Theoretical Study on the Reaction Mechanism of Vinyl Radical with Formaldehyde

A detailed computational study is performed on the radical−molecule reaction between the vinyl radical (C₂H₃) and formaldehyde (H₂CO), for which only the direct hydrogen abstraction channel has been considered by previous and very recent theoretical studies. At the Gaussian-3//B3LYP/6-31G(d) and CBS-QB3 levels, the direct H-abstraction forming C₂H₄ + HCO has barriers of 3.9 and 4.7 kcal/mol, respectively. The addition barrier to form H₂CCHCH₂O has barriers of 2.8 and 2.3 kcal/mol, respectively. Subsequently, there are two highly competitive dissociation pathways for H₂CCHCH₂O:  One is the formation of the direct H-extrusion product H₂CCHCHO + H, and the other is the formation of C₂H₄ + HCO via the intermediate H₂CCH₂CHO. Surely, the released energy is large enough to drive the secondary dissociation of HCO to H + CO. Because the involved transition states and intermediates of the H₂CCHCH₂O evolution all lie energetically lower than the entrance addition transition state, the addition−elimination is more competitive than the direct H-transfer for the C₂H₃ + H₂CO reaction, in contrast to previous expectation. The present results can be useful for future experimental investigation on the title reaction.