Eutectic and Non‐Eutectic Behavior of Binary Mixtures of Tetrabutylammonium Bromide With Polybasic Carboxylic Acids: Experimental and DFT Study

ABSTRACT The structural and electronic properties of 1:1 complexes of tetrabutylammonium bromide (TBAB) with oxalic, adipic, succinic, and citric acids were investigated using density functional theory (DFT) at the B3LYP‐D3/6‐311++G(d,p) level. The molecular electrostatic potential surface analysis was employed to identify electrophilic and nucleophilic regions, revealing favorable sites for intermolecular interactions. Non‐covalent interaction analysis was used to visualize and characterize weak interactions within the complexes. The results indicate that O─H···Br − hydrogen bonds are the primary stabilizing forces in the formation of these molecular complexes. Geometrical optimization shows short, nearly linear hydrogen bonds with O─H···Br distances ranging from 2.089 to 2.170 Å. The TBAB–oxalic acid complex exhibits the strongest interaction energy (−18.44 kcal/mol), consistent with the shortest hydrogen bond. Experimental studies, including melting point determination, thermal analysis, FTIR spectroscopy, and XRD, support the formation of new phases with altered thermal behavior. While mixtures with adipic and succinic acids show eutectic behavior with reduced melting points, those with oxalic and citric acids exhibit increased melting points, suggesting the formation of more stable crystalline complexes. The combined experimental and theoretical approach confirms the critical role of hydrogen bonding in the stabilization of TBAB‐based binary systems and provides insight into their potential as DES.

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