Helical versus planarcis-polyacetylene

An established quantum chemical self-consistent-field method, the modified neglect of diatomic overlap approach, is used to study the equilibrium geometry of cis-polyacetylene. This study was stimulated by very recent experimental results suggesting the existence of helical cis-polyacetylene. For a single chain of this material we do find that the planar structure is unstable towards formation of a helical conformation. Owing to a remarkable flatness in the potential-energy surface over a wide range of carbon-carbon single-bond twist angles, the results account for the experimentally implied existence of both planar and helical $\frac{2*3}{1}$ cis-polyacetylene.