Bijvoet difference

Table 3. Determination of the Absolute Configuration by Analysis of Bijvoet Differences for Compounds Containing Only Oxygen Atoms as Anomalous Scatterers...

Kraut, J. Bijvoet-difference Fourier function. J. Molec. Biol. 35, 511-512 (1968). 

Hooft RWW, Straver LH, Spek AL. Determination of absolute structure using Bayesian statistics on Bijvoet differences. J. Appl. Cryst. 2008 41 96-103. 

From which the p s can be determined by fitting the measured Bijvoet and dispersive differences, along with the knowledge of the / and /" values, as described in Krishna Murthy (1996) and... 

The Friedel pairs are a subset of the so-called Bijvoet pairs, i.e., pairs of any two reflections which are equivalent by Laue symmetry but not by the point group symmetry of the crystal (see Table 2 a full listing of these reflections is available4 1). All Bijvoet pairs may be used for a comparison, but since the measured intensity differences between general Bijvoet pairs are more likely to be affected by systematic error, the conclusion based on their comparison may be less reliable than in the case of Friedel pairs. 

If two isomorphous derivatives in which the replaceable atoms have considerably different diffracting powers are available, an improved form of this method can be used. Strychnine sulphate pentahydrate and the corresponding selenate form isomorphous monoclinic crystals of space group C2 Bokhoven, Schoone, and Bijvoet (1951) solved the centrosymmetric b projection by the straightforward method first used... 

It can be calculated in this way that in NaCl 9.98 electrons are to be found around a Na+ ion and 17.72 electrons round a Cl- ion, almost equal to what would be expected for ions, namely 10 and 18 electrons. For a crystal consisting of light atoms and in which the differences in numbers of electrons between an atom and an ion are relatively larger, as in LiF and a fortiori in LiH, the existence of ions has been deduced in a simpler way from the X-ray intensities. The reasoning, however, is open to serious criticism (Bijvoet). 

The overall effect of resonant scattering is to cause the break down of Friedel s law that the Bijvoet pairs of reflections S(/i) and S(—h) are unequal The difference... 

A third difficulty is the question of absolute polarity. The structure is too nearly centric for there to be any dramatic differences in the Bijvoet pairs, and the differences in agreement indices ate not very decisive. It is even more difficult to exclude the possibility of two orientations mixed by twinning. Had the measurements been confined to one octant, as is often the custom, the polar dispersion effect A would have caused a serious uncertainty. With full data that effect disappears, and nearly the same results are obtained with either assumption of polarity. 

---