Photohydration of Benzophenone in Aqueous Acid

Abstract

Why is the triplet state of aromatic ketones quenched by protons? The long-known but unexplained quenching process was investigated in detail for benzophenone (1). Adiabatic protonation of triplet benzophenone, 31, encounters a state symmetry-imposed barrier, because the electronic structure of 31 is n,π*, while that of its conjugate acid, 31H+, is π,π*. Hence, the rate of protonation of 31, kH+ = 6.8 × 108 M-1 s-1, is well below the diffusion-controlled limit. The short-lived transient intermediate formed by protonation of 31 in 0.1−1 M aqueous HClO4 (λmax = 320 and 500 nm, τ = 50 ns) is not 31H+, as was assumed in previous studies. The latter (λmax = 385 nm) is detectable only in acidified acetonitrile or in highly concentrated aqueous acid (>5 M HClO4), where water activity is low. In moderately concentrated aqueous acids, adiabatic protonation of 31 is the rate-limiting step preceding rapid adiabatic hydration of a phenyl ring, 31H+ + H2O → 31·H2O, k0 = 1.5 × 109 s-1. These findings lead to a revised ...