The intra‐ and inter‐chain structural analyses of proposed poly (acetylene) (PA) valence geometries were performed. The semi‐empirical molecular methods CNDO/2, MINDO/3, and NDDO were used in the intramolecular conformational analysis. A molecular‐mechanics based lattice packing formalism was used to compute the interchain energies. The structures proposed by Baughman and coworkers are predicted to be most stable in observed orthorhombic lattices. The all‐trans chain packs with the lowest energy and the trans‐cisoid structure is more stable than the cis‐transoid. However, a monoclinic set of lattices, not yet observed, are predicted to be about 50 lower in energy than the corresponding orthorhombic lattices for all three chain conformations.
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