Bond alternation defects in long polyene molecules

Recent theoretical work suggests that there is an alternation in carbon-carbon bond lengths throughout a sufficiently long polyene chain. Such alternation implies the possibility of defects in which even-numbered bonds along the chain change over from short to long. In this paper, the electronic properties of such defects are examined by Hückel molecular orbital theory, with the parameters used by Longuet-Higgins and Salem 2. It is shown that there is a localized non-bonding molecular orbital associated with each defect (if they are well separated). In the neutral molecule, there will be an odd electron in each such molecular orbital, which should be detectable by electron spin resonance. An approximate estimate of the energy required to form a defect suggests that there should be about one per seventy carbon atoms.