Synthesis, XRD, spectral, photoluminescence, optical, electrical conductivity, and dielectric studies on melamine – melaminium 2-acetyl benzoate dihydrate

Abstract

A novel nitrogen rich energetic salt Melamine-melaminium 2-acetyl benzoate dihydrate (MMH⁺AB⁻2) has been grown by slow evaporation method at room temperature. Single crystal X-ray diffraction studies reveals that the grown synthesized salt crystallizes in the centrosymmetric space group `c 2yc’ of the monoclinic system. The lattice parameters were established as a = 12.286(3) (Å), b = 18.934(4) (Å), c = 18.515(4) (Å), α = γ = 90 \(^{\circ} {\text{\AA}}\), and β = 95.26 \((3)^{\circ} {\text{\AA}}\), volume of the unit cell V = 4288.9 (15) (Å)³. The intermolecular hydrogen bond N–H···N, N–H···O, C-H···O, and O–H···O type interactions stabilize the structure and lead to the three-dimensional network. Fourier Transform Infrared and Raman Spectra were used to study the compound's structural groups. FT-NMR studies confirm the structure of the grown MMH⁺AB⁻² crystal. The optical properties of the crystal were investigated using UV–Visible spectroscopy measurements. In the UV region, excitation begins at 200 nm, with the first absorption peak observed at a wavelength of 272.53 nm, followed by a second peak at 281.41 nm. The cut-off wavelength is 300 nm. The theoretical and experimental refractive index values are 2.84 and 1.69, respectively. The energy band gap value obtained from Tauc’s plot from UV–Vis data is 4.070 eV. Optical studies, such as refractive index and band gap measurements, were conducted to assess the material’s transparency. HOMO–LUMO transitions and its energy gap are computed to be 5.614 eV and other related molecular properties have also been calculated to reveal the optical properties. In the PL study, excitation starts at a wavelength of 350 nm and the emission peak occurs at 387.94 nm, located in the ultraviolet region. Natural Bond Orbital (NBO) analysis has been performed on MMH⁺AB⁻² compound to analyze the stability of the molecule arising from hyperconjugative interactions and charge delocalization. The highest stabilization energy, calculated to be 173.12 kcal/mol, is for the interaction between the lone pair on the N11 atom and the antibonding orbital of the C5 atom and 124.34 kcal/mol is for the interaction between the lone pair on the N3 atom and the antibonding orbital of the C5 atom. Molecular Electrostatic Potential (MEP) has been performed by DFT -B3LYP/6-311+G (d, p) basis set and the total electron density of the molecule ranges from -7.529 × 10^–2 to + 7.529 × 10^–2 e.s.u.. Dielectric studies further supported its suitability for electronic applications by highlighting its dielectric constant and loss factor. Electrical conductivity tests demonstrated the compound's potential as a semiconductor material. Additionally, the crystal packing behavior of MMH⁺AB⁻² was analyzed statistically using Hirshfeld Surface Analysis and the molecular packing of the crystal strongly depends on H–H hydrogen bond interactions, which account for 39.8% of all intermolecular interactions in the studied system. The computed first order hyperpolarizability β_tot = 377.570 × 10⁻³¹ e,s.u reveals that the present compound MMH⁺AB⁻² has SHG efficiency 101.25 times of Urea and 55.11 times that of KDP.