Spectral Properties and Reactivity of Diarylmethanol Radical Cations in Aqueous Solution. Evidence for Intramolecular Charge Resonance

Spectral properties and reactivities of ring-methoxylated diarylinethane and diarylmethanol radical cations, generated in aqueous solution by pulse and gamma-radiolysis and by the one-electron chemical oxidant potassium 12-tungstocobalt(III)ate, have been studied, The radical cations display three oands in the UV, visible, and vis-NIR regions of the spectrum. The vis-NIR band is assigned to an intramolecular charge resonance interaction (CR) between the neutral donor and charged acceptor rings, as indicated by the observation that the relative intensity of the vis-NIR band compared to that of the UV and visible bands does not increase with increasing substrate concentration and that the position and intensity of this band is influenced by the ring-substitution pattern. In acidic solution (pH 4), monomethoxylated diarylmethanol radical Cations 1a(.+)1e(.+) decay by C(a)-H deprotonation [k (1.7-1.9) x 10(4) s(-1)] through the intermediacy of a ketyl radical, which is further oxidized in the reaction medium to give the corresponding benzophenones, as evidenced by both time-resolved spectroscopic and product studies. With the dimethoxylated radical cation 2(.+) C(a)-H deprotonation is instead significantly slower (k = 6.7 x 102 s 1). In basic solution, 1a(.+)-1e(.+) undergo (-)OH-induced deprotonation from the alpha-OH group with k(OH) approximate to1.4 x 10(10) M (1) S(-1), leading to a ketyl radical anion, which is oxidized in the reaction medium to the corresponding benzophenone.