Theoretical and Solution Studies of a Proton Transfer Compound of o-Phenylenediamine with Pyridine Dicarboxylic Acid

The characterization of a proton-transfer compound, [(OPDH)+(dipicH)−.H۲O] (۱), between o-phenylenediamine, OPD, and ۲,۶-pyridine dicarboxylic acid, dipicolinic acid (dipicH۲), in water was investigated by ۱H, ۱۳C NMR and IR spectroscopes. In the crystal structure, in addition to the H-bonding interactions, the edge to face C–H···π and C–O···π stacking interactions between the dipicH۲ and OPD rings also play roles in the formation of the proton transfer compound. Quantum mechanical calculations were employed to investigate the proton transfer in the reaction between dipicH۲ and OPD. Thermodynamic parameters such as ΔH#, ΔG#, ΔS#, were calculated for this process. The intramolecular hydrogen bonding (IMHB) interaction has been recognized by calculating the electron density ρ(r) and Laplacian ۲(r) at the bond critical point (BCP) using Atoms-In-Molecule (AIM) method and also the interaction between electron acceptor (σ*) of OH with the lone pair of the nitrogen atom as an electron donor using Natural Bond Orbital (NBO) analysis. The protonation constants of dipicH۲ and OPD and the equilibrium constants for the dipic-OPD (۱:۱) proton transfer system were calculated by the potentiometric pH titration method using the Hyperquad ۲۰۰۸ program. The stoichiometries of the proton transfer species in the solution confirm the solid state result.

Keywords:

۲ , ۶-pyridinedicarboxylic acid , o-Phenylenediamine , Proton transfer , Theoretical study , NMR , Solution studyINTROD