o-Benzyne fragmentation and isomerization pathways. A CASPT2 study.

The mechanisms of the fragmentation and isomerization pathways of o-benzyne have been 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 one (already subject to several studies) and a stepwise, studied here with the appropriate method for the first time. While the concerted mechanism is characterized by a single closed shell transition structure, the stepwise pathway is more complex and sees structures with strong diradical character. A third diradicaloid fragmentation pathway of o-benzyne yields, as final product, C6H2. Alternatively to fragmentations, o-benzyne can undergo to rearrangements to its meta and para isomers and to the open-chain cis and trans isomers of hexa-3-en-1,6-diyne (HED). These are found to easily fragment to C2H2 + C4H2 or C6H2. The kinetic modelling, performed at several temperatures (800 - 3000 K), found that the thermal decomposition of o-benzyne is predicted to yield C2H2, C4H2 and C6H2 as main products. Small amounts of HEDs can accumulates only below 1200 K because they rapidly decompose. Between 1000 and 1400 K, C2H2 + C4H2 form exclusively from decomposition of trans-HED. Above 1400 K, C2H2 + C4H2 also form from the direct fragmentation of o-benzyne. The formation of C2H2 + C4H2 prevails up to 1600 K then, above this temperature, the formation of C6H2 prevails. However, above 2400 K, the direct fragmentation of o-benzyne makes the formation of C2H2 + C4H2 to prevail, again. The formation of hydrogen atoms is also justified.