Thermodynamics of Hydronium, Zundel and Eigen ions

The heterolytic dissociation of water gives the cation of proton, which has fleeting existence in reality. It exists in aqueous solution in various levels of hydration, as hydronium (H3O+), Zundel (H5O2+) and Eigen (H9O4+) ions. Herein, we present the thermodynamic parameters involved in the overall treatment. These values are crucial in understanding the behaviour of water and protons in cellular physiology at interfaces and bulk.

A Combined Computational and Experimental Study of Methane Activation during Oxidative Coupling of Methane (OCM) by Surface Metal Oxide Catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4 catalytic active sites for CH4 heterolytic dissociation...

Gold catalysts containing interstitial carbon atoms boost hydrogenation activity

Supported gold nanoparticles are emerging catalysts for heterogeneous catalytic reactions, including selective hydrogenation. The traditionally used supports such as silica do not favor the heterolytic dissociation of hydrogen on the surface of gold, thus limiting its hydrogenation activity. Here we use gold catalyst particles partially embedded in the pore walls of mesoporous carbon with carbon atoms occupying interstitial sites in the gold lattice. This catalyst allows improved electron transfer from carbon to gold and, when used for the chemoselective hydrogenation of 3-nitrostyrene, gives a three times higher turn-over frequency (TOF) than that for the well-established Au/TiO2 system. The d electron gain of Au is linearly related to the activation entropy and TOF. The catalyst is stable, and can be recycled ten times with negligible loss of both reaction rate and overall conversion. This strategy paves the way for optimizing noble metal catalysts to give an enhanced hydrogenation catalytic performance.

Modeling the effect of solvent on the possibility of the formation the HSO3+ cation

The possibility of describing the reactions of heterolytic dissociation sulfuric acid and the further conversion of the H3SO4+ cation to HSO3+ by the density functional theory method was studied. By comparing with the calculated and experimental data, a functional and a basic set were selected for modeling these reactions. The influence of the solvent on the course of the studied reactions was studied: based on COSMO continuum model (implicit solvent accounting) and explicit solvent accounting (introduction of particles participating in the reaction into the solvation shell of four sulfuric acid molecules). It was shown that individually neither the continuum model nor the explicit solvent accounting can adequately describe the reactions under study from the aspect of energy effects. It was determined that the reactions studied can occur only in strongly polar solvents. The effect of the content of sulfuric trioxide in oleum on the particle formation energy of H3SO4+ and HSO3+ was studied. It was shown that an increase in the concentration of sulfuric trioxide from 0 to 15% reduces the energy spent on the formation of H3SO4+ and HSO3+ particles by 2.5 and 3.5 kJ/mol, which leads to an increase in the equilibrium constants of the formation reactions of these products by 2.74 and 4.11 times. An approximate estimate is made of the equilibrium constant of the reaction of formation of the HSO3+ cation, equal to ~10-7-10-8. It was shown that particles of HSO3+ and SO3 in sulfuric acid solutions exist in the form of associates with sulfuric acid, through the sulfur atom SO3/HSO3+ and oxygen H2SO4, in which the S···O distance is 1.98 Å for SO3 and 1.9 Å for HSO3+.

Organic thiocyanates - glucosinolate enzymatic degradation products or artefacts of the isolation procedure?

Glucosinolates are abundant in plants of the order Brassicales, and they are degraded by myrosinases into various organic breakdown products: isothiocyanates, thiocyanates, nitriles, etc., depending on their structure, conditions of hydrolysis, the presence of certain protein cofactors. Their most common hydrolysis products are isothiocyanates, while simple nitriles, epithionitriles, and thiocyanates are produced occasionally. Organic thiocyanates are described from a very limited number of Brassicales taxa. Up to now benzyl, (4-hydroxyphenyl)methyl, (4-methoxyphenyl)methyl, 4- methylthiobutyl, and allyl thiocyanates were reported as products of glucosinolates autolysis. The present review summarizes the knowledge on the mechanism of organic thiocyanate formation from the corresponding thioglucosides. The enzymatic formation of organic thiocyanates is believed to be enabled by thiocyanate-forming protein (TFP), but they could be formed via metabolic routes that do not involve TFP. All of the reported thiocyanates are produced from stable (carbo)cationic species that allow an isomerization of an isothiocyanate to thiocyanate, and vice versa. Although the possibility that thiocyanates can be biosynthesized in plats under certain conditions cannot be dismissed, allyl thiocyanate can be a thermal isomerization artefact of the original isothiocyanate that is formed in the heated zones of the gas chromatograph, while other thiocyanates could form in an aqueous medium via heterolytic dissociation to ambident nucleophilic SCN- and its recapture. One should always be aware of this analytical shortcoming when concluding on the presence and quantity of these specific (iso)thiocyanantes in the analyzed sample.

X-ray Photospectroscopy and Electronic Studies of Reactor Parameters on Photocatalytic Hydrogenation of Carbon Dioxide by Defect-Laden Indium Oxide Hydroxide Nanorods

In the study reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) and in situ Hall charge carrier measurements provide new insights into the surface physical chemistry of gaseous H2, CO2, and H2+CO2 combined with nanostructured In2O(3−x)(OH)y nanorods, which ensue under photochemical and thermochemical operating conditions. Heterolytic dissociation of H2 in H2-only atmosphere appears to occur mainly under dark and ambient temperature conditions, while the greatest amount of OH shoulder expansion in H2+CO2 atmosphere appears to mainly occur under photoilluminated conditions. These results correlate with those of the Hall measurements, which show that the prevalence of homolytic over heterolytic dissociation at increasing temperatures leads to a steeper rate of increase in carrier concentrations; and that H2 adsorption is more prevalent than CO2 in H2+CO2 photoillumination conditions.

Understanding the Role of Rutile TiO2 Surface Orientation on Molecular Hydrogen Activation

Titanium oxide (TiO2) has been widely used in many fields, such as photocatalysis, photovoltaics, catalysis, and sensors, where its interaction with molecular H2 with TiO2 surface plays an important role. However, the activation of hydrogen over rutile TiO2 surfaces has not been systematically studied regarding the surface termination dependence. In this work, we use density functional theory (PBE+U) to identify the pathways for two processes: the heterolytic dissociation of H2 as a hydride–proton pair, and the subsequent H transfer from Ti to near O accompanied by reduction of the Ti sites. Four stoichiometric surface orientations were considered: (001), (100), (110), and (101). The lowest activation barriers are found for hydrogen dissociation on (001) and (110), with energies of 0.56 eV and 0.50 eV, respectively. The highest activation barriers are found on (100) and (101), with energies of 1.08 eV and 0.79 eV, respectively. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.40 to 1.86 eV). Our results indicate that the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. We discuss the implications in the stability of the hydride–proton pair, and provide structures, electronic structure, vibrational analysis, and temperature effects to characterize the reactivity of the four TiO2 orientations.

A valence bond study of the activation of methyl halides bonds by electric fields

In the present work, the activation of methyl halides bonds under experience of an external electric field (EEF) is explained from the Valence Bond theory perspective. The dissociation mechanism of C–X bonds (X [Formula: see text] Cl, Br, I) influenced by a homogeneous and a heterogeneous field placed parallel to the bond axis is presented. For all examples, an increase in the electric field strength have similar consequences: (i) the decrease of the energy depth along the dissociation path, (ii) an increase of the equilibrium interatomic distance (at high EEFs), and (iii) the transition from a homolytic to a heterolytic dissociation after some field magnitude. These general behaviors are explained through the curve crossing between the ionic and the covalent structure at some field strength.