Molecular structure and vibrational spectroscopic investigation of melamine using DFT theory calculations

•Spectroscopic properties of melamine were examined by FT-IR, FT-Raman techniques and DFT methods.•NBO analysis used to explain the interaction between electron donors and acceptors.•HOMO and LUMO energies, MEP distribution of the molecule were calculated.•The atomic charges on the various atoms of molecule were obtained by Mulliken population analysis.

The FT-Raman and FT-IR spectra for melamine have been recorded in the region 4000–100 cm−1 and 4000–400 cm−1, respectively compared with the harmonic vibrational frequencies calculated using density functional theory method (B3LYP) by employing 6-31G(d,p) and 6-311++G(d,p) basis set with appropriate scaling factors. Optimized geometries of the molecule have been interpreted and compared with the reported experimental values. The experimental geometrical parameters prove satisfactory concurrence with the theoretical prediction from DFT. The scaled vibrational frequencies seem to coincide with the experimentally observed values with acceptable deviations. The theoretical spectrograms have been constructed and compared with the experimental FT-Raman and FT-IR spectra. The calculated Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energies show that charge transfer occurs in the molecule. The first order hyperpolarizability βtotal of this molecular system and related properties (α, β, μ and Δα) are calculated using DFT/B3LYP/6-31G(d,p) and 6-311++G(d,p) basis set based on the finite-field approach. Stability of the molecule arising from hyper conjugative interaction, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. Thermodynamic properties like entropy, heat capacity and zero-point energy have been calculated for the molecule.

Graphical abstractMelamine crystals block the small tubes in the kidney potentially stopping the production of urine, causing kidney failure. The equilibrium geometry harmonic vibrational frequencies, IR intensities and Raman scattering activities were calculated by density functional (B3LYP) method with 6-31G(d,p) and 6-311++G(d,p) basis sets, using Gaussian 03W program. HOMO and LUMO energies are calculated that these energies confirm charge transfer occurs within the molecule.Figure optionsDownload full-size imageDownload as PowerPoint slide