Theoretical study of the reaction 1[:CH2]+CHO+→CH3++CO

An ab initio study was performed of the reactions of formyl and isoformyl cations with singlet methylene (1)[:CH2], which plays an important role in the ionic mechanisms for the formation of soot in flames. The corresponding potential energy surface (PES) was studied at the MP2/6-311++G(d,p) level of theory, and single-point calculations on the MP2 geometries were carried out at the CCSD(T)/6-311++G(d,p) and MP2/6-311++G(3df,3pd) levels. According to our results, the interaction of (1)[:CH2] with both HCO+ and COH+ cations directly leads to two C2H3O+ cyclic intermediates, about 83 and 22 kcal/mol, respectively, more stable than methylene and the formyl cation. A linear CH2COH+ structure may also be formed from the interaction between (1)[:CH2] and COH+ fragments that is 133 kcal/mol more stable than the reactants. Different transition structures for the 1,2-H or 1,2-CH3+ shift and/or ring-opening of these intermediates were located, thus allowing us to predict that open-chain structures such as CH3CO+ and CH3+... OC (similar to 138 and 85 kcal/mol more stable than reactants, respectively) may dissociate into the CO + CH3+ products. Proton-transfer mechanisms are also possible for this process through hydrogen-bonded CH2-H ... CO and CH2-H ... OC complexes characterized as minima on the PES. The transition structures for the H-shift corresponding to the isomerization of the CHO+ moiety from formyl into isoformyl are considerably stabilized by the attachment of (1)[:CH2] to the pi CO bond compared with the transition structures for the uncatalyzed process HCO+ --> COH+.