Energetics of cresols and of methylphenoxyl radicals

Combustion calorimetry studies were used to determine the standard molar enthalpies of formation of o-, m-, and p-cresols, at 298.15 K, in the condensed state as Delta H-f(m)degrees(o-CH3C6H4OH,cr) = -204.2 +/- 2.7 kJ center dot mol(-1), Delta H-f(m)degrees(m-CH3C6H4OH,l) = -196.6 +/- 2.1 kJ center dot mol(-1), and Delta fH(m)degrees(p-CH3C6H4OH,cr) = -202.2 +/- 3.0 kJ center dot mol(-1). Calvet drop calorimetric measurements led to the following enthalpy of sublimation and vaporization values at 298.15 K: Delta H-sub(m)degrees(o-CH3C6H4OH) = 73.74 +/- 0.46 kJ center dot mol(-1), Delta H-vap(m)degrees(m-CH3C6H4OH) = 64.96 +/- 0.69 kJ center dot mol(-1), and Delta H-sub(m) degrees(p-CH3C6H4OH) = 73.13 +/- 0.56 Kj center dot mol(-1). From the obtained Delta H-f(m)degrees(1/cr) and Delta H-vap(m)degrees/ Delta H-sub(m)degrees values, it was possible to derive Delta H-f(m)degrees(o-CH3C6H4OH,g) = -130.5 +/- 2.7 kJ center dot mol(-1), Delta H-f(m)degrees(m-CH3C6H4OH,g) = -131.6 +/- 2.2 kJ center dot mol(-1), and Delta H-f(m)degrees(p-CH3C6H4OH,g) = -129.1 +/- 3.1 kJ center dot mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by the B3LYP/ cc-pVDZ, B3LYP/cc-pVTZ, B3P86/cc-pVDZ, B3P86/cc-pVTZ, MPWIPW91/cc-pVTZ, CBS-QB3, and CCSD/cc-pVDZ//B3LYP/cc-pVTZ methods, were used to obtain the differences between the enthalpy of formation of the phenoxyl radical and the enthalpies of formation of the three methylphenoxyl radicals: Delta H-f(m)degrees(C6H5O center dot,g) - Delta H-f(m)degrees(o-CH3C6H4O center dot,g) = 42.2 +/- 2.8 kJ center dot mol(-1), Delta H-f(m)degrees(C6H5O center dot,g) - Delta H-f(m)degrees(m-CH3C6H4O center dot,g) - 36.1 +/- 2.4 kJ center dot mol(-1), and Delta H-f(m)degrees(C6H5O center dot,g) - Delta H-f(m) degrees(p-CH3C6H4O center dot,g) = 38.6 +/- 3.2 kJ center dot mol(-1). The corresponding differences in O-H bond dissociation enthalpies were also derived as DH degrees(C6H5O-H) - DH degrees(o-CH3C6H4O-H) = 8.1 +/- 4.0 kJ center dot mol(-1), DH degrees(C6H5O-H) - DH degrees(m-CH3C6H4O-H) = 0.9 +/- 3.4 kJ center dot mol(-1), and DH degrees(C6H5O-H) - DH degrees(p-CH3C6H4O-H) = 5.9 +/- 4.5 kJ center dot mol(-1). Based on the differences in Gibbs energies of formation obtained from the enthalpic data mentioned above and from published or calculated entropy values, it is concluded that the relative stability of the cresols varies according to p-cresol < m-cresol < o-cresol, and that of the radicals follws the trend m-methylphenoxyl < p-methylphenoxyl < omethylphenoxyl. It is also found that these tendencies are enthalpically controlled.