Molecular modeling of the kinetic isotope effect on the intramolecular hydrogen atom transfer in triplet 6,9-dimethylbenzosuberone

Photoenolization of 6,9-dimethylbenzosuberone (2) has been theoretically studied. DFT(B3LYP) calculations give geometries for the keto form in very good accord with X-ray diffraction data. The energies for the minima and transition-states reveal that the tautomerization is only possible in the T1 state. Dynamics of the tautomerization in T1 are studied for 2 using a modified RRKM formalism including tunneling. Results point to tautomerization at the ultralow temperatures of the experiments taking place entirely through tunneling at energies slightly below the adiabatic energy barrier. The calculated kinetic isotope effect (KIE) is predicted to increase at low energies, a usual result which contrasts with the experimental findings of an almost constant KIE of 1.1 between 4 and 50 K that increases steadily up to ∼5.1 at 100 K. In contrast to the previous assumption that the anomalous KIE is governed by a vibrational assisted tunneling mechanism, theoretical results are interpreted here in terms of the total decay rate of the triplet state being dominated at very low temperatures by the phosphorescence and thermal decay rates.