Tautomeric forms of N-(5-nitrosalicylidene)-2-butylamine: Experimental and theoretical DFT study

Tautomeric forms of N-(5-nitrosalicylidene)-2-butylamine in various solvents were studied with the IR and NMR (1H, 13C) spectroscopy. Also computational studies of keto-amine (O···HN) and enol-imine (OH···N) tautomers of N-(5-nitrosalicylidene)-2-butylamine molecule in the ground state were carried out with the DFT and SCRF theory at B3LYP/6-31G(d,p) level. Experimental data show that the solvent can invert the keto-amino: enol-imine equilibrium. The enol-imine tautomer prevails in non polar CCl4 and weakly polar CDCl3 or CD2Cl2 solvents, while in polar CD3CN solvent the keto-amine form dominates. Theoretical results agree qualitatively with the experimental findings and suggest also that predominance of particular tautomers can be influenced by the aliphatic chain geometry. Theoretical enol-imine model structure corresponding to the vacuum or non polar environment e.g.CCl4 is more stable than the keto-amino counterpart. On the contrary, the keto-amino form is predicted to be more stable than the enol-imine form in aqueous or in CH3CN polar environments. A satisfactory correlation between theoretical 1H and 13C shielding constants and experimental 1H and 13C chemical shifts becomes an additional argument that the three-dimension structures predicted theoretically should appear in the experimental conditions.