Ab initio and DFT computational studies on molecular conformations and strength of the intramolecular hydrogen bond in different conformers of 3-amino-2-iminomethyl acryl aldehyde

The intramolecular hydrogen bonding in 3-amino 2-iminomethyl acryl aldehyde (AIA) has been studied by ab initio and DFT calculations. All possible conformers of the two tautomeric structures of the respective compound were fully optimized at HF, MP2 and B3LYP levels with 6-311++G∗∗ basis set. From 33 different obtained conformers, the most stable one was planar and could be stabilized by the intramolecular NH⋯N hydrogen bonding. In addition, harmonic vibrational frequencies were evaluated at the same levels to confirm the nature of the stationary points found and also to account for the zero point vibrational energy (ZPVE) correction. Furthermore the topological properties of the electron density distributions for NH⋯N and NH⋯O intramolecular hydrogen bond bridges were analyzed in terms of the Bader theory of atoms in molecules (AIM). The electron density (ρ) and Laplacian (∇2ρ) properties, estimated by AIM calculations, indicate that N⋯H and O⋯H bond possess low ρ and positive (∇2ρ) values and are in agreement with electrostatic character of the HBs, whereas NH bonds have covalent character (∇2ρ < 0). The theoretical calculations for AIA conformers in water solution were also carried out at B3LYP/6-311++G∗∗ level of theory. Our data indicate that the order of stability of AIA conformers is similar to their stability order in the gas phase.