STEREOCHEMICAL PATTERNS IN C60XN

A theoretical approach is used to recognise the stereochemical pattern s which emerge when X-2 (X = H, halogen, etc.) is added to C-60 to for m C60Xn. Addition to form C60Xn occurs either at a hex-hex edge, or ac ross the para-positions of one of the hexagonal faces. Both these stru ctural elements may occur in subsequent additions. An important featur e is the nonplanarity of the hexagonal C6 rings resulting from some te trahedrally coordinated carbon atoms, and the way this nonplanarity in fluences the conformations of surrounding rings. Further addition lead s to the formation of chains of edge-shared hexagonal faces. Particula rly stable structures are formed if these chains eliminate the chain e nds to form cyclic structures. Examples include the skew pentagonal py ramids in C60X6, C60X12 and C60X18, and the crowns in C60X18 and C60X3 6. At higher levels of addition, in order to decrease X-X repulsions, it becomes necessary to spread the X atoms as evenly as possible over the surface, and/or to place some X atoms in endo-positions, and/or to disrupt the C-60 cluster. (C) 1997 Elsevier Science Ltd.