Search

The ring closure reaction of C4H3 with acetylene to give phenyl radical is one proposed mechanism for the formation of the first aromatic ring in hydrocarbon combustion. There are two low-lying isomers of C4H3; 1-dehydro-buta-1-ene-3-yne (n-C4H3) and 2-dehydro-buta-1-ene-3-yne (iso-C4H3). It has been proposed that only n-C4H3 reacts with acetylene to give phenyl radical, and since iso-C4H3 is more stable than n-C4H3, formation of phenyl radical by this mechanism is unlikely. We report restricted Hartree–Fock (RHF) plus singles and doubles configuration interaction calculations with a Davidson’s correction (RHF+1+2+Q) using the Dunning correlation consistent polarized valence double zeta basis set (cc-pVDZ) for stationary point structures along the reaction pathway for the reactions of n-C4H3 and iso-C4H3 with acetylene. n-C4H3 plus acetylene (9.4) has a small entrance channel barrier (17.7) (all energetics in parentheses are in kcal/mol with respect to iso-C4H3 plus acetylene) and the subsequent closure steps leading to phenyl radical (-91.9) are downhill with respect to the entrance channel barrier. Iso-C4H3 plus acetylene also has an entrance channel barrier (14.9) and there is a downhill pathway to 1-dehydro-fulvene (-55.0). 1-dehydro-fulvene can rearrange to 6-dehydro-fulvene (-60.3) by a 1,3-hydrogen shift over a barrier (4.0), which is still below the entrance channel barrier, from which rearrangement to phenyl radical can occur by a downhill pathway. Thus, both n-C4H3 and iso-C4H3 can react with acetylene to give phenyl radical with small barriers. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reactions of CH(2Π) and singlet methylene (1CH2) with acetylene lead to intermediates which may be important in soot formation. CH(2Π)+acetylene leads to CHCHCH (C3H3), CHCCH, CH2CC, and propargyl (CH2CCH). 1CH2+acetylene leads to cyclopropene, allene, and propyne. Except for CHCCH, which involves a barrier of 7.5 kcal/mol with respect to reactants, all of these reaction products are formed with no barrier. Miller and Melius have previously discussed the dimerization of propargyl to give benzene. C3H3 and CHCCH are shown to dimerize with no barrier to give benzene and para-benzyne, respectively. C3H3, CHCCH, and CH2CC can also add to smaller polycyclic aromatic hydrocarbons (PAH), and may be important species in forming larger PAH or fullerenes. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reaction of vinylidene (CH2C) with acetylene may be an initiating reaction in soot formation. We report minimum energy paths and accurate energetics for a pathway leading to vinylacetylene and for a number of isomers of C4H4. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contacted configuration interaction (ICCI) and/or coupled cluster singles and doubles with a perturbational estimate of triple excitations [CCSD(T)] to determine the energetics. We find an entrance channel barrier of about 5 kcal/mol for the addition of vinylidene to acetylene, but no barriers above reactants for the reaction pathway leading to vinylacetylene. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]