Enthalpy of Formation and O–H Bond Dissociation Enthalpy of Phenol: Inconsistency between Theory and Experiment

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• 作者：Olga V. Dorofeeva ; Oxana N. Ryzhova
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：April 21, 2016
• 年：2016
• 卷：120
• 期：15
• 页码：2471-2479
• 全文大小：444K
• 年卷期：0
• ISSN：1520-5215

文摘

Gas-phase O–H homolytic bond dissociation enthalpy in phenol, DH₂₉₈^°(C₆H₅O–H), is still disputed, despite a large number of experimental and computational studies. In estimating this value, the experimental enthalpy of formation of phenol, Δ_fH₂₉₈^°(C₆H₅OH, g) = −96.4 ± 0.6 kJ/mol (Cox, J. D. Pure Appl. Chem. 1961, 2, 125−128), is often used assuming high accuracy of the experimental value. In the present work a substantially less negative value of Δ_fH₂₉₈^°(C₆H₅OH, g) = −91.8 ± 2.5 kJ/mol was calculated combining G4 theory with an isodesmic reaction approach. A benchmark quality of this result was achieved by using a large number of reliable reference species in isodesmic reaction calculations. Among these are the most accurate Δ_fH₂₉₈^° values currently available from the Active Thermochemical Tables (ATcT) for 36 species (neutral molecules, radicals, and ions), anisole with recently reassessed enthalpy of formation, and 13 substituted phenols. The internal consistency of the calculated Δ_fH₂₉₈^°(C₆H₅OH, g) value with the experimental enthalpies of formation of more than 50 reference species suggests that the reported experimental enthalpy of formation of phenol is in error. Taking into account that the enthalpy of formation of phenol has not been investigated experimentally since 1961, the new measurements would be extremely valuable. Using the accurate enthalpies of formation of C₆H₅OH and C₆H₅O^• calculated in the present work, we obtained DH₂₉₈^°(C₆H₅O–H) = 369.6 ± 4.0 kJ/mol. This value is in satisfactory agreement with that determined from the most precise experimental measurement.