Pairwise Substitution Effects, Inter- and Intramolecular Hydrogen Bonds in Methoxyphenols and Dimethoxybenzenes. Thermochemistry, Calorimetry, and First-Principles Calculations

Abstract

Methoxyphenols are the structural fragments of different antioxidants and biologically active molecules, which are able to form strong intermolecular and intramolecular hydrogen bonds in condensed matter. In the present work, thermochemical, Fourier transform infrared (FTIR)-spectroscopic and quantum-chemical studies of methoxyphenols and its H-bonded complexes in solution and gas phase have been carried out. Thermodynamic properties (standard molar enthalpies of formation, vapor pressure, vaporization enthalpies, sublimation enthalpies, and fusion enthalpies) of 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, and 1,4-dimethoxybenzene have been studied in this work. To verify the experimental data, ab initio calculations of all compounds have been performed using density functional theory (DFT), MP2, and G3 methods. The quantitative analysis of ortho, meta, and para pairwise-substituent effects in methoxyphenols has been performed. Solution enthalpies of methoxyphenols at infinite dilution in proton acceptor solvents have been measured. Calorimetric data shows that intermolecular hydrogen bond strength in complexes of 2-methoxyphenol with organic bases is less than that for 4-methoxyphenol. Two experimental approaches for determination of enthalpy of intramolecular hydrogen bonds in ortho-methoxyphenols were proposed. The new results help to resolve uncertainties in the available thermochemical data on methoxyphenols and dimethoxybenzenes and to realize relations among properties and structures for these compounds.