Vibrational spectroscopic and computational studies on diisopropylammonium bromide

Hydrogen-bonded structures and low temperature transitions of the confined water in subnano channels

The interfacial and confined water have long been attractive objects due to their crucial roles in biological, geological processes, etc. In this paper, we investigate the hydrogen-bonded structures of water and their low temperature transitions in the subnano channels of AlPO4-11 for the first time on the basis of infrared spectroscopy. The number of the adsorbed water molecules is estimated to be 8.45 per channel in one unit cell by thermogravimetric analysis. It is found that the confined water molecules are involved in saturated and unsaturated coordination with different hydrogen bond strengths at ambient temperature. The former refers to ice-like four-coordinated water and the latter includes liquid-like structures, Al-coordinated and relatively free water molecules. Unique coordination between water molecules and framework Al sites is responsible for the ice-like structures in the channels above the ice melting point. The appearance of liquid-like structures is closely related to the strong channel confinement, which does not allow the formation of extensive tetrahedral hydrogen-bonded configuration. As temperature decreases, a structural transformation of confined water happens in the channels of AlPO4-11. Isolated small water oligomers and two new components with stronger hydrogen bonds, such as low-density amorphous ice-like structures and a kind of low-density liquid-like structures are preferred. Our results provide important insights into the structural organizations and thermal-dynamic behaviors of confined water in extreme narrow channels.

High-Pressure Structural Phase Transformation of Ferroelectric Bis-benzylammonium Lead Tetrachloride Studied by Raman Spectroscopy and X-ray Diffraction

Hybrid organic-inorganic 2-D perovskite bis-benzylammonium lead tetrachloride (BALC) is a room-temperature ferroelectric semiconductor. A structural phase transformation from the ambient Cmc2₁ structure is evident at 1.8 GPa from the Raman spectra, and this is confirmed by our high-pressure X-ray diffraction studies that point to a centrosymmetric structure Cmcm at 1.7 GPa. The ambient phase is recoverable on decompression. Using density functional theory calculations, we have studied the intermolecular and intramolecular vibrations to get an idea of the structural changes as a function of pressure. The high-pressure transition is identified to be due to a distortion in the PbCl₆ octahedra and a conformation change in the molecule. There are several discontinuities, broadening, and splitting of the Raman bands, corresponding to NH₃ units above 1.8 GPa that point to rearrangements in the hydrogen bond network in the new phase. The ambient structure shows anisotropic compressibility, with a bulk modulus of 14.5 ± 0.33 GPa. As the new phase is a centrosymmetric structure, BALC is expected to lose its ferroelectricity above ∼1.8 GPa.

Vibrational Dynamics of Crystalline 4-Phenylbenzaldehyde from INS Spectra and Periodic DFT Calculations

The present work emphasizes the value of periodic density functional theory (DFT) calculations in the assessment of the vibrational spectra of molecular crystals. Periodic calculations provide a nearly one-to-one match between the calculated and observed bands in the inelastic neutron scattering (INS) spectrum of crystalline 4-phenylbenzaldehyde, thus validating their assignment and correcting previous reports based on single molecule calculations. The calculations allow the unambiguous assignment of the phenyl torsional mode at ca. 118-128 cm-1, from which a phenyl torsional barrier of ca. 4000 cm-1 is derived, and the identification of the collective mode involving the antitranslational motion of CH···O bonded pairs, a hallmark vibrational mode of systems where C-H···O contacts are an important feature.

Understanding the vibrational spectra of crystalline isoniazid: Raman, IR and INS spectroscopy and solid-state DFT study

This work presents a comprehensive spectroscopic analysis of crystalline isoniazid, one of the main drugs in tuberculosis chemotherapy, using a blend of spectroscopic and computational methods. Mid- and far-infrared, Raman, and inelastic neutron scattering spectroscopies, with contribution of isotopic substitution are combined with discrete and periodic DFT quantum chemical calculations. This combined approach successfully reproduces the whole spectral range, allowing a sound assignment of all the vibrational bands. Previous misassignments have been corrected and several spectral features of isoniazid crystal are reported for the first time. Virtues and limitations of the computational approach (periodic and discrete) are also discussed in light of the present state-of-the-art in the field.