Improper Hydrogen-Bonding CH·Y Interactions in Binary Methanol Systems As Studied by FTIR and Raman Spectroscopy

Mixed-Matrix Organo-Silica-Hydrotalcite Membrane for CO2 Separation Part 1: Synthesis and Analytical Description

Hydrotalcite exhibits the capability to adsorb CO2 at elevated temperatures. High surface area and favorable coating properties are essential to harness its potential for practical applications. Stable alcohol-based dispersions are needed for thin film applications of mixed membranes containing hydrotalcite. Currently, producing such dispersions without the need for delamination and dispersing agents is a challenging task. This work introduces, for the first time, a manufacturing approach to overcoming the drawbacks mentioned above. It includes a synthesis of hydrotalcite nanoparticles, followed by agent-free delamination of their layers and final dispersion into alcohol without dispersing agents. Further, the hydrotalcite-derived sorption agent is dispersed in a matrix based on organo-silica gels derived from 1,2-bis(triethoxysilyl)ethane (BTESE). The analytical results indicate that the interconnection between hydrotalcite and BTESE-derived gel occurs via forming a strong hydrogen bonding system between the interlayer species (OH groups, CO32-) of hydrotalcite and oxygen and silanol active gel centers. These findings lay the foundation for applications involving incorporating hydrotalcite-like compounds into silica matrices, ultimately enabling the development of materials with exceptional mass transfer properties. In part 2 of this study, the gas separation performance of the organo-silica and the hydrotalcite-like materials and their combined form will be investigated.

Optimization of Intercalated 2D BiOCl Sheets into Bi2WO6 Flowers for Photocatalytic NH3 Production and Antibiotic Pollutant Degradation

Photocatalytic N2 fixation is a complex reaction, thereby prompting researchers to design and analyze highly efficient materials. Herein, one-pot hydrothermal Bi2WO6-BiOCl (BW-BiOCl) heterojunctions were synthesized by varying the molar ratio of tungsten: chlorine precursor. Major morphological transformations in BiOCl were observed wherein it turned from thick sheets ∼230 nm in pure BiOCl to ∼30 nm in BW-BiOCl. This was accompanied by extensive growth of {001} facets verified from X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) analyses. A p-n heterojunction was formed between Bi2WO6 and BiOCl evidenced via photoluminescence (PL), time-resolved photoluminescence (TRPL), photocurrent response, and electrochemical impedance spectroscopy (EIS) analyses. The formation of heterojunction between Bi2WO6 and BiOCl led to the reduction of the work function in the BW-BiOCl 0.25 hybrid confirmed via ultraviolet photoelectron spectroscopy (UPS) analysis. BW-BiOCl 0.25 could produce ammonia up to 345.1 μmol·L-1·h-1 owing to the formation of a robust heterojunction with an S-scheme carrier transport mechanism. Recycle tests resulted in no loss in N2 reduction activities with post-catalytic analysis, showcasing the high stability of the synthesized heterojunction. Novel performance was owed to its excellent chemisorption of N2 gas on the heterojunction surface verified by N2-temperature programmed desorption (TPD). BW-BiOCl 0.25 also displayed a superior rate constant of 3.03 × 10-2 min-1 for 90 min CIP degradation time, higher than pristine BiOCl and Bi2WO6. Post-photocatalytic Fourier transform infrared (FTIR) spectroscopy of BW-BiOCl 0.25 revealed the presence of C-H stretching peaks in the range of 2850-2960 cm-1 due to adsorbed CIP and methanol species in CIP degradation and N2 fixation, respectively. This also confirmed the enhanced adsorption of reacting species onto the heterojunction surface.

Hydrogen bond interactions in the binary solutions of formamide with methanol: FTIR spectroscopic and theoretical studies

FTIR spectroscopic studies on the binary solutions of formamide with methanol reveal the presence of "free" O - H in methanol. These "free" O - H groups are found in methanol tetramers which is confirmed from the DFT calculations. DFT calculations on the formamide dimers of five different geometries encompassing one or more of the N - H⋯O, C - H⋯O and N - H⋯N hydrogen bonds tell that N - H⋯N bonds are the strongest. Dissociation of the. N - H⋯N bonds of formamide in the binary solutions with methanol has a major impact on the NH₂ symmetric stretching mode of formamide in the FTIR spectra. In these solutions the formation of 1:4 (formamide:methanol), 1:5 and 2:4 complexes are possible. These complexes are more stable than the formamide dimers, methanol tetramer and pentamer investigated in the present work. Methanol methyl group plays no role in either the self-association or heterointeraction with formamide.

FTIR studies, DFT calculations and time domain reflectometry studies on tetrahydrofuran - methanol binary solutions

FTIR signature of neat tetrahydrofuran (THF), methanol (MeOH) and their binary solutions at various molefractions (0.8:0.2 (THF:MeOH), 0.6:0.4, 0.4:0.6 and 0.2:0.8) have been recorded. Density Functional Theory (DFT) calculations have also been carried out on THF, MeOH multimers and possible THF-MeOH complex molecules. The results of FTIR studies and DFT calculations confirm the formation of 1:3 (THF:MeOH) and 1:4 complex structures with (MeOH)O - H⋯O(THF), (MeOH methyl)C - H⋯O(THF) and (THF methylene)C - H⋯O(MeOH) H-bond interactions. (MeOH)O - H⋯O(MeOH) homointeractions among the MeOH trimers and tetramers are stronger than the THF-MeOH heterointeractions in complexes. But, the stability of 1:4 complexes is more than that of the trimers and tetramers as inferred from the interaction energy values obtained by DFT calculations. Time Domain Reflectometry (TDR) study has also been carried out on the THF-MeOH binary solutions in the frequency range 10 MHz-30 GHz at 298 K. The results of dielectric studies have been correlated with that of FTIR studies and DFT calculations.

Probe of Alcohol Structures in the Gas and Liquid States Using C⁻H Stretching Raman Spectroscopy

Vibrational spectroscopy is a powerful tool for probing molecular structures and dynamics since it offers a unique fingerprint that allows molecular identification. One of important aspects of applying vibrational spectroscopy is to develop the probes that can characterize the related properties of molecules such as the conformation and intermolecular interaction. Many examples of vibrational probes have appeared in the literature, including the azide group (⁻N₃), amide group (⁻CONH₂), nitrile groups (⁻CN), hydroxyl group (⁻OH), ⁻CH group and so on. Among these probes, the ⁻CH group is an excellent one since it is ubiquitous in organic and biological molecules and the C⁻H stretching vibrational spectrum is extraordinarily sensitive to the local molecular environment. However, one challenge encountered in the application of C⁻H probes arises from the difficulty in the accurate assignment due to spectral congestion in the C⁻H stretching region. In this paper, recent advances in the complete assignment of C⁻H stretching spectra of aliphatic alcohols and the utility of C⁻H vibration as a probe of the conformation and weak intermolecular interaction are outlined. These results fully demonstrated the potential of the ⁻CH chemical group as a molecular probe.

Determination of size of molecular clusters of ethanol by means of NMR diffusometry and hydrodynamic calculations

The microscopic structure of ethanol in the liquid state is characterized as a dynamic equilibrium of hydrogen-bonded clusters of different sizes and topologies. We have developed a novel method for determination of the average size of the clusters that combines the measurement of diffusion coefficient by means of NMR diffusometry technique and hydrodynamic simulations. The approach includes the use of HydroNMR [J. Garcı̀a de la Torre, M. L. Huertas, and B. Carrasco, J. Magn. Reson. 147, 2000, 138] for small molecules, which is attained here by the calibration procedure using a dilute solution of tetramethylsilane. It is thus possible to correlate the experimentally determined diffusion coefficient of ethanol with calculated diffusion coefficients of the modeled clusters of different sizes. We found that average size of the clusters in 0.16 M solution of ethanol in n-hexane corresponds to the monomer above 300 K and to the pentamer/hexamer below 240 K. The clusters in the case of 0.44 M solution are generally slightly larger, from the average size corresponding to the dimer at 320 K and the hexamer at 210 K.

Computational study of proper and improper hydrogen bonding in methanol complexes

The bulk properties of alcohols, like those of aqueous solutions, are governed mostly by hydrogen bonding; however, in contrast with water molecules, the chemical structure of a simple alcohol such as methanol offers an opportunity to explore the effects of both proper and improper hydrogen bonding on a single hydrogen donor. The presence of the hydroxyl group generally gives rise to a strong proper hydrogen bond, while the methyl group of methanol is likely involved in the weaker improper hydrogen bond, among other weak non-covalent interactions. The effects of the two types of hydrogen bonds on the stability, geometric parameters, and properties of electron density of methanol complexes are examined while considering different geometrical arrangements of the methanol dimer and the binary complexes of methanol with water, acetonitrile, and chloromethane. Subsequently, potential conclusions about the nature of improper hydrogen bonding and the origin of the C–H bond contraction that results upon complex formation are discussed. Quantum theory of atoms in molecules and natural bond orbital methods were used in the analysis; all calculations were performed at the MP2(full)/6-311++G(d,p) level of theory.

Identification of spectroscopic patterns of CH...O H-bonds in proteins

Ab initio calculations are used to identify characteristics of vibrational and NMR spectra that signal the involvement of a protein backbone in a CH...O H-bond and that distinguish this sort of interaction from other H-bonds in which a protein might participate. Glycine and alanine dipeptides, in both their C7 and C5 minimum-energy structures, are paired with formamide in a number of different H-bonding arrangements. The CH...O H-bond is characterized by a small contraction of the C-H bond length, along with a blue shift in its stretching frequency, accompanied by an intensification of this vibrational band. In the context of NMR spectra, the bridging CH proton's chemical shift is moved downfield by 1-2 ppm. The aforementioned features are not produced by other H-bonds in which the protein backbone might participate, such as NH proton donation or accepting a proton via the peptide C=O.