Thermodynamic study of nicotinamide, N-methylnicotinamide and N,N-dimethylnicotinamide: Vapour pressures, phase diagrams, and hydrogen bonds

Abstract

Using the Knudsen-effusion method and/or a diaphragm manometer static method, the vapour pressures of the condensed phases (crystalline and liquid) of nicotinamide, N-methylnicotinamide and N,N-dimethylnicotinamide were measured, respectively, over the following temperatures ranges (341.2 to 433.7)K, (329.2 to 426.6)K and (308.2 to 370.5)K. The experimental results enabled the determination of the standard molar enthalpies, entropies and Gibbs free energies of sublimation and of vaporisation, at T=298.15K, as well as phase diagram representations of the (p,T) experimental results, including the triple point, for the three compounds studied. The temperatures and molar enthalpies of fusion were determined using differential scanning calorimetry and were compared with the values derived indirectly from the vapour pressure measurements. The enthalpy of the intermolecular hydrogen bonds N−H…O present in crystalline N-methylnicotinamide was calculated and the sum of the enthalpies of the intermolecular hydrogen bonds N−H⋯O and N−H⋯N, [ΔHB(N−H⋯O)H+ΔHB(N−H⋯N)H], formed in the crystalline phase of nicotinamide was also estimated.