Serge Desgreniers

University of Ottawa, Ottawa, Ontario, Canada

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Publications (54)298.88 Total impact

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    ABSTRACT: A complex exhibiting valence delocalization was prepared from 3,5-bis(2-pyridyl)-1,2,4,6-thiatriazinyl (Py2TTA·), an inherently redox active pincer-type ligand, coordinated to iron (Fe(Py2TTA)Cl2 (1)). Complex 1 can be prepared via two routes, either from the reaction of the neutral radical with FeCl2 or by treatment of the anionic ligand (Py2TTA-) with FeCl3, demonstrating its unique redox behaviour. Electrochemical studies, solution absorption and solid-state diffuse reflectance measurements along with X-ray crystallography were carried out to elucidate the molecular and solid-state properties. Temperature- and field-dependent Mössbauer spectroscopy coupled with magnetic measurements revealed that 1 exhibits an isolated S = 5/2 ground spin state for which the low-temperature magnetic behaviour is dominated by exchange interactions between neighbouring molecules. This ground state is rationalized on the basis of DFT calculations that predict the presence of strong electronic interactions between the redox active ligand and metal. This interaction leads to the delocalization of β electron density over the two redox active centres and highlights the difficulty in assigning formal charges to 1.
    Dalton Transactions 05/2015; DOI:10.1039/C5DT01374G · 4.10 Impact Factor
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    ABSTRACT: Cyanuric triazide (CTA), a nitrogen-rich energetic material, was compressed in a diamond anvil cell up to 63.2 GPa. Samples were characterized by x-ray diffraction, Raman, and infrared spectroscopy. A phase transition occurring between 29.8 and 30.7 GPa was found by all three techniques. The bulk modulus and its pressure derivative of the low pressure phase were determined by fitting the 300 K isothermal compression data to the Birch-Murnaghan equation of state. Due to the strong photosensitivity of CTA, synchrotron generated x-rays and visible laser radiation both lead to the progressive conversion of CTA into a two dimensional amorphous C=N network, starting from 9.2 GPa. As a result of the conversion, increasingly weak and broad x-ray diffraction lines were recorded from crystalline CTA as a function of pressure. Hence, a definite structure could not be obtained for the high pressure phase of CTA. Results from infrared spectroscopy carried out to 40.5 GPa suggest the high pressure formation of a lattice built of tri-tetrazole molecular units. The decompression study showed stability of the high pressure phase down to 13.9 GPa. Finally, two CTA samples, one loaded with neon and the other with nitrogen, used as pressure transmitting media, were laser-heated to approximately 1100 K and 1500 K while compressed at 37.7 GPa and 42.0 GPa, respectively. In both cases CTA decomposed resulting in amorphous compounds, as recovered at ambient conditions.
    The Journal of Chemical Physics 12/2014; 141(23):234506. DOI:10.1063/1.4902984 · 3.12 Impact Factor
  • The Journal of Chemical Physics 08/2014; 141(8):089901. DOI:10.1063/1.4893215 · 3.12 Impact Factor
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    ABSTRACT: We present a method to characterize pressure induced magnetic high to low spin transition in iron sulphide using x-ray Raman scattering spectroscopy at the iron M2,3-edge. The advantage of this method is that the observed spectral changes between pressures of 1.7 GPa and 10.1 GPa can be used with the help of atomic multiplet calculations to determine the crystal field splitting parameters associated with the spin transition. We discuss the potential of this M2,3-edge spectroscopy to investigate the irons electronic spin state in-situ at the conditions of the inner Earth, i.e., at high temperature and high pressure, providing exciting opportunities for geophysical and materials science applications.
    Applied Physics Letters 07/2014; 104(26):262408. DOI:10.1063/1.4886971 · 3.52 Impact Factor
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    ABSTRACT: The heterocyclic bisdithiazolyl radical 1b (R1 = Me, R2 = F) crystallizes in two phases. The α-phase, space group P21/n, contains two radicals in the asymmetric unit, both of which adopt slipped π-stack structures. The β-phase, space group P21/c, consists of cross-braced π-stacked arrays of dimers in which the radicals are linked laterally by hypervalent 4-center 6-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility measurements on α-1b indicate Curie-Weiss behavior (with Θ = -14.9 K), while the dimer phase β-1b is diamagnetic, showing no indication of thermal dissociation below 400 K. High-pressure crystallographic measurements indicate that the cross-braced π-stacked arrays of dimers undergo a wine-rack compression, but the dimer remains intact up to 8 GPa (at ambient temperature). The resistance of β-1b to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer β-1a (R1 = Et, R2 = F), which readily dissociates into a pair of radicals at 0.8 GPa. The different response of the two dimers to pressure has been rationalized in terms of differences in their linear compressibilities occasioned by changes in the degree of cross-bracing of the π-stacks. Dissociation of both dimers can be effected by irradiation with visible (λ = 650 nm) light; the transformation has been monitored by optical spectroscopy, magnetic susceptibility measurements, and single crystal X-ray diffraction. The photoinduced radical pairs persist up to temperatures of 150 K (β-1b) and 242 K (β-1a) before reverting to the dimer state. Variable-temperature optical measurements on β-1b and β-1a have afforded Arrhenius activation energies of 8.3 and 19.6 kcal mol(-1), respectively, for the radical-to-dimer reconversion. DFT and CAS-SCF calculations have been used to probe the ground and excited electronic state structures of the dimer and radical pair. The results support the interpretation that the ground-state interconversion of the dimer and radical forms of β-1a and β-1b is symmetry forbidden, while the photochemical transformation is symmetry allowed.
    Journal of the American Chemical Society 05/2014; 136(22). DOI:10.1021/ja502753t · 11.44 Impact Factor
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    ABSTRACT: 5,5(')-(1H-tetrazolyl)amine (BTA), a nitrogen rich molecular solid has been investigated under compression at room temperature. Powder x-ray diffraction using synchrotron radiation and micro-Raman spectroscopy were carried out to pressures up to 12.9 GPa. BTA conserves the crystalline structure of its room condition phase up to the highest pressure, i.e., an orthorhombic unit cell (Pbca). A fit of the isothermal compression data to the Birch-Murnaghan equation of state reveals the high compressibility of BTA. An analysis of the volume change with pressure yields a bulk modulus and its derivative similar to that of high-nitrogen content molecular crystals. Upon laser heating to approximately 1100 K, the sample decomposed while pressurized at 2.1 GPa, resulting in a graphitic compound. Finally, numerical simulations demonstrate that the minimum energy conformation is not experimentally observed since a higher energy conformation allows for a more stable dense packing of the BTA molecules.
    The Journal of Chemical Physics 05/2014; 140(18):184701. DOI:10.1063/1.4870830 · 3.12 Impact Factor
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    ABSTRACT: We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find that a 6-fold increase in impurity scattering does not shift Pc. From a study of X-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of these lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry.
    Physical Review B 03/2014; 89(13). DOI:10.1103/PhysRevB.89.134502 · 3.66 Impact Factor
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    ABSTRACT: The shape of the Ba 4d–4f giant dipole resonance is studied for Ba atoms embedded inside complex Si networks covering structures consisting of Si nanocages and nanotubes, i.e. the clathrate Ba8Si46, the complex compound BaSi6, and the semiconducting BaSi2. Here, non-resonant x-ray Raman scattering is used to investigate confinement effects on the shape of the giant resonance in the vicinity of the Ba N_IV,V-edge. The distinct momentum transfer dependence of the spectra is analyzed and discussed. The measurements are compared to calculations of the giant resonance within time-dependent local density approximation in the dipole limit. No modulation of the giant resonance’s shape for Ba atoms confined in different local environments was observed, in contrast to the calculations. The absence of such shape modulation for complex Ba/Si compounds is discussed providing important implications for further studies of giant resonance phenomena utilizing both theory and experiment.
    Journal of Physics B Atomic Molecular and Optical Physics 01/2014; 47(4):045102. DOI:10.1088/0953-4075/47/4/045102 · 1.92 Impact Factor
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    ABSTRACT: The crystal structure and charge transport properties of the prototypal oxobenzene-bridged 1,2,3-bisdithiazolyl radical conductor 3a are strongly dependent on pressure. Compression of the as-crystallized α-phase, space group Fdd2, to 3-4 GPa leads to its conversion into a second or β-phase, in which F-centering is lost. The space group symmetry is lowered to Pbn21, and there is concomitant halving of the a and b axes. A third or γ-phase, also space group Pbn21, is generated by further compression to 8 GPa. The changes in packing that accompany both phase transitions are associated with an "ironing out" of the ruffled ribbon-like architecture of the α-phase, so that consecutive radicals along the ribbons are rendered more nearly coplanar. In the β-phase the planar ribbons are propagated along the b-glides, while in the γ-phase they follow the n-glides. At ambient pressure 3a is a Mott insulator, displaying high but activated conductivity, with σ(300 K) = 6 × 10(-3) S cm(-1) and Eact = 0.16 eV. With compression beyond 4 GPa, its conductivity is increased by 3 orders of magnitude, and the thermal activation energy is reduced to zero, heralding the formation of a metallic state. High pressure infrared absorption and reflectivity measurements are consistent with closure of the Mott-Hubbard gap near 4-5 GPa. The results are discussed in the light of DFT calculations on the molecular and band electronic structure of 3a. The presence of a low-lying LUMO in 3a gives rise to high electron affinity which, in turn, creates an electronically much softer radical with a low onsite Coulomb potential U. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs upon pressurization, so that a metallic state is readily achieved at relatively low applied pressure.
    Journal of the American Chemical Society 01/2014; 136(3). DOI:10.1021/ja411057x · 11.44 Impact Factor
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    ABSTRACT: X-ray diffraction experiments at 80 K show that when silicon is compressed under hydrostatic conditions the intermediate high-pressure phases are bypassed leading to a direct transformation to the simple hexagonal structure at 17 GPa. A maximum entropy analysis of the diffraction patterns reveals dramatic alterations in the valence electron distribution from tetrahedral covalent bonding to localization in the interstitial sites and along the 1-D silicon atom chain running along adjacent hexagonal layers. Changes in the orbital character of the unoccupied states are confirmed using x-ray Raman scattering spectroscopy and theoretical Bethe-Salpeter Equation calculations. This is the first direct observation indicating that the silicon valence electrons in 3s and 3p orbitals are transferred to the 3d orbitals at high density which proofs that electrons of compressed elemental solids migrate from their native bonding configuration to interstitial regions.
    The Journal of Physical Chemistry C 12/2013; 118(2):1161. DOI:10.1021/jp408666q · 4.84 Impact Factor
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    ABSTRACT: A cubic metal-organic framework with an unprecedented octanuclear secondary building unit (SBU) was isolated. The obtained SBU is composed of 8 Co(II) ions at each vertex, 6 μ4-OH groups at each face, and 12 cpt(-) ligands framing the metal core. The cuboctahedra arrange in a ubt framework topology, eliciting a highly symmetrical MOF structure. Magnetic measurements as well as DFT calculations on this crystalline MOF reveal intramolecular antiferromagnetic coupling between Co(II) ions in the octanuclear SBU.
    Chemical Communications 10/2013; 50(40). DOI:10.1039/c3cc46396f · 6.72 Impact Factor
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    ABSTRACT: The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [1b]2 has been investigated over the range 0–11 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the x-direction of the monoclinic (space group P21/c) unit cell. Pressurization to 11 GPa causes the unit cell dimensions a and c to undergo a slow but uniform compression, while the b-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped π-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [1a]2, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4–5 GPa, characterized by a sharp contraction in a and c, and a correspondingly large increase in b. At the molecular level, the transition is associated with a buckling of the σ-bonded dimer to a more conventional π-bonded arrangement. Geometry optimized DFT band structure calculations on [1b]2 replicate the observed structural changes and indicate that compression widens both the valence and conduction bands but does not induce band gap closure until >13 GPa. This result is consistent with the measured thermal activation energy for conduction Eact, which indicates that a metallic state requires pressures > 10 GPa.
    Crystal Growth & Design 08/2012; 12(9):4676–4684. DOI:10.1021/cg3009255 · 4.56 Impact Factor
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    J S Tse, S Desgreniers, Y Ohishi, T Matsuoka
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    ABSTRACT: Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the "simple cubic" (SC) unit cell adopted by calcium over a broad pressure ranging from 30-90 GPa and at temperature as low as 40 K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the [110] plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s → d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes.
    Scientific Reports 04/2012; 2:372. DOI:10.1038/srep00372 · 5.08 Impact Factor
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    ABSTRACT: A combined experimental and theoretical study of hydrogen-rich ammonium borohydride (NH(4)BH(4)) subjected to pressures up to 10 GPa indicates two phase transitions, detected by synchrotron radiation powder X-ray diffraction, Raman spectroscopy and Car-Parrinello molecular dynamics calculations, at 1.5 and 3.4 GPa. The ambient pressure, face-centred cubic phase of NH(4)BH(4) transforms into a highly disordered intermediate structure which then evolves upon increasing pressure into an orthorhombic, distorted CsCl structure. The structure of the latter phase was solved using ab initio computational techniques and from a Rietveld full pattern refinement of the powder X-ray diffraction data.
    Physical Chemistry Chemical Physics 04/2012; 14(19). DOI:10.1039/c2cp40160f · 4.20 Impact Factor
  • Jesse S. Smith, Serge Desgreniers
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    ABSTRACT: A technique to improve the ratio of diffraction signal to Compton background has been examined in the context of diamond anvil cell crystallography carried out using imaging plates. Specifically, imaging plates exhibit an abrupt discontinuity in absorption as a function of energy due to barium, the primary compositional element of the imaging plate phosphor. By carrying out diffraction at some energy just above the Ba K-edge, coherent scattering is efficiently absorbed by the detector while less-energetic Compton scattering is less efficiently absorbed. This yields an improvement in the signal-to-background ratio of approximately 33%. Data are presented detailing the absorption characteristics of imaging plates over a broad energy range of the continuous synchrotron radiation spectrum. Diffraction data are presented for Si powder contained in a diamond anvil cell. Diffraction images taken with incident radiation at three distinct energies near the Ba K-edge are analyzed to evaluate the effectiveness of the technique.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 03/2012; 668:9–13. DOI:10.1016/j.nima.2011.11.056 · 1.32 Impact Factor
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    ABSTRACT: Ba(8)Si(46) is the archetype of the Si clathrates family. X-ray diffractions have revealed an unusual homothetic isostructural transition at similar to 14-16 GPa. Raman experiments, however, suggested even more transitions at lower pressure. We present evidence showing that successive electronic topological transitions are responsible for the transformations. It is shown that the electronic structure of Ba(8)Si(46) is easily perturbed by the environment. Reverse Monte Carlo calculations and in-situ resistivity measurements revealed continual changes in the structure and electrical properties upon compression. This finding is corroborated by results of x-ray Raman scattering study in the vicinity of the Ba N(4,5) and Si L(2,3) absorption edges.
    Physical Review B 11/2011; 84(18):184105. DOI:10.1103/PhysRevB.84.184105 · 3.66 Impact Factor
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    ABSTRACT: The Fmm crystal structure of NH4BH4 including the H atoms is refined by powder XRD, ab initio dynamics calculations, and solid state 1H and 2H NMR spectroscopy.
    ChemInform 03/2011; 42(10). DOI:10.1002/chin.201110006
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    ABSTRACT: The bromo-substituted bisdiselenazolyl radical 4b (R(1) = Et, R(2) = Br) is isostructural with the corresponding chloro-derivative 4a (R(1) = Et, R(2) = Cl), both belonging to the tetragonal space group P(4)2(1)m and consisting of slipped π-stack arrays of undimerized radicals. Variable temperature, ambient pressure conductivity measurements indicate a similar room temperature conductivity near 10(-4) S cm(-1) for the two compounds, but 4b displays a slightly higher thermal activation energy E(act) (0.23 eV) than 4a (0.19 eV). Like 4a, radical 4b behaves as a bulk ferromagnet with an ordering temperature of T(C) = 17.5 K. The coercive field H(c) (at 2 K) of 1600 Oe for 4b is, however, significantly greater than that observed for 4a (1370 Oe). High pressure (0-15 GPa) structural studies on both compounds have shown that compression reduces the degree of slippage of the π-stacks, which gives rise to changes in the magnetic and conductive properties of the radicals. Relatively mild loadings (<2 GPa) cause an increase in T(C) for both compounds, that of 4b reaching a maximum value of 24 K; further compression to 5 GPa leads to a decrease in T(C) and loss of magnetization. Variable temperature and pressure conductivity measurements indicate a decrease in E(act) with increasing pressure, with eventual conversion of both compounds from a Mott insulating state to one displaying weakly metallic behavior in the region of 7 GPa (for 4a) and 9 GPa (for 4b).
    Journal of the American Chemical Society 03/2011; 133(15):6051-60. DOI:10.1021/ja200391j · 11.44 Impact Factor
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    ABSTRACT: We report the results of synchrotron X-ray and neutron diffraction studies of methane, argon, nitrogen, and xenon clathrate hydrates at high pressure and room temperature. The results reveal common features in the structural transition sequences. All phases transform initially to the hexagonal clathrate structure and all except xenon hydrate eventually form an orthorhombic dihydrate. Argon and nitrogen hydrates adopt the type-II clathrate structure at low pressures and have a tetragonal cage structure between the hexagonal and orthorhombic phases. At normal compression rates, the hexagonal form of xenon hydrate decomposes into ice and xenon at ~2.5 GPa. PACS Nos.: 61.50Ks, 61.10-i, 61.12ExNous rapportons ici les r�sultats d'une �tude par rayonnement X synchrotron et par diffusion de neutrons sur des hydrates clathrates de m�thane, d'argon, d'azote et de x�non � haute pression et � la temp�rature de la pi�ce. Les r�sultats r�v�lent des caract�ristiques communes dans les s�quences de transition de structure. Toutes les phases se transforment d'abord en structure clathrate hexagonale et toutes, sauf l'hydrate de x�non, �ventuellement forment des dihydrates orthorhombiques. Les hydrates d'argon et d'azote adoptent la structure clathrate de type II � basse pression et ont une structure de cage t�tragonale entre les phases hexagonale et orthorhombique. � des taux normaux de compression, l'hydrate hexagonal de x�non se d�compose en glace plus du x�non � ~2,5 GPa.[Traduit par la R�daction]
    Canadian Journal of Physics 02/2011; 81(1-2). DOI:10.1139/p03-040 · 0.93 Impact Factor
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    ABSTRACT: The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4̅2(1)m and consists of π-stacked radicals, tightly clustered about 4̅ points and running parallel to c. The β-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = (1)/(2) centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S-S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-1a behaves as a Mott insulator, but the ambient-temperature conductivity σ(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of σ(RT) for β-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ(RT) increases in a manner which parallels the behavior of α-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S-S···S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S···Se-Se···S σ-bonds.
    Journal of the American Chemical Society 10/2010; 132(45):16212-24. DOI:10.1021/ja106768z · 11.44 Impact Factor