Polymorphism

Hydrogen/deuterium (H/D)-exchange, the smallest possible modification of a molecule, is generally seen as a non dominating parameter in the formation of crystal  structures of chemical compounds. On the other hand, the aggregation of specific molecules in the solid state is very sensitive on small changes of  the  isotopic substitution pattern of the molecules. Therefore, ROY was deuterated at its amine function, which leads to a seemingly small but effective modification of the hydrogen bond (HB) strength. In contrast to the crystallization of the non-deuterated ROY in methanol or ethanol which leads simultaneous to the formation of two forms (OP and Y-polymorph), so-called concomitant polymorphs, the crystallization of d1-ROY leads to a selective formation of exclusively the Y-polymorphic form. The preferred aggregation behavior of the Y-form of d1-ROY is assigned to the weakening of an intramolecular HB and a consequently strengthened intermolecular HB after deuteration.

J. Falk, D. W. M. Hofmann, K. Merz, IUCrJ, 5 (2018) 569.