Role of Defects in Radiation Chemistry of Crystalline Organic Materials. 3. Geometrical and Electronic Structures of Alkene Radical Anion and Cation in Alkene/n-Alkane Mixed Crystals As Studied by ESR Spectroscopy

Article abstract of DOI:10.1021/j100176a082

An ESR study has been made in order to elucidate the electronic structures of alkene radical anion and cation, the former radical being first detected in the hexene/n-hexane mixed crystals irradiated at 4.2 K along with the cation.The present work extended to the hexene and butene isomers has resulted in evidence that both anions with vinylene and vinylidene groups have pyramidal structures with ?-character, which differ from the planar or twisted structures of corresponding cations.The proton hyperfine couplings of their anions were only about one-third as large asthose of the cations: <*>A<*>(two α-H) = <*>0.45, 0.1, -0.25<*> mT; a(two pairs of β-H) = 1.38 and 0.56 mT for the 3-hexene anion, and a(two α-H) = 1.3 mT and a(two pairs of β-H) = 4.6 and 2.9 mT for the cation.The differences in the geometrical structures and in the sizes of the proton couplings of the anion and cation radicals were discussed on the basis of a simple molecular orbital calculation.It has been found that the anion is stabilized by admixing <*>2s;C<*> atomic orbitals (AO) with a lower core integral than <*>2p;C<*> AO to the unpaired electron orbital and that the small β-proton couplings mainly originate from low extent of hyperconjugation due to a wide energy separation of C=C ?-antibonding and C-H pseudo-?-bonding orbitals.