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.     

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.     

Satellite structure of DNP enhanced 13C NMR spectra in diamond

The P1 nitrogen defect centres in natural type Ia diamonds have been exploited in dynamic nuclear polarization (DNP) of the ¹³C nuclei (1.1% abundant) via the solid-effect. We have achieved sufficiently large ¹³C polarizations in some diamonds to observe weak satellite lines in the ¹³C NMR spectra by using long microwave irradiation times. A study of the dependence of the NMR spectra on the crystal orientation with respect to the applied magnetic field reveals that the satellite structure originates from the small fraction of ¹³C nuclei that are involved in nearest neighbour dipole interactions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Transfer of1H and13C dynamic nuclear polarization from immobilized nitroxide radicals to flowing liquids

In this study,¹H and¹³C dynamic nuclear polarization (DNP) was generated at a magnetic field strength of 0.33 T utilizing silica phase immobilized nitroxide (SPIN) samples. The polarization was subsequently transferred to flowing liquids and monitored at a magnetic field strength of 4.7 T. These solid/liquid intermolecular transfer (SLIT) experiments provide efficient polarization transfer without the necessity of the free radical system present in the monitoring fluid. Specifically, ultimate¹H SLIT DNP Overhauser enhancements of ?56 and ?110 have been observed for benzene and chloroform in the presence of SPIN system 2, respectively. The¹³C SLIT DNP enhancement for benzene is dominated by three-spin effects and poor leakage factors (f _c). However, a particularly favorable case is the chloroform/SPIN 2 system which exhibits a scalar dominated enhancement. For this case, positive enhancements 40–60 times the¹³C thermal Boltzmann magnetization at 4.7 T have been observed. The large scalar dominated¹³C DNP enhancement for this system represents one of the largest experimental enhancements reported to date. The¹³C DNP spectra for other samples which exhibit favorable scalar¹³C dominated enhancements (e.g., Freon 113) are also presented. Three different SPIN systems were also prepared and characterized in the present study.     

Fast Dissolution Dynamic Nuclear Polarization NMR of 13C-Enriched 89Y-DOTA Complex: Experimental and Theoretical Considerations

The yttrium complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(1′-¹³C-acetic acid) [¹³C]DOTA was synthesized. Fast dissolution dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) studies revealed that the ⁸⁹Y, ¹³C, and ¹⁵N nuclei present in the complex could be co-polarized at the same optimum microwave irradiation frequency. The liquid-state spin–lattice relaxation time T ₁ of these nuclei were found to be reasonably long to preserve some or most of the DNP-enhanced polarization after dissolution. The hyperpolarized ¹³C and ⁸⁹Y NMR signals were optimized in different glassing mixtures. The overall results are discussed in light of the thermal mixing model of DNP.     

A dynamic nuclear polarization strategy for multi-dimensional Earth's field NMR spectroscopy

Dynamic nuclear polarization (DNP) is introduced as a powerful tool for polarization enhancement in multi-dimensional Earth’s field NMR spectroscopy. Maximum polarization enhancements, relative to thermal equilibrium in the Earth’s magnetic field, are calculated theoretically and compared to the more traditional prepolarization approach for NMR sensitivity enhancement at ultra-low fields. Signal enhancement factors on the order of 3000 are demonstrated experimentally using DNP with a nitroxide free radical, TEMPO, which contains an unpaired electron which is strongly coupled to a neighboring ¹⁴N nucleus via the hyperfine interaction. A high-quality 2D ¹⁹F–¹H COSY spectrum acquired in the Earth’s magnetic field with DNP enhancement is presented and compared to simulation.     

Selective homonuclear Hartmann–Hahn for 13C→13C polarization transfer in solution state NMR

Polarization transfer has become a commonplace technique for the enhancement of a variety of nuclei in high field nuclear magnetic resonance (NMR). In this paper the homonuclear Hartmann-Hahn method for polarization transfer is revisited and it is shown that a 90% transfer of polarization can be achieved experimentally between a pair of scalar coupled ¹³C nuclei in a sample of isotopically enriched glycine. This may show particular utility in the field of dynamic nuclear polarization (DNP) and could be used as an addendum to already established DNP techniques allowing the favourable enhancement to be ‘stored’ on long-lived nuclei and subsequently transferred to shorter-lived nuclei prior to observation.     

A model for establishing the ultimate enhancements (A∞) in the low to high magnetic field transfer dynamic nuclear polarization experiment

In a preliminary report, we have demonstrated transfer of a flowing bolus enhanced in low magnetic fields (e.g., 0.33 T) with dynamic nuclear polarization (DNP), but monitored in a high magnetic field (4.7 T). The advantages of the high magnetic field monitoring approach include: 1) greater chemical shift dispersion, and 2) improved signal strength in comparison with static low field DNP experiments. In the present study, a model is developed to predict ultimate DNP enhancements (A_∞) in this experiment for flow liquid/liquid intermolecular transfer (L²IT). L²IT¹H and¹³C data is obtained for benzene and chloroform in order to test the validity of the model. The ultimate¹H and¹³C DNP enhancements obtained for benzene/TEMPO are ?150 and ?220, respectively. For a chloroform/TEMPO (L²IT) sample, the ultimate enhancements are close to the¹H dipolar (?330) and the¹³C scalar (+2660) limit, respectively. In the latter case, the observed¹³C DNP enhancement exceeds the thermal Boltzmann magnetization at 4.7 T by a factor of 21. For a 1-chlorobutane/TEMPO sample selective enhancements were observed at different sites in the molecule. For example, the C-1 carbon exhibits a large scalar enhancement, whereas, the other carbons exhibit dipolar enhancements. Data illustrating the importance of three-spin effects in¹³C DNP studies is also presented. Alternative methods of sample transfer from the low to high magnetic field are also discussed.     

Nearly 10-fold enhancements in intermolecular H double-quantum NMR experiments by nuclear hyperpolarization

Intermolecular Multiple-Quantum Coherences (iMQCs) can yield interesting NMR information of high potential usefulness in spectroscopy and imaging – provided their associated sensitivity limitations can be overcome. A recent study demonstrated that ex situ dynamic nuclear polarization (DNP) could assist in overcoming sensitivity problems for iMQC-based experiments on ¹³C nuclei. In the present work we show that a similar approach is possible when targeting the protons of a hyperpolarized solvent. It was found that although the DNP procedure enhances single-quantum ¹H signals by about 600, which is significantly less than in optimized low-γ liquid-state counterparts, the non-linear dependence of iMQC-derived signals on polarization can yield very large enhancements approaching 10⁶. Cleary no practical amount of data averaging can match this kind of sensitivity gains. The fact that DNP endows iMQC-based ¹H NMR spectra with a sensitivity that amply exceeds that of their thermally polarized single-quantum counterpart, is confirmed in a number of simple single-scan 2D imaging experiments.     

DYNAMIC NUCLEAR POLARIZATION OF ORGANIC COMPOUND DOPED WITH FREE RADICALS

For those organic compounds which lack free radicals, a doping approach named melting-liquid-nitrogen-quench was developed in order to perform dynamic nuclear polarization(DNP)-nuclear magnetic resonance experiments. By using this method, the ¹H, ¹³C, ¹⁵N DNP enhancements were observed with free-radical-doped dibenzofuran and benzamide. The enhancement mechanism and relation between the concentration of unpaired electrons and the maximum DNP enhancement are discussed.     

High-frequency dynamic nuclear polarization in the nuclear rotating frame

A proton dynamic nuclear polarization (DNP) NMR signal enhancement (epsilon) close to thermal equilibrium, epsilon = 0.89, has been obtained at high field (B(0) = 5 T, nu(epr) = 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spin-lock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T(1rho)), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T(1n)). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T(1rho) and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T(1n)). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time(1/2) of epsilon(t) = 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for high-field DNP/NMR studies of biological systems.     

Defects and impurities in nanodiamonds: EPR, NMR and TEM study

EPR, ¹³C NMR and TEM study of ultradisperse diamond (UDD) samples is reported. The compounds show a high concentration of paramagnetic centers (up to 10²⁰ spin/g), which are due to structural defects (dangling C-C bonds) on the diamond cluster surface. The anomalous reduction in the spin-lattice relaxation time of ¹³C (from several hours in natural diamond to ∼150 ms in UDD clusters) is attributed to the interaction between the unpaired electrons of the paramagnetic centers and nuclear spins. ¹³C NMR line-width reflects the fact that the structure of the UDD surface is distorted in comparison to the ‘bulk’ diamond structure.     

High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR

Dynamic nuclear polarization (DNP) transfers electron spin-polarization to nuclear spins in close proximity, increasing sensitivity by two-to-three orders of magnitude. This enables nuclear magnetic resonance (NMR) experiments on samples with low concentrations of analyte. The requirement of using cryogenic temperatures in DNP-enhanced solid-state NMR (ssNMR) experiments may impair the resolution and hence limit its broad application to biological systems. In this work, we introduce a “High-Temperature DNP” approach, which aims at increasing spectral resolution by performing experiments at temperatures of around 180?K instead of?~100?K. By utilizing the extraordinary enhancements obtained on deuterated proteins, still sufficiently large DNP enhancements of 11–18 are obtained for proton and carbon, respectively. We recorded high sensitivity 2D ¹³C–¹³C spectra in?~9?min with higher resolution than at 100?K, which has similar resolution to the one obtained at room temperature for some favorable residues.     

Dynamic nuclear polarization at 9T using a novel 250 GHz gyrotron microwave source. 2003

In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9 T (250 GHz for g = 2 electrons, 380 MHz for ¹H). In these experiments, ¹H enhancements of up to 170 ± 50 have been observed in 1-¹³C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20 K; in addition, we have observed significant enhancements in ¹⁵N spectra of unoriented pf1-bacteriophage. Finally, enhancements of ∼17 have been obtained in two-dimensional ¹³C–¹³C chemical shift correlation spectra of the amino acid U–¹³C, ¹⁵N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250 GHz gyrotron oscillator used in these experiments.     

Quantum computation using the 13C nuclear spins near the single NV defect center in diamond

We discuss the possibility of realizing quantum computation on the basis of a cluster of single interacting nuclear spins in solids. This idea seems to be feasible because of the combination of two techniques—Single Molecule Spectroscopy and Optically Detected Electron Nuclear Double Resonance. Compared to the well-known bulk Nuclear Magnetic Resonance (NMR), the proposed method of quantum computation has the advantage that quantum computation is performed with pure spin states and the quantum processor is more easily scalable. At the same time, the advantages of NMR quantum computation are kept: long coherence time and easy construction of quantum gates. As a specific system to implement the above idea, we discuss the ¹³C-nuclear spins in the nearest vicinity of a single nitrogen-vacancy (NV) defect center in diamond, which can be optically detected using the technique of scanning confocal microscopy. Owing to the hyperfine coupling of the ground state electron paramagnetic spin S=1 of the center to ¹³C nuclear spins in a diamond lattice, the states of nuclear spins in the vicinity of the defect-center can be addressed individually. Preliminary consideration shows that it should be possible to address up to 12 individual ¹³C nuclear spins. The dephasing time of the nuclear spin states at low temperatures allows realization up to 10⁵ gates.     

Fourier transform microwave spectroscopy of isotopically substituted diacetylenes: rs-structure, quadrupole coupling, and anisotropic nuclear spin-spin interaction

Monodeuterated diacetylene (HCCCCD) and its ¹³C-substituted species H¹³CCCCD, HC¹³CCCD, HCC¹³CCD, and HCCC¹³CD were investigated by Fourier transform microwave spectroscopy. The D nuclear quadrupole splittings were almost completely resolved. For H¹³CCCCD hyperfine splittings caused by the anisotropic nuclear spin-spin interaction between the H and ¹³C nuclei were also observed. The analysis yielded rotational constants, centrifugal distortion constants, and the constants for the nuclear quadrupole coupling and anisotropic nuclear spin-spin interaction. The substitution structure of HCCCCD was calculated as follows: r_s(C-H) = 1.056054(39) Å, r_s(CC) = 1.208631(4) Å, r_s(C-C) = 1.374117(6) Å, r_s(CC) = 1.208116(4) Å, and r_s(C-D) = 1.056231(17) Å, in the order of the arrangement of the bonds. A rough estimate of the equilibrium structure is also presented. The eQq constant for the deuterium nucleus is 0.2061(4) MHz. The anisotropic ¹³C-H spin-spin interaction constant was experimentally determined for the first time as b = −29.2(15) kHz, which is defined as the coefficient of (3I_2zI_3z − I₂ · I₃), where I₂ and I₃ denote the H and ¹³C nuclear spins, respectively, and I_2z and I_3z their components along the molecular axis. The observed b constant is not accounted for by the direct magnetic dipole-dipole interaction only, suggesting a significant contribution from indirect anisotropic interaction.     

Cross Polarization for Dissolution Dynamic Nuclear Polarization Experiments at Readily Accessible Temperatures 1.2?<?T?<?4.2?K

Cross polarization can provide significant enhancements with respect to direct polarization of low-γ nuclei such as ¹³C. Substantial gains in sample throughput (shorter polarization times) can be achieved by exploiting shorter build-up times τ_DNP(¹H)?<?τ_DNP(¹³C). To polarize protons rather than low-γ nuclei, nitroxide radicals with broad ESR resonances such as TEMPO are more appropriate than Trityl and similar carbon-based radicals that have narrow lines. With TEMPO as polarizing agent, the main Dynamic Nuclear Polarization (DNP) mechanism is thermal mixing (TM). Cross polarization makes it possible to attain higher polarization levels at 2.2?K than one can obtain with direct DNP of low-γ nuclei with TEMPO at 1.2?K, thus avoiding complex cryogenic technology.     

<Superscript>13</Superscript>C Dynamic Nuclear Polarization Using Derivatives of TEMPO Free Radical

The nitroxide-based 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical is widely used in ¹³C dynamic nuclear polarization (DNP) due to its relatively low cost, commercial availability, and effectiveness as polarizing agent. While a large number of TEMPO derivatives are available commercially, so far, only few have been tested for use in ¹³C DNP. In this study, we have tested and evaluated the ¹³C hyperpolarization efficiency of eight derivatives of TEMPO free radical with different side arms in the 4-position. In general, these TEMPO derivatives were found to have slight variations in efficiency as polarizing agents for DNP of 3 M [1-¹³C] acetate in 1:1 v/v ethanol:water at 3.35 T and 1.2 K. X-band electron paramagnetic resonance (EPR) spectroscopy revealed no significant differences in the spectral features among these TEMPO derivatives. ²H enrichment of the ethanol:water glassing matrix resulted in further improvement of the solid-state ¹³C DNP signals by factor of 2 to 2.5-fold with respect to the ¹³C DNP signal of non-deuterated DNP samples. These results suggest an interaction between the nuclear Zeeman reservoirs and the electron dipolar system via the thermal mixing mechanism.     

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.     

Neutron diffraction study on the hyperfine-induced nuclear spin order in the antiferromagnet PrSn3

At several $\text{(h+}\text{1}\text{2}\text{0 0)}$ reciprocal lattice positions of PrSn₃, we observed Bragg scattering of neutrons which is purely due to the nuclear spin polarization of ¹⁴¹Pr, and could measure the temperature dependence of the polarization between 10 mK and 4.2 K. From the perfectly polarized state of the nuclear spins, the difference between the spin-dependent nuclear scattering lengths b⁺ and b^- has been determined to be b⁺ - b^- = -0.110 ± 0.006 × 10^-12 cm, which is significantly larger in magnitude than the previously reported value. An additional electronic polarization which is linearly proportional to the nuclear spin polarization has also been observed.