Unimolecular Thermal Fragmentation of Ortho-Benzyne

The ortho-benzyne diradical, o- C6 H4 has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o- C6 H4 +Δ→ products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o -benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o- C6 H4 +Δ→HCCH+HCC-CCH. The experimental Δrxn H298 (o- C6 H4 →HCCH+HCC-CCH) is found to be 57±3 kcal mol-1. Further experiments with the substituted benzyne, 3,6- (C H3) 2 -o- C6 H2, are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation of o -benzyne has been investigated by rigorous ab initio electronic structure computations. These calculations used basis sets as large as [C (7s6p5d4f3g2h1i) H (6s5p4d3f2g1h)] (cc-pV6Z) and electron correlation treatments as extensive as full coupled cluster through triple excitations (CCSDT), in cases with a perturbative term for connected quadruples [CCSDT(Q)]. Focal point extrapolations of the computational data yield a 0 K barrier for the concerted, C2v -symmetric decomposition of o -benzyne, Eb (o- C6 H4 →HCCH+HCC-CCH) =88.0±0.5 kcal mol-1. A barrier of this magnitude is consistent with the experimental results. A careful assessment of the thermochemistry for the high temperature fragmentation of benzene is presented: C6 H6 →H+ [C6 H5] →H+ [o- C6 H4] →HCCH+HCC-CCH. Benzyne may be an important intermediate in the thermal decomposition of many alkylbenzenes (arenes). High engine temperatures above 1500 K may crack these alkylbenzenes to a mixture of alkyl radicals and phenyl radicals. The phenyl radicals will then dissociate first to benzyne and then to acetylene and diacetylene.