Unimolecular thermal fragmentation of ortho-benzyne

作者：Zhang Xu^*; Maccarone Alan T; Nimlos Mark R; Kato Shuji; Bierbaum Veronica M; Ellison G Barney; Ruscic Branko; Simmonett Andrew C; Allen Wesley D; Schaefer Henry F III

来源：Journal of Chemical Physics, 2007, 126(4): 044312.

DOI：10.1063/1.2409927

The ortho-benzyne diradical, o-C6H4 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-C6H4+Delta -> 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-C6H4+Delta -> HC CH+HC C-C CH. The experimental Delta H-rxn(298)(o-C6H4 -> HC CH+HC C-C CH) is found to be 57 +/- 3 kcal mol(-1). Further experiments with the substituted benzyne, 3,6-(CH3)(2)-o-C6H2, 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, C-2v-symmetric decomposition of o-benzyne, E-b(o-C6H4 -> HC CH+HC C-C CH)=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: C6H6 -> H+[C6H5]-> H+[o-C6H4]-> HC CH+HC C-C CH. 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.

• 出版日期2007-1-28