Characterization of intramolecular hydrogen bonds by atomic charges and charge fluxes

The electronic charge redistribution and the infrared intensities of the two types of intramolecular hydrogen bonds, O-H···O and O-H···π, of o-hydroxy- and o-ethynylphenol, respectively, together with a set of related intermolecular hydrogen bond complexes are described in terms of atomic charges and charge fluxes derived from atomic polar tensors calculated at the B3LYP/cc-pVTZ level of theory. The polarizable continuum model shows that both the atomic charges and charge fluxes are strongly dependent on solvent. It is shown that their values for the OH bond in an intramolecular hydrogen bond are not much different from those for the "free" OH bond, but the changes are toward the values found for an intermolecular hydrogen bond. The intermolecular hydrogen bond is characterized not only by the decreased atomic charge but also by the enlarged charge flux term of the same sign producing thus an enormous increase in IR intensity. The overall behavior of the charges and fluxes of the hydrogen atom in OH and ≡CH bonds agree well with the observed spectroscopic characteristics of inter- and intramolecular hydrogen bonding. The main reason for the differences between the two types of the hydrogen bond lies in the molecular structure because favorable linear proton donor-acceptor arrangement is not possible to achieve within a small molecule. The calculated intensities (in vacuo and in polarizable continuum) are only in qualitative agreement with the measured data.     

Synthesis and Stability of Lanthanum Superhydrides

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen‐rich simple compounds having new clathrate‐like structures and remarkable electronic properties including room‐temperature superconductivity. X‐ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH₁₀ having cages of thirty‐two hydrogen atoms surrounding each La atom. Upon decompression, the fcc‐based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H?H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized.     

Visualizing internal stabilization in weakly bound systems using atomic energies: hydrogen bonding in small water clusters

Atomic energies are used to visualize the local stabilizing and destabilizing energy changes in water clusters. Small clusters, (H(2)O)(n), from n = 2-5, at MP2/aug-cc-pVTZ geometries are evaluated using energies defined by the quantum theory of atoms in molecules (QTAIM). The atomic energies reproduce MP2 total energies to within 0.005 kcal mol(-1). Oxygen atoms are stabilized for all systems and hydrogen atoms are destabilized. The increased stability of the water clusters due to hydrogen bond cooperativity is demonstrated at an atomic level. Variations in atomic energies within the clusters are correlated to the geometry of the waters and reveal variations in the hydrogen bond strengths. The method of visualization of the energy changes applied here is especially suited for application to large biomolecules.     

Dynamics of Atomic and Molecular Hydrogen Elimination from Hydrocarbons at VUV Excitation

Elimination of atomic hydrogen (H) and molecular hydrogen (H₂) are important elementary chemical processes in photochemistry and combustion chemistry. Recently, unique and sensitive detection techniques for atomic and molecular hydrogen detection were developed in our laboratory. Using the advanced molecular beam methods, we have studied the photodissociation of a few typical hydrocarbons at 157 nm excitation, especially their atomic and molecular hydrogen elimination processes. In this report, we will briefly describe the results from photodissociation of propane, ethylene, propyne and methanol at 157 nm excitation. These molecules represent different classes of hydrocarbons such as alkane, alkene, alkyne and alcohol. Through careful studies on differently deuterated compounds, clear pictures of selective atomic and molecular hydrogen elimination processes can be constructed for all of the above compounds. These results will help us to understand the dissociation dynamics of the small hydrocarbon molecules.     

Growth of a vanadium oxide layer on V(100) and its reactivity towards atomic hydrogen

The influence of a surface oxide on V(100) on the sticking of atomic hydrogen was studied. The oxide was produced by dosing with oxygen at different surface temperatures and characterized by AES and XPS. Thermal desorption spectroscopy was used to determine the sticking coefficient of atomic hydrogen. The kinetics of oxidation and the stability of the formed oxide have been studied in detail. The highest oxidation rate was obtained at a surface temperature of 473 K, producing a V2O3 over-layer. Although the sticking coefficient for atomic hydrogen decreases from 0.14 on the oxide free V(100) surface to 0.015 on the V2O3 surface, the sticking coefficient for atomic hydrogen was still several orders of magnitude higher than that for molecular hydrogen. This is relevant in respect of the use of the hydrogen-vanadium system for energy storage.     

A crossed beam and ab initio study of the reaction of atomic boron with ethylene

The reaction of atomic boron, B(2P), with the simplest alkene, C2H4, has been investigated under single collision conditions in crossed beam experiments with mass spectrometric detection. Our experimental data clearly showed that the atomic boron versus hydrogen exchange reaction led to molecule(s) of gross formula C2H3B via bound intermediate(s). According to the experimentally derived fraction of the available energy released as product translational energy, we propose that an important reaction pathways is the one leading to the borirene plus atomic hydrogen and/or the one leading to ethynylborane plus atomic hydrogen. The experimental results are accompanied by electronic structure calculations of the relevant potential energy surface and RRKM estimates of the product branching ratio. According to RRKM calculations, within the limit of complete energy randomization, the three isomers borirene, BH=C=CH2 and BH2-CCH, are all formed, with BH2-CCH being the dominant one. The discrepancies between the trend of the product translational energy distributions and the picture emerging from RRKM estimates are a symptom that a statistical treatment is not warranted for this system.     

Doppler-broadened line shapes of atomic hydrogen in a parallel-plate radio frequency discharge

Doppler-shifted atomic hydrogen emission (Balmer ) is observed from a low-frequency rf discharge through molecular hydrogen by collecting the light through a hole in one of the electrodes. Doppler shifts as large as 0.7 nm, corresponding to an energy of 540 eV or 85% of the peak applied voltage, are observed when ions are accelerated by the sheath electric field. The mechanisms for hot atom production are discussed in terms of both gas-phase and surface ion-impact phenomena. Hot atoms are produced via gas-phase ion-atom and ion-molecule collisions, as a result of simultaneous neutralization and reflection of ions at the electrode surface, and/or by sputtering of adsorbed hydrogen. As much as 30% of the atomic hydrogen emission is substantially Doppler shifted, indicating that most of the atomic emission in the sheaths is actually produced by ion impact and not by electron impact.     

Atomic hydrogen concentrations in pulsed microwave discharges used for diamond synthesis

Pulsing of a microwave discharge is a promising new technique to improve the control of the diamond deposition process. A small amount of argon (5% Ar) was added to the usual feed stock (0.5% methane in hydrogen) to act as an actinometer. Plasma emission was observed during the pulse on-times for pulse repetition rates from 50 Hz to 10 kHz by time-resolved optical spectroscopy. Determined relaxation time constants for atomic hydrogen are on the order of 1 ms. Thus, the atomic hydrogen concentrations do not vary very much at a pulse repetition rate of 20 kHz, whereas at 50 Hz the radical concentrations vary between a saturation value and zero. The saturation value of atomic hydrogen increases with increasing peak power, but mean values are generally lower as in the continuous wave mode with the same average power and decrease with decreasing duty cycles and pulse repetition rates. The results are consistent with deposition experiments.     

Electrothermal atomization of calcium and strontium in a molybdenum micro-tube

The excitation and ionization of calcium and strontium in a molybdenum micro-tube atomizer and their use in atomic-absorption spectrometry are described. Increasing hydrogen flow led to complex calcium atomization profiles for absorption measurements, but decreasing hydrogen flow resulted in higher atomic emission. Ionization of calcium and its suppression by potassium were also observed. Strontium was measured effectively by atomic absorption because of the higher sensitivity. Increasing hydrogen flow resulted in a lower atomization temperature and higher absorption for strontium, while decreasing hydrogen flow resulted in higher atomic emission. No interference from 100-fold amounts of magnesium, calcium and sodium was found for atomization of strontium, but 100-fold amounts of aluminium shifted the peak temperature for strontium though with no variation of appearance temperature and peak absorption. A small shift in peak temperature was observed for strontium in the presence of 100-fold amounts of phosphate.     

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP _{H 2}/P _Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar⁺ and H₂ which lead to ArH⁺ and H ₂ ⁺ ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H⁺, H ₂ ⁺ , and H ₃ ⁺ ) or excited species H(n=2) with H₂ producing H atoms.     

“Superpermeability” and “pumping” of atomic hydrogen through palladium membranes

Permeation of atomic as well as molecular hydrogen through palladium membranes has been investigated experimentally in the temperature range from room temperature to 200 °C and at a higher incident flux of hydrogen atoms on palladium surface than in previous studies. The results demonstrate that phenomena of ‘superpermeability’ and ‘pumping’ of atomic gases through metal membranes are of a common nature. A theoretical model based on chemical thermodynamics and diffusion theory adequately describes the quantitative relationships observed in experiments. It was found that permeability of atomic hydrogen depends strongly on the magnitude of surface incident flux and membrane temperature.     

Kinetic calculation for the reaction of H with Si_2H_6 using the variational transition state theory

The reaction of disilane with atomic hydrogen has been studied. This reaction involves both substitution and abstraction. Calculations show that the hydrogen abstraction is the strongest competing channel. The canonical variational transition state theory with a small curvature tunneling correction (SCT) has been used for the kinetic calculation. The theoretical results are in good agreement with the available experimental data. Comparing the reactions of atomic hydrogen with disilane and silane, it can be seen that the reactivity of the Si-H bond is higher in Si2H6than that in SiH4.     

Die Raumchemie des Wasserstoffs in Metallhydriden im Vergleich mit entsprechenden Fluoriden und Chloriden

The Atomic Volume of Hydrogen in Metal Hydrides in Comparison with Corresponding Fluorides and Chlorides The atomic volume of hydrogen in metal hydrides is calculated by using the atomic volumes of the metal cations as given by Biltz. The exceptional polarizability of hydrogen ligands is the reason for its adaptability when forming different bond structures in metal hydrides. The atomic volume of hydrogen decreases from 13.7 cm³mol^?1 in salt-like caesium hydride to 3.9 cm³mol^?1 in metallic palladium hydride. This variation is significantly higher for metal hydrides than for metal chlorides, although the volume of a hydrogen ion is comparable to that of a fluoride ion, that shows an almost constant value in its compounds. For structurally related hydrides an examination of the atomic volume of hydrogen allows the re-examination of a given composition and therefore the disclosure of a wrong atomic arrangement.     

Fast Reactions of Atom Substitution from Polyatomic Molecules and Solid Salts by Thermally Equilibrated Atomic Reactants

A new class of fast thermal gas-phase reactions of the direct substitution of atoms in polyatomic molecules by atomic reactants is discovered. The Arrhenius parameters are determined for the reactions of atomic deuterium with a number of hydrogen-containing compounds, occurring via the direct substitution of hydrogen atoms and via the abstraction of hydrogen atoms. Fast substitution of alkali metal atoms from the crystals of their salts for the reaction chain carriers of hydrogen flames is found. The importance of reactions of these types in chain combustion is demonstrated. The kinetic isotope effects of hydrogen atom abstraction from hydrocarbon molecules by hydrogen and deuterium atoms are studied. A method for the kinetic studies of free atoms and radicals is developed, which takes into account the role of longitudinal diffusion in the jet and does not require the knowledge of the concentrations of atoms and radicals or the rate constants of other reactions.     

Water formation on Pd(111) by reaction of oxygen with atomic and molecular hydrogen

In this work we have studied the steady-state reaction of molecular and atomic hydrogen with oxygen on a Pd(111) surface at a low total pressure (<10(-7) mbar) and at sample temperatures ranging from 100 to 1100 K. Characteristic features of the water formation rate Phi(pH2; pO2; TPd) are presented and discussed, including effects that are due to the use of gas-phase atomic hydrogen for exposure. Optimum impingement ratios (OIR) for hydrogen and oxygen for water formation and their dependence on the sample temperature have been determined. The occurring shift in the OIR could be ascribed to the temperature dependence of the sticking coefficients for hydrogen (SH2) and oxygen (SO2) on Pd(111). Using gas-phase atomic hydrogen for water formation leads to an increase of the OIR, suggesting that hydrogen abstraction via hot-atom reactions competes with H2O formation. The velocity distributions of the desorbing water molecules formed on the Pd(111) surface have been measured by time-of-flight spectroscopy under various conditions, using either gas-phase H atoms or molecular H2 as reactants. In all cases, the desorbing water flux could be represented by a Maxwellian distribution corresponding to the surface temperature, thus giving direct evidence for a Langmuir-Hinshelwood mechanism for water formation on Pd(111).     

First principle study of hydrogen storage on the graphene-like aluminum nitride nanosheet

Employing density functional calculations including an empirical dispersion term, we investigated the hydrogenation of an aluminum nitride nanosheet (h-AlN) with atomic and molecular hydrogen. It was found that atomic H prefers to be adsorbed on an N atom rather than Al, releasing energy of 21.1 kcal/mol. The HOMO/LUMO energy gap of the sheet is dramatically reduced from 107.9 to 44.5 kcal/mol, upon the adsorption of one hydrogen atom. The adsorption of atomic H on the h-AlN presents properties which are promising for nanoelectronic applications. The molecular H₂ was found to be adsorbed collinearly on an N atom and dissociated to two H atoms on Al–N bond. Calculated barrier and adsorption energies for this dissociation process are about +18.9 and ?1.9 kcal/mol. We predict that each nitrogen atom in an AlN sheet can adsorb two hydrogen molecules on opposite sides of the sheet, and thus the gravimetric density for hydrogen storage on AlN sheet is evaluated to be about 8.9 wt%.     

Selective adsorption of atomic hydrogen on a h-BN thin film

The adsorption of atomic hydrogen on hexagonal boron nitride (h-BN) is studied using two element-specific spectroscopies, i.e., near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy (XPS). B K-edge NEXAFS spectra show a clear change in the energy region of the π* band before and after reaction with atomic deuterium. On the other hand, N K-edge NEXAFS spectra show only a little change. B 1s XPS spectra show a distinct component at the low binding energy side of a main component, while N 1s XPS spectra show peak broadening at the high binding energy side. These experimental results are analyzed by the discrete variational Xα method with a core-hole effect and are explained by a model in which hydrogen atoms are preferentially adsorbed on the B sites of h-BN. Based on the experimental and theoretical results, we propose a site-selective property of BN material on adsorption of atomic hydrogen.     

Atomic hydrogen interaction with Ru(1010)

The interaction of atomic hydrogen with clean and deuterium precovered Ru(1010) was studied by means of temperature-programmed desorption (TPD) spectroscopy. Compared to molecular hydrogen experiments, after exposure of the clean surface to gas-phase atomic hydrogen at 90 K, two additional peaks grow in the desorption spectra at 115 and 150 K. The surface saturation coverage, determined by equilibrium between abstraction and adsorption reactions, is 2.5 monolayers. Preadsorbed deuterium abstraction experiments with gas-phase atomic hydrogen show that a pure Eley-Rideal mechanism is not involved in the process, while a hot atom (HA) kinetics describes well the reaction. By least-squares fitting of the experimental data, a simplified HA kinetic model yields an abstraction cross section value of 0.5 +/- 0.2 angstroms2. The atomic hydrogen interaction with an oxygen precovered surface was also studied by means of both TPD and x-ray photoelectron spectroscopy: oxygen hydrogenation and water production take place already at very low temperature (90 K).     

The structure, properties, and nature of HArF-benzene complex: redshift and blueshift of Ar-H stretch frequency and rare gas atomic number dependence of hydrogen bonds

Ab initio calculations have been performed for the complexes of benzene with HArF, HKrF, and HXeF. The computed results indicate that the complexes of benzene-HArF exist in different conformations and among them those with π-hydrogen bonds are the more stable than those with C-H···F hydrogen bonds. Interestingly, the Ar-H stretching frequency is redshifted in the more stable isomer and blueshifted in the less stable form. The Ng (Ng=Ar, Kr, and Xe) atomic number dependence of the Ng-H···π and C-H···F hydrogen bonds has been explored. The result indicates that the strength of Ng-H···π and C-H···F hydrogen bonds is weakened with the increase of Ng atomic number. Natural bond orbital analysis has been performed to understand the interaction nature, frequency shift of H-Ng stretch, and dependence of Ng-H···π and C-H···F hydrogen bonds on the Ng atomic number.     

Hydrogen adsorption on carbon-doped boron nitride nanotube

The adsorption of atomic and molecular hydrogen on carbon-doped boron nitride nanotubes is investigated within the ab initio density functional theory. The binding energy of adsorbed hydrogen on carbon-doped boron nitride nanotube is substantially increased when compared with hydrogen on nondoped nanotube. These results are in agreement with experimental results for boron nitride nanotubes (BNNT) where dangling bonds are present. The atomic hydrogen makes a chemical covalent bond with carbon substitution, while a physisorption occurs for the molecular hydrogen. For the H(2) molecule adsorbed on the top of a carbon atom in a boron site (BNNT + C(B)-H(2)), a donor defect level is present, while for the H(2) molecule adsorbed on the top of a carbon atom in a nitrogen site (BNNT + C(N)-H(2)), an acceptor defect level is present. The binding energies of H(2) molecules absorbed on carbon-doped boron nitride nanotubes are in the optimal range to work as a hydrogen storage medium.