# Depression of the Melting Point in Naturally Grown Circular Crystals

## TL;DR

This paper shows that curled crystals of a compound have a lower melting point due to structural deformations and lattice distortions.

## Contribution

The study reveals a novel connection between crystal shape deformation and melting point depression in organic compounds.

## Key findings

- Curled crystals of ROY exhibit a 1.4 K lower melting point compared to straight crystals.
- Melting point depression correlates with crystal curvature and is inversely related to crystal width for narrow crystals.
- Lattice distortions and conformational changes in curled crystals are linked to lower mechanical stiffness and hardness.

## Abstract

The melting point is one of the most fundamental properties
of
crystalline materials and has been commonly used to determine the
chemical and phase purity of organic compounds. Here, we report a
significant depression in the melting point of about 1.4 K in naturally
grown crystals with a circular shape obtained for one polymorph of
the tetradecamorphic compound 5-methyl-2-((2-nitrophenyl)-amino)-3-thiophenecarbonitrile
(ROY). Crystals of ROY grown by microspace sublimation have peculiar
habits, with irregular, elongated, occasionally bent, or curled habits,
and with one or both of their ends closed into loops. In contrast
to the straight crystals, the Mueller matrix microscopic analysis
suggested the continuous reorientation of electrical dipole moments
in the curled crystals. When heated, these curly crystals often start
to melt at the kink and exhibit a lower melting onset point and a
broader endothermic peak in the thermal (DSC) fingerprint compared
with the regular crystals. The decrease in the melting point was found
to be proportional to the deformation expressed as the curvature of
the crystals; it is also inversely proportional to the crystal width
for narrow crystals, but it is independent of the width for wider
crystals. The bent section of the crystals is mechanically softer
than the straight part, and both the stiffness and hardness are inversely
proportional to the degree of curling, presumably due to defects.
In the three-dimensional reciprocal space, the curled crystals show
diffuse diffraction or streaks due to lattice distortion. Nanoinfrared
spectroscopic signatures indicate that the lattice distortion is related
to the conformational changes of the molecule. The results highlight
the dependence of the extent of crystal deformation on melting properties,
which may have broad implications for modulating properties of pharmaceutical
crystals.