Ammonium carboxylate salts: the additivity of intermolecular interaction energies in charged organic compounds

Abstract

The supramolecular organization of organic salts has been widely researched, revealing recurring patterns in crystalline lattices that describe their supramolecular properties. In recent years, our research group has underscored the importance of considering the crystalline structures as a whole, incorporating all the necessary energetic and topological information for a comprehensive understanding of the crystalline system. Given this context, we investigated a series of ammonium mono- and dicarboxylate salts (1–12) to determine whether subtle structural modifications in the anionic organic component lead to relevant energy and topology changes in the crystalline lattice. To achieve this, we selected structures whose carboxylate anion only possesses an alkyl chain and is devoid of other functional groups. The ammonium cation (NH₄⁺) was fixed to determine the effect of variations in the alkyl chains of the selected mono- and dicarboxylates, such as length and degree of unsaturation. Additionally, probable crystallization mechanisms were proposed to elucidate some of the topological and energetic aspects involved in the crystallization of these compounds. Destabilizing interactions were observed in 10 crystalline structures, and the MEP data showed that the most destabilizing interactions occur by the proximity of portions with the same type of charge. Some dimers have unexpectedly low intermolecular interaction energies despite having large contact areas. Based on these data we demonstrate the additivity of intermolecular interactions, that is, the low intermolecular interaction energy in these dimers is the result of the sum of destabilizing energies and stabilizing energies. The cluster energy efficiency data revealed that most crystal lattices display typical characteristics of uncharged organic compounds. The proposed crystallization mechanisms showed a gradual increase in nucleus complexity in the initial stages and the total number of nucleation stages, resulting in five main patterns: monomer → 1D → 3D (1–2), monomer → dimer → 2D → 3D (3), monomer → 1D → 2D → 3D (4–5), monomer → dimer → 3D (6), and monomer → 2D → 3D (7–12).