CH 2 + O 2 : reaction mechanism, biradical and zwitterionic character, and formation of CH 2 OO, the simplest Criegee intermediate.

The singlet and triplet potential surfaces for the title reaction were investigated using the CBS-QB3 level of theory. The wave functions for some species exhibited multireference character and required the CASPT2/6-31+G(d,p) and CASPT2/aug-cc-pVTZ levels of theory to obtain accurate relative energies. A Natural Bond Orbital Analysis showed that triplet 3 CH 2 OO (the simplest Criegee intermediate) and 3 CH 2 O 2 (dioxirane) have mostly polar biradical character, while singlet 1 CH 2 OO has some zwitterionic character and a planar structure. Canonical variational transition state theory (CVTST) and master equation simulations were used to analyze the reaction system. CVTST predicts that the rate constant for reaction of 1 CH 2 + 3 O 2 is more than ten times as fast as the reaction of 3 CH 2 (X 3 B 1 ) + 3 O 2 and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the 1 CH 2 O + 3 O product set is dominant at all temperatures and the primary yield of OH radicals is negligible below 600 K, due to competition with other primary reactions in this complex system.