2 CHANNELS OF ELECTRON-TRANSFER OBSERVED FOR THE REACTION OF N-BUTYL CHLORIDE PARENT RADICAL CATIONS WITH NAPHTHOLS AND HYDROXYBIPHENYLS

Pulse radiolysis of naphthols (NpOH) and hydroxybiphenyls (ByOH) in n- butyl chloride (BuCl) at room temperature exhibits electron transfer a t a bimolecular rate constant of (1.0-2.8) x 10(10) dm(3) mol(-1) s(-1 ). The experiments reveal the direct formation of two types of transie nts: phenol type radical cations (NpOH.+, ByOH(.+)) and phenoxyl type radicals (NpO., ByO(.)). This is explained by a mechanism involving tw o different electron-transfer channels. The solute radical cations exh ibit two optical absorption bands in the 570-650 and 360-460 nm region s and undergo electron transfer with triethylamine and proton transfer with ethanol with bimolecular rate constants of (4-12) x 10(9) and (3 -6) x 10(8) dm(3) mol(-1) s(-1), respectively. NpO. and ByO(.) have re latively long lifetimes and show absorption bands in the 340-400 and 4 70-540 nm regions. By way of comparison, these phenoxyl type radicals are separately generated by pulse radiolysis in aqueous alkaline solut ion containing sodium azide, i.e., by oxidation of the solutes with N- 3(.) radicals. Under these conditions, the phenoxyl radicals decay by second-order kinetics with 2k = (1.2-4.5) x 10(8) dm(3) mol(-1) s(-1). The various modes of formation and decay of the phenolic radical cati ons are analyzed over a wide range of dose rate and solute concentrati ons: In comparison to radical cations of one-ring phenols, the increas ed stability of NpOH.+ and ByOH(.+) is explained by the delocalization of the positive charge over the whole aromatic system, a postulate su pported by open-shell quantum chemical calulations.