SOLITONS IN POLYACETYLENE - MAGNETIC HYPERFINE CONSTANTS FROM ABINITIO CALCULATIONS

The spin distribution of the neutral soliton in trans-polyacetylene is controversial, as evidenced by the markedly different interpretations given by the various active experimental groups to their ENDOR measurements of proton hyperfine coupling (hfc) constants. To help resolve the discrepancies, ab initio electronic structure calculations were carried out for proton and carbon-13 hfc on the series of model polyene radicals C5H7,C9H11,...,C21H23 and the most important characteristics of the spin distribution were extrapolated to the long chain limit. Here sigma-spin polarization was treated through single excitation configuration interaction, both with respect to Hartree-Fock and to multiconfiguration reference spaces that include the most significant effects of pi-electron correlation. Particular attention was paid to basis set requirements necessary for proper treatment of hfc. It turns out that the ab initio hfc does not agree with that obtained by any of the major experimental groups. In particular, the calculated width of the bell-shaped distribution for the central and other even proton sites is much narrower than all experimental reports, and the anisotropic coupling calculated for the central proton along the fibril axis is much larger. Several assumptions commonly made in analyzing the ENDOR spectra are shown not to be quantitatively justifiable, but their improvement is unlikely to resolve the large discrepancies we have found between theory and current experimental interpretations. Thus, it appears likely that the experimental spectra are significantly influenced by environmental effects and cannot be adequately modeled by an isolated long chain polyene radical as heretofore assumed.