CH2+CO2 -> CH2O+CO, one-step oxygen atom abstraction or addition/fragmentation via alpha-lactone?

Ab initio G2 calculated pathways are presented for the reaction CH2 + CO2 - -> CH2O + CO in which net transfer of a double bonded oxygen atom occurs fr om CO2 to the carbene. Of particular interest are the electronic state of t he attacking methylene, the structure of the possible intermediates, and th e lowest energy path(s) available for this reaction. As expected, our resul ts support the assignment of alpha -lactone I 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 b y Sander et al. We obtain DeltaH(f)(1) = -43.3 kcal/mol based on the G2 ato mization energy, while a variety of isodesmic reactions point to slightly h igher values (averaged -42.7 kcal/mol). Acyclic . CH2O(CO). (methylene-oxyc arbonyl) and . CH2CO2. (acetoxyl) biradicals 2 and 3, respectively, were al so considered on both singlet and triplet potential energy surfaces: Accord ing to the calculations, the singlet reaction proceeds with little or no ba rrier to form 1; subsequent ring fragmentation (DeltaH double dagger = 27.5 kcal/mol) yields the products CH2O + CO. Collision orientation must play a role, however; Wagner et al. have reported that reaction is only half as f ast as collisional deactivation of (1):CH2 to (3):CH2 which presumably occu rs via nonproductive encounter geometries. An activated channel (DeltaH dou ble dagger = 23.2 kcal/mol) was also located in which 1:CH2 directly abstra cts oxygen from CO2 via an ylide-like TS (1)2. The lowest energy (3):CH2 CO2 attack is endothermic by 7.8 kcal/mol, forming, the triplet acetoxyl di radical (3)3; a higher energy path leads to methylene-oxycarbonyl diradical (3)2. Barriers for these two processes are DeltaH double dagger = 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 th e singlet crosses to become the lower energy spin state were complicated by the difficulty of optimizing geometries within the composite G2 model. Pre liminary efforts, however, indicate that such crossings occur at geometries higher in energy than separated (1):CH2 + CO2, suggesting that their role should be relatively unimportant in this reaction.