A quantum Monte Carlo study of energy differences in C4H3 and C4H5 isomers*

Authors

  • Xénophon Krokidis,

    1. Accelrys, Inc., Parc Club Orsay Université, 20 rue Jean Rostand, 91898 Orsay Cedex, France
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  • Nigel W. Moriarty,

    1. Division of Physical Biosciences, Lawrence Berkeley National Laboratory, Berkeley, California 94720
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  • William A. Lester Jr.,

    1. Department of Chemistry, University of California at Berkeley, Berkeley, California 94720-1640
    2. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
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  • Michael Frenklach

    Corresponding author
    1. Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720–1740
    2. Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
    • Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720–1740
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  • *

    This article is a US Government work and, as such, is in the public domain in the United States of America.

Abstract

Quantum Monte Carlo and a series of other ab initio as well as density functional theory calculations were performed for the enthalpy of formation of C4H3 and C4H5 radicals. The computed ΔfH2980 values, in kcal/mol, are 126.0 for n-C4H3, 119.4 for i-C4H3, 83.4 for n-C4H5, and 76.2 for i-C4H5, all with one standard deviation of 0.6 kcal/mol. The enthalpy differences between the n and i isomers of C4H3 and C4H5 are predicted to be substantially lower than those obtained in recent theoretical studies. The nature of the middle C[BOND]C bond in these radicals was examined using the electron localization function topological analysis performed by bonding evolution theory for partitioning the molecular space into regions with clear chemical meaning. This analysis shows that the n isomers are represented by a unique Lewis structure while the i isomers are represented by a resonance description. For the latter systems, the middle C[BOND]C bond is only mildly conjugated and the corresponding degree of conjugation is calculated. These results signify higher prominence of the even-carbon-atom reaction pathways in the formation of the first aromatic ring in hydrocarbon pyrolysis and oxidation, consistent with the past kinetic modeling and recent experimental studies. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 808–820, 2001

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