SOLITONS IN FINITE-LENGTH AND INFINITE-LENGTH NEGATIVE-DEFECT TRANSPOLYACETYLENE AND THE CORRESPONDING BROOKER (POLYMETHINECYANINE) CATIONS .1. GEOMETRY

The geometries of both small and large (in the infinite chain limit) Brooker cations C2n-1H2n+3N2+ and the corresponding polyene ions are studied using ab initio SCF, AM1, and empirical techniques such as a soliton model. While all methods predict that polyene ions are symmetrical (C2v), AM1 and possibly also ab initio calculations predict that longer Brooker ions have an equilibrium structure in which the soliton is located near one end of the chain, with a very low energy C2v transition state separating the two possible configurations. The Brooker C2v structures are shown to exhibit quite different behaviour from the analogous polyene ions for chain lengths less than 40 CH units because the end effects in the Brooker ions are stronger and of longer range than the end effects in the polyene ions. For chain lengths greater than 40 CH units, the structures of both the polyene and the C2v Brooker ions resemble that of charge-defect trans-polyacetylene, due to the onset of Peierls distortion. Possible applications of the C2v Brooker ions in molecular electronics are discussed: the shorter ions are expected to show considerably greater conductivity than the corresponding polyenes, while the asymmetric Brooker structures may be useful as molecular switches.