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Theoretical Study on the Reaction Mechanism of Vinyl Radical with Formaldehyde

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journal contribution
posted on 22.09.2005, 00:00 by Hong-bin Xie, Yi-hong Ding, Chia-chung Sun
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.