Structural information of undoped trans-polyacetylene obtained by 13C 2D NMR combined with dynamic nuclear polarization

Abstract By means of 13C 2D NMR in combination with dynamic nuclear polarization at room temperature, the orientation of the chemical-shift tensor and the carbon-carbon bond lengths in undoped trans -polyacetylene are determined. It is proven that the tensor element σ 33 has its principal axis perpendicular to the molecular plane, and that the σ 11 axis makes an angle of 43 ± 5° with respect to the single bond and 78 ± 6° with respect to the double bond. It is found that the single bond length is 1.45 ± 0.01 A and the double bond length 1.38 ± 0.01 A. The C−C=C bond angle is 120.7 ± 1.5°. No evidence is found for fast bond alternation caused by mobile solitons, which indicates that the soliton motion is largely restricted to an area close to trap sites.