Radiation Damage in Organic Crystals. III. Long Polyene Radicals

The distribution of electronic spin in a series of linear odd-alternant polyene radicals is calculated using valence-bond, Hückel, and the ``extended'' Hartree—Fock methods. These theoretical results are compared with the spin distributions in a series of free radicals produced by irradiation of polyene acids, CH3 (CH=CH)m-1COOH, m=2–5, 7. The experimental spin distributions are deduced from the line shape of the ESR spectra of polycrystalline samples by making use of a method relating the second moments of the ESR spectra to the spin distribution. Experimental evidence is adduced for the presence of negative spin density on the ``unstarred'' atoms of these radicals. The relation between these results and the problem of bond alternation in long polyene radicals is discussed.