Enthalpy of formation of the cyclohexadienyl radical and the c - h bond enthalpy of 1,4-cyclohexadiene: an experimental and computational re-evaluation


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 6H 7), (b) the homolytic C - H bond enthalpy of 1,4-cyclohexadiene (C 6H 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 6H 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 6H 7 enthalpy of formation. © 2009 American Chemical Society.

Publication Title

Journal of Physical Chemistry A