Unimolecular Isomerizations and Oxygen Atom Loss in Formaldehyde and Acetaldehyde Carbonyl Oxides. A Theoretical Investigation

The lowest singlet and triplet potential energy surfaces of formaldehyde carbonyl oxide (<b>1</b>) and acetaldehyde carbonyl oxide (<b>2</b>) have been investigated in the regions concerning the most relevant unimolecular reactions by means of CASSCF and MRDCI ab initio quantum-chemical calculations. The questions related to the mechanism of O-atom loss from carbonyl oxides, as well as the competition between the cyclization to dioxirane and the tautomerization to hydroperoxide in methyl-substituted carbonyl oxides are addressed in this investigation. The theoretical predictions are consistent with experimental findings obtained from stopped-flow studies of the gas-phase ozonation of both <i>trans</i>-butene and tetramethylethylene. An unexpected result is that the most reasonable pathway for O-atom loss from “hot” singlet carbonyl oxides <b>1</b> and <b>2</b> involves internal rotation about the CO bond axis, followed by intersystem crossing to the lowest triplet state and subsequent scission of the OO bond.