Ordered atomic positions assignment

In contrast to the r -methcxl, where unsubstituted atomic positions are indeterminable, r0-derived methods can locate also unsubstituted atoms, although with less accuracy than for substituted positions. In practical cases, this capacity is limited to one or two unsubstituted atoms and yields useful results only when the SDS is otherwise large enough. Coordinates that are (for whatever reason) not included in a r0-type fit must be assigned estimated values, which are kept fixed in the fit but affect the result. In these cases, one should not do so without also setting error limits to these coordinates in order to find out, by Eq. 30, which additional errors of the positions are due to the error afflicted fixed positions. If symmetrically equivalent atomic positions are among those kept fixed, the relations established between the independent and the expanded set of variables (Eq. 56b) now denoted as B(A) and B, respectively, are applicable in the form ... 

If the same alkyl group occurs more than once as a side chain, this is indicated by the prefixes di-, tri-, tetra-, etc. Side chains are cited in alphabetical order (before insertion of any multiplying prefix). The name of a complex radical (side chain) is considered to begin with the first letter of its complete name. Where names of complex radicals are composed of identical words, priority for citation is given to that radical which contains the lowest-numbered locant at the first cited point of difference in the radical. If two or more side chains are in equivalent positions, the one to be assigned the lowest-numbered locant is that cited first in the name. The complete expression for the side chain may be enclosed in parentheses for clarity or the carbon atoms in side chains may be indicated by primed locants. 

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