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Abstract

The electronic structure of Ba_xC₆₀ fullerides was studied theoretically under special consideration of π electronic effects in the C₆₀ molecule. Band structure data were derived by an intermediate neglect of differential overlap (INDO) crystal orbital (CO) approach. Different electronic configuration were evaluated in the Ba-doped C₆₀ fullerides. Ba_xC₆₀ solids with x=0, 3, 4, 6 are insulators. For a Ba₅C₆₀ model extrapolated from the crystal structure of Ba₆C₆₀, a finite band gap is also predicted. For a Ca₅C₆₀-like structure of Ba₅C₆₀, a quasi-degeneracy between a metallic configuration and an insulating Mott-like state was found. With an increasing Ba-to-C₆₀ charge transfer (CT), sizable changes in the π system of C₆₀ occur. In the neural molecule and for not too high an electron count, the π electrons form more or less electronically isolated hexagon–hexagon (6–6) “double” bonds with only minor hexagon–pentagon (6–5) “double-bond” admixtures. In the vicinity of C¹²⁻₆₀, the 6–6 bonds have lost most of their double-bond character while it is enhanced for the 6–5 bonds. In highly charged anions, the π electron system of the soccer ball approaches a configuration with 12 decoupled 6π electron pentagons. For electron numbers between C¹²⁻₆₀, the net π bonding is not weakened. The INDO CO results of the Ba_xC₆₀ solids are supplemented by INDO MO and ab initio (3-21 G* split-valence basis) calculations of molecular C₆₀ and some highly charged anions. Ab initio geometry optimizations show that the bond alternation of C₆₀ with short 6–6 and long 6–5 bonds is inverted in C¹²⁻₆₀. The high acceptor capability of C₆₀ is explained microscopically on the basis of quantum statistical arguments. In the π electron configurations of C¹²⁻₆₀, the influence of the Pauli antisymmetry principle (PAP) is minimized. The quantum statistics of (π) electron ensembles with a deactivated PAP is of the so-called hard-core bosonic (hcb) type. In these ensembles, the on-site interaction is fermionic while the intersite interaction is bosonic. Energetic consequences of the quantum statistical peculiarities of π systems are explained with the aid of simple model systems; we selected annulenes and polyenes. Computational tools in this step are Green's function quantum Monte Carlo (GF QMC) and full configuration interaction (CI) calculations for the π electrons of the model systems. These many-body techniques were combined with a Pariser–Parr–Pople (PPP) Hamiltonian