Ba x C 60 fullerides: π Electronic peculiarities of the C 60 molecule and their consequences for the solid state

The electronic structure of Ba x C 60 fullerides was studied theoretically under special consideration of π electronic effects in the C 60 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 C 60 fullerides. Ba x C 60 solids with x =0, 3, 4, 6 are insulators. For a Ba 5 C 60 model extrapolated from the crystal structure of Ba 6 C 60 , a finite band gap is also predicted. For a Ca 5 C 60 ‐like structure of Ba 5 C 60 , a quasi‐degeneracy between a metallic configuration and an insulating Mott‐like state was found. With an increasing Ba‐to‐C 60 charge transfer (CT), sizable changes in the π system of C 60 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 C 60 12− , 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 C 60 and C 60 12− , the net π bonding is not weakened. The INDO CO results of the Ba x C 60 solids are supplemented by INDO MO and ab initio (3‐21 G* split‐valence basis) calculations of molecular C 60 and some highly charged anions. Ab initio geometry optimizations show that the bond alternation of C 60 with short 6–6 and long 6–5 bonds is inverted in C 12− 60 . The high acceptor capability of C 60 is explained microscopically on the basis of quantum statistical arguments. In the π electron configurations of C 60 and C 60 12− , 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. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65 : 333–373, 1997