Migration Of Hydrogen Atom Research Articles

Hydrogen storage performance of palladium nanoparticles decorated graphitic carbon nitride

Pd on nitrogen rich carbon material g-C3N4 has been synthesized by a simple cost-effective method. From the study of the hydrogen storage properties of host g-C3N4 matrix and Pd-g-C3N4 by pressure reduction method using Sievert's apparatus in the ranges 0.1 ≤ PH2(MPa) ≤ 4 and 0 ≤ T(°C) ≤ 100, it has been demonstrated that the hydrogen storage capacity of Pd-g-C3N4 reaches 2.6 wt% at 25 °C and 4 MPa. At room temperature, an increase of 66% in the hydrogen uptake capacity is accounted by Pd nanoparticles (Pd-NPs) decoration over host g-C3N4 matrix, clearly demonstrating the hydrogen spillover process. The good dispersion of Pd-NPs over host g-C3N4 matrix and strengthened interaction between the host g-C3N4 matrix and Pd-NPs catalyze the dissociation of hydrogen molecules and subsequent migration of hydrogen atoms to the host g-C3N4 matrix due to spill over mechanism. From the present study, it can be concluded that the nexus between the catalyst support and catalyst particles results in the high hydrogen uptake capacity.

Using positive-ion electrospray ionization mass spectrometry and H/D exchange study phosphoryl group transfer reactions involved in amino acid ester isopropyl phosphoramidates of Brefeldin A

As mini-chemical models, amino acid ester isopropyl phosphoramidates of Brefeldin A (compounds 2a–2d) were synthesized and investigated by electrospray ionization tandem mass spectrometry in combination with H/D exchange. To further confirm the fragments’s structures, off-line Fourier transform resonance tandem mass spectrometry (FT-ICR-MS/MS) was also performed. The fragmentation rules of compounds 2a–2d have been summarized and the plausible schemes for the fragmentation pathways were proposed. In this study, one dephosphorylated ion and two phosphorylated ions were observed in ESI-MS2 spectra of [M+Na]+ ions for compounds 2a–2d. The possible mechanisms about phosphorylation and dephosphorylation were proposed and confirmed by H/D exchange. For the “dephosphorylation” rearrangement, a nitrogen atom was migrated from the phosphoryl group to the carbon atom of Brefeldin A’s backbone with losing a molecule of C3H7PO3 (122Da). For the “phosphorylation” rearrangement, an oxygen atom of one phosphoryl group attacked the sideward phosphorus atom to form a nine-member ring intermediate, then two steps of CH covalent bond cleavage with consecutive migration of hydrogen atom to lose a molecule of C16H20O2 (244Da). The two proposed rearrangement mechanisms about phosphoryl group transfer might be valuable for the structure analysis of other analogs and provide insights into elucidating the dynamic process of the phosphorylation–dephosphorylation of proteins.

Reactivity of Bis(organoamino)phosphanes with Aluminum(III) Compounds: Straightforward Access to Diiminophosphinates by Means of Hydrogen‐Atom Migration – An Experimental and Theoretical Study

The reactivity of bis(organoamino)phosphanes PhP(NHR)(NHR′) (1a–1c, in which R, R′ = tBu for 1a; tBu, Dip for 1b; and Ph for 1c; Dip = C6H3–2,6-iPr2) and tBuP(NHDip)2 (1d) with Me3Al was investigated. The reaction of 1a or 1b gave in the first step compounds [PhP(NHR)(NR′)]AlMe2 (in which R, R′ = tBu for 2a; tBu, Dip for 2b) as a result of methane elimination that upon heating underwent nitrogen-to-phosphorus hydrogen-atom migration under the formation of diiminophosphinates [Ph(H)P(NR)(NR′)]AlMe2 (in which R, R′ = tBu for 3a; tBu, Dip for 3b). In contrast, phosphane 1c showed a reversed reaction sequence that yielded an intermediate [Ph(H)P(NHPh)(=NPh)]AlMe3 (2c) first as a consequence of hydrogen-atom migration followed by the methane elimination and formation of diiminophosphinate [Ph(H)P(NPh)2]AlMe2 (3c). The partial deprotonation of 1a,b,d using one molar equivalent of nBuLi followed by the treatment with AlCl3 smoothly produced compounds [Ph(H)P(NR)(NR′)]AlCl2 (in which R, R′ = tBu for 4a; tBu, Dip for 4b) and [tBu(H)P(NDip)2]AlCl2 (4d), in which the hydrogen atom was again shifted from the nitrogen to the phosphorus atom. All studied compounds were characterized with the help of elemental analysis; 1H, 13C{1H}, 31P, and 31P{1H} NMR spectra; and in the case of 3c, 4a, 4b, and 4d by using single-crystal X-ray diffraction analysis. The phenomenon of the hydrogen-atom migration was subjected also to a theoretical survey with particular emphasis on the influence of the phosphane used.

Hydrogen migration in formation of NH(A³Π) radicals via superexcited states in photodissociation of isoxazole molecules.

Formation of the excited NH(A(3)Π) free radicals in the photodissociation of isoxazole (C3H3NO) molecules has been studied over the 14-22 eV energy range using photon-induced fluorescence spectroscopy. The NH(A(3)Π) is produced through excitation of the isoxazole molecules into higher-lying superexcited states. Observation of the NH radical, which is not a structural unit of the isoxazole molecule, corroborates the hydrogen atom (or proton) migration within the molecule prior to dissociation. The vertical excitation energies of the superexcited states were determined and the dissociation mechanisms of isoxazole are discussed. The density functional and ab initio quantum chemical calculations have been performed to study the mechanism of the NH formation.

Cleavage of phosphorus-carbon (P-C) bonds of α-amino phosphonates with intramolecular hydrogen migration in the gas phase using electrospray ionization tandem mass spectrometry.

α-Amino phosphonates with intrinsic biological activities have been used in a wide variety of applications. Because of the widespread existence of natural organophosphorus compounds containing P-C bonds such as the α-amino phosphonates, it is important to investigate the gas-phase chemistry of P-C bonds in order to determine their basic properties, which might provide some insights into their biosynthesis and catalytic cleavage. Twenty α-amino phosphonates were successfully synthesized and their fragmentation behavior was systematically investigated using in-solution deuterium labeling in combination with high-resolution Fourier transform ion cyclotron resonance (FTICR) electrospray ionization tandem mass spectrometry. The fragmentation pathways of twenty α-amino phosphonates with different chemical structures were systematically studied. In general, P-C bonds could be easily cleaved via a novel intramolecular hydrogen atom migration from the amino group to the phosphoryl group through a five-membered-ring intermediate in the gas phase. A possible mechanism of the rearrangement of α-amino phosphonates is proposed. An interesting intramolecular hydrogen atom migration between the amino and phosphoryl groups was observed with cleavage of the P-C bond in the molecule through a five-membered-ring intermediate. This characteristic fragmentation pathway not only provides some insights into the basic chemistry of compounds with P-C bonds, but could also have some applications in the structural determination of the α-amino phosphonate analogues.

Directed gas-phase formation of the ethynylsulfidoboron molecule.

As a member of the organo sulfidoboron (RBS) family, the hitherto elusive ethynylsulfidoboron molecule (HCCBS) has been formed via the bimolecular reaction of the boron monosulfide radical (BS) with acetylene (C2H2) under single collision conditions in the gas phase, exploiting the crossed molecular beams technique. The reaction mechanism follows indirect dynamics via a barrierless addition of the boron monosulfide radical with its boron atom to the carbon atom of the acetylene molecule, leading to the trans-HCCHBS intermediate. As predicted by ab initio electronic structure calculations, the initial collision complex either isomerizes to its cis-form or undergoes a hydrogen atom migration to form H2CCBS. The cis-HCCHBS intermediate either isomerizes via hydrogen atom shift from the carbon to the boron atom, leading to the HCCBHS isomer, or decomposes to ethynylsulfidoboron (HCCBS). Both H2CCBS and HCCBHS intermediates were predicted to fragment to ethynylsulfidoboron via atomic hydrogen losses. Statistical (RRKM) calculations report yields to form the ethynylsulfidoboron molecule from cis-HCCHBS, H2CCBS, and HCCBHS to be 21%, 7%, and 72%, respectively, under current experimental conditions. Our findings open up an unconventional path to access the previously obscure class of organo sulfidoboron molecules, which are difficult to access through "classical" formation.

Excursion, Roaming and Migration of Hydrogen Atom during Dissociation of Formaldehyde

Hyung-Rae KimDepartment of Chemistry, Hankuk University of Foreign Studies, Kyonggi-do 449-791, Korea. E-mail: hyung2@hufs.ac.krReceived August 13, 2013, Accepted January 1, 2014Several interesting features in trajectory were observed in the direct dynamics study of formaldehydedissociation above radical dissociation limit. The hydrogen atom deliberately placed on the radical dissociationpath can turn around at some distance from C without completion of dissociation and return to HCO moiety,colliding with it just as in a radical-radical recombination and producing a highly energized molecule.Excursion of a hydrogen atom to a distance of 6-8 bohrs and migration of a hydrogen atom back and forthbetween C and O are two of the most interesting features exhibited by the energized molecule. A series ofexcursions is seen to lead to a different kind of dissociation resembling roaming-like dissociation characterizedby high vibrational excitation of H

Hydrogen and fluorine migration in photo-double-ionization of 1,1-difluoroethylene (1,1-C2H2F2) near and above threshold

We have studied the nondissociative and dissociative photo-double-ionization of 1,1-difluoroethylene using single photons of energies ranging from 40 to 70 eV. Applying a coincident electron-ion three-dimensional momentum imaging technique, kinematically complete measurements have been achieved. We present the branching ratios of the six reaction channels identified in the experiment. Electron-ion energy maps and relative electron emission angles are used to distinguish between direct and indirect photo-double-ionization mechanisms at a few different photon energies. The influence of selection and propensity rules is discussed. Threshold energies of double ionization are extracted from the sum of the kinetic energies of the electrons, which hint to the involvement of different manifolds of states. The dissociative ionization channels with two ionic fragments are explored in detail by measuring the kinetic energy release of the fragment ions, sum of the kinetic energies, as well as the energy sharing of the two emitted electrons. We investigate the migration of hydrogen and fluorine atoms and compare the experimental results to the photo-double-ionization of centrosymmetric linear and planar hydrocarbons (${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$ and ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$) whenever possible.

Direct imaging of monovacancy-hydrogen complexes in a single graphitic layer

Understanding how foreign chemical species bond to atomic vacancies in graphene layers can advance our ability to tailor the electronic and magnetic properties of defective graphenic materials. Here, we use ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and density functional theory to identify the precise structure of hydrogenated single atomic vacancies in a topmost graphene layer of graphite and establish a connection between the details of hydrogen passivation and the electronic properties of a single atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of the graphite surface layer with low-energy ions and then exposing it briefly to an atomic hydrogen environment. High-resolution experimental UHV-STM imaging allows us to determine unambiguously the positions of single missing atoms in the defective graphene lattice and, in combination with the ab initio calculations, provides detailed information about the distribution of low-energy electronic states on the periphery of the monovacancy-hydrogen complexes. We found that a single atomic vacancy where each \ensuremath{\sigma} dangling bond is passivated with one hydrogen atom shows a well-defined signal from the nonbonding \ensuremath{\pi} state, which penetrates into the bulk with a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}$ periodicity. However, a single atomic vacancy with full hydrogen termination of \ensuremath{\sigma} dangling bonds and additional hydrogen passivation of the extended \ensuremath{\pi} state at one of the vacancy's monohydrogenated carbon atoms is characterized by complete quenching of low-energy localized states. In addition, we discuss the migration of hydrogen atoms at the periphery of the monovacancy-hydrogen complexes, which dramatically change the vacancy's low-energy electronic properties, as observed in our low-bias, high-resolution STM imaging.

Competitive hydrogen atom migrations accompanying cascade dissociations of peptide cation-radicals of the z+[rad] type

We report a combined experimental and computational study of energy-resolved collision-induced dissociation (ER-CID) and time-resolved infrared multiphoton dissociation (TR-IRMPD) of z4 ions prepared by electron transfer dissociation of peptide (Ala-Ala-Asn-Ala-Arg+2H)2+ ions. The z4 cation-radicals, ANAR+, undergo competitive dissociations by backbone cleavage and loss of a CONH2 radical from the Asn side chain. The backbone cleavage proceeds by radical-assisted dissociation of the Asn Cα-CO bond, forming an x2 ion intermediate which rapidly dissociates by HNCO elimination to yield a stable z2 fragment ion, AR+. The ER-CID and TR-IRMPD data were consistent with the consecutive nature of the backbone dissociation but showed different branching ratios for the two major fragmentations. The ER-CID data showed branching ratios 0.6–1.0 for the side-chain and backbone cleavages whereas the TR-IRMPD data showed an earlier onset for the latter dissociation. Computational analysis of the potential energy surface with density functional theory and ab initio calculations was carried out to provide structures and energies for the reactant ions as well as several intermediates, products, and transition states. Dissociation pathways for cis and trans amide conformers were distinguished and their energies were evaluated. The threshold dissociation energies for the backbone and side-chain dissociations were similar in accordance with the experimental ER-CID branching ratio. The TR-IRMPD data were interpreted by different absorbances of intermediates produced by hydrogen atom migrations along the dissociation pathways.

Simulation of hydrogen diffusion and boron passivation in crystalline silicon

The previously developed model of hydrogen migration and reactions of hydrogen atoms with electrically active impurity is applied to simulate the hydrogen diffusion and passivation process during plasma deuteration of silicon substrates doped with boron. The calculated deuterium concentration profiles agree well over the length of the passivated region with the experimental data obtained upon treatment in hydrogen plasma at a temperature of 200 °C for 5, 10 and 15 min. On the other hand, to achieve a good fit to the abruptness of the calculated profiles between the passivated and unpassivated regions, it is necessary to suppose that the values of the parameters that describe the absorption of hydrogen interstitials by electrically active dopant atoms decrease with increase in the depth of the passivated region. For example, nonuniform spatial distributions of nonequilibrium point defects generated during plasma treatment can lead to a spatial dependence of hydrogen absorption.

The effect of pre-strain on hydrogen embrittlement in 310S stainless steel

The effect of pre-strain on hydrogen embrittlement (HE) was investigated using STS 310S, considering its application to a ferrous cylinder liner for hydrogen storage. Whereas tensile strength was insignificantly influenced by hydrogen charging, elongation was decreased due to a ductile to brittle fracture transition. However, the degree of the reduction in elongation by hydrogen charging was decreased with increasing pre-strain, indicating that pre-straining improved the resistance to HE by suppressing the fracture transition. The cause for suppression of the fracture transition by pre-strain was investigated through the analyses of strain hardening behavior and thermal desorption of hydrogen. The strain hardening rate curves of annealed and pre-strained specimens was divided into several stages, which were greatly affected by primary and secondary twinning, regardless of hydrogen charging. This strain hardening analysis showed that pre-straining suppressed mechanical twinning during tensile deformation. The thermal desorption analysis revealed that the migration of diffusible hydrogen atoms to twin boundaries was hindered by pre-strain. Therefore, the reason why pre-strain suppressed the fracture transition to improve the resistance to HE was because pre-strain hindered both mechanical twinning during tensile deformation and hydrogen delivery to twin boundaries not to form highly hydrogen-concentrated twin boundaries.

Fragmentation of Hydrocarbon Molecules in Intense Laser Fields Studied by Coincidence Momentum Imaging: A Review

Fragmentation of polyatomic molecules is a fundamental chemical reaction in intense laser fields when the electric field strength of the laser pulse is comparable to or exceeds the intra-molecular Coulomb binding fields. For hydrocarbon molecules, because of the migration of light hydrogen atom (or proton) that can lead to chemical bond rearrangement of molecules, fragmentation of hydrocarbon molecules may result in new fragment species that could not be realized from the initial geometry of molecules by direct bond breaking, making the full characterization of their dynamical fragmentation processes difficult. In this article, we will review recent progress in the study of fragmentation of hydrocarbon molecules induced by intense laser fields using the coincidence momentum imaging method that can provide definite information on the fragmentation channels of molecules. The effect of hydrogen migration on fragmentation of hydrocarbon molecules is reviewed, based on which new possible schemes for controlling chemical bond breaking/formation by controlling the motion of protons within a hydrocarbon molecule is discussed.

Comprehensive Analysis of Gly-Leu-Gly-Gly-Lys Peptide Dication Structures and Cation-Radical Dissociations Following Electron Transfer: From Electron Attachment to Backbone Cleavage, Ion–Molecule Complexes, and Fragment Separation

Experimental data from ion mobility measurements and electron transfer dissociation were combined with extensive computational analysis of ion structures and dissociation energetics for Gly-Leu-Gly-Gly-Lys cations and cation radicals. Experimental and computational collision cross sections of (GLGGK + 2H)(2+) ions pointed to a dominant folding motif that is represented in all low free-energy structures. The local folding motifs were preserved in several fragment ions produced by electron transfer dissociation. Gradient optimizations of (GLGGK + 2H)(+•) cation-radicals revealed local energy minima corresponding to distonic zwitterionic structures as well as aminoketyl radicals. Both of these structural types can isomerize to low-energy tautomers that are protonated at the radical-containing amide group forming a new type of intermediates, -C(•)O(-)NH2(+)- and -C(•)(OH)NH2(+)-, respectively. Extensive mapping with B3LYP, M06-2X, and MP2(frozen core) calculations of the potential energy surface of the ground doublet electronic state of (GLGGK + 2H)(+•) provided transition-state and dissociation energies for backbone cleavages of the N-Cα and amide C-N bonds leading to ion-molecule complexes. The complexes can undergo facile prototropic migrations that are catalyzed by the Lys ammonium group and isomerize enolimine c-type fragments to the more stable amide tautomers. In contrast, interfragment hydrogen atom migrations in the complexes were found to have relatively high transition energies and did not compete with fragment separation. The extensive analysis of the intermediate and transition-state energies led to the conclusion that the observed dissociations cannot proceed competitively on the same potential energy surface. The reactive intermediates for the dissociations originate from distinct electronic states that are accessed by electron transfer.

MECHANICAL PROPERTIES AND TOPOLOGICAL CHANGES IN A Fe-CRYSTAL WITH FIXED AND MIGRATING H-ATOM: A NUMERICAL STUDY

A numerical study, within the framework of molecular statics, is conducted to assess the effect of a fixed and migrating hydrogen atom on the stress–strain data and the topological changes taking place in a 3D BCC α-iron crystal subjected to a tensile elongation. A pair-wise Morse potential is used to describe the interatomic forces. The results for a crystal with 2324 iron atoms show that the presence of a hydrogen atom, either migrating or fixed, produces relatively small effect on the stress–strain data within the elastic and some plastic range of loading. However, the topological changes, represented by the changes in the nearest neighbors for each atom within the cut-off distance after each displacement increment, are substantially different. Similarly, a comparison of the changes in the atomic potential energies when the sudden drop in stresses takes place shows differences between the two cases.

On the degenerate tautomerism

Tautomerism is a phenomenon of dynamic equilibrium between structural isomers (tautomers). This type of isomerism is characterized by fast reversible spontaneous interconversion of structural isomers. This interconversion is facilitated by low or permeable energy barriers between the molecule energy minima. Under the conditions of thermodynamic equilibrium, tautomers co-exist in comparable concentrations if their energy difference is low enough. If the energy difference is zero, tautomerism is referred to as a degenerate tautomerism [1]. The migration of a mobile hydrogen atom between the oxygen atoms of unsymmetrical intramolecular hydrogen bond in cis-enol of malonic dialdehyde (3

Comparison of the Mechanism of Borane, Silane, and Beryllium Hydride Ring Insertion into N-Heterocyclic Carbene C–N Bonds: A Computational Study

A computational investigation has been carried out on the mechanism and energetics of the experimentally observed insertion/ring expansion of N-heterocyclic carbenes (NHCs) by boranes (H2BNHR, BH3; R = Me, Ph) and beryllium hydrides (BeH2) in comparison with silanes (SiH2R2; R = Me, Ph). The results suggest that the ring insertion mechanisms are similar for boranes, beryllium hydrides, and silanes. The principal mechanism components are (1) hydrogen atom migration to the carbene carbon, (2) C–N bond expansion of the NHC with insertion of the main-group hydride into the ring, and (3) migration of a second hydrogen atom to the carbene carbon. The synthetically important NHC·BH3 adduct is also predicted to be thermodynamically unstable with respect to this transformation but is kinetically stabilized with a high barrier to the first hydrogen atom migration. The BeH2 insertion product provides a rare example of a Be–N π interaction.

Glutathione--hydroxyl radical interaction: a theoretical study on radical recognition process.

Non-reactive, comparative (2×1.2 μs) molecular dynamics simulations were carried out to characterize the interactions between glutathione (GSH, host molecule) and hydroxyl radical (OH•, guest molecule). From this analysis, two distinct steps were identified in the recognition process of hydroxyl radical by glutathione: catching and steering, based on the interactions between the host-guest molecules. Over 78% of all interactions are related to the catching mechanism via complex formation between anionic carboxyl groups and the OH radical, hence both terminal residues of GSH serve as recognition sites. The glycine residue has an additional role in the recognition of OH radical, namely the steering. The flexibility of the Gly residue enables the formation of further interactions of other parts of glutathione (e.g. thiol, α- and β-carbons) with the lone electron pair of the hydroxyl radical. Moreover, quantum chemical calculations were carried out on selected GSH/OH• complexes and on appropriate GSH conformers to describe the energy profile of the recognition process. The relative enthalpy and the free energy changes of the radical recognition of the strongest complexes varied from −42.4 to −27.8 kJ/mol and from −21.3 to 9.8 kJ/mol, respectively. These complexes, containing two or more intermolecular interactions, would be the starting configurations for the hydrogen atom migration to quench the hydroxyl radical via different reaction channels.

H2CN+ and H2CNH+: New insight into the structure and dynamics from mass-selected threshold photoelectron spectra

In this paper, we reinvestigate the photoionization of nitrogen containing reactive intermediates of the composition H2CN and H2CNH, molecules of importance in astrochemistry and biofuel combustion. In particular, H2CN is also of considerable interest to theory, because of its complicated potential energy surface. The species were generated by flash pyrolysis, ionized with vacuum ultraviolet synchrotron radiation, and studied by mass-selected threshold photoelectron (TPE) spectroscopy. In the mass-selected TPE-spectrum of m/z = 28, contributions of all four isomers of H2CN were identified. The excitation energy to the triplet cation of the methylene amidogen radical H2CN was determined to be 12.32 eV. Considerable activity in the C-N mode of the cation is visible. Furthermore, we derived values for excitation into the triplet cations of 11.72 eV for cis-HCNH, 12.65 eV for trans-HCNH, and 11.21 eV for H2NC. The latter values are probably accurate to within one vibrational quantum. The spectrum features an additional peak at 10.43 eV that corresponds to excitation into the C(2v)-symmetric H2CN(+). As this structure constitutes a saddle point, the peak is assigned to an activated complex on the singlet potential energy surface of the cation, corresponding to a hydrogen atom migration. For methanimine, H2CNH, the adiabatic ionization energy IE(ad) was determined to be 9.99 eV and the vibrational structure of the spectrum was analyzed in detail. The uncertainty of earlier values that simply assigned the signal onset to the IE(ad) is thus considerably reduced. The spectrum is dominated by the H-N-C bending mode ν1(+) and the rocking mode ν3(+). All experimental data were supported by calculations and Franck-Condon simulations.

From the mobile proton to wandering hydride ion: mechanistic aspects of gas‐phase ion chemistry

Structural characterization of molecular species by mass spectrometry supposes the knowledge of the type of ions generated and the mechanism by which they dissociate. In this context, a need for a rationalization of electrospray ionization(+)(-) mass spectra of small molecules has been recently expressed. Similarly, at the other end of the mass scale, efforts are currently made to interpret the major fragmentation processes of protonated and deprotonated peptides and their reduced forms produced in electron capture or electron transfer experiments. Most fragmentation processes of molecular and pseudo-molecular ions produced in the ion source of a mass spectrometer may be described by a combination of several key mechanistic steps: simple bond dissociation, formation of ion-neutral complex intermediates, hydrogen atom, hydride ion or proton migrations and nucleophilic attack. Selected crucial aspects of these elementary reactions, occurring inside positively charged ions, will be recalled and illustrated by examples taken in recent mass spectrometry literature. Emphasis will be given on the protonation process and its consequence in terms of structure and energetic.