DIABATIC PROTONATION OF EXCITED SINGLET STYRENES

The rate-determining step for the general acid-catalysed photohydration of styrenes and phenylacetylenes in solution is described as a diabatic proton transfer from the ground state of the acid to the singlet excited state of the base, generating the ground states of the conjugated base and of the carbocation acid. This follows from the application of transition state concepts to the kinetics of the forward and reverse reactions, together with the known ground and excited state pK(a) values of olefins and acetylenes. The application of the intersecting state model to these diabatic proton transfers yields transition state bond orders close to 0.5, consistent with previous results. This model also shows that such highly exothermic reactions can be dynamically controlled, which explains the experimental observation of a small decrease in the rates with an increase in exothermicity.