Kinetics of proton transfer reactions involving carbon

Enol protomers of ketones and aldehydes, carboxylic acids and esters, ketenes, as well as keto protomers of phenols are generated by flash photolysis to investigate their reaction kinetics in aqueous solution, pH-Rate profiles and buffer dilution plots provide absolute rate constants for acid and base catalysis of keto-enol protomeric reactions. Equilibrium constants of enolization spanning a range of 30 orders of magnitude are determined as the ratio of the rate constants of forward and backward reaction. Structure-reactivity and free energy relationships exhibit the systematic and predictable reactivity of transient protomers. The intrinsic barrier for proton transfer from oxygen to carbon, Delta G(0)(double dagger) as defined by Marcus theory, amounts to 57 +/- 2 kJ mol(-1); thus, the rates for thermoneutral reactions are nearly ten orders of magnitude less than those expected for proton transfer reactions of "normal" acids. The high intrinsic barrier is held responsible for the lack of excited-state adiabatic proton transfer reactions 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.