Low-frequency (10 – 300 cm-1) Raman spectroscopy (LFRS) is an increasingly common method for investigating the molecular and intermolecular vibrations of complex crystalline solids. These vibrations are generally large-amplitude internal torsions and hindered external rotations/translations that collectively result in material-specific LFRS fingerprints. A common application is the rapid detection of crystalline polymorphs in solid-state drug manufacturing, where non-destructive techniques like LFRS offer analytical advantages. A complication of LFRS of solid samples is the difficulty associated with the interpretation of the data because the observed spectral features cannot be assigned by the usual group frequency tables employed at higher frequencies. In LFRS, solid-state density functional theory (ss-DFT) is the current standard for interpreting and assigning these spectra since the use of periodic boundary conditions yields both the internal and external vibrations of the molecular crystals being simulated. While these spectral fingerprints can be successfully reproduced using ss-DFT, the calculations are not trivial in terms of time and resources, easily taking weeks of time over hundreds of computer processors. Raman scattering intensities are also highly sensitive to the computational parameters used because of their reliance on polarizability tensor quality, rather than the dipole moment derivatives used for infrared-active vibrations. Consequently, establishing a reliable ss-DFT methodology for the accurate and practical simulation of LFRS data is important for its broader usability. In this work, the experimental LFRS spectrum of crystalline carbazole serves as the target for these simulations due to the numerous well-resolved peaks it exhibits and the relatively small size of the molecule (C12H9N). The effects of several ss-DFT parameters have been explored, ranging from basis set size and composition, to density functional form including both generalized gradient approximation and hybrid approaches. It is found that the incorporation of Hartree-Fock exchange in hybrid functionals is particularly important to computing realistic Raman scattering intensities.

Computational methodologies for the accurate simulation of Raman intensities in the low-frequency vibrational spectra of molecular crystals

2019 Fall National Meeting

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