Biradical Polarizing Agents at High Fields.

The sensitivity enhancements available from dynamic nuclear polarization (DNP) are rapidly reshaping the research landscape and expanding the field of nuclear magnetic resonance (NMR) spectroscopy as a tool for solving complex chemical and structural problems. The past decade has seen considerable advances in this burgeoning method, while efforts to further improve its capabilities continue along many avenues. In this report, we examine the influence of static magnetic field strength and temperature on the reported 1H DNP enhancements from three conventional organic biradicals: TOTAPOL, AMUPol, and SPIROPOL. In contrast to the conventional wisdom, our findings show that at liquid nitrogen temperatures and 700 MHz/460.5 GHz, these three bisnitroxides all provide similar 1H DNP enhancements, ε ≈ 60. Furthermore, we investigate the influence of temperature, microwave power, magnetic field strength, and protein sample deuteration on the NMR experimental results.

Icosahedral m-Carboranes Containing Exopolyhedral B-Se and B-Te Bonds.

Chalcogen-containing carboranes have been known for several decades and possess stable exopolyhedral B(9)-Se and B(9)-Te σ bonds despite the electron-donating ability of the B(9) vertex. While these molecules are known, little has been done to thoroughly evaluate their electrophilic and nucleophilic behavior. Herein, we report an assessment of the electrophilic reactivity of m-carboranylselenyl(II), -tellurenyl(II), and -tellurenyl(IV) chlorides and establish their reactivity pattern with Grignard reagents, alkenes, alkynes, enolates, and electron-rich arenes. These electrophilic reactions afford unique electron-rich B-Y-C (Y = Se, Te) bonding motifs not commonly found before. Furthermore, we show that m-carboranylselenolate, and even m-carboranyltellurolate, can be competent nucleophiles and participate in nucleophilic aromatic substitution reactions. Arene substitution chemistry is shown to be further extended to electron-rich species via palladium-mediated cross-coupling chemistry.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS-PIETA NMR Spectroscopy.

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands-phase incremented echo-train acquisition (PASS-PIETA). The improved sensitivity and resolution of DNP enhanced PASS-PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the 111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin-orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.

Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy.

Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long-standing goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.

The ß-Agostic Structure in (C5 Me5 )2 Sc(CH2 CH3 ): Solid-State NMR Studies of (C5 Me5 )2 Sc-R (R=Me, Ph, Et).

Multinuclear solid-state NMR studies of Cp*2 Sc-R (Cp*=pentamethylcyclopentadienyl; R=Me, Ph, Et) and DFT calculations show that the Sc-Et complex contains a ß-CH agostic interaction. The static central transition 45 Sc NMR spectra show that the quadrupolar coupling constants (Cq ) follow the trend of Ph≈Me>Et, indicating that the Sc-R bond is different in Cp*2 Sc-Et compared to the methyl and phenyl complexes. Analysis of the chemical shift tensor (CST) shows that the deshielding experienced by Cß in Sc-CH2 CH3 is related to coupling between the filled σC-C orbital and the vacant πSc⋯HC* orbital.

Protein-nucleotide contacts in motor proteins detected by DNP-enhanced solid-state NMR.

DNP (dynamic nuclear polarization)-enhanced solid-state NMR is employed to directly detect protein-DNA and protein-ATP interactions and identify the residue type establishing the intermolecular contacts. While conventional solid-state NMR can detect protein-DNA interactions in large oligomeric protein assemblies in favorable cases, it typically suffers from low signal-to-noise ratios. We show here, for the oligomeric DnaB helicase from Helicobacter pylori complexed with ADP and single-stranded DNA, that this limitation can be overcome by using DNP-enhanced spectroscopy. Interactions are established by DNP-enhanced 31P-13C polarization-transfer experiments followed by the recording of a 2D 13C-13C correlation experiment. The NMR spectra were obtained in less than 2 days and allowed the identification of residues of the motor protein involved in nucleotide binding.

Active Sites in Supported Single-Site Catalysts: An NMR Perspective.

Development of well-defined heterogeneous catalysts requires detailed structural characterization of active sites, an essential step toward establishing structure-activity relationships and promoting rational designs of catalysts. Solid-state NMR has emerged as a powerful approach to provide key molecular-level information about active-site structures and dynamics in heterogeneous catalysis. Here, we describe how one can apply solid-state NMR, ranging from 1D chemical shift assignments (and additional parameters, CQ and η, for quadrupolar nuclei, I > 1/2) to 2D correlations, to analysis of chemical shift anisotropy, providing unprecedented structural information about a broad range of materials. We also describe how modern hyperpolarization techniques like dynamic nuclear polarization can be used to improve the sensitivity of NMR, and make various challenging 1D/2D NMR experiments feasible, thus paving the way to determine the structures of surface sites.

Cage-Walking: Vertex Differentiation by Palladium-Catalyzed Isomerization of B(9)-Bromo-meta-Carborane.

We report the first observed Pd-catalyzed isomerization ("cage-walking") of B(9)-bromo-meta-carborane during Pd-catalyzed cross-coupling, which enables the formation of B-O and B-N bonds at all boron vertices (B(2), B(4), B(5), and B(9)) of meta-carborane. Experimental and theoretical studies suggest this isomerization mechanism is strongly influenced by the steric crowding at the Pd catalyst by either a biaryl phosphine ligand and/or substrate. Ultimately, this "cage-walking" process provides a unique pathway to preferentially introduce functional groups at the B(2) vertex using B(9)-bromo-meta-carborane as the sole starting material through substrate control.

Dendritic polarizing agents for DNP SENS.

Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an effective method to significantly improve solid-state NMR investigation of solid surfaces. The presence of unpaired electrons (polarizing agents) is crucial for DNP, but it has drawbacks such as leading to faster nuclear spin relaxation, or even reaction with the substrate under investigation. The latter can be a particular problem for heterogeneous catalysts. Here, we present a series of carbosilane-based dendritic polarizing agents, in which the bulky dendrimer can reduce the interaction between the solid surface and the free radical. We thereby preserve long nuclear T'2 of the surface species, and even successfully enhance a reactive heterogeneous metathesis catalyst.

Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS.

Obtaining detailed structural information of reaction intermediates remains a key challenge in heterogeneous catalysis because of the amorphous nature of the support and/or the support interface that prohibits the use of diffraction-based techniques. Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid-state NMR spectroscopy (SENS) towards surface species in heterogeneous alkene metathesis catalysts; this in turn allows direct determination of the bond connectivity and measurement of the carbon-carbon bond distance in metallacycles, which are the cycloaddition intermediates in the alkene metathesis catalytic cycle. Furthermore, this approach makes possible the understanding of the slow initiation and deactivation steps in these heterogeneous metathesis catalysts.

Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy.

Understanding the chemistry of colloidal quantum dots (QDs) is primarily hampered by the lack of analytical methods to selectively and discriminately probe the QD core, QD surface and capping ligands. Here, we present a general concept for studying a broad range of QDs such as CdSe, CdTe, InP, PbSe, PbTe, CsPbBr3, etc., capped with both organic and inorganic surface capping ligands, through dynamic nuclear polarization (DNP) surface enhanced NMR spectroscopy. DNP can enhance NMR signals by factors of 10-100, thereby reducing the measurement times by 2-4 orders of magnitude. 1D DNP enhanced spectra acquired in this way are shown to clearly distinguish QD surface atoms from those of the QD core, and environmental effects such as oxidation. Furthermore, 2D NMR correlation experiments, which were previously inconceivable for QD surfaces, are demonstrated to be readily performed with DNP and provide the bonding motifs between the QD surfaces and the capping ligands.

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.

Biosilica-Entrapped Enzymes Studied by Using Dynamic Nuclear-Polarization-Enhanced High-Field NMR Spectroscopy.

Enzymes are used as environmentally friendly catalysts in many industrial applications, and are frequently immobilized in a matrix to improve their chemical stability for long-term storage and reusability. Recently, it was shown that an atomic-level description of proteins immobilized in a biosilica matrix can be attained by examining their magic-angle spinning (MAS) NMR spectra. However, even though MAS NMR is an excellent tool for determining structure, it is severely hampered by sensitivity. In this work we provide the proof of principle that NMR characterization of biosilica-entrapped enzymes could be assisted by high-field dynamic nuclear polarization (DNP).

Formation of Organic Molecular Nanocrystals under Soft Confinement.

Methods to produce nano-sized organic molecular crystals in thin films are of great interest in the pharmaceutical industry due to the potential benefit of increased solubility of poorly soluble drugs and the advantages of film-based dosage forms over traditional tablet/capsule-based dosage form. One method to directly form organic nanocrystals is by crystallization in confined environments where the overall crystallization volume is constrained. We report the use of a novel solution impregnation method to form nanocrystals in polymer matrices with various microstructures in order to study the structure of the confined nanocrystals and the role of soft confinement and polymer chemistry on the nucleation process of nano-sized crystals. The particle diameter correlates with the microstructure of the polymer matrices and the nucleation kinetics. In addition, by carefully choosing the experimental conditions and the polymer matrix, polymorph control of nanocrystals can be achieved. Solid-state nuclear magnetic resonance (ssNMR) was used to examine the local structure of nanocrystals inside the polymer matrices and crystal polymer interactions. This method may serve as a novel formulation method to obtain nanocrystals of poorly soluble active pharmaceutical ingredients (APIs) for pharmaceutical industry.

Structural Insights into Bound Water in Crystalline Amino Acids: Experimental and Theoretical (17)O NMR.

We demonstrate here that the (17)O NMR properties of bound water in a series of amino acids and dipeptides can be determined with a combination of nonspinning and magic-angle spinning experiments using a range of magnetic field strengths from 9.4 to 21.1 T. Furthermore, we propose a (17)O chemical shift fingerprint region for bound water molecules in biological solids that is well outside the previously determined ranges for carbonyl, carboxylic, and hydroxyl oxygens, thereby offering the ability to resolve multiple (17)O environments using rapid one-dimensional NMR techniques. Finally, we compare our experimental data against quantum chemical calculations using GIPAW and hybrid-DFT, finding intriguing discrepancies between the electric field gradients calculated from structures determined by X-ray and neutron diffraction.

Dynamic DMF Binding in MOF-5 Enables the Formation of Metastable Cobalt-Substituted MOF-5 Analogues.

Multinuclear solid-state nuclear magnetic resonance, mass spectrometry, first-principles molecular dynamics simulations, and other complementary evidence reveal that the coordination environment around the Zn(2+) ions in MOF-5, one of the most iconic materials among metal-organic frameworks (MOFs), is not rigid. The Zn(2+) ions bind solvent molecules, thereby increasing their coordination number, and dynamically dissociate from the framework itself. On average, one ion in each cluster has at least one coordinated N,N-dimethylformamide (DMF) molecule, such that the formula of as-synthesized MOF-5 is defined as Zn4O(BDC)3(DMF) x (x = 1-2). Understanding the dynamic behavior of MOF-5 leads to a rational low-temperature cation exchange approach for the synthesis of metastable Zn4-x Co x O(terephthalate)3 (x > 1) materials, which have not been accessible through typical high-temperature solvothermal routes thus far.

Zeolite Y Adsorbents with High Vapor Uptake Capacity and Robust Cycling Stability for Potential Applications in Advanced Adsorption Heat Pumps.

Modular and compact adsorption heat pumps (AHPs) promise an energy-efficient alternative to conventional vapor compression based heating, ventilation and air conditioning systems. A key element in the advancement of AHPs is the development of adsorbents with high uptake capacity, fast intracrystalline diffusivity and durable hydrothermal stability. Herein, the ion exchange of NaY zeolites with ingoing Mg2+ ions is systematically studied to maximize the ion exchange degree (IED) for improved sorption performance. It is found that beyond an ion exchange threshold of 64.1%, deeper ion exchange does not benefit water uptake capacity or characteristic adsorption energy, but does enhance the vapor diffusivity. In addition to using water as an adsorbate, the uptake properties of Mg,Na-Y zeolites were investigated using 20 wt.% MeOH aqueous solution as a novel anti-freeze adsorbate, revealing that the MeOH additive has an insignificant influence on the overall sorption performance. We also demonstrated that the labscale synthetic scalability is robust, and that the tailored zeolites scarcely suffer from hydrothermal stability even after successive 108-fold adsorption/desorption cycles. The samples were analyzed using N2 sorption, 27Al/29Si MAS NMR spectroscopy, ICP-AES, dynamic vapor sorption, SEM, Fick's 2nd law and D-R equation regressions. Among these, close examination of sorption isotherms for H2O and N2 adsorbates allows us to decouple and extract some insightful information underlying the complex water uptake phenomena. This work shows the promising performance of our modified zeolites that can be integrated into various AHP designs for buildings, electronics, and transportation applications.

One-pot Solvothermal Synthesis of Well-ordered Layered Sodium Aluminoalcoholate Complex: A Useful Precursor for the Preparation of Porous Al2O3 Particles.

One-pot solvothermal synthesis of a robust tetranuclear sodium hexakis(glycolato)tris(methanolato)aluminate complex Na3[Al4(OCH3)3(OCH2CH2O)6] via a modified yet rigorous base-catalyzed transesterification mechanism is presented here. Single crystal X-ray diffraction (SCXRD) studies indicate that this unique Al complex contains three penta-coordinate Al3+ ions, each bound to two bidentate ethylene glycolate chelators and one monodentate methanolate ligand. The remaining fourth Al3+ ion is octahedrally coordinated to one oxygen atom from each of the six surrounding glycolate chelators, effectively stitching the three penta-coordinate Al moieties together into a novel tetranuclear Al complex. This aluminate complex is periodically self-assembled into well-ordered layers normal to the [110] axis with the intra-/inter-layer bindings involving extensive ionic bonds from the three charge-counterbalancing Na+ cations rather than the more typical hydrogen bonding interactions as a result of the fewer free hydroxyl groups present in its structure. It can also serve as a valuable precursor toward the facile synthesis of high-surface-area alumina powders using a very efficient rapid pyrolysis technique.

Designed single-step synthesis, structure, and derivative textural properties of well-ordered layered penta-coordinate silicon alcoholate complexes.

The controllable synthesis of well-ordered layered materials with specific nanoarchitecture poses a grand challenge in materials chemistry. Here the solvothermal synthesis of two structurally analogous 5-coordinate organosilicate complexes through a novel transesterification mechanism is reported. Since the polycrystalline nature of the intrinsic hypervalent Si complex thwarts the endeavor in determining its structure, a novel strategy concerning the elegant addition of a small fraction of B species as an effective crystal growth mediator and a sacrificial agent is proposed to directly prepare diffraction-quality single crystals without disrupting the intrinsic elemental type. In the determined crystal structure, two monomeric primary building units (PBUs) self-assemble into a dimeric asymmetric secondary BU via strong Na(+)-O(2-) ionic bonds. The designed one-pot synthesis is straightforward, robust, and efficient, leading to a well-ordered (10i)-parallel layered Si complex with its principal interlayers intercalated with extensive van der Waals gaps in spite of the presence of substantial Na(+) counter-ions as a result of unique atomic arrangement in its structure. However, upon fast pyrolysis, followed by acid leaching, both complexes are converted into two SiO2 composites bearing BET surface areas of 163.3 and 254.7 m(2) g(-1) for the pyrolyzed intrinsic and B-assisted Si complexes, respectively. The transesterification methodology merely involving alcoholysis but without any hydrolysis side reaction is designed to have generalized applicability for use in synthesizing new layered metal-organic compounds with tailored PBUs and corresponding metal oxide particles with hierarchical porosity.