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 h ave been studied by ab initio calculations at the UHF, MP2, and SDCI l evels, as well as experimentally, using low-temperature matrix isolati on electron spin resonance (ESR). The ESR spectra, recorded in halocar bon matrices at temperatures between 77 and 145 K, are reported togeth er with calculated structural parameters, total energies, and isotropi c hyperfine (hf) coupling constants. For the 1-pentene radical cation, the ESR spectrum is explained by a rigid, nonplanar structure where t he 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 th e whole allylic fragment. The radical cation of 2-pentene, on the othe r hand, shows experimentally as well as theoretically a hf structure t ypical for a localized pi-bond ionization, as normally expected in alk ene radical cations. Deprotonation reactions of the radical cations in CF2-ClCFCl2 matrices at 120 K resulted in the appearance of alkyl rad icals.