Thermochemical and theoretical study of tert-butyl and di-tert-butylphenolisomers

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 phas e were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry and the standa rd molar enthalpies of evaporation at 298.15 K, measured by Calvet microcal orimetry: 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 dis ubstituted phenols as well as those of the corresponding radicals were obta ined, respectively, by ab initio restricted Hartree-Fock (RHF) and restrict ed Hartree-Fock open shell (ROHF) methods with the 6-31G* basis set. The re sulting 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 forma tion enthalpies of all substituted phenols.