Exchange Of Hydrogen Atoms Research Articles

Neutron diffraction studies on aqueous solutions of glucose

Neutron diffraction experiments were carried out at ambient temperature on aqueous solutions of glucose at concentrations of 1, 3, and 5 molal. The difference methods of neutron diffraction and isotopic substitution were used to determine various structure factors associated with all the exchangeable hydrogen atoms of the solutions. The results imply that the hydration of the glucose molecule is not concentration sensitive. It is also found that the average water structure itself is relatively unperturbed by the presence of glucose.

Ruthenium dihydrogen complexes with wide bite angle diphosphines.

The wide bite angle diphosphines homoxantphos (10,11-dihydro-4,5,-bis(diphenylphosphino)dibenzo[b,f]oxepine), sixantphos (4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin), and thixantphos (2,8-dimethyl-4,6-bis(diphenylphosphino)phenoxathiin) were used to prepare cis[MH(2)(diphosphine)(2)] complexes (1a-f) by reaction of [Ru(cod)(cot)] (cod = cyclo-octa-1,5-diene, cot = cyclo-octa-1,3,5-triene) with 2 equiv of the diphosphine under dihydrogen pressure. The electronic properties of the thixantphos ligand were varied. Complexes 1a-f can be protonated with HBF(4) or CF(3)COOH to yield hydrido(dihydrogen) complexes cis[MH(H(2))(diphosphine)(2)](+) (2a-f), which were characterized by VT (variable temperature) NMR and T(1) measurements. These complexes show fast hydrogen atom exchange between the eta(2)-H(2) and the terminal hydride at all temperatures studied. They are thermally unstable toward dihydrogen loss yielding the cationic monohydride complexes cis[MH(diphosphine)(2)](+) (3a-f). Coordination of the eta(2)-H(2) is dominated by sigma --> d donation, and hence, the H-H distance is hardly influenced by the electronic properties of the ligands.

Hydrogen isotope exchange kinetics between H 2O and H 4SiO 4 from ab initio calculations

Hydrogen isotope exchange between water and orthosilicic acid (H 4SiO 4) was modeled using B3LYP calculations and classical transition-state theory. Configurations of 1, 2, 3 and 7 water molecules and H 4SiO 4 were used to investigate energetically viable reaction pathways. An upper-bound of 71 kJ/mol was assumed for the zero-point energy corrected barrier (ZPECB) because this is the experimentally determined activation energy for Si-O bond breaking (Rimstidt and Barnes, 1980) and ZPECB is expected to be close to this value. Long range solvation forces were accounted for using the integral equation formalism polarized continuum model (IEFPCM; Cancès et al., 1997). Primary and secondary isotope effects were computed by exchanging hydrogen atoms with deuterium. Results show that reaction mechanisms involving 3 and 7 water molecules have ZPECB of 34 to 38 kJ/mol, whereas those involving 1 and 2 water molecules have ZPECB in excess of the set upper-bound. The lower range of ZPECB with 3 or 7 water molecules is reasonable to explain rapid hydrogen isotope exchange with silicates. Rate constant calculations accounting for tunneling, anharmonicity and scaling factors indicate that the reaction is fast and equilibrium can be assumed under most geologic conditions.

Complementary use of ion trap/time-of-flight mass spectrometry in combination with capillary high-pressure liquid chromatography: early characterization of in vivo metabolites of the cathepsin K inhibitor NVP-AAV490 in rat

Cathepsin K is a cysteine proteinase, primarily expressed in osteoclasts, which has a strong collagenolytic activity and plays an essential role involved in bone matrix degradation. Its inhibition could provide a novel approach to the treatment and prevention of osteoporosis. One structural class of lead compounds in our cathepsin K inhibitors program is based on an arylaminoethyl amide scaffold, which has potential metabolic weak points that might be stabilized by appropriate chemical modification(s). For the identification of potential metabolic “soft spots” and the rational design of improved derivatives, early biotransformation of a potent arylaminoethyl amide cathepsin K inhibitor (NVP-AAV490-NX) was investigated in plasma, urine and liver homogenates of rats after intravenous bolus administration of 10 mg/kg. The detection and identification of metabolites was achieved by high-resolution mass spectrometry (time-of-flight MS) and multi-dimensional mass spectrometry (ion trap MS). Both mass spectrometers were combined with reversed-phase capillary high-performance liquid chromatography columns. It was demonstrated that both mass analyzers complement each other and that, even in the sub-nanogram range, the resulting set of MS data can be successfully used to elucidate most of the metabolic changes unambiguously, solely by mass spectrometric techniques. The proposed metabolite structures were additionally corroborated by exact mass measurement of the protonated molecular ions to confirm the predicted elemental composition, by determination of the number of the exchangeable hydrogen atoms replacing water against deuterium oxide as mobile phase and, in one case, by an MS 3 product ion experiment in order to elucidate the site of conjugation.

Proton-coupled electron transfer versus hydrogen atom transfer in benzyl/toluene, methoxyl/methanol, and phenoxyl/phenol self-exchange reactions.

Degenerate hydrogen atom exchange reactions have been studied using calculations, based on density functional theory (DFT), for (i) benzyl radical plus toluene, (ii) phenoxyl radical plus phenol, and (iii) methoxyl radical plus methanol. The first and third reactions occur via hydrogen atom transfer (HAT) mechanisms. The transition structure (TS) for benzyl/toluene hydrogen exchange has C(2)(h)() symmetry and corresponds to the approach of the 2p-pi orbital on the benzylic carbon of the radical to a benzylic hydrogen of toluene. In this TS, and in the similar C(2) TS for methoxyl/methanol hydrogen exchange, the SOMO has significant density in atomic orbitals that lie along the C-H vectors in the former reaction and nearly along the O-H vectors in the latter. In contrast, the SOMO at the phenoxyl/phenol TS is a pi symmetry orbital within each of the C(6)H(5)O units, involving 2p atomic orbitals on the oxygen atoms that are essentially orthogonal to the O.H.O vector. The transferring hydrogen in this reaction is a proton that is part of a typical hydrogen bond, involving a sigma lone pair on the oxygen of the phenoxyl radical and the O-H bond of phenol. Because the proton is transferred between oxygen sigma orbitals, and the electron is transferred between oxygen pi orbitals, this reaction should be described as a proton-coupled electron transfer (PCET). The PCET mechanism requires the formation of a hydrogen bond, and so is not available for benzyl/toluene exchange. The preference for phenoxyl/phenol to occur by PCET while methoxyl/methanol exchange occurs by HAT is traced to the greater pi donating ability of phenyl over methyl. This results in greater electron density on the oxygens in the PCET transition structure for phenoxyl/phenol, as compared to the PCET hilltop for methoxyl/methanol, and the greater electron density on the oxygens selectively stabilizes the phenoxyl/phenol TS by providing a larger binding energy of the transferring proton.

Investigation of NO Reduction by H 2 on Pd Monolith with Transient and Isotopic Exchange Techniques : I. H 2/D 2 Exchange with H 2O and NH 3

Isotope exchange of hydrogen atoms in water, ammonia, and hydrogen with deuterium, as well as adsorption of ammonia and water on an alumina-supported Pd monolith, were studied with transient step-response experiments. The investigation provided new insights into reaction mechanisms. Deuterium underwent an exchange reaction with hydrogen in ammonia and water. The step-response experiments yielded information about the surface reaction mechanisms, indicating adsorption-assisted desorption of hydrogen, decomposition of ammonia, and stepwise formation of deuterated water and ammonia. Detailed reaction pathways explaining isotope exchange of the hydrogen atoms in water, ammonia, and hydrogen with deuterium were proposed. The work forms the basis for the understanding of reaction mechanisms between nitrogen oxide and hydrogen, where ammonia is formed on Pd as a by-product.

Hydrogen/deuterium exchange and aggregation of a polyvaline and a polyleucine alpha-helix investigated by matrix-assisted laser desorption ionization mass spectrometry.

The membrane-associated pulmonary surfactant protein C (SP-C), containing a polyvaline alpha-helix, and a synthetic SP-C analogue with a polyleucine helix (SP-C(Leu)) were studied by hydrogen/deuterium exchange matrix-assisted laser desorption ionization (MALDI) mass spectrometry. SP-C, but not SP-C(Leu), formed abundant amyloid fibrils under experimental conditions. In CD(3)OD/D(2)O, 91:9 (v/v), containing 2 mM ammonium acetate, SP-C(Leu) and SP-C exchanged 40% of their exchangeable hydrogens within 1 min. This corresponds to exchange of labile side-chain hydrogen atoms, hydrogens on the N- and C-terminal heteroatoms, and amide hydrogen atoms in the unstructured N-terminal regions. After approximately 300 h, four exchangeable hydrogen atoms in SP-C(Leu) and 10 in SP-C remained unexchanged. During this time period the ion current corresponding to singly charged SP-C decreased to <10% of the initial value due to the formation of insoluble aggregates that are not detected by MALDI mass spectrometry. In contrast, the ion current for SP-C(Leu) was maintained over this time period, although the peptides were incubated together. In combination, hydrogen/deuterium exchange and aggregation data indicate that the polyleucine peptide refolds into a helix after opening, while the unfolded polyvaline peptide forms insoluble beta-sheet aggregates rather than refolding into a helix. The SP-C helix, but not the SP-C(Leu) helix, is thus in a metastable state, which may contribute to the recently observed tendency of SP-C and its precursor to misfold and aggregate in vivo.

Raman and infrared spectral study of meso-sulfonatophenyl substituted porphyrins (TPPS n, n=1, 2A, 2O, 3, 4)

Raman and IR spectra of the free base p-sulfonatophenyl and phenyl meso-substituted porphyrins [5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS 4); 5,10,15-tris(4-sulfonatophenyl)-20-phenyl-porphyrin (TPPS 3); 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS 2A); 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS 2O); and 5-(4-sulfonatophenyl)-10, 15,20-trisphenylporphyrin (TPPS 1)] and their N-diprotonated derivatives (porphyrin diacids) were studied. The Raman spectra of the deuterated analogues of these porphyrins, in which the central hydrogen atoms were substituted with deuterium, were also measured. The observed vibrational bands were assigned on the basis of the deuteration shifts and compared with the structural analogues of these compounds. In IR spectra of the free-base porphyrins, the p-sulfonation of phenyl groups results in evident alteration for the phenyl modes and the porphyrin skeleton modes that are strongly coupled with phenyl vibrations. While the p-sulfonation of phenyl groups causes only slight changes for the high-frequency Raman bands (>850 cm −1), dramatic shifts and band splitting were observed in the low-frequency region (<500 cm −1) of Raman spectra. The observed differences of low-frequency Raman spectra were attributed to the alteration of the structure of the porphyrin ring, especially the C αC mC α bond-angles, by different meso-sulfonatophenyl substitutions. In addition, different packing style of TPPS n molecules in the aggregates is also responsible for the alteration of the vibrational spectra of the aggregated TPPS n .

The reactivity of ground-state carbon atoms with unsaturated hydrocarbons in combustion flames and in the interstellar medium

This article reviews recent crossed-beams and ab-initio studies of reactions of ground-state carbon atoms C(3P j ) with unsaturated hydrocarbons and their radicals. All reactions have no entrance barrier and are initiated via an addition of the carbon atom to the ~ system. With the exception of the carbon atom reaction with acetylene, which also shows a significant fraction of molecular hydrogen loss, these bimolecular collisions are dominated by an atomic carbon versus hydrogen atom exchange mechanism. In some systems, homolytic cleavages of carbon-carbon bonds present additional decomposition routes of typically a few per cent at the most. The impact-parameter-dependent chemical dynamics are interpreted in terms of statistical and non-statistical decomposition of reaction intermediates involved. The polyatomic reaction products are highly hydrogend-eficient resonance-stabilized free radicals. The latter are strongly suggested as suitable precursors to form the first (substituted) aromatic ring molecule, polycyclic aromatic hydrocarbon-like species and carbonaceous nanoparticles in combustion processes, chemical vapour deposition and the outflows of carbon stars.

Hydrogen/deuterium exchange for higher specificity of protein identification by peptide mass fingerprinting

Genome sequencing projects produce large amounts of information that could be translated into potential protein sequences. Such amounts of material continuously increase protein database sizes. At present, 22 times more protein sequences are available in the SWISS-PROT and TrEMBL databases than 8 years ago in SWISS-PROT. One of the methods of choice for protein identification makes use of specific endoproteolytic cleavage followed by matrix-assisted laser desorption/ionisation mass spectrometric (MALDI-MS) analysis of the digested product. Since 1993, when this technique was first demonstrated, the conditions required for a correct identification have changed dramatically. Whilst 4–5 peptides with an uncertainty of 2–3 Da were sufficient for a correct identification in 1993, 10–13 peptides with less than 60 ppm mass error are now required for human and E. coli proteins. This evolution is directly related to the continuous increase in protein database sizes, which causes an increase in the number of false positive matches in identification results. Use of an information complement deduced from the primary protein sequence, in the process of identification by peptide mass fingerprints, can help to increase confidence in the identification results. In this article, we propose the exchange of labile hydrogen atoms with deuterium atoms to provide an alternative information complement. The exchange reaction with optimised techniques has shown an average 95% of hydrogen/deuterium (H/D) exchange on tryptic peptides. This level of exchange was sufficient to single out one or more peptides from a list of potential candidate proteins due to the dependence of H/D exchange on the peptide primary structure. This technique also has clear advantages in the identification of small proteins where direct protein identification is impaired by the limited number of endoproteolytic peptides. Then, information related to primary sequence obtained with this technique could help to identify proteins with high confidence without any expensive tandem mass spectrometry instruments. Copyright © 2002 John Wiley & Sons, Ltd.

Hydrogen/deuterium exchange for higher specificity of protein identification by peptide mass fingerprinting

Genome sequencing projects produce large amounts of information that could be translated into potential protein sequences. Such amounts of material continuously increase protein database sizes. At present, 22 times more protein sequences are available in the SWISS-PROT and TrEMBL databases than 8 years ago in SWISS-PROT. One of the methods of choice for protein identification makes use of specific endoproteolytic cleavage followed by matrix-assisted laser desorption/ionisation mass spectrometric (MALDI-MS) analysis of the digested product. Since 1993, when this technique was first demonstrated, the conditions required for a correct identification have changed dramatically. Whilst 4-5 peptides with an uncertainty of 2-3 Da were sufficient for a correct identification in 1993, 10-13 peptides with less than 60 ppm mass error are now required for human and E. coli proteins. This evolution is directly related to the continuous increase in protein database sizes, which causes an increase in the number of false positive matches in identification results. Use of an information complement deduced from the primary protein sequence, in the process of identification by peptide mass fingerprints, can help to increase confidence in the identification results. In this article, we propose the exchange of labile hydrogen atoms with deuterium atoms to provide an alternative information complement. The exchange reaction with optimised techniques has shown an average 95% of hydrogen/deuterium (H/D) exchange on tryptic peptides. This level of exchange was sufficient to single out one or more peptides from a list of potential candidate proteins due to the dependence of H/D exchange on the peptide primary structure. This technique also has clear advantages in the identification of small proteins where direct protein identification is impaired by the limited number of endoproteolytic peptides. Then, information related to primary sequence obtained with this technique could help to identify proteins with high confidence without any expensive tandem mass spectrometry instruments.

Studies on phosphatidylglycerol with triple quadrupole tandem mass spectrometry with electrospray ionization: fragmentation processes and structural characterization

Negative-ion low-energy collisionally activated dissociation (CAD) tandem mass spectrometry of electrospray-produced ions permits structural characterization of phosphatidylglycerol (PG). The major ions that identify the structures arise from neutral loss of free fatty acid substituents ([M − H − R x CO 2H] −) and neutral loss of the fatty acids as ketenes ([M − H − R x CH = C = O] −), followed by consecutive loss of the glycerol head group. The abundances of the ions arising from neutral loss of the sn-2 substutient as a free fatty acid ([M − H − R 2CO 2H] −) or as a ketene ([M − H − R 2CH = C = O] −) are greater than those of the product ions from the analogous losses at sn-1. Nucleophilic attack of the anionic phosphate site on the C-1 or the C-2 of the glycerol to which the carboxylates attached expels the sn-1 (R 1CO 2 −) or the sn-2 (R 2CO 2 −) carboxylate anion, resulting in a greater abundance of R 2COO − than R 1COO −. These features permit assignments of fatty acid substituents and their position in the glycerol backbone. The results are also consistent with our earlier findings that pathways leading to those losses at sn-2 are sterically more favorable than those at sn-1. Fragment ions at m/ z 227, 209 and 171 reflect the glycerol polar head group and identify the various PG molecules. Both charge-remote fragmentation (CRF) and charge-drive fragmentation (CDF) processes are the major pathways for the formation of [M − H − R x COOH] − ions. The CRF process involves participation of the hydrogen atoms on the glycerol backbone, whereas the CDF process involves participation of the exchangeable hydrogen atoms of the glycerol head group. The proposed fragmentation pathways are supported by CAD tandem mass spectrometry of the analogous precursor ions arising from the H–D exchange experiment, and further confirmed by source CAD in combination with tandem mass spectrometry.

Kinetic aspects of the growth of platelets and voids in H implanted Si

We have undertaken a systematic and quantitative study of the extended defects formed after high-dose proton implantation in silicon. This study is based on the transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS) experiments to “follow” the thermal evolution of platelets and voids for a large variety of annealing conditions up to 900°C. Up to about 500°C, only platelets are observed and, as the anneal proceeds, they grow in size and reduce their density through the conservative exchange of hydrogen (H) atoms. On the contrary, above 500°C, H starts to diffuse out of the defect-rich region and this out-diffusion can be completed after 700°C anneals. Concurrently, platelets tend to disappear and voids appear. Above 700°C anneals, hydrogen cannot be detected anymore in the layers and only voids remain. Upon time, they also grow in size and reduce their density. This is again attributed to the Ostwald ripening of voids which involves now vacancy diffusion from small voids to large ones. In summary, we have shown that platelets and voids both undergo quasi-conservative ripening upon annealing; at low-temperature (LT) platelets exchange the H atoms they are composed of while at high-temperature voids exchange vacancies.

The standard enthalpies of formation of 1- and 2-Adamantyl cations and radicals. An ab initio study

The results of a G2(MP2) computational study involving 1- and 2-Adamantyl cations ( 1 +, 2 + ) as well as 1- and 2-Adamantyl radicals ( 1 , 2 ) are presented. They provide purely computational thermodynamic data for the following processes: (i) Ionization of 1 and 2 , (ii) Exchange of hydrogen atoms or hydride anions between Adamantyl radicals or cations and alkyl radicals or cations, respectively. These data, once combined with the experimental enthalpies of formation of iso -C 3 H 7 ,iso -C 3 H 7 + ,tert -C 4 H 9 and tert -C 4 H 9 + , allowed us to screen the available experimental data and to define a self-consistent set of experimentally-based standard enthalpies of formation, Δ f H 0 m , for Adamantyl species, namely: Δ f H 0 m ( 1 +, g)=162.0±2.0 ; Δ f H 0 m ( 2 +, g)=171.9±2.0 ; Δ f H 0 m ( 1 , g)=17.9±2.1 , Δ f H 0 m ( 2 , g)=16.6±2.0 kcal mol −1 .

The Formation of C5H5Isomers as Potential Key Intermediates to Polycyclic Aromatic Hydrocarbon–like Molecules

The reactions of ground-state atomic carbon with three C4H6 isomers, dimethylacetylene, 1,2-butadiene, and 1,3-butadiene, are studied at relative collision energies between 19.3 and 48.6 kJ mol-1 in crossed molecular beam experiments to elucidate the reaction products and chemical dynamics to form C5H5 isomers?possible precursor radicals to PAH-like species?in the circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars. Our combined experimental and computational studies show that reactions are dominated by the C(3Pj) versus hydrogen atom exchange to form the 1-methylbutatrienyl radical, H2CCCCCH3 (X 2A''), as well as 1- and 3-vinylpropargyl radicals, HCCCHC2H3 (X 2A'') and H2CCCC2H3 (X 2A''), under single-collision conditions. The methyl (CH3) and vinyl radical (C2H3) loss channels to yield the propargyl radical C3H3 (X 2B2) and n-C4H3 (X 2A') are only minor pathways. All reactions have no entrance barrier and are dominated by an addition of the carbon atom to the ? electron density of the second reactant via a complex-forming reaction mechanism on the triplet potential energy surface (PES). The decomposing complexes are long lived and reside in a deep potential energy well bound by 360-410 kJ mol-1 with respect to the separated reactants. Tight exit transition states of about 20 kJ mol-1 above the products and strong exothermic reactions of 180-240 kJ mol-1 are common features of the involved PESs. These characteristics make the reactions of atomic carbon with C4H6 isomers a compelling candidate to form C5H5 isomers in the outflow of AGB stars.

Hydrogen in nanostructured vanadium-hydrogen systems

Nanostructured vanadium-hydrides, ${\ensuremath{\beta}}_{2}\ensuremath{-}{\mathrm{VH}}_{x},$ with typical grain sizes of 80 nm $(x=0.82),$ 30 nm $(x=0.73),$ and 10 nm $(x=0.67)$ were prepared by mechanical milling under hydrogen atmosphere. The final grain size, about 10 nm, does not change any more with increasing milling time, and a homogeneous amorphous phase is not formed in this system, even after milling for 300 min. The hydrogen concentration in the grains ${x}_{\mathrm{G}}$ decreases with decreasing grain size from ${x}_{\mathrm{G}}=\phantom{\rule{0ex}{0ex}}0.82$ in 80-nm grains to 0.72 in 10-nm grains. This indicates a modification of the ${\ensuremath{\beta}}_{2}\ensuremath{-}\ensuremath{\gamma}$ phase boundary in the V-H system with nanometer-scale grains. The hydrogen concentration in the intergrains, ${x}_{\mathrm{IG}}$ $\ensuremath{\approx}0.5--0.6,$ is smaller than in the grains, and was found to be nearly independent of the grain size. The hydrogen diffusivity has been studied by NMR measurements of the proton spin-lattice relaxation ${\ensuremath{\Gamma}}_{1}.$ Generally, the measured ${\ensuremath{\Gamma}}_{1}$ consists of contributions that result from both hydrogen in the grains and in the intergrain regions. Due to the smaller spin-spin relaxation rate ${\ensuremath{\Gamma}}_{2}$ of the protons in the intergrain regions, their contribution to ${\ensuremath{\Gamma}}_{1}$ could be measured separately by the spin-echo technique. The relaxation data indicate that, at a given temperature, the hydrogen diffusivity in the intergrain regions is substantially higher than inside the grains. The frequency dependence of the dipolar contribution ${\ensuremath{\Gamma}}_{1,\mathrm{d}\mathrm{i}\mathrm{p}}$ reveals a distribution in the activation enthalpy for hydrogen in the intergrain regions. This distribution was found to be the broader the smaller the grain size. A change in the diffusion mechanisms, presumably arising from the ${\ensuremath{\beta}}_{2}\ensuremath{-}\ensuremath{\delta}$ phase transition, takes place at about 200 K. The exchange of hydrogen atoms between the grains and the intergrain regions occurs very slowly and is negligible on the time scale given by ${\ensuremath{\Gamma}}_{1}.$

Highly Excited Bound and Low-Lying Resonance States of H2O

A theoretical study of the highest bound states and some of the lowest energy resonance states of water in its ground electronic state, X = 11A‘, and with zero total angular momentum, is presented. The majority of our calculations correspond to the even symmetry block with respect to hydrogen atom exchange. An accurate ab initio based potential surface is employed. The Lanczos method, applied to a large grid representation of the Hamiltonian, is shown to be a straightforward method for obtaining the bound state energy levels. Selected eigenfunctions are also determined and several of those near the dissociation threshold are quite extended. Resonance states just above the dissociation threshold are characterized with the aid of damped Chebyshev iterations. Among the resonances observed are those with hyperspherical and local mode character.

An efficient one-pot route to an aza-bridged bis-phenanthroline macrocyclic compound

A facile synthesis of an aza-bridged bis-1,10-phenanthroline macrocycle is described. X-ray crystallographic analysis of the aza-bridged bis-1,10-phenanthroline revealed that the two central hydrogen atoms were localized on two of the opposite central nitrogen atoms similar to those of free base porphyrins.

Intrinsic Barriers for Electron and Hydrogen Atom Transfer Reactions of Biomimetic Iron Complexes

Self-exchange reactions between high-spin iron complexes of 2,2'-bi-imidazoline (H2bim) have been investigated by the dynamic NMR line-broadening technique. Addition of the ferric complex (Fe III (H2bim)3) 3+ causes broadening of the 1 H NMR resonances of the ferrous analogue, (Fe II (H2bim)3) 2+ . This indicates electron self-exchange with ke - ) (1.7 ( 0.2) 10 4 M -1 s -1 at 298 K in MeCN-d3 (I ) 0.1 M). Similar broadening is observed when the deprotonated ferric complex (Fe III (Hbim)(H2bim)2) 2+ is added to (Fe II (H2bim)3) 2+ . Because these reactants differ by a proton and an electron, this is a net hydrogen atom exchange reaction. Kinetic and thermodynamic results preclude stepwise mechanisms of sequential proton and then electron transfer, or electron and then proton transfer. Concomitant electron and proton (H ¥ ) transfer occurs with bimolecular rate constant kH ¥ ) (5.8 ( 0.6) 10 3 M -1 s -1 . This is a factor of 3 smaller than ke - under the same conditions. The H-atom exchange reaction exhibits a primary kinetic isotope effect kNH/kND ) 2.3 ( 0.3 at 324 K, whereas no such effect is detected in the electron exchange reaction. Proton self-exchange between the two ferric complexes, (Fe III (Hbim)(H2bim)2) 2+ and (Fe III (H2bim)3) 3+ , has also been investigated and is found to be faster than both the electron and H-atom transfer reactions. From kinetic analyses and the application of simple Marcus theory, an order of intrinsic reaction barriers IH ¥ > Ie - > IH + is derived. The reorganization energies are discussed in terms of their inner-sphere and outer-sphere components.

Hydrogen/deuterium exchange using a coaxial sheath-flow interface for capillary electrophoresis/mass spectrometry.

The interfacing of capillary electrophoresis (CE) with mass spectrometry (MS) is well established and may be accomplished by use of either a coaxial arrangement or by employing a liquid T-junction. In both these interfaces a make-up flow is introduced. This is required because of the mismatch in flow rates for capillary electrophoresis approximately nL/min and 'true' electrospray approximately 2-10 microL/min. Electrical connectivity may also be established where the liquid flows meet (the introduction of nanospray renders the use of make-up flow unnecessary). Hydrogen/deuterium (H/D) exchange occurs in solution when there are labile hydrogen atoms present in a molecule. The establishment of the presence and the number of such exchangeable hydrogen atoms may be of importance in the identification and differentiation of compounds. It may also be an aid in the structural elucidation of unknown materials. We have investigated the feasibility of carrying out H/D exchange via a CE/MS interface. This involved the addition of D2O to the sheath flow and our preliminary results showing the separations of drug substances, subsequently undergoing exchange, are presented.