A molecular dynamics study of the rotational dynamics and polymerization of C 60 in C 60 -cubane crystals
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1130-W06-31
A molecular dynamics study of the rotational dynamics and polymerization of C60 in C60cubane crystals Vitor R. Coluci1,2, Fernando Sato2, Scheila F. Braga2, Munir S. Skaf 3, and Douglas S. Galvão2 1
Center for High Education on Technology, University of Campinas - UNICAMP 13484-332, Limeira, SP, Brazil 2 Applied Physics Department, Institute of Physics P.O.Box 6165, University of Campinas UNICAMP 13083-970, Campinas, SP, Brazil 3 Institute of Chemistry P.O.Box 6154, University of Campinas - UNICAMP 13084-862, Campinas, SP, Brazil ABSTRACT Recently, heteromolecular crystals of fullerene C60 and cubane (C8H8) have been synthesized. For some temperatures the C60 molecules are free to rotate whereas cubanes behave like a static bearing in a so-called rotor-stator phases. In this work we report classical and tightbinding molecular dynamics simulations in order to investigate the rotor-stator dynamics and polymerization processes. Our results show that, for 200 K and 400 K, cubane molecules remain basically fixed, presenting only thermal vibrations within the timescale of our simulations, 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). Random copolymerization among cubanes and fullerenes were observed when temperature is increased, leading to the formation of a disordered structure. INTRODUCTION Recently, heteromolecular crystals of C60 fullerene and cubane (C8H8) [1] have been prepared from aromatic solutions by evaporating solvent and adding isopropyl alcohol as precipitant [2]. These crystals present an interesting phase diagram showing orientationalordering phase transitions. At atmospheric pressures and temperatures below 140 K, the crystal exhibits an orthorhombic symmetry. From 140 K up to 470 K, the C60 molecules are located on the lattice sites of an fcc crystal with the cubane molecules on the octahedral voids. In this temperature range, the C60 molecules are free to rotate whereas cubanes behave like static bearings in a so-called rotor-stator phase [2]. Above 470 K, solid-state reactions take place among cubanes and fullerenes leading to different crystalline and, eventually, amorphous phases due to copolymerization [3]. Similar phase diagrams are also observed for other fullerenes, such as C70 and C84 [4]. Despite experimental studies about the polymerization process that takes place during high temperature treatments [3,4] and under pressure [5], providing information of the possible conformations of the resulting polymer, the molecular aspects involved in these processes are still largely unknown. For instance, experimental results suggest that the resulting compound after thermal treatment (470 K) is a copolymer of C60 with a decomposition product of cubane (dihydropentalene) [4]. However, other possibilities for the copolymer can not be ruled out even when other decomposition products of
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