Pulsed ENDOR and TRIPLE Resonance on trans-Polyacetylene à 1a Durham Route

Abstract

Electron Nuclear Double Resonance (ENDOR) has proven to be very important in the investigation of neutral soliton and polaron structures [1–3]. Dalton and co-workers [4–6] have demonstrated in a series of publications that ENDOR provides much more decisive information on soliton structure in polyacetylene than ESR or any other type of spectroscopy. Others followed this route and have presented similar ENDOR results with partially different interpretations [7-10]. However, the interpretation of ENDOR spectra of powders or amorphous materials is rather complicated, although much simpler than ESR spectra. At the IWEPP 85 [11] and in a later publication [12] it was shown that virtually any distribution function of spin-density p _j at position j, i.e. soliton structure, fit the observed ESR spectra, whereas the ENDOR spectra vary significantly. Although ENDOR spectra represent the direct distribution of hyperfine interactions, thus spin-density distributions, their analysis is hampered by several complications: (a) The hyperfine interaction of the soliton spin with the surrounding nuclei is not a scalar quantity but a second-rank tensor. Powder spectra are therefore a superposition of different spectra for different orientations. (b) The connection between the hyperfine interaction and the spin-density is only approximately known within the limits of the McConnell relation, (c) Spin dynamical effects due to different relaxation mechanisms can severely alter the ENDOR spectra, (d) Hyperfine enhancement up to second order influences the transition probabilities.