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

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, DeltaDH(sln)(o)(ArO-H) = DHslno(ArO-H) - DHslno(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, DHo(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 DeltaDH(sln)(o)(ArO-H) approximate to DeltaDH(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.