FLATTENED FULLERENES

Quantum-chemical calculations of giant flattened fullerenes C-n (lenti l-shaped) have been carried out. The topology, molecular and electroni c structure of these fullerenes have been studied. Such molecules cons ist of two identical coronenoid fragments of a graphite layer, which a re arranged one above the other, and a system of polycondensed five- a nd six-membered cycles, which form a side surface of the cluster. Poly hedral structures with isolated pentagons of three symmetry types (D-6 h, D-6d, and D-3h) have been considered. The topology of these structu res is described in terms of planar molecular graphs. Electronic struc tures of eleven flattened lentil-shaped C-n clusters (n = 72-216) have been studied in the pi approximation. Most of the considered systems have closed or quasi-closed electron shells (according to Huckel) and rather large energy gaps separating the highest occupied and lowest un occupied MO, which is indicative of their kinetic stability. Fragments of the potential energy surfaces of the C-72 and C-96 fullerenes have been studied by the MNDO, AM1, and MNDO/PM3 methods. For the C-96 clu ster, two local energy minima, which correspond to the lentil-shaped i somers with D-6h and D-6d symmetry, have been determined. As a result of optimization of geometric parameters, it was found that all three m ethods give close values of heights (H = 6.7 Angstrom) and diameters ( D = 9.8 Angstrom) for both isomers. The clusters change to quasi-two-d imensional systems (H much less than D) with increasing sizes of coron enoid fragments.