Proton Fermi-contact coupling constants from local-density–functional theory: Application to the soliton in polyacetylene

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Abstract
We show that the first-principles local-density–functional Xα method reliably predicts proton Fermi-contact coupling constants for planar, neutral, organic π radicals. Results for the radical soliton in polyacetylene yield -0.39 for the ratio of the two observed proton Fermi-contact constants, close to the experimental value of -0.33. These studies also justify use of McConnell’s relation to analyze the polyacetylene proton ENDOR spectrum. Our theoretical value of McConnell’s constant for polyacetylene is -26 G, in good agreement with classical estimates of this quantity.