Thermochemical and theoretical study of tert-butyl and di-tert-butylphenol isomers

The standard (p(o) = 0.1MPa) molar enthalpies of formation for 2-, 3-, and 4-tert-butylphenol and 2,4- and 2,6-di-tert-butylphenol in the gaseous phase were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry and the standard molar enthalpies of evaporation at 298.15 K, measured by Calvet microcalorimetry: 2-tert-butylphenol, -184.7 +/- 2.6 kJ mol(-1); 3-tert-butylphenol, -198.0 +/- 2.1 kJ mol(-1); 4-tert-butylphenol, -187.3 +/- 3.3 kJ mol(-1); 2,4-di-tert-butylphenol -283.3 +/- 3.8 kJ mol(-1), 2,6-di-tert-butylphenol -272.0 +/- 4.0 kJ mol(-1). The most stable geometries of all mono- and disubstituted phenols as well as those of the corresponding radicals were obtained, respectively, by ab initio restricted Hartree-Fock (RHF) and restricted Hartree-Fock open shell (ROHF) methods with the 6-31G* basis set. The resulting geometries were then used to obtain estimates of the effect of the tert-butyl substituent on the O-H bond dissociation energy and on the formation enthalpies of all substituted phenols.