Authors: X. Zhao, Z. Slanina and E. Osawa
Affilation: Toyohashi University of Technology, Japan
Pages: 122 - 125
Keywords: isomeric fullerenes, enthalpy, entropy, interplay, computing
Early results from the eighties for small carbon clusters like C4 or C6 have suggested that the combined quantum-chemical and statistical-mechanical computations could show some interesting temperature effects also for isomeric fullerenes. This feature was first demonstrated on some C50 isomers. The fullerene research has indeed supplied several sets of isomeric higher fullerenes. At present over twenty stable fullerenes Cn have been identified with n varying from 60 to 96. Several such mixtures of fullerene isomers have been computed (C76 till C94 ) and an agreement with observations found. Hence, the computations have demonstrated that the presumption of partial, inter-isomeric thermodynamic equilibrium is actually well working. One of the biggest systems computed within the partial thermodynamic-equilibriumtreatment so far is C92 with 86 IPR isomers. The reported computations are based on semiempirical (SAM1), ab initio SCF (HF/4-31G), and DFT (B3LYP/6-31G*) approaches and RRHO partition functions. The entropy considerations also indicate an interesting way for non-IPR structures stabilization, illustrated on the C72 system. The approach is also applied to smaller fullerenes like C36 . As fullerenes are formed at high temperatures, entropy effects should be important owing to the temperature enhancement. It turns out for C36 that in the most sophisticated computational approximation used, B3LYP/6-31G*, just two structures are controlling the region of higher temperatures: the conventional fullerenes D6 h and D2 d. Although the D2 d ground state has to prevail at low temperatures, the stability order is reversed by entropy effects already at a temperature of 1500 K. The case of non-isomeric species is also discussed. Fullerenes and nanotubes have been considered as new agents and materials for molecular electronics including molecular memories. Optimized preparation and understanding of the material properties is not possible without this new type of enthalpy/entropy evaluations.