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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.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Resonant Inelastic X-Ray Scattering Reveals Hidden Local Transitions of the Aqueous OH Radical

Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities to interrogate ultrafast dynamics in liquids. In this work we use RIXS to study the fundamentally and practically important hydroxyl radical in liquid water, OH ( a q ) . Impulsive ionization of pure liquid water produced a short-lived population of OH ( a q ) , which was probed using femtosecond x-rays from an x-ray free-electron laser. We find that RIXS reveals localized electronic transitions that are masked in the ultraviolet absorption spectrum by strong charge-transfer transitions—thus providing a means to investigate the evolving electronic structure and reactivity of the hydroxyl radical in aqueous and heterogeneous environments. First-principles calculations provide interpretation of the main spectral features.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Solvent oligomerization pathways facilitated by electrolyte additives during solid-electrolyte interphase formation

The solid–electrolyte interphase (SEI) layer formation is known to play an important role in determining the lifetime of lithium-ion batteries. A thin, stable SEI layer is linked to overall improved battery performance and longevity, however, the factors and mechanisms that lead to optimal SEI morphology and composition are not well understood. Inclusion of electrolyte additives (fluoroethylene carbonate, FEC; and vinylene carbonate, VC) is often necessary for improving SEI characteristics. To understand how these electrolyte additives impact SEI formation, we employed molecular dynamics (MD) and density functional theory (DFT) simulations to study the reaction networks and oligomerization pathways, respectively, for three systems containing ethylene carbonate (EC), a lithium ion, and FEC or VC. MD simulations suggest radical oligomerization pathways analogous to traditional oligomerization with nucleophilic alkoxide species via SN1 reaction mechanisms. Both S N 1 and S N 2 mechanisms were studied for all three systems using DFT. Oligomerization reactions were studied with both a standard alkoxide species and a ring-opened EC radical as the nucleophiles and EC, FEC, and VC as the electrophiles. For all cases, FEC and VC exhibited lower free energy barriers and more stable adducts when compared with EC. We conclude that one of the role of additives is to modify the oligomerization process of EC by introducing branching points (FEC) or termination points (VC).

25 ENERGY STORAGE↗
DOE OSTI2023

Mechanism of Radical Initiation in the Radical SAM Enzyme Superfamily

Radical S-adenosylmethionine (SAM) enzymes use a site-differentiated [4Fe-4S] cluster and SAM to initiate radical reactions through liberation of the 5'-deoxyadenosyl (5'-dAdo•) radical. They form the largest enzyme superfamily, with more than 700,000 unique sequences currently, and their numbers continue to grow as a result of ongoing bioinformatics efforts. The range of extremely diverse, highly regio- and stereo-specific reactions known to be catalyzed by radical SAM superfamily members is remarkable. The common mechanism of radical initiation in the radical SAM superfamily is the focus of this review. Most surprising is the presence of an organometallic intermediate, Ω, exhibiting an Fe–C5'-adenosyl bond. Regioselective reductive cleavage of the SAM S–C5' bond produces 5'-dAdo• to form Ω, with the regioselectivity originating in the Jahn–Teller effect. Ω liberates the free 5'-dAdo• as the catalytically active intermediate through homolysis of the Fe–C5' bond, in analogy to Co–C5' bond homolysis in B 12 , which was once viewed as biology's choice of radical generator.

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

Electron Transfer Theory Elucidates the Hidden Role Played by Triethylamine and Triethanolamine during Photocatalysis

Triethylamine (TEA) and triethanolamine (TEOA) are renowned, in part, for their ability to reductively quench excited states by outer-sphere electron transfer with vast and still growing applications as sacrificial electron donors for photocatalytic systems. Upon amine oxidation, the resulting TEA •+ and TEOA •+ radical cations undergo proton transfer (or hydrogen atom transfer), resulting in the formation of a chemical reductant that has an α-carbon centered radical adjacent to the nitrogen center (TEA • and TEOA • ). In this contribution, we have electrochemically and spectroscopically characterized a set of electron acceptors which, upon accepting an electron, are a series of photocatalysts, [ReCl(R 1 R 2 -bpy)(CO) 3 ] •– , where R 1 and R 2 are electron-donating and electron-withdrawing groups in the 4,4′- and 5,5′-positions on the bipyridyl ligand. Here, we substantiated the formation of the electron donors, TE(O)A • , by spin trapping electron paramagnetic resonance spectroscopy, where TE(O)A • reacts with 2,4,6-tri-tert-butylnitrosobenzene to generate N-centered and O-centered radical adducts. Having established the chemical behaviors of the electron acceptors and donors individually, the electron transfer rate constants were determined across a 1.43 V range in driving force. The redox potential of TEA • was benchmarked to within ±80 mV on an absolute scale in V vs Fc + /Fc in CH 3 CN by using an empirical rate vs free-energy correlation, electron transfer theory, and density functional theory calculations. The equilibrium potentials for TEA • and TEOA • were determined to be −1.98 V and −1.76 V, respectively. Based on the kinetic and thermochemical analysis presented for TEA • and TEOA • , these transient radicals can be broadly considered strong homogeneous chemical reductants within the wider context of photoredox potentials. Thus, this work clarifies a frequently unnoticed secondary function for these sacrificial electron donors during photocatalysis and rationalizes the possibility of a one-photon/two-electron conversion process that is dependent on the free-energy exchange between TE(O)A • and photocatalysts.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2023

Dual Tunability for Uncatalyzed N‐Alkylation of Primary Amines Enabled by Plasma‐Microdroplet Fusion

Abstract The fusion of non‐thermal plasma with charged microdroplets facilitates catalyst‐free N‐alkylation for a variety of primary amines, without halide salt biproduct generation. Significant reaction enhancement (up to >200×) is observed over microdroplet reactions generated from electrospray. This enhancement for the plasma‐microdroplet system is attributed to the combined effects of energetic collisions and the presence of reactive oxygen species (ROS). The ROS (e.g., O 2 ⋅ − ) act as a proton sink to increase abundance of free neutral amines in the charged microdroplet environment. The effect of ROS on N‐alkylation is confirmed through three unique experiments: (i) utilization of radical scavenging reagent, (ii) characterization of internal energy distribution, and (iii) controls performed without plasma, which lacked reaction acceleration. Establishing plasma discharge in the wake of charged microdroplets as a green synthetic methodology overcomes two major challenges within conventional gas‐phase plasma chemistry, including the lack of selectivity and product scale‐up. Both limitations are overcome here, where dual tunability is achieved by controlling reagent concentration and residence time in the microdroplet environment, affording single or double N‐alkylated products. Products are readily collected yielding milligram quantities in eight hours. These results showcase a novel synthetic strategy that represents a straightforward and sustainable C−N bond‐forming process.

Chemistry↗
DOE OSTI2020

A Pd III Sulfate Dimer Initiates Rapid Methane Monofunctionalization by H Atom Abstraction

An electrogenerated Pd III 2 species in fuming sulfuric acid is competent for rapid and concurrent methane monohydroxylation to methyl bisulfate (CH 3 OSO 3 H) and methane sulfonation to methanesulfonic acid (CH 3 SO 3 H). In situ NMR at 50 °C is used to track methane transformation exclusively to CH 3 OSO 3 H and CH 3 SO 3 H at high conversions. Integrating a set of kinetic and computational studies, the mechanism of methane monofunctionalization by Pd III 2 is examined. Here, experimental rate laws and common kinetic isotope effects for CH 3 OSO 3 H and CH 3 SO 3 H formation suggest that both transformations proceed via a common rate-limiting C-H activation step. Introduction of O 2 or Pd II,III 2 suppresses CH 3 SO 3 H generation, indicating a radical chain sequence. Although the metal-metal bonded Pd III 2 complex is a net two-electron oxidant, our aggregate kinetic data point to a mechanistic model that features rate-limiting H atom abstraction by the Pd III 2 complex to generate a methyl radical intermediate. The CH 3 • intermediate then recombines with Pd II,III 2 to furnish a CH 3 Pd III 2 intermediate that reductively eliminates CH 3 OSO 3 H. Alternatively, the CH 3 intermediate can enter a chain reaction with SO 3 to generate CH 3 SO 3 H. DFT computations support the radical-based C-H activation by Pd III 2 and delineate H atom abstraction pathways with computed reaction barriers and kinetic isotope effects (KIEs) that are consistent with experimental data. These mechanistic investigations challenge the paradigm of electrophilic C-H activation and highlight H atom abstraction as a potent pathway for selective methane C-H oxidative functionalization at high reaction rates.

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

Toward Metal-free Photocatalysis: Photochemical Regeneration of Organic Hydride Donors Using Phenazine-Based Photosensitizers

Earth-abundant chromophores and catalysts are important molecular building blocks for artificial photosynthesis applications. Our team previously reported that metal-free hydride donors, such as biomimetic benzoimidazole-based motifs, can reduce CO 2 selectively to the formate ion and that they can be electrochemically regenerated using the proton-coupled mechanism. To enable direct utilization of solar energy, we report here the photochemical regeneration of a benzoimidazole-based hydride donor using a phenazine-based metal-free chromophore. The photochemical regeneration was investigated under different experimental conditions involving varying sacrificial donors, proton donors, solvents, and component concentrations. The best hydride regeneration yield of 50% was obtained with phenol as a proton source and thiophenolate as a sacrificial electron donor. The mechanism of photochemical regeneration was studied using steady-state and time-resolved UV/Vis spectroscopies. Based on the results of these studies, we hypothesize that the initial photoinduced electron transfer from photoexcited phenazine chromophores involves the benzoimidazole cation and that this process is likely coupled with proton transfer to generate protonated benzoimidazole-based radical cation. The second photoinduced electron transfer is hypothesized to generate the hydride form. Furthermore, our findings provide the requisite information for the future development of reductive photocatalysts for solar energy and light-harvesting applications utilizing earth-abundant metal-free materials.

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

Oxidative Functionalization of Long-Chain Liquid Alkanes by Pulsed Plasma Discharges at Atmospheric Pressure

In this work, we introduce the oxidation of long aliphatic alkanes using non-thermal, atmospheric plasma processing as an eco-friendly route for organic synthesis. A pulsed dielectric barrier discharge in He/O 2 gas mixtures was employed to functionalize n-octadecane. C 18 secondary alcohols and ketones were the main products, with an optimal molar yield of ~29.2%. Prolonged treatment resulted in the formation of dialcohols, diketones, and higher molecular weight oxygenates. Lighter hydrocarbon products and decarboxylation to CO 2 were also observed at longer treatment times and higher power inputs. A maximum energy yield of 5.48 × 10 –8 mol/J was achieved at short treatment times and high powers, associated with higher selectivity to primary oxygenates. Direct hydroxylation of alkyl radicals, as well as disproportionation reactions, are proposed as the main pathways to alcohols and ketones. The results hold promise for functionalizing long hydrocarbon molecules at ambient conditions using catalyst-free plasma discharges.

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

[FeFe]‐Hydrogenase: Defined Lysate‐Free Maturation Reveals a Key Role for Lipoyl‐H‐Protein in DTMA Ligand Biosynthesis

Abstract Maturation of [FeFe]‐hydrogenase (HydA) involves synthesis of a CO, CN − , and dithiomethylamine (DTMA)‐coordinated 2Fe subcluster that is inserted into HydA to make the active hydrogenase. This process requires three maturation enzymes: the radical S‐adenosyl‐ l ‐methionine (SAM) enzymes HydE and HydG, and the GTPase HydF. In vitro maturation with purified maturation enzymes has been possible only when clarified cell lysate was added, with the lysate presumably providing essential components for DTMA synthesis and delivery. Here we report maturation of [FeFe]‐hydrogenase using a fully defined system that includes components of the glycine cleavage system (GCS), but no cell lysate. Our results reveal for the first time an essential role for the aminomethyl‐lipoyl‐H‐protein of the GCS in hydrogenase maturation and the synthesis of the DTMA ligand of the H‐cluster. In addition, we show that ammonia is the source of the bridgehead nitrogen of DTMA.

Pagnier, Adrien↗
DOE OSTI2022

[FeFe]-Hydrogenase: Defined Lysate-Free Maturation Reveals a Key Role for Lipoyl-H-Protein in DTMA Ligand Biosynthesis

Maturation of [FeFe]-hydrogenase (HydA) involves synthesis of a CO, CN - , and dithiomethylamine (DTMA)-coordinated 2Fe subcluster that is inserted into HydA to make the active hydrogenase. This process requires three maturation enzymes: the radical S-adenosyl-l-methionine (SAM) enzymes HydE and HydG, and the GTPase HydF. In vitro maturation with purified maturation enzymes has been possible only when clarified cell lysate was added, with the lysate presumably providing essential components for DTMA synthesis and delivery. We report maturation of [FeFe]-hydrogenase using a fully defined system that includes components of the glycine cleavage system (GCS), but no cell lysate. Our results reveal for the first time an essential role for the aminomethyl-lipoyl-H-protein of the GCS in hydrogenase maturation and the synthesis of the DTMA ligand of the H-cluster. In addition, we show that ammonia is the source of the bridgehead nitrogen of DTMA.

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

Probing the Kinetic Origin of Varying Oxidative Stability of Ethyl- vs. Propyl-spaced Amines for Direct Air Capture

Amine-based adsorbents are promising for direct air capture of CO 2 , yet oxidative degradation remains a key unmitigated risk hindering wide-scale deployment. Borrowing wisdom from the basic auto-oxidation scheme, insights are gained into the underlying degradation mechanisms of polyamines by quantum chemical, advanced sampling simulations, adsorbent synthesis, and accelerated degradation experiments. The reaction kinetics of polyamines are contrasted with that of typical aliphatic polymers and they elucidate for the first time the critical role of aminoalkyl hydroperoxide decomposition in the oxidative degradation of amino-oligomers. The experimentally observed variation in oxidative stability of polyamines with different backbone structures is explained by the relationship between the local chemical structure and the free energy barrier of aminoalkyl hydroperoxide decomposition, suggesting that its energetics can be used as a descriptor to screen and design new polyamines with improved stability. In conclusion, the developed computational capability sheds light on radical-induced degradation chemistry of other organic functional materials.

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

Formation of Gas-Phase Allyl Radicals from Glycerol on Rutile TiO2(110)

The reaction pathways of glycerol on partially reduced TiO2(110) have been studied by a combination of molecular beam dosing and temperature-programmed desorption (TPD). Glycerol is found to bind more strongly to TiO2(110) than glycols due to an additional hydroxyl group in the molecule. When the Ti surface sites are saturated by glycerol, the majority of glycerol molecules (~ 95%) undergo further reactions to yield carbon-free (water and hydrogen) and carbon-containing products that are desorbed during the TPD ramp. In contrast to glycols for which both water and hydrogen arise exclusively from the cleavage of glycol O-H bonds, we identify two distinct reaction channels resulting from the cleavage of O-H and C-H bonds, respectively, for both water and hydrogen evolution from glycerol on TiO2(110). Quantitative determination of the desorption yields of water and hydrogen reveals that the C-H bond scission channel which occurs at higher temperature than the O-H bond scission channel, is a minor reaction channel for water formation, while it is a major reaction channel responsible for hydrogen evolution. For the carbon-containing products, we exclude both carbon deposition and carbon oxides (CO and CO2) formation, and identify the allyl radical as the major carbon-containing product. The formation of the allyl radical is accompanied by the formation of various minor carbon-containing products including propylene, acrolein, allyl alcohol, acetone, formaldehyde and ethylene. The different surface chemistries observed between glycerol and simpler alcohols and glycols on TiO2(110), suggest that smaller oxygenates cannot serve as models for larger oxygenates in probing their reaction pathways on oxide surfaces. The new reaction channels observed for glycerol on TiO2(110) may open up a new way to convert bio-renewable glycerol directly to pure hydrogen and hydrocarbons on TiO2. This work was supported by the US Department of Energy, Office of Sciences, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences, and performed in EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle.

Chen, Long↗
DOE OSTI2024

Broadband Rotational Spectroscopy in Uniform Supersonic Flows: Chirped Pulse/Uniform Flow for Reaction Dynamics and Low Temperature Kinetics

ConspectusThe study of gas-phase chemical reactions at very low temperatures first became possible with the development and implementation of the CRESU (French acronym for Reaction Kinetics in Uniform Supersonic Flows) technique. CRESU relies on a uniform supersonic flow produced by expansion of a gas through a Laval (convergent-divergent) nozzle to produce a wall-less reactor at temperatures from 10 to 200 K and densities of 1016-1018 cm-3 for the study of low temperature kinetics, with particular application to astrochemistry. In recent years, we have combined uniform flows with revolutionary advances in broadband rotational spectroscopy to yield an instrument that affords near-universal detection for novel applications in photodissociation, reaction dynamics, and kinetics. This combination of uniform supersonic flows with chirped-pulse Fourier-transform microwave spectroscopy (Chirped-Pulse/Uniform Flow, CPUF) permits detection of any species with a modest dipole moment, thermalized to the uniform temperature of the gas flow, with isomer, conformer, and vibrational state specificity. In addition, the use of broadband, high-resolution, and time-dependent (microsecond time scale) micro- and mm-wave spectroscopy makes it an ideal tool for characterizing both transient and stable molecules, as well as studying their spectroscopy and dynamics.In this Account, we review recent advances made using the CPUF technique, including studies of photodissociation, radical-radical reaction dynamics, and low temperature kinetics. These studies highlight both the strength of universal and multiplexed detection and the challenges of coupling it to a high-density collisional environment. Product branching and product evolution as a function of time have been measured for astrochemically relevant systems, relying on the detailed characterization of these flow conditions via experiments and fluid dynamics simulations. In the photodissociation of isoxazole, an unusual heterocyclic molecule with a very low-energy conical intersection, we have identified 7 products in 5 reaction channels and determined the product branching, pointing to both direct and indirect pathways. We have also approached the same system from separated NO and C3H3 reactants to explore a broader range of the potential energy surface, demonstrating the power of multichannel branching measurements for complex radical-radical reactions. We determined the product branching in the C3H2 isomers in the photodissociation of the propargyl radical and identified the importance of a hydrogen atom catalyzed isomerization to the lowest energy cyclic form. This then motivated a study of direct D-H exchange reaction in radicals, in which we demonstrate that it is an important and overlooked pathway for deuterium fractionation in astrochemical environments. Recently, we have shown the measurement of low temperature kinetics inside an extended Laval nozzle, after which a shock-free secondary expansion to low temperature and density affords an ideal environment for detection by rotational spectroscopy. These results highlight the power and potential of the CPUF approach, and future prospects will also be discussed in light of these developments.

Dias, Nureshan↗
DOE OSTI2020

Losses of CO and CO 2 upon collision-activated dissociation of substituted 2-methoxyphenoxides after methyl radical loss

Some deprotonated, substituted 2-methoxyphenols fragment by methyl radical loss followed by competing losses of CO and CO 2 upon collisional activation in linear quadrupole ion trap mass spectrometers. These reactions were examined experimentally and computationally in order to determine their mechanisms. For deprotonated vanillin, the CO loss was found to involve ring contraction, with the highest free energy barrier of 58.0 kcal mol -1 (for the syn rotamer; here, syn refers to the relationship between the aldehyde oxygen atom and the oxygen atom of the methoxy group) for the entire process. The atoms lost in this fragmentation are the oxygen atom that was bound to the first eliminated methyl group and the aromatic carbon atom bound to this oxygen. Examination of carbon-13 labeled vanillin supports these assignments. Examination of several model compounds revealed that this reaction requires the presence of an electron-withdrawing substituent in the para-position relative to the phenol moiety. In contrast, the CO 2 loss from deprotonated vanillin occurs via ring opening followed by re-cyclization and then ring contraction, leading to the loss of CO 2 in a process wherein the highest free energy barrier is 84.5 kcal mol -1 (for the syn isomer). The atoms lost in this fragmentation are the carbon and oxygen atoms from the phenoxide group and the oxygen atom that was bound to the first eliminated methyl group, which is supported by examination of carbon-13 labeled vanillin. Despite the higher total free energy requirement, the CO 2 loss is competitive with CO loss, possibly due to favorable entropy and low activation energy (free energy barrier of 48.9 kcal mol -1 ) for the first reaction step (the analogous value for CO loss is 58.0 kcal mol -1 ). The extent of CO 2 loss is strongly affected by substituents – it either is not observed or is very slow for the other compounds studied here. For example, it is substantially less favorable than CO loss (highest free energy barrier 60.9 kcal mol -1 ) for deprotonated acetovanillone for which the first reaction step for CO 2 loss has a substantially greater free energy barrier (60.6 kcal mol-1) than for vanillin.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Impact of lanthanide ion complexation and temperature on the chemical reactivity of N , N , N ', N '-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation

The impact of trivalent lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the n-dodecane radical cation (RH˙+) has been measured by electron pulse radiolysis and evaluated by quantum mechanical calculations. Additionally, Arrhenius parameters were determined for the reaction of the non-complexed TODGA ligand with the RH˙ + from 10–40 °C, giving the activation energy (E a = 17.43 ± 1.64 kJ mol –1 ) and pre-exponential factor (A = (2.36 ± 0.05) × 10 13 M –1 s –1 ). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [LnIII(TODGA)3(NO3)3] complexes that exhibited significantly increased reactivity (up to 9.3× faster) with the RH˙ + , relative to the non-complexed ligand: k([Ln III (TODGA) 3 (NO 3 ) 3 ] + RH˙ + ) = (8.99 ± 0.93) × 10 10 , (2.88 ± 0.40) × 10 10 , and (1.53 ± 0.34) × 10 10 M –1 s –1 , for Nd(III), Gd(III), and Yb(III) ions, respectively. The rate coefficient enhancement measured for these complexes exhibited a dependence on atomic number, decreasing as the lanthanide series was traversed. Preliminary reaction free energy calculations—based on a model [Ln III (TOGDA)] 3+ complex system—indicate that both electron/hole and proton transfer reactions are energetically unfavorable for complexed TODGA. Furthermore, complementary average local ionization energy calculations showed that the most reactive region of model N,N,N',N'-tetraethyl diglycolamide (TEDGA) complexes, [Ln III (TEGDA) 3 (NO 3 ) 3 ], toward electrophilic attack is for the coordinated nitrate (NO 3 – ) counter anions. Furthermore, it is possible that radical reactions with the complexed NO 3 – counter anions dominate the differences in rates seen for the [Ln III (TODGA) 3 (NO 3 ) 3 ] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2022

Elucidating Instabilities Contributing to Capacity Fade in Bipyridine‐Based Materials for Non‐aqueous Flow Batteries

Abstract Metal‐based non‐aqueous redox flow batteries have the potential for long‐term energy storage if stability requirements can be achieved. One such stability issue in metal coordination complexes arises from ligand shedding in the anolyte upon reduction. Recognizing that the free ligands are relatively more stable than the metal coordination complex under highly reducing conditions, we evaluated a family of metal‐free bipyridinium materials as flow battery anolytes with the corresponding iron coordination complex as the catholyte. Bipyridinium compounds were functionalized for increased electrochemical stability, cycled in flow cells to understand efficiencies, and analyzed for degradation products. Methylation of the bipyridine nitrogens increased electrochemical stability yet left the reduced molecule susceptible to radical‐induced bond cleavage. Subsequent functionalization of bipyridine with carbomethoxy groups resulted in good battery performance with 96 % Coulombic and 90 % voltage efficiency, and improved cycling stability over a methoxy‐substituted anolyte with 14 % vs 36 % capacity fade in the first charge‐discharge cycle.

Kolesnichenko, Claudina X.↗
DOE OSTI2022

Ni/Photoredox-Catalyzed C(sp 3 )–C(sp 3 ) Coupling between Aziridines and Acetals as Alcohol-Derived Alkyl Radical Precursors

Aziridines are readily available C(sp 3 ) precursors that afford valuable β-functionalized amines upon ring opening. In this article, we report a Ni/photoredox methodology for C(sp 3 )–C(sp 3 ) cross-coupling between aziridines and methyl/1°/2° aliphatic alcohols activated as benzaldehyde dialkyl acetals. Orthogonal activation modes of each alkyl coupling partner facilitate cross-selectivity in the C(sp 3 )–C(sp 3 ) bond-forming reaction: the benzaldehyde dialkyl acetal is activated via hydrogen atom abstraction and β-scission via a bromine radical (generated in situ from single-electron oxidation of bromide), whereas the aziridine is activated at the Ni center via reduction. Here, we demonstrate that an Ni(II) azametallacycle, conventionally proposed in aziridine cross-coupling, is not an intermediate in the productive cross-coupling. Rather, stoichiometric organometallic and linear free energy relationship studies indicate that aziridine activation proceeds via Ni(I) oxidative addition, a previously unexplored elementary step.

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

Alternant Hydrocarbon Diradicals as Optically Addressable Molecular Qubits

High-spin molecules allow for bottom-up qubit design and are promising platforms for magnetic sensing and quantum information science. Optical addressability of molecular electron spins has also been proposed in first-row transition-metal complexes via optically detected magnetic resonance (ODMR) mechanisms analogous to the diamond-nitrogen-vacancy color center. However, significantly less progress has been made on the front of metal-free molecules, which can deliver lower costs and milder environmental impacts. At present, most luminescent open-shell organic molecules are π-diradicals, but such systems often suffer from poor ground-state open-shell characters necessary to realize a stable ground-state molecular qubit. In this work, we use alternancy symmetry to selectively minimize radical–radical interactions in the ground state, generating π-systems with high diradical characters. We call them m-dimers, referencing the need to covalently link two benzylic radicals at their meta carbon atoms for the desired symmetry. Through a detailed electronic structure analysis, we find that the excited states of alternant hydrocarbon m-diradicals contain important symmetries that can be used to construct ODMR mechanisms leading to ground-state spin polarization. Furthermore, the molecular parameters are set in the context of a tris(2,4,6-trichlorophenyl)methyl (TTM) radical dimer covalently tethered at the meta position, demonstrating the feasibility of alternant m-diradicals as molecular color centers.

Chemistry↗