Volume 209, Issue 7

A brief survey is provided of some recent developments in the application of photon beams to study magnetic properties of solids. The chosen examples of experimental work are ones performed at electron synchroton facilities, and include the use of polarization analysis, spectroscopy, Bragg scattering and dichroism.

Based on the assumption that the structure factor components X = [unk] n j cos 2 π ( hx j + ky j + lZ j )and Y = [unk] n j sin 2 π ( hx j + ky j + lz j ) obey a statistics given by a truncated Edgeworth series, the probability distribution for the standardised intensity P ( Z ) and the cumulative probability for the standardised intensity N ( Z ) for the spacegroup P 2 1 / c are calculated. Also N ( z ) for a structure containing heavy atoms has been derived. Two specific cases: 1) an all light atom structure and 2) a structure containing one or more heavy atoms have been considered. P + ( E [unk]), the probability that E [unk] is positive has been calculated and a modified tangent formula developed in case (1).

A survey of the results of the theory of phase equations is given. Using an algorithm described by Woolfson, the spectrum of convolutional phase equations in the neighborhood of Sayre's equation is derived in a most general way. A non statistical extension of the tangent formula is developed.

The crystal structures of Hg 2 P 3 Br(I) and Hg 2 P 3 Cl(II) have been refined by least-squares full-matrix procedures to R = 0.037 (I) and R = 0.035 (II). I is orthorhombic, space group Pbcn , a = 8.014(1) Å, b = 8.902(1) Å, c = 7.823(1) Å, Z = 4. II is monoclinic, space group C 2/ c , a = 7.834(2) Å, b = 8.844(1) Å, c = 7.591(1) Å, β = 98.68(1), Z = 4. Both structures consist of [unk](P 1− ) helical chains running along the c -direction. The conformation of the chains is the same in I and II but the arrangement of the chains within the unit cells is different. The structures are also compared with those of Cd 2 Z 3 X (X = Cl, Br, I; Z = P, As).

The crystal structure of BaCuO 2 has been determined at 193 K by a single crystal X-ray study. The crystal used was the same as that in an earlier study at room temperature and was obtained as a parasitic phase during preparation of YBa 2 Cu 3 O 7 . The compound crystallizes in the cubic space group Im 3 m with a = 18.319(3) Å. The non stoichiometric formula now got is Ba 0.92 Cu 1.06 O 2.14 , that of the part with completely occupied positions Ba 0.92 Cu 0.88 O 1.63 , that of the part with partially occupied positions Cu 0.18 O 0.51 . 8806 reflections were collected and the structure was refined to R w = 0.074 based on F 2 of 922 unique reflections, R 1 = 0.037 (for 557 reflections with | F | > 4 σ ( F )). The results for the part with completely occupied positions confirm earlier structure determinations. The structure for the partially occupied atomic positions is different from all earlier given results in a few details.

URuAl, M r = 366.1, hexagonal, P [unk]2 m (I.T., nb. 189), a = 6.866(5) Å, c = 4.026(3) Å, V = 164.4(2) Å 3 , Z = 3, D x = 11.1 g cm −3 , neutron diffraction, λ = 0.8308(2) Å, μ = 0.0188 mm −1 ,| F (000)| = 5.630×10 −14 m, T = 120(2) K, R ( F 2 ) = 0.0196, w R ( F 2 ) = 0.0291, S = 1.077 for 429 reflections with I > 3 σ ( I ) out of 442 measured reflections. HoNiAl type of structure, U-Ru layers alternating with Ru-Al layers. Two distinct atomic sites are occupied by the transition metal atom. A refinement of the neutron coherent scattering length for the ruthenium element ( b Ru = 6.94(3) fm) is reported.

The structure of the synthetic, protected tetrapeptide Z-Leu-Aib-Pro-Leuol(C 29 H 46 O 6 N 4 , M w = 546.71) which contains the conformationally constrained residue α -aminoisobutyric acid (Aib), has been determined by X-ray crystallography. The title compound comprises the C-terminal tetrapeptide of trichovirin and has been chosen as a first step in a systematic analysis of the trichovirin structure-sequence relationships. Furthermore, the tetrapeptide has an interesting amino acid sequence, containing a strong helix former (Aib) and a helix breaker (Pro). The title compound crystallizes in P 2 1 with a = 10.523(1) Å, b = 23.033(5) Å, c = 13.462(4) Å, β = 91.81(1)°, V = 3261(2) Å 3 , Z = 4, D calc = 1.135 g cm −3 ( R = 0.054 for 3572 observed reflections). Both independent molecules in the asymmetric unit form a β -turn of type I with a 4 → 1 intramolecular hydrogen bond; the overall folding of each molecule fits approximately the pattern of a right-handed 3 10 -helix, although the conformational angles φ , ψ of the Leu residues deviate significantly from the standard helical values. In the crystal the molecules are hydrogen-bonded head-to-tail forming infinitely long, helical columns along the a -axis; these columns are associated by hydrogen bonds forming layers parallel to the ac -plane.

1,1′-dinapthoxymethanephenyloxyphosphinesulphide (C 27 H 19 O 3 SP) crystallizes in the triclinic system, P [unk] with a = 11.532(1), b = 11.931(1), c = 8.210(1) Å, α = 94.11(1), β = 110.78(1), γ = 77.40(1)°, V = 1080.6(2) Å 3 , Z = 2, D x = 1.40 Mg m −3 , λ (Cu K α ) = 1.5418 Å and T = 292 K. The structure was solved by direct methods and refinement by full-matrix least-squares method led to a final R = 0.045 for 3525 observed reflections. The heterocyclic phosphorous eight membered ring assumes a distorted boat conformation. The packing of the molecules are by van der Waals forces.

The crystal structure of the title compound, (C 14 H 13 N 3 O 3 PbS) 2 , has been determined from three-dimensional X-ray diffraction data. The crystals are monoclinic, space group P 2 1 / c with a = 10.523(1) Å, b = 8.9922(4) Å, c = 16.6720(7) å, β = 104.142(4)° and Z = 2. The structure was solved by the heavy-atom method and refined by least-squares to a final R = 0.040 ( R w = 0.031) for 2393 observed unique reflections [ F o > 5 σ ( F o )]. The lead atom is bonded to the 1-(2-thenoyl)-2-[1′(2″-pyridyl)ethylidenc]hydrazine ligand through the carbonyl oxygen, the pyridinyl nitrogen and the azomethinic nitrogen [Pb–O(thenoyl) 2.341(4) Å, Pb—N(azomethine) 2.410(6) Å, Pb—N (pyridine) 2.566(6) Å]. The lead atom is further chelated by the acetate ligand [Pb—O(2)2.290(6) Å, Pb—O(3)2.868(6) Å], while an additional weak interaction occurs between this atom and an acetate oxygen of another molecule [Pb—O(3) i 3.002(6) Å]. In this way dimer units are formed, the coordination geometry of the lead atom being that of a distorted pentagonal pyramid. The dimensions of the two organic ligands are as expected. The IR and electronic spectra are in accordance with the established structure from X-ray data. Conductivity measurements indicate the non-electrolytic character of the complex.

The title compound was synthesized by photolysis of hexa- tert -butylcyclotrisilane in the presence of N -cyclohexyl-2,3-diphenyl- α -ketoimine. It crystallizes from n-pentane monoclinically in the space group P2 1 /a with a = 1823.2(1)pm, b = 1041.1(1)pm, c = 1366.3(1) pm and β = 96.52(1)°.

Dimethyltin dichloride crystallizes with 15-crown-5 from methanol as the diaqua complex, (CH 3 ) 2 SnCl 2 · 2H 2 O · 15-crown-5, C 12 H 30 Cl 2 O 7 Sn, which is monoclinic in the C 2/ c space group ( a = 9.313(2) Å, b = 17.266(3) Å, c = 13.525(3) Å, β = 107.37(2)°; Z = 4). The tin atom occupies a distorted ( trans -CH 3 , cis -Cl, cis -H 2 O octahedral site (C–Sn–C = 157.5(3)°, Sn—Cl = 2.474(1), Sn—O = 2.460(4) Å). The oxygen atoms of the polyether point away from the center of the ring and the O water [unk]O crown ether distances all exceed 3.5 Å.