Aminonitrone–iminohydroxamic acid tautomerism: Theoretical and spectroscopic study

Abstract

The comprehensive theoretical study of the tautomerism of aminonitrone species employing the С-methylaminonitrone MeC(NH2) = N+(Me)O− and the С-phenylaminonitrone PhC(NH2) = N+(Me)O− species as models and direct experimental observation of the tautomeric forms in a chloroform solution by infrared spectroscopy have been reported. All possible tautomeric forms including different (E)/(Z)-configurations of each species and the nature of solvent on their relative energies have been considered at the M06-2X/6-311 + G(d,p) level of theory. Results of theoretical calculations reveal that the most stable tautomeric form in chloroform, dimethyl sulfoxide, and methanol solutions is the (Z)-aminonitrone. The protic solvent (MeOH) significantly stabilizes both zwitter-ionic (E)- and (Z)-aminonitrone forms, whereas all tautomers related to the iminohydroxamic acid remain almost unaffected upon variation of the solvent nature. The phenomenon of intramolecular NH⋯O and OH⋯N hydrogen bonding in (Z)-aminonitrone and (E,Z)-iminohydroxamic acid tautomeric forms was studied by the topological analysis of the electron density distribution (QTAIM method), and estimated energies of these non-covalent interactions vary from 6 to 10 kcal/mol.