CASSCF and CAS+1+2 Studies on the Potential Energy Surface and the Rate Constants for the Reactions between CH₂ and O₂

CAS(14,12)/cc-pvdz calculations are reported for the reaction of 3 CH 2 + 3 O 2 →products. On the singlet potential energy surface, a transition state has been located with an energy barrier of 1.65 kcal/mol, which is in good agreement with the experimental estimation of 1.0−1.5 kcal/mol. The rearrangement and metathesis of the singlet intermediates have been also investigated at the same level of theory. For the triplet case, the formation of CH 2 OO has an energy barrier of 5.79 kcal/mol, and the formed triplet CH 2 OO could be further decomposed into CH 2 O+O( 3 P) with an energy barrier of 2.92 kcal/mol. The geometries of some key points have been relocated at the CAS(8,6)+1+2/cc-pvdz level of theory for comparison. The present theoretical results for the total reaction rates, at the CAS(8,6)+1+2/cc-pvdz level, can be expressed by the three-parameter expression: k ( T ) = 4.273×10 -18 T 2.245 exp(−185/ T ) within ±5% error at the temperature range 295−2600 K.