Experimental and computational thermodynamic study of ortho-, meta-, and para-methylbenzamide

The Knudsen mass-loss effusion technique was used to measure the vapour pressures of the three crystalline isomers of methylbenzamide. From the temperature dependence of the vapour pressures, the standard molar enthalpies of sublimation and the enthalpies of the intermolecular hydrogen bonds N−H⋯O were calculated. The temperature and molar enthalpy of fusion of the studied isomers were measured using differential scanning calorimetry. The values of the standard (p° = 0.1 MPa) molar enthalpy of formation in the crystalline phase, at T = 298.15 K, of the compounds studied were derived from their standard massic energies of combustion measured by static-bomb combustion calorimetry. From the experimental values, the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were calculated and compared with the values estimated by employing computational calculations that were conducted using different quantum chemical methods: G3(MP2), G3, and CBS-QB3. Good agreement between experimental and theoretical results is verified. The aromaticity of the compounds has been evaluated through nucleus independent chemical shifts (NICS) calculations.

► Vapour pressures, ΔcrgHm∘(g), and N–H⋯O hydrogen bond enthalpies were determined for 3 methylbenzamides.
► ΔfHm∘(g) was calculated from the enthalpies of sublimation and from combustion calorimetry experiments.
► ΔfHm∘(g) was also estimated by computational calculations using different quantum chemical methods.
► Experimental and computational results are in good agreement being the ortho isomer the less stable.
► The methyl group does not seem to affect the enthalpy of the intermolecular enthalpy of the N–H⋯O bond.