Calorimetric and theoretical determination of standard enthalpies of formation of dimethoxy- and trimethoxybenzene isomers

The standard (p degrees = 0.1MPa) molar enthalpies of formation for 1,2-, 1,3-, and 1,4-dimethoxybenzene and 1,2,3- and 1,3,5-trimethoxybenzene in the gaseous phase were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry and the standard molar enthalpies of evaporation at T = 298.15 K, measured by Calvet microcalorimetry: 1,2-dimethoxybenzene, -(202.4 +/- 3.4) kJ.mol(-1); 1,3-dimethoxybenzene -(221.8 +/- 2.4) kJ.mol(-1); I,4-dimethoxybenzene, -(211.5 +/- 3.0) kJ.mol(-1); I,2,3-trimethoxybenzene, -(339.0 +/- 2.2) kJ.mol(-1); 1,3,5-trimethoxybenzene: -(371.4 +/- 3.0) kJ.mol(-1). Ab initio geometry optimizations for the three dimethoxybenzene and the three trimethoxybenzene isomers were performed using the 3-21G* and the 6-31G* basis sets. Single-point MP2/6-31G* and DFT energy calculations allowed the estimation of the enthalpies of formation of all methoxy-substituted benzenes.