| Résumé | Dynamic EPR spectroscopy has been employed to investigate the inversion of the oxiranyl and [2- 2H]oxiran-2-yl radicals. For the deuterium-substituted radical the inversion can be "frozen" at low temperature, so that the two β-hydrogen atoms can be distinguished (a Hβ = 5.23 and 4.74 G at 120 K). For the oxiranyl radical the two β-hydrogens do not become distinguishable, but there is some broadening of the central line of the CH 2 triplets at low temperatures. On the basis of the observed difference in a Hβ values, the spectra can be simulated, and the simulation can be used to derive temperature-dependent inversion rate constants for the two isotopomers. The results show a large primary isotope effect (factor 5-40, dependent on the temperature) and a curved Arrhenius plot for oxiranyl leading to an invariant low-temperature rate constant for T ≤ 140 K. It is therefore concluded that the inversion proceeds by quantum-mechanical tunneling. To investigate this transition quantitatively, an ab initio UHF-MO calculation has been carried out, yielding the molecular geometry, the normal-mode frequencies, and the inversion barrier height. The angle between the C α-H α bond and the plane of the ring is calculated to be much larger than in cyclopropyl (56.5 vs 39.3°) in agreement with the larger a 13Cα (121 G for oxiranyl vs 95.9 G for cyclopropyl) and with the slower inversion. The observed rate constants can be fitted accurately to a modified quartic double-minimum potential, which compares favorably with the effective one-dimensional inversion potential deduced from the quantum-chemical results. The barrier height is estimated to be 6.8 kcal/mol. |
|---|
