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_f H_m~(deg)(o-CH_3C_6H_4OH,cr) = -204.2 ± 2.7 kJ centre dot mol~(-1), DELTA_f H_m~(deg)(m-CH_3C_6H_4OH,1) = -196.6 ± 2.1 KJ centre dot mol~(-1), and DELTA_f H_m~(deg)(p-CH_3C_6H_4OH,cr) = -202.2 ± 3.0 kJ centre dot mol~(-1). Calvet drop calorimetric measurements led to the following enthalpy of sublimation and vaporization values at 298.15 K: DELTA_(sub) H_m~(deg)(o-CH_3C_6H_4OH) = 73.74 ± 0.46 kJ centre dot mol~(-1), DELTA_(vap) H_m~(deg)(m-CH_3C_6H_4OH) = 64.96 ± 0.69 kJcentre dot mol~(-1), and DELTA_(sub) H_m~(deg)(p-CH_3C_6H_4OH) = 73.13 ± 0.56 kJ centre dot mol~(-1). From the obtained DELTA_f H_m~(deg)(1/cr) and DELTA_(vap) H_m~(deg)/ DELTA_(sub) H_m~(deg) values, it was possible to derive DELTA_f H_m~(deg)(o-CH_3C_6H_4OH,g) = -130.5 ± 2.7 kJ centre dot mol~(-1), DELTA_f H_m~(deg)(m-CH_3C_6H_4OH,g) = -131.6 ± 2.2 kJ centre dot mol~(-1), and DELTA_f H_m~(deg)(p-CH_3C_6H_4OH,g) = -129.1 ± 3.1 kJ centre 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, MPWlPW91/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_f H_m~(deg)-(C_6H_5O~(centre dot),g) - DELTA_f H_m~(deg)(o-CH_3C_6H_4O~(centre dot),g) = 42.2 ± 2.8 kJ centre dot mol~(-1), DELTA_f H_m~(deg)(C_6H_5O~(centre dot),g) - DELTA_f H_m~(deg)(m-CH_3C_6H_4O~(centre dot),g) = 36.1 ± 2.4 kJ centre dot mol~(-1), and DELTA_f H_m~(deg)(C_6H_5O~(centre dot),g) - DELTA_f H_m~(deg)(p-CH_3C_6H_4O~(centre dot),g) = 38.6 ± 3.2 kJ centre dot mol~(-1). The corresponding differences in O-H bond dissociation enthalpies were also derived as DH~(deg)(C_6H_5O-H) - DH~(deg)(o-CH_3C_6H_4O-H) = 8.1 ± 4.0 kJ centre dot mol~(-1), DH~(deg)(C_6H_5O-H) - DH~(deg)(m-CH_3C_6H_4O-H) = 0.9 ± 3.4 kJ centre dot mol~(-1), and DH~(deg)(C_6H_5O-H) - DH~(deg)(p-CH_3C_6H_4O-H) = 5.9 ± 4.5 kJ centre 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 follows the trend m-methylphenoxyl < p-methylphenoxyl < o-methylphenoxyl. It is also found that these tendencies are enthalpically controlled.
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