RESUMEN
Fullerenes and cubane (C(8)H(8)) can be arranged to form heteromolecular crystals that exhibit interesting crystal phases. Experimental measurements indicate a rotor-stator phase for C(60)-cubane crystals in which the C(60) molecules rotate freely whereas cubane molecules are essentially static. A similar phase is found for C(70)-cubane crystals but, due to C(70)'s asymmetry, hindered rotations can be observed in specific crystal phases. Details of the rotational dynamics of the fullerenes in these heteromolecular crystals are difficult to be completely assessed by experiments. To this end, we have performed classical molecular dynamics simulations of C(70)-cubane crystals to investigate the behavior of C(70) fullerenes and cubanes in the face-centered cubic and body-centered tetragonal crystallographic phases. Our simulations show that, in the cubic phase, C(70) molecules are allowed to freely rotate whereas cubanes act as molecular bearings. In the tetragonal phase, the cubane molecules also remain practically fixed and the rotation of C(70) fullerenes becomes hindered. In this phase, C(70) molecules rotate around the fivefold axis, which in turn precesses about the c crystallographic direction of the unit cell. Details regarding the dynamics (e.g., energy barriers, reorientational relaxation processes, and phonon-libration coupling) of the C(70) molecules in both crystal phases are discussed. In general, our results agree with previous experimental findings for C(70)-cubane crystals.
RESUMEN
We developed a simple pair-additive Lennard-Jones plus Coulomb potential for molecular simulations of the trivalent cation Al(3+) in water which accounts reasonably well for the behavior of aluminum aqueous solutions. The model predicts an octahedral first hydration shell containing 6 water molecules and a trigonal second shell with 12 molecules on average, in good agreement with the available experimentally determined structure. The peak positions of the cation-oxygen radial distribution function are only slightly compressed compared to the x-ray structure, the hydration enthalpy is 10% too low, and the cation self-diffusion coefficient and the single-particle second rank reorientational time are in excellent agreement with inelastic neutron scattering and NMR spectroscopy data, respectively. The model also captures the essential vibrational features of the hydrated [Al(H(2)O)(6)](3+) complex. It predicts the main O-Al-O bending mode frequency to within approximately 5%, but significantly overestimates the frequency of the totally symmetric Al-O stretching mode. Overall, the accuracy of the proposed model is as good as the best available classical potentials, if not better in some aspects, with a much simpler functional form, which makes it an attractive alternative for computer simulations of Al(3+) in more complex aqueous and biomolecular systems.
