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, 3 1, encounters a state symmetry-imposed barrier, because the electronic structure of 3 1 is n, π*, while that of its conjugate acid, 3 1 H+, is π, π*. Hence, the rate of protonation of 3 1, k H+= 6.8× 108 M-1 s-1, is well below the ...
