Molecular speciation controlling stereoselectivity of additives:: Impact on the habit modification in α-glycine crystals

The demonstration of stereoselective habit modification in molecular crystals using "tailor-made" ( or structurally related) additives, over the last two decades, has been a milestone in understanding the phenomenon of molecular recognition at crystal interfaces. The centrosymmetric alpha-glycine crystal has provided a classic example in earlier studies for elucidating the mechanisms of such stereoselective processes. In these previous studies, with chirally resolved ( L or D-amino acids) and racemic alpha-amino acids (DL-amino acids) as tailor-made additives, habit modification was observed to be along the enantiopolar b-axis of alpha-glycine crystals. Revisiting this work has, however, revealed additional habit modification along the c-axis of the alpha-glycine crystal with certain alpha-amino acids as additives ( viz. aspartic acid and glutamic acid). In the presence of L-Asp and L-Glu, the (0 (1) over bar1) and (0 (1) over bar(1) over bar) faces were of morphological importance and with the D-amino acids, the ( 011) and (01 (1) over bar) faces were well-developed. On the basis of the fact that these amino acids exist in two charged states (zwitterion and anion) and building on the stereoselectivity mechanism, it is surmised that the zwitterionic species interact along the b-axis of the alpha-glycine crystal and the anions alter their interaction from the crystallographic b-axis to the c-axis, due to electronic repulsive forces acting at the docking sites on the ( 010) and (0 (1) over bar0) faces. In this paper, we have used optical microscopy, molecular modeling, and IR spectroscopy to demonstrate and explain the newly observed habit modification.