DFT Study on Structural, Electronic, and Nonlinear Optical Properties of Melaminium Glutarate Monohydrate

Single crystals of melaminium glutarate monohydrate (MGM) were
grown by slow evaporation at room temperature. Single-crystal X-ray dif
fraction analysis confirmed that the crystal belongs to the monoclinic sys
tem with a centrosymmetric space group, P21/c. Density functional
theory (DFT) optimization was performed at the RB3LYP/6-311G(d,p)
level using the Gaussian 09 program. The calculated FT-IR and Raman
spectra are in good agreement with the observed vibrational features,
while the ¹H and ¹³C NMR chemical shifts were computed using the
Gauge-Including Atomic Orbitals (GIAO) method. The HOMO–LUMO
energy gap was found to be 6.2439 eV, indicating good molecular stabil
ity and moderate chemical reactivity. Mulliken charge and natural bond
orbital (NBO) analyses were carried out to identify the reactive sites and
elucidate intramolecular charge-transfer interactions. The strongest intra
molecular stabilization energies, 71.64 and 54.58 kcal mol−1, arise from
hyperconjugative interactions involving donation from the lone pair of
the nitrogen atom (N11) to the antibonding π∗(N3−C6) orbital. Fukui
function analysis distinguished the electrophilic and nucleophilic regions
of the molecule. The first-order hyperpolarizability was computed to be 118.303×10−31 esu, which is approximately 32 times greater than that of
urea. Hirshfeld surface analysis was employed to investigate the intermo
lecular interactions present in the compound, revealing that the major
contribution arises from O–H interactions (38.2%), followed by H–H
(37.1%) and N–H (11.8%) interactions. Topological analysis provided
further insights into the molecular structure and reactivity. The CVB
index result (−0.00464) indicates the presence of strong hydrogen-bond
ing interactions.
Keywords DFT, Vibrations, HOMO-LUMO, Dipole moment calculation, NBO,
Hirshfeld Surface, Fukui function, Multiwfn