TITLE

Unimolecular thermal fragmentation of ortho-benzyne

AUTHOR(S)
Xu Zhang; 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.
PUB. DATE
January 2007
SOURCE
Journal of Chemical Physics;1/28/2007, Vol. 126 Issue 4, p044312
SOURCE TYPE
Academic Journal
DOC. TYPE
Article
ABSTRACT
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+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-C6H4+Δ→HC=CH+HC=C–C=CH. The experimental ΔrxnH298(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, C2v-symmetric decomposition of o-benzyne, Eb(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.
ACCESSION #
23923534

 

Related Articles

  • Dissociation of singly and multiply charged fullerenes: Emission of C[sub 4], or sequential... Foltin, M.; Echt, O.; Scheier, P.; Dunser, B.; Worgotter, R.; Muigg, D.; Matt, S.; Mark, T.D. // Journal of Chemical Physics;10/22/1997, Vol. 107 Issue 16, p6246 

    Reports on the achievement of direct mass spectrometric evidence that fullerene ions undergo unimolecular dissociation by sequential emission of two C2 units. Comparison of experimental and theoretical breakdown graphs; Reaction channels accessible to excited fullerenes; Importance of thermal...

  • Low-energy helium nozzle beam. Kern, Klaus; David, Rudolf; Comsa, George // Review of Scientific Instruments;Mar85, Vol. 56 Issue 3, p369 

    A high-intensity supersonic nozzle beam source is described, whose beam energy can be varied between 2 and 170 meV (helium). Experimental performance and time-of-flight measurements on cold helium beams are reported. Condensation in the free jet expansions of helium has been studied by mass...

  • State selected unimolecular dissociation of HOCI. Barnes, Rhett James; Sinha, Amitabha // Journal of Chemical Physics;9/1/1997, Vol. 107 Issue 9, p3730 

    Studies the state selected unimolecular dissociation of hypochlorous acid. Characterization of the intermediate vibrational state; Rotational assignment based on an analysis using an asymmetric rotor fitting program; Linewidth measurement.

  • Comment on the rate of vibrational predissociation of some RgCl2 and RgICl molecules. Zhao, Meishan; Rice, Stuart A. // Journal of Chemical Physics;5/15/1992, Vol. 96 Issue 10, p7483 

    We report the results of calculations of the rates of predissociation of HeCl2, NeCl2, HeICl, and NeICl using a version of the classical theory of unimolecular reaction rate which includes a balanced treatment of both the intramolecular energy transfer and fragmentation processes. The agreement...

  • The unimolecular dissociation of the OH stretching states of HOCl: Comparison with experimental data. Weiß, J.; Hauschildt, J.; Schinke, R.; Haan, O.; Skokov, S.; Bowman, J. M.; Mandelshtam, V. A.; Peterson, K. A. // Journal of Chemical Physics;11/15/2001, Vol. 115 Issue 19, p8880 

    The unimolecular dissociation of the (v[sub 1],0,0) pure OH stretching states of hypochlorous acid (HOCl) in the ground electronic state is investigated for v[sub 1] = 6–9. The dynamics calculations are performed on an accurate potential energy surface and employ filter diagonalization in...

  • The unimolecular dissociation of HCO. V. Mixings between resonance states. Brandt-Pollmann, Ulrich; Weiß, Jan; Schinke, Reinhard // Journal of Chemical Physics;11/15/2001, Vol. 115 Issue 19, p8876 

    The unimolecular dissociation of HCO in its ground electronic state is considered. By performing calculations (filter diagonalization, absorbing potential) for nonzero angular momentum quantum numbers, it is demonstrated how mixings with near-by ”background“ states affect the...

  • A semiclassical study of the effects of rotation on the unimolecular dissociation of HN[sub 2] and DN[sub 2]. Guo, Yin; Thompson, Donald L. // Journal of Chemical Physics;2/15/2003, Vol. 118 Issue 7, p3096 

    The influence of rotation on the unimolecular dissociation of HN[sub 2] and DN[sub 2] is investigated. The semiclassical method we have been using for treating tunneling within classical trajectory simulations is employed to compute the level widths of selected states in both low and high energy...

  • Unimolecular dissociation in the regular regime. Yurtsever, Ersin; Günay, Hülya; Uzer, T. // Journal of Chemical Physics;7/15/1993, Vol. 99 Issue 2, p1135 

    The unimolecular dissociation of a model triatomic molecule is analyzed. The model, which is loosely based on the radical HO2, shows mostly regular dynamical behavior although it has enough energy to dissociate. By analyzing the progress towards dissociation, we find that dissociation takes...

  • CH3CH2OD/D2O binary condensation in a supersonic Laval nozzle: Presence of small clusters inferred from a macroscopic energy balance. Tanimura, Shinobu; Wyslouzil, Barbara E.; Wilemski, Gerald // Journal of Chemical Physics;4/14/2010, Vol. 132 Issue 14, p144301 

    We determined the heat released in the condensing flow of a CH3CH2OD/D2O/carrier gas mixture (EtOD/D2O for brevity) through a supersonic Laval nozzle by integrating the equations for supersonic flow with condensation, using the static pressure, temperature, and mole fractions of EtOD and D2O...

Share

Read the Article

Courtesy of VIRGINIA BEACH PUBLIC LIBRARY AND SYSTEM

Sorry, but this item is not currently available from your library.

Try another library?
Sign out of this library

Other Topics