title: Synchrotron and neutron diffraction study of 4-methylpyridine-N-oxide at low temperature.
abstract: The structure of 4-methylpyridine-N-oxide has been determined at 250, 100 and 10 K by combined synchrotron (C(6)H(7)NO) and neutron (C(6)D(7)NO) powder diffraction experiments. At 250 K the space group is I4(1)/amd and the tetragonal unit cell [a = b = 7.941 (2), c = 19.600 (5) A] 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) A. The 16 equivalent molecules are rotated by about 8 degrees 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 P4(1), a = b = 15.410 (2) A and c = 19.680 (3) A. 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.
authors.affiliation: Laboratoire Léon Brillouin, CEA-CNRS UMR12, CE Saclay, 91191 Gif sur Yvette Cedex, France. damay@llb.saclay.cea.fr
authors.forename: Françoise
authors.initials: F
authors.lastname: Damay
authors.forename: Adrian
authors.initials: A
authors.lastname: Carretero-Genevrier
authors.forename: Alain
authors.initials: A
authors.lastname: Cousson
authors.forename: Wouter
authors.initials: W
authors.lastname: Van Beek
authors.forename: Juan
authors.initials: J
authors.lastname: Rodriguez-Carvajal
authors.forename: François
authors.initials: F
authors.lastname: Fillaux