Showing papers in "Acta Crystallographica in 1963"

On the crystal structure of the manganese(III) trioxides of the heavy lanthanides and yttrium

Abstract: The preparation, determination of optical properties, and approximate crystal structure of the ferroelectric manganese (III) trioxides of Ho, Er, Tm, Yb, Lu, and Y are reported. The compounds crystallize in the hexagonal system, probable space group P6₃ cm, and their structure is characterized by unusual five- and sevenfold coordination polyhedra about manganese and rare-earth atoms, respectively. The unacceptable values assumed by the temperature factors of two oxygen ions throw doubt on the exact determination of atom positions by least- squares refinement. Possible causes of this difficulty are reviewed. (auth)

The crystal and molecular structure of a hydrogen-bonded complex between 1-methylthymine and 9-methyladenine

Abstract: Crystals of a 1:1 hydrogen-bonded complex between 1-methylthymine and 9-methyladenine can be grown from an aqueous solution containing equimolecular quantities of the two compounds. The crystals are monoclinic, with a--8-304, b = 6.552, c = 12.837 A, and fl--106 ° 50'. The space group is P21/m, with two base-pair complexes in the unit cell. The structure was refined with three-dimensional data taken with copper radiation. The positional coordinates and anisotropic temperature factors of the heavy atoms were obtained by least-squares analyses. The hydrogen atoms, except those of two methyl groups, were located from a three-dimensional difference Fourier synthesis. The 1-methylthymine and 9-methyladenine molecules form a planar base pair lying in a mirror plane and are connected to each other by two nearly linear hydrogen bonds, from the NH 2 group of 9-methyladenine to 0(9) of 1-methylthymine (2.846 A) and from N(3) of 1-methylthymine to N(7) of 9-methyladenine (2.924 A). This structure differs from the adenine-thymine pairing proposed by Watson & Crick, where N(3) of thymine is hydrogen bonded to N(i) of adenine. The distance between the methyl group at N(1) of 1-methylthymine and the one at N(9) of 9-methyladenine is 8-645 A, whereas this distance is 11.1 /~ in the pairing proposed by Watson & Crick.

The secondary extinction correction

Abstract: It is shown that Darwin's formula for the secondary extinction correction, which has been universally accepted and extensively used, contains an appreciable error in the x-ray diffraction case. The correct formula is derived. As a first order correction for secondary extinction, Darwin showed that one should use an effective absorption coefficient mu + gQ where an unpolarized incident beam is presumed. The new derivation shows that the effective absorption coefficient is mu + 2gQ(1 + cos/sup 4/2 theta )/(1 plus or minus cos/sup 2/2 theta )/s up 2/, which gives mu + gQ at theta =0 deg and theta = 90 deg , but mu + 2gQ at theta = 45 deg . Darwin's theory remains valid when applied to neutron diffraction. (auth)

The structure of dimolybdenum carbide by neutron diffraction technique

Abstract: : It is generally believed that Mo2C has either the hexagonal C6 Cadmium iodide anti-type structure or the related hexagonal L'3 structure. A neutron diffraction study showed that this is not the case. Mo2C crystallizes with an orthorhombic unit cell with a = 4.72 sub 4A, b = 6.00 sub 4A, and c = 5.19 sub 9A. The atomic arrangement of Mo2C presents a new structure type. Space group is D(14)2h - Pbcn. Eight molybdenum atoms are in 8d) with x = 1/4, y = 1/8, z = 1/12 and four carbon atoms are in 4c) with y = 3/8. The carbon atoms in Mo2C arrange themselves in such a way, that each molybdenum atom has three nearly planar carbon neighbours. (Author)

A nuclear quadrupole resonance and X-ray study of the crystal structure of 2,5-dichloroaniline

Abstract: : The crystal structure of 2,5-dichloroaniline, C6H3Cl2NH2, was determined by the combined application of nuclear quadrupole resonance and X-ray diffraction The molecule is planar, except for the nitrogen which deviates slightly, by 006 A The benzene ring is slightly deformed from the regular hexagon and the observed bond lengths agree well with the result of a simple molecular orbital calculation There is no hydrogen bonding in the structure The shortest intermolecular ClCl distance, 337 A, is shorter than the usually quoted van der Waals distance of 36 A (Author)

Analytic constants for atomic scattering factors

Abstract: New and revised values of the analytical constants A, a, B, b, and C for both CuK alpha and MoK alpha radiation are presented. Analytical constants for atomic scattering factors derived from self-consistent or variational wave functions and from Thomas-Fermi-Dirac statistics are tabulated. (P.C.H.)