Errors, crystal structures

The parameters in the original parameterization are adjusted in order to reproduce the correct results. These results are generally molecular geometries and energy differences. They may be obtained from various types of experimental results or ah initio calculations. The sources of these correct results can also be a source of error. Ah initio results are only correct to some degree of accuracy. Likewise, crystal structures are influenced by crystal-packing forces. 

The next step is to obtain geometries for the molecules. Crystal structure geometries can be used however, it is better to use theoretically optimized geometries. By using the theoretical geometries, any systematic errors in the computation will cancel out. Furthermore, the method will predict as yet unsynthesized compounds using theoretical geometries. Some of the simpler methods require connectivity only. 

Looking at the crystal structures of the monofluorophosphates, it is surprising, that the three PO bonds are of different lengths. These differences are out of the limits of error at least in case of the very exactly calculated data of Ca[P03F] 2 H2O and (NH4)2[P03F] H2O (26) (see Table 3). In fact this leads to a Cj symmetry for the POgF ion. 

Fig. 5 Scatterplot derived from the CSD reporting the N- I-C angle (deg) vs. the N- I distance (A) for crystal structures containing intermolecular N- I contacts only error-free and non-polymeric structures containing single-bonded iodine atoms and showing no disorder with R < 0.06 are considered. The scatterplot clearly demonstrates the high directionality of the N- I XB...

Triorganoboron complexes of Af,0-heterocyclic carbenes are also known (36). Thus, triphenylboron-carbene complex 48 can be isolated from the reaction of methanol with an isocyanide adduct of BPh3 (47) in the presence of a catalytic amount of KF. The X-ray crystal structure of 48 demonstrated that the boron atom is tetrahedrally coordinated and that all four B-C bond distances are equal within experimental error. 

Considering an estimated experimental error of 10-20% for the STD data as well as some systematic errors in the estimates from resonance overlap, the analysis suggests a generally reasonable correspondence between the crystal structure and the solution state. At the same time, our data suggest that the crystallographic structure of the complex may not be entirely compatible... 

To solve a crystal structure by direct methods, difficult data are those which are incomplete in the sampling of reciprocal space, have non-atomic i.e. < 1.3A resolution) and are noisy with large (systematic) errors in the data measurements. As we have seen, this definition spans many electron diffraction data sets, but there are some of sufficient quality that they can be solved routinely using conventional direct methods packages. Often these are of inorganic materials or intermetallic compounds that are relatively resistant to radiation damage. 

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