Kinetic isotope effect on the photoenolization of o-methylanthrone. A microcanonical transition state theory calculation

The photoenolization of 1,4-dimethylanthrone (1,4-MAT) and 1,4-dimethylanthrone-d8 (1,4-DMAT) in the gas phase and in 2,2,2-trifluoroethanol (TFE) has been studied theoretically in this work. An electronic energy profile with minima and saddle-point structures is determined by density functional theory methods in the ground state (So) and in the triplet state (T1). This study reveals that in the excited state an inversion of stability of the tautomers and a lower energy barrier to overcome makes the proton transfer feasible. Our molecular orbital analysis shows that upon proton transfer, T1 changes from n,π* to π,π* so that a diabatic crossing takes place along the reaction coordinate. This crossing is not influenced by the presence of TFE. Also, upon the photoexcitation to the first excited singlet state (S1) an intersystem crossing must occur to access T1. Pure electronic calculations cannot tell us the exact point of the triplet potential energy surface that will be accessed. For this reason we have performed a microcanonical dynamic calculation taking into account the tunneling effect of the hydrogen and deuterium transfer rate constants (kH and k D, respectively) on T1. After the rate-constant calculation we have also calculated the kinetic isotope effect (KIE) to compare with the experimental value. Our results indicate that the predicted large KIE for this reaction can be explained if the proton transfer takes place through tunneling at an energy slightly below the barrier.