In the present work the 3D-supramolecular network is stabilized by N–H…O and O–H…O hydrogen bonds, by O…N interactions involving the organic cation and inorganic anion as checked by Hirshfeld surface analysis. The van der Waals contacts play a key role in the consolidation of the packing of 4-chloro-2-methylanilinium dihydrogenphosphate[4-CMDHP] structure. In order to support experimental results, density functional theory calculations have been performed using B3LYP functional with 3-21 G basis set. All of the obtained theoretical results are in a perfect agreement with the experimental ones. Furthermore, nonlinear optical behavior of 4-CMDHP has been investigated by determining the Hirshfeld surface, density of states and HOMO-LUMO energy gap using the same basis set. Finally, the molecular electrostatic potential surface of 4-CMDHP molecule was simulated and discussed.

This present work undertakes the study of organic-inorganic hybrid material, which has been obtained successfully by an acid-base reaction at room tem-perature and structurally studied by the single crystal X-ray diffraction method. N-(Dicyclopropylmethylamino)-4,5-dihydro-1,3-oxazolium dihydrogenphosphate [10-CN@DP] crystallizes in the triclinic system with the space group P-1. The X-ray structural analysis supported by a Hirshfeld surface analysis of the crystal structure indicates that the most significant contributions to the crystal packing are from H…H (63.3%), H…O/O…H (32.2%) and H…C/C…H (2.5%) contacts. Density functional theory geometry-optimized calculations were compared to the experimentally determined structure. Using the same level of theory to imagine the chemical reactivity and charge distribution on the molecule, used to determine the HOMO-LUMO energy gap and density of state (DOS) range, the molecular electrostatic potential (MEP) image was drawn. Keywords: HOMO–LUMO, density of state, Hirshfeld surface analysis, electrostatic potential surface.

ABSTRACT. In this present study, Density fuctional theoretical computations are performed at (B3LYP, PBEPBE), HF and MP2 level with 3-21G basis sets to derive optimized geometry and bonding features. The calculated geometrical parameters have been compared with experimental data. The intermolecular O–H…O and N–H…O hydrogen bonds are involved in crystal structure stabilization. An intramolecular O–H…N hydrogen bond is observed between the phosphate OH group and the N atom. Further, the molecular Hirshfeld surface analysis was carried out to reveal the nature of intermolecular contacts and the fingerprint plot provides the information about the percentage contribution and reveals that the O…H (44.1%) interactions have the major contribution to form the crystal. The Mulliken population analysis and the HOMO-LUMO energy gaps show that the molecule has a good stability and from the calculation, we conclude that the molecule is really hard materials.                     KEY WORDS: Energy gaps, Hydrogen bonds, Hirshfeld surface, Mulliken population, Phosphate   Bull. Chem. Soc. Ethiop. 2021, 35(3), 625-638. DOI: https://dx.doi.org/10.4314/bcse.v35i3.13