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.

Ph(i-PrO)SiH2: An Exceptional Reductant for Metal-Catalyzed Hydrogen Atom Transfers

“…Unless there is a very high inverse KIE from the metal hydride, this scrambling must occur between the intermediate substrate radical and substrate 33 since we see 100% D incorporation at C4 (Scheme 9) and a metal hydride formed by reverse HAT would label C4 with H. Abstraction of the benzylic hydrogen from 33 would not be surprising since its BDE should lie somewhere between diphenylmethane (81.4 kcal/mol) and 1,4-pentadienes (76.4 kcal/mol), similar to the BDE of 1,4-cyclohexadiene (76.9 kcal/mol), used previously as a stoichiometric reductant. 25 …”

Activation of Carbon−Hydrogen and Hydrogen−Hydrogen Bonds by Copper−Nitrenes: A Comparison of Density Functional Theory with Single- and Multireference Correlation Consistent Composite Approaches

“…In general, the largest difference between DFT and ccCA methods is for those processes that involve radical species and homoloytic bond dissociation. Other research in our group indicates that spin contamination can be problematic in open-shell organic 62 and inorganic compounds, 63 making the use of restricted open-shell methodologies a prudent choice. However, no evidence for spin contamination was seen in the present research.…”

Hydrogen‐Atom Tunneling Could Contribute to H2 Formation in Space

“…[28,29] The B3LYP, PBE, and PW91 calculated classical transition-state theory (TST) reaction rates are always higher than the experimental rates, [30,31] while the CCSD(T) and MPWB1K reaction rates are lower at low temperatures (Supporting Information). The CCSD(T)/CBS , [30] while the other functionals have slightly lower barriers.…”