Enthalpy of Formation of the Cyclohexadienyl Radical and the C−H Bond Enthalpy of 1,4-Cyclohexadiene: An Experimental and Computational Re-Evaluation

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2009-06, Vol.113 (25), p.6955-6963
• Main Authors: Gao, Yide, DeYonker, Nathan J, Garrett, E. Chauncey, Wilson, Angela K, Cundari, Thomas R, Marshall, Paul
• Format: Article
• Language: English
• Subjects: A: Kinetics, Spectroscopy; Carbon - chemistry; Computer Simulation; Cyclohexenes - chemistry; Free Radicals - chemistry; Hydrogen - chemistry; Models, Chemical; Thermodynamics
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp901314y

A quantitative understanding of the thermochemistry of cyclohexadienyl radical and 1,4-cyclohexadiene is beneficial for diverse areas of chemistry. Given the interest in these two species, it is surprising that more detailed thermodynamic data concerning the homolytic C−H bond enthalpies of such entities have not been forthcoming. We thus undertook an experimental and computational evaluation of (a) the enthalpy of formation of cyclohexadienyl radical (C6H7), (b) the homolytic C−H bond enthalpy of 1,4-cyclohexadiene (C6H8), and (c) the enthalpy of the addition of a hydrogen atom to benzene. Using laser photolysis experiments coupled with highly accurate ab initio quantum mechanical techniques, a newly recommended enthalpy of formation for C6H7 is determined to be 208.0 ± 3.9 kJ mol−1, leading to a homolytic bond dissociation enthalpy of 321.7 ± 2.9 kJ mol−1, almost 9 kJ mol−1 higher than previously determined enthalpies that used less certain experimental values for the C6H7 enthalpy of formation.