The Same but Different: Isostructural Polymorphs and the Case of 3‑Chloromandelic Acid

The expression “isostructural polymorphs” would appear to be an impossible combination of two mutually incompatible words. “Isostructural” implies a high degree of structural similarity; however, conversely, “polymorph” implies structural distinguishability. The structures of two newly determined polymorphs of 3-chloromandelic acid nevertheless justify the use of this expression, for they differ only in crystal symmetry and hardly at all in molecular position or conformation within the crystalline lattice. We demonstrate that parameters derived by the XPac program can be useful in establishing the limits of isostructurality.