Structure refinement, full-matrix least-squares

Data collection on an Enraf Nonius CAD-4 diffractometer (7796 measured reflections, T = 298 K, 1851 observed reflections, with > 3.0cr(/net))- Program used to solve the structure MULTAN 80 [7]. Program to refine the structure by full matrix least squares SHELX [8]. Molecular graphics ORTEPII [9], GenMol [10]. 

X-ray structural analysis. Suitable crystals of compound 14 were obtained from toluene/ether solutions. X-ray data were collected on a STOE-IPDS diffractometer using graphite monochromated Mo-Ka radiation. The structure was solved by direct methods (SHELXS-86)16 and refined by full-matrix-least-squares techniques against F2 (SHELXL-93).17 Crystal dimensions 0.3 0.2 0.1 mm, yellow-orange prisms, 3612 reflections measured, 3612 were independent of symmetry and 1624 were observed (I > 2ct(7)), R1 = 0.048, wR2 (all data) = 0.151, 295 parameters. 

The chitobiose unit has been treated as a rigid body, and by using the full-matrix, least-squares, rigid-body, refinement procedure, the structure was refined to an R factor of 40.7%. Visually estimated intensities were used. The structure was found to be free from short contacts, and to be stabilized by an intrachain OH-3—0-5 hydrogen-bond and one interchain N-H—O hydrogen-bond. 

Full-matrix least-squares refinement of the structure model was carried out with programs orfls (9). Since this program in the form we used refines structure factors Fhki rather than the intensities of the powder lines, it was necessary to decompose the intensity of each line having more than one component into contributions from the individual component reflections. This was done by assuming that the F°mz 2 for the several components of a powder line were in the same ratios as the corresponding FcftfcZ 2 obtained from a structure model—the original trial structure or the previous... 

The trial structure was refined by using a modified version of the full-matrix least-squares fortran program SFLS (31). The quantity minimized was... 

Sorum [12] to a final R of 0.10. Thus, the reinvestigation of the structure of acetylcholine bromide, [C7H1602N] + Br, with X-ray diffraction intensities collected from two untwinned crystals showed that the crystals are monoclinic, and are characterized by a space group of P21/ra, with a = 10.966 (4), b = 13.729 (7), c = 7.159 (4) A, p = 108.18 (7)°, and Z = 4. The structure was refined by full-matrix least squares calculations using 1730 observed reflections, and anisotropic temperature factors for all non-hydrogen atoms. The final R was found to be 0.041. Atomic coordinates, thermal parameters, bond lengths, and angles were compared with those from a previous work on acetylcholine derivatives. 

A preliminary knowledge of the crystal structure is important prior to a detailed charge density analysis. Direct methods are commonly used to solve structures in the spherical atom approximation. The most popular code is the Shelx from Sheldrick [26] which provides excellent graphical tools for visualization. The refinement of the atom positional parameters and anisotropic temperature factors are carried out by applying the full-matrix least-squares method on a data corrected if found necessary, for absorption and diffuse scattering. Hydrogen atoms are either fixed at idealized positions or located using the difference Fourier technique. 

The crystal structures of 4-(6-nitro-2-benzoxazolyl)phenyl 4-(acryloyloxyhexyloxy) benzoate, 2-[4-A,iV-bis(2-hydroxyethyl)] phenyl-6-nitrobenzoxazole monohydrate (A) and 2- 4-/V-(6-hydroxyhexyl)-A-mcthyl] phenyl-6-nitrobenzoxazole (B) have been determined at room temperature by direct methods and refined by full-matrix least-squares method [210], These compounds are monomer precursors of polymers with nonlinear optical properties of the second order. 

Structural Solution and Refin ent. The structure was solved by analysis of similar structuies and expanded using Fourier maps. All atoms were refined anisotropically. The final cycle of full-matrix least-squares refinement was based on 555 observed reflections (7 > 3.00o(7)) and 70 variable parameters and converged (largest parameter shift was 0.00 times its esd) with unweighted and weighted agreement foctors of... 

The structure was solved by heavy-atom methods at the U.C. Berkeley CHEXRaY facility using full-matrix least-squares refinement procedures detailed elsewhere. Systematically absent reflections were eliminated from the data set, and those remaining were corrected for absorption by means of the calculated absorption coefficient. A three-dimensional Patterson synthesis gave peaks that were consistent with Xe atoms in Wyckoff position 4c and Ge atoms in 4a in space group Pnmb (see Pnma, No. 62). Three cycles of... 

The crystal of compound 1 was grown in ethanol solution at room temperature. A yellow crystal of 1, irregularly shaped, of approximate dimensions 0.45x0.50x0.56 mm, was used for crystal and intensity data collection. The crystal structure of 1 was solved by direct method with SHELXTL [17] and refined by full-matrix least squares on F with anisotropic displacement parameters for all non-H atoms. All H-atoms were generated in idealized positions and refined in a riding model. 

All of the structures tackled have been refined by least squares methods and, with few exceptions, by full-matrix least squares methods. The total computing investment over the past three years amounts to approximately 300 hours of central-processor time on a CDC-6400. When the necessary calculations have exceeded the capacity of the CDC-6400 (about 250 variables), we have turned to a remote hookup with the CDC-7600 at Lawrence Berkeley Laboratory. 

---