Discovery of three polymorphs of 7-fluoroisatin reveals challenges in using computational crystal structure prediction as a complement to experimental screening

Abstract

A combined computational and experimental polymorph search was undertaken to establish the crystal forms of 7-fluoroisatin, a simple molecule with no reported crystal structures, to evaluate the value of crystal structure prediction studies as an aid to solid form discovery. Three polymorphs were found in a manual crystallisation screen, as well as two solvates. Form I (P2₁/c, Z′ = 1), found from the majority of solvent evaporation experiments, corresponded to the most stable form in the computational search of Z′ = 1 structures. Form III (P2₁/a, Z′ = 2) is probably a metastable form, which was only found concomitantly with form I, and has the same dimeric R²₂(8) hydrogen bonding motif as form I and the majority of the computed low energy structures. However, the most thermodynamically stable polymorph, form II (P, Z′ = 2), has an expanded four molecule R⁴₄(18) hydrogen bonding motif, which could not have been found within the routine computational study. The computed relative energies of the three forms are not in accord with experimental results. Thus, the experimental finding of three crystalline polymorphs of 7-fluoroisatin illustrates the many challenges for computational screening to be a tool for the experimental crystal engineer, in contrast to the results for an analogous investigation of 5-fluoroisatin.