Magnetic resonance properties of polyacetylene

Electron-nuclear double resonance (ENDOR) spectra have been recorded for samples of pristine cis- and trans-polyacetylene at temperatures ranging from 15°K to 290°K. The ENDOR spectra of cis-polyacetylene consist of resolved contributions arising from distant and local ENDOR mechanisms. The temperature dependence of the peak positions of the local ENDOR spectra is analyzed in terms of a simple two-site electron jump model resulting in modulation of a 1.8 MHz isotropic hyperfine coupling. This analysis leads to an activation energy of 34°K. The ENDOR spectra of trans-polyacetylene at temperature above 70°K consist of a single line centered at the free proton Larmor frequency and exhibiting a temperature-dependent linewidth. These spectra are assumed to be composed of distant and local ENDOR contributions with motional narrowing of the local ENDOR structure accounting for the temperature dependence of the ENDOR linewidths. This assumption is supported by observation of two-component ENDOR spectra below 40°K. Analysis of the linewidth variations of the high temperature spectra suggests that the linewidths are determined by modulation of an approximately 1.4 MHz hyper-fine interaction by a dynamic process characterized by a 400-K activation energy. These numbers are in excellent agreement with values obtained from analysis of electron spin echo (ESE) spin-spin relaxation data establishing that the same process determines both ENDOR and ESE data. No evidence is obtained from ENDOR spectra for the existence of large hyperfine interactions. Thus, the EPR, ESE, and ENDOR results for trans-polyacetylene are dominated by an intermediate motion process (τ = 10-6 to 10-9 sec) modulating approximately 2 MHz hyperfine couplings and characterized by a relatively high activation energy (400°K) and not by a fast, highly directional motion modulating large hyperfine interactions as suggested by recent soliton theory analysis of EPR spectra.