We investigate the
effect of crystal growth within a magnetic field
for three polymorphic pharmaceuticals, using an experiment where the
magnetic field can be varied in strength without altering other crystallization
conditions. In the case of carbamazepine, fields above 0.6 T produce
metastable form I, and for flufenamic acid, there is an increased
propensity to crystallize metastable form I around 1 T. In contrast,
the magnetic field has no effect on the crystallization of mefenamic
acid, a closely related molecule. The growth of the metastable β
polymorph of coronene within a magnetic field at ambient temperature
is difficult to reproduce but has been seen as a minor component,
consistent with this transformation to the more stable form being
facile, depending on the particle size. Calculations of the diamagnetic
susceptibility tensors of the polymorphs and their morphologies provide
semiquantitative estimates of how the diamagnetic susceptibilities
of crystallites differ between polymorphs and explain why mefenamic
acid crystallization is unaffected. As the onset of crystallization
of carbamazepine and coronene, as defined by changes in turbidity,
occur at lower temperatures and hence greater supersaturations in
certain ranges of magnetic field strength, this suggests that the
field causes precipitation of the metastable form through Ostwald’s
rule of stages.