Examines both structural optimization and spectroscopic analysis of C₁₆H₁₃N₃O and its brominated and chlorinated derivative compounds through infrared and ultraviolet-visible spectroscopy techniques.

The investigation combines theoretical research with practical testing to analyze the indole-based hydrazone derivatives A1 A2 and A3. The researchers achieved successful compound creation through condensation reactions which produced crystalline solids in high yield. We conducted our research by using DFT calculations together with experimental spectroscopy methods to study the molecular structures and physicochemical properties of the compounds. The DFT/B3LYP level required geometry optimizations to use both the STO-3G* and 3-21G basis sets for its calculations. The optimization process created stable structures which sustained their most stable state throughout the entire evaluation period. The 3-21G basis set produced bond lengths which were shorter and bond angles which reached higher accuracy compared to the STO-3G*. The method provides improved accuracy for both electron delocalization and molecular polarization effects according to this result. Heteroatom bonds with halogen substituents showed more pronounced bond length measurement differences during analysis. The halogenated compounds A2 and A3 exhibit lengthened C–Br and C–Cl bonds because additional halogen elements produce stronger polarizing effects. The optimized geometries showed systems which maintained their near-planar structure because their π-electron system extended over a wider area. The Mulliken population analysis showed a strong relationship between the analysis results and the basis-set size of the study. The negative charges of nitrogen and oxygen atoms reached their maximum strength because the atoms held their lone pairs in fixed positions. Bromine substitution caused a greater increase in electronic polarization than chlorine substitution did. The calculated vibrational frequencies showed strong similarity to the results of experimental FT-IR spectroscopy analysis. The study demonstrates that the combination of experimental methods with DFT calculations delivers precise structural and spectroscopic data for the tested compounds.