Addition of one and two units of C2H to styrene: a theoretical study of the C10H9 and C12H9 systems and implications toward growth of polycyclic aromatic hydrocarbons at low temperatures.

Various mechanisms of the formation of naphthalene and its substituted derivatives have been investigated by ab initio G3(MP2,CC)∕B3LYP∕6-311G∗∗ calculations of potential energy surfaces for the reactions of one and two C(2)H additions to styrene combined with RRKM calculations of product branching ratios under single-collision conditions. The results show that for the C(2)H + styrene reaction, the dominant routes are H atom eliminations from the initial adducts; C(2)H addition to the vinyl side chain of styrene is predicted to produce trans or cis conformations of phenylvinylacetylene (t- and c-PVA), whereas C(2)H addition to the ortho carbon in the ring is expected to lead to the formation of o-ethynylstyrene. Although various reaction channels may lead to a second ring closure and the formation of naphthalene, they are not competitive with the direct H loss channels producing PVAs and ethynylstyrenes. However, c-PVA and o-ethynylstyrene may undergo a second addition of the ethynyl radical to ultimately produce substituted naphthalene derivatives. α- and β-additions of C(2)H to the side chain in c-PVA are calculated to form 2-ethynyl-naphthalene with branching ratios of about 30% and 96%, respectively; the major product in the case of α-addition would be cis-1-hexene-3,5-diynyl-benzene produced by an immediate H elimination from the initial adduct. C(2)H addition to the ethynyl side chain in o-ethynylstyrene is predicted to lead to the formation of 1-ethynyl-naphthalene as the dominant product. The C(2)H + styrene → t-PVA + H∕c-PVA + H∕ o-ethynylstyrene, C(2)H + c-PVA → 2-ethynyl-naphthalene + H, and C(2)H + o-ethynylstyrene → 1-ethynyl-naphthalene + H reactions are calculated to occur without a barrier and with high exothermicity, with all intermediates, transition states, and products lying significantly lower in energy than the initial reactants, and hence to be fast even at very low temperature conditions prevailing in Titan's atmosphere or in the interstellar medium. If styrene and C(2)H are available and overlap, the sequence of two C(2)H additions can result in the closure of a second aromatic ring and thus provide a viable route to the formation of 1- or 2-ethynyl-naphthalene. The analogous mechanism can be extrapolated to the low-temperature growth of polycyclic aromatic hydrocarbons (PAH) in general, as a step from a vinyl-PAH to an ethynyl-substituted PAH with an extra aromatic ring.