Concentration-dependent SERS, DFT, NCI, and molecular dynamics investigation of...

In this work, the structural, vibrational, electronic, and adsorption properties of N-(5-chloropyrazin-2-yl)-4-ethylbenzamide (CEB) were investigated through a combined experimental and theoretical approach. FT-IR, Raman, and concentration-dependent SERS spectra were analyzed with the support of density functional theory calculations. The optimized geometry, vibrational assignments, frontier molecular orbitals, molecular electrostatic potential, density of states, and UV-Vis transitions were examined to understand the intrinsic electronic nature of CEB and its interaction with silver clusters. Two adsorption models, involving Ag₆ interaction near the carbonyl and NH sites, were explored. The calculated adsorption and thermodynamic parameters revealed that both complexes are stable, with the NH-bound configuration showing stronger interaction and greater spontaneity. The reduced HOMO-LUMO energy gaps of the adsorbed systems indicated enhanced charge-transfer behavior and increased chemical reactivity relative to the isolated molecule. Concentration-dependent SERS analysis showed that the 10^-3 M system exhibits the strongest enhancement, with clear shifts and intensity modulation in ring and functional-group vibrations due to the molecule-surface interactions. Non-covalent interaction and RDG analyses confirmed that the stabilization of the complexes arises mainly from van der Waals interactions together with localized attractive contacts. MD simulations in water, methanol, and DMSO further demonstrated solvent-dependent stability, with DMSO providing the most compact and stable environment. Overall, the study provides a detailed molecular-level understanding of the adsorption behavior, spectral enhancement, and reactivity of CEB on silver surfaces, highlighting its relevance for SERS-based sensing and spectroscopic characterization.