Time-Domain Terahertz Spectroscopy and Solid-State Density Functional Theory Analysis of p -Nitrophenol Polymorphs
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Time-Domain Terahertz Spectroscopy and Solid-State Density Functional Theory Analysis of p-Nitrophenol Polymorphs Thiago H. da Silva 1 & Neilson R. Rexrode 1 & Matthew D. King 1,2 Received: 13 September 2019 / Accepted: 4 December 2019/ # Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
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. Keywords Terahertz spectroscopy . Solid-state DFT . Density functional theory . Frequency scalar . p-Nitrophenol
* Matthew D. King [email protected]
1
Micron School of Materials Science and Engineering, Boise State University, 1910 University Drive, Boise, ID 83725, USA
2
Department of Chemistry and Biochemistry, Boise State University, 1910 University Drive, Boise, ID 83725, USA
Journal of Infrared, Millimeter, and Terahertz Waves
1 Introduction Low-frequency terahertz (THz) spectra of molecular crystals can be rich with information on structure and dynamics of a crystal system, but the proper analysis requires accurate computational treatment of the periodic systems. Interpretation of THz spectral data through unambiguous vibrational mode assignment can provide valuable information about the crystal system in regards to polymorph contamination [1–3], identification of impurities [4, 5], hydration/ solvation processes [6–8], structural transitions [9, 10], and anisotropic behavior of structural, mechanical, and vibrational properties [11, 12]. The computational treatment, however, is dependent on the availability of known or predicted crystal structures to serve as the starting point for solid-state computations. The requirement of ancillary structural data is often times problemati
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