5-(4-Chlorophenyl) -3H-pyrazol-3-one (CPP) has been synthesized by the reaction of 2-(piperidin-1-yl)acetamide, p-chlorobenzaldehyde and hydrazine hydrate. Structural characterizations were achieved by means of Mass, 1HNMR (0–20 ppm), 13CNMR (0–200 ppm), IR (3500–400 cm−1) and Raman (3500–100 cm−1) spectral measurements. In addition, the regioselectivity of the above synthetic route / mechanistic path was investigated using B3LYP/6–311++G(2d,p) DFT calculations, which is supported by frontier molecular orbital (FMO), olecular lectrostatic otential (MEP) and natural bond orbital (NBO) analysis. Good agreement between the theoretical analysis and experimental synthetic route has been achieved. Molecular interactions, charge delocalization and electrophilic/nucleophilic sites within the reactant species have been also investigated. CPP can undergo internal rotation around C-C single bond; hence it could be either planar or non-planar configurations. Aided by Density Functional Theory (DFT) optimizations implementing a tight convergence criteria, vibrational frequencies and potential energy surface scan were carried out using B3LYP/6–311++G(2d,p), in favor of planar configuration while a saddle point was obtained for the non-planar geometry. A complete vibrational interpretations were proposed herein, assisted by normal coordinate analysis (NCA) and Potential Energy Distributions (PEDs). Finally, global reactivity descriptors; ionization potential (IP), electron affinity (EA), electronegativity (χ), chemical potential (µ), chemical hardness (η), and electrophilicity index were also calculated and found well correlated to the chemical reactivity of the reactant molecules.
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