Volume 209, Issue 5

In the past decade pulsed-neutron sources have become firmly established as being complementary to reactor sources for work in crystallography. We describe some of the special advantages of pulsed-neutron crystallography, such as the presence of a high flux of short-wavelength neutrons and the exceptionally good resolution for powder diffraction studies. A few examples are then given of how these advantages have been exploited in practice, including the measurement of thermal diffuse scattering in materials with the fluorite structure.

Multiple diffraction experiments have been performed on a microcrystal and a quasicrystal of the Al–Cu–Fe alloy for the purpose of extracting phase information. The microcrystal used in this work is a twinned rhombohedral crystal exhibiting overall icosahedral symmetry. The experiments are performed by rotating the specimen around the scattering vector, ( ψ -scans) and by observing the peaks due to the Renninger effect, and the asymmetric features on the wings of the peaks, source of phase information. By fitting theoretical plots to the experimental ψ -scans we conclude, within the resolution limits of this experiment, that the atomic arrangement in a microcrystal seems to be different from that of the corresponding quasicrystal and that the structure may be centrosymmetric. The measurements on the quasicrystal support a previous finding that the structure is not centrosymmetric.

Refractive indices, Faraday effect, dielectric constants, and elastic constants have been studied on crystals grown from aqueous solutions. The physical properties are closely related to the chemical composition of the crystals. In the ammonium salts the first transition cycle of a virgin crystal yields a different behaviour compared to subsequent cycles. Copper salts and the isotypic manganese compound exhibit transitions of different type. The transition temperature is correlated to the void occupied by the [NH 4 ] + -ions.

(NH 4 ) 2 Be 4 Cl 10 crystallizes in the cubic space group Fd [unk] m ( Z = 8) with a = 14.705(3) Å, D rö = 1.784 g/cm 3 . The structure was solved from 111 symmetry independent reflections and refined to R = 0.030. The structure is characterised by [Be 4 Cl 10 ] 2− -units, analogous to P 4 O 10 , held together by the [NH 4 ] + -ions. The compounds A 2 Be 4 Cl 10 with A = Cs, Rb, K, Tl are isotypic.

In the scope of providing new chemical insight of the oxidation states of copper and vanadium, crystal structures of VOB's β′ -Cu x V 2 O 5 with x = 0.29, 0.47 and 0.59, have been carefully investigated. The structures are monoclinic, space group C 2/ m , with a = 15.233(3) Å, 15.218(1) Å, 15.2007(7) Å, b = 3.6133(9) Å, 3.6271(6) Å, 3.6375(3) Å, c = 10.0927(9) Å, 10.087(8) Å, 10.0925(4) Å, β = 107.30(1) Å, 106.55(4) Å, 106.13(2)°, R = 0.0467, 0.0290, 0.0315. A thorough examination of bond distances and angles allows to give a precise description of the structural evolution versus increasing values of copper insertion x in the tunnels of the [V 2 O 5 ] n framework. The three independent vanadium atoms exhibit distorted oxygen coordination polyhedra, octahedra for V(1) and V(2), square pyramid for V(3). The copper, as revealed by successive Fourier map differences, is spreaded over a sort of toroidal section inserted in a huge oxygen trigonal bipyramid; more precisely the copper coordination changes, in its asymmetric site, from CN4 for Cu(1) (in the mirror plane) to CN3 for Cu(2) (apart the mirror plane, Δy ∼± 0.12) which rarely occurs in sulphides but, up to now never in oxides. The multiplication of Cu sites as well as their various coordination let us think that copper oxidation states could be not unique; such hypothesis is confirmed by ESCA measurements, which clearly indicate that simultaneously Cu(I) and Cu(II) are present. Such results establish that β′ ′-Cu x V 2 O 5 phases are doubly mixed valence vanadium oxide bronzes and can be formally written: Cu xt + Cu x (1− t ) 2+ V 2− x (2− t ) 5+ V x (2− t ) 4+ O 5 ( t stands for Cu(I) rate).

Fe 0.33 V 2 O 5 crystallizes in the monoclinic space group C 2/ m with a = 15.307(8) Å, b = 3.624(2) Å, c = 10.123(5) Å, β = 107.30(3)°, V = 536.12 Å 3 , and Z = 6. The structure was determined from single crystal X-ray diffraction data and it is related to the β -M x V 2 O 5 structure type. There are some indications of partial cationic order between V 4+ and V 5+ within the three vanadium sites. Fe 2+ occupies two crystallographic sites with tetrahedral and 3 + 1 coordination, respectively.

Single crystals of Sr 1− x Ca x CuO 2 with x = 0–0.573 have been grown by a modified Bi 2 O 3 flux technique using different ratios of the earth alkali carbonates. Upto x = 0.573 the SrCuO 2 structure type was found. The changes of the lattice constants and of the atomic parameters are given. The different temperature dependence of the atomic parameters comparing undoped and doped crystals is discussed. From reaction mixtures with an excess of Ca (Ca:Sr ≥ 2:1) crystals with the composition Ca 1.866 Sr 0.134 · CuO 3 could be obtained. They crystallize in the Ca 2 CuO 3 structure type.

The title compound C 12 H 15 O 9 PS has been synthesized. Its structure has been determined by X-ray single crystal methods. Crystals are monoclinic, space group C 2 with a = 20.902(9) Å, b = 12.029(4) Å, c = 13.853(7) Å, β = 103.06(4)°, Z = 8, V = 3392 Å 3 , ϱ c = 1.43 g/cm 3 , μ (Mo K α ) = 3.13 cm −1 , F (000) = 1520. The two crystallographic independent molecules are related by pseudo symmetry ( x , 1/2 − y , 1/2 + z ). The conjugation between the C–C double bond and the carbonyl atom of the ester is poor because they are not in the same plane. The relationship between structure and reactivity is discussed.

C 8 H 10 N 4 O 3 , M = 210.18, triclinic space group P [unk], a = 10.193(1), b = 12.401(1), c = 8.061(1) Å, α = 97.80(1), β = 98.28(1), γ = 107.89(1)°, V = 941.9(2) Å 3 , Z = 4, D x = 1.581 g cm −3 , λ (Mo K α = 0.71073 Å, μ = 1.2 cm −1 , F (000) = 384. The crystal structure was determined by direct methods and refined by least-squares procedure to a discrepancy factor R = 0.035. The two molecules of the asymmetric unit are tautomeric forms characterized by an ketamine-hydroxyimine tautomerism.

The result of crystal structure analysis of two Zn–Cu bimetallic complexes are reported. Poly-[diammine-zinc(II)-tricyanocuprate(I)], C 3 H 6 N 5 CuZn, [Zn(NH 3 ) 2 Cu(CN) 3 ] ∞ , M r = 241.03, monoclinic, space group P 2 1 / n , a = 6.395(2), b = 6.827(2), c = 17.296(5) Å, β = 98.55(2)°, V = 746.7(3) Å 3 , Z = 4, D m = 2.12, D x = 2.14M gm −3 , λ (Cu K α ) = 1.54178 Å, μ = 6.72 mm −1 , F (000) = 472, T = 293 K, R = 0.0487 for 856 observed reflections ( F ≧ 4 σ ( F )). The polymeric structure is formed by [(NH 3 ) 2 Zn(NC)Cu(CN)] 2 rings linked together by bridging cyano groups bonded to the Cu(I) atoms. The Zn(II) atom is tetrahedrally coordinated by two ammonia molecules and two N-oriented cyano groups. The Cu(I) atom is deformed tetrahedrally coordinated by one N-bonded and three C-bonded cyano groups. One of the Cu–C≡N angles exhibits the unusual low value of 156.2(6)°. Hexakis-[tris(ethylenediamine)-zinc(II)-tricyanocuprate bis[heptacyanodicuprate)- n -hydrate ( n = 8.4), C 53 H 160.8 Cu 5 N 53 O 8.4 Zn 6 , [Zn(en) 3 ] 6 [Cu 2 (CN) 7 ] 2 [Cu(CN) 3 ] · n H 2 O ( n = 8.4), M r = 2385.40 rhombohedral, space group R 32, a = 15.475(9) Å, α = 59.14(4)°, V = 2569(3) Å 3 , Z = 1, (hexagonal setting: a = 15.273 Å, c = 38.150 Å, V = 7707 Å 3 , Z = 3), λ (Mo K α ) = 0.71069 Å, μ = 2.5 mm −1 , F (000) = 1242, T = 293 K, wR = 0.0772 for 1519 observed reflections. The structure is ionic and is formed by three crystallographically independent [Zn(en) 3 ] 2+ cations, one planar [Cu(CN) 3 ] 2- anion and two [(NC) 3 Cu(CN)Cu(CN) 3 ] 5- anions. Two crystallographically independent water molecules arc disordered.

The title compound, Pd 0.5 S 2 C 16 H 23 ON 4 , crystallizes in the monoclinic space group P 2 1 / n with a = 9.917(8) Å, b = 15.382(5) Å, c = 13,218(2) Å, β = 100.12(3)°, Z = 4. The structure was solved by SHELXS-86 using 2077 out of 5778 reflections with I > 3 σ ( I ). Full-matrix refinement gave a final residual of R f = 0.048. The coordination polyhedron is undistorted and it does not show any hydrogen bonding. The packing of the molecules shows a layered arrangement.

The title compound, Ni 0.5 S 2 C 16 H 24 O 2 N 4 , crystallizes in the monoclinic space group P 2 1 / a with a = 15.062(3) Å, b = 7.829(2) Å, c = 18.527(2) Å, β = 110.179(9)°, Z = 4. The structure was solved by SHELXS-86 using 1772 out of 4524 reflections with I > 3 σ ( I ). Full-matrix refinement gave a final residual of R f = 0.0573. The nickel is in a distorted octahedral coordination and it does not show any hydrogen bonding. The packing of the molecules shows a layered arrangement down b .

Dark-blue colored transluscent rhombic plates of diaquabissarcosinatocopper(II) were crystallised from an aqueous solution of sarcosine and freshly prepared cupric hydroxide. The crystals are monoclinic with the cell dimensions a = 5.598(2) Å, b = 8.380(3) Å, c = 12.346(3) Å, β = 100.25(2)°, space group P 2 1 / n and Z = 2. Least-squares refinement using 658 X-ray (Mo K α ) reflections led to an R -factor of 0.039. Each of the centrosymmetrically related sarcosine molecules are coordinated to copper in the basal plane and the oxygens of the two water molecules occupy the weak axial coordination sites. Two O–H…O and one N–H…O hydrogen bonds are present in the structure.

N-Trichloroacetyldimesylamine, C 4 H 6 Cl 3 NO 5 S 2 , crystallises in P 2 1 / n with a = 8.485(3) Å, b = 13.330(5) Å, c = 20.028(6) Å, β = 95.97° (at 178 K), Z = 8. The structure ( R ( F ) 0.031) involves two independent but essentially identical molecules. The coordination at the nitrogen atoms is non-planar, and the C–N and N–S bonds are long (av. 1.446, 1.707 Å). Short intermolecular Cl…O contacts are observed.

N-[2,6-dichlorophenyl]-2,2,2-trichloroacetamide (2,6-Cl 2 C 6 H 3 NHCOCCl 3 ) shows a bleaching out of certain 35 Cl-NQR frequencies at a temperature of 159 K. The 35 Cl-NQR spin lattice relaxation time ( T 1 ) was determined as function of temperature. The data of the trichloromethyl resonances lead to an activation energy of 23.7 kJ/mole for the bleaching out process. The chlorine atoms bonded to the phenyl ring show a normal temperature dependence of T 1 . In addition crystal structure determinations were made at three temperatures using neutron diffraction (117 K, 195 K, 373 K). The space group of the compound is P 2 1 / c = C [unk] h with four molecules per unit cell. The cell parameters are a = 1007(1) pm, b = 1168(1) pm, c = 1002(1) pm, β = 101.79(7)° (117 K), a = 1017(3) pm, b = 1194(3) pm, c = 1007(3) pm, β = 101.9(2)° (295 K), a = 1018.5(9) pm, b = 1208(1) pm, c = 1009(1) pm, β = 101.63(8)° (373 K). The molecules form intermolecular hydrogen bonds of the N–H…O–C type along [001]. An anharmonic behaviour of two of the trichloromethyl chlorine atoms was observed and studied as a function of temperature. The intramolecular distances and angles show no significant temperature variations.

The title compound was obtained by hydrolysis of the corresponding amide with 2-aminobenzophenone. It crystallizes from dichloromethane monoclinically in the space group P 2 1 / c with a = 1156.9(2) pm, b = 1027.7(2) pm, c = 901.2(1) pm and β = 103.92(2)°.