Potential Energy Curves for Formation of the CH 2 O 2 Criegee Intermediate on the 3 CH 2 + 3 O 2 Singlet and Triplet Potential Energy Surfaces.

The potential energy curves (PECs) for the interaction of ³ CH ₂ with ³ O ₂ in singlet and triplet potential energy surfaces (PESs) leading to singlet and triplet Criegee intermediates (CH ₂ OO) 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 ³ CH ₂ with ³ O ₂ 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 ³ CH ₂ OO is less stable than the ¹ CH ₂ OO. 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 ³ CH ₂ OO has a larger biradical character than ¹ CH ₂ OO, and because of the larger bond order of ¹ CH ₂ OO, the C-O covalent bond becomes harder to break, thereby stabilizing ¹ CH ₂ OO. Thus, this study provides insights into the shape of the PEC obtained from the reaction between ³ CH ₂ and ³ O ₂ 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.