PHYS 534 |
| Both the cyclohexadienyl radical and 1,4-cyclohexadiene stand as important paradigms in several, diverse application areas of chemistry. For example, the addition of H· to benzene to produce the cyclohexadienyl radical is a model of the reactions that have been proposed to lead to polycyclic aromatic hydrocarbons, which are important constituents of soot formation mechanisms. 1,4-Cyclohexadiene, with its weak bis-allylic C-H bond, has been a popular substrate in the experimental study and design of novel catalysts. Given the foregoing interest in 1,4-cyclohexadiene and cyclohexadienyl radical, it is surprising that more detailed thermodynamic data concerning the crucial homolytic C-H bond enthalpies has not been forthcoming. A quick perusal of the literature makes it apparent that the homoloytic C-H bond dissociation enthalpy, despite its relevance to several applications areas, is known neither accurately nor precisely. We thus undertook a computational evaluation of (a) the enthalpy of formation of cyclohexadienyl radical (C6H7), (b) the homolytic C-H bond enthalpy of 1,4-cyclohexadiene, and (c) the enthalpy of the addition of a hydrogen atom to benzene. An enthalpy of formation for the cyclohexadienyl radical is obtained through use of the n-homodesmotic reaction model, allowing us to predict with high accuracy both (b) and (c) above. Equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies and enthalpies were computed using density functional theory, second-order perturbation theory, and two composite methods, G3B3 and the correlation consistent composite approach (ccCA). The DFT functional utilized is Becke's three-parameter hybrid functional in conjunction with the LYP correlation functional. Basis sets of at least triple-zeta quality, at a minimum, are employed on valence electrons. We gratefully acknowledge support from the National Science Foundation – Research Experience for Undergraduates (NSF-REU) site at the Department of Chemistry, University of North Texas (grant, CHE-0648843). CASCaM is supported by the U.S. Department of Education. |
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PHYS Poster Session - General Theory
7:30 PM-10:00 PM, Wednesday, April 9, 2008 Morial Convention Center -- Hall A, Poster
Division of Physical Chemistry |