficients are compared with empirical values derived from diffraction data. The angular dependence of the crystal field, resulting from an anticlockwise rotation of the molecule around the [111] axis, exhibits an absolute and a secondary minimum at angles of 98° and 38° respectively. The self interaction of the molecule in a deformable lattice is investigated.     

Coupling of orientational and translational modes in solid C60 and C70

The orientational phase transitions in solid C₆₀ and C₇₀ are accompanied by quite different anomalies in the crystalline strains. In solid C₆₀ the phase transition Fm3m→Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X₅ ⁺ irreducible representation (irreps) of Fm3m. These modes are the primary order parameters (oops) and their number is equal to the number of irreps of T_2g and T_1g symmetry within the manifolds under consideration. Taking into account irreps up to the manifold 1=12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c₁₁ at the phase transition. In solid C₇₀ (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of T_2g symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m → R3m. Finally we investigate the transition from the rhombohedral phase to a low temperature monoclinic phase. This transition in antiferro-rotational.     

Neutron scattering study of intermolecular excitations in solid C60 and C70

The spectrum of intermolecular excitations in solid C₆₀ and C₇₀ is investigated by neutron time-of-flight spectroscopy. The quasielastic scattering above the ordering transition is studied in detail and compared to a rotational diffusion model. Below the transition the scattering spectra have essentially a three peak structure with a prominent libronic excitation near ±2 meV for both fullerenes. A detailed analysis gives evidence for a broader distribution of librational modes with an additional pronounced maximum near 5 meV.     

Free energy and orientational phase transition in solid C60

On the basis of a microscopic model, the free energy of the orientationally disordered fcc crystalline phase of C₆₀-fullerite is derived up to fourth order in the rotational density fluctuations. The order parameter variables are symmetry adapted rotator functions which belong to thel=6 andl=10 manifolds. They are basis functions ofT _2g representations of the cubic group. The phase transition to thePa3 structure is discussed, and is found to be of first order. The orientationally ordered phase is described in terms of condensed rotator functions.     

Electron delocalization and dimerization in solid C59N doped C60 fullerene

Electron spin resonance and ab initio electronic structure calculations show an intricate relation between molecular rotation and chemical bonding in the dilute solid solution. The unpaired electron of C59N is delocalized over several C60 molecules above 700 K, while at lower temperatures it remains localized within short range. The data suggest that below 350 K rigid C59N-C60 heterodimers are formed in thermodynamic equilibrium with dissociated rotating molecules. The structural fluctuations between heterodimers and dissociated molecules are accompanied by simultaneous electron spin transfer between C60 and C59N molecules. The calculation confirms that in the C59N-C60 heterodimer the spin density resides mostly on the C60 moiety, while it is almost entirely on C59N in the dissociated case.