Covalently-bonded ''onion type'' double fullerenic carbon cages

Three ''onion type'' double carbon cages (consisting of an inner sp(3)-hybridized C-60 buckminsterfullerene skeleton and an outer larger carbon cage with icosahedral symmetry, with covalent bonds between the carbon atoms of the 5-membered rings) are described and their strain energies are discussed : (i) when the outer cage is a C-120 Ca derived from buckminsterfullerene by replacing each bond between two six-membered rings by an acetylenic group, the strain is smallest; (ii) when the same derivation concerns each bond between 5- and 6-membered rings leading to a C-180 Outer cage, the strain is largest; (iii) when the outer cage is C-180 With benzenoid rings, the strain is intermediate.