Resonance interactions in the N-nitramino group. X-ray study of alpha-, beta- and gamma-nitraminopyridines

The crystal and molecular structure of three isomers (alpha, beta and gamma) of nitraminopyridine have been solved by X-ray diffraction at room temperature with Cu K alpha radiation, All isomers exist as the imino tautomeric form with strong resonance interactions in the nitramino group. NN and NO bond lengths and ONO bond angles are interrelated as found from consideration of 61 structures taken from the Cambridge Structural Database (release April 1995). Changes in geometry of the nitramino group are explained by differences in the s/p-character of the bond. Stabilization of the imino form seems to be due to the I-I-bonding net in the crystal, a phenomenon not taken into account in the nb initio calculation of alpha-isomers, which favors the amino form.