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Molecular crystals are known for the delicate interplay of non-covalent interactions governing the molecular packing and cohesion. This work exploits first-principles methods to predict, quantify and interpret structural and thermodynamic phenomena related to crystalline biogenic carboxylic acids. Going beyond the established methods for treatment of molecular crystals focusing on sublimation or polymorphism, this works focuses also on phenomena such as local disorder and related configurational entropy, and local anisotropy.
Crystalline carboxylic acids are known to exhibit a considerable anisotropy of their thermal expansion, which is interpreted in this work using fragment-based calculations of the cohesion of these crystals. SAPT calculations are then used to decompose the pair interactions, and to identify the directions of the strongest cohesion, imposed by the site-specific and spatially-oriented hydrogen bonding, restricting the thermal expansion in particular directions.
In addition, two polymorphs are currently known for each of succinic acid, fumaric acid and malic acid. Various ambiguities have prevailed in the literature on their experimental ranking of the relative thermodynamic stability. Quasi-harmonic density functional theory calculations, refined with the ab initio fragment-based cohesive energies and a first-principles model of the configurational entropy, related to the static disorder of carboxyl hydrogen atoms is used to rank these polymorphs in silico. The configurational entropy is demonstrated to shift the polymorph ranking in the correct direction, mostly yielding qualitatively correct ranking. Still, the tiny sub-kilojoule per mole differences of the chemical potentials of the polymorph pairs render reaching a semi-quantitative agreement of theory and experiment in this field extremely challenging.
