skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Unimolecular thermal fragmentation of ortho-benzene.

Abstract

The ortho-benzyne diradical, o-C{sub 6}H{sub 4} 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-C{sub 6}H{sub 4}{sup +} {Delta} {yields} 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-C{sub 6}H{sub 4}{sup +}{Delta}{yields} HC {triple_bond} CH+HC {triple_bond} C-C {triple_bond} CH. The experimental {Delta}{sub rxn}H{sub 298}(o-C{sub 6}H{sub 4} {yields} HC {triple_bond} CH+HC {triple_bond} C-C {triple_bond} CH) is found to be 57 {+-} 3 kcal mol{sup -1}. Further experiments with the substituted benzyne, 3,6-(CH{sub 3}){sub 2}-o-C{sub 6}H{sub 2}, 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 connectedmore » quadruples [CCSDT(Q)]. Focal point extrapolations of the computational data yield a 0 K barrier for the concerted, C{sub 2v}-symmetric decomposition of o-benzyne, E{sub b}(o-C{sub 6}H{sub 4} {yields} HC {triple_bond} CH+HC {triple_bond} C-C {triple_bond} CH) = 88.0 {+-} 0.5 kcal mol{sup -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: C{sub 6}H{sub 6} {yields} H+[C{sub 6}H{sub 5}] {yields} H+[o-C{sub 6}H{sub 4}] {yields} HC {triple_bond} CH-HC {triple_bond} C-C {triple_bond} 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.« less

Authors:
; ; ; ; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC); National Science Foundation (NSF)
OSTI Identifier:
939552
Report Number(s):
ANL/CHM/JA-57209
Journal ID: ISSN 0021-9606; JCPSA6; TRN: US200823%%151
DOE Contract Number:  
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: J. Chem. Phys.; Journal Volume: 126; Journal Issue: 4 ; 2007
Country of Publication:
United States
Language:
ENGLISH
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; AROMATICS; BENZYL RADICALS; DISSOCIATION; ELECTRONIC STRUCTURE; FRAGMENTATION; PHENYL RADICALS; PYROLYSIS; CHEMICAL REACTION KINETICS

Citation Formats

Zhang, X., Maccarone, A. T., Nimlos, M. R., Kato, S., Bierbaum, V. M., Ellison, G. B., Ruscic, B., Simmonett, A. C., Allen, W. D., Schaefer, H. F., Chemistry, Univ. of Colorado, NREL, and Univ. of Georgia. Unimolecular thermal fragmentation of ortho-benzene.. United States: N. p., 2007. Web. doi:10.1063/1.2409927.
Zhang, X., Maccarone, A. T., Nimlos, M. R., Kato, S., Bierbaum, V. M., Ellison, G. B., Ruscic, B., Simmonett, A. C., Allen, W. D., Schaefer, H. F., Chemistry, Univ. of Colorado, NREL, & Univ. of Georgia. Unimolecular thermal fragmentation of ortho-benzene.. United States. doi:10.1063/1.2409927.
Zhang, X., Maccarone, A. T., Nimlos, M. R., Kato, S., Bierbaum, V. M., Ellison, G. B., Ruscic, B., Simmonett, A. C., Allen, W. D., Schaefer, H. F., Chemistry, Univ. of Colorado, NREL, and Univ. of Georgia. Mon . "Unimolecular thermal fragmentation of ortho-benzene.". United States. doi:10.1063/1.2409927.
@article{osti_939552,
title = {Unimolecular thermal fragmentation of ortho-benzene.},
author = {Zhang, X. and Maccarone, A. T. and Nimlos, M. R. and Kato, S. and Bierbaum, V. M. and Ellison, G. B. and Ruscic, B. and Simmonett, A. C. and Allen, W. D. and Schaefer, H. F. and Chemistry and Univ. of Colorado and NREL and Univ. of Georgia},
abstractNote = {The ortho-benzyne diradical, o-C{sub 6}H{sub 4} 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-C{sub 6}H{sub 4}{sup +} {Delta} {yields} 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-C{sub 6}H{sub 4}{sup +}{Delta}{yields} HC {triple_bond} CH+HC {triple_bond} C-C {triple_bond} CH. The experimental {Delta}{sub rxn}H{sub 298}(o-C{sub 6}H{sub 4} {yields} HC {triple_bond} CH+HC {triple_bond} C-C {triple_bond} CH) is found to be 57 {+-} 3 kcal mol{sup -1}. Further experiments with the substituted benzyne, 3,6-(CH{sub 3}){sub 2}-o-C{sub 6}H{sub 2}, 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{sub 2v}-symmetric decomposition of o-benzyne, E{sub b}(o-C{sub 6}H{sub 4} {yields} HC {triple_bond} CH+HC {triple_bond} C-C {triple_bond} CH) = 88.0 {+-} 0.5 kcal mol{sup -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: C{sub 6}H{sub 6} {yields} H+[C{sub 6}H{sub 5}] {yields} H+[o-C{sub 6}H{sub 4}] {yields} HC {triple_bond} CH-HC {triple_bond} C-C {triple_bond} 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.},
doi = {10.1063/1.2409927},
journal = {J. Chem. Phys.},
number = 4 ; 2007,
volume = 126,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}