SEARCH · Search NASA
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.
To investigate whether the vascular collapse in tumors by conventional dose rate (CONV) irradiation (IR) would also occur by the ultra-high dose rate FLASH IR.
Nanoscale coatings of AlO x were deposited onto SiO 2 using atomic layer deposition (ALD) to synthesize amorphous silica-alumina (SiAl) catalysts, and these catalysts were investigated for levoglucosanol (Lgol) hydrogenolysis. With decreasing Al 2 O 3 loading, the ratio of Brønsted to Lewis acid sites, measured by NH 3 -TPD and pyridine-FTIR, systematically increases, while the Al coordination, measured by solid state 27 Al NMR, decreases. These structural changes correspond to an increasing mass-normalized rate of Lgol hydrogenolysis. We model the mass-normalized reaction rate as the sum of independent contributions from Brønsted and Lewis sites, showing that Brønsted acid sites on ALD-AlO x /SiO 2 catalysts have a 6-times higher turnover frequency (TOF) than Lewis acid sites on these catalysts. Additionally, Lewis acid sites on ALD-AlO x /SiO 2 catalysts (potentially related to Al(V) species) have a 4-times higher TOF than Lewis acid sites on bulk γ-Al 2 O 3 . The overall mass-normalized reactivity of ALD-AlO x /SiO 2 catalysts is due to Lewis acid sites at the highest Al 2 O 3 loading, while it is predominantly due to Brønsted acid sites at the lowest Al 2 O 3 loadings. In conclusion, this work provides a new approach to synthesize amorphous SiAls with tunable Brønsted/Lewis acid site ratio and reveals differences in the reactivity of Brønsted and Lewis acid sites on these materials.
Proteinaceous aggregates containing ?-synuclein protein called Lewy bodies in the substantia nigra is a hallmark of Parkinson’s disease. The molecular mechanisms of Lewy body formation and associated neuronal loss remain largely unknown. To gain insights on proteins and pathways associated with Lewy body pathology, we performed quantitative profiling of the proteome. We analyzed substantia nigra tissue from 51 subjects arranged into three groups: cases with Lewy body pathology, Lewy body-negative controls with matching neuronal loss and controls with no neuronal loss. Using label-free LC-MS/MS approach we quantified the 2,963 most abundant proteins. Statistical testing for differential protein abundance, followed by pathway enrichment and Bayesian learning of the causal network structure were performed to identify likely drivers of Lewy body formation and dopaminergic neuronal loss. The identified pathways include (1) Arp2/3 complex-mediated actin nucleation; (2) synaptic function; (3) poly(A) RNA binding; (4) basement membrane and endothelium; and (5) hydrogen peroxide metabolic process. According to the data, the endothelial/basement membrane pathway is tightly connected with both pathologies and likely to be one of the drivers of the neuronal loss. The poly(A) RNA-binding proteins, including the ones relevant to other neurodegenerative disorders (e.g. TDP-43 and FUS) have strong inverse correlation with Lewy bodies and may reflect alternative mechanism of nigral neurodegeneration.
Lewis acid-base interactions are common in chemical processes presented in diverse applications, such as synthesis, catalysis, batteries, semiconductors, and solar cells. The Lewis acid-base interactions allow precise tuning of material properties from the molecular level to more aggregated and organized structures. This review will focus on the origin, development, and prospects of applying Lewis acid-base interactions for the materials design and mechanism understanding in the advancement of battery materials and chemistries. The covered topics relate to aqueous batteries, lithium-ion batteries, solid-state batteries, alkali metal-sulfur batteries, and alkali metal-oxygen batteries. In this review, the Lewis acid-base theories will be first introduced. Thereafter the application strategies for Lewis acid-base interactions in solid-state and liquid-based batteries will be introduced from the aspects of liquid electrolyte, solid polymer electrolyte, metal anodes, and high-capacity cathodes. The underlying mechanism is highlighted in regard to ion transport, electrochemical stability, mechanical property, reaction kinetics, dendrite growth, corrosion, and so on. Last but not least, perspectives on the future directions related to Lewis acid-base interactions for next-generation batteries are like to be shared.
The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in non-aqueous conditions, commonly used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a 31 P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. Plots of the pK a values of the corresponding metal-aqua species, [M(H 2 O) m ] n+ , measured in H 2 O vs. the 31 P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d3-MeCN) and deuterated dichloromethane (CD 2 Cl 2 ) display tightly co-linear relationships, suggesting similar behavior for these ions in water, d 3 -MeCN, and CD 2 Cl 2 . This collinearity reinforces the utility of the common approach of using the aqueous pK a values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d 3 -MeCN suggest 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Altogether, these data suggest that both aqueous pK a values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.
Doping is required to increase the electrical conductivity of organic semiconductors for uses in electronic and energy conversion devices. The limited number of commonly used p-type dopants suggests that new dopants or doping mechanisms could improve the efficiency of doping and provide new means for processing doped polymers. Drawing on Lewis acid–base pair chemistry, we combined Lewis acid dopant B(C 6 F 5 ) 3 (BCF) with the weak Lewis base benzoyl peroxide (BPO). The detailed behavior of p-type doping of the model polymer poly(3-hexylthiophene) (P3HT) with this Lewis acid–base pair in solution was examined. Solution 19F-NMR spectra confirmed the formation of the expected counterion, as well as side products from reactions with solvent. BCF : BPO was also found to efficiently dope a range of semiconducting polymers with varying chemical structures demonstrating that the BCF : BPO combination has an effective electron affinity of at least 5.3 eV. In thin films of regioregular P3HT cast from the doped solutions, delocalized polarons formed due to the large counterions leading to a large polaron-counterion distance. At and above 0.2 eq. BCF : BPO doping, amorphous areas of the film became doped, disrupting the structural order of the films. Despite the change in structural order, thin films of regioregular P3HT doped with 0.2 eq. BCF : BPO had a conductivity of 25 S cm -1 . This study demonstrates the effectiveness of a two-component Lewis acid–base doping mechanism and suggests additional two-component Lewis acid–base chemistries should be explored.
Herein it is well known that catalytic centers containing the zinc(II) ion can act as both Lewis and Bronsted-Lowry acids. In addition to coordination number, the Zn coordination geometry can also strongly impact the acidity of the active site, no matter what measure of acidity is considered. Herein, we report the first pentacoordinate, zinc-ammonia complex containing a pyridine based tetraazamacrocycle, [(pyclen)Zn(NH 3 )](PF 6 ) 2 , that has applications in Lewis acid catalysis. From this structure, we obtain binding energies and acidities for a series of related pyclen and cross-bridged cyclen type tetraazamacrocycles comprising the pentacoordinate N 4 Zn(II)–OH 2 entity, collectively referred to as [(R-cyclen)Zn–OH 2 ] 2+ . Results from gas- and aqueous-phase density functional theory (M05-2X) and ab initio (MP2) calculations reported herein demonstrate that molecular geometry has a substantial influence on both the Zn–OH x binding strengths and the deprotonation energy of coordinated H 2 O, but not necessarily in a predicable way. While generally more constrained N–Zn–N coordination leads to greater Zn–OH 2 binding energies (Lewis acidities), the corresponding Lewis acidities of the complexes don’t always correlate with the Zn–OH 2 bond lengths nor the (Bronsted) acidity of the coordinated H 2 O. Additionally, the order of the Lewis acid strength of the [(R-cyclen)Zn(II)] complexes changes as the basic –OH 2 ligand is replaced with its –OH counterpart.
While the bis(trifluoromethylsulfonyl)imide anion (TFSI – ; formula [N(SO 2 CF 3 ) 2 ] – ) has been noted for its practical utility, the use of TFSI – salts as sources of Lewis acidic metal cations for studies of cation-driven tuning effects has not been reported. Here, the effective Lewis acidity of mono-, di-, and trivalent cations (namely, K + , Na + , Li + , Ba 2+ , Ca 2+ , Mg 2+ , Zn 2+ , La 3+ , Y 3+ , Lu 3+ , and Sc 3+ ) in the form of their TFSI – salts is described, along with quantitative comparisons to salts of several other weakly coordinating anions (namely, SO 3 CF 3 – , PF 6 – , and BArF 24 – ). Triphenylphosphine oxide (TPPO) was used as a 31 P NMR probe in titration experiments for quantification of key parameters describing the effective Lewis acidity of the salts in acetonitrile (CH 3 CN) solutions. Notably, the TFSI – salts of di- and trivalent cations were found to display strong binding to TPPO with significant speciation and were found to be quite hygroscopic. Taken together, the measurements demonstrate that TFSI – salts are systematically better/stronger effective Lewis acids than their triflate analogues. And, considering the excellent solubility of TFSI – salts, these materials appear attractive for further use and development in Lewis-acidity-dependent applications, including catalysis and tuning of multimetallic materials.
In the fabrication of FAPbI 3 -based perovskite solar cells, Lewis bases play a crucial role in facilitating the formation of the desired photovoltaic α-phase. However, an inherent contradiction exists in their role: they must strongly bind to stabilize the intermediate δ-phase, yet weakly bind for rapid removal to enable phase transition and grain growth. To resolve this conflict, we introduced an on-demand Lewis base molecule formation strategy. This approach utilized Lewis-acid-containing organic salts as synthesis additives, which deprotonated to generate Lewis bases precisely when needed and could be reprotonated back to salts for rapid removal once their role is fulfilled. This method promoted the optimal crystallization of α-phase FAPbI 3 perovskite films, ensuring the uniform vertical distribution of A-site cations, larger grain sizes and fewer voids at buried interfaces. Perovskite solar cells incorporating semicarbazide hydrochloride achieved an efficiency of 26.1%, with a National Renewable Energy Laboratory-certified quasi-steady-state efficiency of 25.33%. These cells retained 96% of their initial efficiency after 1,000 h of operation at 85 °C under maximum power point tracking. Additionally, mini-modules with an aperture area of 11.52 cm 2 reached an efficiency of 21.47%. This strategy is broadly applicable to all Lewis-acid-containing organic salts with low acid dissociation constants and offers a universal approach to enhance the performance of perovskite solar cells and modules.
The intramolecular “inverse” frustrated Lewis pairs (FLPs) of general formula 1-BR 2 -2-[(Me 2 N) 2 C=N]-C 6 H 4 (3-6) [BR 2 = BMes 2 (3), BC 12 H 8 , (4), BBN (5), BBNO (6)] were synthesized and structurally characterized by multinuclear NMR spectroscopy and X-ray analysis. Furthermore, these novel types of pre-organized FLPs, featuring strongly basic guanidino units rigidly linked to weakly Lewis acidic boryl moieties via an ortho-phenylene linker, are capable of activating H-H, C-H, N-H, O-H, Si-H, B-H and C=O bonds. 4 and 5 deprotonated terminal alkynes to form the zwitterionic borates 1-(RC≡C-BR 2 )-2-[(Me 2 N) 2 C=NH]-C 6 H 4 (R = Ph, H) and reacted with ammonia, BnNH 2 and pyrrolidine, to generate the adducts 1-(R 2 HN→BR 2 )-2-[(Me 2 N) 2 C=NH]-C 6 H 4 , where the N-H functionality is activated by intramolecular H-bond interactions. In addition, 5 was found to rapidly add across the double bond of H 2 CO, PhCHO and PhNCO to form cyclic zwitterionic guanidinium borates in excellent yields. Likewise, 5 is capable of cleaving H 2 , HBPin and PhSiH 3 to form various amino boranes. Collectively, the results demonstrate that these new types of intramolecular FLPs featuring weakly Lewis acidic boryl and strongly basic guanidino moieties are as potent as conventional intramolecular FLPs with strongly Lewis acidic units in activating small molecules.
Olefin oligomerization by γ-Al 2 O 3 has recently been reported, and it was suggested that Lewis acid sites are catalytic. The goal of this study is to determine the number of active sites per gram of alumina to confirm that Lewis acid sites are indeed catalytic. Addition of an inorganic Sr oxide base resulted in a linear decrease in the propylene oligomerization conversion at loadings up to 0.3 wt %; while, there is a >95 % loss in conversion above 1 wt % Sr. Additionally, there was a linear decrease in the intensity of the Lewis acid peaks of absorbed pyridine in the IR spectra with an increase in Sr loading, which correlates with the loss in propylene conversion, suggesting that Lewis acid sites are catalytic. Characterization of the Sr structure by XAS and STEM indicates that single Sr 2+ ions are bound to the γ-Al 2 O 3 surface and poison one catalytic site per Sr ion. The maximum loading needed to poison all catalytic sites, assuming uniform surface coverage, was ~0.4 wt % Sr, giving an acid site density of ~0.2 sites per nm2 of γ-Al 2 O 3 , or approximately 3 % of the alumina surface.
Hydroxymethylfurfural (HMF) is one of the important renewable platform compounds that can be obtained from biomass feedstocks through glucose conversion catalyzed by Brønsted and Lewis acids. However, it is challenging to enhance the HMF yield due to side reactions. In this study, a systematic approach combining theory and experiment was performed to investigate the influence of Lewis acids and organic solvents on the HMF yield. For the Lewis acid effect, a relationship between chemical hardness and experimental HMF yields was found in the rate-limiting step of glucose-to-fructose isomerization for six metal chlorides; HMF production was promoted when the metal chloride and a substrate had a similar chemical hardness. To study the organic solvent effect, a multivariate model was developed based on the insights gained from the mechanistic study of fructose dehydration, to predict HMF yields in a given water-organic cosolvent system. It showed a reliable accuracy in evaluating HMF yields with a mean absolute error (MAE) of 3.0% with respect to experimental HMF yields for 13 solvents, and also predicted HMF yields with a MAE of 10.7% for four new solvents. Chemical interpretation of the model revealed that it is desirable to use a solvent capable of stabilizing the carbocation intermediates with low proton transfer activity and high hydrogen bond basicity, to maximize the HMF yield. This multivariate model informs experimentalists about rational selection of solvents with very low computational costs needed to calculate only six variables for each solvent. It can be expanded to other catalytic systems such as heterogeneous Brønsted–Lewis bifunctional catalysts and enables optimization of reaction conditions to obtain other useful platform molecules through biomass conversion.
This work demonstrates how the stability of a Ru(II) formyl complex (cis-[Ru(bpy) 2 (CO)(CHO)][PF 6 ]), which is known to decarbonylate to form the metal hydride complex, is greatly enhanced by the addition of a Lewis acidic cation (Li + or Mg 2+ ). Multinuclear NMR is used to measure equilibrium constants of adduct formation, and then ultrafast 2DIR spectroscopy is applied to determine how the electronic structure and local molecular dynamics change upon complexation of the formyl ligand with the Lewis acid. We attribute the stabilization of the complex to the formation of a carbene-like structure where the bond strength between the Ru and the formyl group increases. The change in electronic structure is evidenced by steady-state NMR spectroscopy and DFT calculations. 2DIR measurements further confirm both the complexation of the formyl ligand with the Lewis acid and the change in electronic structure, where changes in vibrational frequency, relaxation kinetics, and a slowdown in the time scale of rotation about the metal-formyl bond are observed in the presence of Lewis acids.
A good catalyst for semihydrogenation of alkynes must preclude both over-hydrogenation of alkene to alkane and isomerization to the other alkene isomer. In addition, it should balance the trade-off between selectivity and activity. In 2013, the Repo and Pápai groups reported a frustrated Lewis pair (FLP) (1-NMe 2 -2-B(C 6 F 5 ) 2 -C 6 H 4 ), 1, which is a metal-free catalyst and for the first time shows excellent reactivity for the hydrogenation of internal alkynes. However, it is unreactive for terminal alkynes. In this work, we have designed 13 FLPs, a–m, based on 1 by varying the Lewis base site with N and P and the Lewis acid site with B, Al, Ga, and In and replacing pentafluorophenyl with 1,3,5-trifluorophenyl, phenyl, or trifluoromethyl. We apply density functional theory to study the activity, selectivity, and deactivation of FLP 1-m for acetylene semihydrogenation. The catalytic cycle consists of three steps: (1) alkyne insertion, (2) H 2 heterolysis, and (3) intramolecular protonation. We found the activity does not change much by the modification of bulky ligands, while it decreases with the direct replacement of LA and LB sites. The overall activity depends on steps 1 and 3, which are, respectively, positively and negatively linear correlated with the charge of the Lewis acid site. Most of the FLPs in this work show comparable or better selectivity for semihydrogenation of acetylene than 1. FLP deactivation is due to the strong binding of acetylene and the elimination of electron-withdrawing bulky ligands at the preactivated catalyst rather than at activated catalysts. Furthermore, taking the selectivity and stability of FLPs into account, we predict d and k are potentially active for terminal alkynes.
C-Nitroso compounds (RNO, R = alkyl and aryl) are byproducts of drug metabolism and bind to heme proteins, and their heme-RNO adducts are isoelectronic to ferrous nitroxyl (NO-/HNO) complexes. Importantly, heme-HNO compounds are key intermediates in the reduction of NO to N 2 O and nitrite to ammonium in the nitrogen cycle. Ferrous heme-RNO complexes act as stable analogs of these species, potentially allowing for the investigation of the vibrational and electronic properties of unstable heme-HNO intermediates. In this paper, a series of six-coordinate ferrous heme-RNO complexes (where R = iPr and Ph) were prepared using the TPP 2- and 3,5-Me-BAFP 2- co-ligands, and tetrahydrofuran, pyridine, and 1-methylimidazole as the axial ligands (bound trans to RNO). These complexes were characterized using different spectroscopic methods and X-ray crystallography. The complex [Fe(TPP)(THF)(iPrNO)] was further utilized for nuclear resonance vibrational spectroscopy (NRVS), allowing for the detailed assignment of the Fe–N(R)O vibrations of a heme-RNO complex for the first time. The vibrational properties of these species were then correlated with those of their HNO analogs, using DFT calculations. Our studies support previous findings that RNO ligands in ferrous heme complexes do not elicit a significant trans effect. In addition, the complexes are air-stable, and do not show any reactivity of their RNO ligands towards NO. So although ferrous heme-RNO complexes are suitable structural and electronic models for their HNO analogs, they are unsuitable to model the reactivity of heme-HNO complexes. We further investigated the reaction of our heme-RNO complexes with different Lewis acids. Here, [Fe(TPP)(THF)(iPrNO)] was found to be unreactive towards Lewis acids. In contrast, [Fe(3,5-Me-BAFP)(iPrNO) 2 ] is reactive towards all of the Lewis acids investigated here, but in most cases the iron center is simply oxidized, resulting in the loss of the iPrNO ligand. In the case of the Lewis acid B 2 (pin) 2 , the reduced product [Fe(3,5-Me-BAFP)(iPrNH 2 )(iPrNO)] was identified by X-ray crystallography.
We report the construction of heterogeneous frustrated Lewis pairs (FLPs) with performance comparable to or surpassing the homogeneous counterparts in H 2 activation is a long-standing challenge. Herein, sterically hindered Lewis acid (“B” center) and Lewis base (“N” center) sites were anchored within the rigid lattice of highly crystalline hexagonal boron nitride (h-BN) scaffolds. The active sites were created via precision defect regulation during the molten-salt-involved (NaNH 2 and NaBH 4 ) h-BN construction procedure. The as-afforded h-BN scaffolds achieved highly efficient H 2 /D 2 activation and dissociation under ambient pressure via FLP-like behavior, and attractive catalytic efficiency in hydrogenation reactions surpassing the current heterogeneous analogues.
Molecular functionalization of 2D transition metal dichalcogenides (TMDs), which unites tailorability of organic molecules and robustness of inorganic solids, is important for tuning their surface properties. While it is common to leverage the electronic effect of organic ligands to functionalize TMDs, the corresponding steric hinderance has remained elusive and underexplored. Herein N‐heterocyclic carbene (NHC) functionalization of WS 2 is demonstrated via a frustrated Lewis pair (FLP) route, which exploits both the electronic and steric effect for TMD functionalization. The Lewis base, 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes), is anchored to the sulfur vacancy sites (Lewis acid) of the WS 2 nanostructures with the hybrid‐interface‐mediated steric hinderance, mimicking the FLP chemistry and enabling dihydrogen cleavage at room temperature and atmospheric pressure. Solid‐state nuclear magnetic resonance (ssNMR) results reveal the local chemical environment of the activated hydrogen species, which can be transferred for room‐temperature hydrogenation reactions. Finally, the insights are useful for designing weak, non‐covalent bonds to modify the 2D surface of TMDs for a broad scope of applications.
Penicillin G acylase (PGA) has been immobilized on a lanthanum-incorporated mesostructured cellular foam (La-MCF) support by using the interaction between the strong Lewis acid sites on the surface of La-MCF and the free amino groups of lysine residues of PGA. The La-MCF support was successfully synthesized in situ through the addition of a citric acid (CA) complexant. The results of pyridine-IR spectroscopy show the presence of strong Lewis acid sites on the surface of the prepared La-MCF (with CA), attributed to the incorporation of lanthanum species into the framework of MCF. Through interaction with the strong Lewis acid sites, the enzymes can be firmly immobilized on the surface of the support. The results indicate that PGA/La-MCF (with CA) exhibits a high specific activity and greatly enhanced operational stability. For the hydrolysis of penicillin G potassium salt, the initial specific activity of PGA/La-MCF (with CA) reaches 10023 U/g. Finally, even after being recycled 10 times, PGA/La-MCF (with CA) retains 89 % of its initial specific activity, much higher than the 77 % of PGA/Si-MCF.