Naphthalene formation pathways from phenyl radical via vinyl radical (C2H3) and vinylacetylene (C4H4): computational studies on reaction mechanisms and kinetics

The reaction mechanisms of PAH formation from phenyl radical (C6H5) to naphthalene via C2H3 (C2H3-Path) and C4H4 (C4H4-Path) were investigated by the G3(MP2, CC) method. The hydrogen abstraction, ring closure, cis-trans isomerization, and disproportionation reactions were considered, as well as their occurred sequence. The results showed that H-abstraction reactions occurred more easily than H-dissociation reactions. The cis-trans conversion reactions in sub-routes of C2H3-Path and C4H4-Path provided the largest barriers of 51, 53, and 36 kcal/mol along their routes, which illustrated that the cis-trans isomerization was energetically costly in the PAH formation process. The entrance barriers of C2H2-Path, C2H3-Path, and C4H4-Path are 6, 8, and 3 kcal/mol, respectively, which indicates that it is easier to add C4H4 to C6H5 compared with adding C2H2 to C2H3. C2H3 additions were highly exothermic with reaction energies greater than 110 kcal/mol, and compared with C2H2 additions, C2H3 additions were irreversible. However, C2H2-Path, C2H3-Path and C4H4-Path involved energy barriers of 20, 32, and 36 kcal/mol, respectively. Considering the high temperature in combustion and the approximate concentrations of C2H3 and C4H4, all three of these pathways could lead to naphthalene in some combustion flames.