Proton hyperpolarisation preserved in long-lived states

The polarisation of abundant protons, rather than dilute nuclei with low gyromagnetic ratios, can be enhanced in less than 10 min using dissolution DNP and converted into a long-lived state delocalised over an ensemble of three coupled protons. The process is more straightforward than the hyperpolarisation of heteronuclei followed by magnetisation transfer to protons.     

Hyperpolarization of Nitrogen‐15 Schiff Bases by Reversible Exchange Catalysis with para‐Hydrogen

NMR with thermal polarization requires relatively concentrated samples, particularly for nuclei with low abundance and low gyromagnetic ratios, such as ¹⁵N. We expand the substrate scope of SABRE, a recently introduced hyperpolarization method, to allow access to ¹⁵N‐enriched Schiff bases. These substrates show fractional ¹⁵N polarization levels of up to 2 % while having only minimal ¹H enhancements.     

Distance measurements between paramagnetic centers and a planar object by matrix Mims electron nuclear double resonance

Frequency-domain electron nuclear double resonance (ENDOR), two time-domain electron nuclear double resonance techniques, and electron spin echo envelope modulation spectroscopy are compared with respect to their merit in measurements of small hyperfine couplings to nuclei with intermediate gyromagnetic ratio such as 31P. The frequency-domain Mims ENDOR experiment is found to provide the most faithful line shapes. In the limit of long electron-nuclear distances of more than 0.5 nm, sensitivity of this experiment is optimized by matching the first interpulse delay to the transverse relaxation time of the electron spins. In the same limit, Mims ENDOR efficiency scales inversely with the sixth power of distance. Hyperfine splittings as small as 33 kHz can be detected, corresponding to an electron-31P distance of 1 nm. In systems, where a certain kind of nuclei is distributed in a plane, measurements of intermolecular hyperfine couplings can be analyzed in terms of a distance of closest approach of a paramagnetic center to that plane. By applying this technique to spin-labeled lipids in a fully hydrated lipid bilayer it is found that for a fraction of lipids, chain tilt angles can be 25 degrees larger than the mean tilt angle of the lipid chains. This model of all-trans hydrocarbon chains with a broad distribution of tilt angles is also consistent with orientation selection effects in high-field ENDOR spectra.     

Cross-Polarisation Applied to the Study of Liquid Crsytalline Ordering

 Cross polarisation is extensively used in solid state NMR for enhancing signals of nuclei with low gyromagnetic ratio. However, the use of the method for providing quantitative structural and dynamics information is limited. This arises due to the fact that the mechanism which is responsible for cross polarisation namely, the dipolar interaction, has a long range and is also anisotropic. In nematic liquid crystals these limitations are easily overcome since molecules orient in a magnetic field. The uniaxial ordering of the molecules essentially removes problems associated with the angular dependence of the interactions encountered in powdered solids. The molecular motion averages out intermolecular dipolar interaction, while retaining partially averaged intramolecular interaction. In this article the use of cross polarisation for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment were considered and their utility illustrated. A method for obtaining proton–proton dipolar couplings, by utilizing cross polarisation from the dipolar reservoir, is also presented.     

Cross-Polarisation Applied to the Study of Liquid Crsytalline Ordering

Summary.  Cross polarisation is extensively used in solid state NMR for enhancing signals of nuclei with low gyromagnetic ratio. However, the use of the method for providing quantitative structural and dynamics information is limited. This arises due to the fact that the mechanism which is responsible for cross polarisation namely, the dipolar interaction, has a long range and is also anisotropic. In nematic liquid crystals these limitations are easily overcome since molecules orient in a magnetic field. The uniaxial ordering of the molecules essentially removes problems associated with the angular dependence of the interactions encountered in powdered solids. The molecular motion averages out intermolecular dipolar interaction, while retaining partially averaged intramolecular interaction. In this article the use of cross polarisation for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment were considered and their utility illustrated. A method for obtaining proton–proton dipolar couplings, by utilizing cross polarisation from the dipolar reservoir, is also presented. Corresponding author. E-mail: kvr@sif.iisc.ernet.in Received May 28, 2002; accepted June 19, 2002     

High-resolution magic-angle spinning (13)C spectroscopy of brain tissue at natural abundance

High-resolution magic-angle spinning (MAS) (1)H and (13)C magnetic resonance spectroscopy (MRS) has recently been applied to study the metabolism in intact biological tissue samples. Because of the low natural abundance and the low gyromagnetic ratio of the (13)C nuclei, signal enhancement techniques such as cross-polarization (CP) and distortionless enhancement by polarization transfer (DEPT) are often employed in MAS (13)C MRS to improve the detection sensitivity. In this study, several sensitivity enhancement techniques commonly used in liquid- and solid-state NMR, including CP, DEPT and nuclear Overhauser enhancement (NOE), were combined with MAS to acquire high-resolution (13)C spectra on intact rat brain tissue at natural abundance, and were compared for their performances. The results showed that different signal enhancement techniques are sensitive to different classes of molecules/metabolites, depending on their molecular weights and mobility. DEPT was found to enhance the signals of low-molecular weight metabolites exclusively, while the signals of lipids, which often are associated with membranes and have relatively lower mobility, were highly sensitive to CP enhancement.     

Rapid Acquisition of 14N Solid‐State NMR Spectra with Broadband Cross Polarization

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR‐active isotopes, ¹⁴N and ¹⁵N, solid‐state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio (γ) and broad powder patterns arising from first‐order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality ¹⁴N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen‐containing systems. We demonstrate that it is possible to dramatically enhance ¹⁴N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., ¹H). The BR oadband A diabatic IN version C ross‐ P olarization ( BRAIN–CP ) pulse sequence is combined with other elements for efficient acquisition of ultra‐wideline SSNMR spectra, including W ideband U niform‐ R ate S mooth‐ T runcation ( WURST ) pulses for broadband refocusing, C arr– P urcell M eiboom– G ill ( CPMG ) echo trains for T₂‐driven S/N enhancement, and frequency‐stepped acquisitions. The feasibility of utilizing the BRAIN–CP/WURST–CPMG sequence is tested for ¹⁴N, with special consideration given to (i) spin‐locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN–CP experiments are shown to provide increases in signal‐to‐noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where ¹⁴N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments.     

Bound particle with magnetic moment in a space with topological defect

We consider a bounded quantum mechanical particle with spin ?1/2 and a gyromagnetic ratio g, which is placed in a uniform magnetic field, in a space with a linear topological defect. We obtain the exact expressions for eigenfunctions and eigenvalues, using the approach of the continuum theory of defects, and show the dependence on the topological parameters and potential harmonic. Besides, we study the limits case and obtained the results described in the literature. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Measurement of Angstrom to Nanometer Molecular Distances with 19F Nuclear Spins by EPR/ENDOR Spectroscopy

Spectroscopic and biophysical methods for structural determination at atomic resolution are fundamental in studies of biological function. Here we introduce an approach to measure molecular distances in bio‐macromolecules using ¹⁹F nuclear spins and nitroxide radicals in combination with high‐frequency (94 GHz/3.4 T) electron–nuclear double resonance (ENDOR). The small size and large gyromagnetic ratio of the ¹⁹F label enables to access distances up to about 1.5 nm with an accuracy of 0.1–1 Å. The experiment is not limited by the size of the bio‐macromolecule. Performance is illustrated on synthesized fluorinated model compounds as well as spin‐labelled RNA duplexes. The results demonstrate that our simple but strategic spin‐labelling procedure combined with state‐of‐the‐art spectroscopy accesses a distance range crucial to elucidate active sites of nucleic acids or proteins in the solution state.     

Preparation of single-crystalline platinum nanowires with small diameters under mild conditions

We report a convenient method to synthesize single-crystalline platinum nanowires with high aspect ratio of ca. 2.0 nm diameter by sophisticated and precise control of Pt(0) nuclei and their growth.     

Theoretical investigations on the defect structures and g factors for V4+ in 30PbO-5Bi2O3-(65-x)SiO2 glasses at different concentrations

For 3d¹ (V⁴⁺) impurity in 30PbO-5Bi₂O₃-(65-x)SiO₂ glass systems with different concentrations x of V2O5, the defect structures and gyromagnetic factors are theoretically investigated by using the perturbation formulas of g factors for a tetragonally compressed octahedral 3d¹ group. The concentration dependences of d-d transition band and g factors are suitably explained from the Fourier type concentration functions of the cubic crystal field parameter Dq, covalency factor N and relative tetragonal compression ratio ρ. The above concentration dependences of these quantities are suitably illustrated by the modifications of the local crystal field strength and electron cloud distribution with increasing x. The concentration variations of the electrical conductivity and dielectric relaxation are further analyzed from the stability of the systems in view of two competitive factors (increasing network polymerization and bulk stability at low concentrations and decreasing former SiO₂ and stability at high concentrations).     

Microscopic anisotropy revealed by NMR double pulsed field gradient experiments with arbitrary timing parameters

We consider a general double pulsed field gradient experiment with arbitrary experimental parameters and calculate an exact expression for the NMR signal attenuation from restricted geometries, which is valid at long wavelengths, i.e., when the product of the gyromagnetic ratio of the spins, the pulsed gradients' duration, and their magnitude is small compared to the reciprocal of the pore size. It is possible to observe microscopic anisotropy within the pore space induced by the boundaries of the pore, which can be used to differentiate restricted from free or multicompartmental diffusion and to estimate a characteristic pore dimension in the former case. Explicit solutions for diffusion taking place between parallel plates as well as in cylindrical and spherical pores are provided. In coherently packed cylindrical pores, it is possible to measure simultaneously the cylinders' orientation and diameter using small gradient strengths. The presence of orientational heterogeneity of cylinders is addressed, and a scheme for differentiating microscopic from ensemble anisotropy is proposed.     

¹H-nuclear magnetic resonance analysis of the triacylglyceride composition of cold-pressed oil from Camellia japonica

Camellia japonica (CJ) has oil-rich seeds, but the study of these oils has received little attention and has mainly focused only on their health properties. In the present work the relative composition of the fatty acid (FA) components of the triglycerides in cold-pressed oil from CJ is studied by 1H-NMR. The results obtained were: 75.75%, 6.0%, 0.17% and 18.67%, for oleic, linoleic, linolenic and saturated FA respectively. Levels of C?? unsaturated FA found in CJ oil were similar to those reported for olive oils. We also checked the possibility of using 13C-NMR spectroscopy; however, the results confirmed the drawback of 13C over 1H-NMR for the study of FA components of CJ triglycerides due to its low gyromagnetic ratio and its very low natural abundance.     

Is it cubic? Ice crystallization from deeply supercooled water

Ice crystallized below 200 K has the diffraction pattern of a faulty cubic ice, and not of the most stable hexagonal ice polymorph. The origin and structure of this faulty cubic ice, presumed to form in the atmosphere, has long been a puzzle. Here we use large-scale molecular dynamics simulations with the mW water model to investigate the crystallization of water at 180 K and elucidate the development of cubic and hexagonal features in ice as it nucleates, grows and consolidates into crystallites with characteristic dimensions of a few nanometres. The simulations indicate that the ice crystallized at 180 K contains layers of cubic ice and hexagonal ice in a ratio of approximately 2 to 1. The stacks of hexagonal ice are very short, mostly one and two layers, and their frequency does not seem to follow a regular pattern. In spite of the high fraction of hexagonal layers, the diffraction pattern of the crystals is, as in the experiments, almost identical to that of cubic ice. Stacking of cubic and hexagonal layers is observed for ice nuclei with as little as 200 water molecules, but a preference for cubic ice is already well developed in ice nuclei one order of magnitude smaller: the critical ice nuclei at 180 K contain approximately ten water molecules in their core and are already rich in cubic ice. The energies of the cubic-rich and hexagonal-rich nuclei are indistinguishable, suggesting that the enrichment in cubic ice does not have a thermodynamic origin.     

Long-range 1H-15N heteronuclear shift correlation across wide F1 spectral windows

Long‐range ¹H? ¹⁵N heteronuclear shift correlation experiments at natural abundance are becoming more routinely utilized in the characterization of unknown chemical structures from a diverse range of sources including natural products and pharmaceuticals. Apart from the inherent challenges of the low gyromagnetic ratio and natural abundance of ¹⁵N, investigators are also occasionally hampered by having to deal with the wide spectral range inherent to various nitrogen functional groups, which can exceed 500 ppm. Earlier triple resonance cryoprobe designs typically provided 90° ¹⁵N pulses in the range of 35–40 µs, which did not allow the uniform excitation of wide F₁ spectral ranges for ¹H? ¹⁵N GHMBC spectra. We report the results obtained with a newly designed Bruker 600 MHz triple resonance TCI Micro CryoProbe? using methyl orange as a model compound, in which the ¹⁵N resonances are separated by > 450 ppm. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of the temperature dependence of the dynamic nuclear polarisation enhancement of water protons at 3.4 Tesla

It is shown that the temperature dependence of the DNP enhancement of the NMR signal from water protons at 3.4 T using TEMPOL as a polarising agent can be obtained provided that the nuclear relaxation, T(1I), is sufficiently fast and the resolution sufficient to measure the (1)H NMR shift. For high radical concentrations (～100 mM) the leakage factor is approximately 1 and, provided sufficient microwave power is available, the saturation factor is also approximately 1. In this situation the DNP enhancement is solely a product of the ratio of the electron and nuclear gyromagnetic ratios and the coupling factor enabling the latter to be directly determined. Although the use of high microwave power levels needed to ensure saturation causes rapid heating of the sample, this does not prevent maximum DNP enhancements, ε(0), being obtained since T(1I) is very much less than the characteristic heating time at these concentrations. It is necessary, however, to know the temperature variation of T(1I) to allow accurate modelling of the behaviour. The DNP enhancement is found to vary linearly with temperature with ε(0)(T) = -2 ± 2 - (1.35 ± 0.02)T for 6 °C ≤ T ≤ 100 °C. The value determined for the coupling factor, 0.055 ± 0.003 at 25 °C, agrees very well with the molecular dynamics simulations of Sezer et al. (Phys. Chem. Chem. Phys., 2009, 11, 6626) who calculated 0.0534, however the experimental values increase much more rapidly with increasing temperature than predicted by these simulations. Large DNP enhancements (|ε(0)| > 100) are reported at high temperatures but it is also shown that significant enhancements (e.g.～40) can be achieved whilst maintaining the sample temperature at 40 °C by adjusting the microwave power and irradiation time. In addition, short polarisation times enable rapid data acquisition which permits further enhancement of the signal, such that useful liquid state DNP-NMR experiments could be carried out on very small samples.     

The growth of single crystal silver wires at the nitrobenzene|water interface

Single crystal silver wires can be grown at the nitrobenzene|water interface when silver ions dissolved in the aqueous phase are reduced by decamethyl ferrocene dissolved in the nitrobenzene phase. The successful growth of these wires depends on a number of experimental conditions, most prominently on the concentration ratio of reactants, nucleation rates, shape of formed nuclei, and wettability of nuclei. The size-time dependence can be modeled on the basis of microelectrode behavior of the silver nuclei and wire. AFM, SEM, light microscopy and single crystal X-ray diffraction has been applied to study the morphology of the silver nuclei and wires.     

Introducing krypton NMR spectroscopy as a probe of void space in solids

A wealth of information about porous materials and their void spaces has been obtained from the chemical shift data in (129)Xe NMR spectroscopy during the past decades. In this contribution, the only NMR active, stable krypton isotope (83)Kr (spin I = (9)/(2)) is explored as a novel probe for porous materials. It is demonstrated that (83)Kr NMR spectroscopy of nanoporous or microporous materials is feasible and straightforward despite the low gyromagnetic ratio and low abundance of the (83)Kr isotope. The (83)Kr line width in most of the studied cases is quadrupolar dominated and field-strength independent. A significant exception was found in calcium-exchanged zeolites where the field dependence of the line width indicates a distribution of isotropic chemical shifts that may be caused by long-range disorder in the zeolite structure. The (83)Kr chemical shifts observed in the investigated materials display a somewhat different behavior than that of their (129)Xe counterparts and should provide a great resource for the verification or refinement of current (129)Xe chemical shift theory. In contrast to xenon, krypton with its smaller atomic radius has been demonstrated to easily penetrate the porous framework of NaA. Chemical shifts and line widths of (83)Kr are moderately dependent on small fluctuations in the krypton loading but differ strongly between some of the studied samples.     

Growth of polydisperse aerosol in an adiabatic condensation nuclei counter (CNC) with finite expansion time

The diffusional growth of polydisperse aerosol was calculated as a function of expansion time, i.e.; time in which the temperature drop necessary for supersaturation-induced condensational growth is produced. At high nuclei concentrations and/or long expansion time only fractions of the ideal supersaturation (produced with small concentrations and fast expansion) are obtained. Large nuclei are activated earlier than smaller ones and consume so much vapor that smaller nuclei cannot possibly grow. This may cause a strong influence on the particle size detection limit. Therefore, great care is necessary to interpret condensation nuclei counter (CNC) measurements with concentrated polydisperse aerosols correctly.