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

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journal contribution
posted on 20.07.2001, 00:00 by Dalila Kovacs, James E. Jackson
Ab initio G2 calculated pathways are presented for the reaction CH2 + CO2CH2O + CO in which net transfer of a double bonded oxygen atom occurs from CO2 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 ΔHf(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 ·CH2O(CO)· (methylene-oxycarbonyl) and ·CH2CO2· (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 (ΔH = 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 fast as collisional deactivation of 1:CH2 to 3:CH2 which presumably occurs via nonproductive encounter geometries. An activated channel (ΔH = 23.2 kcal/mol) was also located in which 1:CH2 directly abstracts oxygen from CO2 via an ylide-like TS 12. The lowest energy 3:CH2 + CO2 attack is endothermic by 7.8 kcal/mol, forming the triplet acetoxyl diradical 33; a higher energy path leads to methylene-oxycarbonyl diradical 32. Barriers for these two processes are ΔH = 19.3 and 57.7 kcal/mol, respectively. No path for isomerization of 33 to 32 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:CH2 + CO2, suggesting that their role should be relatively unimportant in this reaction.