Rotational dynamics and polymerization of C60 in C60-cubane crystals: A molecular dynamics study (original) (raw)
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The Journal of Chemical Physics, 2008
We report classical and tight-binding molecular dynamics simulations of the C 60 fullerene and cubane molecular crystal in order to investigate intermolecular dynamics and polymerization processes. Our results show that, for 200 K and 400 K, cubane molecules remain basically fixed, presenting only thermal vibrations, while C 60 fullerenes show rotational motions. Fullerenes perform "free" rotational motions at short times (< ∼ 1 ps), small amplitude hindered rotational motions (librations) at intermediate times, and rotational diffusive dynamics at long times (> ∼ 10 ps). The mechanisms underlying these dynamics are presented. Random copolymerization among cubanes and fullerenes were observed when temperature is increased, leading to the formation of a disordered structure. Changes in the radial distribution function and electronic density of states indicate the coexistence of amorphous and crystalline phases. The different conformational phases that cubanes and fullerenes undergo during the copolymerization process are discussed.
Lattice dynamics of a rotor-stator molecular crystal: Fullerene-cubane C_{60}⋅C_{8}H_{8}
Physical Review B, 2010
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.
Stability of C60 chains: molecular dynamics simulations
Journal of Molecular Graphics and Modelling, 2005
A linearly aligned structure of three C 60 fullerene, interconnected by two benzorods of same size, have been investigated under heat treatment. The overall structure resembles a section of a beaded string. Nine different lengths of benzorods have been considered, and the effect on the thermal stability have been investigated by means of molecular dynamics method. It has been found that the structure is thermally stable up to elevated temperatures, and the linear alignment of the structure is persistent, up to the temperature of decomposition.
13C NMR investigation of fullerene-cubane C60·C8H8cocrystals
Physica Status Solidi B-basic Solid State Physics, 2009
The rotor–stator molecular cocrystal C60·C8H8 (fullerene–cubane) has been investigated by 13C nuclear magnetic resonance (NMR). The room-temperature spectrum obtained using 1H–13C 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 C60 and C8H8. 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 C60 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 C60 compounds. The 13C spectrum remains narrow down to 115 K indicating that similarly to pristine C60, the molecular reorientational motion is still fast in the ordered phase on the NMR time scale.
Pressure-induced polymerization of fullerenes: A comparative study of C60 and C70
Physical Review B, 1996
To investigate the polymerization of fullerenes, solid C 60 and C 70 have been subjected simultaneously to high pressures and temperatures, with pressures up to 7.5 GPa and temperatures up to 800°C. X-raydiffraction measurements indicate that the fcc solid C 60 transforms to an orthorhombic structure consisting of a polymerized linear chain of C 60 molecules. The associated changes in the intramolecular vibrational modes have been probed through Raman and infrared measurements. In contrast to solid C 60 , under similar conditions, polymerization is not observed in the case of solid C 70 , although there is a structural transition from the initial hcp structure to a rhombohedral structure. This lack of polymerization in C 70 is discussed in terms of the structure of solid C 70 under high pressure and temperature conditions, coupled with the topochemical features of the C 70 molecule.
Structural transformation in fullerene solids C60 and C70 at low temperatures—new insights
Journal of Physics and Chemistry of Solids, 2000
This paper reviews several recent investigations by the authors' group concerning the structural transitions at low temperatures in the fullerene solids C 60 and C 70. In the case of C 60 it has been shown that the heat capacity variation with temperature in the ordered phase reflects the contribution due to configurational reorientation of the molecules in the solid between two energetically favourable orientations. Evidence is presented for the possible occurrence of an intermediate phase in solid C 60 in a narrow temperature range just below the ordering transition. In the case of C 70 the nature of transition as well as resulting lowtemperature structure is kinetic dependent. The equilibrium structure is obtained by the simultaneous freezing of all orientations of the molecules which brings to the fore the importance of polar cap interaction between the molecules of the basal plane and those in the midplane. It has been shown by a potential energy calculation that the structure obtained by slow cooling in experiment corresponds to the minimum energy configuration.
physica status solidi (b), 2006
We present the formation, structure and properties of a new family of molecular crystals of distinct rotating and static components: the derivatives of fullerenes with cubane. The new crystals consist of alternating arrays of the constituents, stabilized by the recognition of the complementary molecular surfaces. The concave cubanes match six rotating fullerenes in octahedral configuration, while the six nearest neighbours of the convex fullerenes may form either octahedral or trigonal prismatic configurations. Fullerenes rotate freely in the 'molecular bearing' of cubanes. Lower symmetry crystals of similar properties form when cubane is replaced by 1,4-diethynyl cubane. The following properties will be discussed: (i) The significant size and shape recognition makes possible the design of the crystal structure of new higher fullerenes and substituted cubane derivatives. (ii) The phase diagram of the rotor -stator crystals can be constructed in terms of the ball diameter of rotating fullerenes. (iii) The molecular geometry of the constituents has an influence on the structure and dynamics of the lower symmetry variants. (iv) At elevated temperatures the decomposition of cubane or diethynyl cubane induces topochemical copolymerization with the surrounding fullerenes.
We have simulated (by the molecular dymanics (MD) method) the dynamics of fullerenes (C 60) in an extremely small cluster composed of only as many as seven C 60 molecules. The interaction is taken to be the full 60-site pairwise additive Lennard-Jones (LJ) potential which generates both translational and anisotropic rotational motions of each molecule. Our atomically detailed MD simulations discover the plastic phase (no translations but active reorientations of fullerenes) at low energies (temperatures) of the (C 60) 7 cluster. We provide the in-depth evidence of the dynamical solid-liquid bistability region in the investigated cluster. Moreover, we confirm the existence of the liquid phase in (C 60) 7 , the finding of Gallego et al (1999 Phys. Rev. Lett. 83 5258) obtained earlier on the basis of Girifalco's model, which assumes single-site only and spherically symmetrical interaction between C 60 molecules. We have calculated the translational and angular velocity autocorrelation functions and estimated the diffusion coefficient of fullerene in the liquid phase.