Thiabendazole: Fumaric acid as a case study on the salt cocrystal continuum: A local vibrational mode perspective

Cocrystals are of tremendous significance for the pharmaceutical industry, due to their favorable properties when compared to their salt analogs. The Δ pK a rule, commonly used in the identification of a cocrystal, meets shortcomings in the 0 ≤ Δ pK a ≤ 3 range, where salts and cocrystals ought to emerge. In this project, we probed the salt vs. cocrystal nature, using the strength of hydrogen bondings through the local mode analysis, in the case example of the product formed between thiobendazole and fumaric acid. Moreover, we employed a topological analysis of the electronic density to quantify the covalency of such interactions. • Quantification of H bonds in binary crystals via local vibrational mode theory. • Application of QTAIM in the assessment of H bonds covalency in molecular crystals. • Distinction between salt and cocrystal through local vibrational mode theory.