Markus Ramseier, Paul Senn and Jakob Wirz, "Photohydration of Benzophenone in Aqueous Acid", J. Phys. Chem. A, 2003, 107, 3305-3315.

Abstract: Why is the triplet state of aromatic ketones quenched by protons? The quenching process was investigated in detail for benzophenone (1). Adiabatic protonation of triplet benzophenone, 3-1, encounters a symmetry-imposed barrier, because the electronic structure of 3-1 is n,pi*, and that of 3-1-H+ is pi,pi*. Hence, the rate protonation of 3-1 on the oxygen atom, k(H+) = 6.8 x 1e8 M-1 s-1, is well below the diffusion-controlled limit. The short-lived transient intermediate observed after protonation of 3-1 in 0.1-1 M aqueous HClO4 (lambda_max = 500 nm, tau = 50 ns) is not the conjugate acid, 3-1-H+, as was assumed in previous studies. 3-1-H+ (lambda_max = 385 nm) is observed only in acidified acetonitrile or in strongly acidic aqueous solutions (>5 M HClO4), where water activity is low. In moderately concentrated aqueous acids, adiabatic protonation of 3-1 is the rate-limiting step preceding rapid adiabatic hydration of a phenyl ring, 3-1-H+ + H2O --> 3-1.H2O, k(H2O) = 1.5 x 1e9 s-1. These findings lead to a revised value of the acidity constant of protonated 3-1, pKa(3-1-H+) = -0.4 +- 0.1. Following intersystem crossing of 3-1.H2O to the ground state, dehydration of 1.H2O (lambda_max = 315 nm) regenerates 1 with a rate constant k = 0.21 s-1. Acetophenone (2) and several derivatives of 1 and 2 were found to undergo a similar sequence of reactions in aqueous acid. The photohydration reaction results in substitution of fluoride by hydroxide in meta-fluorinated derivatives of 1 and 2.