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The mechanisms of the fragmentation and isomerization pathways of o-benzyne were studied at the multi-configurational second-order perturbative level [CAS(12,12)-PT2]. The direct fragmentation of o-benzyne to C2H2 + C4H2 follows two mechanisms: a concerted mechanism and a stepwise mechanism. Although the concerted mechanism is characterized by a single closed-shell transition structure, the stepwise pathway is more complex and structures with a strong diradical character are seen. A third diradicaloid fragmentation pathway of o-benzyne yields C6H2 as the final product. As an alternative to fragmentation, o-benzyne can also undergo rearrangement to its meta and para isomers and to the open chain cis and trans isomers of hexa-3-en-1,6-diyne (HED). These easily fragment to C2H2 + C4H2 or C6H2. Kinetic modelling at several different temperatures between 800 and 3000 K predicted that the thermal decomposition of o-benzyne should yield C2H2, C4H2 and C6H2 as the main products. Small amounts of the HED isomers accumulated at temperatures1200 K, but they rapidly decompose at higher temperatures. Between 1000 and 1400 K, C2H2 + C4H2 are formed exclusively from the decomposition of trans-HED. At temperatures1400 K, C2H2 + C4H2 also form from the direct fragmentation of o-benzyne. The formation of C2H2 + C4H2 prevails up to 1600 K but above this temperature the formation of C6H2 prevails. At temperatures2400 K, the direct fragmentation of o-benzyne again leads to the formation of C2H2 + C4H2. The formation of hydrogen atoms is also explained by our proposed mechanisms.

The synthesis of polycyclic aromatic hydrocarbons (PAHs) and the formation of soot platelets occur both during combustion at relatively low [O(2)], or under pyrolysis conditions. When the PAH size grows beyond the number of three-four condensed cycles, the partitioning of PAHs between the gas and particle phases favours the latter (i.e. adsorption). This study aims to assess which role the soot particle plays during PAH synthesis, in particular if catalytic or template effects of some sort can be exerted by the soot platelet on the adsorbed growing PAH-like radical. Our theoretical calculations indicate that chain elongation by ethyne addition cannot compete with cyclization when both can take place in the growing PAH-like radical, already in the gas phase. When it is adsorbed, cyclization is found to become easier than in the gas phase (more so, in terms of Gibbs free energy barriers, at higher temperatures), hinting at some sort of template effect, while chain elongation by ethyne addition becomes somewhat more difficult. The underlying soot platelet can assist (at lower temperatures) the formation of a larger aromatic hydrocarbon, by a final hydrogen abstraction from that endocyclic saturated carbon the newly formed cycle still bears. As an alternative (at higher temperature), a spontaneous hydrogen atom loss can take place. Finally, at rather low temperatures, the addition of the growing radical to the underlying soot platelet might occur and cause some reticulation, form more disordered structures, i.e. soot precursors instead of PAHs.