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Spin distribution in neutral polyene radicals: Pariser-Parr-Pople model studied with the density matrix renormalization group method.

The geometries and pi electron spin distributions induced by neutral soliton defects in trans-polyacetylene radicals (from C(7)H(9) to C(49)H(51)) are studied using Pariser-Parr-Pople (PPP) model, solved by the density matrix renormalization group (DMRG) method. Comparisons with other quantum chemical methods as well as the experimental observations on heptatrienyl (C(7)H(9)) and nonatetraenyl (C(9)H(11)) radical species show that the semiempirical PPP method is in the list of the very few theories that can give correct description of the spin distributions for such extended pi-conjugated systems. By virtue of DMRG's power in dealing with large one-dimensional systems, we predicted that the half-width of a neutral spin soliton in polyacetylene is about 14 atoms, and the spin distributions in the center of the soliton is calculated as rho(0)=0.25, rho(1)=-0.12 with rho(1)rho(0)=-0.48, rho(-)rho(+)=-0.52, which agree well with the results from electron-nuclear double resonance experiments.