La. Eriksson et al., EFFECTS OF IONIZATION IN LINEAR ALKENES - A STUDY OF THE RADICAL CATIONS OF 1-PENTENE AND 2-PENTENE, Journal of the American Chemical Society, 115(8), 1993, pp. 3244-3249
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.