title: Enthalpy of formation of the cyclohexadienyl radical and the C-H bond enthalpy of 1,4-cyclohexadiene: an experimental and computational re-evaluation.
abstract: 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 (C(6)H(7)), (b) the homolytic C-H bond enthalpy of 1,4-cyclohexadiene (C(6)H(8)), 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 C(6)H(7) 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 C(6)H(7) enthalpy of formation.
authors.affiliation: The Department of Chemistry, Center for Advanced Scientific Computing and Modeling, University of North Texas, 1155 Union Circle No. 305070, Denton, Texas 76203-5017, USA.
authors.forename: Yide
authors.initials: Y
authors.lastname: Gao
authors.forename: Nathan J
authors.initials: NJ
authors.lastname: DeYonker
authors.forename: E Chauncey
authors.initials: EC
authors.lastname: Garrett
authors.forename: Angela K
authors.initials: AK
authors.lastname: Wilson
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Cundari
authors.forename: Paul
authors.initials: P
authors.lastname: Marshall
mesh_terms: D002244:Carbon / Q000737:chemistry*; D003198:Computer Simulation; D053138:Cyclohexenes / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D008956:Models, Chemical; D013816:Thermodynamics
chemical_list: D053138:Cyclohexenes; D005609:Free Radicals; C048401:1,4-cyclohexadiene; D002244:Carbon; D006859:Hydrogen