Structures and isomerism in C 70X 36, X=H, F. A comparison with C 60X 36

Abstract

The structures and stabilities of isomers of C 70H 36 and C 70F 36 have been investigated using the AM1 Hamiltonian and the program MOPAC 6.0. There are many structurally related isomers of similar stability. All these isomers have ten bare carbon atoms in the equatorial plane of the molecule separating two C 30 hemispheres which do not interact strongly with each other. Each hemisphere contains 18 hydrogen atoms and six pairs of unhydrogenated carbon atoms which are isolated CC units or part of isolated C 6 units, these being separated by CH or CF groups. There are 62 possible hemispheres of which 51 exist as an optically active pair. Each hemisphere can be rotated relative to the other hemisphere by 72, 144, 216 or 288° leading to a large number of related structures. For C 70H 36 the stable isomers are constructed from only three types of hemisphere, labeled L, Aa and Ea, with two, two and one C 6 unit, respectively. There are two similarities between the most stable isomers of C 60X 36 and C 70X 36. Firstly, both contain a tetrahedral arrangement of four C 6 units, which in C 60X 36 form a regular tetrahedron and in C 70X 36 are as close to a regular tetrahedron as possible within the constraints of the C 70 skeleton. Secondly, in both C 60X 36 and C 70X 36 the 36 X atoms are distributed on twelve 1,2,3-C 5X 3 faces. Factors effecting the separation and characterization of C 70X 36 isomers are discussed.