Structural and spectral analysis of 3- and 4-methylated nitraminopyridine N-oxides

Abstract

The X-ray structures of 3-methyl-2-nitraminopyridine N-oxide (3M) and 4-methyl-2-nitraminopyridine N-oxide (4M) has been determined and experimental electronic absorption spectra investigated. The X-ray results show that both compounds occur as dimers either in the molecular form (3M) or in the proton transferred (PT) form (4M). A ‘driving force’ for protonation depends on the relative position of the two monomers in the crystal lattice (parallel for 3M and perpendicular for 4M) and on the degree of electronic conjugation of –NH–NO2 substituent with the π-system of the pyridine ring. Position of the methyl group does not affect the direction of the CT process. Quantum-chemical semi-empirical PM3-CI calculations allowed an interpretation of experimental electronic absorption spectra and suggested the prototropic amino–imino equilibria which possible to occur in the solution.