Exchange Of Hydrogen Atoms Research Articles

The feasibility of formation and kinetics of NMR signal amplification by reversible exchange (SABRE) at high magnetic field (9.4 T).

1H NMR signal amplification by reversible exchange (SABRE) was observed for pyridine and pyridine-d5 at 9.4 T, a field that is orders of magnitude higher than what is typically utilized to achieve the conventional low-field SABRE effect. In addition to emissive peaks for the hydrogen spins at the ortho positions of the pyridine substrate (both free and bound to the metal center), absorptive signals are observed from hyperpolarized orthohydrogen and Ir-complex dihydride. Real-time kinetics studies show that the polarization build-up rates for these three species are in close agreement with their respective 1H T1 relaxation rates at 9.4 T. The results suggest that the mechanism of the substrate polarization involves cross-relaxation with hyperpolarized species in a manner similar to the spin-polarization induced nuclear Overhauser effect. Experiments utilizing pyridine-d5 as the substrate exhibited larger enhancements as well as partial H/D exchange for the hydrogen atom in the ortho position of pyridine and concomitant formation of HD molecules. While the mechanism of polarization enhancement does not explicitly require chemical exchange of hydrogen atoms of parahydrogen and the substrate, the partial chemical modification of the substrate via hydrogen exchange means that SABRE under these conditions cannot rigorously be referred to as a non-hydrogenative parahydrogen induced polarization process.

Synthesis of deuterium-labelled fluorobenzoic acids to be used as internal standards in isotope dilution mass spectrometry

A method for the deuterium labelling of fluorobenzoic acids (FBAs) via acidic H/D exchange of the aromatic hydrogen atoms in concentrated D2SO4 is described. The synthesis is shown to be easy, fast, low-priced and without the use of catalysts or further purification. The use of at least double-deuterated FBAs as internal standards in organic isotope dilution mass spectrometry allows the determination of FBAs in complex matrices with highest possible accuracy in combination with a simplified analytical evaluation.

Characteristics of the Multi-Hydrogen Bonded Systems: DFT Description on the Solvated Electrons

The multi-hydrogen bonded systems with the solvated electrons are investigated at the B3LYP/6-311++G** basis set level. The symmetrical linear geometrical characteristic is common for the dimer systems, while for the tetramer system, the tetrahedron configuration is generated. The NBO charge analyses demonstrate that the multi-hydrogen-multi-electron (mH-ne) coupling exist in these anion systems, as is supported by the electrostatic potential and the molecular orbital analyses. The positive chemical shift value of the central hydrogen (Hc) and the negative chemical shift value of the terminal hydrogen (Ht) indicate that the Hc is electronegative while the Ht is electropositive, respectively. Strong coupling between two central hydrogen atoms is demonstrated by the large spin-spin coupling constants. The solvated electron donates significant contributions for the stability of these systems.

Gas-Phase Synthesis of Phenyl Oxoborane (C6H5BO) via the Reaction of Boron Monoxide with Benzene

Organyl oxoboranes (RBO) are valuable reagents in organic synthesis due to their role in Suzuki coupling reactions. However, organyl oxoboranes (RBO) are only found in trimeric forms (RBO3) commonly known as boronic acids or boroxins; obtaining their monomers has proved a complex endeavor. Here, we demonstrate an oligomerization-free formation of organyl oxoborane (RBO) monomers in the gas phase by a radical substitution reaction under single-collision conditions in the gas phase. Using the cross molecular beams technique, phenyl oxoborane (C6H5BO) is formed through the reaction of boronyl radicals (BO) with benzene (C6H6). The reaction is indirect, initially forming a van der Waals complex that isomerizes below the energy of the reactants and eventually forming phenyl oxoborane by hydrogen emission in an overall exoergic radical-hydrogen atom exchange mechanism.

Cp2TiCH2CHMe(SiMe3)]+, an Alkyl–Titanium Complex Which (a) Exists in Equilibrium between a β-Agostic and a Lower Energy γ-Agostic Isomer and (b) Undergoes Hydrogen Atom Exchange between α-, β-, and γ-Sites via a Combination of Conventional β-Hydrogen Elimination–Reinsertion and a Nonconventional CH Bond Activation Process Which Involves Proton Tunnelling

The compound [Cp2Ti(Me)(CD2Cl2)][B(C6F5)4] reacts with trimethylvinylsilane (TMVS) to form the 1,2-insertion product [Cp2TiCH2CHMe(SiMe3)](+) (III), which exists in solution as equilibrating β- and γ-agostic isomers. In addition, while free rotation of the β-methyl group results in a single, averaged γ-H atom resonance at higher temperatures, decoalescence occurs below ~200 K, and the resonance of the γ-agostic hydrogen atom at δ ~ -7.4 is observed. Reaction of [Cp2Ti(CD3)(CD2Cl2)](+) with TMVS results in the formation of [Cp2TiCH2CH(CD3)(SiMe3)](+), which converts, via reversible β-elimination, to an equilibrium mixture of specifically [Cp2TiCH2CH(CD3)(SiMe3)](+) and [Cp2TiCD2CD(CH3)(SiMe3)](+). Complementing this conventional process, exchange spectroscopy experiments show that the β-H atom of [Cp2TiCH2CHMe(SiMe3)](+) undergoes exchange with the three hydrogen atoms of the β-methyl group (β-H/γ-H exchange) but not with the two α-H atoms. This exchange process is completely shut down when [Cp2TiCH2CH(CD3)(SiMe3)](+) is used, suggesting an H/D kinetic isotope effect much larger (apparently >16,000) than the maximum possible for an over-the-barrier process. It is proposed that β-H/γ-H exchange is facilitated by quantum mechanical proton tunnelling in which a hydrogen atom of the 2-methyl group of the alkene-hydride deinsertion product [Cp2TiH{CH2═CMe(SiMe3)}](+) undergoes reversible exchange with the hydride ligand via the allyl dihydrogen species [Cp2TiH2{(η(3)-CH2C(SiMe3)CH2}](+). Complementing these findings, DFT calculations were carried out to obtain energies and NMR parameters for all relevant species and thence to obtain better insight into the agostic preference(s) of complex III and the observed exchange processes. In all cases where comparisons between experimental and calculated data were possible, agreement was excellent.

Evaluation of the efficiency of Pd/H2‐catalyzed benzylic H/D exchange of dehydroabietinal with D2O and synthesis of a tritium‐labeled analogue

Dehydroabietinal (DA) has been identified as an important signaling molecule in systemic acquired resistance in plants. Deuterium and tritium-labeled DA were synthesized to confirm its role in signaling and to further elucidate the mechanism by which DA induces systemic acquired resistance. Pd/H2 -catalyzed exchange of benzylic hydrogen atoms of DA with (2) H-H2 O or (3) H-H2 O was conducted with >97% label incorporation for (2) H-DA and a specific activity of 12.6 mCi/mmol for (3) H-DA synthesized from 90 mCi/mmol (3) H-H2 O. The extent of deuterium labeling at each benzylic position was determined via an inverse-gated (13) C NMR experiment. C7 and C15 were 87% and 81% labeled, respectively. Isotope-induced chemical shift changes at C6 were used to approximate the amount of singly (66%) and doubly (17%) labeled (2) H-DA at C7. Results also indicated that two of the three benzylic protons in DA underwent facile exchange. Exchange at the remaining position was likely hampered by steric interactions of nearby methyl groups at the surface of the Pd catalyst.

Multivalency in the Gas Phase: H/D Exchange Reactions Unravel the Dynamic “Rock ’n’ Roll” Motion in Dendrimer-Dendrimer Complexes

Noncovalent dendrimer-dendrimer complexes were successfully ionized by electrospray ionization of partly protonated amino-terminated polypropylene amine (POPAM) and POPAM dendrimers fully functionalized with benzo[21]crown-7 on all branches. Hydrogen/deuterium exchange (HDX) experiments conducted on dendrimer-dendrimer complexes in the high vacuum of a mass spectrometer give rise to a complete exchange of all labile NH hydrogen atoms. As crown ethers represent noncovalent protective groups against HDX reactions on the ammonium group to which they are coordinated, this result provides evidence for a very dynamic binding situation: each crown is mobile enough to move from one ammonium binding site to another. Schematically, one might compare this motion with two rock 'n' roll dancers that swirl around each other without completely losing all contact at any time. Although the multivalent attachment certainly increases the overall affinity, the "microdynamics" of individual site binding and dissociation remains fast.

COLLISIONLESS SHOCKS IN A PARTIALLY IONIZED MEDIUM. III. EFFICIENT COSMIC RAY ACCELERATION

In this paper we present the first formulation of the theory of non-linear particle acceleration in collisionless shocks in the presence of neutral hydrogen in the acceleration region. The dynamical reaction of the accelerated particles, the magnetic field amplification and the magnetic dynamical effects on the shock are also included. The main new aspect consists however in accounting for charge exchange and ionization of neutral hydrogen, which profoundly change the structure of the shock, as discussed in our previous work. This important dynamical effect of neutrals is mainly associated to the so-called neutral return flux, namely the return of hot neutrals from the downstream region to the upstream, where they deposit energy and momentum through charge exchange and ionization. We also present the self-consistent calculation of Balmer line emission from the shock region and discuss how to use measurements of the anomalous width of the different components of the Balmer line to infer the cosmic ray acceleration efficiency in supernova remnants showing Balmer emission: the broad Balmer line, which is due to charge exchange of hydrogen atoms with hot ions downstream of the shock, is shown to become narrower as a result of the energy drainage into cosmic rays, while the narrow Balmer line, due to charge exchange in the cosmic-ray--induced precursor, is shown to become broader. In addition to these two well-known components, the neutral return flux leads to the formation of a third component with intermediate width: this too contains information on ongoing processes at the shock.

Metal complex analogues of crown ethers as the preorganized motif to stabilize aquated proton in solid state

The work in this report presents the syntheses, characterization and crystal structures of [{CuIILop}2(H5O2)](ClO4) (1), [{CuIILhex}2(H5O2)](ClO4) (2) and [{CuIILen}2(H5O2)](ClO4) (3) derived from three 3-ethoxysalicylaldehyde-diamine Schiff base compartmental ligands H2Lop, H2Lhex and H2Len, in which the diimine moieties come from ortho-phenylenediamine, trans-1,2-diaminocyclohexane and ethylenediamine, respectively. Compounds 1 and 2 crystallize in triclinic crystal system having P space group, while the crystal system and space group of compound 3 is monoclinic P21/c. In the [{CuIILop/hex/en}2(H5O2)]+ cation in 1–3, the two OH2 sites of the aquated proton, H5O2+, interact with two O(phenoxo)2O(ethoxy)2 compartments of the two [CuIILop/hex/en] moieties by forming bifurcated hydrogen bonds and thus [{CuIILop/hex/en}2(H5O2)]+ may be considered as a supramolecular dimer of two mononuclear moieties, self-assembled by H5O2+ as the tecton. On the basis of the extent of the difference in the two O–H distances involving the central hydrogen atom, H5O2+ moieties in 1–3 may be considered as H2O+–H⋯OH2 (in 1) or as the resonance hybrid of H2O+–H⋯OH2 and [H2O⋯H⋯OH2]+ (in 2 and 3). A unique aspect in the composition of 1–3 in terms of the interaction of metal complexes with an aquated proton has been discussed.

Photoelectrochemical biofuel cells based on H2-mesoporphyrin IX or Zn-mesoporphyrin IX sensitizer on titanium dioxide film electrode

H2-mesoporphyrin IX and Zn-mesoporphyrin IX have been investigated as sensitizers for the titanium dioxide (TiO2) film electrode to construct a new two-compartment photoelectrochemical biofuel cell (PEBFC). The PEBFC can convert the light and chemical energy to electricity by consuming photons and oxidizing glucose. The two sensitizers are similar with two carboxyl anchoring groups, except that the central hydrogen atoms in the H2-mesoporphyrin IX are replaced by zinc. To determine how cell performance is affected by the sensitizer, we analyze the photochemical and photoelectrochemical properties of the two sensitizers by physical characterization and photoelectrochemical experiments. The UV–Vis absorption spectra and X-ray photoelectron spectra (XPS) indicate that the interactions between the sensitizer and TiO2 decrease in the order of H2-mesoporphyrin IX > Zn-mesoporphyrin IX. The interactions are also determined by Fourier transform infrared (FTIR) spectra which indicate that the two sensitizers are adsorbed on the TiO2 film through the carboxyl groups. The photovoltaic characteristics show the Zn-mesoporphyrin IX is less effective in comparison with the H2-mesoporphyrin IX, since the Zn-mesoporphyrin IX enhances the back electron-transfer process, producing lower IPCE and current. These results reveal that the H2-mesoporphyrin IX is a more efficient sensitizer compared with the Zn-mesoporphyrin IX for the PEBFC.

Activation and Coordination of Ammonia at [Cp*Ir(H)2]: NMR and Matrix Isolation Studies

(1)H NMR exchange spectroscopy of a reaction mixture of [Cp*Ir(H)(4)] (1; Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) and ammonia suggests an exchange of hydrogen atoms between the hydrido ligands and ammonia. Treatment of 1 with ND(3) led to an H/D exchange between ND(3) and the hydrido ligands of 1. Subsequent studies showed that photolysis of 1 isolated in frozen argon matrices leads to the formation of the iridium compounds [Cp*Ir(H)(2)] (2) and [Cp*Ir(H)(3)] (4), as it was confirmed by IR spectroscopy. In the presence of water the aqua complex [Cp*Ir(H)(2)(OH(2))] (3) was generated simultaneously. Accordingly, photolysis of 1 in an argon matrix doped with ammonia gave rise to the ammine complex [Cp*Ir(H)(2)(NH(3))] (5). IR assignments were supported by calculations of the gas-phase IR spectra of 1-5 by DFT methods.

The internal cavities of pike α-parvalbumin probably contain water

The kinetics of hydrogen exchange of pike a-parvalbumin was investigated using the method of infrared spectroscopy (sensitive to the amide hydrogen atoms in the peptide) and radioisotope method (sensitive to all labile hydrogen atoms). Ultraslow exchangeable hydrogen atoms were found to be substantially less in the first case than in the second one. Taking into account that the internal cavities in the parvalbumin are formed by hydrophobic amino acid residues, devoid of labile hydrogen atoms, it is possible to make the most appropriate assumption, namely, these cavities contain water molecules, hydrogen atoms of which are ultraslow exchangeable.

Stability of hydrogenation states of graphene and conditions for hydrogen spillover

The hydrogen spillover mechanism has been discussed in the field of hydrogen storage and is believed to have particular advantage over the storage as metal or chemical hydrides. We investigate conditions for practicality realizing the hydrogen spillover mechanism onto carbon surfaces, using first-principles methods. Our results show that contrary to common belief, types of hydrogenation configurations of graphene (the aggregated all-paired configurations) can satisfy the thermodynamic requirement for room-temperature hydrogen storage. However, the peculiarity of the paired adsorption modes gives rise to a large kinetic barrier against hydrogen migration and desorption. It means that an extremely high pressure is required to induce the migration-derived hydrogenation. However, if mobile catalytic particles are present inside the graphitic interstitials, hydrogen migration channels can open and the spillover phenomena can be realized. We suggest a molecular model for such a mobile catalyst which can exchange hydrogen atoms with the wall of graphene.

Growing Fragmented Potentials for Gas–Surface Reactions: The Reaction between Hydrogen Atoms and Hydrogen-Terminated Silicon (111)

Systematic molecular fragmentation of nonconducting crystals has been combined with modified Shepard interpolation to develop an automated procedure to study the dynamics of gas–surface reactions. Any level of molecular electronic structure theory can be used to construct the gas–surface potential energy surface. The new methodology has been applied to the interaction of hydrogen atoms with the hydrogen-terminated silicon (111) surface at normal incidence. Direct hydrogen abstraction, hydrogen atom exchange, hydrogen atom sorption, and inelastic scattering processes were all found to be significant.

First principles calculations on the effect of pressure on SiH4(H2)2

The effect of pressure on the strength of H2 covalent bond in the molecular solid SiH4(H2)2 has been investigated using quantum molecular dynamics simulations and charge density analysis. Our calculations show, in agreement with the implications of the experimental results, that substantial elongation of H2 bond can be achieved at low pressures, with the onset of rapid changes close to 40 GPa. Model calculations show redistribution of charge from bonding to antibonding states to be responsible for the behavior. Our computed Raman spectra confirm the dynamic exchange of hydrogen atoms speculated to be operative in SiH4–D2 mixture by experiments. This exchange is shown to be a three step process driven by thermal fluctuations.

Metabolite identification of triptolide by data‐dependent accurate mass spectrometric analysis in combination with online hydrogen/deuterium exchange and multiple data‐mining techniques

Triptolide (TP), the primary active component of the herbal medicine Tripterygium wilfordii Hook F, has shown promising antileukemic and anti-inflammatory activity. The pharmacokinetic profile of TP indicates an extensive metabolic elimination in vivo; however, its metabolic data is rarely available partly because of the difficulty in identifying it due to the absence of appropriate ultraviolet chromophores in the structure and the presence of endogenous interferences in biological samples. In the present study, the biotransformation of TP was investigated by improved data-dependent accurate mass spectrometric analysis, using an LTQ/Orbitrap hybrid mass spectrometer in conjunction with the online hydrogen (H)/deuterium (D) exchange technique for rapid structural characterization. Accurate full-scan MS and MS/MS data were processed with multiple post-acquisition data-mining techniques, which were complementary and effective in detecting both common and uncommon metabolites from biological matrices. As a result, 38 phase I, 9 phase II and 8 N-acetylcysteine (NAC) metabolites of TP were found in rat urine. Accurate MS/MS data were used to support assignments of metabolite structures, and online H/D exchange experiments provided additional evidence for exchangeable hydrogen atoms in the structure. The results showed the main phase I metabolic pathways of TP are hydroxylation, hydrolysis and desaturation, and the resulting metabolites subsequently undergo phase II processes. The presence of NAC conjugates indicated the capability of TP to form reactive intermediate species. This study also demonstrated the effectiveness of LC/HR-MS(n) in combination with multiple post-acquisition data-mining methods and the online H/D exchange technique for the rapid identification of drug metabolites.

Identification of a drug degradation product found in a stressed dosage form using LC/MS n, LC/TOF MS and on-line H/D exchange MS

An unknown degradation product was found when a dosage form was stressed under base and heat. Comprehensive LC–MS studies have been conducted to identify this degradation product. The approach included the use of an ion-trap MS for MS n ion fragmentation patterns, a time-of-flight (TOF) MS to measure the accurate mass for its potential chemical formula, and on-line hydrogen/deuterium (H/D) exchange LC–MS to determine the number of exchangeable hydrogen atoms in the gradation product. Based upon the above LC–MS results, the unknown was identified to result from the conversion of a trifluoromethyl moiety in the drug substance to a carboxylic acid under the combination of thermal and base stress. Different ion fragmentation pathways between the drug substance and its degradation product were discussed. The reaction mechanism was proposed to be nucleophilic substitution through S N2 mechanism.

C–H⋯O interaction and water tunneling in the CHClF 2–H 2O dimer

The rotational spectrum of the CHClF 2–H 2O weakly bound dimer has been measured using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 5–18 GHz range. The structure of the complex has been determined by analysis of the moments of inertia of five isotopologues of the dimer. The primary interaction between the two monomers is a weak C–H⋯O contact ( R H⋯O = 2.332(3) Å) with a C–Cl⋯H–O contact also present ( R Cl⋯H = 2.749(13) Å). The observed structure is in reasonable agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level, although these predict a Cl⋯H distance that is significantly longer than the experimental results indicate. The rotational transitions of all isotopologues containing H 2O or D 2O were doubled, with relative intensities of the observed transitions consistent with an internal rotation of the water molecule leading to exchange of equivalent hydrogen atoms. Fitting the upper and lower components of the transitions using an effective Hamiltonian with the ERHAM program has yielded an energy difference between the tunneling states of 16.0(4) GHz, resulting in an estimate of the barrier to internal rotation of 195(5) cm −1 (to be compared with an ab initio estimate of ∼117 cm −1). The binding energy of the complex is estimated to be ∼5.5(2) kJ/mol (∼460 cm −1) from a pseudo-diatomic approximation and assumption of a Lennard–Jones intermolecular potential.

Reversible redox reaction and water configuration on a positively charged platinum surface: first principles molecular dynamics simulation

The water dissociation reaction and water molecule configuration on a positively charged platinum (111) surface were investigated by means of first principles molecular dynamics under periodic boundary conditions. Water molecules on the Pt surface were mostly in the O-down orientation but some H-down structures were also found. OH(-) ion, generated by removing H from H(2)O in the bulk region, moved to the Pt surface, on which a positive charge is induced, by a Grotthuss-like proton-relay mechanism and adsorbed on it as OH(Pt). Hydrogen atom exchange between OH(Pt) and a near-by water molecule frequently occurred on the Pt surface and had a low activation energy of the same order as room temperature energy. When a positive charge (7 μC cm(-2)) was added to the Pt surface, H(3)O(+) and OH(Pt) were generated from 2H(2)O on the Pt. This may be coupled with an electron transfer to the Pt electrode [2H(2)O → H(3)O(+) + OH(Pt) + e(-)]. The opposite reaction was also observed on the same charged surface during a simulation of duration about 10 ps; it is a reversible redox reaction. When further positive charge (14 μC cm(-2)) was added, the reaction shifted to the right hand side completely. Thus, this one-electron transfer reaction, which is a part of the oxygen electrode reaction in fuel cells and water electrolysis, was confirmed to be a low activation energy process.

Conformational transmission in proinsulin and its derivatives: A study using H/D exchange

Buffalo proinsulin cDNA was isolated, sequenced and shown to differ from that of its bovine counterpart in six nucleotides. However, at the protein level the predicted sequences of the two species are identical. Buffalo M-proinsulin, containing the initiation methionine, was produced in Escherichia coli and purified to give Mr of 8812. Following the replacement of 99% of the exchangeable hydrogen atoms with deuterons a preparation containing 131 D atoms was obtained. Buffer exchange of the latter into a protio medium led to, the immediate release of 109 (±1) D atoms into the medium and the retention of 22 (±1) D atoms in the protein. The slow exchange of these D atoms was studied at 0°C/pH 2.8. Insulin derived from buffalo proinsulin as well as bovine when deuteriated and buffer exchanged, similarly, gave the retention of 25 (±1) D atoms. The data show that the secondary structure of the insulin core present within buffalo/bovine proinsulin contains 5 (±1) fewer slow exchanging hydrogen atoms than are present in the final hormone. This effect is attributed, predominantly, to the long range influence of the C-peptide, composed of 26 residues, on the insulin core of buffalo proinsulin. In contrast, in the case of human proinsulin, comprising 31 amino acids in the C-peptide, the secondary structure of the insulin core within human proinsulin is closer to that of insulin itself.