Random versus Partially Ordered Solid Solutions

Random versus Partially Ordered Solid Solutions.—One of the points that needed to be clarified in order to understand the nature of the interactions at the microscopic level within the disordered phase is whether the heterochiral molecules are randomly distributed in the crystals, in which case the contacts are of (RR), (SS), and RS) natures, or if they are ordered in homochiral stacks, where the contacts are mainly of (RR) and (S5) natures.  

In order to measure the parameter r, we applied a chemical mass-spectrometric method. The racemates were constructed by mixing equimolar amounts of fully protiated enantiomer (-)-(/ ) (2) and deuterium-labelled ( + )-(5) (3). 

Polycrystalline samples of these racemates were irradiated, and the diastereomeric dimers were isolated and submitted to a careful mass-spectrometric analysis. The values of r were deduced from the ratios of the intensities of peaks corresponding to doubly deuteriated (55), partially deuteriated (RS + SR), and non-deuteriated dimers (RR). Parallel investigations of the dependence of r on temperature and time of irradiation have shown that no differential kinetic effect is operative in the dimerization of (RR) [or (55)] with respect to (RS). By applying this method to monomer (2) rapidly cooled from the melt we obtained values of r = I, suggesting a totally random distribution of the enantiomers along the polymerization axis.  

This chemical technique is therefore unique in its advantages, because it affords very detailed information on the microstructure of the disordered phase, even as a function of sample preparation. 

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