9α-Fluorocortisol is a steroid containing 27 nonhydrogen atoms which crystallizes in space group P212121 with one molecule per asymmetric unit in a cell having the dimensions a = 10.087, b = 23.710, and c = 7.660Å. The procedure used to obtain an initial set of 49 phases for this structure will be described in detail because it illustrates the use of many different phase determining formulas which have been discussed in this monograph. It is an especially pleasing example because all of the deductions based on the predicted cosine seminvariants proved to be correct, and this resulted in a set of 49 phases for two-dimensional reflections which were all correct. This set of 49 phases was input to the tangent formula which was used to determine 201 additional phases, and 26 of the nonhydrogen atoms were found among the highest 35 peaks on the E map based on all 250 phases. The strength of the particular set of base phases which were used is demonstrated by the fact that, throughout the tangent cycles, multiple minima were encountered in the modified tangent minimization function only twice. Wherever possible, enough data are given that alternative methods of phasing this structure can be investigated, and although it is undoubtedly true that many such approaches would be ultimately successful, the merit of an approach which requires inspection of a single Fourier map, in which nearly the entire structure can be found among the strongest peaks, is obvious.
KeywordsLarge Contributor Triple Product Nonhydrogen Atom Space Group Symmetry Pair Average
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