For a series of 61 2-substituted phenols, 2-X−PhOH, forming a total of 73 intramolecular hydrogen bonds, the intramolecular hydrogen bond enthalpy, ΔH intra - HB, has been determined by density functional theory (DFT) calculations on the B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) level. The ΔH intra - HB was defined as the enthalpy difference between the hydrogen-bonded (HB) form and the lowest-energy conformer in which the OH is rotated into the “away” position. The correlation of ΔH intra - HB with geometrical factors such as r(O−H), or r(OH···A), with A as the hydrogen bond accepting atom, was generally very poor, showing that none of these parameters can be used as an universal descriptor for the hydrogen bond strength. The relation between ΔH intra - HB and ν(O−H) intra - HB is also insignificant, in contrast with previous estimates. The data clearly demonstrate that the genuine ΔH intra - HB of a phenolic compound cannot be unequivocally derived by simple rotation of the OH group into the “away” orientation, because additional steric and/or electronic 1,2 interactions may take place which are difficult or even impossible to be separated from the sole H-donor/acceptor interaction. Nevertheless, a good correlation has been found between computed and experimental liquid phase ν(O−H)s obeying ν(O−H)DFT = 1.0097ν(O−H)exp + 159.5. It could be established with the use of solute acidity, , and solvent basicity, , parameters, that the strength of an intramolecular hydrogen bond is noticeably fortified for electron withdrawing groups. Furthermore, it was found that with the proper non-HB geometries the bond dissociation enthalpy, BDE(O−H), in the series of 25 2-X-PhOHs correlates quite well with σ+, in the same way as has been found for 4-X-substituted phenols.

The effect of methoxy substitution on the phenolic-hydrogen bond dissociation enthalpy has been established by a photoacoustic calorimetric method and by means of density functional theory (DFT) calculations. Experimentally, the relative BDE(O−H) in kcal mol-1 with respect to phenol are found to be as follows:  2-methoxyphenol (−4.0), 4-methoxyphenol (−4.9), 2,6-dimethoxyphenol (−10.6), 2,4-dimethoxyphenol (−9.0), 2,4,6-trimethoxyphenol (−13.6), and ubiquinol-0 (−12.0). The intramolecular hydrogen-bond enthalpy in o-methoxy-substituted phenol is −4.3 kcal mol-1; the intramolecularly hydrogen-bonded molecule forms an additional hydrogen bond with HBA solvents. The low BDE(O−H) for ubiquinol-0 of 78.5 kcal mol-1 and the protective intramolecular hydrogen bond make it a good antioxidant.

The effect of methoxy substitution on the abstraction of the phenolic hydrogen atom involved in intramolecular hydrogen bonding by tert-butoxyl and cumyloxyl radicals has been investigated by laser flash photolysis. Also transition state calculations for methoxyl radical and 2-methoxyphenol have been carried out by a density functional theory (DFT) method. Hydrogen atom abstraction is surprisingly easy from intramolecularly hydrogen bonded methoxyphenols, in contrast to intermolecularly hydrogen bonded molecules. The kinetic solvent effect, investigated in six solvents with different hydrogen bond accepting properties, on the hydrogen atom abstraction reaction from o-methoxy phenols was shown to be smaller than for non-hydrogen bonded phenols, and is independent of further methoxy substitution. The high rate constant for hydrogen atom abstraction from ubiquinol-0 (2.8 × 10 9 M -1 s -1 in CCl 4 ) and the small kinetic solvent effect make it a good antioxidant, even in a polar environment.

In the presence of hydrogen donor solvents and at elevated temperatures, aromatic ketones can be selectively deoxygenated to the corresponding hydroaromatic compounds. The kinetics for reduction of 7H-benz[d,e]anthracen-7-one (benzanthrone, 6) into 7H-benz[d,e]anthracene (benzanthrene, 1) in 9,10-dihydroanthracene (3) solvent has been investigated in detail. The relatively slow hydrogenation of 6 is due to reversibility of the initial hydrogen-transfer step according to a reverse radical disproportionation (RRD). The dynamics could well be rationalized using the energetics of species computed by density functional theory (DFT). The application of hydrogen donors such as 1 as a hydrogen-transfer agent, although favorable in terms of a low benzylic carbon-hydrogen bond dissociation enthalpy, is limited due to the slow self-hydrogenation, which in case of 1 gives 5,6-dihydro-4H-benz[d,e]anthracene (7).