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.