Low-angle data

Clearly the first three of these criteria do not apply to most electron diffraction data sets where a resolution of iK is common, low angle data may be missing and where accuracy is limited by poor crystalline specimens. 

Inspection of the residuals ordered in ranges of (sin )/X, Fob and parity and values of (he individual indexes showed no unusual features or trends, other than those caused by (he strong pseudotranslation of V2C in (he structure. There was no indication of secondary extinction in the high-intensity low-angle data. 

Fcaic( )> such as occurs for example in low-angle terms when the solvent has not been included in the calculations (i.e., F aicfli) is considerably overestimated), then the map will contain errors. Omission of the low-angle data will destroy the continuity of the electron density and the distinction between the solvent and the protein. Secondly, although the phases acomb have been suitably weighted to minimise bias from the calculated structure, no such weighting has been applied to the amplitude coefficients. These problems have been considered by Stuart and Artymiuk [147], who have shown that a map based on coefficients... 

While a neutron study is preferred, the large crystal size required and the limited number of groups involved in these studies have meant that in most cases structural determinations have been carried out by X-ray methods. Unfortunately, the hydrogens often escape detection or, even when detected, cannot often be reliably located by X-ray methods, especially in the neighborhood of one of the heavier transition metals, or when disorder is present. Collection of data at low temperature, and use of low angle data, seem to improve one s chances. From X-ray data collected at 200 K, for example, Howard and co-workers were able to locate all six hydrogens in WHs(PPr2Ph)3. 

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