Silicon carbide has attracted much interest as hard material and semiconductor with wide band-gap. In both crystalline and amorphous phases, SiC bonding was described in terms of tetrahedral sp3 lattices with more or less chemical order. A completely new SiC bonding has been proposed, where silicon atoms bridge pure C60 molecules. Abundance and photofragmentation mass spectroscopy revealed the relatively high stability of (C60)nSin+, (C60)nSin-1+, and (C60)nSin-2+ species. Their corresponding structures were postulated to be polymers that form rings, chains, or branch structures, but there is no direct experimental evidence that would support this type of bonding interaction. As of yet, no calculation has been published on this novel type of chemical bonding between silicon and fullerenes. This paper presents the first systematic ab initio integrated molecular orbital study on singlet and triplet states of C60-Si and (C60)2Si as smallest building blocks for the larger, experimentally observered polymers.
We used the multi-layer ONIOM method we have developed in the past five years as our "nanotheoretical" tool of the study. ONIOM not only allows the integration of molecular orbital (MO) with molecular mechanics (MM) methods (IMOMM), but also the integration of molecular orbital with molecular orbital methods (IMOMO), which is a unique feature of the ONIOM method. In particular, we employed a two-layer ONIOM(B3LYP/6-31G(d):HF/STO-3G) strategy for geometry optimizations and frequencies, after carefully selecting chemical models for the Si-C60 interaction region. It was found that the systems are essentially charge transfer complexes with silicon donating nearly one electron to the fullerene(s) molecule. However, the interaction is not strictly ionic, as analysis of the electronic wave function shows that there is substantial covalent binding, for the singlet as well as for the triplet states. The C60Si system is stable against dissociation by about 38 kcal/mol, with Si being located on top over a formal C=C double bond. Other minima and transition states between them were located as well, but the binding energy never drops below 30 kcal/mol. Surprisingly, adding a second C60 unit actually reduces the binding energy by 20 kcal/mol, making the postulated polymeric structures metastable against dissociation into C60Si units and neutral C60. Our poster will carefully discuss the reason for this interesting finding on the basis of molecular orbitals and energetic differences between ONIOM layers.
Stephan Irle, Associate Scientist and System Manager
Cherry L. Emerson Center for Scientific Computation and Chemistry Department, Emory University
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