Dissociation Constant (pK_a) and Thermodynamic Properties of 1,4-Bis(3-aminopropyl) Piperazine, 1,3-Bis(aminomethyl) Cyclohexane, Tris(2-aminoethyl) Amine, and 1‑Amino-4-methyl Piperazine: Study of the Protonation Mechanism Using the Density Function Theory

This study focuses on determining the dissociation constants as well as other thermodynamic properties for four amines of importance in the area of CO₂ capture, namely, 1,4-bis­(3-aminopropyl) piperazine, 1,3-bis­(aminomethyl)­cyclohexane, tris­(2-aminoethyl) amine, and 1-amino-4-methylpiperazine. All experiments were done at atmospheric pressure and at seven temperatures in the range of 288.15–323.15 K. The dissociation constants (pK_as) were measured using a potentiometric titration. The van’t Hoff equation was used to determine thermodynamic properties such as the enthalpy, entropy, and free energy (ΔH⁰, ΔS⁰, and ΔG⁰) based on the pK_a values measured at different temperatures. In addition, Gaussian was used to help explain the influence that the tertiary amine has on the second pK_a of 1-amino-4-methylpiperazine. The density functional theory (DFT) was used to optimize the geometries after each protonation step and calculate the bond lengths between the nitrogen to the proton, which can be used as a comparison of the pK_a of the amines. pK_a values for the four studied amines were also predicted using the Perrin–Dempsey–Serjeant (PDS) model.