CH2 + CO2 → CH2O + CO, One-Step Oxygen Atom Abstraction or Addition/Fragmentation via α-Lactone?

Ab initio G2 calculated pathways are presented for the reaction CH 2 + CO 2 → CH 2 O + CO in which net transfer of a double bonded oxygen atom occurs from CO 2 to the carbene. Of particular interest are the electronic state of the attacking methylene, the structure of the possible intermediates, and the lowest energy path(s) available for this reaction. As expected, our results support the assignment of α-lactone 1 as the intermediate observed by IR in the matrix isolation experiments of Milligan and Jacox; analogous reactions involving substituted carbenes have more recently been reported by Sander et al. We obtain Δ H f ( 1 ) = −43.3 kcal/mol based on the G2 atomization energy, while a variety of isodesmic reactions point to slightly higher values (averaged −42.7 kcal/mol). Acyclic ·CH 2 O(CO)· (methylene-oxycarbonyl) and ·CH 2 CO 2 · (acetoxyl) biradicals 2 and 3, respectively, were also considered on both singlet and triplet potential energy surfaces. According to the calculations, the singlet reaction proceeds with little or no barrier to form 1; subsequent ring fragmentation (Δ H ⧧ = 27.5 kcal/mol) yields the products CH 2 O + CO. Collision orientation must play a role, however; Wagner et al. have reported that reaction is only half as fast as collisional deactivation of 1:CH 2 to 3:CH 2 which presumably occurs via nonproductive encounter geometries. An activated channel (Δ H ⧧ = 23.2 kcal/mol) was also located in which 1:CH 2 directly abstracts oxygen from CO 2 via an ylide-like TS 1 2 . The lowest energy 3:CH 2 + CO 2 attack is endothermic by 7.8 kcal/mol, forming the triplet acetoxyl diradical 3 3; a higher energy path leads to methylene-oxycarbonyl diradical 3 2 . Barriers for these two processes are Δ H ⧧ = 19.3 and 57.7 kcal/mol, respectively. No path for isomerization of 3 3 to 3 2 was found. Attempts to locate regions on the triplet approach surface where the singlet crosses to become the lower energy spin state were complicated by the difficulty of optimizing geometries within the composite G2 model. Preliminary efforts, however, indicate that such crossings occur at geometries higher in energy than separated 1:CH 2 + CO 2, suggesting that their role should be relatively unimportant in this reaction.