Thermochemical study of dichloromethylpyrimidine isomers

•Enthalpies of formation were derived by rotating-bomb combustion calorimetry.•Enthalpies of phase transition were determined by Calvet microcalorimetry.•Gaussian-3 theory for calculation of enthalpies of formation for all the isomers.•Structure and energy correlations were established.

The standard (po = 0.1 MPa) molar enthalpies of formation in the condensed phase, ΔfHmo, 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, ΔcUmo, in oxygen, to yield CO2 (g), N2 (g) and HCl·600H2O (l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation, Δcr,lgHmo, 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.