Charge-Density and Ring-Size Dependent Dimerization of 1-Arylcycloalkene Cation Radicals

Abstract

Abstract The γ-radiolysis of 1-arylcyclopentenes [Ar = Ph (1a); 4-ClC6H4 (1b); 4-MeC6H4 (1c); 4-MeOC6H4 (1d)] at 77 K initially gave transient absorption bands attributable to their monomer cation radicals (1a·+—1d·+), with λmax around 400 and 650—730 nm. In the case of 1a, 1b, and 1c, simultaneous formation of new absorption bands with λmax around 480—500 nm, attributable to the dimer cation radicals, was clearly observed upon warming along with a decrease in the monomer cation radical absorption bands. Using 355-nm laser photolysis at room temperature and an N-methylquinolinium salt as an electron acceptor, the rate constants for the formation of the dimer cation radicals (kdimer) and the reaction-rate constants for 1·+ with methanol (kMeOH) were determined. The reactivities of 1·+ with 1 and with methanol are discussed on the basis of the results obtained by a semiempirical molecular orbital calculation using the PM3 method. The results indicate that the reactivities of 1·+ depend not on its structure but on the charge density of the ethylene bond. In addition, dimerization of the cycloalkene cation radicals with larger ring size than 1, such as 1-phenylcyclohexene, 1-phenylcycloheptene, and 1-phenylcyclooctene cation radicals, was found to be remarkably dependent on the ring size.