Coupling Interaction of Baclofen with Ninhydrin in Aqueous Acidic Medium: Kinetics and Computational Studies

Abstract—Kinetics of the reaction of ninhydrin (Nin) with baclofen (Bac) has been studied spectrophotometrically in an aqueous acidic medium under pseudo order conditions over 30–50°C range, 0.1–0.4 × 10^–4 M of Bac, 0.5–5.0 × 10^–2 M Nin, and ionic strength 0.3–0.9 M. The thermodynamics activation parameters involving ∆H* and ∆S* have been calculated. The UV-visible spectroscopic measurements were carried out to confirm the coupling between Nin and Bac. The reaction is first order with respect to [Nin] and [Bac], decreases as pH increases in the range (3.90–5.06). The experimental rate law is consistent with a mechanism in which the protonated and deprotonated form of Nin are involved in the rate-determining step and the deprotonated species is the more reactive one. The product of the reaction was examined spectroscopically using ¹H-NMR and IR spectra in addition to an ultra-performance liquid chromatograph (UPLC). Density functional theory (DFT) was performed to search the geometries of the final product result from the reaction between Nin and Bac. Also, enthalpy of the reaction was calculated theoretically with DFT. Interaction region indicator (IRI) calculations is used to reveal chemical bonding and weak interaction in the coupled compound of Bac–Nin.