Intramolecular hydrogen bondings (IHB) in some derivatives of 4-(aminomethylene)-1-methylpyrazolin-5-ones (AMP) and its imine tautomers were investigated using density functional theory (DFT) calculations at B3LYP/6- 311+G(d,p) level of theory. The relative stability calculations showed that AMP forms are more stable than IMP ones. The strength of the IHB were investigated by means of several quantum descriptors such as geometrical structural (bond lengths and angles), topological parameters based of Atoms in Molecules theory (AIM) of Bader (total electronic density and its Laplacian at bond critical point (BCP) and ring critical point (RCP)), hyper conjugative interaction energy (ECT), related to the LP(O)→σ*(N–H) and LP(N)→σ* (O–H) overlaps, chemical shifts of the bridged hydrogen atom, σ1H, frequency shifts of the N-H and O-H stretching vibration modes. The combined results strongly illustrate that the O-H×××N exist for IMP forms is stronger that the N-H×××O in AMP tautomers. Good linear correlation coefficients between IHB energy, EHB, based on Espinosa’s equation and the abovementioned descriptors were reported.
