PI-ELECTRON FORCE-FIELD IN INTERNAL COORDINATES FOR TRANSPOLYACETYLENE AND CIS-POLYACETYLENE

We report the force fields F(int) and the transferable electron-phonon (e-ph) coupling constants for trans- and cis-polyacetylene (PA) in internal coordinates, modeling the polymers as Huckel chains with alternating transfer integrals t(1 +/- delta). The valence force field of trans-PA reproduces the experimental Raman and infrared frequencies of all the isotopes as well as the force field F in symmetry coordinates obtained from linear response theory. Either case yields linear e-ph coupling constants t'(R(d)) = 6.4 eV/angstrom and t'(R(s)) = 4.1 eV/angstrom for double and single bonds, respectively, and decisively indicates exponential rather than linear t(R) in a Huckel description of PA. The same procedure with the same e-ph coupling constants lead to F(int) for cis-PA. The Raman frequencies of cis-PA and deuterated analogue are fit with a susceptibility eta = 0.56 relative to trans-PA, providing direct and surprising evidence for third-neighbor interactions.