Relationship between crystal structure and thermal properties of polymorphic system methylethanolammonium 2-chloro-4-nitrobenzoate

The crystal structures and thermal characteristics of two polymorphs of methylethanolammonium 2-chloro-4-nitrobenzoate were investigated by a combined experimental and computational approach. The main attention was paid to the synthesis of polymorphs, structural characterization associated with computational studies on the nature of intermolecular interactions, and thermodynamic stability relationship between the polymorphs and thermal decomposition kinetics. Analysis of the polymorphs structure revealed distinct crystal packing arrangements and different numbers of independent molecules pairs in the asymmetric crystal unit. Depending on the conformation of cations, polymorphs α and β exhibit the formation of different charge-assisted and normal hydrogen bonds, while in β, non-covalent interactions of halogens are also manifested. The calculated values of the lattice energies indicated that α has a more stable lattice than β at high temperatures. The thermodynamic stability relationship between the polymorphs pair is monotropic and is determined by measuring melting points and heats of fusion. The kinetics of thermal decomposition processes has been comparatively studied by two different kinetic methods. The results showed higher activation energies for α compared to β . According to the modified nonparametric kinetic method, the decomposition mechanism revealed two distinct stages in both polymorphs, indicating one predominant chemical degradation process followed by a physical transformation process.