In situ Raman spectroscopy and synchrotron X-ray diffraction studies on maleic acid under high pressure conditions

Maleic acid was studied by Raman spectroscopy and powder synchrotron X-ray diffraction (XRD) under high pressure conditions by using a diamond anvil cell. The Raman spectroscopy measurements were performed from ambient pressure up to 9.2 GPa in the 100-3200 cm-1 spectral range. While the XRD measurements were performed up to 10.1 GPa. Here we present the pressure-dependence behavior from both the Raman modes and cell parameters. Maleic acid lattice parameters decrease anisotropically as a function of pressure and a reduction of 27% in the volume of the unit cell was observed. Modifications in the material's compressibility were observed at around 2 and 6 GPa.

Raman Evidence of Multiple Adsorption Sites and Structural Transformation in ZIF-4

The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10^-6 Torr) conditions, facilitated by C-H···π interactions. In the gaseous environment of N₂ and CO₂ around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C-H stretching modes and low-frequency modes (<200 cm^-1). In contrast to a single vibrational mode generally reported for entrapped N₂, we show three Raman modes of adsorbed N₂ in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C-Hs are the preferred adsorption sites on ZIF-4 in the following order: C4-H, C5-H > C2-H > center of the Im ring (interacting with C-H centers) > center of the cavity. We also show that with an increasing concentration of N₂ adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.

Mechanism of Polysulfone-Based Anion Exchange Membranes Degradation in Vanadium Flow Battery

The stability of hydrocarbon ion exchange membranes is one of the critical issues for a flow battery. However, the degradation mechanism of ion exchange membranes has been rarely investigated especially for anion exchange membranes. Here, the degradation mechanism of polysulfone based anion exchange membranes, carrying pyridine ion exchange groups, under vanadium flow battery (VFB) medium was investigated in detail. We find that sp(2) hybrid orbital interactions between pyridinic-nitrogen in 4,4'-bipyridine and benzylic carbon disrupt the charge state balance of pristine chloromethylated polysulfone. This difference in electronegativity inversely induces an electrophilic carbon center in the benzene ring, which can be attacked by the lone pair electron on the vanadium(V) oxygen species, further leading to the degradation of polymer backbone, while leaving the 4,4'-bipyridine ion exchange groups stable. This work represents a step toward design and construction of alternative type of chemically stable hydrocarbon ion exchange membranes for VFB.

p-Aminobenzoic acid polymorphs under high pressures

The effect of high pressure on two forms (α, β) of p-aminobenzoic acids (PABA) is studied in a diamond anvil cell using in situ Raman spectroscopy.

High-pressure study of lithium amidoborane using Raman spectroscopy and insight into dihydrogen bonding absence

One of the major obstacles to the use of hydrogen as an energy carrier is the lack of proper hydrogen storage material. Lithium amidoborane has attracted significant attention as hydrogen storage material. It releases ∼10.9 wt% hydrogen, which is beyond the Department of Energy target, at remarkably low temperature (∼90 °C) without borazine emission. It is essential to study the bonding behavior of this potential material to improve its dehydrogenation behavior further and also to make rehydrogenation possible. We have studied the high-pressure behavior of lithium amidoborane in a diamond anvil cell using in situ Raman spectroscopy. We have discovered that there is no dihydrogen bonding in this material, as the N-H stretching modes do not show redshift with pressure. The absence of the dihydrogen bonding in this material is an interesting phenomenon, as the dihydrogen bonding is the dominant bonding feature in its parent compound ammonia borane. This observation may provide guidance to the improvement of the hydrogen storage properties of this potential material and to design new material for hydrogen storage application. Also two phase transitions were found at high pressure at 3.9 and 12.7 GPa, which are characterized by sequential changes of Raman modes.

High-pressure study of tetramethylsilane by Raman spectroscopy

High-pressure behavior of tetramethylsilane, one of the Group IVa hydrides, was investigated by Raman scattering measurements at pressures up to 142 GPa and room temperature. Our results revealed the phase transitions at 0.6, 9, and 16 GPa from both the mode frequency shifts with pressure and the changes of the full width half maxima of these modes. These transitions were suggested to result from the changes in the inter- and intra-molecular bonding of this material. We also observed two other possible phase transitions at 49-69 GPa and 96 GPa. No indication of metallization in tetramethylsilane was found with stepwise compression to 142 GPa.

High-pressure Raman spectroscopic study of the ammonia-borane complex. Evidence for the dihydrogen bond

The Raman spectra of the ammonia-borane complex, NH(3)BH(3), have been investigated as a function of pressure up to 40 kbar. Vibrational modes involving the NH(3) group show negative pressure dependences, supporting the existence of the dihydrogen bond, but the vibrations of the BH(3) group have a positive dependence. Two transitions were observed in the solid phase under pressure, in contrast to the temperature behavior, where a single transition occurs. Factor group splitting occurs for the degenerate vibrations, and this allows the correct assignment of the observed vibrations.

Effect of high external pressures on the vibrational spectra of biomedical materials: calcium hydroxyapatite and calcium fluoroapatite

Infrared and Raman spectra of the principal mineral component of human hard tissue, calcium hydroxyapatite, Ca10(PO4)6(OH)2, or HAP, and the analogous calcium fluoroapatite, Ca10(PO4)6F2, or FAP, have been recorded, using a diamond-anvil cell, at pressures ranging from ambient to 30 kbar. For FAP, the absence of any discontinuities in the slopes of the nu (cm-1) versus P (kbar) plots for the observed bands indicates that no pressure-induced structural transition occurs in this material throughout the pressure range investigated. For the internal vibrational modes of HAP, however, there are distinct breaks at approximately 20 kbar in the nu versus P plots, suggesting the occurrence of a structural change at this pressure. The OH stretching mode of HAP shifts to higher wave numbers with increasing pressure while the associated OH librational mode shifts in the opposite direction. The pressure-induced structural transition in HAP is reversible and occurs at approximately 22 kbar upon decompression. Further evidence for a structural change taking place at approximately 20 kbar was provided by a parallel pressure-tuning Raman study. Hydrogen bonding does not occur, or is very weak, in HAP.