Twinned Crystals of Enantiomorphous Morphology of Racemic Alanine Induced by Optically Resolved .alpha.-Amino Acids; A Stereochemical Probe for the Early Stages of Crystal Nucleation

Polymer‐Induced Alignment of dl‐Alanine Nanocrystals to Crystalline Mesostructures

“…The dl-alanine morphologies observed for the single crystal, based on the work of Lahav and co-workers [59][60][61] (Figure 9a), and the suggested morphology of the primary building unit of the mesocrystals (Figure 9b), demonstrate the morphology changes to the latter. Instead of the needlelike structure obtained in the absence of polymer (Figure 9a), the lowering of interface energies by polymer adsorption, as discussed, results in a platelet with strongly exposed (001) and (00-1) faces.…”

“…It is already known that dl-alanine is a polar crystal along the c axis; it displays well-developed "capped (201) and (011) faces" at one end of the polar c axis where ÀNH 3 + groups are exposed, and at the opposite flat (00-1) face, ÀCOO À groups are exposed. [59][60][61] We have depicted the molecular surface structure of dl-alanine, as viewed along the b and c axis, by using the program Cerius www.chemeurj.org 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim tion were not considered; the surfaces were constructed simply from the well-known unit cell of dl-alanine.…”

“…The (201) and (011) faces can be easily identified as polar surfaces, from which four amino units per unit cell extrude. [59][60][61] The most polar surface, however, is the (001) face. Because the c axis in dl-alanine is the dipole axis, the (001) face is polar, with one side rich in carboxylic acids and the counterface rich in amines.…”

Understanding and control of nucleation, growth, habit, dissolution and structure of two- and three-dimensional crystals using `tailor-made' auxiliaries

“…The other system involved preferential crystallization from aqueous solution of the "),-form of glycine, when grown in the presence of the additive racemic hexafluorovaline, (CF3)2CHCH(NH~)COf (Weissbuch, Leiserowitz & Lahav, 1994). As already mentioned, glycine usually crystallizes in the centrosymmetric aform from aqueous solutions, but Iitaka (1961) reported that glycine can precipitate in a polar -)'-form from acetic acid or ammonia solutions.…”

Aspects of spontaneous separation of enantiomers in two- and three-dimensional crystals

“…The formation of the racemic nuclei appeared to be inhibited and the chiral nuclei were formed, but secondary epitaxial growth on the chiral nuclei eventually gave rise to twinned racemic crystals. 25 All in all, it would appear, therefore, that the lattice energy of a pair of enantiomorphs must be significantly lower than that of the corresponding racemate for spontaneous separation of enantiomers to occur.…”