Spectral Properties and Reactivity of Diarylmethanol Radical Cations in Aqueous Solution. Evidence for Intramolecular Charge Resonance¹

Abstract

Spectral properties and reactivities of ring-methoxylated diarylmethane 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 bands 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(alpha)-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(alpha)-H deprotonation is instead significantly slower (k = 6.7 x 10(2)s(-1)). In basic solution, 1a.(+)-1e.(+) undergo (-)OH-induced deprotonation from the alpha-OH group with k(OH.)approximately equal to 1.4 x 10(10)M(-1)s(-1), leading to a ketyl radical anion, which is oxidized in the reaction medium to the corresponding benzophenone.

Anomalous reactivity of radical cations produced by photosensitized oxidation of 4-methoxybenzyl alcohol derivatives: role of the sensitizer

“…The mechanism is the same regardless of the way in which the radical cation is produced, however in aqueous solutions, it is only operative at pH r 4. 5,6 The mechanism has been supported by intermediate characterization and kinetics studies performed by laser flash photolysis, pulse radiolysis, and steady-state experiments.…”

“…The absorption at 550 nm was again ascribed to NMQ , while the one at 420 nm can be reasonably assigned to 1c + , the partner of NMQ in the electron transfer reaction. Actually, the absorption spectrum of 1c + generated by pulse radiolysis in aqueous solution 6 shows three absorp-tion bands at 290, 440 and 980 nm. The last one is typical of these systems and is ascribed to an intramolecular charge resonance interaction.…”

“…This spectral behavior is explained in the Discussion section. Moreover, the absorption at 350 nm is made up of two components, a shortlived one, probably due to the ketyl radical An 2 COH , that reacts quickly with molecular oxygen, 6 and a longer-lived one coupled with the absorption at 420 nm. The kinetics analysis gave a lifetime of 6.6 ms for the long component, which is very close to the 6.8 ms value obtained at 420 nm.…”

Photochemistry for the Synthesis of Noble Metal Nanoparticles

“…The diphenyl methanol cation deprotonates to regenerate the BP (Scheme 5). 99 The Au + species disproportionate to form Au 0 (eq 6). The formed Au 0 are associated to form Au NPs.…”

“…The diphenylmethanol cation quickly deprotonates to regenerate MeOBP (state f). 99,120,122,132134 In the presence of an appropriate electron acceptor, the formed MePI…”