Crystallographic Structure and Density Functional Theory (DFT) Study of a Novel Crystalline Molecule, Diisopropylammonium Hydrogen Maleate

Abstract

Maleates are present in large amounts in foods and medicines. These are important pharmacophores in modern medicines because they can improve the physical and chemical properties of drugs, including their water solubility. Enhancing the physicochemical properties of new maleate-based compounds and enriching the literature largely depend on the synthesis method of multicomponent solid forms. In this research, we analyzed the synthesis, theoretical vibrational spectra, and geometric parameters of the diisopropylammonium hydrogen maleate molecule (iPr₂NH₂·OC-C₂H₂-CO₂H) using both experimental and theoretical methods. Parallel zigzag chains are formed through O-H···O and N-H···O hydrogen bonds between cations and the interconnected acidic anions HO₂C-C₂H₂-CO₂, as demonstrated by the crystallographic analysis of this compound. An experimental study was conducted to investigate the characteristics of a new crystal, assessing the applicability of various Density Functional Theory (DFT) methods and adjustments to describe its structural and spectroscopic properties. Using the DFT/M06-2X and DFT/B3LYP methods with the basis sets 6-31+G(d,p), 6-311G(d,p), 6-311++G(d,p), and 6-311++G(d), theoretical vibrational frequencies and geometric parameters (bond lengths and bond angles) were calculated for the first time. The calculated frequency values were corrected using an appropriate scaling factor to obtain Infrared (IR) spectra consistent with IR data from the literature. A good agreement was observed when the optimized geometric parameters were compared with the corresponding experimental data. The dipole moment, as well as the energies of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), were also determined.