Hydrogen-bond effect, spectroscopic and molecular structure investigation of sulfamethazine Schiff-base: Experimental and quantum chemical calculations

Comprehensive theoretical and experimental structural studies on N-(4,6-dimethyl-pyrimidin-2-yl)-4-[(2-hydroxy-benzylidene)-amino] benzenesulfonamide (SMS) have been carried out by elemental analysis, FT IR, ¹H NMR, UV-Vis. and MS. Optimized molecular structure and harmonic vibrational frequencies have been investigated by DFT/B3LYP and HF methods combined with 6-31G(d) basis set. Stability of the molecule arises from hyperconjugative interactions, charge delocalization and intramolecular hydrogen bond has been analyzed using natural bond orbital (NBO) analysis. Electronic structures were discussed by TD-DFT method and the descriptions of frontier molecular orbitals and the relocation of the electron density were determined. ¹H NMR chemical shifts were computed by Gauge-invariant atomic orbital (GIAO) method in both gas and DMSO media, using the polarizable continuum model (PCM). Structure-activity relationship has been used to correlate biological activity with some appropriate quantum descriptors such as E_HOMO, E_LUMO, energy gap, dipole moment (μ), global hardness (η), softness (S), electrophilicity index (ω), molecular polarizability (α), Mulliken electronegativity (χ), Mulliken charge (Q_i) and molecular electrostatic potential (MEP).