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This review presents a critical assessment of the available experimental information ͑contained in ϳ90 literature references͒ on the thermochemistry of the O-H bond in phenol and substituted phenols. The analysis led to a set of recommended values for the O-H bond dissociation enthalpies, which in turn allowed us to discuss several empirical and theoretical methodologies used to estimate these data.

The energetics of intramolecular hydrogen bonds (H-bonds) is a subject of fundamental importance in chemistry and biochemistry. In contrast with intermolecular H-bonds, whose enthalpy can be determined by experiment or accurately evaluated through a supermolecular approach, there is no general accepted procedure to determine the enthalpy of an intramolecular H-bond. In this work, different ways for assessing the energetics of intramolecular H-bonds of selected aromatic systems were applied and compared. They include the widely used conformational analysis approach (cis−trans method), a recently proposed isodesmic reaction method, and a new procedure that we designate as the ortho−para method. Energy calculations were carried out at several theory levels, including a modified complete basis set extrapolation method (CBS-QMPW1), in which the geometries are based on MPW1PW91/aug-cc-pVDZ density functional theory optimizations. The obtained results, together with a simple dipole−dipole interaction model, help to explain why the enthalpies of intramolecular H-bonds are often overestimated by the cis−trans method. The results also show that intramolecular H-bond enthalpies based on the isodesmic reaction method may be unreliable. The ortho−para method, which can be applied when accurate theoretical or experimental standard enthalpies of formation are available, is probably the best way of estimating the enthalpies of intramolecular hydrogen bonds. Finally, our results illustrate the important role played by intramolecular H-bonds in the energetics of homolytic dissociation reactions involving di-substituted benzenes.

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The hydration of the hydroxyl OH radical has been investigated by microsolvation modeling and statistical mechanics Monte Carlo simulations. The microsolvation approach was based on density functional theory ͑DFT͒ calculations for OH-(H 2 O) 1-6 and (H 2 O) 1-7 clusters. The results from microsolvation indicate that the binding enthalpies of the OH radical and water molecule to small water clusters are similar. Monte Carlo simulations predict that the hydration enthalpy of the OH radical, ⌬ hyd H(OH,g), is Ϫ39.1 kJ mol Ϫ1. From this value we have estimated that the band gap of liquid water is 6.88 eV, which is in excellent agreement with the result of Coe et al. ͓J. Chem. Phys. 107, 6023 ͑1997͔͒. We have compared the structure of the hydrated OH solution with the structure of pure liquid water. The structural differences between the two systems reflect the strong role played by the OH radical as a proton donor in water. From sequential Monte Carlo/DFT calculations the dipole moment of the OH radical in liquid water is 2.2Ϯ0.1 D, which is ϳ33% above the experimental gas phase value ͑1.66 D͒.