Volume 208, Issue 2

A rigorous analytical expression is derived for the electrostatic potential originating from aspherical atomic multipole electron densities according to the Stewart or Hansen-Coppens charge density model with a Slater type radial electron distribution. Such models are widely used to determine charge distributions from high resolution single crystal diffraction experiments.

The crystal structure of the title compound, C 13 H 13 N 2 F 3 O 2 , (NTFAAI hereafter) has been determined from three-dimensional X-ray diffraction data. The structure is monoclinic, C2, with a = 19.023(5), b = 7.574(2), c = 10.405(2), Å, β = 114.17(2)°, V = 1368(1) Å 3 and Z = 4. The structure was solved by direct methods and refined by full-matrix least-squares calculations to a final residual of 0.051 for 1030 observed reflections [ I ≥3 σ ( I )]. The molecule comprises an indole ring bound to a trifluoracetyl group via an alanine. Both the six- and five-membered rings are coplanar to a very good approximation, since they form a dihedral angle of 1.6(2) degrees. The trifluorocarbon group is tetrahedrally coordinated with the two carbonyl oxygens located on their peptide plane. All bond distances and angles demonstrate no significant deviations from standard values.

Triphenylphosphine oxide oxalic acid benzene (TPPO · OA · B), 2(C 6 H 5 ) 3 PO · (COOH) 2 · C 6 H 6 , MW = 724.77, triclinic, P [unk], a = 9.102(7), b = 13.593(6), c = 8.757(4) Å, α = 102.54(4), β = 106.84(6), γ = 70.74(5)°, V = 970.06 Å 3 , Z = 1, D c = 1.241 Mg · m −3 , λ (Mo K α ) = 0.71069 Å, μ = 1.19 cm −1 , F (000) = 380, R = 0.057 for 2270 observed reflections and 234 variables. The presence of benzene in TPPO · OA · B has the effect of limiting the rotation freedom of phenyl groups and reducing lattice symmetry compared to that in TPPO · OA.

In order to study the phase transition of the incommensurate phase of NbTe 4 to the commensurate low temperature phase X-ray diffraction experiments have been carried out down to 10 K. The q -vectors and intensity profiles of selected satellite reflections have been studied as a function of temperature. The macroscopic crystals used do not show the lock-in transition which was observed by electron diffraction. Apparently the incommensurate distortion is pinned below the expected transformation temperature.

The crystal structure of (Mn 0.6 Ti 0.4 )(Ga 1.6 Mn 0.4 )S 4 has been solved from X-ray single crystal data and refined to R = 0.036 ( R w = 0.040). The compound crystallizes in the FeGa 2 S 4 type structure of space group P [unk] m 1 [ a = 370.7(1), c = 1215.9(2) pm, V = 0.1447 nm 3 , Z = 1, D x = 3.598 Mg · m −3 ].

An apparent discrepancy (arithmetic crystal class or dichromatic ‘colour’ group?) between two descriptions of the symmetry of the complex Patterson synthesis is reconciled, and attention is drawn to gaps in the current series of International Tables for Crystallography . The symmetry of reciprocal space in the complex case is discussed, and a table of generalized Laue groups is given.

The crystal structure of GeI 4 was refined using 100 K and 297 K X-ray data. GeI 4 crystallizes in the cubic space group P 2 1 / a [unk] (No 205) with a = 11.861(1) Å at 100 K and 12.0303(6) Å at 297 K. Analysis of the displacement parameters gives no hint for rigid body motion.

The title compound contains three independent formula units. There is a large spread of values for chemically equivalent dimensions, notably in the cations, which display tetrahedral coordination at silver; Ag–N 2.228(4)–2.315(5) [av.2.266] Å, N–Ag–N 104.6(2)–115.7(2) [av. 109.6]°.

Optical activity in crystal class [unk]2 m was observed with simple experimental methods for the first time in 1968 with AgGaS 2 because, at a certain wavelength, n e equals n 0 . The present study introduces a further example where this phenomenon can be observed. The title compound has the refractive indices n e = n 0 = 1.543(1) at 512 nm.

The title compound was prepared by esterification of the corresponding dicarboxylic acid with methanol/SOCl 2 . It crystallizes from diethyl ether monoclinically in the space group P 2 1 with a = 1004.2(7), b = 665.0(3), c = 980.3(5) pm and β = 115.06(4)°.