Thermodynamic study of alkane-α,ω-diamines – Evidence of odd–even pattern of sublimation properties

Abstract

Calculated ideal-gas heat capacities and experimental data for vapor pressure, condensed phase heat capacities, and calorimetric vaporization enthalpies of alkane-α,ω-diamines, NH2-(CH2)n-NH2 (2≤n≤8), were treated simultaneously to obtain a consistent thermodynamic description. Vapor pressures for the members with 3≤n≤8 were measured using the static method in the temperature range from 233 to 314K. The thermodynamic properties in the ideal gaseous state for the members with n=2 and 3 were obtained using the fundamental vibrational frequencies, molecular structure data, and potential energy profiles of internal rotations calculated using the density functional theory (DFT) at the B3LYP/6-311+G(d,p) level of theory and four statistical thermodynamic models of different complexity. Based on the results of the simultaneous correlation, the sublimation and vaporization thermodynamic properties for 2≤n≤8 and the triple-point coordinates for n=3, 4, 5 and 7 were calculated. The thermodynamic properties for liquid–vapor equilibrium display a linear dependence on number of methylene groups n while the sublimation thermodynamic properties show an evident odd–even effect with higher values for even members of a homologous series. The alternating pattern is interpreted on the basis of the literature crystallographic data and corresponding intermolecular interactions. Based on the observed trends, the correlations allowing the prediction of the thermodynamic properties for the crystal–vapor and liquid–vapor equilibria and crystalline and ideal-gas heat capacities within the homologous series are suggested.