Sensitivity-enhanced NMR reveals alterations in protein structure by cellular milieus.

Biological processes occur in complex environments containing a myriad of potential interactors. Unfortunately, limitations on the sensitivity of biophysical techniques normally restrict structural investigations to purified systems, at concentrations that are orders of magnitude above endogenous levels. Dynamic nuclear polarization (DNP) can dramatically enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and enable structural studies in biologically complex environments. Here, we applied DNP NMR to investigate the structure of a protein containing both an environmentally sensitive folding pathway and an intrinsically disordered region, the yeast prion protein Sup35. We added an exogenously prepared isotopically labeled protein to deuterated lysates, rendering the biological environment "invisible" and enabling highly efficient polarization transfer for DNP. In this environment, structural changes occurred in a region known to influence biological activity but intrinsically disordered in purified samples. Thus, DNP makes structural studies of proteins at endogenous levels in biological contexts possible, and such contexts can influence protein structure.

Are Selenides the Same as Sulfides? Structure, Spectroscopy, and Properties of Narrow-Gap Rare-Earth Semiconductors RE2Sn(S1-xSex)5 (RE = La, Ce; x = 0-0.8).

The ternary rare-earth sulfides RE2SnS5 (RE = La-Nd) and the partial solid solutions RE2Sn(S1-xSex)5 (RE = La, Ce; x = 0-0.8) were prepared in the form of polycrystalline samples by reaction of the elements at 900 °C and as single crystals in the presence of KBr flux. They adopt the La2SnS5-type structure (orthorhombic, space group Pbam, Z = 2) consisting of chains of edge-sharing SnCh6 octahedra separated by RE atoms. Although the cell parameters evolve smoothly in RE2Sn(S1-xSex)5, detailed structural analysis by single-crystal X-ray diffraction revealed a pronounced preference for the Se atoms to occupy two out of the three chalcogen sites, which offers a rationalization for why the all-selenide end-members RE2SnSe5 do not form. Solid-state 119Sn NMR spectra confirmed the nonrandom distribution of SnS6-nSen local environments, which could be resolved into individual resonances. The Raman spectra of RE2SnS5 compounds show an intense peak at 307-320 cm-1 assigned to a symmetric A1g mode, which is dominated by Sn-S bonds; the Raman peak intensities varied with Se substitution in La2Sn(S1-xSex)5. Optical diffuse reflectance spectra, band structure calculations, and electrochemical impedance spectra indicated that these compounds are narrow band gap semiconductors; the optical band gaps are insensitive to RE substitution in RE2SnS5 (0.7 eV) but they gradually decrease with greater Se substitution in RE2Sn(S1-xSex)5 (0.7-0.4 eV).

Unraveling Sodium-Ion Dynamics in Honeycomb-Layered Na2MgxZn2-xTeO6 Solid Electrolytes with Solid-State NMR.

All-solid-state sodium-ion batteries (SIBs) have the potential to offer large-scale, safe, cost-effective, and sustainable energy storage solutions by supplementing the industry-leading lithium-ion batteries. However, for the enhanced bulk properties of SIB components (e.g., solid electrolytes), a comprehensive understanding of their atomic-scale structure and the dynamic behavior of sodium (Na) ions is essential. Here, we utilize a robust multinuclear (23Na, 125Te, 25Mg, and 67Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide Na2MgxZn2-xTeO6 solid solution series. These new intermediate compounds exhibit tailorable bulk Na-ion conductivity (σ) with the highest σ = 0.14 × 10-4 S cm-1 for Na2MgZnTeO6 at room temperature suitable for SIB solid electrolyte applications as observed by powder electrochemical impedance spectroscopy (EIS). A combination of powder X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, and field emission scanning electron microscopy (FESEM) reveals highly crystalline phase-pure compounds in the P6322 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using 125Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) 23Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of 25Mg and 67Zn NMR, complemented by electronic structure calculations using density functional theory (DFT). Furthermore, we expose very fast local Na-ion hopping processes (hopping rate, 1/τNMR = 0.83 × 109 Hz) by using a laser to achieve variable high-temperature (∼860 K) 23Na NMR, which are sensitive to different Mg/Zn ratios. The Na2MgZnTeO6 with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.

Triangulating Dopant-Level Mn(II) Insertion in a Cs2NaBiCl6 Double Perovskite Using Magnetic Resonance Spectroscopy.

Lead-free metal halide double perovskites are gaining increasing attention for optoelectronic applications. Specifically, doping metal halide double perovskites using transition metals enables broadband tailorability of the optical bandgap for these emerging semiconducting materials. One candidate material is Mn(II)-doped Cs2NaBiCl6, but the nature of Mn(II) insertion on chemical structure is poorly understood due to low Mn loading. It is critical to determine the atomic-level structure at the site of Mn(II) incorporation in doped perovskites to better understand the structure-property relationships in these materials and thus to advance their applicability to optoelectronic applications. Magnetic resonance spectroscopy is uniquely qualified to address this, and thus a comprehensive three-pronged strategy, involving solid-state nuclear magnetic resonance (NMR), high-field dynamic nuclear polarization (DNP), and electron paramagnetic resonance (EPR) spectroscopies, is used to identify the location of Mn(II) insertion in Cs2NaBiCl6. Multinuclear (23Na, 35Cl, 133Cs, and 209Bi) one-dimensional (1D) magnetic resonance spectra reveal a low level of Mn(II) incorporation, with select spins affected by paramagnetic relaxation enhancement (PRE) induced by Mn(II) neighbors. EPR measurements confirm the oxidation state, octahedral symmetry, and low doping levels of the Mn(II) centers. Complementary EPR and NMR measurements confirm that the cubic structure is maintained with Mn(II) incorporation at room temperature, but the structure deviates slightly from cubic symmetry at low temperatures (<30 K). HYperfine Sublevel CORrelation (HYSCORE) EPR spectroscopy explores the electron-nuclear correlations of Mn(II) with 23Na, 133Cs, and 35Cl. The absence of 209Bi correlations suggests that Bi centers are replaced by Mn(II). Endogenous DNP NMR measurements from Mn(II) → 133Cs (<30 K) reveal that the solid effect is the dominant mechanism for DNP transfer and supports that Mn(II) is homogeneously distributed within the double-perovskite structure.

Structure and Optical Properties of LixAg1-xGaSe2 and LixAg1-xInSe2.

Complete substitution of Li atoms for Ag atoms in AgGaSe2 and AgInSe2 was achieved, resulting in the solid solutions LixAg1-xGaSe2 and LixAg1-xInSe2. The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state 7Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS2-type structure (space group I4̅2d) was retained within the entirety of the Ga-containing solid solution LixAg1-xGaSe2, which is noteworthy because the end-member LiGaSe2 normally adopts the orthorhombic ß-NaFeO2-type structure (space group Pna21). These structures are closely related, being superstructures of the cubic sphalerite and hexagonal wurtzite prototypes adopted by diamond-like semiconductors. For the In-containing solid solution LixAg1-xInSe2, the structure transforms from the tetragonal to orthorhombic forms as the Li content increases past x = 0.50. The optical band gaps increase gradually with higher Li content, from 1.8 to 3.4 eV in LixAg1-xGaSe2 and from 1.2 to 2.5 eV in LixAg1-xInSe2, enabling control to desired values, while the second harmonic generation responses become stronger or are similar to those of benchmark infrared nonlinear optical materials such as AgGaS2. All members of these solid solutions remain congruently melting at accessible temperatures between 800 and 900 °C. Electronic structure calculations support the linear trends seen in the optical band gaps and confirm the mostly ionic character present in Li-Se bonds, in contrast to the more covalent character in Ga-Se or In-Se bonds.

Lysozyme is Sterically Trapped Within the Silica Cage in Bioinspired Silica-Lysozyme Composites: A Multi-Technique Understanding of Elusive Protein-Material Interactions.

Lysozyme is widely known to promote the formation of condensed silica networks from solutions containing silicic acid, in a reproducible and cost-effective way. However, little is known about the fate of the protein after the formation of the silica particles. Also, the relative arrangement of the different components in the resulting material is a matter of debate. In this study, we investigate the nature of the protein-silica interactions by means of solid-state nuclear magnetic resonance spectroscopy, small-angle X-ray scattering, and electron microscopy. We find that lysozyme and silica are in intimate contact and strongly interacting, but their interaction is neither covalent nor electrostatic: lysozyme is mostly trapped inside the silica by steric effects.

Ultrahigh Size Exclusion Selectivity for Carbon Dioxide from Nitrogen/Methane in an Ultramicroporous Metal-Organic Framework.

Separations based on molecular size (molecular sieving) are a solution for environmental remediation. We have synthesized and characterized two new metal-organic frameworks (MOFs) (Zn2M; M = Zn, Cd) with ultramicropores (<0.7 nm) suitable for molecular sieving. We explore the synthesis of these MOFs and the role that the DMSO/H2O/DMF solvent mixture has on the crystallization process. We further explore the crystallographic data for the DMSO and methanol solvated structures at 273 and 100 K; this not only results in high-quality structural data but also allows us to better understand the structural features at temperatures around the gas adsorption experiments. Structurally, the main difference between the two MOFs is that the central metal in the trimetallic node can be changed from Zn to Cd and that results in a sub-Å change in the size of the pore aperture, but a stark change in the gas adsorption properties. The separation selectivity of the MOF when M = Zn is infinite given the pore aperture of the MOF can accommodate CO2 while N2 and/or CH4 is excluded from entering the pore. Furthermore, due to the size exclusion behavior, the MOF has an adsorption selectivity of 4800:1 CO2/N2 and 5 × 1028:1 CO2/CH4. When M = Cd, the pore aperture of the MOF increases slightly, allowing N2 and CH4 to enter the pore, resulting in a 27.5:1 and a 10.5:1 adsorption selectivity, respectively; this is akin to UiO-66, a MOF that is not able to function as a molecular sieve for these gases. The data delineate how subtle sub-Å changes to the pore aperture of a framework can drastically affect both the adsorption selectivity and separation selectivity.

Mercurial possibilities: determining site distributions in Cu2HgSnS4 using 63/65Cu, 119Sn, and 199Hg solid-state NMR spectroscopy.

Chalcogenides are an important class of materials that exhibit tailorable optoelectronic properties accessible through chemical modification. For example, the minerals kesterite, stannite, and velikite (Cu2MSnS4, where M = Zn, Cd, or Hg, respectively) are a series of Group 12 transition metal tin sulfides that readily exhibit optical bandgaps spanning the Shockley-Queisser limit; however, achieving consensus on their structure (space group I4̄ vs. I4̄2m) has been difficult. This study explores the average long-range and local structure of Cu2HgSnS4 and evaluates the parallels of M = Zn and Cd sister compounds using complementary X-ray diffraction and solid-state nuclear magnetic resonance (NMR) spectroscopy. The 63/65Cu NMR spectra were acquired at multiple magnetic field strengths (B0 = 7.05, 11.75, and 21.1 T) to assess the unique chemical shift anisotropy and quadrupolar coupling contributions. They reveal two inequivalent sets of Cu sites in Cu2ZnSnS4, in contrast to only one set of sites in Cu2CdSnS4 and Cu2HgSnS4, clarifying structural assignments previously proposed through X-ray diffraction methods. The presence of these Cu sites was further supported by DFT calculations. The 119Sn and 199Hg NMR spectra suggest that an ordering phenomenon takes place in Cu2HgSnS4 when it undergoes annealing treatments. The trend in measured optical band gaps (1.5 eV for Cu2ZnSnS4, 1.2 eV for Cu2CdSnS4, and 0.9 eV for Cu2HgSnS4) was confirmed by electronic structure calculations, which show that the band gap narrows as the difference in electronegativity is diminished and that Hg-S bonds in Cu2HgSnS4 have greater covalent character.

High-Capacitance Pseudocapacitors from Li+ Ion Intercalation in Nonporous, Electrically Conductive 2D Coordination Polymers.

Electrochemical capacitors (ECs) have emerged as reliable and fast-charging electrochemical energy storage devices that offer high power densities. Their use is still limited, nevertheless, by their relatively low energy density. Because high specific surface area and electrical conductivity are widely seen as key metrics for improving the energy density and overall performance of ECs, materials that have excellent electrical conductivities but are otherwise nonporous, such as coordination polymers (CPs), are often overlooked. Here, we report a new nonporous CP, Ni3(benzenehexathiolate) (Ni3BHT), which exhibits high electrical conductivity of over 500 S/m. When used as an electrode, Ni3BHT delivers excellent specific capacitances of 245 F/g and 426 F/cm3 in nonaqueous electrolytes. Structural and electrochemical studies relate the favorable performance to pseudocapacitive intercalation of Li+ ions between the 2D layers of Ni3BHT, a charge-storage mechanism that has thus far been documented only in inorganic materials such as TiO2, Nb2O5, and MXenes. This first demonstration of pseudocapacitive ion intercalation in nonporous CPs, a class of materials comprising thousands of members with distinct structures and compositions, provides important motivation for exploring this vast family of materials for nontraditional, high-energy pseudocapacitors.

Na3 SO4 H-The First Representative of the Material Class of Sulfate Hydrides.

The first representative of a novel class of mixed-anionic compounds, the sulfate hydride Na3 SO4 H, and the corresponding deuteride Na3 SO4 D were obtained from the solid-state reaction of NaH or NaD with dry Na2 SO4 . Precise reaction control is required, because too harsh conditions lead to the reduction of sulfate to sulfide. A combined X-ray and neutron diffraction study revealed that the compound crystallizes in the tetragonal space group P4/nmm with the lattice parameters a=7.0034(2) Šand c=4.8569(2) Å. The sole presence of hydride and absence of hydroxide ions is proven by vibrational spectroscopy and comparison with spectra predicted from quantum chemical calculations. 1 H and 23 Na MAS NMR spectra are consistent with the structure of Na3 SO4 H: a single 1 H peak at 2.9 ppm is observed, while two peaks at 15.0 and 6.2 ppm for the inequivalent 23 Na sites are observed. Elemental analysis and quantum chemical calculations further support these results.

Tailorable Indirect to Direct Band-Gap Double Perovskites with Bright White-Light Emission: Decoding Chemical Structure Using Solid-State NMR.

Efficient white-light-emitting single-material sources are ideal for sustainable lighting applications. Though layered hybrid lead-halide perovskite materials have demonstrated attractive broad-band white-light emission properties, they pose a serious long-term environmental and health risk as they contain lead (Pb2+) and are readily soluble in water. Recently, lead-free halide double perovskite (HDP) materials with a generic formula A(I)2B'(III)B″(I)X6 (where A and B are cations and X is a halide ion) have demonstrated white-light emission with improved photoluminescence quantum yields (PLQYs). Here, we present a series of Bi3+/In3+ mixed-cationic Cs2Bi1-xInxAgCl6 HDP solid solutions that span the indirect to direct band-gap modification which exhibit tailorable optical properties. Density functional theory (DFT) calculations indicate an indirect-direct band-gap crossover composition when x > 0.50. These HDP materials emit over the entire visible light spectrum, centered at 600 ± 30 nm with full-width at half maxima of ca. 200 nm upon ultraviolet light excitation and a maximum PLQY of 34 ± 4% for Cs2Bi0.085In0.915AgCl6. Short-range structural insight for these materials is crucial to unravel the unique atomic-level structural properties which are difficult to distinguish by diffraction-based techniques. Hence, we demonstrate the advantage of using solid-state nuclear magnetic resonance (NMR) spectroscopy to deconvolute the local structural environments of these mixed-cationic HDPs. Using ultrahigh-field (21.14 T) NMR spectroscopy of quadrupolar nuclei (115In, 133Cs, and 209Bi), we show that there is a high degree of atomic-level B'(III)/B″(I) site ordering (i.e., no evidence of antisite defects). Furthermore, a combination of XRD, NMR, and DFT calculations was used to unravel the complete atomic-level random Bi3+/In3+ cationic mixing in Cs2Bi1-xInxAgCl6 HDPs. Briefly, this work provides an advance in understanding the photophysical properties that correlate long- to short-range structural elucidation of these newly developed solid-state white-light emitting HDP materials.

Solvent-Induced Bond-Bending Isomerism in Hexaphenyl Carbodiphosphorane: Decisive Dispersion Interactions in the Solid State.

Previous reports in the literature describe that the crystallization of hexaphenyl carbodiphosphorane (CDPPh) from a variety of solvents gives a "bent" geometry for the P-C-P moiety as the solid-state molecular structure. However, a linear structure is observed when CDPPh is crystallized from benzene. Here, we report detailed spectroscopic and theoretical studies on the linear and bent structures. X-ray powder diffraction examinations show a phase transition of linear CDPPh upon the loss of co-crystallized benzene molecules, which is accompanied by the bending of the P-C-P unit. Studies on the linear and bent structures (i.e., X-ray powder diffraction, solid-state NMR, UV-vis spectroscopy, and IR spectroscopy) show significant differences in their properties. Investigations of the solid-state structures with density functional theory-based methods (PBE-D3) point toward subtle dispersion effects being responsible for this solvent-induced bond-bending isomerism in CDPPh.

Combining DNP NMR with segmental and specific labeling to study a yeast prion protein strain that is not parallel in-register.

The yeast prion protein Sup35NM is a self-propagating amyloid. Despite intense study, there is no consensus on the organization of monomers within Sup35NM fibrils. Some studies point to a ß-helical arrangement, whereas others suggest a parallel in-register organization. Intermolecular contacts are often determined by experiments that probe long-range heteronuclear contacts for fibrils templated from a 1:1 mixture of 13C- and 15N-labeled monomers. However, for Sup35NM, like many large proteins, chemical shift degeneracy limits the usefulness of this approach. Segmental and specific isotopic labeling reduce degeneracy, but experiments to measure long-range interactions are often too insensitive. To limit degeneracy and increase experimental sensitivity, we combined specific and segmental isotopic labeling schemes with dynamic nuclear polarization (DNP) NMR. Using this combination, we examined an amyloid form of Sup35NM that does not have a parallel in-register structure. The combination of a small number of specific labels with DNP NMR enables determination of architectural information about polymeric protein systems.

Lead-207 NMR spectroscopy at 1.4 T: Application of benchtop instrumentation to a challenging I = ½ nucleus.

The practicality of obtaining liquid- and solid-state 207 Pb nuclear magnetic resonance (NMR) spectra with a low permanent-field magnet is investigated. Obtaining 207 Pb NMR spectra of salts in solution is shown to be viable for samples as dilute as 0.05 M. The concentration dependence of the 207 Pb chemical shifts for lead nitrate was investigated; the results are comparable with those obtained with high-field instruments. Likewise, the isotope effect of substituting D2 O for H2 O as the solvent was investigated and found to be comparable with those reported previously. Obtaining solid-state 207 Pb NMR spectra is challenging, but we demonstrate the ability to obtain such spectra for three unique solid samples. An axially symmetric 207 Pb powder pattern for lead nitrate and the powder pattern expected for lead chloride reveal linewidths dominated by shielding anisotropy, while 207 Pb-35/37 Cl J-coupling dominates in the methylammonium lead chloride perovskite material. Finally, recent innovations and the future potential of the instruments are considered.

C3N5: A Low Bandgap Semiconductor Containing an Azo-Linked Carbon Nitride Framework for Photocatalytic, Photovoltaic and Adsorbent Applications.

Modification of carbon nitride based polymeric 2D materials for tailoring their optical, electronic and chemical properties for various applications has gained significant interest. The present report demonstrates the synthesis of a novel modified carbon nitride framework with a remarkable 3:5 C:N stoichiometry (C3N5) and an electronic bandgap of 1.76 eV, by thermal deammoniation of the melem hydrazine precursor. Characterization revealed that in the C3N5 polymer, two s-heptazine units are bridged together with azo linkage, which constitutes an entirely new and different bonding fashion from g-C3N4 where three heptazine units are linked together with tertiary nitrogen. Extended conjugation due to overlap of azo nitrogens and increased electron density on heptazine nucleus due to the aromatic π network of heptazine units lead to an upward shift of the valence band maximum resulting in bandgap reduction down to 1.76 eV. XRD, He-ion imaging, HR-TEM, EELS, PL, fluorescence lifetime imaging, Raman, FTIR, TGA, KPFM, XPS, NMR and EPR clearly show that the properties of C3N5 are distinct from pristine carbon nitride (g-C3N4). When used as an electron transport layer (ETL) in MAPbBr3 based halide perovskite solar cells, C3N5 outperformed g-C3N4, in particular generating an open circuit photovoltage as high as 1.3 V, while C3N5 blended with MA xFA1- xPb(I0.85Br0.15)3 perovskite active layer achieved a photoconversion efficiency (PCE) up to 16.7%. C3N5 was also shown to be an effective visible light sensitizer for TiO2 photoanodes in photoelectrochemical water splitting. Because of its electron-rich character, the C3N5 material displayed instantaneous adsorption of methylene blue from aqueous solution reaching complete equilibrium within 10 min, which is significantly faster than pristine g-C3N4 and other carbon based materials. C3N5 coupled with plasmonic silver nanocubes promotes plasmon-exciton coinduced surface catalytic reactions reaching completion at much low laser intensity (1.0 mW) than g-C3N4, which showed sluggish performance even at high laser power (10.0 mW). The relatively narrow bandgap and 2D structure of C3N5 make it an interesting air-stable and temperature-resistant semiconductor for optoelectronic applications while its electron-rich character and intrasheet cavity make it an attractive supramolecular adsorbent for environmental applications.

Endogenous dynamic nuclear polarization NMR of hydride-terminated silicon nanoparticles.

Silicon nanoparticles (SiNPs) are intriguing materials and their properties fascinate the broader scientific community; they are also attractive to the biological and materials science sub-disciplines because of their established biological and environmental compatibility, as well as their far-reaching practical applications. While characterization of the particle nanostructure can be performed using 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy, poor sensitivity due to low Boltzmann population and long acquisition times hinder in-depth studies of these potentially game-changing materials. In this study, we compare two dynamic nuclear polarization (DNP) NMR protocols to boost 29Si sensitivity in hydride-terminated SiNPs. First, we assess a traditional indirect DNP approach, where a nitroxide biradical (AMUPol or bCTbk) is incorporated into a glassing agent and transferred through protons (e- → 1H → 29Si) to enhance the silicon. In this mode, electron paramagnetic resonance (EPR) spectroscopy demonstrated that the hydride-terminated surface was highly reactive with the exogenous biradicals, thus decomposing the radicals within hours and resulting in an enhancement factor, ε, of 3 (TB = 15 s) for the 64 nm SiNP, revealing the surface components. Secondly, direct DNP NMR methods were used to enhance the silicon without the addition of an exogenous radical (i.e., use of dangling bonds as an endogenous radical source). With radical concentrations <1 mM, 29Si enhancements were obtained for the series of SiNPs ranging from 3 to 64 nm. The ability to use direct 29Si DNP transfer (e- → 29Si) shows promise for DNP studies of these inorganic nanomaterials (ε = 6 (TB = 79 min) for 64 nm SiNPs) with highly reactive surfaces, showing the sub-surface and core features. These preliminary findings lay a foundation for future endogenous radical development through tailoring the surface chemistry, targeting further sensitivity gains.

Nickel as a Lewis Base in a T-Shaped Nickel(0) Germylene Complex Incorporating a Flexible Bis(NHC) Ligand.

Flexible, chelating bis(NHC) ligand 2, able to accommodate both cis- and trans-coordination modes, was used to synthesize (2)Ni(η2 -cod), 3. In reaction with GeCl2 , it produced (2)NiGeCl2 , 4, featuring a T-shaped Ni0 and a pyramidal Ge center. Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)2 , in a reaction that formally involved Ni-Ge transmetalation, followed by coordination of the extruded GeCl2 moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ-donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)2 Ni(cod) complexes 9 and 10, as well as (NHC)2 GeCl2 derivative 11, incorporating ligands that cannot accommodate a wide bite angle, failed to produce isolable Ni-Ge complexes. The isolation of (2)Ni(η2 -Py), 12, provides further evidence for the reluctance of the (2)Ni0 fragment to act as a σ-Lewis acid.

17O MAS NMR Correlation Spectroscopy at High Magnetic Fields.

The structure of two protected amino acids, FMOC-l-leucine and FMOC-l-valine, and a dipeptide, N-acetyl-l-valyl-l-leucine (N-Ac-VL), were studied via one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy. Utilizing 17O magic-angle spinning (MAS) NMR at multiple magnetic fields (17.6-35.2 T/750-1500 MHz for 1H) the 17O quadrupolar and chemical shift parameters were determined for the two oxygen sites of each FMOC-protected amino acids and the three distinct oxygen environments of the dipeptide. The one- and two-dimensional, 17O, 15N-17O, 13C-17O, and 1H-17O double-resonance correlation experiments performed on the uniformly 13C,15N and 70% 17O-labeled dipeptide prove the attainability of 17O as a probe for structure studies of biological systems. 15N-17O and 13C-17O distances were measured via one-dimensional REAPDOR and ZF-TEDOR experimental buildup curves and determined to be within 15% of previously reported distances, thus demonstrating the use of 17O NMR to quantitate interatomic distances in a fully labeled dipeptide. Through-space hydrogen bonding of N-Ac-VL was investigated by a two-dimensional 1H-detected 17O R3-R-INEPT experiment, furthering the importance of 17O for studies of structure in biomolecular solids.

Thiophene-based covalent organic frameworks.

We report the synthesis and characterization of covalent organic frameworks (COFs) incorporating thiophene-based building blocks. We show that these are amenable to reticular synthesis, and that bent ditopic monomers, such as 2,5-thiophenediboronic acid, are defect-prone building blocks that are susceptible to synthetic variations during COF synthesis. The synthesis and characterization of an unusual charge transfer complex between thieno[3,2-b]thiophene-2,5-diboronic acid and tetracyanoquinodimethane enabled by the unique COF architecture is also presented. Together, these results delineate important synthetic advances toward the implementation of COFs in electronic devices.

Methane to acetic acid over Cu-exchanged zeolites: mechanistic insights from a site-specific carbonylation reaction.

The selective low temperature oxidation of methane is an attractive yet challenging pathway to convert abundant natural gas into value added chemicals. Copper-exchanged ZSM-5 and mordenite (MOR) zeolites have received attention due to their ability to oxidize methane into methanol using molecular oxygen. In this work, the conversion of methane into acetic acid is demonstrated using Cu-MOR by coupling oxidation with carbonylation reactions. The carbonylation reaction, known to occur predominantly in the 8-membered ring (8MR) pockets of MOR, is used as a site-specific probe to gain insight into important mechanistic differences existing between Cu-MOR and Cu-ZSM-5 during methane oxidation. For the tandem reaction sequence, Cu-MOR generated drastically higher amounts of acetic acid when compared to Cu-ZSM-5 (22 vs 4 µmol/g). Preferential titration with sodium showed a direct correlation between the number of acid sites in the 8MR pockets in MOR and acetic acid yield, indicating that methoxy species present in the MOR side pockets undergo carbonylation. Coupled spectroscopic and reactivity measurements were used to identify the genesis of the oxidation sites and to validate the migration of methoxy species from the oxidation site to the carbonylation site. Our results indicate that the Cu(II)-O-Cu(II) sites previously associated with methane oxidation in both Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive. In turn, combined UV-vis and EPR spectroscopic studies showed that a novel Cu(2+) site is formed at Cu/Al <0.2 in MOR. These sites oxidize methane and promote the migration of the product to a Brønsted acid site in the 8MR to undergo carbonylation.