EFFECTS OF IONIZATION IN LINEAR ALKENES - A STUDY OF THE RADICAL CATIONS OF 1-PENTENE AND 2-PENTENE

The electronic ground states of the 1- and 2-pentene radical cations have been studied by ab initio calculations at the UHF, MP2, and SDCI levels, as well as experimentally, using low-temperature matrix isolation electron spin resonance (ESR). The ESR spectra, recorded in halocarbon matrices at temperatures between 77 and 145 K, are reported together with calculated structural parameters, total energies, and isotropic hyperfine (hf) coupling constants. For the 1-pentene radical cation, the ESR spectrum is explained by a rigid, nonplanar structure where the two terminal aliphatic carbon atoms are rotated out of the plane of the allylic fragment by approximately 25-degrees. The singly occupied molecular orbital (SOMO) is found to be partially delocalized over the whole allylic fragment. The radical cation of 2-pentene, on the other hand, shows experimentally as well as theoretically a hf structure typical for a localized pi-bond ionization, as normally expected in alkene radical cations. Deprotonation reactions of the radical cations in CF2-ClCFCl2 matrices at 120 K resulted in the appearance of alkyl radicals.