Thermochemical study of dichloromethylpyrimidine isomers

Abstract The standard ( p o  = 0.1 MPa) molar enthalpies of formation in the condensed phase, Δ f H m o , of 2,4-dichloro-5-methylpyrimidine, 2,4-dichloro-6-methylpyrimidine, 4,6-dichloro-2-methylpyrimidine and 4,6-dichloro-5-methylpyrimidine were derived from the standard molar energies of combustion, Δ c U m o , in oxygen, to yield CO 2 (g), N 2 (g) and HCl·600H 2 O (l), at T  = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation, Δ cr, l g H m o , for these compounds, at T  = 298.15 K were determined by high temperature Calvet microcalorimetry. Combining these values, the following enthalpies of formation in the gas phase, at T  = 298.15 K, were then derived: 2,4-dichloro-5-methylpyrimidine, (79.6 ± 4.1) kJ mol −1 , 2,4-dichloro-6-methylpyrimidine, (70.5 ± 3.0) kJ mol −1 , 4,6-dichloro-2-methylpyrimidine, (68.7 ± 3.3) kJ mol −1 , and 4,6-dichloro-5-methylpyrimidine, (78.1 ± 2.5) kJ mol −1 . The gas-phase enthalpies of formation were also estimated by G3 theoretical calculations, which were extended to the computation of gas-phase enthalpies of formation of the other dichloromethylpyrimidine isomers, namely, 2,5-dichloro-4-methylpyrimidine, 4,5-dichloro-2-methylpyrimidine and 4,5-dichloro-6-methylpyrimidine, whose experimental study was not performed.