PARISER-PARR-POPLE FARCE FIELD FOR PI-ELECTRONS - RAMAN AND INFRARED SHIFTS OF TRANSPOLYACETYLENE

The transfer integrals t(R) and Coulomb potential V(R) of pi-electron Hamiltonians H-e define linear electron-phonon (e-ph) coupling constants t'(R) and V'(R) for the equilibrium structure. We generalize linear response (LR) theory for Raman and ir shifts due to pi-electron delocalization in Hamiltonians with arbitrary t(R) and spin independent V(R). pi-electron contributions Delta F-ij to the force field of trans-polyacetylene (PA) are obtained in the symmetry coordinates S-i, with 1-5, for k = 0 phonons. We compare Delta F-ij for Huckel chains with alternating transfer integrals t(1+/-delta) and for Pariser-Parr-Pople (PPP) models with hydrocarbon parameters derived from pi-pi* spectra to a phenomenological Delta F-ij for trans-PA and its isotopes. An exponential rather than linear t(R) is found. The molecular PPP potential V(R) accounts quantitatively for pi-electron coupling to CCC bends and for the length dependence of the Raman shifts of finite polyenes. The dominant but not exclusive pi-electron coupling remains the dimerization coordinate singled out in previous treatments, with substantially larger t'(R) than V'(R) contributions in the PPP force field. We comment on extensions of LR theory to PA models with interacting pi-electrons and several electronic susceptibilities.