Computational investigations of the influence of polymorphism on the lattice dynamics of molecular organic crystals

Abstract

The teraHertz range (60 GHz 4 THz = 2-130 cm) is a relatively unexplored, but information-rich part of the vibrational spectrum of molecular crystals. The motions in this frequency range are at lower energies than most of the internal vibrations ofmolecules (e.g., bond stretching and angle bending), but correspond instead to translations and librations of molecules. Thus, spectroscopic and computational investigations of this frequency range are a source of information on the intermolecular interactions in molecular crystals. Lattice dynamics calculations are being developed within the crystal structure modelling program DMAREL[1-2], for the use of elaborate model intermolecular potentials. Such calculations have been performed to characterise the measured teraHertz spectra of several molecular organic crystals, correlating regions of the spectra to distortions of certain intermolecular interactions. One of the systems studied is the polymorphic pharmaceuticalmolecule carbamazepine, whose basic hydrogen bonding amide dimers remains unchanged between polymorphs (Figure).