Theoretical calculation of accurate absolute and relative gas- and liquid-phase O-H bond dissociation enthalpies of 2-mono- and 2,6-disubstituted phenols, using DFT/B3LYP

DFT/B3LYP is used to calculate the gas-phase absolute and relative phenolic O-H bond dissociation enthalpies (BDEs) in hydroxy/methoxy ortho substituted phenols. The PCM and SCIPCM continuum models are applied to calculate the liquid-phase BDEs. This is the first theoretical determination of liquid-phase BDEs of phenols, the corresponding experimental data of which is rare. The solvated-phase optimized structures of both the parent phenols and their respective radicals are also presented for the first time. A systematic study on a series of 17 different basis sets on phenol, 2-hydroxyphenol (catechol), and 2-methoxyphenol (guaiacol) leads to the optimum 6-31+G(,3pd) basis set. Derived BDEs are among the most accurate of any gas-phase ones (deviations of the absolute gas-phase BDEs do not exceed 0.20 kcal/mol, relative to experiment, and those of the relative ones do not exceed 0.24 kcal/mol). Use of the optimum basis set to obtain the absolute gas-phase BDEs of 2,6-dimethoxyphenol (syringol) and 2,6-dihydroxyphenol (pyrogallol), the liquid-phase BDEs, the solvent, and substituent effects of phenols shows the usefulness of this approach. Seven solvents, differing in their H-bonding ability and polarity, n-heptane, benzene, acetone, acetonitrile, ethanol, methanol, and water, are used to model different environmental situations. Only the PCM model describes well the "bulk" solvent effects, which, depending on the E-N(T) and/or alpha polarity parameter values of the solvent, modify the structure of the solute. Calculated liquid-phase BDEs are in close agreement with the experimental ones, where available, exceeding those in the gas-phase by as much as ca. 8 kcal/mol in some media. Solvent effects are common for catechol and phenol and different for guaiacol. Close agreement is derived between the theoretical and the experimental solvent effects for known phenolic antioxidants, namely, ubiquinols and flavonoids. The different ortho groups in catechol and guaiacol lead to different substituent effects in accordance with experimental findings.