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

Secondary Kinetic Isotope Effect on the Photoenolization of Triplet O-Methylanthrones. A Microcanonical Transition State Theory Calculation

“…As a whole, the results presented here, together with the ones previously published using a similar methodology (refs and ), provide a global explanation for the primary and secondary KIE on the photoenolization of different o -methyl aryl ketones in solid glass at extremely low temperatures so that it can be concluded that all these processes are taking place at energies just slightly below the adiabatic potential energy barrier.…”

“…Recently we have analyzed from a theoretical point of view the kinetics of the hydrogen-atom transfer in the lowest triplet state of several o -methyl aryl ketones. , Through the use of the well-known RRKM kinetic formalism, modified to take into consideration the possibility of tunneling of the transferring hydrogen, we found that the experimentally measured rate constants for 1,4-MAT when all the methyl-group hydrogens were deuterated were in accordance with the reaction taking place entirely by tunneling at energies shortly below the energy barrier . As expected for a tunneling-dominated reaction, a curved Arrhenius plot was observed at the lowest temperatures, which, in our microcanonical treatment, corresponds to the lowest energies.…”

“…Now it was also possible to directly compare the obtained and measured primary kinetic isotope effects (KIE) for the deuterium substitution. The anomalous behavior of the KIE at the lowest end of the temperature range found in the experimental data was theoretically explained in terms of the total decay rate of the triplet state of 6,9-dimethylbenzosuberone being dominated by the phosphorescence and thermal decay rates at very low temperatures …”

“…The dynamic method used here is analogous to the one used for the study of 6,9-dimethylbenzosuberone so that just a brief outline is given here. Full details are to be found elsewhere …”

“…Our results were also in accordance with the absence of phosphorescence observed for the original (nondeuterated) 1,4-MAT as the rate measured at the same energies was high enough (∼10 8 s -1 ) to prevent phosphorescence to compete with triplet decay by hydrogen transfer (this fact poses an experimental lower limit of 10 6 s -1 to the H-atom transfer reaction). A second theoretical work was devoted to another member of the o -methyl aryl ketones family: 6,9-dimethylbenzosuberone . In that case, the rate constants were clearly lower than the ones found for 1,4-MAT, so that now phosphorescence was confirmed to occur for both the original and deuterated molecules.…”