Showing papers by "Éva Kováts published in 2009"
TL;DR: The low-temperature specific heat was analyzed in terms of the Debye-Einstein model to estimate the librational energies of the fullerenes and Debye temperatures and found very similar values for the two cocrystals, approximately Elib and TDebye values.
Abstract: Cocrystals of cubane and fullerenes, C60-cubane and C70-cubane, show distinct rotational ordering transitions. We studied the corresponding structural changes with temperature-dependent X-ray diffraction and the thermodynamics of the phase transitions with adiabatic microcalorimetry and differential scanning calorimetry. C60-cubane has one phase transition around 130 K from a high-temperature fcc phase with freely rotating C60 to a low-temperature orthorombic phase in which the fullerene rotation is frozen. The corresponding enthalpy change is approximately 1170 J/mol, and the entropy change is 9.6 J/(mol K). C70-cubane has two phase transitions. Around 380 K, the high-temperature fcc phase with freely rotating C70 transforms into a bct phase in which the C70 rotates uniaxially around an axis that precesses around the c direction with a full opening angle of 40°. Around 170 K, the uniaxial rotation also freezes out, with an accompanying structural transition to monoclinic and enthalpy and entropy changes ...
12 citations
TL;DR: In this article, the rotor-stator molecular cocrystal C 60 ·C 8 H 8 (fullerene-cubane) has been investigated by 13 C nuclear magnetic resonance (NMR).
Abstract: The rotor-stator molecular cocrystal C 60 ·C 8 H 8 (fullerene-cubane) 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.
2 citations