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.

Modelling of crystal structure of cis-1,2,3,6 and 3,4,5,6-tetrahydrophthalic anhydrides using lattice energy calculations

“…This finding argues against the suggestion that many high Z' polymorphs tend to be metastable fossil [49,50] relics or forms of arrested crystallisations [51]. This example joins the other many cases of crystals where a higher Z' structure has been shown to be more stable than a Z'=1 polymorph [51][52][53][54]. This study confirms that high Z' crystals are also not ''on the way'' or incompletely crystallized, they are fully crystalline, periodic ordered structures as also indicated by several researchers [55].…”

Charge flipping combined with histogram matching to solve complex crystal structures from powder diffraction data

“…This structure had previously been solved by Damay et al (2006) using the simulated annealing algorithm implemented in FullProf (Rodriguez-Carvajal, 1993). It crystallizes in the space group P4 1 (a ¼ 15.454 A, c ¼ 16.695 A) and has eight 4MPNO molecules (i.e.…”

Validation of missed space-group symmetry in X-ray powder diffraction structures with dispersion-corrected density functional theory

“…For those 215 energy-minimized structures, eight compounds showed additional symmetry [BUTYNE01 (Ibberson & Prager, 1995); HMBENZ06, HMBENZ07, HMBENZ08, HMBENZ09, HMBENZ10, HMBENZ11 (Stride, 2005) Chemical structure of RUDREN and ADPs of VAVHEH. 2007); TMAPCL05, TMAPCL06 (Palacios et al, 2003); ZZZUXA02, ZZZUXA03 (Damay et al, 2008); ZZZVCO03 (Damay et al, 2006); QIKZAN02 (Bekö et al, 2014)]. Here we re-evaluated the possible space-group changes in these 215 structures with the recipe validated on the 200 reinterpreted structures.…”