Kinetics of proton transfer reactions involving carbon

Enol protomers of ketones and aldehydes, carboxylic acids and esters, keten es, as well as keto protomers of phenols are generated by flash photolysis to investigate their reaction kinetics in aqueous solution, pH-Rate profile s and buffer dilution plots provide absolute rate constants for acid and ba se catalysis of keto-enol protomeric reactions. Equilibrium constants of en olization spanning a range of 30 orders of magnitude are determined as the ratio of the rate constants of forward and backward reaction. Structure-rea ctivity and free energy relationships exhibit the systematic and predictabl e reactivity of transient protomers. The intrinsic barrier for proton trans fer from oxygen to carbon, Delta G(0)(double dagger) as defined by Marcus t heory, amounts to 57 +/- 2 kJ mol(-1); thus, the rates for thermoneutral re actions are nearly ten orders of magnitude less than those expected for pro ton transfer reactions of "normal" acids. The high intrinsic barrier is hel d responsible for the lack of excited-state adiabatic proton transfer react ions involving carbon. Such reactions generally proceed directly to ground state products through avoided crossings or conical intersections. Evidence for protonation of n,pi*-excited triplet ketones at the carbonyl carbon is presented.