Structure and properties of N-methyl-N-(4-pyridyl)-nitramine and 1,4-dihydro-1-methyl-4-nitriminopyridine

Abstract

The molecular structure and spectral properties of N-methyl-N-(4-pyridyl)-nitramine (1) and 1,4-dihydro-1-methyl-4-nitriminopyridine (3) were investigated by the X-ray diffraction, FTIR, nuclear magnetic resonance (NMR) methods and DFT theoretical calculations. N-methyl-N-(4-pyridyl)-nitramine (1) crystallises in the orthorhombic Pbca space group. It is composed of two planar fragments; nitramino (N2O2) group and the pirydyl ring. These two groups are twisted at about 70° with respect to each other, along the Ar–N bond. On the contrary, in compound 3, they are coplanar to each other. Twist of the nitramino group in 1 with respect to the pirydyl ring leads to a significant increase in C–N bond length ca. 0.07Å in comparison to the analogous bond in the nitrimine 3. The true nitramines, like 1, give strong bands in the infrared spectra, in the regions 1252–1276 and 1488–1563cm−1. In the spectra of nitrimine 3, the frequencies characteristic of the N-nitro group, appear at 1245–1255 and 1420–1450cm−1. The proton and carbon NMR spectra of 3 indicate that the ortho positions of the ring, in the liquid state, are magnetically equivalent, due to the fast rotation along the CN bond, which was formally a double bond. The nitramine 1 rearranges to 4-(N-methylamino)-3-nitropyridine, in 95% sulphuric acid, whereas the nitrimine 3 cannot rearrange in any way. The difference in the susceptibility towards rearrangement was explained by the electron deficient character of the migration terminus, i.e. the pyridine ring (3a).