| Hydrogen and deuterium abstraction rates in the photoenolization of 5,8-dimethyl-1-tetralone (3), 6,9-dimethyl-1-benzosuberone (6), 4,7-dimethyl-1-indanone (7), and several related compounds were determined from 120-4 K by comparing their phosphorescence decays with those of similar ketones containing no abstractable hydrogens. The results indicate that quantum mechanical tunneling is important to the reaction of 3, 6 and several related ketones below 100 K, but tunneling could not be confirmed for 4,7-dimethyl-l-indanone (7). Phosphorescence measurements suggest that reaction of deuterated (d6)-5,8-dimethyl-6,7-(d2)-1-tetralone ( D3) occurs exclusively by tunneling between 40 K and 20 K at a rate (kD(QMT) = 9 +/- 2 x 102 s-1 ) that is consistent with the laser flash photolysis results reported by Al Soufi and coworkers [Al-Soufi, W.; Eychmuer, A.; Grellmann, K. H. J. Phys. Chem. 1991, 95, 2022---2026]. Hydrogen tunneling rates of 3 were not obtained because the reaction rate was too fast to measure by phosphorescence, but a lower limit for the isotope effect is estimated to be kH/k D > 103.*; While the addition of methyl groups a to the carbonyl of 3 had little effect on the hydrogen tunneling rate in the photoenolization reaction, changing the six-membered benzocycloalkanone ring in 5,8-dimethyl-l-tetralone (3) to a seven-membered ring in 6,9-dimethyl-l-benzosuberone (6) or a five-membered ring in 4,7-dimethyl-l-indanone ( 7) dramatically reduces the hydrogen tunneling rate and the isotope effect. The hydrogen (and deuterium) tunneling rates of 6 are reduced relative to 3 to kH = 80 +/- 40 s -1 and kD = 65 + 40 s-1, with an isotope effect of kH/kD = 1.2 +/- 0.2. The lower reaction rate and decreased isotope effects are interpreted to be a result of a change in the reaction barrier and a less favorable geometry for hydrogen tunneling. An unexpected increase in quantum mechanical deuterium tunneling rates in the photoenolization reaction is observed below 20 K, and is interpreted in terms of a temperature dependence of the equilibration rate between the triplet sublevels. The role of the n,pi* and pi,pi* triplet states in photoenolization reaction is also discussed.; *Please refer to dissertation for diagrams. |
