O–H Bond dissociation enthalpies in hydroxyphenols. A time-resolved photoacoustic calorimetry and quantum chemistry study

Abstract

Time-resolved photoacoustic calorimetry (TR-PAC) was used to investigate the energetics of O–H bonds of phenol, catechol, pyrogallol, and phloroglucinol. Values of −27.1 ± 3.9, −44.1 ± 4.4 and −1.6 ± 3.8 kJ mol⁻¹, respectively, were obtained for the solution-phase (acetonitrile) O–H bond dissociation enthalpies of the last three compounds relative to the O–H bond dissociation enthalpy in phenol, ΔDH^o_sln(ArO–H) = DH^o_sln(ArO–H) − DH^o_sln(PhO–H). A value of 388.7 ± 3.7 kJ mol⁻¹ was determined for the PhO–H bond dissociation enthalpy in acetonitrile. Density functional theory (MPW1PW91/aug-cc-pVDZ) calculations and complete basis set (CBS-4M) calculations were carried out to analyse intramolecular hydrogen bonding and to predict gas-phase O–H bond dissociation enthalpies, DH^o(ArO–H). A microsolvation model, based on the DFT calculations, was used to study the differential solvation of the phenols and their radicals in acetonitrile and to bridge solution- and gas-phase data. The results strongly suggest that ΔDH^o_sln(ArO–H) ≈ ΔDH^o(ArO–H). Hence, to calculate absolute gas-phase O–H bond dissociation enthalpies in substituted phenols from the corresponding solution-phase values, the solvation enthalpies of the substituted phenols and their radicals are not required.