DFT-based theoretical investigation of structural stability, electronic, vibrational, NMR (1H and 13C), NLO, and thermodynamic properties of substituted 2-hydroxy naphthoyl chalcones

The structural, electronic, and nonlinear optical (NLO) properties of three isomeric forms of 2-hydroxy naphthoyl chalcone (2-HNC) were systematically investigated to elucidate the influence of molecular configuration and substituent effects on stability and optical response. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the B3LYP/6-311++G(d,p) level were employed to examine the geometric, electronic, spectroscopic, and thermodynamic characteristics of the studied systems. Potential energy surface (PES) scans revealed that the s-cis conformers are generally more stable than the s-trans and chromene forms, while electron-withdrawing substituents preferentially stabilize the s-trans geometry. The calculated relative Gibbs free energies (ΔΔG) indicate that the 3-Br and 4-Br substituents exert a pronounced stabilizing effect, with the chromene conformer exhibiting greater thermodynamic stability than both s-cis and s-trans forms. TD-DFT results showed that nitro- and hydroxy-substituted derivatives display bathochromic shifts and enhanced oscillator strengths, reflecting stronger intramolecular charge transfer (ICT). Simulated infrared (IR) and nuclear magnetic resonance (NMR) spectra agree well with experimental trends, confirming substituent-induced deshielding and vibrational frequency shifts. Frontier molecular orbital (FMO) and global reactivity analyses revealed that chromene isomers possess smaller HOMO–LUMO gaps and higher softness, implying greater electronic delocalization. NLO analysis further indicated that dipole moment ( μ ), polarizability ( α ), and first-order hyperpolarizability ( β ) increase with π-conjugation, with chromene forms exhibiting the highest β values. These findings establish clear structure–property correlations and provide theoretical insight into substituent- and conformation-dependent NLO behavior in chalcone-based organic materials.