| Abstract
| - The standard (p° = 0.1 MPa) molar enthalpies of formation for 2-, 3-, and 4-methoxyphenol and 2,3-,2,6-, and 3,5-dimethoxyphenol in the gaseous phase were derived from the standard molar enthalpies ofcombustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry, and the standardmolar enthalpies of evaporation at 298.15 K, measured by Calvet microcalorimetry: 2-methoxyphenol,−(246.1 ± 1.9) kJ mol-1; 3-methoxyphenol, −(240.4 ± 2.1) kJ mol-1; 4-methoxyphenol, −(229.7 ± 1.8) kJmol-1; 2,3-dimethoxyphenol, −(386.0 ± 2.2) kJ mol-1; 2,6-dimethoxyphenol, −(381.7 ± 1.9) kJ mol-1;3,5-dimethoxyphenol, −(399.4 ± 3.0) kJ mol-1. Density functional theory calculations for all the methoxy-and dimethoxyphenols and respective phenoxyl radicals and phenoxide anions were performed usingextended basis sets, which allowed the estimation of the gas-phase enthalpies of formation for allcompounds. The good agreement of the calculated and experimental gas-phase enthalpies of formationfor the closed-shell systems gives confidence to the estimates concerning the isomers which were notexperimentally studied and to the estimates concerning the radicals and the anions. Substituent effectson the homolytic and heterolytic O−H bond dissociation energies have been analyzed, the results beingin good agreement with available experimental data. Detailed analysis of these effects suggests thatelectronic exchange phenomena between the substituents dominate the effect the substituents have onthese systems.
|
