Non-merohedral twins

In case of a two-domain non-merohedral twin, two orientation matrices must be determined. The programs DIRAX (Duisenberg, 1992) and GEMINI (Sparks, 1997 Bruker-AXS, 1999) take into account that only a certain fraction of the reflections will fit an individual solution. As a result, a list of possible solutions is presented. If a solution is accepted, all the reflections that do not fit this first solution are placed in a new reflection list to re-run the cell determination process. After the determination of both orientation matrices the twin law can be calculated in a separate step  

In the general case of a non-merohedral twin, there are three different types of reflections reflections that do not overlap with any reflection of the second domain, reflections that overlap exactly with a reflection of the second domain, and finally reflections that overlap partially with a reflection of the second domain. The last type is the most problematic, because usually the degree of overlap is not known and differs from reflection to reflection. When oifly one orientation matrix is used to integrate the data, part of the intensity of the reflection of the second domain is added to the intensity of the reflection of the main domain. 

Using the second orientation matrix, it is possible to integrate the whole intensity of the overlapping reflection. Such an integration is available with SAINT (Bruker, 2001) or EvalCCD (Duisenberg et al. 2003). 

The procedure for SAINT is as follows the program checks whether there is an overlap and tries to integrate the intensity of the individual reflections. The raw reflection file consists of non-overlapped reflections and overlapped reflections split 

Absorption correction, scaling, merging and generation of twin reflection file 

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For non-merohedral twins, the twin law does not belong to the crystal class of the stmcture nor to the metric symmetry of the cell. Therefore the different reciprocal lattices do not overlap exactly. There are some reflections, which overlap or cannot be distinguished from each other, but the majority of the reflections are not affected by the twinning. As shown in Figure 7.8, the diffraction pattern should normally reveal this type of twiiming. 

Fig. 7.9 Typical diffraction pattern of a non-merohedral twin. Next to normal looking reflections there is one reflection, which is split. The intensity profile has been drawn along a line across this reflection.

If only part of the reflections have a contribution from the second domain (twinning by reticular merohedry and non-merohedral twins), a special reflection file is necessary, which is read in by the command... 

The following features are typical of non-merohedral twins, where the reciprocal lattices do not overlap exactly and only some of the reflections are affected by the twinning ... 

In the following sections we present examples of how to refine twinned structures with SHELXL. All files you may need in order to perform the refinements yourself are given on the CD-ROM that accompanies this book. The first example is a case of merohedral twinning that will acquaint you with the basics of practical twin refinement. The second example describes a typical pseudo-merohedral twin such as every crystallographer will encounter sooner or later. Two different examples for twinning by reticular merohedry are given next and the chapter ends with two cases of non-merohedral twinning. 

The data for the structure of 2-(chloro-methyl)p3Tidinium chloride (Jones et al., 2002) was collected on a Bruker SMART 1000 CCD area detector. The normal indexing program failed and split reflections profiles combined with nice profiles and reflections very close to each other indicated that this was a non-merohedral twin. The program CELL NOW easily finds two orientation matrices (see nonm2 cn) ... 

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