Critical re-evaluation of the O-H bond dissociation enthalpy in phenol

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Journal titleJournal of Physical Chemistry A
Pages26472655; # of pages: 9
SubjectBond dissociation enthalpy (BDE); Gas-phase kinetics; Isodesimic reactions; Photoacoustic calorimetry (PAC); Benzene; Calorimetry; Chemical bonds; Computational methods; Dissociation; Enthalpy; Mathematical models; Paramagnetic resonance; Pyrolysis; Thermoanalysis; Thermodynamics; Phenols
AbstractThe gas-phase O-H bond dissociation enthalpy, BDE, in phenol provides an essential benchmark for calibrating the O-H BDEs of other phenols, data which aids our understanding of the reactivities of phenols, such as their relevant antioxidant activities. In a recent review, the O-H BDE for phenol was presented as 90 ± 3 kcal mol -1 (Acc. Chem. Res. 2003, 36, 255-263). Due to the large margin of error, such a parameter cannot be used for dynamic interpretations nor can it be used as an anchor point in the development of more advanced computational models. We have reevaluated the existing experimental gas-phase data (thermolyses and ion chemistry). The large errors and variations in thermodynamic parameters associated with the gas-phase ion chemistry methods produce inconsistent results, but the thermolytic data has afforded a value of 87.0 ± 0.5 kcal mol -1. Next, the effect of solvent has been carefully scrutinized in four liquid-phase methods for measuring the O-H BDE in phenol: photoacoustic calorimetry, one-electron potential measurements, an electrochemical cycle, and radical equilibrium electron paramagnetic resonance (REqEPR). The enthalpic effect due to solvation, by, e.g., water, could be rigorously accounted for by means of an empirical model and the difference in hydrogen bond interactions of the solvent with phenol and the phenoxyl radical. For the REqEPR method, a second correction is required since the calibration standard, the O-H BDE in 2,4,6-tri-tert-butylphenol, had to be revised. From the gas-phase thermolysis data and three liquid-phase techniques (excluding the electrochemical cycle method), the present analysis yields a gas-phase BDE of 86.7 ± 0.7 kcal mol -1. The O-H BDE was also estimated by state-of-the-art computational approaches (G3, CBS-APNO, and CBS-QB3) providing a range from 86.4 to 87.7 kcal mol -1. We therefore recommend that in the future, and until further refinement is possible, the gas-phase O-H BDE in phenol should be presented as 86.7 ± 0.7 kcal mol -1.
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AffiliationNational Institute for Nanotechnology; National Research Council Canada
Peer reviewedYes
NPARC number21276566
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Record identifieraa7ce62d-3907-4ccf-a5e3-7c7b415ac871
Record created2015-10-13
Record modified2016-05-09
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