Electron-spin-resonance experiments and theory of the butatriene-to-acetylene transition in short-chain polydiacetylene molecules

Abstract
We report on electron-spin-resonance experiments and theoretical calculations concerning the electronic structure of short-chain polymerization intermediates in diacetylene crystals. The structural transition from diradicals to dicarbenes observed at a critical chain length of six repeat units is quantitatively explained by the generation of ‘‘up’’ and ‘‘down’’ radical electrons at the chain ends of the reaction intermediates. The best fit of the fine-structure and hyperfine-structure parameters is obtained by an energy difference of 0.4 eV per unit cell between the butatriene and acetylene structure and by a transfer integral of 0.5 eV.

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