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

The FT-Raman and FT-IR spectra for melamine have been recorded in the region 4000-100cm(-1) and 4000-400cm(-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.