KINETICS OF THE PHOTOENOLIZATION OF 5,8-DIMETHYL-1-TETRALONE - HYDROGEN-TRANSFER TUNNEL EFFECTS IN THE EXCITED TRIPLET-STATE

The kinetics of the photoenolization of 5,8-dimethyl-1-tetralone (DMT) has been studied laser-flash photolytically in a polar solvent at different temperatures between 80 and 290 K. The transient absorption spectra of the tautomeric keto and enol triplet states of DMT were determined between 320 and 490 nm by analyzing absorbance decay traces kinetically. It could be shown that the hydrogen-transfer reaction in the excited triplet states of DMT is largely governed by tunnel effects. Nonlinear Arrhenius plots and large isotope effects provide convincing evidence for these tunnel contributions to the rate of the keto-enol tautomerization in the excited triplet state. The hydrogen and deuterium transfer rate data of DMT are very similar to those of 2-(2'-hydroxyphenyl)benzoxazole, where tunnel effects in the excited triplet states also play an important role.