Rate Acceleration below 20 K in the H-Atom Tunneling of Triplet <i>ortho</i>-Methyltetralones

Johnson, Brent A.

doi:10.1021/ja990803q

Cited by 7 publications

(1 citation statement)

References 21 publications

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“…The study of chemical reactivity in electronically excited states is quite a complicated task both from experimental and theoretical aspects. Over the last 12 years, Garcia-Garibay's group has extensively studied the photoinduced hydrogen-atom transfer that takes place at ultralow temperatures (3−50 K) in different o -methyl aryl ketones. − From these studies, on the basis of phosphorescence lifetimes and total intensity measurements, it is now well-established that the more stable keto form of the molecule in the ground (singlet) electronic state ( 1 K ) can be electronically excited up to a (singlet) excited electronic state. From there a rapid intersystem crossing leads to the lowest triplet electronic state ( 3 K ).…”

Section: Introductionmentioning

confidence: 99%

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

Moreno

Lluch

2007

J. Phys. Chem. A

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The energy profile for the tautomerization reaction of 1,4-dimethylanthrone in the first triplet electronic state obtained through electronic calculations (B3LYP/ 6-31G(d)) is used to calculate the rate constants for the process at a wide range of energies using a modified RRKM microcanonical statistical formalism that takes into account tunneling. Through partial or total substitution of the hydrogen atoms of the methyl groups by deuterium atoms, it is possible to evaluate different primary and secondary kinetic isotope effects (KIE). These results can be compared with experimental data for these processes taking place in solid matrix at extremely low temperatures (4-50 K). Such a comparison allows us to conclude that the reaction is taking place at energies just slightly below (around 0.5 kcal/mol) the adiabatic potential energy barrier, a result that was previously found for other related molecules so that this mechanism may be extended to the photoenolization of other o-aryl methyl ketones. Analysis of the different factors contributing to the primary and secondary KIEs discloses that at energies not far below the adiabatic barrier, the tunneling effect is not the only factor that accounts for the large KIE but the differences in the energy level distribution upon isotopic substitution may be the predominant factor at a certain range of negative energies (this is especially so for the case of primary KIE). At positive energies (above the barrier) the levels factor is always the dominant factor in the total KIE.

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