Conductivity in polyacetylene. I. <i>Ab initio</i> calculation of charge localization, bond distances, and reorganization energy in model molecules

The electronic structure and geometry of neutral or charged molecules of the type trans-H(CH)nH is calculated using ab initio methods. The reorganization energy λ for adding or subtracting electrons is obtained for the cases with n=10, 11, and 12 using (U)MP2 with a 6-31G basis set. We find contributions to λ not only from C–C bond distance changes but also from C–C–C bond angle changes, whereas the contribution from C–H bond length and C–C–H bond angle changes are unimportant. The change of bond length when electrons are added to or subtracted from a neutral molecule with an even number of carbon atoms is typically 0.05–0.08 Å and occurs over a large part of the molecule, even if the donated charge is concentrated near the positive alkali ion. λ for one-electron reduction or oxidation is large in an even C chain but small in an odd C chain. The introduction of electronic correlation at (at least) the level of second-order Mo/ller–Plesset perturbation theory is important in the calculation of bond lengths and reorganization energies.