Spin Density Distribution of the Soliton in Pristine Polyacetylene Detected by ENDOR

2 Spin Density Distribution of the Soliton in Pristine Polyacetylene Detected by ENDOR 

The detailed structure of the soliton in PA can be clarified by using the ENDOR technique as shown by Kuroda et al. [45,46]. 

The anisotropy of the line width is clearly manifested. When the external magnetic field is applied parallel to the stretching direction of the film, the signal intensity has a higher intensity in most fi equency regions than that in the case of the field being perpendicular to the direction. The spectra seem to be composed of more than two components. The ENDOR features at low temperatures can be interpreted as the direct evidence of the soliton like spin density by the simulation of the anisotropic spectrum. The maximum frequency of the ENDOR spectrum is related to the spin density, p(0) at the central carbon of the soliton as indicated in Fig. 7.41. 

The anisotropy of the ENDOR spectra is attributed to the corresponding anisotropies of the C—H protons of the soliton. The directions of the principal tensor axes are parallel to the C—axis (x), the chain axis (y), and the axis of the ji-orbital (z), respectively in accordance with the n-electron radical structure of the soliton. The ENDOR frequency due to the proton of the n-th C—unit with the magnetic field along a principal direction is given by a simple equation. For the magnetic field along the y-axis one obtains for instance Eq. (7.33)  

Ayy is the principal value for the C—proton in a Jt-electron radical with spin density p = 1, and vo is the free proton frequency. When the external field is applied parallel fo the stretching direction (y) the maximum ENDOR frequency v+ is estimated to be c.a. 21 MHz at which point the signal changes its slope. By substituting the experimental values of Ayy and (v -vo) in Eq. (7.33), p(0) is evaluated. Due to the uncertainty of the exact value for Ayy the value of p(0) ranges between 0.11 and 

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