Charge-density and ring-size dependent dimerization of 1-arylcycloalkene cation radicals

The gamma-radiolysis of 1-arylcyclopentenes [Ar = Ph (1a); 4-ClC6H4 (1b); 4 -MeC6H4 (1c); 4-MeOC6H4 (1d)] at 77 K initially gave transient absorption b ands attributable to their monomer cation radicals (1a(.+)-1d(.+)), with la mbda(max) around 400 and 650-730 nm. In the case of 1a, 1b, and 1c, simulta neous formation of new absorption bands with lambda(max) around 480-500 nm, attributable to the dimer cation radicals, was clearly observed upon warmi ng along with a decrease in the monomer cation radical absorption bands. Us ing 355-nm laser photolysis at room temperature and an N-methylquinolinium salt as an electron acceptor, the rate constants for the formation of the d imer cation radicals (k(dimer)) and the reaction-rare constants for 1(.+) w ith methanol (k(MeOH)) were determined. The reactivities of 1(.+) with 1 an d with methanol are discussed on the basis of the results obtained by a sem iempirical 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 c ycloalkene cation radicals with larger ring size than 1, such as 1-phenylcy clohexene, 1-phenylcycloheptene, and 1-phenylcyclooctene cation radicals, w as found to be remarkably dependent on the ring size.