Theoretical Study on the Mechanism of the Dissociation of Benzene. The C5H3 + CH3 Product Channel

Potential energy surfaces for the various mechanisms of the dissociation of C 6 H 6 producing different isomers of the C 5 H 3 radical have been investigated using the ab initio modified Gaussian-2 (G2M) method. The most stable structures of C 5 H 3, H 2 CCCCCH ( II1 ) and HCCC(H)CCH ( II2 ), can be formed from benzene and fulvene through the pathways involving a 1,2-H shift in the carbon ring and the ring opening followed by the series of hydrogen shifts in the open chain structures of C 6 H 6 . The reaction is completed by the elimination of CH 3 from C 5 H 3 −CH 3 . All the transition states along the reaction pathways are found to lie below ∼143 kcal/mol relative to benzene, while the heats of the reactions, benzene → H 2 CCCCCH + CH 3 and benzene → HCCC(H)CCH + CH 3, are calculated to be about 150 kcal/mol. On this basis, the C 5 H 3 + CH 3 product channel of the photofragmentation of benzene at 248 nm is expected to be a two-photon process. In total, 25 different isomers of C 5 H 3 have been calculated. The most stable, II1 and II2, are predicted to have the enthalpies of formation of 136.5 and 137.2 kcal/mol, respectively. Contrary to C 5 H 3 +, the ethynyl cyclopropenyl isomer of neutral C 5 H 3 is only the third in stability with a Δ H f (298) of 166.9 kcal/mol.