Acta Cryst. (2006). B62, 627-633 [ doi:10.1107/S0108768106011608 ]
Abstract: The structure of 4-methylpyridine-N-oxide has been determined at 250, 100 and 10 K by combined synchrotron (C6H7NO) and neutron (C6D7NO) powder diffraction experiments. At 250 K the space group is I41/amd and the tetragonal unit cell [a = b = 7.941 (2), c = 19.600 (5) Å] contains eight equivalent molecules. At 100 K the structure is orthorhombic, with space group Fddd, a = 12.138 (2), b = 10.237 (2) and c = 19.568 (3) Å. The 16 equivalent molecules are rotated by about 8° around the c axis with respect to positions at high temperature. At 10 K the best structural model corresponds to a tetragonal unit cell with the space group P41, a = b = 15.410 (2) Å and c = 19.680 (3) Å. The 32 molecules (eight molecules in the asymmetric unit) show complex reorientations around the three cell axes. Whereas at 250 and 100 K the deuterated methyl groups are largely disordered, at 10 K they are ordered in-phase along infinite chains parallel to a and b. Face-to-face methyl groups along c are in an eclipsed configuration. The structure at 10 K suggests that the manifold of rotational tunnelling transitions could be due to inequivalent lattice sites for crystallographically independent methyl groups.
Keywords: 4-methylpyridine-N-oxide molecular systems; neutron powder diffraction; synchrotron powder diffraction; methyl rotational dynamics.
 |   Rietveld powder data file (CIF format) (167.2 kbytes) |
| Rietveld powder data file (CIF format) (152.5 kbytes) |
| Rietveld powder data file (CIF format) (152.5 kbytes) |
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