DFT-Assisted Polymorph Identification from Lattice Raman Fingerprinting

A combined
experimental and theoretical approach, consisting of
lattice phonon Raman spectroscopy and density functional theory (DFT)
calculations, is proposed as a tool for lattice dynamics characterization
and polymorph phase identification. To illustrate the reliability
of the method, the lattice phonon Raman spectra of two polymorphs
of the molecule 2,7-dioctyloxy[1]­benzothieno­[3,2-b]­benzothiophene are investigated. We show that DFT calculations of
the lattice vibrations based on the known crystal structures, including
many-body dispersion van der Waals (MBD-vdW) corrections, predict
experimental data within an accuracy of ≪5 cm–1 (≪0.6 meV). Due to the high accuracy of the simulations,
they can be used to unambiguously identify different polymorphs and
to characterize the nature of the lattice vibrations and their relationship
to the structural properties. More generally, this work implies that
DFT-MBD-vdW is a promising method to describe also other physical
properties that depend on lattice dynamics like charge transport.