Study of natural diamonds by dynamic nuclear polarization-enhanced C nuclear magnetic resonance spectroscopy

The results of a study of two types of natural-diamond crystals by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state ¹³C nuclear magnetic resonance (NMR) are reported. The home-built DNP magic-angle spinning (MAS) ¹³C NMR spectrometer operates at 54 GHz for electrons and 20.2 MHz for carbons. The power of the microwave source was about 30 W and the highest DNP enhancement factor came near to 10³. It was shown that in the MAS spectra the ¹³C NMR linewidths of the I_b-type diamond were broader than those of I_aB3-type diamond. From the hyperfine structure of the DNP enhancement as a function of frequency, four kinds of nitrogen-centred and one kind of carbon-centred free radicals could be identified in the I_b-type diamond. The hyperfine structures of the DNP enhancement curve that originated from the anisotropic hyperfine interaction between electron and nuclei could be partially averaged out by MAS. The ¹³C polarization time of DNP was rather long, i.e. 1500 s, and the spin—lattice relaxation time (without microwave irradiation) was about 300 s, which was somewhat shorter than anticipated. Discussions on these experimental results have been made in this report.     

Study of diamond film by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Three chemical vapor deposited diamond films were studied by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state¹³C nuclear magnetic resonance (NMR) spectroscopy. Enhanced¹³C direct-polarization spectra of diamond films were obtained by irradiating the samples with microwaves at or near electron spin resonance Larmor frequency of carbon center free radicals. No NMR signal for sp² hybridized carbons could be observed. From the curve of the DNP enhancement as a function of frequency, it is found that the dominant DNP mechanism is the solid-state effect. The¹³C cross-polarization spectrum, which is an evidence for existence of the proton defect in the lattice of diamond films, is much broader than the¹³C single pulse spectrum. The reason is discussed shortly.     

Chemisorption and surfaces studied by nuclear magnetic resonance spectroscopy

This paper presents an introduction to the study of surfaces and chemically adsorbed species with nuclear magnetic resonance (NMR) spectroscopy. The analysis is based on nuclear magnetic interactions in the solid state: dipole-dipole couplings, chemical shift anisotropy, Knight shifts, and quadrupolar splitting. The physical origins and characteristics of each interaction, as well as relative intensities for different nuclei, are discussed. In particular, emphasis is placed on the relation of these interactions to quantities of interest to studies in adsorption and catalysis: motional properties of the adsorbate, the distribution of adsorption sites, the chemical state of atoms adsorbed at the surface, electrostatic field gradients, and the metallic character of surface atoms. Techniques to observe these interactions are described; subdivided by the type of nucleus: strongly coupled nuclei (e.g. ¹H, ¹⁹F), weakly coupled nuclei (e.g. ¹³C, ¹⁵N, ²⁹Si, ¹⁹⁵Pt), and quadrupolar nuclei (e.g. ²H, ¹⁴N, ²⁷Al). The techniques described to isolate and identify the overlapping effects in the spectra include multiple-pulse spin echoing and decoupling, double-resonance irradiation, multiple-quantum excitation, and mechanical sample spinning. A review of the recent application of these techniques to studies of adsorption and surfaces illustrates the potentials and limitations. Finally, a procedure for formulating a NMR study of surface samples is proposed, with respect to sample composition and character, and the type of information desired.     

Double-resonance circuit for nuclear magnetic resonance spectroscopy

A new double-resonance probe circuit design is described. The circuit contains no quarter-wavelength elements or equivalents, yet nonetheless achieves adequate isolation between the two input channels. It contains relatively few components, and so is both compact and efficient. It has been incorporated in two solid-state nuclear magnetic resonance (NMR) probes, with excellent results.     

Imaging of (13)C-labeled glucose and sorbitol in bovine lens by (1)H-detected (13)C nuclear magnetic resonance spectroscopy

Bovine lenses were incubated in a solution containing [1-(13)C]glucose (50 mM) for 1, 2 and 4 days. Spectroscopic images of [1-(13)C]glucose and [1-(13)C]sorbitol were constructed using (1)H-detected gradient-enhanced heteronuclear multiple-quantum coherence (GE-HMQC) in a 2.0-tesla magnetic field. Accumulations of [1-(13)C]glucose and [1-(13)C]sorbitol were mainly observed at the periphery of the lens. Their distributions corresponded to the cortex. (1)H-detected (13)C nuclear magnetic resonance (NMR) spectroscopic imaging by GE-HMQC successfully demonstrated the distribution of [1-(13)C]glucose and [1-(13)C]sorbitol at the periphery of bovine lenses.     

Reconstruction of randomly under-sampled spectra for in vivo 13C magnetic resonance spectroscopy

PurposeOver the past decade, many techniques have been developed to reduce radiofrequency (RF) power deposition associated with proton decoupling in in vivo Carbon-13 (¹³C) magnetic resonance spectroscopy (MRS). In this work we propose a new strategy that uses data under-sampling to achieve reduction in RF power deposition.Materials and methodsEssentially, proton decoupling is required only during randomly selected segments of data acquisition. By taking advantage of the sparse spectral pattern of the carboxylic/amide region of in vivo ¹³C spectra of brain, we developed an iterative algorithm to reconstruct spectra from randomly under-sampled data. Fully sampled data were used as references. Reconstructed spectra were compared with the fully sampled references and evaluated using residuals and relative signal intensity errors.ResultsNumerical simulations and in vivo experiments at 7 Tesla demonstrated that this novel decoupling and data processing strategy can effectively reduce decoupling power deposition by greater than 30%.ConclusionThis study proposes and evaluates a novel approach to acquire ¹³C data with reduced proton decoupling power deposition and reconstruct in vivo ¹³C spectra of carboxylic/amide metabolite signals using randomly under-sampled data. Because proton decoupling is not needed over a significant portion of data acquisition, this novel approach can effectively reduce the required decoupling power and thus SAR. It opens the possibility of performing in vivo ¹³C experiments of human brain at very high magnetic fields.     

Therapeutic response of tumors by in-vitro proton nuclear magnetic resonance spectroscopy

Proton NMR spectra of perchloric acid extracts of methyl cholantherene induced tumors grown in rats have been analyzed and compared with the normal and the treated tumor tissue samples. Well-resolved resonances from numerous low-molecular weight compounds including various amino acids, nucleotides, choline, creatine, phosphocreatine etc. were observed and assigned using pH titration, 2D NMR and by comparison with the spectra of model compounds. Significant differences were noticed in the spectra of the tumor and the normal tissue samples. Ratios of metabolite levels were calculated for the normal, tumor and treated tumor tissues which are shown as good markers to assess the state of the tumor and their response to treatment.     

Nuclear magnetic resonance study of ultrananocrystalline diamonds

We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique. Analysis of the ¹³C and ¹H NMR spectra, spin-spin and spin-lattice relaxation times in purified UNCD samples is presented. Our measurements show that UNCD particles consist of a diamond core that is partially covered by a sp ²-carbon fullerene-like shell. The uncovered part of outer diamond surface comprises a number of hydrocarbon groups that saturate the dangling bonds. Our findings are discussed along with recent calculations of the UNCD structure. Significant increase in the spin-lattice relaxation rate (in comparison with that of natural diamond), as well as stretched exponential character of the magnetization recovery, are attributed to the interaction of nuclear spins with paramagnetic centers which are likely fabrication-driven dangling bonds with unpaired electrons. We show that these centers are located mainly at the interface between the diamond core and shell.     

Carbon-13 CP-MAS nuclear magnetic resonance studies of teas

13C CP-MAS NMR spectra of green and black tea were obtained and assigned based on the solid-state NMR spectra of tropolone, (+)-catechin hydrate, gallic acid, caffeine and flavone derivatives. The peak shape and chemical shifts observed for carbonyl carbons in CP-MAS spectra of teas indicate the existence of different chemical species, mainly free phenollic acids and ester derivatives of flavonoids. The peak patterns allow to establish differences between both teas.     

New developments in nuclear magnetic resonance using noise spectroscopy

In the 30 years since Ernst and Kaiser introduced the idea of incoherent radiation fields and their application to NMR spectroscopy, relatively few researchers have exploited the advantages of noise spectroscopy. Some recent applications of one-dimensional noise spectroscopy in NMR are presented which display a versatility which commonly is not appreciated. Excitation schemes are discussed which demonstrate both broadband and narrowband features, and demonstrate both theoretically and experimentally how noise spectroscopy allows for the observation of distortion-free broadline spectra in solids whichmay not be amenable to techniques more traditionally used in pulsed NMR experiments. It is argued that these applications of noise spectroscopy deserve a more common place in the experimentalists arsenal.     

Magnetic field distribution in superconducting niobium by nuclear magnetic resonance fourier spectroscopy

Nuclear magnetic resonance measurements of the superconducting mixed-state field distribution in niobium metal are reported. It is demonstrated that pulsed NMR Fourier spectroscopy (defined in the text) can resolve considerable detail in the field distribution functionf(h). A simple analytic model forf(h) was used for numerical calculation of the expected spectrum shape. An ordered vortex lattice was always observed. Far from the transition the spectra were most consistent with triangular lattice symmetry. The temperature variation of the Maki parameter? ₂ was measured. The shift of spectrum position is compatible with appreciable reduction of spin susceptibility in the superconducting state.     

1H and13C nuclear magnetic relaxation and local dynamics of lysozyme and synthetic polypeptides

The combined analysis of¹H and¹³C NMR relaxation data in solid lysozyme and some typical homopolypeptides was carried out by using “model-free” approach. Three types of relaxation transitions (γ’, γ and β) were revealed in the temperature range investigated. The microdynamical parameters of these motions were determined. From the comparison of these parameters with those of selected synthetic polymers it follows that the molecular motions in proteins and synthetic polymers are of the same nature. All these motions show pronounced anisotropic character. In the investigated temperature range no molecular motions corresponding to α-relaxation (liquid-like) transition were revealed. The hydration effects on parameters of the motions in proteins were considered. The most pronounced effect takes place for β-transition. The effect of Brownian rotation of protein molecule in solution on measured correlation function of local motions was also discussed.     

Transport properties of CsHSO4 investigated by impedance spectroscopy and nuclear magnetic resonance

Transport properties of the superprotonic conductor, CsHSO₄, have been investigated by impedance spectroscopy and nuclear magnetic resonance (NMR). It has been found that both, conductivity (σ) and NMR diffusion (D _NMR) are practically isotropic in the high-conductive (superprotonic) phase (above 414 K). The NMR diffusion coefficient, D _NMR , increases rapidly and discontinuously at the melting point (~490 K). The temperature change of D _NMR in the superprotonic phase is characterized by a smaller activation energy compared to that in the liquid state. The values calculated from the Nernst-Einstein relation practically coincide with D _NMR in the superprotonic phase, i.e., the Haven ratio is close to unity. This indicates that in this phase the proton motion is rather uncorrelated.