Insight into Solvent-Dependent Conformational Polymorph Selectivity: The Case of Undecanedioic Acid

Here we report the polymorph nucleation of undecanedioic acid (UDA) having high solvent-dependent selectivity. Solvents with high hydrogen bond donating (HBD) ability preferred to produce form II, while form I was obtained from solvents with no HBD ability. Cooling experiments in a series of binary solvents with an HBD ability value from 0 to 86 were designed to affirm the strong correlation between solute–solvent interactions and polymorph formation verified by solution Fourier transform infrared (FTIR) spectroscopy and quantitative powder X-ray diffraction. Two crystal structures, evaluated as conformational dimorphs assisted by FTIR, solid-state NMR, and quantum chemistry calculations, were reported. Furthermore, the crystal structures suggest that UDA molecules from various solutions went through different conformation rearrangements resulting in two forms during nucleation. Meanwhile, we reveal that there is no direct connection between the solution chemistry of UDA in solvents with various HBD strengths and the corresponding forms. The results imply that the difficulty of desolvation, closely linked with solute–solvent hydrogen-bonding interaction, markedly affected the degree of conformation rearrangement and the nucleation outcome.