Direct structure analysis of a paraffin solid solution.

Single microcrystals of a 1:1 solid solution of n-C32H66/n-C36H74 have been grown by epitaxial nucleation on benzoic acid, and 0kl electron diffraction patterns from them can be obtained with a lamellar spacing characteristic of the intermediate n-paraffin n-C34H70. This average structure is also indicated by indices (space group Pca21) for major intensities in this zone. Direct phasing of the intensity data is carried out by combined use of low-dose electron microscope lattice images (to assign values to the "lamellar" reflections) and three-phase structure invariant relationships (to find values for the "polyethylene" reflections). The computed electrostatic potential map closely resembles the crystal structure of n-C34H74, for which all 34 atom positions can be found. It is apparent, however, that lower atomic occupancies at the chain ends correspond to anticipated disorder at the lamellar interface. Structural refinement based on this occupancy results in a good match to the observed intensity data.