Showing papers in "Acta Crystallographica in 1961"

Hartree–Fock atomic scattering factors for the iron transition series

Abstract: Atomic scattering factors for thirty-five atoms and ions of the iron group transition series (Sc through Cu) were determined from Watson's calculation of Hartree-Fock wave functions and compared with previous calculations. Principal scattering factors were computed, following the methods of Freeman, for the 3d electrons of those atoms containing nonspherical charge distributions. As was shown by Weiss and Freennan, these factors are all that are needed for determining (from the measured neutron form factor) the spatial symmetry of the outer electrons in magnetic materials. Argon core scattering factors were calculated, and, on the basis of these new results, the question of the constancy of the argon core is discussed quantitutively. (auth)

The crystal structure of myoglobin: Phase determination to a resolution of 2 Å by the method of isomorphous replacement

Abstract: The method of multiple isomorphous replacement has been used to solve the structure of the protein myoglobin to a resolution of 2 A. EDSAC II has been programmed to calculate the phase angles, and the statistical treatment of errors proposed by Blow & Crick has been used throughout, leading not to the most probable electron-density map but to the one with the least mean square error over the entire unit cell. Problems inherent in adapting the process for a digital computer are discussed, and examples are given of probability curves for typical phase determinations. A method of refining heavy-atom parameters in the course of phase determination is presented. Comparisons of the mostprobable and least-error electron-density maps are presented in the region of the haem group, and it is shown that the latter map gives slightly better results. In an appendix a means of trial-anderror least-squares refinement of heavy-atom parameters using data from a centrosymmetric projection is discussed.

A theoretical study of pendellösung fringes. I. General considerations

Abstract: A dynamical theory of diffraction is formulated by Kato for a general type of monochromatic incident wave.

La structure des colloïdes d'association. III. Description des phases mésomorphes des savons de sodium purs, rencontrées au‐dessus de 100°C

Abstract: We are indebted to Prof. A. B. Burg for supplying us with crystals of the compound and Prof. Donohue for some of the Xray diffraction photographs. We wish to thank the Univers i ty of Minnesota for t ime on the Univac 1103 computer, and the Li t tauer Statistical Labora to ry of H a r v a r d Universi ty for assistance in the use of the IBM 704 computer. This research was supported in par t by the Office of Nava l Research and the Office of Ordnance Research. We also wish to thank the Up.john Company and the Dupont Corporation for fellowships to L.M.T. and the Dut)ont Corporation for a fellowship to F .S .M.

The crystal structure of thymine monohydrate

Abstract: The crystal structure was refined on the basis of three-dimensional intensity data obtained with copper radiation. The structural parameters for the heavy atoms, including anisotropic temperature factors, were refined by the least-squares method; the positions of the hydrogen atoms were located from a three-dimensional difference synthesis. The final R factor was 0.078 for 1068 observed reflections and the standard deviations in the positional parameters for the heavy atoms are about 0-0023 A. A surprising feature is the amount of the water of crystallization, which is about 83% of the theoretical value. An arrangement of the water molecules is proposed which accounts for the observed X-ray data.

A neutron diffraction refinement of the low temperature phase of NaNO2

Abstract: The structural nature of the spontaneous polarization of NaNO/sub 2/ below its transition point was investigated by neutron diffraction. A set of (Okl) single crystal data was collected at room temperature. The positional coordinates and their standard deviations were compared with those of Truter and Carpenter. The spontaneous polarization calculated on the basis of simple Na/sup +/, N/sup 3+/ and O/sup 2-/ ions was 74 mu coulombs/ cm/sup 2/ which was off by an order of magnitude from the observed value of 7 mu coulombs/cm/sup 2/. This was a result of the highly covalent nature of the nitrite group. (M.C.G.)

Gadolinium and dyprosium intermetallic phases. II. Laves phases and other structure types

Abstract: Unit-cell dimensions and interatomic distances are given for the following phases: MgCu/sub 2/ type: GdX/sub 2/ and DyX/sub 2/ where X = Pt, Mn, Fe, Co, Ni and Al; CsCl type: GdX and DyX where X = Ag, Cu, In, Tl and Al; AuCu/ sub 3/ type: GdIn/sub 3/, DyIn/sub 3/, DyTl/sub 3/, DyPt/sub 3/; MoSi/sub 2/ type: GdAg/sub 2/, DyAg2/sub 2/ i GdAu/sub 2/, DyAu/sub 2/; CuTi/sub 3/ type: Dy/ sub 3/In; CaZn/sub 5/ type: GdNi/sub 5/, DyNi/sub 5/, GdCo/sub 5/, DyCo/sub 5/; AlB/sub 2/ type: GdGa/sub 2/, DyGa/sub 2/; SnNi/sub 3/ type: GdAl/sub 3/. (auth)

The single isomorphous replacement method

Abstract: 1195 and refinement was usually complete after ten to fifteen cycles. The refined values of the parameters compared well with the Rossmann programme values, except for the effective atomic numbers, which were uniformly smaller than indicated by the Rossmann programme. The Hart hO1 values were used in the 2 /~. Fourier synthesis, although there may be some question remaining about the proper Z values to use, as evidenced by negative regions in the myoglobin Fourier synthesis at the three heavy-atom sites and some blurring of detail in the immediate neighbourhood. This problem is currently being investigated. Thanks are due to Dr R. G. Hart, whose heavy-atom refinement programme proved invaluable, and to Dr IV[. G. Rossmann, whose least-squares refinement provided an independent check on the parameters. We should also like to express our appreciation to Miss Mary Pinkerton, whose assistance was invaluable at all stages of the work. Despite the phase ambiguity which arises when one isomorphous pair is used to determine phases in a non-centrosymmetric structure, a single pair of compounds can be used to give an interpretable Fourier synthesis. Two conditions must be satisfied: the replacing atoms must themselves form a non-centrosymmetric array, and a sufficient number of terms must be available. The method has been applied to the crystalline proteins haemoglobin and myoglobin. Examples are given which show the improvements which can be made by the use of weighting functions and by the introduction of anomalous-dispersion data. The isomorphous-replacement method, when applied in its conventional form to a non-centrosymmetrie structure, leads to an ambiguous result for the phase angle (Bokhoven et al., 1951). A general method of removing the ambiguity is to employ a series of compounds with isomorphous replacements at different sites, and this method has been used successfully with the proteins myoglobin and haemoglobin (Kendrew et al., 1960; Perutz et al., 1960). However, the preparation of suitable isomorphous protein derivatives has been a matter of great difficulty, and is likely to remain the most time-consuming step in a protein structure determination. It is therefore important to find methods which use the minimum number of iso-morpheus derivatives. Rogers (1951) suggested a procedure applicable when only one isomorphous pair is available. Although in the form proposed it only applies to different atoms substituting at the same site, it may readily be generalized. A synthesis is calculated in which each term is given the phase of the scattering …

Magnetic structures of 3d transition metal double fluorides, KMeF3

Abstract: The antiferromagnetic structures of KCrF3, KMnF3, eF3, KCoF3, KNiF3, and KCuF3 were obtained by neutron fraction. The manganese, iron, cobalt, and nickel doufluonides exhibited an order in which the divalent 3d ion as coupled antiferromagnetically to its six nearest neighbors. In the chromium double fluonide, the magnetic structure consisted of ferromagnetic (001) sheets, coupled antiferromagnetically along the STA001! direction. No magnetic ning was observed in the neutron d fraction pattern copper double fluoride. The magnetic structures and served crystallographic distortions are discussed in of an indirect exchange mechanism and in terms of the crystal field theory, respectively. (auth)

The crystal structure of gadolinium trichloride hexahydrate

Abstract: GdCl/sub 3/ - 6 H/sub 2/O is monoclinic with space group P2/n and two molecules in the cell; a = 9.651, b = 6.525, c = 7.923 A; beta = 93.65 deg . All atomic positions (except for hydrogen) were determined with high precision. The structure contains complexes STACl/sub 2/Gd(OH/sub 2/)/sub 6/! which are held together by 0-H... Cl bonds. Thus one third of the chlorine atoms are not bonded to gadolinium atoms. The only symmetry element of a complex is a twofold axis. The bond lengths are: Gd-C1=2.768, Gd-0 =2.39-2.42, 0-H... C1=3.143.24 A. (auth)