Charge transfer, polarizability and stability of Li–C60 complexes
Introduction
Complexation between C60 and alkali metals can be divided into two types: endohedral and exohedral complexes. The endohedral complexes are ferroelectric 1, 2while the exohedral complexes have a wide range of properties varying from superconductors to insulators, thus depending strongly on the ratio of metal and C60. The M–C60 complexes, where M is Na, K or Cs, have been widely synthesized. Their properties have been intensively elucidated 3, 4, 5, 6, 7, 8, 9, 10, but Li–C60 complexes have been left open for new discovery both experimentally and computationally 11, 12.
Many outstanding features of the M–C60 complexes are rooted in electron donation to C60 by the metal atom(s). This charge transfer determines the stability and electronic properties, e.g. dipole moment, of the complexes. In our study, the charge transfer was investigated both quantitatively, in terms of localized charge distribution of the complexes, and qualitatively, in terms of the stability and dipole moment of the complexes.
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Methods and calculation details
STO-3G [13], 6-31G 14, 15, 16and DZP [17]basis sets have been examined for the Li–C60 system and reported in detail in our previous works 18, 19. It was found that the 6-31G basis set is most appropriate for this system in terms of accuracy and computational cost. Therefore, we have selected this basis set for the ab initio calculations throughout this Letter.
Unrestriced Hartree–Fock SCF calculations, using the GAUSSIAN 92 program [20], have been performed for a number of selected configurations
Electron distribution of Li–C60 endohedral and exohedral complexes
One major advantage of quantum chemical calculations over an experimental approach is that they allow the electronic structure to be computed, especially the charge distribution in molecular systems, which is the key to many physical and chemical properties. Previous experimental and theoretical studies have found electron transfer from a metal to C60 in metal-doped C60, even to the degree that the metal is completely ionized in the case of endohedral metal–C60 complexes 22, 23, 24. Although
Acknowledgements
The Austrian–Thai Center (ATC) for Computer-Assisted Chemical Education and Research in Bangkok is acknowledged for provision of computational resources. We would like to thank Dr. David Ruffolo for proofreading the manuscript. T. K. acknowledges the Faculty of Science, Mahidol University for supporting his work through a Research Initiative Grant.
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