NMR investigation on molecular dynamics of C60 and C70 solids (original) (raw)
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Physical Review B
Solid C 60 was stored in NO under high pressure and the gas molecules NO were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Its 13 C nuclear magnetic resonance magic angle spinning spectra composed of a primary resonance at 143.7 ppm, accompanied by two minor peaks shifted 0.4 and 0.8 ppm downfield, respectively. The dopant was found to depress its phase-transition temperature at 260 K in pure C 60 , and to reduce substantially the drop ⌬Ј at the phase-transition temperature. Furthermore, the spectral features associated with relaxation during glass transition at lower temperature, as observed in impedance spectra, were smeared. The fraction of P orientation below T c has been calculated from the impedance data. These results show that a nearly completely P oriented phase had occurred in (NO) 0.1 C 60 , and this phase is favored by a negative pressure on C 60 lattice exerted by NO, as well as the electrostatic interaction between the two.
13C NMR spectroscopy of chloroform-solvated C60
By means of pulsed Nuclear Magnetic Resonance and Magic Angle Spinning we have studied crystalline C60 × 0.2 C70 x 1.5(CHC13). We present data of the spin lattice relaxation (Tl) of 13C nuclei as a function of temperature between 140 and 310K. Their analysis has been made in terms of the chemical shift anisotropy model. Our results show that the high temperature free rotation of C60 molecules is not suppressed and that there is no structural phase transition near 260 K as observed in pure crystals of C60.
Structural phase transitions and orientational ordering in C70
Chemical Physics, 1993
The thermal behavior of solid CT0 has been studied by synchrotron X-ray powder diffraction and differential scanning calorimetry. The equilibrium solid state structures formed by C,a were solved by full profile refinement techniques in which orientational and packing disorder were explicitly accounted for. Above 345 R, CT0 forms a plastic crystal, with an equilibrium faceGentere.d cubic structure. At lower temperatures, orientational freezing occurs in two stages. Between 295 and 345 K, disorder persists only about the long axis of the molecule, and the lattice undergoes a rhombohedral distortion. Below 295 R, the rhombohedral lattice undergoes a further distortion, resulting in a previously unobserved monoclinic structure, in which the molecules are presumed to be essentially static. At all temperatures, however, the structure of CT0 retains an ABC packing sequence.
The formation of dimerized molecules of C60 and their solids
Carbon, 2008
The formation of dimerized molecules of C 60 was studied using the Brenner potential. Several structures are obtained which have been classified into three categories, viz., dumbbells, and fused and coalesced dimers, similar to those obtained earlier, using the Tersoff potential. However, there are differences in the binding energies obtained using these two potentials. From these formations, we chose four forms of the composite dimer molecule as cyclo dumbbell, peanut, capped armchair (5,5) and zigzag nanotubes (10,0) to form crystalline solids. Calculations have been performed by placing them in various crystal structures, i.e. monoclinic, hexagonal and cubic close packed. To obtain stable dimerized crystal structures, the dimer molecules are considered to be rigid, interacting via atomatom interaction of 6-exponential form. The monoclinic phase has been found to be energetically most stable for each of the dimers. Various structural, thermodynamic and phonon related properties of the stable dimer phases were investigated.
Crystal structure of solid C70
Pramana, 1993
Detailed analysis of the room temperature X-ray powder diffraction data of pure solid C7o is reported. C7o prepared by slow evaporation from toluene solution adopts an hcp structure (space group P63/mmc ) with a = 10.53(1) A and c = 17.24(1) A. C7o sublimed on to Si wafer adopts an fcc structure with a = 14.89(I) A. The occurrence of both the hcp and fcc phases is rationalized in terms of cohesive energy calculations. Theoretical calculations of the diffraction pattern for the hcp structure, taking into account (a) orientational disorder amongst the molecules (b) presence of stacking faults and (c) a fraction of the sample to be amorphous/microcrystailine is seen to provide very good agreement with the experimental diffraction pattern.
Phase transitions in solid C70 studied by dynamical mechanical analysis
Solid State Communications, 1994
The linear thermal expansion of hop single-crystal Cv0 was measured in the temperature range 95-770 K, using a Dynamical Mechanical Analyzer. A 5% change in the length L of the samples in c-direction occurs at 360 K on heating and at 305 K on cooling. Applying a uniaxial force along the hexagonal c-axis to the samples shifts the transition tolower temperatures and deforms the shape of the hysteresis loop. Another transition, with a hysteresis of 4 K, was observed at 283 K. There is some evidence of an additional anomaly in L of a virgin sample around 300 K. This anomaly shows up more clearly in low frequency elastic constant measurements.
Phase diagram, structure, and disorder in C60 below 300 K and 1 GPa
Solid State Communications, 1995
Eariier structural studies have shown that the pentagon-to-hexagon orientation ratio in the orientationally ordered simple cubic phase of Cbo decreases under pressure From anomalies observed in the compressibility and thermal conductivity of C60 under pressure we have deduced a pressure-temperature phase diagram for this substance in the range below 300 K and I GPa (10 kbar). We conclude that C6" forms a new, completely "hexagon" ordered structural phase above about 0 G GPa at 150 K (1.2 GPa at 300 K), and that the glass transition shifts upwards in T under pressure by 54 K GPa-' However, above 0 1 GPa, pentagon-to-hexagon orientation relaxation seems to occur on heating at an almost pressure independent temperature near 100 K
Solid State Communications, 2000
In this work, we present 13 C MAS NMR results of the rhombohedral (rh-2D) and tetragonal (tet-2D) two-dimensional polymerised C 60 , under high pressure, using the paramagnetic O 2 as a chemical shift and relaxation agent. The 13 C MAS NMR spectrum of the rh-2D polymer (under P O 2 2400 Torr) shows two resonances, a broad and intense line around 152.8 and a small one at 78.3 ppm. The spectrum of the tetragonal tet-2D polymer (under P O 2 2500 Torr), shows two isotropic lines around 150.6 and 77.9 ppm. For both polymers, the intense line around 150 ppm has several components, related to the deformation of the spherical shape of the C 60 molecules. We attribute the observed group of lines to the inequivalent sp 2 carbons on the C 60 molecules, and the small line around 78 ppm to the intermolecular sp 3 bonding carbons.
Rotational dynamics and polymerization of C60 in C60-cubane crystals: A molecular dynamics study
Chemical Physics, 2008
We report classical and tight-binding molecular dynamics simulations of the C60 fullerene and cubane molecular crystal in order to investigate the intermolecular dynamics and polymerization processes. Our results show that, for 200 and 400 K, cubane molecules remain basically fixed, presenting only thermal vibrations, while C60 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 copolymerizations 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.