The dynamics of fullerene-cubane ͑C 60 ·C 8 H 8 ͒ cocrystal is studied combining experimental ͓x-ray diffuse scattering, quasielastic and inelastic neutron scattering ͑INS͔͒ and simulation ͑molecular dynamics͒ investigations. Neutron scattering gives direct evidence of the free rotation of fullerenes and of the libration of cubanes in the high-temperature phase, validating the "rotor-stator" description of this molecular system. X-ray diffuse scattering shows that orientational disorder survives the order/disorder transition in the low-temperature phase, although the loss of fullerene isotropic rotational diffusion is featured by the appearance of a 2.2 meV mode in the INS spectra. The coupling between INS and simulations allows identifying a degeneracy lift of the cubane librations in the low temperature phase, which is used as a tool for probing the environment of cubane in this phase and for getting further insights into the phase transition mechanism.

The rotor-stator molecular cocrystal C 60 ÁC 8 H 8 (fullerenecubane) has been investigated by 13 C nuclear magnetic resonance (NMR). The room-temperature spectrum obtained using 1 H-13 C cross-polarization technique shows two lines with chemical shifts identical with the shifts of the original molecular constituents demonstrating the lack of a strong electronic interaction between C 60 and C 8 H 8 . The temperature dependence of the spin-lattice relaxation time of the fullerene component confirms the existence of a first-order orientational ordering transition around 145 K. The activation energies of large-angle C 60 reorientations above and below the ordering transitions are 260 K and 570 K, respectively. The transition temperature and the activation energies are significantly lower than in other C 60 compounds. The 13 C spectrum remains narrow down to 115 K indicating that similarly to pristine C 60 , the molecular reorientational motion is still fast in the ordered phase on the NMR time scale.

High-pressure infrared transmission measurements up to 9.5 GPa were carried out on the rotor-stator molecular cocrystal C 60 ÁC 8 H 8 . Helium served as pressure transmitting medium, which intercalates into the C 60 ÁC 8 H 8 lattice. Thus, we investigated the pressure effects and effect of intercalation of helium into the C 60 ÁC 8 H 8 lattice. The pressure-induced shift of the vibrational modes of C 60 ÁC 8 H 8 shows an anomaly around 3 GPa. This anomaly can be interpreted in terms of the orientational ordering transition of fullerene molecules accompanied by a change in the crystal symmetry, which causes the splitting of the vibrational modes. We compare the value of the critical pressure to that obtained earlier [Thirunavukkuarasu et al.,

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The limited success in the prediction of structure is one of the most serious problems in the engineering of molecular crystals. Here we show that the packing of high-symmetry molecules such as ball-shaped rotating fullerenes, cube-shaped cubane, and octahedral-shaped mesitylene dimers give rise to the formation of cubic cocrystals with easily predictable lattice parameters. We present the synthesis and structure determination of Sc 3 N@C 80 -I h cocrystals with cubane (C 8 H 8 ) and mesitylene (C 9 H 12 ) and compare the new materials with related C 60 -and C 70 -based structures. In this family of materials, most atom-to-atom interactions are averaged out by the symmetry, and the crystal structures can be described in terms of classical molecule-to-molecule interactions. Size-dependent homoand heteromolecular contacts control the stability of the ball-cube and ball-octahedron systems creating several host−guest and recognition-controlled regions. The analysis of the global phase diagrams explains not only the stability of the observed materials, but also the instability of a missing derivative.