Time-Domain Terahertz Spectroscopy and Solid-State Density Functional Theory Analysis of p-Nitrophenol Polymorphs

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Accurate interpretations of THz vibrational spectra using density functional theory (DFT) must account for the often significant thermal expansion exhibited by molecular crystal systems. In this study, the THz spectra of two p-nitrophenol polymorphs were obtained in the spectral bandwidth of 20–95 cm−1, and solid-state DFT was used to assign the observed absorptions to specific vibrational mode characters. Computational treatments of the crystal systems involved comparison of phonon frequencies produced from both fixed- and full-unit cell geometry optimizations using a variety of functional and basis set combinations. Arbitrary frequency scaling was applied to overcome the temperature dependence of unit cell volumes and associated phonon vibrational frequencies. Fully relaxed cell geometries producing contracted cell volumes resulted in a large overestimation of THz vibrational frequencies. Employing appropriate frequency scaling factors to the computed mode frequencies allowed for proper vibrational mode assignments and enhanced the quality of spectral simulations versus fixed-cell calculations. Optimal frequency scalars in the range of 0.675 to 0.827 were determined to best reproduce the THz spectra of the p-nitrophenol polymorphs. This treatment of calculated phonon modes offers the key advantage of eliminating the temperature correlation between the crystal structure data and the THz data acquisition.