Vibronic states of bond alternation defect in polyacetylene and ESR spectrum

Abstract

An origin of the temperature dependence of the ESR line shape in trans -(CH) X is investigated on the vibronic structure model of a bond alternation defect in a linear conjugated carbon chain. For this purpose first the vibronic energy levels of a bond alternation defect is calculated from the variational principle for a chain with the odd number of carbon atoms. Secondly the rate of transitions between vibronic levels due to anharmonic terms of the adiabatic potential is calculated. It is shown that these transitions cause the motional narrowing in the ESR spectrum. Finally on the above vibronic structure model the temperature dependence of the ESR line width is calculated from a general formula for the ESR line shape, and a rival effect of transitions between vibronic levels and hyperfine interactions with magnetically inequivalent protons in a vibronic level is clarified. The present theoretical results are in good agreement with experiments.