Understanding the Metastability of Theophylline FIII by Means of Low-Frequency Vibrational Spectroscopy

 Paiva, Eduardo Maia, Qi Li, Adam J. Zaczek, Claudete F. Pereira, Jarbas José Rodrigues Rohwedder, and J. Axel Zeitler. "Understanding the Metastability of Theophylline FIII by Means of Low-Frequency Vibrational Spectroscopy." Molecular Pharmaceutics (2021).

Abstract
While theophylline has been extensively studied with multiple polymorphs discovered, there is still currently no conclusive structure for the metastable theophylline form III. In this present work, by combining more widely used techniques such as X-ray diffraction and thermogravimetric analysis with more emerging techniques like low-frequency Raman and terahertz time-domain spectroscopy, to analyze the structure and dynamics of a crystalline system, it was possible to provide further evidence that the form III structure has a theophylline monohydrate structure with the water molecules removed. Solid-state density functional theory simulations were paramount in proving that this proposed structure is correct and explain how vibrational modes within the crystal structures feature and govern polymorphic transitions and the metastable form III. Through the insight provided by both simulated and experimental results, it was possible to decisively conclude the elusive crystal structure of theophylline form III. It was also shown that the correct space group for theophylline monohydrate is not P21/n but, in fact, Pc.

for full paper see https://pubs.acs.org/doi/full/10.1021/acs.molpharmaceut.1c0047

.......THz-TDS experiments were used to track the entire dehydration process of FM by placing the sample pellet under vacuum or by purging the sample chamber that contained the sample with a dry nitrogen gas overnight, both of which achieved the same results. In these cases, the terahertz transmission spectra were acquired with the TeraPulse 4000 spectrometer (TeraView Ltd.) at a continuous measurement interval of 10 s per spectrum for measurements acquired during a dehydration under vacuum and 5 min.....

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Terahertz-infrared dielectric properties of lead-aluminum double-cation substituted single-crystalline barium hexaferrite

 Ahmed, Asmaa, Anatoly S. Prokhorov, Vladimir Anzin, Denis Vinnik, Sergei Ivanov, Adam Stash, Y. S. Chen, Alexander Bush, Boris Gorshunov, and Liudmila Alyabyeva. "Terahertz-infrared dielectric properties of lead-aluminum double-cation substituted single-crystalline barium hexaferrite." arXiv preprint arXiv:2108.08571 (2021).

Abstract 

Hexaferrite materials are highly demanded to develop and manufacture electronic devices operating at radio- and microwave frequencies. In the light of the prospects for their use in the forthcoming terahertz electronics, here, we present our results on the terahertz and infrared dielectric response of a typical representative of hexaferrites family, lead-substituted M-type barium hexaferrite doped with aluminum, Ba0.2Pb0.8AlxFe12-xO19, x(Al)=0.0, 3.0, and 3.3. We studied uniquely large and high-quality single crystals of the compounds prepared by spontaneous crystallization growth technique. Our aim was to explore the effect of aluminum substitution on the dielectric response of the compounds. Systematic and detailed investigations of the dependences of terahertz-infrared (frequencies 8 - 8000 cm-1 ) spectra of complex dielectric permittivity on the temperature, 4 - 300 K, and on the chemical composition, x(Al)=0.0, 1.2, 3.0, 3.3, were performed for two principal polarizations of the electric field E-vector of the probing radiation relative to the crystallographic c-axis, namely E||c and Ec. Furthermore, infrared phonon resonances are recorded and discussed. In contrast to undoped BaFe12O19, no softening of the lowest frequency A2u phonon is observed, indicating suppression of a displacive phase transition in substituted compounds. A number of resonance absorption bands are discovered at terahertz frequencies and assigned to transitions between energy levels of the fine-structured ground state of Fe2+ ( 5 E) ions. The temperature and aluminum-doping dependences of the resonances are analyzed with an account taken of disorder introduced by aluminum. Basing on dielectric data and detailed X-ray experiments, we find that for all concentrations of Al3+ ions, x(Al)=0.0, 1.2, 3.0, and 3.3, they mainly occupy the 2a and 12k octahedral site positions and that the degree of substitution of iron in tetrahedral positions is not substantial. Along with fundamental findings, the obtained data on broad-band dielectric properties of Ba0.2Pb0.8AlxFe12-xO19 crystals provides the information that can be used for development and manufacture of electronic devices with operating frequencies lying in the terahertz spectral band. Keywords M-type hexaferrite, multiferroics, dielectric properties, site occupancy of Al in BaM, Fourier transform infrared spectroscopy, terahertz time-domain spectroscopy.

for full paper see https://arxiv.org/ftp/arxiv/papers/2108/2108.08571.pdf

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In-situ Observation of the Structure of Crystallising Magnesium Sulfate by Terahertz Spectroscopy

 

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