Resorcinol Crystallization from the Melt: A New Ambient Phase and New “Riddles”

Structures of the α and β
phases of resorcinol, a major
commodity chemical in the pharmaceutical, agrichemical, and polymer
industries, were the first polymorphic pair of molecular crystals
solved by X-ray analysis. It was recently stated that “no additional
phases can be found under atmospheric conditions” (Druzbicki,
K. et al. J. Phys. Chem. B 2015, 119,
1681). Herein is described the growth and structure of a new ambient
pressure phase, ε, through a combination of optical and X-ray
crystallography and by computational crystal structure prediction
algorithms. α-Resorcinol has long been a model for mechanistic
crystal growth studies from both solution and vapor because prisms
extended along the polar axis grow much faster in one direction than
in the opposite direction. Research has focused on identifying the
absolute sense of the fast directionthe so-called “resorcinol
riddle”with the aim of identifying how solvent controls
crystal growth. Here, the growth velocity dissymmetry in the melt
is analyzed for the β phase. The ε phase only grows from
the melt, concomitant with the β phase, as polycrystalline,
radially growing spherulites. If the radii are polar, then the sense
of the polar axis is an essential feature of the form. Here, this
determination is made for spherulites of β resorcinol (ε,
point symmetry 222, does not have a polar axis) with additives that
stereoselectively modify growth velocities. Both β and ε
have the additional feature that individual radial lamellae may adopt
helicoidal morphologies. We correlate the appearance of twisting in
β and ε with the symmetry of twist-inducing additives.