Coupling of molecular orientation with the hydrogen-bond dimensions and H-sites in carboxylic acids

Chitosan-based composite hydrogel with a rigid-in-flexible network structure for pH-universal ultra-efficient removal of dye

Polysaccharide-based hydrogel adsorbents become popular because of their high adsorption capacity and fast adsorption rate, but their low removal rate and poor pH resistance have always been fatal shortcomings. Herein, a feasible strategy was proposed to strengthen the ability of hydrogel adsorbent to remove organic pollutants (i.e., dye) by incorporating natural rectorite (REC) into chitosan-g-poly (2-acrylamido-2-methyl-propane-sulfonic-acid) hydrogel network to form a rectorite-in-polymer network structure. The introduction of less dosage of REC (1.2 wt%) into the hydrogel facilitates to improve its adsorption capacities toward methylene blue (MB) in deionized water, tap water, seawater, Yangtze River water, and Yellow River water (1083.39-1303.49 mg/g); while incorporating higher content of REC (15.8 wt% REC) helps to improve the removal rate (99.6 % for MB in real waters), which are greatly superior to commercial activated carbons. The adsorbent keeps high adsorption efficiency in a broad pH range (2-11), and can be reused for >4 times.

Potential energy barrier for proton transfer in compressed benzoic acid

Benzoic acid (BA) is a model system for studying proton transfer (PT) reactions. The properties of solid BA subject to high pressure (exceeding 1 kbar = 0.1 GPa) are of particular interest due to the possibility of compression-tuning of the PT barrier. Here we present simulations aimed at evaluating the value of this barrier in solid BA in the 1 atm – 15 GPa pressure range. We find that pressure-induced shortening of O⋯O contacts within the BA dimers leads to a decrease in the PT barrier, and subsequent symmetrization of the hydrogen bond. However, this effect is obtained only after taking into account zero-point energy (ZPE) differences between BA tautomers and the transition state. The obtained results shed light on previous experiments on compressed benzoic acid, and indicate that a common scaling behavior with respect to the O⋯O distance might be applicable for hydrogen-bond symmetrization in both organic and inorganic systems.

Pressure-induced shortening of O⋯O contacts within the dimers of solid benzoic acid leads to a decrease in the PT barrier and subsequent symmetrization of the hydrogen bond (an effect obtained only after taking into account the ZPE correction).

Influence of Oligopeptide Length and Distribution on Polyisoprene Properties

The tuning of binding modes of polar groups is the key step to mimicking the structure and properties of natural rubber through the molecular design of synthetic polyisoprenes. Herein, the ordering and binding distances of oligopeptides could be altered systematically by changing their lengths and distribution along the polyisoprene chain, which impose huge impacts on the mechanical properties and chain dynamics of green rubber. In detail, a series of peptide-functionalized polyisoprenes with terminal blocks (B-2A-PIP, B-3A-PIP) or random sequences (R-2A-PIP, R-3A-PIP) are fabricated by using dipeptides (2A) or tripeptides (3A) as crosslinkers to explore the mechanism of terminal interaction on mechanism properties and chain dynamics. B-4A-PIP and R-4A-PIP served as control samples. It is found that the increased oligopeptide length and the block distribution improves the mechanical properties and confine the chain movement by elevate the contents of ordered and compact microstructures, which is indicated by XRD, broadband dielectric spectroscopy (BDS) and consistent with the result of molecular dynamics simulation. New relaxation signals belonging to oligopeptide aggregates are found which showed elevated dielectric strengths upon temperatures increase. Additionally, it also reveals that the binding modes of oligopeptide do not significantly influence the entanglements of polyisoprene.

Pressure-imposed changes of benzoic acid crystals

Structural and energetic properties of benzoic acid crystals at pressure elevated from ambient condition up to 2.21 GPa were characterized. The directly observed variations of cell parameters and consequently cell volume are associated with many other changes including energetic, geometric, and electronic characteristics. First of all the non-monotonous change of lattice energy are noticed with the rise of pressure since the increase of stabilization up to 1GPa is followed by systematic decrease of lattice energies after extending the hydrostatic compression. There is also an observed increase of C₂²(8) synthon stabilization interaction with increase of pressure. The lattice response rather than interaction within synthons are source of observed pressure-related trend of lattice energy changes. The energy decomposition analysis revealed that the total steric interactions determine the overall trend of lattice energy change with the rise of pressure. Besides geometric aromaticity index was used as a measure of geometric changes. Serious discrepancies were noticed between HOMA values computed with the use of experimental and optimized geometries of the ring. Even inclusion of uncertainties of experimental geometries related to limited precision of X-ray diffraction measurements does not cancel mentioned discrepancies. Although HOMA exhibit similar trends at modest pressures the diversity became surprisingly high at more extreme conditions. This might suggest limitations of periodic DFT computations at elevated pressures and the experimentally observed breaking of molecules at very high pressures will probably not be accounted properly in this approach. Also limitation of direct use of experimental geometries were highlighted.

Structural, spectroscopic and theoretical studies of dimethylphenyl betaine complex with two molecules of 2,6-dichloro-4-nitro-phenol

The 1:2 complex (1) of dimethylphenyl betaine (DMPB) with two molecules of 2,6-dichloro-4-nitro-phenol (DCNP) was prepared and characterized by X-ray diffraction, B3LYP/6-311++G(d,p) and B3LYP-D3/6-311++G(d,p)calculations, FTIR and NMR spectroscopies. The crystal is monoclinic, space group P21/c with Z=4. The protons at the oxygen atoms of phenols are bonded to each oxygen atoms of the DMPB carboxylate group by two nonequivalent H-bonds with the OH⋯O distances of 2.473(5) and 2.688(4)Å. Both H-bonds in the optimized structures 2 (in vacuum), 3 (in DMSO solution) and dispersion-correlated functional (D3) 4 (in vacuum) are comparable and are slightly shorter than O(6)H(O6)⋯O(2) in the crystal. The FTIR spectrum of 1 shows a broad absorption in the 3400-2000cm(-1) region corresponding to a longer hydrogen bond and a broad absorption in the 1800-500cm(-1) region caused by the shorter H-bond. The relations between the experimental (13)C and (1)H chemical shifts (δexp) of the investigated compound 1 in DMSO solution and GIAO/B3LYP/6-311++G(d,p) magnetic isotropic shielding constants (σcalc) obtained by using the screening solvation model (COSMO) for 3 are linear and reproduce well the experimental chemical shifts described by the equation: δexp=a+b σcalc.

Chemical aspects of the primary ionization mechanisms in matrix-assisted laser desorption ionization

It has been proposed that the primary ionization mechanism occurring in matrix-assisted laser desorption ionization (MALDI) experiments originates from the presence, in the solid-state matrix-analytes sample, of matrix dimers. These species are formed by the interaction of carboxylic groups present in the matrix molecules with the formation of strong hydrogen bonds. Theoretical calculations proved that the laser irradiation of these structures leads to one or two H-bridge cleavages, giving rise to an "open" dimer structure or to disproportionation with the formation of MH(+) and [M-H](-) species. The ions so formed can be considered highly effective in their reaction with analyte ions, leading to their protonation (or deprotonation). To achieve further evidence for these proposals, in the present study the energetics of the reactions of ions from different aromatic carboxylic acids with two amino acids (glycine and lysine) and three multipeptides (gly-gly, gly-gly-gly and gly-gly-gly-gly) was investigated. The lowest ∆G values were obtained for 2,5- dihydroxybenzoic acid, widely employed as the MALDI matrix. Also, for p-nitrobenzoic acid the reaction is slightly exothermic, while for the other aromatic carboxylic acids derivatives positives values of ∆G are present.

On the Primary Ionization Mechanism(s) in Matrix-Assisted Laser Desorption Ionization

A mechanism is proposed for the first step of ionization occurring in matrix-assisted laser desorption ionization, leading to protonated and deprotonated matrix (Ma) molecules ([Ma + H](+) and [Ma - H](-) ions). It is based on observation that in solid state, for carboxyl-containing MALDI matrices, the molecules form strong hydrogen bonds and their carboxylic groups can act as both donors and acceptors. This behavior leads to stable dimeric structures. The laser irradiation leads to the cleavage of these hydrogen bonds, and theoretical calculations show that both [Ma + H](+) and [Ma - H](-) ions can be formed through a two-photon absorption process. Alternatively, by the absorption of one photon only, a heterodissociation of one of the O-H bonds can lead to a stable structure containing both cationic and anionic sites. This structure could be considered an intermediate that, through the absorption of a further photon, leads to the formation of matrix ions. Some experiments have been performed to evaluate the role of thermal ionization and indicate that its effect is negligible. Some differences have been observed for different matrices in the formation of analyte molecule (M) ion [M + H](+), [M - H](-), M(+•), and [M - 2H](-•), and they have been explained in terms of ionization energies, pKa values, and thermodynamic stability.