Probing the solvent effects and protonation sites of 1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amine (PDPA): DFT calculations meet pKa profiling

5-methyl-2-phenyl-4-(2-phenylhydrazono)-2,4-dihydro-3H-pyrazol-3-one (PDPA) was recently synthesized as a promising nuclear magnetic resonance (NMR) reference in studying the tautomeric structures of azo dyes. In this study, the interaction of PDPA derivatives with a single water molecule was studied using the density functional theory (DFT) method. The protonation reaction was proposed to identify the most basic center in the investigated species by computing the proton affinity (PA) and gas phase basicity (GB) values. The nature of the intermolecular hydrogen bond (IMHB) due to the interaction of the hydrogen atom of the water molecule and the nitrogen atom of the PDPA molecule was investigated using several aspects, including geometrical structures, quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO) analysis and the noncovalent interactions (NCIs) using the reduced density gradient (RDG) MAPS. Results revealed that the PDPA derivatives are medium potent and basic with average PA and GB values of 219.7 and 213.0 kcal/mol, respectively. The maximum PA and GB values of 228.5 and 221.7 were found for the PDPA-NH 2 derivative, respectively, while the smallest ones of 213.5 and 206.7 for the PDPA-CN derivative, reflecting the effect of the nature of the substituted group. Our results revealed that IMHB has moderate strength and is strongly dependent on the nature of the substituted group. The results of QTAIM indicated that the average IMHB strength is 5.0 kcal/mol and that between the water molecule and thiazolidine-2,4-dione and its derivatives is 4.8 kcal/mol. Moreover, NBO analysis showed that the average IMHB strength is 6.7 kcal/mol. Results also revealed that substitution of the electron donating groups strengthens the IMHB, while that of the electron withdrawing groups weakens it.