Unimolecular Thermal Fragmentation of Ortho-Benzyne

Xu Zhang, Alan T. MacCarone, Mark R. Nimlos, Shuji Kato, Veronica M. Bierbaum, G. Barney Ellison, Branko Ruscic, Andrew C. Simmonett, Wesley D. Allen, Henry F. Schaefer

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Abstract

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.

Original languageAmerican English
Article numberArticle No. 044312
Number of pages20
JournalThe Journal of Chemical Physics
Volume126
Issue number4
DOIs
StatePublished - 2007

NREL Publication Number

  • NREL/JA-510-41462

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