Distribution of molecular pairs in Z' = 2 structures.

The positions of pairs of independent molecules in Z' = 2 structures have been surveyed for six of the most populated space groups for that class of structure. These results have been compared with the Z' = 1 situation to reveal whether there are any fundamental differences in the construction of the asymmetric units in these cases. The results indicate that, broadly speaking, the packing of the molecular pairs in Z' = 2 structures resembles that of single molecules in structures with Z' = 1; this similarity may be chiefly attributed to the constraints imposed by the symmetry operators of the space group. However, there are key differences, which are particularly marked in the space groups with higher symmetry, that indicate that the asymmetric units in Z' = 1 and Z' = 2 structures are not directly comparable. In those cases where the positions of the pair centroids in Z' = 2 structures are similar to the positions of molecular centroids for Z' = 1 structures, it follows that the molecular centroids in Z' = 2 structures must follow a different distribution. A different pattern is produced if the independent molecules in Z' = 2 structures behave like the individual molecules in Z' = 1 structures. These two scenarios combine to form the observed distributions of pair centroid positions.