The application of direct methods to centrosymmetric structures containing heavy atoms. II

Abstract

813 |040|[~oI] the proposed structure for the high temperature form will be about 1.78 A for the shorter and 2.83 A for the longer bond. The important transformation vectors can be seen in Fig. 5. This mechanism will lead to smaller and more regular IO3 pyramids compared to the room-temperature form. As the contracting I-O bond is nearly parallel to b, it must cause a small shrinkage parallel to b. (Received 12 May 1975; accepted23 June 1975) Direct methods are applied to the difference structure factors for a structure containing one or more heavy atoms in known positions. For those reflexions whose sign is determined by the heavy atoms, the known heavy-atom contribution is subtracted from the observed structure factor to obtain the magnitude and the sign of the light-atom contribution. The signs of the reflexions that do not have an appreciable heavy-atom contribution are found using the Y.2 sign relationship. For reflexions with an intermediate heavy-atom contribution the sign and magnitude ambiguity is also solved by the Y.2 formula. Thus it is possible to maximize the number of correct signs, and correct some of the amplitudes, before a difference Fourier map is calculated.

The application of direct methods to non-centrosymmetric structures containing heavy atoms

“…In paper I (Beurskens & Noordik, 1971) and in paper II (Gould, van den Hark & Beurskens, 1975) direct methods were used to solve the phase problem, or to speed up the solution, for centrosymmetric structures containing one or more heavy atoms on known positions. The present paper deals with the non-centrosymmetric case.…”

Statistical properties of normalized difference-structure factors for non-centrosymmetric structures

“…The procedure used in our present programs is: (a) The rest structure is centrosymmetric, lEvi must be equal to either I EII or I E zl. For all reflections, calculate the probabilities P1 and P2 (Gould, van den Hark & Beurskens, 1975) and sum P~ and P2 in the appropriate ranges of I E~ I. …”

“…The statistical properties of difference structure factors in centrosymmetric structures have been described (Woolfson, 1956;Gould, van den Hark & Beurskens, 1975). The discussion here is for space group P1, but is applicable to general reflections in other space groups if the symmetry enhancement factor e is included.…”

Structures of α- and β-copper(II) pipecolinate dihydrate, diaquabis(2-piperidinecarboxylato)copper(II), [Cu(C6H10NO2)2(H2O)2]

“…Density of a bulk sample was determined pycnometrically in toluene. (Beurskens & Noordik, 1971;Gould, van den Hark & Beurskens, 1975;van den Hark, Prick & Beurskens, 1976) for the non-hydrogen atoms. Leastsquares refinement on F, initially with block-diagonal approximation and isotropic to anisotropic thermal parameters.…”