Role Of Hydrogen Atoms Research Articles

Bonding and Acidity of the Formal Hydride in the Prototypical Au9 (PPh3 )8 H2+ Nanocluster.

The role of hydrogen atoms as surface ligands on metal nanoclusters is of profound importance but remains difficult to directly study. While hydrogen atoms often appear to be incorporated formally as hydrides, evidence suggests that they donate electrons to the cluster's delocalized superatomic orbitals and may consequently behave as acidic protons that play key roles in synthetic or catalytic mechanisms. Here we directly test this assertion for the prototypical Au9 (PPh3 )8 H2+ nanocluster, formed by addition of a hydride to the well-characterized Au9 (PPh3 )8 3+ . Using gas-phase infrared spectroscopy, we were able to unambiguously isolate Au9 (PPh3 )8 H2+ and Au9 (PPh3 )8 D2+ , revealing an Au-H stretching mode at 1528 cm-1 that shifts to 1038 cm-1 upon deuteration. This shift is greater than the maximum expected for a typical harmonic potential, suggesting a potential governing cluster-H bonding that has some square-well character consistent with the hydrogen nucleus behaving as a metal atom in the cluster core. Complexing this cluster with very weak bases reveals a redshift of 37 cm-1 in the Au-H vibration, consistent with those typically seen for moderately acidic groups in gas phase molecules and providing an estimate of the acidity of Au9 (PPh3 )8 H2+ , at least with regard to its surface reactivity.

Bonding and Acidity of the Formal Hydride in the Prototypical Au9(PPh3)8H2+ Nanocluster

AbstractThe role of hydrogen atoms as surface ligands on metal nanoclusters is of profound importance but remains difficult to directly study. While hydrogen atoms often appear to be incorporated formally as hydrides, evidence suggests that they donate electrons to the cluster's delocalized superatomic orbitals and may consequently behave as acidic protons that play key roles in synthetic or catalytic mechanisms. Here we directly test this assertion for the prototypical Au9(PPh3)8H2+ nanocluster, formed by addition of a hydride to the well‐characterized Au9(PPh3)83+. Using gas‐phase infrared spectroscopy, we were able to unambiguously isolate Au9(PPh3)8H2+ and Au9(PPh3)8D2+, revealing an Au−H stretching mode at 1528 cm−1 that shifts to 1038 cm−1 upon deuteration. This shift is greater than the maximum expected for a typical harmonic potential, suggesting a potential governing cluster‐H bonding that has some square‐well character consistent with the hydrogen nucleus behaving as a metal atom in the cluster core. Complexing this cluster with very weak bases reveals a redshift of 37 cm−1 in the Au−H vibration, consistent with those typically seen for moderately acidic groups in gas phase molecules and providing an estimate of the acidity of Au9(PPh3)8H2+, at least with regard to its surface reactivity.

A New Understanding to the High-Pressure Hydride Superconductors— Role of Hydrogen Atoms and Effect of Pressure

A New Understanding to the High-Pressure Hydride Superconductors— Role of Hydrogen Atoms and Effect of Pressure

The role of hydrogen atoms in redox catalysis by the flavoenzyme cholesterol oxidase.

Flavoenzymes comprise a large class of proteins that carry out a diverse range of important redox chemistry. Although X-ray crystal structures of many flavoenzymes have been determined, there are still unresolved questions regarding the actual oxidation state of the flavin cofactors in these structures due to photoreduction by the ionizing radiation of the X-ray beam during the diffraction experiment. Additionally, the ability to visualize hydrogen atoms in X-ray structures is difficult due to the weak scattering capability of these atoms. Since hydrogen atoms affect the electrostatic nature of enzyme active sites and play important roles in the chemistry of key amino acid residues, visualizing the precise positions of these atoms provides a more detailed understanding of their role in enzyme catalysis. Single crystal neutron diffraction is an alternative method to structure determination, circumventing problems associated with photoreduction of the sample thus providing a clearer view of the structural features of a flavoenzyme in different redox states. Additionally, the larger neutron scattering factors for hydrogen and deuterium atoms enables one to visualize these atoms much more easily than from X-ray scattering measurements. In this chapter we give an overview of neutron and X-ray crystallography studies on the flavoenzyme, cholesterol oxidase and how the observations of unusual hydrogen atom positions provide insight into the redox chemistry of the flavin cofactor.

Atomic and Electronic Properties of a 155 H2S Cluster under Pressure.

This is an all-electron density functional study of a cluster with 155 H2S molecules subjected to pressures between 0.2 and 681.2 GPa. For modeling pressure, the cluster was in a container made of 500 He atoms. As the pressure increased, the bond length between the atoms decreased. This decrease changed the atomic distribution of the cluster. Initially, the H2S molecules interacted weakly through hydrogen bonds. Then, the pressure moved the H atoms along the axis connecting two sulfur atoms, with S–H bond lengths between 1.4 and 1.6 Å. At high pressures, the atomic distribution consisted of interleaved layers of H and S atoms. The energy density of states of the valence band had two sub-bands with an energy gap between them. The overlapping of the 2a1 molecular orbitals of the H2S molecules determined the molecular orbitals in the low-energy sub-band. In this sub-band, the molecular orbital with the lowest energy has no nodes; at high pressures, it has non-zero values for all the internuclear regions of the cluster. The overlapping of the molecular orbitals 1b2, 3a1, and 1b1 of the H2S molecules determined the orbitals in the high-energy sub-band. The energy band gap (lowest unoccupied molecular orbital–highest occupied molecular orbital) decreased with the pressure, from 5.3906 eV for 0.2 GPa to 0.4980 eV for 681.2 GPa, whereas the gap between the sub-bands decreased from 4.7729 eV for 0.2 GPa to 0.03 eV for pressures higher than 125.5 GPa. The present study provides, from first principles, an idea on the role of hydrogen atoms in the evolution of solid phases of H2S with pressure, which is difficult to obtain from experiments.

First-principles insights into role of hydrogen atom in black titania

We employ first-principles calculations to investigate hydrogen doping at the (101) surface of the anatase TiO2, as the most stable surface of black titania. Interstitial sites at the surface as well as substitutional sites at the surface and subsurface are considered as possible doping configurations. In the framework of first-principles thermodynamics, it is shown that surface adsorption is more probable in normal environment, while substitutional doping at the surface is feasible in a low oxygen pressure environment, in agreement with experimental observations. It is found that the adsorbed hydrogen impurity creates delocalized electron carriers with Ti-3d character at the bottom of the conduction band, which may enhance light absorption in the infrared and visible regions. On the contrary, the substituted hydrogen impurity results in a localized mid gap state, which reduces oxidation states of the neighboring Ti atom from 4+ to 3+. This localized state may act as a trapping center for electron-hole recombination and thus reducing the photocatalytic efficiency of the system.

Experimental Study of Desulfurization of Crude Fraction by In Situ Generated Hydrogen- The Role of Hydrogen Atoms

not Available.

ChemInform Abstract: The Role of Hydrogen Atoms in Interactions Involving Imidazolium‐Based Ionic Liquids

AbstractReview: 102 refs.

The role of hydrogen atoms in interactions involving imidazolium-based ionic liquids

In the first part of this report experimental results are discussed which focus onto the importance of hydrogen atoms in the interaction of imidazolium-based ionic liquids. These include examples for the cation–anion interaction in neat ionic liquids as well as the interactions between ionic liquids and their molecular environment, water in particular. Most of the studies emphasize the importance of the C(2)–H group of the imidazolium ring for the intra- and intermolecular interactions; commonly, the interactions of the type C–H … X (X =: O, halide) are attributed to “hydrogen bonding”. In the second part it is analyzed whether these interactions and their consequences fulfill the criteria set by standard definitions of hydrogen bonding. Two cation–anion co-conformations at the C(2)–H group are found. One co-conformer (in-plane) often resembles a hydrogen bond while the other one (on-top) points to a non-hydrogen bonding behavior. Furthermore, the degree of hydrogen bonding for the in-plane structure is very dependent on the anion. Spatial distribution functions show that, in general, both co-conformations are occupied. However, the question of how long a particular co-conformer is populated in the liquid state has yet to be answered. Therefore, it is concluded that the term “hydrogen bond” should, at present, be treated with care to characterize the cation–anion contacts, because of the above-mentioned difficulties. Once more it must be stressed that oversimplifications and generalizations, even for this subclass of ionic liquids have to be avoided, because these liquids are more complicated than it appears from first sight.

Effect of the vapors of organometallic compounds on the processes of ignition and combustion of hydrogen, propylene, and natural gas

It is established that small additions (∼10−1%) of hexacarbonyls of chromium and molybdenum promote combustion of the stoichiometric 2H2 + O2 mixture, which manifests itself in a decrease in the lower limit of initiated ignition by the pressure and in an increase in the visible propagation rate of the flame. However, inhibition of oxidation of propylene by these additives takes place. This fact means that the role of hydrogen atoms in the development of chains during flame propagation in hydrocarbon-air mixtures is not determining. Therefore, the kinetic mechanism inhibiting combustion of hydrocarbons by carbonyls based on accounting for termination of hydrogen atoms, which is suggested in the literature, should be refined. In the flame of oxidation of hydrogen in the presence of chromium hexacarbonyl, excited chromium atoms are found. Comparison of the inhibiting efficiency of the vapors of ferrocene Fe(C5H5)2, butyrylferrocene C14H16FeO, carbonyls of metals and their mixture with the vapors of sulfur hexafluoride SF6, and a series of halogen-substituted hydrocarbons (CF2Cl2, C4H9J, CCl4, CHCl3) on combustion of mixtures of natural gas with air is performed.

Effects of Hydrogen Plasma Treatment on Hysteresis Phenomenon and Electrical Properties for Solid Phase Crystallized Silicon Thin Film Transistors

AbstractWe have investigated the effects of hydrogen plasma treatment on the hysteresis phenomenon and electrical properties of solid phase crystallized silicon thin film transistors (SPC-Si TFTs) employing alternating magnetic field crystallization (AMFC). We employed H2 plasma treatment on the SPC-Si active layer before SiO2 gate insulator deposition. By increasng the power and time duration of H2 plasma treatment, it was observed that hysteresis phenomenon of SPC-Si TFT was suppressed and electrical properties such as threshold voltage, field effect mobility was improved considerably. This is due to role of hydrogen atom by passivating the defects and grain boundary trap states in SPC-Si film. However, relatively high power and long hydrogen plasma treatment (100W, 5 minutes) could degrade the electrical characteristics of the device. SPC-Si TFT for 100W power of PECVD and 3 minutes with the H2 plasma treatment exhibit the significant improvement of electrical characterics (VTH = - 3.85V, μFE = 21.16cm2/Vs), and a smaller hysteresis phenomenon (∆VTH = -0.30V) which is suitable for high quality AMOLED Display.

Different effects of active minor admixtures on hydrogen and methane ignitions

The methane combustion inhibitor CCl4 exerts no effect on the first ignition limit of hydrogen; therefore, the role of hydrogen atoms in hydrocarbon oxidation consists at least of participating in longer reaction chains than are observed in hydrogen oxidation. The upper limits of the rate constants of the reactions of hydrogen atoms with propylene and isobutylene molecules were estimated by the self-ignition limit method to be (1.0 ± 0.3) × 10−11 exp(−1450 ± 400/T) and (0.8 ± 0.3) × 10−11 exp(−550 ± 200/T) cm3 molecule−1 s−1, respectively, in the temperature range of 840–950 K. These data are evidence that the stronger inductive effect of the two methyl groups in isobutylene lowers the energy barrier to the H + iso-C4H8 reaction. It has been demonstrated experimentally that chemiluminescence in the hydrocarbon flame front at atmospheric pressure precedes heat evolution. Throughout the pressure and temperature ranges examined (5–750 Torr, 298–950 K), the chain mechanism determines the basic laws of combustion.

A Model for Separation and Melting of Deoxyribonucleic Acid in Replication and Transcription Processes

In order to explain the denaturation and melting in replication and transcription of deoxyribonucleic acid (DNA) at physiological temperature, we propose a dynamical model on the basis of structure and motion of DNA under the actions of energy released in hydrolysis of adenosine triphosphate (ATP) and enzymes. The model admits three degrees of freedom per base pair: two displacement variables associated with the vibrations of hydrogen atom in the hydrogen bonds and base (nucleotide), respectively, and an angular variable describing the rotation of base. The important role of hydrogen atom in the hydrogen bonds is stressed in this model. The Hamiltonian of the intact double helix system is given first, subsequently the equations of motion and solutions of a melting system are developed. The solutions represent the excited states formed by the displacements of hydrogen atoms and bases and the rotations of bases, arising from the energy absorbed by DNA, respectively. The results obtained show that the displacements of hydrogen atoms increase with time and along the helix. Once the displacements of hydrogen atoms and rotations of bases reach to certain limits, the hydrogen bonds are disrupted, and the two double helical strands of DNA are separated, a melting state in the replication or transcription process occurs. The properties of thermal motion of hydrogen atoms in this state at physiological temperature is further studied by using a transfer integral method and the thermodynamic properties such as free energy and entropy of the thermally excited state are obtained in the process. Values of characteristic parameters and critical temperature of melting in transcription process are derived. Finally the validity of the theory was demonstrated through comparison between experimental and theoretical data of specific heat and force of phase transition. This investigation is helpful to understand mechanism and essence of the replication or transcription process and promote quantitative description of these processes.

Recent results on hydrogen and hydration in biology studied by neutron macromolecular crystallography.

Neutron diffraction provides an experimental method of directly locating hydrogen atoms in proteins, a technique complimentary to ultra-high-resolution [1, 2] X-ray diffraction. Three different types of neutron diffractometers for biological macromolecules have been constructed in Japan, France and the United States, and they have been used to determine the crystal structures of proteins up to resolution limits of 1.5-2.5 A. Results relating to hydrogen positions and hydration patterns in proteins have been obtained from these studies. Examples include the geometrical details of hydrogen bonds, H/D exchange in proteins and oligonucleotides, the role of hydrogen atoms in enzymatic activity and thermostability, and the dynamical behavior of hydration structures, all of which have been extracted from these structural results and reviewed. Other techniques, such as the growth of large single crystals, the preparation of fully deuterated proteins, the use of cryogenic techniques, and a data base of hydrogen and hydration in proteins, will be described.

Hydrogen and hydration in proteins.

Neutron diffraction provides an experimental method of directly locating hydrogen atoms in proteins. High-resolution neutron diffractometers dedicated to biological macromolecules (BIX-type diffractometer) have been constructed at the Japan Atomic Energy Research Institute and they have been used in the 1.5A-resolution crystal structure analyses of several proteins. Interesting topics relevant to hydrogen and hydration in proteins, such as (1) the detailed geometry of hydrogen bonds; (2) information regarding hydrogen/deuterium exchange behavior; (3) the acidities of certain H atoms; (4) the role of hydrogen atoms in enzyme mechanisms and thermostability; (5) the location methyl hydrogen atoms; and (6) dynamical behavior of hydration structures that include H positions have been extracted from these structural results. In addition, a method for the systematic growth of large single crystals based on phase diagrams has been introduced and will be briefly described in this article.

The Role of Hydrogen Atoms and Molecules in the Plasma Boundary

A status report on recent spectroscopic investigations on hydrogen atoms and molecules is given. The interplay between atoms, molecules and their interaction with the surrounding surfaces is described and the resulting balances of particles, energy and momentum for the development of the different fluxes presented.

Hydrogen-terminated Si Surfaces. Role of Hydrogen Atoms in the Initial Oxidation Processes of H-Terminated Si(100) Surfaces.

Initial oxidation processes and local bonding structures of hydrogen-terminated Si(100)-2×1 and H2O-terminated Si(100)-2×1 surfaces have been examined by high-resolution energy loss spectroscopy (HREELS). The hydrogen adsorption on Si(100) surfaces suppress the oxidation of dimer bond sites, and oxygen atoms preferentially adsorb on one of the two back-bond sites of surface Si atoms. On the other hand, oxidation proceeds randomly up to an oxygen coverage of 3 ML on H-terminated Si(111) surfaces. The Si-O-Si bonds formed on H-terminated Si(100) surfaces are more relaxed than those on clean Si(100) surfaces, which is considered to originate from the change in bond angles of Si-O-Si bonds. The H2O-terminated Si(100)-2×1 surfaces are also stable for the adsorption of oxygen molecules. However, the uptake of oxygen atoms of Si-OH species into back-bond sites occurs even at room temperature by the reaction of H2O-terminated Si(100) surfaces with atomic hydrogen.

The role of hydrogen atoms in CIDNP effects in the reaction of diisobutylaluminum hydride with CCl4

Integral polarization of chloroform, methylene dichloride, and pentachloroethane was observed in the1H NMR spectra during the exothermal reaction of a 1M solution of Bu2 i in 1,4-dioxane with CCI4. CIDNP was shown to appear in the diffusion radical pair of the hydrogen atom and trichloromethyl radical.

The roles of deposition pressure and rf power in opto-electronic properties of a-SiO:H films

Hydrogenated amorphous silicon oxide (a-SiO:H) films have been prepared in a conventional single-chamber radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition system with a carbon dioxide , silane and hydrogen gas mixture. Variation in optical gap from 1.96 to 2.12 eV has been obtained by controlling the deposition pressure and radio frequency (rf) power. The photoconductivities of films in each set are similar to each other for similar optical gaps. High rf power does not have any remarkable deteriorative effect on material but the rate of deposition of film and optical gap are enhanced. At rf power density the and are and 2.11 eV respectively whereas under similar other conditions but with rf power density the corresponding and are and 1.96 eV respectively. By lowering the deposition pressure the optical gap of films has also been enhanced. At 1.5 Torr pressure, the optical gap is 1.96 eV and at 0.13 Torr it becomes 2.12 eV. Here variation of the dilution of hydrogen is adopted in order to look into the role of hydrogen atoms in plasma and characteristics of the deposited films. A model in which incorporation of oxygen is described through plasma kinetics is proposed in this regard. With a higher optical gap the photoconductivity reduces but this reduction is mostly due to there being a lower optical absorption coefficient , partly due to there being a lower value of the product of the mobility , lifetime and quantum efficiency and also partly due to the mid-gap defect density . For the power variation set of samples, with optical gap up to about 1.99 eV, and the photoconductivity remain around and respectively and, with this optical gap, the dark conductivity is . When the material is degraded by soaking in light, a modification of mid-gap states and also of the valence band tail state takes place and falls, together with the photoconductivity. For material of 1.95 eV optical gap the initial and are reduced to and respectively after 3 h of soaking in light.

G2ab initio calculations on three-membered rings: Role of hydrogen atoms

G2 ab initio calculations on all ABX three-membered rings (TMRs) that can be derived from cyclopropane by systematic substitution of the (SINGLE BOND)CH2 groups by (SINGLE BOND)NH or (SINGLE BOND)O groups have been performed. Our results show that the decrease in the A(SINGLE BOND)B bond length as the electronegativity of X increases is significantly larger than that found for the corresponding acyclic analogs. In general, a systematic substitution of the (SINGLE BOND)CH2 groups of cyclopropane by (SINGLE BOND)NH or (SINGLE BOND)O groups implies significant geometric changes that are not reflected in a parallel change of the corresponding conventional ring strain energy (CRSE). When the electronegativity of the groups forming the TMR increases the effect on the CRSE of the system is small, although the charge delocalization inside the ring decreases. The near constancy of the CRSE along the series can be explained in terms of the charge redistribution of the system where the (SINGLE BOND)CH2 groups play a crucial role. There are, however, significant changes in the hydrogenation energies of the TMR investigated; our results show that, when in an ABX three-membered ring, the electronegativity of X increases the hydrogenation energy of A(SINGLE BOND)B bond decreases and vice versa. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1072–1086, 1998