Vladimir Michaelis | University of Alberta (original) (raw)

publications by Vladimir Michaelis

17O solid-state NMR was used to study oxygen connectivity in ternary cesium borosilicate glasses.... more 17O solid-state NMR was used to study oxygen connectivity in ternary cesium borosilicate glasses. 17O multiple-quantum magic-angle spinning (MAS) NMR and ultrahigh-field 17O MAS NMR provide resolution of non-bridging and bridging oxygens, permitting their quantification for modeling short-range order. 17O {11B} rotational-echo double-resonance (REDOR) NMR is used to verify peak assignments for B–O–B, Si–O– B and Si–O–Si species, which agree well with a body of data from other alkali borosilicate glasses. The resulting bridging oxygen populations are in poor agreement with a model based on random network mixing, suggesting some degree of macroscopic segregation into silica-rich and borate-rich phases. Electron microprobe analysis reveals significant compositional variation between the batch and final products due to cesium volatilization at high temperatures. Both phase separation and elemental boil-off must be considered to account fully for the 17O MAS NMR results.

Papers by Vladimir Michaelis

Nature Chemistry, 2015

Gels formed via metal-ligand coordination typically have very low branch functionality, f, as the... more Gels formed via metal-ligand coordination typically have very low branch functionality, f, as they consist of ∼2-3 polymer chains linked to single metal ions that serve as junctions. Thus, these materials are very soft and unable to withstand network defects such as dangling ends and loops. We report here a new class of gels assembled from polymeric ligands and metal-organic cages (MOCs) as junctions. The resulting 'polyMOC' gels are precisely tunable and may feature increased branch functionality. We show two examples of such polyMOCs: a gel with a low f based on a M2L4 paddlewheel cluster junction and a compositionally isomeric one of higher f based on a M12L24 cage. The latter features large shear moduli, but also a very large number of elastically inactive loop defects that we subsequently exchanged for functional ligands, with no impact on the gel's shear modulus. Such a ligand substitution is not possible in gels of low f, including the M2L4-based polyMOC.

ABSTRACT Structure–property relationships of alkaline earth borosilicate glasses were determined ... more ABSTRACT Structure–property relationships of alkaline earth borosilicate glasses were determined using solid state NMR, the glass transition temperature and density. A sharing of alkaline earth oxide amongst the borate and silicate networks was observed and modelled. Density and Tg data were compared with the constructed models providing support to structural ideas. Comparisons to structure–property relations of alkali borosilicate glasses were also made.

Chemical Communications, 2009

Inorganic Chemistry, 2015

A variety of crystalline alkali molybdate phases are characterized by (23)Na, (133)Cs, and (95)Mo... more A variety of crystalline alkali molybdate phases are characterized by (23)Na, (133)Cs, and (95)Mo magic-angle-spinning nuclear magnetic resonance (MAS NMR) to provide spectroscopic handles for studies of devitrification products in borosilicate nuclear waste glasses. The NMR parameters obtained from line-shape simulations are plotted as a function of various structural parameters to discern trends that may prove useful in the determination of unknown phases. These are applied to Cs3Na(MoO4)2, the most common precipitate found in cesium- and molybdenum-bearing model nuclear waste glasses, the crystal structure of which has not yet been determined, to provide structural constraints that may guide the refinement of powder X-ray diffraction data.

Chem. Sci., 2015

A new material MIT-1 comprised of delaminated MWW zeolite nanosheets is made in a one-pot synthes... more A new material MIT-1 comprised of delaminated MWW zeolite nanosheets is made in a one-pot synthesis using a rationally designed organic structure-directing agent (OSDA). The OSDA consists of a hydrophilic head segment that resembles the OSDA used to synthesize the zeolite precursor MCM-22(P), a hydrophobic tail segment that resembles the swelling agent used to swell MCM-22(P), and a di-quaternary ammonium linker that connects both segments. MIT-1 features high crystallinity and surface areas exceeding 500 m 2 g À1 , and can be synthesized over a wide synthesis window that includes Si/Al ratios ranging from 13 to 67. Characterization data reveal high mesoporosity and acid strength with no detectable amorphous silica phases. Compared to MCM-22 and MCM-56, MIT-1 shows a three-fold increase in catalytic activity for the Friedel-Crafts alkylation of benzene with benzyl alcohol.

Cell, Jan 7, 2015

Biological processes occur in complex environments containing a myriad of potential interactors. ... more 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 act...

ChemPhysChem, 2015

Enzymes are used as environmentally friendly catalysts in many industrial applications, and are f... more 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).

Angewandte Chemie (International ed. in English), Jan 12, 2015

Cross-effect (CE) dynamic nuclear polarization (DNP) is a rapidly developing technique that enhan... more Cross-effect (CE) dynamic nuclear polarization (DNP) is a rapidly developing technique that enhances the signal intensities in magic-angle spinning (MAS) NMR spectra. We report CE DNP experiments at 211, 600, and 800 MHz using a new series of biradical polarizing agents referred to as TEMTriPols, in which a nitroxide (TEMPO) and a trityl radical are chemically tethered. The TEMTriPol molecule with the optimal performance yields a record (1) H NMR signal enhancement of 65 at 800 MHz at a concentration of 10 mM in a glycerol/water solvent matrix. The CE DNP enhancement for the TEMTriPol biradicals does not decrease as the magnetic field is increased in the manner usually observed for bis-nitroxides. Instead, the relatively strong exchange interaction between the trityl and nitroxide moieties determines the magnetic field at which the optimum enhancement is observed.

Pure-silica zeolites containing a small amount of tetravalent heteroatoms with open coordination ... more Pure-silica zeolites containing a small amount of tetravalent heteroatoms with open coordination sites have emerged as highly active, water-tolerant solid Lewis acids for many important reactions. The catalytic activity of tin-containing zeolites, such as Sn-Beta, is critically dependent on the successful incorporation of the tin metal center into the zeolite framework. However, synchrotron-based techniques or solid-state nuclear magnetic resonance (ssNMR) of samples enriched with 119Sn isotopes are the only reliable methods to verify framework incorporation. For 119Sn (I = 1/2, natural abundance = 8.6%) magic angle spinning (MAS) NMR, the coupled effects of low natural abundance of the 119Sn isotope, low intrinsic NMR sensitivity, and low Sn loadings in the sample make NMR analysis impractical without 119Sn isotopic enrichment. Unfortunately, the high cost of isotopic enrichment drastically hinders high throughput screening, routine analysis, or analysis of low-yield syntheses with...

The Journal of Physical Chemistry B, 2015

We demonstrate here that the (17)O NMR properties of bound water in a series of amino acids and d... more 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.

CrystEngComm / RSC, Jan 21, 2014

Crystallization in rigid confinement is a promising method to obtain organic molecular nanocrysta... more Crystallization in rigid confinement is a promising method to obtain organic molecular nanocrystals. However, the crystallization behavior and the related characterization methods are not well studied. Here we present a systematic study of the nucleation of organic molecular nanocrystals in rigid pores. Four different compounds were studied, ibuprofen, fenofibrate, griseofulvin, and indomethacin, which range from simple to complex molecules. Solid-state Nuclear Magnetic Resonance (NMR) was employed to analyse the structure of these compounds inside pores which are difficult to characterize by other analytical methods. We successfully demonstrated the production of nano-crystalline ibuprofen, fenofibrate and griseofulvin in porous silica particles with ~ 40 nm pores. These nanocrystals showed significant enhancement in dissolution rates. These results help advance the fundamental understanding of nucleation under rigid confinement and may lead to potential applications in developing ...

CrystEngComm, 2014

ABSTRACT Nuclear magnetic resonance (NMR) parameters of 11B in borates and borosilicates, unlike ... more ABSTRACT Nuclear magnetic resonance (NMR) parameters of 11B in borates and borosilicates, unlike those of many other nuclei such as 29Si and 27Al, vary only in limited ranges for a given polyhedral geometry, but mechanisms for such insensitivity to local structural environments remain poorly understood. In this contribution, ulexite and probertite with the ([B5O6(OH)6]3-) pentaborate polyanion as the fundamental building block have been investigated in detail by ab initio theoretical calculations of the density of states (DOS) as implemented in WIEN2k, including optimization of the structures and determination of contributions to the magnetic shielding at the five distinct B sites each. Calculated 11B NMR parameters of these two pentaborates are compared with high-precision experimental data obtained at high (14 T) and ultrahigh (21 T) fields. Optimized structures using the linearized augmented plane-wave method with additional radial basis functions in the form of local orbitals (i.e., LAPW+LO) not only yield more accurate electric field gradients (EFG) at the distinct three- and four-fold-coordinated B sites (i.e., [BO3] or [3]B and [BO4] or [4]B] but also improve the calculated 11B magnetic shielding. In particular, the magnetic shielding variation trends among the B sites in ulexite and probertite are determined mainly by the valence states and especially by the local p orbitals of B and its nearest-neighbor O atoms. Calculations with the water molecules removed or K+ substituting for Na+ in the structures show that the next-nearest-neighbor cations and water molecules have negligible effects. Theoretical calculations also reveal that the systematic differences in shielding between [3]B and [4]B are caused by multiple factors such as the occupancies and imbalance of the sp hybrid orbitals between B and its nearest-neighbor O atoms.

ChemSusChem, 2014

The coupled TH/etherification strategy was investigated with a series of zeolites containing vari... more The coupled TH/etherification strategy was investigated with a series of zeolites containing various Lewis acid centers. We report the synthesis of a new zeolite, Hf-Beta, in addition to conventionally used Lewis acid zeolites such as Sn-and Zr-Beta. The rationale for studying these different metal centers is that the extent of intermolecular conversion between carbonyl and hydroxyl functionalities during MPV reactions is dictated by the relative thermodynamic stability between products and reactants while reaction kinetics are altered by the ability of the Lewis acid center to appropriately coordinate and polarize the CÀO bonds. Preliminary studies of the catalytic performance of the solid Lewis acids were conducted under batch conditions ( . The proposed network for the coupled TH and etherification of HMF with ethanol is shown in Scheme 1. Pathways a and b proceed through a sequence of one TH and two etherification reactions. In pathway c, acetalization of HMF produces 5-(hydroxymethyl)furfural diethyl acetal (HMFDEA; see in the Supporting Information for the 1 H NMR spectrum), which reacts with ethanol to give the intermediate 5-(ethoxymethyl)furfural diethyl acetal (EMFDEA). The product distribution obtained with ethanol is consistent with the reaction network proposed by Jae et al. for the etherification of HMF with isopropanol. [9a] High BEMF yields are associated with Hf-, Zr-, and Sn-Beta having the highest catalytic activity for the TH step of all catalysts tested . Ta-and Al-Beta are less effective TH catalysts but do promote etherification reactions as evidenced by the higher proportion of 5-(ethoxymethyl)furfural (EMF) and EMFDEA products , entries 5 and 9). Ti-and Nb-Beta were ineffective at catalyzing either the TH or the

Microporous and mesoporous materials : the official journal of the International Zeolite Association, 2015

Modular and compact adsorption heat pumps (AHPs) promise an energy-efficient alternative to conve... more 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 Mg(2+) 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 als...

various trigonal borons present and provided an opportunity to test speciation models in the high... more various trigonal borons present and provided an opportunity to test speciation models in the high- alkali region. Surprisingly, these experiments also found significant differences between the network structure of lithium borates and those of the other alkali borate glasses, where lower-field NMR studies found none. Furthermore, the cation disorder and paramagnetic impurities often present in natural minerals can cause peak broadening which compromises site resolution and quantification in borosilicates. Using the ultrahigh-field NMR facility, well-resolved 11 B MAS NMR spectra could be obtained for tourmalines, allowing straightforward analysis of the total and fractional boron contents, quantities not easily accessible by conventional techniques such as x-ray diffraction and electron- microprobe analysis. Reduced quadrupolar broadening and amplified chemical shift dispersion at high field combine to produce excellent site resolution and quantification required to determine key asp...

The Journal of Physical Chemistry B, 2014

We illustrate the ability to place a water-insoluble biradical, bTbk, into a glycerol/water matri... more We illustrate the ability to place a water-insoluble biradical, bTbk, into a glycerol/water matrix with the assistance of a surfactant, sodium octyl sulfate (SOS). This surfactant approach enables a previously water insoluble biradical, bTbk, with favorable electron-electron dipolar coupling to be used for dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments in frozen, glassy, aqueous media. Nuclear Overhauser enhancement (NOE) and paramagnetic relaxation enhancement (PRE) experiments are conducted to determine the distribution of urea and several biradicals within the SOS macromolecular assembly. We also demonstrate that SOS assemblies are an effective approach by which mixed biradicals are created through an assembly process.

The Journal of Physical Chemistry B, 2013

Dynamic nuclear polarization of (17)O was studied using four different polarizing agents: the bir... more Dynamic nuclear polarization of (17)O was studied using four different polarizing agents: the biradical TOTAPOL and the monoradicals trityl and SA-BDPA, as well as a mixture of the latter two. Field profiles, DNP mechanisms, and enhancements were measured to better understand and optimize directly polarizing this low-gamma quadrupolar nucleus using both mono- and biradical polarizing agents. Enhancements were recorded at <88 K and were >100 using the trityl (OX063) radical and <10 with the other polarizing agents. The >10,000-fold savings in acquisition time enabled a series of biologically relevant small molecules to be studied with small sample sizes and the measurement of various quadrupolar parameters. The results are discussed with comparison to room temperature studies and GIPAW quantum chemical calculations. These experimental results illustrate the strength of high field DNP and the importance of radical selection for studying low-gamma nuclei.

Proceedings of the National Academy of Sciences, 2013

We report the synthesis and characterization of covalent organic frameworks (COFs) incorporating ... more 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. electronic materials | polymers | porous materials C ovalent organic frameworks (COFs) have emerged as an important class of polymeric crystalline materials with potential applications that complement those of metal organic frameworks (MOFs) (1-7). In particular, 2D COFs exhibit unique architectures wherein the monomers that make up their 2D layers stack almost perfectly into infinite 1D columns that are ideal for charge and exciton transport (6-12). Such electronic properties are particularly difficult to engineer in microporous MOFs, where high charge mobility and conductivity have only recently been demonstrated (13-15). Despite their layered structures, the typical hexagonal or square lattices of 2D COFs exhibit large 1D channels with well-defined pores that can be subject to reticular chemistry and postsynthetic modification (16, 17), and where complementary donor or acceptor molecules may in principle be inserted. Clearly, the unprecedented combination of high surface area, crystallinity, chemical and pore size tunability, and unique molecular architecture make COFs tantalizing targets for a new generation of electronic devices, including sustainable batteries, field-effect transistors, and photovoltaics. The first steps toward such applications must involve the incorporation of electroactive monomers within the COFs. This has been beautifully exemplified by the insertion of porphyrins (18), phthalocyanines (9), pyrene (8), naphthalenetetracarboxydiimide (19), and benzothiadiazole in such materials. Notably, however, one of the most popular monomers in conductive organic polymers, thiophene, has not been incorporated in COFs thus far. Herein, we report the synthesis and characterization of thiophene-based COFs, including materials made from thiophene-, bithiophene-, and thienothiophene-diboronic acids. We also advance a hypothesis regarding the more general synthetic tractability of COFs made from bent ligands. Finally, we show that p-type COFs are amenable to doping with electron acceptors and report an unusual charge-transfer complex with tetracyanoquinodimethane (TCNQ).

17O solid-state NMR was used to study oxygen connectivity in ternary cesium borosilicate glasses.... more 17O solid-state NMR was used to study oxygen connectivity in ternary cesium borosilicate glasses. 17O multiple-quantum magic-angle spinning (MAS) NMR and ultrahigh-field 17O MAS NMR provide resolution of non-bridging and bridging oxygens, permitting their quantification for modeling short-range order. 17O {11B} rotational-echo double-resonance (REDOR) NMR is used to verify peak assignments for B–O–B, Si–O– B and Si–O–Si species, which agree well with a body of data from other alkali borosilicate glasses. The resulting bridging oxygen populations are in poor agreement with a model based on random network mixing, suggesting some degree of macroscopic segregation into silica-rich and borate-rich phases. Electron microprobe analysis reveals significant compositional variation between the batch and final products due to cesium volatilization at high temperatures. Both phase separation and elemental boil-off must be considered to account fully for the 17O MAS NMR results.

Nature Chemistry, 2015

Gels formed via metal-ligand coordination typically have very low branch functionality, f, as the... more Gels formed via metal-ligand coordination typically have very low branch functionality, f, as they consist of ∼2-3 polymer chains linked to single metal ions that serve as junctions. Thus, these materials are very soft and unable to withstand network defects such as dangling ends and loops. We report here a new class of gels assembled from polymeric ligands and metal-organic cages (MOCs) as junctions. The resulting 'polyMOC' gels are precisely tunable and may feature increased branch functionality. We show two examples of such polyMOCs: a gel with a low f based on a M2L4 paddlewheel cluster junction and a compositionally isomeric one of higher f based on a M12L24 cage. The latter features large shear moduli, but also a very large number of elastically inactive loop defects that we subsequently exchanged for functional ligands, with no impact on the gel's shear modulus. Such a ligand substitution is not possible in gels of low f, including the M2L4-based polyMOC.

ABSTRACT Structure–property relationships of alkaline earth borosilicate glasses were determined ... more ABSTRACT Structure–property relationships of alkaline earth borosilicate glasses were determined using solid state NMR, the glass transition temperature and density. A sharing of alkaline earth oxide amongst the borate and silicate networks was observed and modelled. Density and Tg data were compared with the constructed models providing support to structural ideas. Comparisons to structure–property relations of alkali borosilicate glasses were also made.

Chemical Communications, 2009

Inorganic Chemistry, 2015

A variety of crystalline alkali molybdate phases are characterized by (23)Na, (133)Cs, and (95)Mo... more A variety of crystalline alkali molybdate phases are characterized by (23)Na, (133)Cs, and (95)Mo magic-angle-spinning nuclear magnetic resonance (MAS NMR) to provide spectroscopic handles for studies of devitrification products in borosilicate nuclear waste glasses. The NMR parameters obtained from line-shape simulations are plotted as a function of various structural parameters to discern trends that may prove useful in the determination of unknown phases. These are applied to Cs3Na(MoO4)2, the most common precipitate found in cesium- and molybdenum-bearing model nuclear waste glasses, the crystal structure of which has not yet been determined, to provide structural constraints that may guide the refinement of powder X-ray diffraction data.

Chem. Sci., 2015

A new material MIT-1 comprised of delaminated MWW zeolite nanosheets is made in a one-pot synthes... more A new material MIT-1 comprised of delaminated MWW zeolite nanosheets is made in a one-pot synthesis using a rationally designed organic structure-directing agent (OSDA). The OSDA consists of a hydrophilic head segment that resembles the OSDA used to synthesize the zeolite precursor MCM-22(P), a hydrophobic tail segment that resembles the swelling agent used to swell MCM-22(P), and a di-quaternary ammonium linker that connects both segments. MIT-1 features high crystallinity and surface areas exceeding 500 m 2 g À1 , and can be synthesized over a wide synthesis window that includes Si/Al ratios ranging from 13 to 67. Characterization data reveal high mesoporosity and acid strength with no detectable amorphous silica phases. Compared to MCM-22 and MCM-56, MIT-1 shows a three-fold increase in catalytic activity for the Friedel-Crafts alkylation of benzene with benzyl alcohol.

Cell, Jan 7, 2015

Biological processes occur in complex environments containing a myriad of potential interactors. ... more 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 act...

ChemPhysChem, 2015

Enzymes are used as environmentally friendly catalysts in many industrial applications, and are f... more 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).

Angewandte Chemie (International ed. in English), Jan 12, 2015

Cross-effect (CE) dynamic nuclear polarization (DNP) is a rapidly developing technique that enhan... more Cross-effect (CE) dynamic nuclear polarization (DNP) is a rapidly developing technique that enhances the signal intensities in magic-angle spinning (MAS) NMR spectra. We report CE DNP experiments at 211, 600, and 800 MHz using a new series of biradical polarizing agents referred to as TEMTriPols, in which a nitroxide (TEMPO) and a trityl radical are chemically tethered. The TEMTriPol molecule with the optimal performance yields a record (1) H NMR signal enhancement of 65 at 800 MHz at a concentration of 10 mM in a glycerol/water solvent matrix. The CE DNP enhancement for the TEMTriPol biradicals does not decrease as the magnetic field is increased in the manner usually observed for bis-nitroxides. Instead, the relatively strong exchange interaction between the trityl and nitroxide moieties determines the magnetic field at which the optimum enhancement is observed.

Pure-silica zeolites containing a small amount of tetravalent heteroatoms with open coordination ... more Pure-silica zeolites containing a small amount of tetravalent heteroatoms with open coordination sites have emerged as highly active, water-tolerant solid Lewis acids for many important reactions. The catalytic activity of tin-containing zeolites, such as Sn-Beta, is critically dependent on the successful incorporation of the tin metal center into the zeolite framework. However, synchrotron-based techniques or solid-state nuclear magnetic resonance (ssNMR) of samples enriched with 119Sn isotopes are the only reliable methods to verify framework incorporation. For 119Sn (I = 1/2, natural abundance = 8.6%) magic angle spinning (MAS) NMR, the coupled effects of low natural abundance of the 119Sn isotope, low intrinsic NMR sensitivity, and low Sn loadings in the sample make NMR analysis impractical without 119Sn isotopic enrichment. Unfortunately, the high cost of isotopic enrichment drastically hinders high throughput screening, routine analysis, or analysis of low-yield syntheses with...

The Journal of Physical Chemistry B, 2015

We demonstrate here that the (17)O NMR properties of bound water in a series of amino acids and d... more 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.

CrystEngComm / RSC, Jan 21, 2014

Crystallization in rigid confinement is a promising method to obtain organic molecular nanocrysta... more Crystallization in rigid confinement is a promising method to obtain organic molecular nanocrystals. However, the crystallization behavior and the related characterization methods are not well studied. Here we present a systematic study of the nucleation of organic molecular nanocrystals in rigid pores. Four different compounds were studied, ibuprofen, fenofibrate, griseofulvin, and indomethacin, which range from simple to complex molecules. Solid-state Nuclear Magnetic Resonance (NMR) was employed to analyse the structure of these compounds inside pores which are difficult to characterize by other analytical methods. We successfully demonstrated the production of nano-crystalline ibuprofen, fenofibrate and griseofulvin in porous silica particles with ~ 40 nm pores. These nanocrystals showed significant enhancement in dissolution rates. These results help advance the fundamental understanding of nucleation under rigid confinement and may lead to potential applications in developing ...

CrystEngComm, 2014

ABSTRACT Nuclear magnetic resonance (NMR) parameters of 11B in borates and borosilicates, unlike ... more ABSTRACT Nuclear magnetic resonance (NMR) parameters of 11B in borates and borosilicates, unlike those of many other nuclei such as 29Si and 27Al, vary only in limited ranges for a given polyhedral geometry, but mechanisms for such insensitivity to local structural environments remain poorly understood. In this contribution, ulexite and probertite with the ([B5O6(OH)6]3-) pentaborate polyanion as the fundamental building block have been investigated in detail by ab initio theoretical calculations of the density of states (DOS) as implemented in WIEN2k, including optimization of the structures and determination of contributions to the magnetic shielding at the five distinct B sites each. Calculated 11B NMR parameters of these two pentaborates are compared with high-precision experimental data obtained at high (14 T) and ultrahigh (21 T) fields. Optimized structures using the linearized augmented plane-wave method with additional radial basis functions in the form of local orbitals (i.e., LAPW+LO) not only yield more accurate electric field gradients (EFG) at the distinct three- and four-fold-coordinated B sites (i.e., [BO3] or [3]B and [BO4] or [4]B] but also improve the calculated 11B magnetic shielding. In particular, the magnetic shielding variation trends among the B sites in ulexite and probertite are determined mainly by the valence states and especially by the local p orbitals of B and its nearest-neighbor O atoms. Calculations with the water molecules removed or K+ substituting for Na+ in the structures show that the next-nearest-neighbor cations and water molecules have negligible effects. Theoretical calculations also reveal that the systematic differences in shielding between [3]B and [4]B are caused by multiple factors such as the occupancies and imbalance of the sp hybrid orbitals between B and its nearest-neighbor O atoms.

ChemSusChem, 2014

The coupled TH/etherification strategy was investigated with a series of zeolites containing vari... more The coupled TH/etherification strategy was investigated with a series of zeolites containing various Lewis acid centers. We report the synthesis of a new zeolite, Hf-Beta, in addition to conventionally used Lewis acid zeolites such as Sn-and Zr-Beta. The rationale for studying these different metal centers is that the extent of intermolecular conversion between carbonyl and hydroxyl functionalities during MPV reactions is dictated by the relative thermodynamic stability between products and reactants while reaction kinetics are altered by the ability of the Lewis acid center to appropriately coordinate and polarize the CÀO bonds. Preliminary studies of the catalytic performance of the solid Lewis acids were conducted under batch conditions ( . The proposed network for the coupled TH and etherification of HMF with ethanol is shown in Scheme 1. Pathways a and b proceed through a sequence of one TH and two etherification reactions. In pathway c, acetalization of HMF produces 5-(hydroxymethyl)furfural diethyl acetal (HMFDEA; see in the Supporting Information for the 1 H NMR spectrum), which reacts with ethanol to give the intermediate 5-(ethoxymethyl)furfural diethyl acetal (EMFDEA). The product distribution obtained with ethanol is consistent with the reaction network proposed by Jae et al. for the etherification of HMF with isopropanol. [9a] High BEMF yields are associated with Hf-, Zr-, and Sn-Beta having the highest catalytic activity for the TH step of all catalysts tested . Ta-and Al-Beta are less effective TH catalysts but do promote etherification reactions as evidenced by the higher proportion of 5-(ethoxymethyl)furfural (EMF) and EMFDEA products , entries 5 and 9). Ti-and Nb-Beta were ineffective at catalyzing either the TH or the

Microporous and mesoporous materials : the official journal of the International Zeolite Association, 2015

Modular and compact adsorption heat pumps (AHPs) promise an energy-efficient alternative to conve... more 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 Mg(2+) 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 als...

various trigonal borons present and provided an opportunity to test speciation models in the high... more various trigonal borons present and provided an opportunity to test speciation models in the high- alkali region. Surprisingly, these experiments also found significant differences between the network structure of lithium borates and those of the other alkali borate glasses, where lower-field NMR studies found none. Furthermore, the cation disorder and paramagnetic impurities often present in natural minerals can cause peak broadening which compromises site resolution and quantification in borosilicates. Using the ultrahigh-field NMR facility, well-resolved 11 B MAS NMR spectra could be obtained for tourmalines, allowing straightforward analysis of the total and fractional boron contents, quantities not easily accessible by conventional techniques such as x-ray diffraction and electron- microprobe analysis. Reduced quadrupolar broadening and amplified chemical shift dispersion at high field combine to produce excellent site resolution and quantification required to determine key asp...

The Journal of Physical Chemistry B, 2014

We illustrate the ability to place a water-insoluble biradical, bTbk, into a glycerol/water matri... more We illustrate the ability to place a water-insoluble biradical, bTbk, into a glycerol/water matrix with the assistance of a surfactant, sodium octyl sulfate (SOS). This surfactant approach enables a previously water insoluble biradical, bTbk, with favorable electron-electron dipolar coupling to be used for dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments in frozen, glassy, aqueous media. Nuclear Overhauser enhancement (NOE) and paramagnetic relaxation enhancement (PRE) experiments are conducted to determine the distribution of urea and several biradicals within the SOS macromolecular assembly. We also demonstrate that SOS assemblies are an effective approach by which mixed biradicals are created through an assembly process.

The Journal of Physical Chemistry B, 2013

Dynamic nuclear polarization of (17)O was studied using four different polarizing agents: the bir... more Dynamic nuclear polarization of (17)O was studied using four different polarizing agents: the biradical TOTAPOL and the monoradicals trityl and SA-BDPA, as well as a mixture of the latter two. Field profiles, DNP mechanisms, and enhancements were measured to better understand and optimize directly polarizing this low-gamma quadrupolar nucleus using both mono- and biradical polarizing agents. Enhancements were recorded at <88 K and were >100 using the trityl (OX063) radical and <10 with the other polarizing agents. The >10,000-fold savings in acquisition time enabled a series of biologically relevant small molecules to be studied with small sample sizes and the measurement of various quadrupolar parameters. The results are discussed with comparison to room temperature studies and GIPAW quantum chemical calculations. These experimental results illustrate the strength of high field DNP and the importance of radical selection for studying low-gamma nuclei.

Proceedings of the National Academy of Sciences, 2013

We report the synthesis and characterization of covalent organic frameworks (COFs) incorporating ... more 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. electronic materials | polymers | porous materials C ovalent organic frameworks (COFs) have emerged as an important class of polymeric crystalline materials with potential applications that complement those of metal organic frameworks (MOFs) (1-7). In particular, 2D COFs exhibit unique architectures wherein the monomers that make up their 2D layers stack almost perfectly into infinite 1D columns that are ideal for charge and exciton transport (6-12). Such electronic properties are particularly difficult to engineer in microporous MOFs, where high charge mobility and conductivity have only recently been demonstrated (13-15). Despite their layered structures, the typical hexagonal or square lattices of 2D COFs exhibit large 1D channels with well-defined pores that can be subject to reticular chemistry and postsynthetic modification (16, 17), and where complementary donor or acceptor molecules may in principle be inserted. Clearly, the unprecedented combination of high surface area, crystallinity, chemical and pore size tunability, and unique molecular architecture make COFs tantalizing targets for a new generation of electronic devices, including sustainable batteries, field-effect transistors, and photovoltaics. The first steps toward such applications must involve the incorporation of electroactive monomers within the COFs. This has been beautifully exemplified by the insertion of porphyrins (18), phthalocyanines (9), pyrene (8), naphthalenetetracarboxydiimide (19), and benzothiadiazole in such materials. Notably, however, one of the most popular monomers in conductive organic polymers, thiophene, has not been incorporated in COFs thus far. Herein, we report the synthesis and characterization of thiophene-based COFs, including materials made from thiophene-, bithiophene-, and thienothiophene-diboronic acids. We also advance a hypothesis regarding the more general synthetic tractability of COFs made from bent ligands. Finally, we show that p-type COFs are amenable to doping with electron acceptors and report an unusual charge-transfer complex with tetracyanoquinodimethane (TCNQ).

Physical Review B, 2009

Magnetic properties of the S=3 / 2 geometrically frustrated double perovskites La 2 LiRuO 6 and B... more Magnetic properties of the S=3 / 2 geometrically frustrated double perovskites La 2 LiRuO 6 and Ba 2 YRuO 6. Tomoko Aharen 1 , John E. Greedan 1,2 , Fanlong Ning 2,3 , Takashi Imai 2,3,4 , Vladimir Michaelis 5 , Scott Kroeker ...