The structures of c i s-polyacetylene and highly conducting derivatives

The structures of cis-polyacetylene and polyacetylene–iodine derivatives have been investigated by x-ray diffraction and crystal packing analysis. Results obtained for cis-polyacetylene are consistent with the space group Pnma with one CH per asymmetric unit and a chain symmetry (m1̄)t. Hence, the structure is analogous to that of orthorhombic polyethylene. For a structure with such a concentration of π electrons there is minimal overlap between parallel π orbitals. The π orbitals are almost orthogonal for four of the six closest neighbors of a given chain. Based on x-ray diffraction and spectroscopic results, a model is proposed for the structure of the highly conducting polyacetylene–iodine complexes. According to this model, linear arrays of polyhalide ions (principally in the form of I−3) substitute in positions of displaced polyacetylene chains. At high dopant concentrations, alternate close-packed layers of polyacetylene are separated by a layer consisting of such polyhalide arrays. This model predicts a limiting charge transfer of 0.11 electron on the average per carbon atom in the polyacetylene chain.