Hydrated Hydronium Ions Research Articles

QM/MM-Based Energy Decomposition Analysis Method for Large Systems.

In this work, a QM/MM-based EDA method, called GKS-EDA(QM/MM), is proposed. As an extension of GKS-EDA, this scheme divides the total interaction energy into electrostatic, exchange-repulsion, polarization, and correlation/dispersion terms. GKS-EDA(QM/MM) can be applied to describe the interactions of large-scale systems combined with various QM/MM platforms. By using the examples of a hydrated hydronium ion complex in water solution, the barnase-barstar complex, and MMP-13-pyrimidinetrione in a metalloprotein, the capability of GKS-EDA(QM/MM) for various interactions in large systems is validated.

Manipulation of Ion Conversion in Dichloromethane-Enhanced Vacuum Ultraviolet Photoionization Mass Spectrometry of Oxygenated Volatile Organic Compounds.

The ion conversion processes in CH2Cl2-enhanced vacuum ultraviolet photoionization of oxygenated volatile organic compounds (OVOCs) have been systematically studied by regulating the pressure, humidity, and reaction time in the ionization source of a time-of-flight mass spectrometer. As the ionization source pressure increased from 100 to 1100 Pa, the main characteristic ions changed from CH2Cl+ to CH2Cl+(H2O), CH2OH+, and C2H4OH+ and then to the hydrated hydronium ions H3O+(H2O)n (n = 1, 2, 3). The total ion current (TIC) almost remained unchanged even if the humidity increased from 44 to 3120 ppmv, indicating interconversion between ions through ion-molecule reactions. The intensity of protonated methanol/ethanol (sample S) ion was almost linearly correlated with the intensity of H3O+(H2O)n, which pointed to the proton transfer reaction (PTR) mechanism. The reaction time was regulated by the electric field strength in the ionization region. The intensity variation trends of different ions with the reaction time indicated that a series of step-by-step ion-molecule reactions occurred in the ionization source, i.e., the primary ion CH2Cl+ reacted with H2O and converted to the intermediate product ions CH2OH+ and C2H4OH+, which then further reacted with H2O and led to the production of H3O+, and finally, the protonated sample ion SH+ was obtained through PTR with H3O+, as the ion-molecule reactions progressed. This study provides valuable insights into understanding the formation mechanism of some unexpected intermediate product ions and hydrated hydronium ions in dopant-enhanced VUV photoionization and also helps to optimize experimental conditions to enhance the sensitivity of OVOCs.

Impact of the Local Concentration of Hydronium Ions at Tungstate Surfaces for Acid-Catalyzed Alcohol Dehydration.

Tungstate domains supported on ZrO2, Al2O3, TiO2, and activated carbon drastically influence the hydronium-ion-catalyzed aqueous-phase dehydration of alcohols. For all catalysts, the rate of cyclohexanol dehydration normalized to the concentration of Brønsted acid sites (turnover frequencies, TOFs) was lower for monotungstates than for polytungstates and larger crystallites of WO3. TOFs were constant when reaching or exceeding the monolayer coverage of tungstate, irrespective of the specific nature of surface structures that continuously evolve with the surface W loading. However, the TOFs with polytungstates and large WO3 crystallites depend strongly on the underlying support (e.g., WOx/C catalysts are 10-50-fold more active than WOx/Al2O3 catalysts). The electrical double layer (EDL) surrounding the negatively charged WOx domains contains hydrated hydronium ions, whose local concentrations change with the support. This varying concentration of interfacial hydronium ions ("local ionic strength") impacts the excess chemical potential of the reacting alcohols and induces the marked differences in the TOFs. Primary H/D kinetic isotope effects (∼3), together with the substantially positive entropy of activation (111-195 J mol-1 K-1), indicate that C-H(D) bond cleavage is involved in the kinetically relevant step of an E1-type mechanistic sequence, regardless of the support identity. The remarkable support dependence of the catalytic activity observed here for the aqueous-phase dehydration of cycloalkanols likely applies to a broad set of hydronium-ion-catalyzed organic reactions sensitive to ionic strength.

Rate enhancement of phenol hydrogenation on Pt by hydronium ions in the aqueous phase

Metal-containing zeolites exhibit remarkable catalytic activity for hydrogenation owing to the synergistic interactions between acid and metal sites. In the aqueous phase, the presence of water and, in particular, hydronium ions, complicate the adsorption of H2 and organic substrates. It is shown how hydrated hydronium ions formed from zeolite Brønsted acid sites promote the rate of hydrogenation of phenol on Pt by modifying reaction pathways in the aqueous phase. Hydrogen is preferentially added to the ortho-C of phenol at low concentrations of hydronium ions, while at high concentrations of hydronium ions hydrogen adds to both ortho- and para-C of phenol with equal probability. A proton coupled electron transfer (PCET) pathway is hypothesized to occur at metal surfaces associated with large concentrations of hydrated hydronium ions and adsorbed H, establishing a (quasi-) equilibrium open circuit potential. In the presence of lower concentrations of hydrated hydronium ions, the reaction follows a Langmuir-Hinshelwood mechanism in which adsorbed H atoms add to co-adsorbed phenol. DFT calculations show a lower activation energy barrier for the PCET pathway in the presence of hydronium ions compared to the pathway following a Langmuir-Hinshelwood type mechanism.

Influence of Intracrystalline Ionic Strength in MFI Zeolites on Aqueous Phase Dehydration of Methylcyclohexanols

AbstractThe impact of the concentration of hydrated hydronium ions and in turn of the local ionic strength in MFI zeolites has been investigated for the aqueous phase dehydration of 4‐methylcyclohexanol (E1 mechanism) and cis‐2‐methylcyclohexanol (E2 mechanism). The E2 pathway with the latter alcohol led to a 2.5‐fold higher activity. The catalytic activity normalized to the hydronium ions (turnover frequency, TOF) passed through a pronounced maximum, which is attributed to the increasing excess chemical potential of the alcohols in the pores, increasing in parallel with the ionic strength and the additional work caused by repulsive interactions and charge separation induced by the bulky alcohols. While the maximum in rate observed is invariant with the mechanism or substitution, the reaction pathway is influencing the activation parameters differently.

Speeding reactions through ionic strength

Zeolite Catalysis Bronsted acidity is introduced into microporous zeolites through the addition of framework aluminum. Pfriem et al. show that in the presence of water, the limited volume in the microchannels of zeolite H-MFI leads to a high concentration of hydrated hydronium ions at aluminum sites. The resulting high charge density creates a highly non-ideal solvation environment and, for cyclohexanol dehydrogenation, the charged carbenium-ion transition state was stabilized. A higher rate was maintained with lower-acidity sodium ion–exchanged zeolites that had the same high ionic strength. Science , abh3418, this issue p. [952][1] [1]: /lookup/doi/10.1126/science.abh3418

Role of the ionic environment in enhancing the activity of reacting molecules in zeolite pores.

Tailoring the molecular environment around catalytically active sites allows for the enhancement of catalytic reactivity through a hitherto unexplored pathway. In zeolites, the presence of water creates an ionic environment via the formation of hydrated hydronium ions and the negatively charged framework aluminum tetrahedra. The high density of cation-anion pairs determined by the aluminum concentration of a zeolite induces a high local ionic strength that increases the excess chemical potential of sorbed and uncharged organic reactants. Charged transition states (carbocations for example) are stabilized, which reduces the energy barrier and leads to higher reaction rates. Using the intramolecular dehydration of cyclohexanol on H-MFI zeolites in water, we quantitatively show an enhancement of the reaction rate by the presence of high ionic strength as well as show potential limitations of this strategy.

Dissociation of H3O+, NO+ and O2+• reagent ions injected into nitrogen carrier gas in SIFT-MS and reactivity of the ion fragments

A comparative study has been made of the utility of helium and nitrogen as the carrier gas in selected ion flow tube mass spectrometry, SIFT-MS. A most important parameter is shown to be the injection energy, Elab, into the nitrogen carrier gas of the reagent cations H3O+, NO+ and O2+• produced in the microwave discharge ion source. If Elab is too high, partial fragmentation of the reagent ions occurs in the hard collisions with nitrogen molecules, which does not occur to a significant extent in their collisions with lighter helium atoms. Then, the H3O+ ions fragment producing highly reactive ions, including OH+ and H2O+•, that can ionise and dissociate the N2 molecules, and the fraction of the non-dissociated H3O+ can associate with N2 molecules producing H3O+N2 adduct ions. These events promote the production of hydrated hydronium ions, H3O+(H2O)1,2,3, which become dominant even when the carrier gas and sample gas are only a modest humidity. The NO+ reagent ions become electronically excited and partially fragment to either O+• or N+ ions. These events result in their reactions with the N2 molecules producing excessive O2+• ions in the carrier gas. Similarly, the O2+• reagent ions become electronically excited and fragment to O+• ions that react with the N2 molecules producing large fractions of NO+ ions. These processes result in larger fractions of “impurity ions” than are desirable for gas phase analysis using SIFT-MS, which is especially serious for H3O+ reagent ions, but less so for NO+ and O2+•, which can still be used, with caution, for trace gas analysis. It is shown by reducing Elab that fragmentation can be minimised, and it is suggested that by increasing the carrier gas temperature or exploiting drift tube methods, adduct ion formation can be inhibited and thus partially “purify” the reagent ions to approach the situation that pertains to helium carrier gas.

Stimulation and inhibition of respiratory burst in neutrophils as a result of action of weak combined magnetic fields adjusted to ICR of protonated water forms

ABSTRACT Weak combined collinear magnetic fields (CMFs) composed of constant component, 60 μT and low-frequency alternating field, 100 nT, used in 40 min pretreatment, were shown to have diverse effects (alleviation/stimulation) depending on the frequency of the alternating component on respiratory burst intensity in neutrophil suspension after activation by N-formyl-Met-Leu-Phe recorded by luminol-dependent chemiluminescence. About 12.6 Hz frequency formally corresponding to ion cyclotron resonance (ICR) frequency of hydrated hydronium ion (Н9О4 +) had notable stimulating effect. On the contrary, treatment with 48.5 Hz frequency corresponding to hydronium ion (Н3О+) was accompanied by significant alleviation of respiratory burst intensity. CMFs-pretreated water conducted only stimulating effect of CMFs when the field was adjusted to ICR of hydrated hydronium ion form, which is direct proof of participation of water in the mechanism of this effect of CMFs.

The Role of Water in the Effect of Weak Combined Magnetic Fields on Production of Reactive Oxygen Species (ROS) by Neutrophils

Various models have been used to demonstrate the pronounced effects of the microwave frequency range electromagnetic fields, as well as weak and very weak combined collinear magnetic fields (CMF) with static and variable components in the order of micro- and nano-tesla. One of such models, previously shown to be sensitive to variations in the parameters of applied magnetic fields, is the neutrophil respiratory burst. Using luminol-enhanced cell chemiluminescence assay, we studied the effects of the CMF exerted directly on neutrophil suspensions and, indirectly, through aqueous solutions. To experimentally create a uniform standard weak CMF with 60 µT static and 100 nT alternating magnetic field components, we engineered a shielded magnetic field induction device. CMF applied directly to neutrophils enhanced reactive oxygen species (ROS) production by more than 36%. The pronounced stimulating effect was observed only when using the signals that included the frequency of 12.6 Hz that corresponds to the ion cyclotron resonance (ICR) frequency of the hydrated hydronium ion. Similarly, to direct exposure, CMF pre-treatment of a water sample subsequently added to the neutrophil suspension increased ROS production by 66%. The effect of CMF pre-treatment was retained after a series of dilutions and mechanical treatment but disappeared in “magnetic vacuum” or without mechanical influence. Therefore, weak and super weak magnetic fields may indirectly, via water, activate ROS production by neutrophils, provided that modulation of super weak component of collinear field corresponds to the ICR frequency of the hydrated hydronium ion.

Water Bridges Conduct Sequential Proton Transfer in Photosynthetic Oxygen Evolution.

Proton transfer using water bridges has been observed in bulk water, acid-base reactions, and several proton-translocating biological systems. In the photosynthetic water-oxidizing enzyme, photosystem II (PSII), protons from substrate water are transferred 35 Å from the Mn4CaO5 catalytic site to the chloroplast lumen. This process leads to acidification of the lumen and ATP synthesis. Water oxidation occurs in a flash-induced, five-step S n state cycle; acetate is a chloride-dependent inhibitor of the S2 to S3 step of this cycle. Here, we study the effect of acetate on a previous step of the cycle, the S1 to S2 transition, using reaction-induced infrared spectroscopy. PSII was isolated from spinach, and experiments were conducted at pH 7.5, using 532 nm laser flashes to advance the cycle from the dark-adapted state S1 to the S2 state. Isotope-editing of acetate reveals direct contributions to the S2-minus-S1 infrared spectrum consistent with protonation of bound acetate in PSII. In the acetate-derived S2-minus-S1 PSII spectra, an accompanying decrease in the intensity of a 2830 cm-1 band is observed when compared to the chloride control. The 2830 cm-1 band has been assigned previously to a stretching vibration of an internal, hydrated hydronium ion, W n+. Density functional studies of a catalytic site model predict the spontaneous transfer of a proton from this internal hydronium ion to acetate, when acetate is substituted at a chloride-binding site. Taken together, the results show that the mechanism of PSII proton transfer at pH 7.5 involves proton hopping through an internal, water-containing network.

Influence of Hydronium Ions in Zeolites on Sorption.

In the presence of sufficient concentrations of water, stable, hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites. For a medium-pore zeolite, such as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of 0.24 nm3 . In their presence, larger organic molecules can only adsorb in the portions of the pore that are not occupied by hydronium ions. As a consequence, the available pore volume decreases proportionally to the concentration of the hydronium ions. The higher charge density (the increasing ionic strength) that accompanies an increasing concentration of hydronium ions leads to an increase in the activity coefficients of the adsorbed substrates, thus, weakening the interactions between the organic part of the molecules and the zeolite and favoring the interactions with polar groups. The quantitative understanding of these interactions makes it possible to link a collective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on molecular level.

Influence of Hydronium Ions in Zeolites on Sorption

AbstractIn the presence of sufficient concentrations of water, stable, hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites. For a medium‐pore zeolite, such as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of 0.24 nm3. In their presence, larger organic molecules can only adsorb in the portions of the pore that are not occupied by hydronium ions. As a consequence, the available pore volume decreases proportionally to the concentration of the hydronium ions. The higher charge density (the increasing ionic strength) that accompanies an increasing concentration of hydronium ions leads to an increase in the activity coefficients of the adsorbed substrates, thus, weakening the interactions between the organic part of the molecules and the zeolite and favoring the interactions with polar groups. The quantitative understanding of these interactions makes it possible to link a collective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on molecular level.

Genesis and Stability of Hydronium Ions in Zeolite Channels.

The catalytic sites of acidic zeolite are profoundly altered by the presence of water changing the nature of the Brønsted acid site. High-resolution solid-state NMR spectroscopy shows water interacting with zeolite Brønsted acid sites, converting them to hydrated hydronium ions over a wide range of temperature and thermodynamic activity of water. A signal at 9 ppm was observed at loadings of 2-9 water molecules per Brønsted acid site and is assigned to hydrated hydronium ions on the basis of the evolution of the signal with increasing water content, chemical shift calculations, and the direct comparison with HClO4 in water. The intensity of 1H-29Si cross-polarization signal first increased and then decreased with increasing water chemical potential. This indicates that hydrogen bonds between water molecules and the tetrahedrally coordinated aluminum in the zeolite lattice weaken with the formation of hydronium ion-water clusters and increase the mobility of protons. DFT-based ab initio molecular dynamics studies at multiple temperatures and water concentrations agree well with this interpretation. Above 140 °C, however, fast proton exchange between bridging hydroxyl groups and water occurs even in the presence of only one water molecule per acid site.

The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase.

The thermodynamic state of H2 adsorbed on Pt in the aqueous phase was determined by kinetic analysis of H2 reacting with D2 O to HDO, HD, and D2 , and by DFT-based ab initio molecular dynamics simulations of H2 adsorption on Pt(111), Pt(110), and Pt nanoparticles. Dissociative adsorption of H2 on Pt is significantly weakened in the aqueous phase compared to adsorption at gas-solid interfaces. Water destabilizes the adsorbed H atoms, decreasing the heat of adsorption by 19-22 kJ while inducing an additional entropy loss of 50-70 J K-1 . Upon dissociative adsorption of H2 , the average distance of water from the Pt surface increases and the liquid adopts a structure that is more ordered than before close to the Pt surface, which limits the translation mobility of the adsorbed H atoms. The presence of hydrated hydronium ions next to the Pt surface further lowers the H-Pt bond strength.

The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase

AbstractThe thermodynamic state of H2 adsorbed on Pt in the aqueous phase was determined by kinetic analysis of H2 reacting with D2O to HDO, HD, and D2, and by DFT‐based ab initio molecular dynamics simulations of H2 adsorption on Pt(111), Pt(110), and Pt nanoparticles. Dissociative adsorption of H2 on Pt is significantly weakened in the aqueous phase compared to adsorption at gas–solid interfaces. Water destabilizes the adsorbed H atoms, decreasing the heat of adsorption by 19–22 kJ while inducing an additional entropy loss of 50–70 J K−1. Upon dissociative adsorption of H2, the average distance of water from the Pt surface increases and the liquid adopts a structure that is more ordered than before close to the Pt surface, which limits the translation mobility of the adsorbed H atoms. The presence of hydrated hydronium ions next to the Pt surface further lowers the H−Pt bond strength.

Molecular-Scale Structure of Electrode-Electrolyte Interfaces: The Case of Platinum in Aqueous Sulfuric Acid.

Knowledge of the molecular composition and electronic structure of electrified solid-liquid interfaces is key to understanding elemental processes in heterogeneous reactions. Using X-ray absorption spectroscopy in the interface-sensitive electron yield mode (EY-XAS), first-principles electronic structure calculations, and multiscale simulations, we determined the chemical composition of the interfacial region of a polycrystalline platinum electrode in contact with aqueous sulfuric acid solution at potentials between the hydrogen and oxygen evolution reactions. We found that between 0.7 and 1.3 V vs Ag/AgCl the electrical double layer (EDL) region comprises adsorbed sulfate ions with hydrated hydronium ions in the next layer. No evidence was found for bisulfate or Pt-O/Pt-OH species, which have very distinctive spectral signatures. In addition to resolving the long-standing issue of the EDL structure, our work establishes interface- and element-sensitive EY-XAS as a powerful spectroscopic tool for studying condensed phase, buried solid-liquid interfaces relevant to various electrochemical processes and devices.

Synthesis and Characterization by a Single-Crystal XRD Study of [H3O]4[Cu7(NH3)2(H2O)4{B24O39(OH)12}]\xb713H2O: An Unusual [{(H2O)2(NH3)Cu}2{B2O3(OH)2}2Cu]2+ Trimetallic Bis(dihydroxytrioxidodiborate) Chain Supported by a [{Cu4O}{B20O32(OH)8}]6\u2212 Cluster

The unusual [Cu7(NH3)2(H2O)4{B24O39(OH)12}]4− cluster anion, isolated in the form of its hydrated hydronium ion salt, has been crystallized in moderate yield (59%) through a templated self-assembly process from [Cu(NH3)4(H2O)2](OH)2 and B(OH)3 (10 equivalents) in aqueous solution. This novel anionic cluster is comprised of two diborate(2-) ligands bridging a linear {Cu3}6+ framework with this Cu3 fragment interacting with, and further supported by, a known larger [{Cu4O}B20O32(OH)8]6− cluster.

Impact of structural defects and hydronium ion concentration on the stability of zeolite BEA in aqueous phase

The presence of fluoride anions in the synthesis gel leads to zeolite BEA with high hydrothermal stability. This longer lifetime is caused by a lower concentration of internal silanol defects that are sites of the initial framework hydrolysis eventually destroying the lattice structure. The lifetime of these zeolites was the higher the lower the concentration of framework bridging hydroxyl groups that form hydronium ions was. While lattice hydrolysis was initiated at defect sites in all cases, the role of defects became less important as the concentration of hydronium ions increased. At higher concentration of hydrated hydronium ions, the presence of water associated with the hydrated hydronium ions limits the useful lifetime of zeolite catalysts in water. For such materials, the selective hydrophobization of the external surface extends the lifetime.

Tracking the Chemical Transformations at the Brønsted Acid Site upon Water-Induced Deprotonation in a Zeolite Pore

The structural changes induced by reversible formation of Bronsted acidic sites and hydronium ions with water in a zeolite with MFI structure are reported as a function of temperature using a combination of physicochemical methods and theory. In the presence of an ample concentration of water, the protons are present as hydrated hydronium ions (H3O+(H2O)n) that are ion-paired to the zeolite. Loss of water molecules hydrating the hydronium ions leads to an unstable free hydronium ion that dissociates to form the hydroxylated T-site. The formation of this SiOHAl species leads to the elongation of one of the four Al–O bonds and causes significant distortion of the tetrahedral symmetry about the Al atom. This distortion leads to the appearance of new pre-edge features in the Al K-edge X-ray absorption near edge structure (XANES) spectra. The pre-edge peak assignment is confirmed by time-dependent density functional theory calculation of the XANES spectrum. The XANES spectra are also sensitive to solutes or so...