ELECTRONIC-STRUCTURE AND REACTIVITY OF DIOXIRANE AND CARBONYL OXIDE

Dioxirane, the diradical dioxymethane, and carbonyl oxide have been studied at a variety of levels including MP2, MP4, CASSCF, QCISD, and QCISD(T). MP2 optimization is sufficient for dioxirane, the various dioxymethane biradicals, and substituted carbonyl oxide, but optimization at the QCISD or QCISD(T) level is needed to obtain the correct structure for carbonyl oxide. Significantly, both levels of theory give essentially the same C-O/O-O bond length ratio for this controversial molecule (1.281/1.369 angstrom and 1.287/1.356 angstrom for QCISD/6-31G* and QCISD(T)/6-31G**, respectively). The unsubstituted carbonyl oxide is probably anomalously sensitive to the level of calculation, since MP2/6-31G* calculations on dimethylcarbonyl oxide afford a C-O/O-O bond ratio of 1.283/1.345 angstrom, consistent with the higher level calculations on the parent carbonyl oxide. Transition structures have been obtained at the MP2/6-31G* level for the isomerization of carbonyl oxide to dioxirane and for the donation of oxygen from carbonyl oxide and dioxirane to ethylene. A scan of the potential energy curves for O-O bond dissociation in hydrogen peroxide and water oxide shows that the MP4SDTQ and QCISD(T) curves are nearly superimposable for bond elongation up to 2.5 angstrom. At the QCISD(T)/6-31G*//MP2/6-31G* level, carbonyl oxide is 28.6 kcal/mol higher than dioxirane and separated from it by a barrier of 19.1 kcal/mol; the barriers for donation of an oxygen atom from dioxirane and carbonyl oxide to ethylene are 16.7 and 11.9 kcal/mol, respectively. The MP4SDTQ level is adequate for determining the barriers for the first two reactions, but QCISD(T) is required for a proper treatment of the barrier for dioxirane plus ethylene (MP4 overestimates the triples contribution). Theoretical evidence is presented that suggests that both the reactants and the transition structures involved in these epoxidation reactions are principally closed shell in nature.