Structural and vibrational analysis of 3-methyl-4-nitropyridine- N-oxide: neutron and X-ray diffraction studies

Abstract

X-Ray and neutron diffraction studies of 3-methyl-4-nitropyridine- N-oxide (3M4NPO) have been carried out at 106 K. 3M4NPO, C 6H 6N 2O 3 crystallizes in the orthorhombic space group P2 12 12 1, a = 20.890 (6), b = 6.094 (3), c = 5.135 (3) Å and Z = 4. Refinements including isotropic extinction correction lead to R w( F) = 0.037 for the X-ray data and 0.034 for the neutron data. The results of an earlier X-ray study at room temperature are confirmed. Atomic parameters from an X-ray high order (HO) refinement are very similar to those from the neutron refinement. A TLS thermal vibration analysis on the non-hydrogen atoms excluding the oxygen atoms of the nitro group indicate rigid body behaviour for this fragment of the molecule. This is confirmed by the rigid bond test. The mean square displacements of the H atoms are estimated from the frequencies measured by IR and Raman spectroscopy. These internal contributions added to the rigid body motion of the skeleton give thermal parameters for the H atoms almost identical to those derived from neutron data. Refinements of the population and radial dependence of the spherical atomic valence shell against the X-ray data provide an estimate of the net atomic charges, and are used to calculate an X-ray molecular dipole moment. The results allow an interpretation of the effects of the 3-methyl substituent on the intramolecular charge transfer. A comparison is made with 4-nitropyridine- N-oxide.