CH + O: reaction mechanism, biradical and zwitterionic character, and formation of CHOO, 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 CHOO (the simplest Criegee intermediate) and CHO (dioxirane) have mostly polar biradical character, while singlet CHOO 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 CH + O is more than ten times as fast as the reaction of CH (XB) + O and the ratio remains almost independent of temperature from 900 K to 3000 K. The master equation simulations predict that at low pressures the CHO + 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.