Hydrogen-hydrogen Interactions Research Articles

General-purpose neural network interatomic potential for the α -iron and hydrogen binary system: Toward atomic-scale understanding of hydrogen embrittlement

To understand the physics of hydrogen embrittlement at the atomic scale, a general-purpose neural network interatomic potential (NNIP) for the $\ensuremath{\alpha}$-iron and hydrogen binary system is presented. It is trained using an extensive reference database produced by density functional theory (DFT) calculations. The NNIP can properly describe the interactions of hydrogen with various defects in $\ensuremath{\alpha}$-iron, such as vacancies, surfaces, grain boundaries, and dislocations; in addition to the basic properties of $\ensuremath{\alpha}$-iron itself, the NNIP also handles the defect properties in $\ensuremath{\alpha}$-iron, hydrogen behavior in $\ensuremath{\alpha}$-iron, and hydrogen-hydrogen interactions in $\ensuremath{\alpha}$-iron and in vacuum, including the hydrogen molecule formation and dissociation at the $\ensuremath{\alpha}$-iron surface. These are superb challenges for the existing empirical interatomic potentials, like the embedded-atom method based potentials, for the $\ensuremath{\alpha}$-iron and hydrogen binary system. In this study, the NNIP was applied to several key phenomena necessary for understanding hydrogen embrittlement, such as hydrogen charging and discharging to $\ensuremath{\alpha}$-iron, hydrogen transportation in defective $\ensuremath{\alpha}$-iron, hydrogen trapping and desorption at the defects, and hydrogen-assisted cracking at the grain boundary. Unlike the existing interatomic potentials, the NNIP simulations quantitatively described the atomistic details of hydrogen behavior in the defective $\ensuremath{\alpha}$-iron system with DFT accuracy.

Investigating the non-classical M-H2 bonding in OsClH3(PPh3)3

Dihydrogen ligation to metal centers results in a wide range of metal-hydrogen interactions. Compounds containing non-classical dihydrogen ligands in which the H–H contacts are significantly longer than the 0.74 Å distance found in solid H2 are classified as Kubas-like compounds, stretched dihydrogen ligands, or compressed dihydrides, depending on the H–H contact length. Compounds containing stretched dihydrogen and compressed dihydride ligands are somewhat rare. Developing a more complete understanding of the metal-hydrogen and hydrogen-hydrogen interactions in these species may provide insights into the mechanisms for the oxidative addition reaction and hydrogen storage. Here, we use diffraction and inelastic neutron scattering (INS) measurements, paired with both molecular and solid-state density functional theory (DFT), to fully characterize the nature of the metal-hydrogen and hydrogen-hydrogen interactions in a previously reported compound, OsClH3(PPh3)3. In that report, the H–H distance was measured as 1.48(2) Å by single-crystal neutron diffraction and was described as a “stretched” dihydrogen. The INS-generated phonon density of states is well described by the DFT calculations, including those normal modes dominated by the vibrational motions of the three Os-ligated hydrogen atoms. The resulting calculated electron charge density landscape indicates that there is no remnant bonding character between the Os-ligated hydrogen atoms in the compound. Based on these findings, we update the understanding of the metal-hydrogen bonding in this complex and reclassify it as a compressed dihydride. As such, OsClH3(PPh3)3 represents a rare example of a non-classical compressed dihydride species.

Polymorphism of Calcium Decahydrido-closo-decaborate and Characterization of Its Hydrates.

Metal closo-borates and their derivatives have shown promise in several fields of application from cancer therapy to solid-state electrolytes partly owing to their stability in aqueous solutions and high thermal stability. We report the synthesis and structural analysis of α- and β-CaB10H10, which are structurally and energetically similar, both showing a tetrahedral coordination of Ca2+ to four closo-borate cages. The main distinctions between the α- and β-polymorph are found in the crystal system (monoclinic or orthorhombic), topology (wurtzite or cag), and the degree of displacement of Ca2+ from the center of the coordination tetrahedron. Neutron vibrational spectroscopy measurements further revealed distinct perturbations in the cation-anion interactions arising from the different crystal structures. We also synthesized and structurally investigated five stoichiometric hydrates, CaB10H10·xH2O, x = 1, 4, 5, 6, and 7, and discovered an order-disorder polymorphic transition, α- to β-CaB10H10·6H2O. The hydrates reveal a rich structural diversity with ordered structures, CaB10H10·xH2O, x = 1, 4, 5, 6, and 7, as well as disordered structures, x = 6 and 8. The latter allow for a continuum of compositions within 7-8 molecules of crystal water. The DFT-optimized experimental crystal structures reveal complex networks of three types of hydrogen interactions: dihydrogen bonds, B-Hδ-···+δH-O; hydrogen-hydrogen interactions, B-H···H-B; and hydrogen bonds, O-Hδ+···-δO-H. A rather short B-H···H-B (2.14 Å) interaction is observed for CaB10H10·5H2O, which is locally stabilized by four hydrogen bonds.

A Monte Carlo simulation study of hydrogen adsorption in slit-shaped pores

Abstract Physisorption of hydrogen inside slit-shaped pores is a simple but promising technique to store hydrogen. Molecular simulations have played an important role in understanding and obtaining hydrogen adsorption isotherms. Here from GCMC simulations, we obtain adsorption isotherms and density profiles of hydrogen inside slit-shaped pores of width 0.7, 1.0 and 1.5 nm at 80 and 298 K. Hydrogen-hydrogen interactions are modeled using classical Silvera-Goldman potential. This potential has electrostatic interaction sites along with Lennard-Jones sites. Wall-fluid interactions are modeled using both Steele potential and an all atom Lennard-Jones potential mimicking Graphene. Adsorption isotherms are similar for both the models and in all pore-widths. At 298 K and 80 K maximum adsorption of hydrogen was obtained at 200 bar and 50 bar respectively. Although our results show that at room temperature and in 1.5 nm pores classically modeled hydrogen adsorption results are somewhat higher than results obtained from quantum corrected potentials but the overall adsorption trends are very well reproduced. At 80 K and 50 bar, gravimetric weight in the 1.5 nm pore was maximum and close to 9 wt%.

Electronic Communication in Binuclear Osmium- and Iridium-Polyhydrides.

Reactions of polyhydrides OsH6(PiPr3)2 (1) and IrH5(PiPr3)2 (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MHn unit of the other in mixed valence binuclear polyhydrides. Hexahydride 1 activates an ortho-CH bond of the heterocyclic moiety of the trihydride metal–ligand compounds OsH3{κ2-C,N-[C5RH2N-py]}(PiPr3)2 (R = H (3), Me (4), Ph (5)). Reactions of 3 and 4 lead to the hexahydrides (PiPr3)2H3Os{μ-[κ2-C,N-[C5RH2N-C5H3N]-N,C-κ2]}OsH3(PiPr3)2 (R = H (6), Me (7)), whereas 5 gives the pentahydride (PiPr3)2H3Os{μ-[κ2-C,N-[C5H3N-C5(C6H4)H2N]-C,N,C-κ3]}OsH2(PiPr3)2 (8). Pentahydride 2 promotes C—H bond activation of 3 and the iridium-dihydride IrH2{κ2-C,N-[C5H3N-py]}(PiPr3)2 (9) to afford the heterobinuclear pentahydride (PiPr3)2H3Os{μ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (10) and the homobinuclear tetrahydride (PiPr3)2H2Ir{μ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (11), respectively. Complexes 6–8 and 11 display HOMO delocalization throughout the metal–heterocycle-metal skeleton. Their sequential oxidation generates mono- and diradicals, which exhibit intervalence charge transfer transitions. This notable ability allows the tuning of the strength of the hydrogen–hydrogen and metal–hydrogen interactions within the MHn units.

Hydrogen hydrate cage occupancy: A key parameter for hydrogen storage and transport

In this work, an accurate model is presented for predicting the hydrogen hydrate cage occupancies. This novel predictive method is based on the hydrate phase stability and hydrogen effective molecular size and configuration. The hydrogen-hydrogen interactions in a hydrate cage are calculated using the Kihara potential and the pairwise additivity rule. The accuracy and reliability of the proposed method are evaluated by comparison with various literature data. The model shows that the small cage double occupancy of structure II is possible if the volume expansion of the cages has been taken into account and a slight change in the volume of small cages leads to a considerable decrease in pressure required for double occupancy of small cages. The model also indicates that the large cages volume expansion has a weaker effect on the required pressure for quadruple occupancy in comparison with the case of double occupancy of small cages. The proposed model confirms the findings of most of the previous experimental observations and theoretical simulations.

Structural Properties and Reactive Site Selectivity of Some Transition Metal Complexes of 2,2′(1E,1′E)-(ethane-1,2-diylbis(azan-1-yl-1-ylidene))bis(phenylmethan-1-yl-1-ylidene)dibenzoic Acid: DFT, Conceptual DFT, QTAIM, and MEP Studies

Herein is presented a density functional theory (DFT) study of reactivity and structural properties of transition metal complexes of the Schiff base ligand 2,2′(1E,1′E)-(ethane-1,2-diylbis(azan-1-yl-1-ylidene))bis(phenylmethan-1-yl-1-ylidene)dibenzoic acid (hereafter denoted EDA2BB) with Cu(II), Mn(II), Ni(II), and Co(II). The quantum theory of atoms-in-molecules (QTAIM), conceptual DFT, natural population analysis (NPA), and molecular electrostatic potential (MEP) methods have been used. Results have revealed a distorted octahedral geometry around the central metal ion in each gas phase complex. In the DMSO solvent, a general axial elongation of metal-oxygen bonds involving ancillary water ligands has been observed, suggestive of loosely bound water molecules to the central metal ion that may be acting as solvent molecules. Weak, medium, and strong intramolecular hydrogen bonds along with hydrogen-hydrogen and van der Waals interactions have been elucidated in the complexes investigated via geometric and QTAIM analyses. From the chemical hardness values, the complex [Co(EDA2BB)(OH2)2] is the hardest, while [Cu(EDA2BB)(OH2)2] is the softest. Based on the global electrophilicity index, the complexes [Ni(EDA2BB)(OH2)2] and [Cu(EDA2BB)(OH2)2] are the strongest and weakest electrophiles, respectively, among the complexes studied. In conclusion, the reactivity of the complexes is improved vis-à-vis the ligand, and stable geometries of the complexes are identified, alongside their prominent electrophilic and nucleophilic sites.

Organocatalytic [6+4] Cycloadditions via Zwitterionic Intermediates: Chemo-, Regio-, and Stereoselectivities.

The mechanisms and origins of chemo- and stereoselectivities of the organocatalytic [6+4] cycloaddition between 2-cyclopentenone and tropone have been investigated by a combined computational and experimental study. In the presence of a cinchona alkaloid primary amine catalyst and an acid additive, 2-cyclopentenone forms a cross-dienamine intermediate that subsequently undergoes a stepwise [6+4] cycloaddition reaction via a zwitterionic intermediate. The rate-determining transition state features a strong hydrogen-bonding interaction between the tropone oxygen atom and the protonated quinuclidine directing the reaction course leading to a highly periselective [6+4] cycloaddition. The importance of the strong hydrogen-bonding interaction is also demonstrated by the influence of the concentration of the acid additive on the yields and enantioselectivities of the reaction. The corresponding [4+2] cycloaddition reaction has a much higher energy barrier. The enantioselectivity of the [6+4] cycloaddition originates from different repulsive hydrogen-hydrogen interactions that distinguish the diastereomeric transition states.

Density functional theory study on the dihydrogen bond cooperativity in the growth behavior of dimethyl sulfoxide clusters

We have carried out a density functional theory study on the structures of DMSO clusters and analysed the structure and their stability using molecular electrostatic potential and quantum theory of atoms-in-molecules (QTAIM). The ground state geometry of the DMSO clusters, prefer to exist in ouroboros shape. Pair wise interaction energy calculation show the interaction between methyl groups of adjacent DMSO molecules and destabilization is created by the methyl groups which are away from each other. Molecular electrostatic potential analysis shows the existence of σ-hole on the odd numbered clusters, which helps in their highly directional growth. QTAIM analysis show the existence of two intermolecular hydrogen bonds, of type SO⋯HC hydrogen bonds and methyl CH⋯HC dihydrogen bonds. The computed ρ and Laplacian values were all positive for the intermolecular bonds, supporting the existence of noncovalent interactions. The computed ellipticity for the dihydrogen bonds have values>2, which confirms the delocalization of electron, are mainly due to the hydrogen-hydrogen interactions of methyl groups. A plot of total hydrogen bonding energy vs the observed total local electron density shows linearity with correlation coefficient of near unity, which indicates the cooperative effects of intermolecular dihydrogen H·H bonds.

Diffusion Monte Carlo simulations of gas phase and adsorbed D2-(H2)n clusters.

We have computed ground state energies and analyzed radial distributions for several gas phase and adsorbed D2(H2)n and HD(H2)n clusters. An external model potential designed to mimic ionic adsorption sites inside porous materials is used [M. Mella and E. Curotto, J. Phys. Chem. A 121, 5005 (2017)]. The isotopic substitution lowers the ground state energies by the expected amount based on the mass differences when these are compared with the energies of the pure clusters in the gas phase. A similar impact is found for adsorbed aggregates. The dissociation energy of D2 from the adsorbed clusters is always much higher than that of H2 from both pure and doped aggregates. Radial distributions of D2 and H2 are compared for both the gas phase and adsorbed species. For the gas phase clusters, two types of hydrogen-hydrogen interactions are considered: one based on the assumption that rotations and translations are adiabatically decoupled and the other based on nonisotropic four-dimensional potential. In the gas phase clusters of sufficiently large size, we find the heavier isotopomer more likely to be near the center of mass. However, there is a considerable overlap among the radial distributions of the two species. For the adsorbed clusters, we invariably find the heavy isotope located closer to the attractive interaction source than H2, and at the periphery of the aggregate, H2 molecules being substantially excluded from the interaction with the source. This finding rationalizes the dissociation energy results. For D2-(H2)n clusters with n≥12, such preference leads to the desorption of D2 from the aggregate, a phenomenon driven by the minimization of the total energy that can be obtained by reducing the confinement of (H2)12. The same happens for (H2)13, indicating that such an effect may be quite general and impact on the absorption of quantum species inside porous materials.

Seeking for ultrashort “non-bonded” hydrogen–hydrogen contacts in some rigid hydrocarbons and their chlorinated derivatives

In this communication a systematic computational survey on some rigid hydrocarbon skeletons, e.g. half-cage pentacyclododecanes and tetracyclododecanes, and their chlorinated derivatives in order to seek for the so-called ultrashort "non-bonded" hydrogen-hydrogen contacts is done. It is demonstrated that upon a proper choice and modifications of the main hydrocarbon backbones, and addition of some chlorine atoms instead of the original hydrogen atoms in parts of the employed hydrocarbons, the resulting strain triggers structural changes yielding ultrashort hydrogen-hydrogen contacts with inter-nuclear distances as small as 1.38 Angstrom. Such ultrashort contacts are clearly less than the world record of an ultrashort non-bonded hydrogen-hydrogen contact, 1.56 Angstrom, very recently realized experimentally by Pascal and coworkers in in,in-bis(hydrosilane) [J. Am. Chem. Soc. 135, 13235 (2013)]. The resulting computed structures as well as the developed methodology for structure design open the door for constructing a proper set of molecules for future studies on the nature of the so-called non-bonded hydrogen-hydrogen interactions that is now an active and controversial area of research.

Stabilizing hydrogen-hydrogen interactions in cationic indopolycarbocyanine dyes

A quantum chemical DFT method with the hybrid B3LYP functional in the 6-31G(d) basis set is employed to calculate the equilibrium geometric parameters of the ground and excited states of cationic symmetric indopolycarbocyanine dyes. Based on the Bader topological analysis of the electron density distribution function, it is found that there are hydrogen-hydrogen bonding interactions in the ground, first singlet, and first triplet states of the studied compounds. These interactions are assumed to have the dispersion character. The effect of the stabilization of the conformational position of methyl groups due to hydrogenhydrogen interatomic interactions on fluorescence deactivation processes is shown. The total stabilization energy of hydrogen-hydrogen interatomic interactions in dye cations is found, which is ≈9 kcal/mol.

Hydrogen - Hydrogen Adsorption Interaction Energy in Zeolites

A general model based on Ono-Kondo lattice theory for hydrogen adsorption, nearest neighbor interaction energy among adsorbate molecules were derived according to thermodynamic principle. A linear form of the above general model was applied to determine the interaction energies among hydrogen molecules inside adsorption layer from the previous experimental data of hydrogen adsorption on A- and X-type zeolites. The results show that the energies of hydrogen-hydrogen interactions inside the adsorption layer are negative values, indicating that the attractions among the adsorbed hydrogen molecules are prominent in the present work. And the influence of hydrogen molecules outside adsorption layer on the adsorbed hydrogen molecule is not important.

Thermodynamic and kinetic properties of hydrogen defect pairs in SrTiO3 from density functional theory

A density functional theory investigation of the thermodynamic and kinetic properties of hydrogen-hydrogen defect interactions in the cubic SrTiO(3) perovskite is presented. We find a net attraction between two hydrogen atoms with an optimal separation of ∼2.3 Å. The energy gain is ca. 0.33 eV compared to two non-interacting H defects. The main cause of the net attractive potential is elastic defect interactions through lattice deformation. Two possible diffusion paths for the hydrogen defect pair are investigated and are both determined to be faster than the corresponding diffusion path for single hydrogen atoms. Finally, we set up a simple model to determine the contribution from the double hydrogen defect to the total hydrogen flux, and find the double defect to be the main diffusing species at temperatures below ca. 400 °C. Post submission infrared absorption experiments show excellent agreement with the proposed properties of the double hydrogen defect.

Probing the binding and spatial arrangement of molecular hydrogen in porous hosts via neutron Compton scattering

The adsorption of molecular hydrogen (H2) in the alkali-graphite intercalate KC24 has been studied using simultaneous neutron diffraction and Compton scattering. Neutron Compton scattering data for the (H2)xKC24 system (x = 0-2.5) were measured at T = 1.5 K as a function of the relative orientation between the neutron beam and the intercalate c-axis. Synchronous with the above proton-recoil measurements, high-resolution diffraction patterns were measured in backscattering geometry. From these diffraction measurements, the intrinsic mosaicity of the Papyex-based intercalate was determined to be approximately 15 degrees half-width-at-half-maximum, in good agreement with previous studies [Finkelstein et al., Physica B, 2000, 291, 213]. Hydrogen uptake by the intercalate leads to a distinct and readily detectable broadening of the isotropic Compton profile compared to bulk H2, indicative of an enhanced interaction of the H2 molecule with the surrounding solid-state environment. Total proton-recoil intensities also scale linearly with the amount of adsorbed hydrogen. Taking as our starting point previous experimental and theoretical results, the isotropic widths of the proton momentum distributions can be explained on the basis of three energy scales, namely, intramolecular H-H vibrations, followed by H-H librations and H2 centre-of-mass translations. From the coverage dependence of these neutron data, we also establish an upper bound of approximately 10 meV for intermolecular hydrogen-hydrogen interactions. Finally, we observe a weak anisotropy of the width of the proton momentum distributions. Comparison of these experimental data with first-principles predictions indicates that subtle quantum mechanical effects associated with particle delocalisation and exchange lie at the heart of the observed behaviour. Overall, these results demonstrate the suitability and largely untapped potential of neutron Compton scattering to explore H2 uptake by solid-state hosts.

Intramolecular noncovalent interactions: Bis(toluene)chromium(0) conformers

The structure of trans- and cis-conformers of bis(toluene)chromium(0) and the intramolecular interactions in them are studied by means of MP2 and density functional theory along with topological and NBO analyses. It was concluded for the first time that the locations of two hydrogen atoms of each methyl group between the phenyl ring planes in the most stable cis-conformer of bis(toluene)chromium(0) are largely determined by the stabilizing interactions of methyl C-H bonds and their hydrogen atoms with the metal atom and chromium-carbon bonds. It was concluded from the obtained data that the C-H...Cr contacts in the studied conformers should be considered as hydrogen bonds rather than agostic interactions. Contrary to the existing conception of the quantum theory of atoms in molecules, repulsive interaction is shown to occur between the pairs of hydrogen atoms of the two methyl groups in bis(toluene)chromium(0) cis-conformers rather than the stabilizing hydrogen-hydrogen interactions.

Influence of Film-Mediated Interactions on the Microwave and Radio Frequency Spectrum of Spin-Polarized Hydrogen on Helium Films

We argue that helium film-mediated hydrogen-hydrogen interactions strongly reduce the magnitude of cold collision shifts in spin-polarized hydrogen adsorbed on a helium film. With plausible assumptions about experimental parameters this can explain (i) the 2 orders of magnitude discrepancy between previous theory and recent experiments and (ii) the anomalous dependence of the cold collision frequency shifts on the film's 3He covering. The mediated interaction is attractive, suggesting that in current experiments the gas will become unstable before reaching the Kosterlitz-Thouless transition.

Materials for hydrogen storage: structure and dynamics of borane ammonia complex

The activation energies for rotations in low-temperature orthorhombic ammonia borane were analyzed and characterized in terms of electronic structure theory. The perdeuterated (11)B-enriched ammonia borane, (11)BD(3)ND(3), sample was synthesized, and the structure was refined from neutron powder diffraction data at 175 K. This temperature has been chosen as median of the range of previously reported nuclear magnetic resonance spectroscopy measurements of these rotations. A representative molecular cluster model was assembled from the refined geometry, and the activation energies were calculated and characterized by analysis of the environmental factors that control the rotational dynamics. The barrier for independent NH(3) rotation, E(a) = 12.7 kJ mol(-1), largely depends on the molecular conformational torsion in the solid-state geometry. The barrier for independent BH(3) rotation, E(a) = 38.3 kJ mol(-1), results from the summation of the effect of molecular torsion and large repulsive intermolecular hydrogen-hydrogen interactions. However, a barrier of E(a) = 31.1 kJ mol(-1) was calculated for internally correlated rotation with preserved molecular conformation. Analysis of the barrier heights and the corresponding rotational pathways shows that rotation of the BH(3) group involves strongly correlated rotation of the NH(3) end of the molecule. This observation suggests that the barrier from previously reported measurement of BH(3) rotation corresponds to H(3)B-NH(3) correlated rotation.

Are recent water models obtained by fitting diffraction data consistent with infrared/Raman and x-ray absorption spectra?

X-ray absorption (XA) spectra have been computed based on water structures obtained from a recent fit to x-ray and neutron diffraction data using models ranging from symmetrical to asymmetrical local coordination of the water molecules [A. K. Soper, J. Phys.: Condens. Matter 17, S3273 (2005)]. It is found that both the obtained symmetric and asymmetric structural models of water give similar looking XA spectra, which do not match the experiment. The fitted models both contain unphysical structures that are allowed by the diffraction data, where, e.g., hydrogen-hydrogen interactions may occur. A modification to the asymmetric model, in which the non-hydrogen-bonded OH intramolecular distance is allowed to become shorter while the bonded OH distance becomes longer, improves the situation somewhat, but the overall agreement is still unsatisfactory. The electric field (E-field) distributions and infrared (IR) spectra are also calculated using two established theoretical approaches, which, however, show significant discrepancies in their predictions for the asymmetric structural models. Both approaches predict the Raman spectrum of the symmetric model fitted to the diffraction data to be significantly blueshifted compared to experiment. At the moment no water model exists that can equally well describe IR/Raman, x-ray absorption spectroscopy, and diffraction data.

Hydrogen and hydride detection in metallic glasses

Different methods for the detection of hydrogen dissolved in the structure of metallic glasses and the detection of hydrides eventually present in these materials are presented and discussed. It includes a conventional procedure, the electrochemical hydrogen permeation test and two alternative ones: a new method herein named proton elastic recoil track analysis (p-ERTA) and another one based on Hall Effect measurements. These methods were applied in samples of Fe40 Ni38Mo4B18 and Ni81P19 metallic glasses. The electrochemical hydrogen permeation tests were described and discussed based on results of the two metallic glasses used so as to support the results obtained with the new methodologies. The p-ERTA technique was proposed as a combination of two well known and established techniques, namely the elastic recoil detection analysis and the solid state nuclear track detection. It permitted the observation that the spatial localization of hydrogen in the structure of the metallic glass possesses a heterogeneous distribution. The easy-to-perform Hall Effect measurements were useful to detect hydride formation in the structure of a metallic glass and the variation of the hydrogen diffusion coefficient over a wide range of hydrogen concentrations, even in regions where hydrogen-hydrogen interactions are likely to occur.