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Issue 4, 2008
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Discovery of three polymorphs of 7-fluoroisatin reveals challenges in using computational crystal structure prediction as a complement to experimental screening

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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[1 with combining macron], 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.

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

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Publication details

The article was received on 24 Sep 2007, accepted on 03 Dec 2007 and first published on 02 Jan 2008


Article type: Paper
DOI: 10.1039/B714566G
Citation: CrystEngComm, 2008,10, 399-404
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    Discovery of three polymorphs of 7-fluoroisatin reveals challenges in using computational crystal structure prediction as a complement to experimental screening

    S. Mohamed, S. A. Barnett, D. A. Tocher, S. L. Price, K. Shankland and C. K. Leech, CrystEngComm, 2008, 10, 399
    DOI: 10.1039/B714566G

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