A low‐temperature neutron diffraction study of Mn12‐acetate

In the low‐temperature region, where the dodecanuclear mixed‐valence manganese carboxyl­ate hexa­deca­acetatotetra­aqua­dodeca­oxo­dodeca­manganese bis­(acetic acid) tetra­hydrate, [Mn₁₂O₁₂(C₂D₃O₂)₁₆(H₂O)₄]·2C₂HD₃O₂·4H₂O, displays unusual magnetic properties, its structure is similar to that previously determined at room temperature [Lis (1980). Acta Cryst. B 36 , 2042–2046], differing only by a small change in the configuration of one of the coordinated acetate groups, related to the formation of additional hydrogen bonds, and by the orientation of the methyl groups. Since most of the magnetization density of this system resides on the Mn atoms, the consequences of these rearrangements for the magnetic properties of the compound are small.     

A new crystal phase of barium nitroprusside trihydrate studied by neutron diffraction at 20 K

The crystal of barium penta­cyano­nitro­syl­ferrate trihydrate {barium nitro­prusside trihydrate, Ba[Fe(CN)₅(NO)]·3H₂O} has been studied by neutron diffraction at 20 K. The study was performed to characterize the structural phase generated by the phase transition undergone by the crystals at 80 K, at which temperature the unit‐cell volume doubles. This crystal phase still exists at 20 K. The crystal structure, in space group P1, is completely ordered. The positional changes of the water mol­ecules in the present structure with respect to those of the compound at 105 K are presented.     

A single‐crystal neutron diffraction study of RbTiOAsO4

A rubidium titanyl arsenate single‐crystal has been studied by neutron diffraction (λ = 1.207 Å). The polished sample used was 5 × 3 × 2 mm and was cut from a crystal made by top‐seeded solution growth. The crystal showed severe extinction. It was, however, possible to obtain a structural model with well defined oxy­gen sites and reasonable anisotropic displacement parameters.     

The low‐temperature phase transition of 9‐methylfluoren‐9‐ol: comparison of the crystal structures at 100 and 200 K

Crystals of 9‐methyl­fluoren‐9‐ol, C₁₄H₁₂O, undergo a reversible phase transition at 176 (2) K. The structure of the high‐temperature α form at 200 K is compared with that of the low‐temperature β form at 100 K. Both polymorphs crystallize in space group P with Z = 4 and contain discrete hydrogen‐bonded R(8) ring tetramers arranged around crystallographic inversion centres. The most obvious changes observed on cooling the crystals to below 176 K are an abrupt increase of ca 0.5 Å in the shortest lattice translation, and a thermal transition with ΔH = 1 kJ mol^?1.     

2,4,5,7‐Tetra­methyl­phenanthrene at 150 K

The title compound, C₁₈H₁₈, crystallized in the centrosymmetric space group P2₁/c with one mol­ecule as the asymmetric unit. The methyl‐group H atoms at the 4 and 5 positions are ordered, while those at the 2 and 7 positions are disordered. The torsion of the bay region of the core is notably similar to that of other 4,5‐di­methyl­phenanthrenes. No substantial C—H?π interaction occurs in this structure.     

Powder neutron diffraction of SrNbO2N at room temperature and 1.5 K

The structure of strontium niobium dioxygen nitride, SrNbO₂N, has been solved by powder neutron diffraction at room temperature and 1.5 K. SrNbO₂N crystallizes in the tetragonal space group I4/mcm, with a = 5.7056 (4) and c = 8.1002 (9) Å at room temperature, and a = 5.6938 (4) and c = 8.0974 (8) Å at 1.5 K. The crystal structure is derived from the cubic perovskite archetype by a slight rotation of the Nb(O,N)₆ octahedra with respect to the tetragonal axis. A partially ordered distribution of oxygen and nitrogen on the anionic sites was found.     

Acetone‐4‐methyl­thio­semicarbazone at 220 K

In the title compound, C₅H₁₁N₃S, the trans conformation is stabilized by a weak intramolecular N—H?N hydrogen bond. Unusually, one N—H bond is not involved in any hydrogen‐bond interactions and instead the mol­ecules form a one‐dimensional polymer via N—H?S intermolecular hydrogen bonds.     

1,4‐Diethynyl‐2,5‐di­methoxy­benzene at ca 150 K

The principal determinants of packing in crystals of the title compound, C₁₂H₁₀O₂, which has crystallographically imposed inversion symmetry, are interactions between the alkyne H atoms and the methoxy O atoms [H⋯O = 2.39 (1) Å].     

1,3,5‐Triiodo‐2,4,6‐trimethylbenzene at 293 K

In the structure of tri­iodo­mesityl­ene (1,3,5‐tri­iodo‐2,4,6‐tri­methyl­benzene), C₉H₉I₃, at 293 K, the benzene ring is found to be significantly distorted from ideal D_6h symmetry; the average endocyclic angles facing the I atoms and the methyl groups are 123.8 (3) and 116.2 (3)°, respectively. The angle between the normal to the molecular plane and the normal to the (100) plane is 5.1°. No disorder was detected at 293 K. The thermal motion was investigated by a rigid‐body motion tensor analysis. Intra‐ and intermolecular contacts are described and topological differences compared with the isomorphous compounds tri­chloro­mesityl­ene and tri­bromo­mesityl­ene are discussed.     

Rotationally disordered phase of 1,3‐dibromo‐5‐iodo‐2,4,6‐trimethylbenzene at 293 K

In the crystal state at room temperature, the molecule of dibromoiodomesitylene (1,3‐dibromo‐5‐iodo‐2,4,6‐trimethylbenzene), C₉H₉Br₂I, is prone to strong disorder, apparently involving only the three halogen sites (occupied identically by 66.7% Br and 33.3% I). This disorder, of the rotational type according to previously published NMR measurements, corresponds to fast 2π/3 stochastic in‐plane reorientations of the whole molecule between three discernable locations. This kind of rotational disorder can be revealed for the first time by diffractometry thanks to the C_2v idealized molecular symmetry of the title compound, although it has been indirectly suspected at room temperature in other trihalogenomesitylenes of similar crystal packing but of D_3h molecular symmetry. The average endocyclic angles facing the Br/I sites and the methyl groups are 124.14 (6) and 115.85 (2)°, respectively. The angle between the normal to the aromatic ring and the normal to the (100) plane is 4.1°. TLS analysis indicates that only the aromatic ring and the methyl groups behave as a rigid body with respect to the thermal librations.     

γ‐Alumina: a single‐crystal X‐ray diffraction study

The structure of γ‐alumina (Al_21+1/3□_2+2/3O₃₂) crystals obtained as a product of a corrosion reaction between β‐sialon and steel was refined in the space group Fdm. The oxygen sublattice is fully occupied. The refined occupancy parameters are 0.83 (3), 0.818 (13), 0.066 (14) and 0.044 (18) for Al ions in 8a, 16d, 16c and 48f positions, respectively. The Al ions are distributed over octa­hedral and tetra­hedral sites in a 63:37 ratio, with 6% of all Al ions occupying non‐spinel positions.     

Low‐temperature study of 3‐acetylindole at 110 K

The title compound, C₁₀H₉NO, contains an acetyl group that is nearly coplanar with the indole ring system, with an angle between the planes of the heterocyclic ring and the acetyl group of 1.75 (17)°. The planes of the benzene and pyrrole rings in the indole system make a dihedral angle of 2.05 (11)°. Each molecule in the unit cell is linked through N—H...O hydrogen bonds to two other molecules, forming hydrogen‐bonded chains in the [101] direction with graph set C(6). The significance of this study lies in the analysis of the interactions occurring via hydrogen bonds in this structure, as well as in the comparison drawn between the molecular structure of the title compound and those of several other indole derivatives possessing a 3‐carbonyl group. The correlation between the IR spectrum of this compound and the structural data is also discussed.     

Diphenyl(2‐thioxo‐1,3‐dithiole‐4,5‐di­thiolato‐S,S′)plumbane at 295 K and diphenyl(2‐thioxo‐1,3‐dithiole‐4,5‐di­thiolato‐S,S′)stannane at 150 K

Molecules of di­phenyl(2‐thio­xo‐1,3‐di­thiole‐4,5‐di­thiol­ato‐S,S′)­plumbane, [Pb(C₃S₅)(C₆H₅)₂], are linked into sheets via two intermolecular Pb?S_thione interactions of 3.322 (4) and 3.827 (4) Å; the Pb centre has a distorted octahedral geometry. In contrast, mol­ecules of ­di­phenyl(2‐thio­xo‐1,3‐di­thiole‐4,5‐di­thiol­ato‐S,S′)­stannane, [Sn(C₃S₅)(C₆H₅)₂], are linked into chains via a single intermolecular Sn—S_thione interaction of 2.8174 (9) Å; the Sn centre has a distorted trigonal‐bipy­ramidal geometry.     

An ordered low‐temperature phase of barium nitro­prusside trihydrate studied by neutron diffraction

Crystals of barium penta­cyano­nitro­syl­ferrate trihydrate (barium nitro­prusside trihydrate), Ba[Fe(CN)₅(NO)]·3H₂O, have been studied by neutron diffraction in order to ex­amine the structural behaviour of the compound in the 20–120 K temperature range and to determine the structure at 105 K. The results show the existence of a new crystal phase of the compound at 80 K (with a duplicated a parameter), which still exists at 20 K. The crystal structure at 105 K shows a rearrangement of the water mol­ecules, which results in an ordered structure with P1 symmetry. Two of the four independent nitro­prusside cations are rotated by 4.5° around the [100] direction.     

Racemic dipeptide glycyl‐dl‐leucine at 120 K

The structure of glycyl‐dl ‐leucine, C₈H₁₆N₂O₃, has been determined at 120 K by single‐crystal X‐ray diffraction. In addition to three N—H?O‐type hydrogen bonds of the positively charged RNH₃⁺ group of the zwitterionic mol­ecule, an intermolecular N—H?O contact exists between the peptide bond and the carboxyl­ate group. Four hydrogen‐bond cycles were identified, giving a complex pattern.     

Powder neutron diffraction of Tl2BeF4 at six temperatures from room temperature to 1.5 K

The structure of thallium fluoro­beryllate, Tl₂BeF₄, has been analysed by the Rietveld method on neutron diffraction patterns collected at 1.5, 50, 100, 150, 200 and 300 K, with the aim of detecting low‐temperature instabilities. Atomic parameters based on the isomorphic β‐K₂SO₄ crystal in the paraelectric phase were used as the starting model at room temperature; no evidence for any phase transition has been detected at lower temperature. The structure was determined in the ortho­rhom­bic space group Pnma. All the atoms (except one F atom) occupy sites with m symmetry. We have compared the structure with those of other compounds of the β‐K₂SO₄ family, at room temperature, in order to gain insight into their observed instabilities. The irregular coordination of the cations may indicate stereochemical activity of the Tl^I lone pair but does not indicate a possible structural instability.     

trans‐4‐Bromo‐ONN‐azoxy­benzene at 100 K

The crystal structure of the α isomer of trans‐4‐bromo­azoxy­benzene [systematic name: trans‐1‐(bromophenyl)‐2‐phenyl­diazene 2‐oxide], C₁₂H₉BrN₂O, has been determined by X‐ray dif­frac­tion. The geometries of the two mol­ecules in the asymmetric unit are slightly different and are within ∼0.02 Å for bond lengths, ∼2° for angles and ∼3° for torsion angles. The azoxy bridges in both mol­ecules have the typical geometry observed for trans‐azoxy­benzenes. The crystal network contains two types of planar mol­ecules arranged in columns. The torsion angles along the Ar—N bonds are only 7 (2)°, on either side of the azoxy group.     

trans‐[1,3‐Bis(2,4‐di­methyl­phenyl)­imidazolidin‐2‐yl­idene]­di­chloro­(tri­phenyl­phosphine‐κP)­palladium(II)

The title complex, [PdCl₂(C₁₉H₂₂N₂)(C₁₈H₁₅P)], shows slightly distorted square‐planar coordination of the palladium(II) metal center. The Pd—C bond distance between the N‐heterocyclic ligand and the metal atom is 2.008 (3) Å. The dihedral angle between the two di­methyl­phenyl ring planes is 33.17 (13)°.     

An anthracene dimer with a fused 1,3‐dithiole ring at 193 K

The first butterfly‐shaped anthracene dimer including S atoms,8,9‐di­hydro‐3a,8[1′,2′]:9,13b[1′′,2′′]­di­benzeno­dibenzo­[3,4:7,8]cyclo­octa­[1,2‐d]‐1,3‐di­thiole, C₂₉H₂₀S₂, contains an exceptionally long Csp³—Csp³ bond of 1.672 (2) Å in the fused 1,3‐di­thiole ring. The length of the other bond bridging the anthracene moieties is 1.604 (3) Å.     

(p‐Nitro­benzoato)­tri­phenyl­tin at 298 K

The structure at 298 K described here, [Sn(C₆H₅)₃(C₇H₄NO₄)], completely confirms the results at 173 K obtained previously [Weng, Das & Robinson (1990), Malays. J. Sci. 12 , 57]. In both structures, weak interaction between Sn and the carbonyl O atom of the benzoate group provides a distorted trigonal‐pyramidal environment at the Sn atom derived from its pseudo‐tetrahedral primary coordination in both mol­ecules of the asymmetric unit.