Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “bonding”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

A Direct Probe of the Hydrogen Bond Network in Aqueous Glycerol Aerosols

The properties of aerosols are of paramount importance in atmospheric chemistry and human health. The hydrogen bond network of glycerol-water aerosols generated from an aqueous solution with different mixing ratios is probed directly with X-ray photoelectron spectroscopy. The carbon and oxygen X-ray spectra reveal contributions from gas and condensed phase components of the aerosol. It is shown that water suppresses glycerol evaporation up to a critical mixing ratio. A dielectric analysis using terahertz spectroscopy coupled with infrared spectroscopy of the bulk solutions provides a picture of the microscopic heterogeneity prevalent in the hydrogen bond network when combined with the photoelectron spectroscopy analysis. The hydrogen bond network is composed of three intertwined regions. At low concentrations, glycerol molecules are surrounded by water forming a solvated water network. Adding more glycerol leads to a confined water network, maximizing at 22 mol %, beyond which the aerosol resembles bulk glycerol. This microscopic view of hydrogen bonding networks holds promise in probing evaporation, diffusion dynamics, and reactivity in aqueous aerosols.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Formation of Carbon–Carbon Interlinkage Bonds under High Pressure

The formation of carbon–carbon interlinkage bonds (CCIBs) via the chemical binding of interlayer carbon atoms of many sp 2 -bonded carbon precursors is an essential step for synthesizing various diamond and diamond-like materials. Although the existence of CCIBs may be reasonably assumed under high-pressure conditions, direct experimental evidence has been scarce. Micro-Raman spectroscopy is here employed to track in situ the evolution of C–C bonds in a pressure range from ambient to 54 GPa. A pressure-induced two-stage (polynomial and linear) shift of the G peak and new generation of the CCIB peak at about 1550 cm –1 are observed in multiple types of layer-structured carbon precursors, including glassy carbon, natural graphite, and carbon nanotubes. In conclusion, the experimental discovery of CCIBs holds significance in comprehending phase transitions of sp 2 -bonded carbon materials and has implications for the advancement of novel carbon structures.

Anode materials↗
DOE OSTI2017

Role of Hydrogen Bonding in Photoinduced Electron-Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand

Electron-proton transfer, EPT, from phenols to a triplet MLCT-excited Ru polypyridine complex containing an uncoordinated nitrogen site, 1(T), can be described by a kinetic model that accounts for the H-bonding of 1(T) to phenol, 1(T) to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and 1(T)-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to an inconsistent data analysis. In conclusion, control experiments replacing either 1(T) by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between 1(T) and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Hydrogen-Bonding Bottlebrush Networks: Self-Healing Materials from Super-Soft to Stiff

The impact of polymer architecture on network dynamics and self-healing is presented using bottlebrushes containing side chains that are end-functionalized with 2-ureido-4[1H]-pyrimidinone (UPy). The synthesis of these materials is straightforward through a three-step process: (1) synthesizing rubbery poly(4-methylcaprolactone) macromonomers (p4MCL–OH) with a norbornene-based initiator, (2) functionalizing the terminal hydroxyl group with UPy–isocyanate (p4MCL–UPy), and (3) statistically copolymerizing p4MCL–OH and p4MCL–UPy via ring-opening metathesis polymerization (ROMP) to form hydrogen-bonding bottlebrushes having a fraction (p) of side chains functionalized with UPy. Attaching UPy to the free end of bottlebrush side chains dilutes the impact of friction from complementary UPy interactions on segmental dynamics, leading to a much weaker dependence of the glass-transition temperature (T g ) on p than observed in linear analogues, while the activation energy to dissociate UPy–UPy bonds (41–47 kJ/mol) remains mostly unchanged. Longer side chains result in a competition between reducing T g and inducing entanglements that influence hydrogen-bonded network dynamics. Increasing the backbone length extends the sticky Rouse region without affecting the network modulus (G x ) or UPy–UPy dissociation time (τ s ). Gx scales linearly with p and ranges from 27 kPa to 1.6 MPa, while τs remains nearly constant in contrast to linear telechelic ionomers, implying a similar self-healability across bottlebrush networks containing different amounts of UPy. Furthermore, these stretchable networks with p ≤ 0.25 undergo self-healing upon repeated rupture and melt pressing at ≤100 °C while retaining similar tensile properties. In summary, decorating bottlebrush polymers with hydrogen bonds creates opportunities to independently manipulate associative network dynamics and mechanical moduli.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Precise Linker Length and Dynamic Bond Exchange Control Penetrant Diffusion in Dense Vitrimers

Polymer networks with dynamic covalent bonds have been investigated for their self-healing ability, recyclability, and potential as more sustainable materials. Recent results have indicated that in some cases, bond exchange can enhance the transport of penetrants in dense networks, pointing to their potential for separations of membranes. Here, imine dynamic bonds in ethylene oxide (EO) networks with precise linker lengths were synthesized to investigate the transport of N,N′-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (BTBP), a large, anisotropic dye molecule. Networks with mesh sizes smaller than, comparable to, and greater than the size of the penetrant axes were investigated to probe the effects of bond exchange and network confinement on transport. Mesh sizes, which ranged from 0.5 to 1.62 nm, were determined from shear rheology, glass transitions by calorimetry, and probe diffusion coefficients by fluorescence recovery after photobleaching. Permanent networks with identical EO chain lengths were prepared as control samples, and up to a 3 orders of magnitude increase in diffusion coefficient is observed in the dynamic systems for short linkers containing 13 backbone atoms. The longest linkers with 71 backbone atoms show no difference between the permanent and dynamic networks. Linkers shorter than 11 backbone atoms, corresponding to a mesh size smaller than the penetrant small axis, diffusion is no longer observable on the experimental time scale, indicating a sharp cutoff attributed to the precise linkers and narrow mesh size distribution. The dynamic imine exchange time scales were compared to the diffusive hopping times of penetrants and indicate that exchange can occur during a diffusive displacement. Furthermore, these findings provide insights into the factors affecting penetrant transport in dense polymers and inspire the development of next-generation selective polymer membranes.

Diffusion↗
DOE OSTI2017

A Discovery of Strong Metal–Support Bonding in Nanoengineered Au–Fe 3 O 4 Dumbbell-like Nanoparticles by in Situ Transmission Electron Microscopy

The strength of metal-support bonding in heterogeneous catalysts determines their thermal stability, therefore, a tremendous amount of effort has been expended to understand metal-support interactions. Herein, we report the discovery of an anomalous “strong metal-support bonding” between gold nanoparticles and “nano-engineered” Fe 3 O 4 substrates by in-situ microscopy. During in-situ vacuum annealing of Au-Fe 3 O 4 dumbbell-like nanoparticles, synthesized by the epitaxial growth of nano-Fe 3 O 4 on Au nanoparticles, the gold nanoparticles transform into the monolayered gold thinfilms and wet the surface of nano-Fe 3 O 4 , as the surface reduction of nano-Fe 3 O 4 proceeds. This phenomenon results from a unique coupling of the size-and shape-dependent high surface reducibility of nano-Fe 3 O 4 and the extremely strong adhesion between Au and the reduced Fe 3 O 4 . This strong-metal support bonding reveals the significance of controlling the metal oxide support size and morphology for optimizing metal-support bonding and, ultimately, for the development of improved catalysts and functional nanostructures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Discrimination of Bond Order in Organic Molecules Using Noncontact Atomic Force Microscopy

Noncontact atomic force microscopy (nc-AFM) with a CO-functionalized tip can image submolecular structures through high-resolution images with the possibility of discriminating bond order. Here, we employ real-space pseudopotential calculations to simulate nc-AFM images of molecules containing double (dibenzo(cd,n)naphtho(3,2,1,8-pqra)perylene (DBNP), hexabenzo(bc,ef,hi,kl,no,qr)coronene (HBC)) and triple (1,2-bis[2-(2-ethynylphenyl)ethynyl]-benzene (BEEB), 6-phenylhexa-1,3,5-triynylbenzene (PHTB)) bonds. We find (1) triple bonds can be unambiguously distinguished from other interatomic interactions based on a characteristic image and (2) the degree of double bond character can be directly determined from the image. We propose that large lateral forces acting on the tip may induce specific image distortions in the cases of DBNP and BEEB.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Ancillary Steric Effects on the Activation of SiH Bonds in Arylsilazido Rare-Earth Compounds

Three new hydridosilazido ligands, –N(SiHMe 2 )Aryl (Aryl = Ph, 2,6-C 6 Me 2 H 3 (dmp), 2,6-C 6 iPr 2 H 3 (dipp)) and their rare-earth complexes Ln{N(SiHMe 2 )Aryl} 3 (THF) n (Ln = Sc, Y, Lu; Aryl = Ph, n = 2; Aryl = dmp, n = 1; Aryl = dipp, n = 0) were synthesized to study the relationships among ligand steric properties, secondary Ln←H–Si bonding, and the reactivity of amide and SiH groups. In these compounds, the steric encumbrance of the aryl group was systematically increased from phenyl to 2,6-dimethylphenyl to 2,6-diisopropylphenyl. NMR, IR, and X-ray diffraction studies of the complexes characterize the number of secondary interactions and additional THF ligands coordinated to the rare-earth centers. The complexes with the smallest phenylsilazido ligands, Ln{N(SiHMe 2 )Ph} 3 (THF) 2 , contain features associated with three nonbridging 2-center-2-electron (2c-2e) Si–H bonds. Characterization of intermediate-sized Ln{N(SiHMe 2 )dmp} 3 THF reveals three and two Ln←H–Si interactions for yttrium and lutetium analogues, respectively, with both metals having one coordinated THF per complex. Ln{N(SiHMe 2 )dipp} 3 is formed solvent-free, and all three ligands adopt Ln←H–Si bonding modes. The reaction between Ln{N(SiHMe 2 )dipp} 3 and ketones provides the hydrosilylated product via addition of C=O and Si–H bonds, which occurs rapidly even at low temperature. Furthermore, this reaction is proposed to occur through an associative mechanism on the basis of negative activation entropy measured for substitution of pyridine in Ln{N(SiHMe 2 )dipp} 3 ·NC 5 H 5 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

C–H Bond Activation Facilitated by Bis(phosphinoamide) Heterobimetallic Zr/Co Complexes

The activation of C–H bonds using first-row transition metals poses a formidable challenge in the development of sustainable catalytic methods. Early/late heterobimetallic complexes provide a Lewis acidic binding site for directing groups, facilitating the activation of C–H bonds at an appended first-row transition metal center. In this work, the reactivity of the ZrIV/Co–I heterobimetallic complexes (THF)(I)Zr(XylNP i Pr 2 ) 2 Co(PR 3 ) (1-PR 3 ; Xyl = 3,5-dimethylphenyl; PR 3 = PMe 3 , PPh 2 Me) toward directed C–H bond activation is explored with pyridine and terminal alkyne derivatives. 1-PMe 3 reacts reversibly with 4-methylpyridine to afford the C–H activated complex (4-Me-C 5 H 4 N)(I)Zr(XylNP i Pr 2 ) 2 (μ-4-Me-C 5 H 3 N)Co(PMe 3 )(H) (3-PMe 3 ). By using the more Lewis basic substrate 4-tert-butylpyridine, (I)Zr(XylNP i Pr 2 )2(μ-4- t Bu-C 5 H 3 N)Co(PMe 3 )(H) (4-PMe 3 ) is formed irreversibly. In addition to pyridine derivatives, 1-PPh 2 Me can activate the C–H bond of terminal alkynes to form (THF)(I)Zr(XylNPiPr 2 ) 2 (μ-R'C≡C)Co(PPh 2 Me)(H) (R' = Ph (5-PPh 2 Me); R' = SiMe 3 (6-PPh 2 Me)).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Synthesis and Hydrolysis of Uranyl, Neptunyl, and Plutonyl Gas-Phase Complexes Exhibiting Discrete Actinide–Carbon Bonds

In this study, gas-phase organoactinyl complexes possessing discrete An–C bonds (An = U, Np, Pu) were synthesized in a quadrupole ion trap by endothermic decarboxylation of [AnO2(O2C–R)3] - anion complexes in which a formally AnO 2 2+ actinyl core is coordinated by three carboxylate ligands, with R = CH 3 (methyl), CH 3 CC (1-propynyl), C 6 H 5 (phenyl), C 6 F 5 (pentafluorophenyl). Decarboxylation and competing ligand loss were studied computationally by density functional theory complementing experiment. Although decarboxylation was computed to be the energetically most favorable process in all cases, reduction from An(VI) to An(V) via neutral ligand loss was often prevalent, particularly for An = Np, Pu, presumably resulting from barriers associated with decarboxylation. Comparative hydrolysis rates of the An–C bonds were experimentally determined, and the chemical properties of these bonds were analyzed by the quantum theory of atoms in molecules. The measured hydrolysis rates differed by up to 3 orders of magnitude: the fastest was for [(CH 3 CC)UO 2 (O 2 C–CCCH 3 ) 2 ] - and the slowest for [(C 6 F 5 )PuO 2 (O 2 C–C 6 F 5 ) 2 ] - . There is a general correlation between hydrolysis exothermicity and hydrolysis rate. Prototypical hydrolysis reaction pathways computed for R = CH 3 (An = U, Np) reveal a mechanism in which an outer-sphere water becomes inner-sphere concomitant with transfer of an H atom to yield an OH ligand and CH 4 , with a net energy release of 170 kJ mol –1 and a transition state barrier of 45 kJ mol –1 for An = U. Infrared multiphoton dissociation spectra of selected complexes were acquired to confirm the predicted structures by agreement between the computed and observed vibrational frequencies. Lastly, the experiment and theory results provide an evaluation of the comparative propensities for formation of the organoactinyls as a function of actinide and carboxylate and an assessment of the nature and stability toward hydrolysis of the primarily ionic An–C bonds.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hydrogen-Bond-Driven Chemical Separations: Elucidating the Interfacial Steps of Self-Assembly in Solvent Extraction

Chemical separations, particularly liquid extractions, are pervasive in academic and industrial laboratories, yet a mechanistic understanding of the events governing their function are obscured by interfacial phenomena that are notoriously difficult to measure. In this work, we investigate the fundamental steps of ligand self-assembly as driven by changes in the interfacial H-bonding network using vibrational sum frequency generation. Our results show how the bulk pH modulates the interfacial structure of extractants at the buried oil/aqueous interface via the formation of unique H-bonding networks that order and bridge ligands to produce self-assembled aggregates. These extended H-bonded structures are key to the subsequent extraction of Co2+ from the aqueous phase in promoting micelle formation and subsequent ejection of the said micelle into the oil phase. The combination of static and time-resolved measurements reveals the events underlying complexities of liquid extractions at high [Co2+]:[ligand] ratios by showing an evolution of interfacially assembled structures that are readily tuned on a chemical basis by altering the compositions of the aqueous phase. The results of this work point to new principles to design-applied separations through the manipulation of surface charge, electrostatic screening, and the associated H-bonding networks that arise at the interface to facilitate organization and subsequent extraction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Corrosion Resistance of Lignin-Based Thermoplastic Adhesive-Bonded Aluminum Joints

Adhesive-bonded joints are essential for lightweight, multimaterial automotive structures. We developed a lignin-based thermoplastic adhesive with adhesion strength comparable to conventional epoxy adhesives (20–23 MPa), while offering exceptional durability, recyclability, and corrosion resistance. Unlike commercial epoxy adhesives requiring refrigerated storage, our adhesive retains its bonding potential after a year of ambient conditioning. Bonded joints can be repeatedly disassembled and rejoined with less than 2% strength loss. After 500 h of salt fog exposure, the adhesive-bonded aluminum joints exhibited a 24% strength reduction, compared to 61% reduction in strength of joints based on a commercial epoxy adhesive.

Yu, Zeyang [ORNL] (ORCID:0000000209002374)↗
DOE OSTI2021

Investigating the Variation in Solvation Interactions of Choline Chloride-Based Deep Eutectic Solvents Formed Using Different Hydrogen Bond Donors

Choline chloride-based deep eutectic solvents (DESs) have gained widespread recognition as green solvents in catalysis and separation science as their designer properties permit solvation properties to be modulated by the choice of the hydrogen bond donor (HBD) as well as its molar ratio. Despite being one of the most popular classes of DESs, very little is currently understood regarding how their solvation characteristics differ among themselves as well as from other classes of DESs. Previous studies have shown that the catalytic activity, extraction efficiency, and solubilities of natural compounds can be influenced by the type of choline salt and HBD, but no study has systematically related these parameters with its solvation properties. In this study, inverse gas chromatography was employed for the first time to study the individual solvation characteristics of a diverse range of choline chloride and acetate-based DESs composed of amides, diols, organic acids, and carbohydrates. Furthermore, hydrogen bond acidity was found to be a dominant interaction for all DESs, especially those composed of organic acid HBDs. DESs composed of isomers of butane diol and hexane diol as HBDs exhibited higher hydrogen bond basicity and dispersive-type interactions compared to those composed of urea, acetamide, and organic acids with the position of the hydroxyl functional group influencing their solvation interactions. Choline acetate-based DESs exhibited lower dipolarity and stronger hydrogen bond basicity and dispersive-type interactions compared to choline chloride DESs. The solvation models developed in this study were used to interpret and explain DES behavior in previously reported studies involving catalysis, organic synthesis, and extractions demonstrating that the measured solute–solvent interactions can serve to predict the performance of choline chloride-based DESs when used in various applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Understanding Boron through Size-Selected Clusters: Structure, Chemical Bonding, and Fluxionality

Conspectus Boron is an interesting element with unusual polymorphism. While three-dimensional (3D) structural motifs are prevalent in bulk boron, atomic boron clusters are found to have planar or quasi-planar structures, stabilized by localized two-center–two-electron (2c–2e) σ bonds on the periphery and delocalized multicenter–two-electron (nc–2e) bonds in both σ and π frameworks. Electron delocalization is a result of boron’s electron deficiency and leads to fluxional behavior, which has been observed in B13+ and B19–. A unique capability of the in-plane rotation of the inner atoms against the periphery of the cluster in a chosen direction by employing circularly polarized infrared radiation has been suggested. Such fluxional behaviors in boron clusters are interesting and have been proposed as molecular Wankel motors. The concepts of aromaticity and antiaromaticity have been extended beyond organic chemistry to planar boron clusters. The validity of these concepts in understanding the electronic structures of boron clusters is evident in the striking similarities of the π-systems of planar boron clusters to those of polycyclic aromatic hydrocarbons, such as benzene, naphthalene, coronene, anthracene, or phenanthrene. Chemical bonding models developed for boron clusters not only allowed the rationalization of the stability of boron clusters but also lead to the design of novel metal-centered boron wheels with a record-setting planar coordination number of 10. The unprecedented highly coordinated borometallic molecular wheels provide insights into the interactions between transition metals and boron and expand the frontier of boron chemistry. Another interesting feature discovered through cluster studies is boron transmutation. Even though it is well-known that B–, formed by adding one electron to boron, is isoelectronic to carbon, cluster studies have considerably expanded the possibilities of new structures and new materials using the B–/C analogy. It is believed that the electronic transmutation concept will be effective and valuable in aiding the design of new boride materials with predictable properties. The study of boron clusters with intermediate properties between those of individual atoms and bulk solids has given rise to a unique opportunity to broaden the frontier of boron chemistry. Understanding boron clusters has spurred experimentalists and theoreticians to find new boron-based nanomaterials, such as boron fullerenes, nanotubes, two-dimensional boron, and new compounds containing boron clusters as building blocks. Here, a brief and timely overview is presented addressing the recent progress made on boron clusters and the approaches used in the authors’ laboratories to determine the structure, stability, and chemical bonding of size-selected boron clusters by joint photoelectron spectroscopy and theoretical studies. Specifically, key findings on all-boron hydrocarbon analogues, metal-centered boron wheels, and electronic transmutation in boron clusters are summarized.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2022

Formation of the N≡N Triple Bond from Reductive Coupling of a Paramagnetic Diruthenium Nitrido Compound

Construction of nitrogen–nitrogen triple bonds via homocoupling of metal nitrides is an important fundamental reaction relevant to a potential Nitrogen Economy. Here, we report that room temperature photolysis of Ru 2 (chp) 4 N 3 (chp– = 2-chloro-6-hydroxypyridinate) in CH 2 Cl 2 produces N 2 via reductive coupling of Ru2(chp)4N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane “zigzag” geometry instead of the anticipated planar zigzag TS. However, with intentional exclusion of dispersion correction, the planar zigzag TS geometry can also be found. Both the out-of-plane and planar zigzag TS geometries feature two important types of orbital interactions: (1) donor–acceptor interactions involving intermolecular donation of a nitride lone pair into an empty Ru–N π* orbital and (2) Ru–N π to Ru–N π* interactions derived from coupling of nitridyl radicals. The relative importance of these two interactions is quantified both at and after the TS. Our analysis shows that both interactions are important for the formation of the N–N σ bond, while radical coupling interactions dominate the formation of N–N π bonds. Comparison is made to isoelectronic Ru 2 -oxo compounds. Formation of an O–O bond via bimolecular oxo coupling is not observed experimentally and is calculated to have a much higher TS energy. As a result, the major difference between the nitrido and oxo systems stems from an extremely large driving force, ~–500 kJ/mol, for N–N coupling vs a more modest driving force for O–O coupling, –40 to –140 kJ/mol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Synergistic Effects of the Electric Field Induced by Imidazolium Rotation and Hydrogen Bonding in Electrocatalysis of CO 2

The roles of the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), and water in controlling the mechanism, energetics, and electrocatalytic activity of CO₂ reduction to CO on silver in non-aqueous electrolytes were investigated. The first electron transfer occurs to CO₂ at reduced overpotentials when it is trapped between the planes of [EMIM]⁺ ring and electrode surface due to cation re-orientation as determined from voltammetry, in-situ surface-enhanced Raman spectroscopy, and density functional theory calculations. Within this interface, water up to 0.5 M does not induce significant faradaic activity, opposing the notion of it as a free proton source. Instead, water acts as a hydrogen bond donor and the proton is sourced from [EMIM]⁺. Furthermore, this study demonstrates that alcohols with varying acidity tune the hydrogen bonding network in the interfacial microenvironment to lower the energetics required for CO₂ reduction. Here, the hydrogen bonding suppresses the formation of inactive carboxylate species, thus preserving the catalytic activity of [EMIM]⁺. The ability to tune the hydrogen bonding network opens new avenues for advancing IL-mediated electrocatalytic reactions in non-aqueous electrolytes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

The Mechanism of N–N Double Bond Cleavage by an Iron(II) Hydride Complex

The use of hydride species for substrate reductions avoids strong reductants, and may enable nitrogenase to reduce multiple bonds without unreasonably low redox potentials. Here in this work, we explore the N=N bond cleaving ability of a high-spin iron(II) hydride dimer with concomitant release of H 2 . Specifically, this diiron(II) reacts with azobenzene (PhN=NPh) to perform the fourelectron reduction to two imido groups, where two electrons come from H 2 reductive elimination and the other two come from iron oxidation. The rate law of the H 2 releasing reaction indicates that diazene binding occurs prior to H 2 elimination, and the negative entropy of activation and inverse kinetic isotope effect indicate that H-H bond formation is the rate-limiting step. Thus, substrate binding causes reductive elimination of H 2 that formally reduces the metals, and the metals use the additional two electrons to cleave the N-N multiple bond.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Light Makes a Surface Banana-Bond Split: Photodesorption of Molecular Hydrogen from RuO 2 (110)

The coordination of H2 to a metal center via polarization of its bond electron density, known as a Kubas complex, is the means by which H2 chemisorbs at Ru4+ sites on the rutile RuO2(110) surface. This distortion of electron density off an interatomic axis is often described as a ‘banana-bond.’ We show that the Ru-H2 banana-bond can be destabilized, and split, using visible light. Photodesorption of H2 (or D2) is evident by mass spectrometry and scanning tunneling microscopy. From time-dependent density functional theory, the key optical excitation splitting the Ru-H2 banana-bond involves an interband transition in RuO2 which effectively diminishes its Lewis acidity, and thereby weakening the Kubas complex. Such excitations are not expected to affect adsorbates on RuO2 given its metallic properties. Therefore, this common thermal co-catalyst employed in promoting water splitting is, itself, photo-active in the visible.

desorption↗