Potential Energy Curves for Formation of the CH₂O₂ Criegee Intermediate on the ³CH₂ + ³O₂ Singlet and Triplet...

The potential energy curves (PECs) for the interaction of ³CH₂ with ³O₂ in singlet and triplet potential energy surfaces (PESs) leading to singlet and triplet Criegee intermediates (CH₂OO) are studied using electronic structure calculations. The bonding mechanism is interpreted by analyzing the ground state multireference configuration interaction (MRCI) wave function of the reacting species and at all points along the PES. The interaction of ³CH₂ with ³O₂ on the singlet surface leads to a flat long-range attractive PEC lacking any maxima or minima along the curve. The triplet surface stems into a maximum along the curve resulting in a transition state with an energy barrier of 5.3 kcal/mol at CASSCF(4,4)/cc-pVTZ level. The resulting ³CH₂OO is less stable than the ¹CH₂OO. In this study, the biradical character (β) is used as a measure to understand the difference in the topology of the singlet and triplet PECs and the relation of the biradical nature of the species with their structures. The ³CH₂OO has a larger biradical character than ¹CH₂OO, and because of the larger bond order of ¹CH₂OO, the C-O covalent bond becomes harder to break, thereby stabilizing ¹CH₂OO. Thus, this study provides insights into the shape of the PEC obtained from the reaction between ³CH₂ and ³O₂ in terms of their bonding nature and from the shape of the curves, the temperature dependence or independence of the rate of the reaction is discussed.