The crystal and molecular structures of 3-methyl-4-nitropyridine N-oxide and 3,5-dimethyl-4-nitropyridine N-oxide

The crystal structures of 3-methyl-4-nitropyridine N-oxide (M4NPO), tetragonal 3,5-dimethyl-4-nitropyridine N-oxide (D4NPO-T) and orthorhombic D4NPO (D4NPO-O) have been determined. M4NPO is orthorhombic, space group P2~2~2~, a=21-359(2) , b6.111(1),c=5.132(1)A, Z = 4 . D4NPO-T is tctragonal, space group P4~2j2, a = 7.443 (1), c = 13.447 (1) A, Z4. D4NPO-O is orthorhombic, space group Pbca, a = 7.329 (1), b = 14.912 (2), c = 13.852 (2) A, Z = 8. The intensity data were collected on a four-circle diffractometer by use of Zr-filtered Mo Ka radiation. The structures were refined by a blockdiagonal least-squares method to R = 0.062 for M4NPO (661 reflexions), 0.051 for D4NPO-T (472) and 0.085 for D4NPO-O (941). The twist angles of the nitro group out ofthe molecular plane are 16.7, 49.4 and 51.1 °, and the N--O distances of the N-oxide group are 1.292 (1.299 after libration corrections), 1.289 (1.293) and 1.302 (1.306) A in the order given above. The contribution of the quinoid structure to the resonance forms is signifcant in the two molecules, as in 4-nitropyridine N-oxide. The intramolecular charge transfer from the N-oxide group O atom to the nitro group plays an important role in stabilizing these molecules in less-twisted conformations than those of their related compounds.