Nuclear Larmor Frequency Research Articles

One- and Two-Dimensional Nuclear Magnetic Resonance Spectroscopy with a Diamond Quantum Sensor

We report on Fourier spectroscopy experiments performed with near-surface nitrogen-vacancy centers in a diamond chip. By detecting the free precession of nuclear spins rather than applying a multipulse quantum sensing protocol, we are able to unambiguously identify the NMR species devoid of harmonics. We further show that, by engineering different Hamiltonians during free precession, the hyperfine coupling parameters as well as the nuclear Larmor frequency can be selectively measured with up to five digits of precision. The protocols can be combined to demonstrate two-dimensional Fourier spectroscopy. Presented techniques will be useful for mapping nuclear coordinates in molecules deposited on diamond sensor chips, en route to imaging their atomic structure.

Liquid state DNP at high magnetic fields: Instrumentation, experimental results and atomistic modelling by molecular dynamics simulations

Dynamic nuclear polarization (DNP) at high magnetic fields has recently become one of the major research areas in magnetic resonance spectroscopy and imaging. Whereas much work has been devoted to experiments where the polarization transfer from the electron spin to the nuclear spin is performed in the solid state, only a few examples exist of experiments where the polarization transfer is performed in the liquid state. Here we describe such experiments at a magnetic field of 9.2 T, corresponding to a nuclear Larmor frequency of 400MHz for proton spins and an excitation frequency of 263GHz for the electron spins. The technical requirements to perform such experiments are discussed in the context of the double resonance structures that we have implemented. The experimental steps that allowed access to the enhancement factors for proton spins of several organic solvents with nitroxide radicals as polarizing agents are described. A computational scheme for calculating the coupling factors from molecular dynamics (MD) simulations is outlined and used to highlight the limitations of the classical models based on translational and rotational motion that are typically employed to quantify the observed coupling factors. The ability of MD simulations to predict enhancements for a variety of radicals and solvent molecules at any magnetic field strength should prove useful in optimizing experimental conditions for DNP in the liquid state.

Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids.

Dynamic nuclear polarization (DNP) is a powerful method to enhance sensitivity especially of solid-state magic-angle spinning (MAS) NMR by up to several orders of magnitude. The increased interest both from a practical as well as theoretical viewpoint has spawned several fields of active research such as the development of new polarizing agents with improved or unique properties and description of the underlying DNP mechanisms such as solid effect (SE) and cross effect (CE). Even though a novel class of unique polarizing agents based on high-spin metal ions such as Gd(iii) and Mn(ii) has already been utilized for MAS DNP a theoretical description of the involved DNP mechanism is still incomplete. Here, we review several aspects of DNP-relevant electron-paramagnetic resonance (EPR) properties of the general class of these half-integer high-spin metal ions with isotropic Zeeman interaction but significant zero-field splitting (ZFS). While the SE can be relatively easily described similar to that of a S = 1/2 system and is assumed to be effective only for polarizing agents featuring a narrow central EPR transitions (i.e., mS = -1/2 → +1/2) with respect to the nuclear Larmor frequency, the CE between two high-spin ions requires a more detailed theoretical investigation due to a multitude of possible transitions and matching conditions. This is especially interesting in light of recent understanding of CE being induced by MAS-driven level anti-crossings (LACs) between dipolar-coupled electron spins. We discuss the requirements of such CE-enabling LACs to occur due to anisotropy of ZFS, the expected adiabaticity, and the resulting possibilities of high-spin metal ion pairs to act as polarizing agents for DNP. This theoretical description serves as a framework for a detailed experimental study published directly following this work.

Time domain DNP with the NOVEL sequence.

We present results of a pulsed dynamic nuclear polarization (DNP) study at 0.35 T (9.7 GHz/14.7 MHz for electron/(1)H Larmor frequency) using a lab frame-rotating frame cross polarization experiment that employs electron spin locking fields that match the (1)H nuclear Larmor frequency, the so called NOVEL (nuclear orientation via electron spin locking) condition. We apply the method to a series of DNP samples including a single crystal of diphenyl nitroxide (DPNO) doped benzophenone (BzP), 1,3-bisdiphenylene-2-phenylallyl (BDPA) doped polystyrene (PS), and sulfonated-BDPA (SA-BDPA) doped glycerol/water glassy matrices. The optimal Hartman-Hahn matching condition is achieved when the nutation frequency of the electron matches the Larmor frequency of the proton, ω(1S) = ω(0I), together with possible higher order matching conditions at lower efficiencies. The magnetization transfer from electron to protons occurs on the time scale of ∼100 ns, consistent with the electron-proton couplings on the order of 1-10 MHz in these samples. In a fully protonated single crystal DPNO/BzP, at 270 K, we obtained a maximum signal enhancement of ε = 165 and the corresponding gain in sensitivity of ε(T1/T(B))(1/2)=230 due to the reduction in the buildup time under DNP. In a sample of partially deuterated PS doped with BDPA, we obtained an enhancement of 323 which is a factor of ∼3.2 higher compared to the protonated version of the same sample and accounts for 49% of the theoretical limit. For the SA-BDPA doped glycerol/water glassy matrix at 80 K, the sample condition used in most applications of DNP in nuclear magnetic resonance, we also observed a significant enhancement. Our findings demonstrate that pulsed DNP via the NOVEL sequence is highly efficient and can potentially surpass continuous wave DNP mechanisms such as the solid effect and cross effect which scale unfavorably with increasing magnetic field. Furthermore, pulsed DNP is also a promising avenue for DNP at high temperature.

Pulsed EPR and ENDOR Study of SiC Nanopowders

The great potential of the micro(mp) nano-particle (np) silicon carbide (SiC) for future applications in spintronics stimulates detailed studies of the effects of impurities and defects in their electronic properties. In particular, mpand np-SiC doped with nitrogen (N) atoms is an important issue to be investigated, since N is a common contaminant in SiC bulk, acting as n-type dopant. Although N has been found to be present in the nanomaterials, there is no experimental evidence indicating for the formation of the N shallow donor state in np-SiC. The np-SiC samples with R 50 nm which was identified by pulsed ENDOR as the N in quasi-cubic (Nk2) site. As the size of the np-SiC decrease, the signal from VC in 6H SiC phase appeared in the FS ESE spectrum, which gives rise to the compensation of the N donors and as a result the FS ESE spectrum intensity of N decrease significantly. Only two signals due to the VC located in 3C and 6H SiC phase were observed in the FS ESE spectrum of the np-SiC samples with the R < 50 nm. The fact that the ENDOR signal centered at the hydrogen (H) nuclear Larmor frequency (n) was observed in the np-SiC with R < 100 nm indicates that H atoms are localized in the vicinity of the VC and can form the H-related defect like (VC+H). As a conclusion, two phases (and -SiC crystalline phases) are presented in np-SiC. The work was supported by GA CR 13-06697P and SAFMAT project CZ.2.16/3.1.00/22132.

All-optical NMR in semiconductors provided by resonant cooling of nuclear spins interacting with electrons in the resonant spin amplification regime

Resonant cooling of different nuclear isotopes manifested in optically induced nuclear magnetic resonances (NMR) is observed in $n$-doped CdTe/(Cd,Mg)Te and ZnSe/(Zn,Mg)Se quantum wells and for donor-bound electrons in ZnSe:F and GaAs epilayers. By time-resolved Kerr rotation used in the regime of resonant spin amplification, we can expand the range of magnetic fields where the effect can be observed up to nuclear Larmor frequencies of 170 kHz. The mechanism of the resonant cooling of the nuclear spin system is analyzed theoretically. The developed approach allows us to model the resonant spin amplification signals with NMR features.

Correlating nuclear frequencies by two-dimensional ELDOR-detected NMR spectroscopy

ELDOR (Electron Double Resonance)-detected NMR (EDNMR) is a pulse EPR experiment that is used to measure the transition frequencies of nuclear spins coupled to electron spins. These frequencies are further used to determine hyperfine and quadrupolar couplings, which are signatures of the electronic and spatial structures of paramagnetic centers. In recent years, EDNMR has been shown to be particularly useful at high fields/high frequencies, such as W-band (∼95GHz, ∼3.5T), for low γ quadrupolar nuclei. Although at high fields the nuclear Larmor frequencies are usually well resolved, the limited resolution of EDNMR still remains a major concern. In this work we introduce a two dimensional, triple resonance, correlation experiment based on the EDNMR pulse sequence, which we term 2D-EDNMR. This experiment allows circumventing the resolution limitation by spreading the signals in two dimensions and the observed correlations help in the assignment of the signals.First we demonstrate the utility of the 2D-EDNMR experiment on a nitroxide spin label, where we observe correlations between 14N nuclear frequencies. Negative cross-peaks appear between lines belonging to different MS electron spin manifolds. We resolved two independent correlation patterns for nuclear frequencies arising from the EPR transitions corresponding to the 14N mI=0 and mI=−1 nuclear spin states, which severely overlap in the one dimensional EDNMR spectrum. The observed correlations could be accounted for by considering changes in the populations of energy levels that S=1/2, I=1 spin systems undergo during the pulse sequence. In addition to these negative cross-peaks, positive cross-peaks appear as well. We present a theoretical model based on the Liouville equation and use it to calculate the time evolution of populations of the various energy levels during the 2D-EDNMR experiment and generated simulated 2D-EDMR spectra. These calculations show that the positive cross-peaks appear due to off resonance effects and/or nuclear relaxation effects. These results suggest that the 2D-EDNMR experiment can be also useful for relaxation pathway studies. Finally we present preliminary results demonstrating that 2D-EDNMR can resolve overlapping 33S and 14N signals of type 1 Cu(II) center in 33S enriched Azurin.

Dynamic nuclear polarization using frequency modulation at 3.34 T

During dynamic nuclear polarization (DNP) experiments polarization is transferred from unpaired electrons to their neighboring nuclear spins, resulting in dramatic enhancement of the NMR signals. While in most cases this is achieved by continuous wave (cw) irradiation applied to samples in fixed external magnetic fields, here we show that DNP enhancement of static samples can improve by modulating the microwave (MW) frequency at a constant field of 3.34T. The efficiency of triangular shaped modulation is explored by monitoring the 1H signal enhancement in frozen solutions containing different TEMPOL radical concentrations at different temperatures. The optimal modulation parameters are examined experimentally and under the most favorable conditions a threefold enhancement is obtained with respect to constant frequency DNP in samples with low radical concentrations. The results are interpreted using numerical simulations on small spin systems. In particular, it is shown experimentally and explained theoretically that: (i) The optimal modulation frequency is higher than the electron spin–lattice relaxation rate. (ii) The optimal modulation amplitude must be smaller than the nuclear Larmor frequency and the EPR line-width, as expected. (iii) The MW frequencies corresponding to the enhancement maxima and minima are shifted away from one another when using frequency modulation, relative to the constant frequency experiments.

Storage of nuclear magnetization as long-lived singlet order in low magnetic field

Hyperpolarized nuclear states provide NMR signals enhanced by many orders of magnitude, with numerous potential applications to analytical NMR, in vivo NMR, and NMR imaging. However, the lifetime of hyperpolarized magnetization is normally limited by the relaxation time constant T(1), which lies in the range of milliseconds to minutes, apart from in exceptional cases. In many cases, the lifetime of the hyperpolarized state may be enhanced by converting the magnetization into nuclear singlet order, where it is protected against many common relaxation mechanisms. However, all current methods for converting magnetization into singlet order require the use of a high-field, high-homogeneity NMR magnet, which is incompatible with most hyperpolarization procedures. We demonstrate a new method for converting magnetization into singlet order and back again. The new technique is suitable for magnetically inequivalent spin-pair systems in weak and inhomogeneous magnetic fields, and is compatible with known hyperpolarization technology. The method involves audio-frequency pulsed irradiation at the low-field nuclear Larmor frequency, employing coupling-synchronized trains of 180° pulses to induce singlet-triplet transitions. The echo trains are used as building blocks for a pulse sequence called M2S that transforms longitudinal magnetization into long-lived singlet order. The time-reverse of the pulse sequence, called S2M, converts singlet order back into longitudinal magnetization. The method is demonstrated on a solution of (15)N-labeled nitrous oxide. The magnetization is stored in low magnetic field for over 30 min, even though the T(1) is less than 3 min under the same conditions.

ENDOR Study on the Dynamic Properties of the First Stable Paramagnetic Center in γ-Irradiated l-Alanine Crystals

Dynamic properties of the first stable l-alanine radical, SAR1, induced by gamma-irradiation of l-alanine crystals, have been investigated by the electron nuclear double resonance technique (ENDOR). The study focuses on the dynamic properties of the alpha-proton hyperfine splitting in the temperature range from 180 to 320 K. In this region the motion of the NH(3)(+) and CH(3) groups exhibits slow and fast motional dynamics in comparison to the nuclear and electron Larmor frequencies, respectively. Evidence for different conformations of the SAR1 center is presented on the basis of thermodynamic properties of the alpha-hyperfine splitting. The activation processes causing the broadening of the ENDOR lines are studied. At room temperature the motional dynamics of the SAR1 center are modulated by the dynamics of the charged, neighboring NH(3)(+) group.

A System for NMR Stark Spectroscopy of Quadrupolar Nuclei

Electrostatic influences on NMR parameters are well accepted. Experimental and computational routes have been long pursued to understand and utilize such Stark effects. However, existing approaches are largely indirect informants on electric fields, and/or are complicated by multiple causal factors in spectroscopic change. We present a system to directly measure quadrupolar Stark effects from an applied electric (E) field. Our apparatus and applications are relevant in two contexts. Each uses a radiofrequency (rf) E field at twice the nuclear Larmor frequency (2omega(0)). The mechanism is a distortion of the E-field gradient tensor that is linear in the amplitude (E(0)) of the rf E field. The first uses 2omega(0) excitation of double-quantum transitions for times similar to T(1) (the longitudinal spin relaxation time). This perturbs the steady state distribution of spin population. Nonlinear analysis versus E(0) can be used to determine the Stark response rate. The second context uses POWER (perturbations observed with enhanced resolution) NMR. Here, coherent, short-time (<<T(2), the transverse relaxation rate) excitation at 2omega(0) is synchronized with an NMR multiple-pulse line-narrowing sequence. Linear analysis of the Stark response is then possible: a quadrupolar multiplet with splitting proportional to E(0). The POWER sequence converts the 2omega(0) interaction from off-diagonal/nonsecular to the familiar diagonal form (I(z)(2)) of static quadrupole interactions. Meanwhile, background contributions to line width are averaged to zero, providing orders-of-magnitude resolution enhancement for correspondingly high sensitivity to the Stark effect. Using GaAs as a test case with well-defined Stark response, we provide the first demonstration of the 2omega(0) effect at high-field (14.1 T) and room temperature. This, along with the simplicity of our apparatus and spectral approach, may facilitate extensions to a wider array of material and molecular systems. The POWER context, which has not previously been tested, is detailed here with new design insights. Several key aspects are demonstrated here, while complete implementation is to be presented at a later time. At present, we (1) account for finite pulse times in pulse sequence design, (2) demonstrate two-channel phase coherence for magnetic (omega(0)) and electric (2omega(0)) excitation, and (3) provide line narrowing by a factor of 10(3). In addition, we find that certain anomalous contributions to the line shape, observed in previous low-field (250 mT) applications, are absent here.

Direct Evidence for Nitrogen Ligation to the High Stability Semiquinone Intermediate in Escherichia coli Nitrate Reductase A

The membrane-bound heterotrimeric nitrate reductase A (NarGHI) catalyzes the oxidation of quinols in the cytoplasmic membrane of Escherichia coli and reduces nitrate to nitrite in the cytoplasm. The enzyme strongly stabilizes a menasemiquinone intermediate at a quinol oxidation site (Q(D)) located in the vicinity of the distal heme b(D). Here molecular details of the interaction between the semiquinone radical and the protein environment have been provided using advanced multifrequency pulsed EPR methods. (14)N and (15)N ESEEM and HYSCORE measurements carried out at X-band ( approximately 9.7 GHz) on the wild-type enzyme or the enzyme uniformly labeled with (15)N nuclei reveal an interaction between the semiquinone and a single nitrogen nucleus. The isotropic hyperfine coupling constant A(iso)((14)N) approximately 0.8 MHz shows that it occurs via an H-bond to one of the quinone carbonyl group. Using (14)N ESEEM and HYSCORE spectroscopies at a lower frequency (S-band, approximately 3.4 GHz), the (14)N nuclear quadrupolar parameters of the interacting nitrogen nucleus (kappa = 0.49, eta = 0.50) were determined and correspond to those of a histidine N(delta), assigned to the heme b(D) ligand His-66 residue. Moreover S-band (15)N ESEEM spectra enabled us to directly measure the anisotropic part of the nitrogen hyperfine interaction (T((15)N) = 0.16 MHz). A distance of approximately 2.2 Abetween the carbonyl oxygen and the nitrogen could then be calculated. Mechanistic implications of these results are discussed in the context of the peculiar properties of the menasemiquinone intermediate stabilized at the Q(D) site of NarGHI.

NMR spectroscopy in the milli-Tesla regime: Measurement of 1H chemical-shift differences below the line width

NMR spectroscopy for chemical analysis at high field employs precision measurements of resonance frequencies governed by chemical shift and nuclear spin interactions. At low field and in the absence of hetero-nuclear J-couplings the natural line width limits the chemical shift measurements. We have performed chemical-shift resolved proton NMR spectra in the milli-Tesla regime, and found that in the presence of hetero-nuclear J-coupling, proton chemical-shift differences smaller than the line width can be measured down to nuclear Larmor frequencies of 41 kHz. The measurement of J-couplings and chemical shifts with simple NMR devices operating in the milli-Tesla regime may develop as an attractive tool for chemical analysis.

1H Spin–Lattice Relaxation Study of Dynamical Inequivalence of Methyl Groups in Solid 1,2-O-(1-Ethylpropylidene)-α-d-Glucofuranose

We investigated the dynamics of methyl groups in organic polycrystalline 1,2-O-(1-ethylpropylidene)-α-d-glucofuranose by the proton spin–lattice relaxation method. The temperature and nuclear magnetic resonance Larmor frequency dependence of relaxation time is presented and interpreted in terms of simple possible dynamical model for the reorientation of methyl groups: the random hopping for methyl groups, which are in a, b, and c sites in the crystal. The energy E a of 13.5 kJ mol−1 for the a-type methyl groups is typical for methyl groups in ethyl groups. In contrast, the b- and c-methyl groups characterized by the lower E a values of 9.5 and 6.5 kJ mol−1 are located in the crystal structure where the intermolecular interactions significantly influence the potential, leading to a decrease in the total energy.

Pulse NMR of 3He in aerogel at temperature 1.5 K

Experimental data of 3He pulse NMR in contact with aerogel (95%) in a nuclear Larmor frequency range of 5 to 20 MHz at T=1.5 K are reported. For the first time spin kinetics of 3He (adsorbed, gaseous and liquid) has been studied systematically at temperatures higher than degeneracy temperature of this quantum liquid. It has been shown, that in all cases the nuclear spin-lattice relaxation time T1 is linearly proportional to the Larmor frequency, while the spin-spin relaxation time T2 is frequency independent. The magnetic relaxation times T1 and T2 are linearly proportional to the ratio of the magnetization of whole spin system to magnetization of the adsorbed layer of 3He.

High-frequency dynamic nuclear polarization using biradicals: A multifrequency EPR lineshape analysis

To date, the cross effect (CE) and thermal mixing (TM) mechanisms have consistently provided the largest enhancements in dynamic nuclear polarization (DNP) experiments performed at high magnetic fields. Both involve a three-spin electron-electron-nucleus process whose efficiency depends primarily on two electron-electron interactions--the interelectron distance R and the correct electron paramagnetic resonance (EPR) frequency separation that matches the nuclear Larmor frequency, /omega(e2)-omega(e1)/ = omega(n). Biradicals, for example, two 2,2,6,6-tetramethyl-piperidine-1-oxyls (TEMPOs) tethered with a molecular linker, can in principle constrain both the distance and relative g-tensor orientation between two unpaired electrons, allowing these two spectral parameters to be optimized for the CE and TM. To verify this hypothesis, we synthesized a series of biradicals--bis-TEMPO tethered by n ethylene glycol units (a.k.a. BTnE)--that show an increasing DNP enhancement with a decreasing tether length. Specifically at 90 K and 5 T, the enhancement grew from approximately 40 observed with 10 mM monomeric TEMPO, where the average R approximately 56 A corresponding to electron-electron dipolar coupling constant omega(d)2 pi = 0.3 MHz, to approximately 175 with 5 mM BT2E (10 mM electrons) which has R approximately 13 A with omega(d)2 pi = 24 MHz. In addition, we compared these DNP enhancements with those from three biradicals having shorter and more rigid tethers-bis-TEMPO tethered by oxalyl amide, bis-TEMPO tethered by the urea structure, and 1-(TEMPO-4-oxyl)-3-(TEMPO-4-amino)-propan-2-ol (TOTAPOL) TOTAPOL is of particular interest since it is soluble in aqueous media and compatible with DNP experiments on biological systems such as membrane and amyloid proteins. The interelectron distances and relative g-tensor orientations of all of these biradicals were characterized with an analysis of their 9 and 140 GHz continuous-wave EPR lineshapes. The results show that the largest DNP enhancements are observed with BT2E and TOTAPOL that have shorter tethers and the two TEMPO moieties are oriented so as to satisfy the matching condition for the CE.

ENDOR study of the dynamic properties of stable paramagnetic centres in γ -irradiated L-alanine crystals

The dynamic properties of the stable L-alanine radicals SAR1 and SAR2 induced by γ-irradiation of a L-alanine crystal have been investigated using the electron nuclear double resonance technique (ENDOR). The study was focused on the dynamic properties of these centres in the temperature range from 180 to 320 K. In this region, the motion of the and CH3 groups exhibits slow and fast motional dynamics in comparison with the nuclear and electron Larmor frequencies, respectively. The correlation rotation times of the CH3 and groups of the SAR2 CH3 group and the neighbouring CH3 group of the SAR1 paramagnetic centre were estimated from the spectral line broadening and spectral intensity.

Microstructure and texture of cementitious porous materials

We have characterized the microstructure of different cementitious materials (white and Portland cement pastes, mortars, concretes) by different magnetic resonance techniques. In particular, we show how the measurement of proton nuclear magnetic spin-lattice relaxation as a function of magnetic field strength (and hence nuclear Larmor frequency) can provide reliable information on the dynamics of proton species at the surface of CSH, the specific surface area and the pore size distribution throughout the progressive hydration of cement-based materials. The measurement does not require any drying temperature modification and is sufficiently fast to be applied continuously during the progressive hydration of the material. Coupling this method with the standard proton nuclear spin relaxation and high-resolution NMR allows us to follow the development of microscale texture within the material.

Microstructure and texture of hydrated cement-based materials: A proton field cycling relaxometry approach

We show how the measurement of proton nuclear magnetic spin-lattice relaxation as a function of magnetic field strength (and hence nuclear Larmor frequency) can provide reliable information on the microstructure (specific surface area and pore size distribution) throughout the progressive hydration of cement-based materials. We present in details the experimental and theoretical characteristic features of the relaxation dispersion to support an interpretation in terms of coupled solid–liquid relaxation at pore interfaces, surface diffusion, and nuclear paramagnetic relaxation. The measurement does not require any drying temperature modification and is sufficiently fast to be applied continuously during the progressive hydration of the material. Coupling this method with the standard proton nuclear spin relaxation and high resolution NMR allows us to follow the development of micro-scale texture within the material.

Inhomogeneous Electronic Structure Probed by Spin-Echo Experiments in the Electron Doped High-TcSuperconductorPr1.85Ce0.15CuO4−y

63Cu nuclear magnetic resonance spin-echo decay rate (T-12) measurements are reported for the normal and superconducting states of a single crystal of Pr(1.85)Ce(0.15)CuO(4-y) in a magnetic field B(0)=9 T over the temperature range 2<T<200 K. The spin-echo decay rate is temperature dependent for T<55 K and has a substantial dependence on the radio frequency (rf) pulse parameters below T approximately 25 K. This dependence indicates that T-12 is strongly effected by a local magnetic field distribution that can be modified by the rf pulses, including ones that are not at the nuclear Larmor frequency. The low-temperature results are consistent with the formation of a static inhomogeneous electronic structure that couples to the rf fields of the pulses.