Overhauser Mechanism Research Articles

Sample volume effects in optical overhauser dynamic nuclear polarization

The optical dynamic nuclear polarization (DNP) method has been proposed as an alternative to microwave pumping as a hyperpolarization method for solution-state NMR studies. Using continuous laser illumination to photogenerate triplet states in the presence of a persistent radical produces chemically-induced dynamic electron polarization (CIDEP) via the radical-triplet pair mechanism (RTPM), with cross-relaxation transferring this to nuclear hyperpolarization via an Overhauser mechanism. Numerical simulations have previously indicated that reducing the sample volume while maintaining a constant optical density can significantly increase the NMR signal enhancement, due to the larger steady-state concentration of triplets obtained. Here we provide the first experimental confirmation of these effects, producing a nearly five-fold increase in the optical DNP enhancement factor just by reducing the sample volume with optimal dye and radical concentrations adjusted for each optical path length. The results are supported with an in depth analysis of volume effects in the numerical model, with which they are in good qualitative agreement. These important observations will impact on the future development of the technique, with particular significance for attempts to apply DNP methods to increase sensitivity for volume-limited biological samples.

Solid-like Dynamic Nuclear Polarization Observed in the Fluid Phase of Lipid Bilayers at 9.4 T.

Dynamic nuclear polarization (DNP) is a powerful method to enhance NMR sensitivity. Much progress has been achieved recently to optimize DNP performance at high magnetic fields in solid-state samples, mostly by utilizing the solid or the cross effect. In liquids, only the Overhauser mechanism is active, which exhibits a DNP field profile matching the EPR line shape of the radical, distinguishable from other DNP mechanisms. Here, we observe DNP enhancements with a field profile indicative of the solid effect and thermal mixing at ∼320 K and a magnetic field of 9.4 T in the fluid phase of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers doped with the radical BDPA (1,3-bis(diphenylene)-2-phenylallyl). This interesting observation might open up new perspectives for DNP applications in macromolecular systems at ambient temperatures.

A table-top PXI based low-field spectrometer for solution dynamic nuclear polarization.

We present the development of a portable dynamic nuclear polarization (DNP) instrument based on the PCI eXtensions for Instrumentation platform. The main purpose of the instrument is for study of 1 H polarization enhancements in solution through the Overhauser mechanism at low magnetic fields. A DNP probe set was constructed for use at 6.7mT, using a modified Alderman-Grant resonator at 241MHz for saturation of the electron transition. The solenoid for detection of the enhanced 1 H signal at 288kHz was constructed with Litz wire. The largest observed 1 H enhancements (ε) at 6.7mT for 14 N-CTPO radical in air saturated aqueous solution was ε~65. A concentration dependence of the enhancement is observed, with maximum ε at 5.5mM. A low resonator efficiency for saturation of the electron paramagnetic resonance transition results in a decrease in ε for the 10.3mM sample. At high incident powers (42W) and long pump times, capacitor heating effects can also decrease the enhancement. The core unit and program described here could be easily adopted for multi-frequency DNP work, depending on available main magnets and selection of the "plug and play" arbitrary waveform generator, digitizer, and radiofrequency synthesizer PCI eXtensions for Instrumentatione cards.

Nanodiamond-enhanced MRI via in situ hyperpolarization

Nanodiamonds are of interest as nontoxic substrates for targeted drug delivery and as highly biostable fluorescent markers for cellular tracking. Beyond optical techniques, however, options for noninvasive imaging of nanodiamonds in vivo are severely limited. Here, we demonstrate that the Overhauser effect, a proton–electron polarization transfer technique, can enable high-contrast magnetic resonance imaging (MRI) of nanodiamonds in water at room temperature and ultra-low magnetic field. The technique transfers spin polarization from paramagnetic impurities at nanodiamond surfaces to 1H spins in the surrounding water solution, creating MRI contrast on-demand. We examine the conditions required for maximum enhancement as well as the ultimate sensitivity of the technique. The ability to perform continuous in situ hyperpolarization via the Overhauser mechanism, in combination with the excellent in vivo stability of nanodiamond, raises the possibility of performing noninvasive in vivo tracking of nanodiamond over indefinitely long periods of time.

ChemInform Abstract: Perspectives on DNP‐Enhanced NMR Spectroscopy in Solutions

AbstractReview: 81 refs.

Towards Overhauser DNP in supercritical CO2

Overhauser Dynamic Nuclear Polarization (ODNP) is a well known technique to improve NMR sensitivity in the liquid state, where the large polarization of an electron spin is transferred to a nucleus of interest by cross-relaxation. The efficiency of the Overhauser mechanism for dipolar interactions depends critically on fast local translational dynamics at the timescale of the inverse electron Larmor frequency. The maximum polarization enhancement that can be achieved for 1H at high magnetic fields benefits from a low viscosity solvent. In this paper we investigate the option to use supercritical CO2 as a solvent for Overhauser DNP. We have investigated the diffusion constants and longitudinal nuclear relaxation rates of toluene in high pressure CO2. The change in 1H T1 by addition of TEMPO radical was analyzed to determine the Overhauser cross-relaxation in such a mixture, and is compared with calculations based on the Force Free Hard Sphere (FFHS) model. By analyzing the relaxation data within this model we find translational correlation times in the range of 2–4ps, depending on temperature, pressure and toluene concentration. Such short correlation times may be instrumental for future Overhauser DNP applications at high magnetic fields, as are commonly used in NMR. Preliminary DNP experiments have been performed at 3.4T on high pressure superheated water and model systems such as toluene in high pressure CO2.

Perspectives on DNP-enhanced NMR spectroscopy in solutions

More than 60years after the seminal work of Albert Overhauser on dynamic nuclear polarization by dynamic cross relaxation of coupled electron–nuclear spin systems, the quest for sensitivity enhancement in NMR spectroscopy is as pressing as ever. In this contribution we will review the status and perspectives for dynamic nuclear polarization in the liquid state. An appealing approach seems to be the use of supercritical solvents that may allow an extension of the Overhauser mechanism towards common high magnetic fields. A complementary approach is the use of solid state DNP on frozen solutions, followed by a rapid dissolution or in-situ melting step and NMR detection with substantially enhanced polarization levels in the liquid state. We will review recent developments in the field and discuss perspectives for the near future.

High-field Overhauser dynamic nuclear polarization in silicon below the metal–insulator transition

Single crystal silicon is an excellent system to explore dynamic nuclear polarization (DNP), as it exhibits a continuum of properties from metallic to insulating as a function of doping concentration and temperature. At low doping concentrations DNP has been observed to occur via the solid effect, while at very high-doping concentrations an Overhauser mechanism is responsible. Here we report the hyperpolarization of (29)Si in n-doped silicon crystals, with doping concentrations in the range of (1-3) × 10(17) cm(-3). In this regime exchange interactions between donors become extremely important. The sign of the enhancement in our experiments and its frequency dependence suggest that the (29)Si spins are directly polarized by donor electrons via an Overhauser mechanism within exchange-coupled donor clusters. The exchange interaction between donors only needs to be larger than the silicon hyperfine interaction (typically much smaller than the donor hyperfine coupling) to enable this Overhauser mechanism. Nuclear polarization enhancement is observed for a range of donor clusters in which the exchange energy is comparable to the donor hyperfine interaction. The DNP dynamics are characterized by a single exponential time constant that depends on the microwave power, indicating that the Overhauser mechanism is a rate-limiting step. Since only about 2% of the silicon nuclei are located within 1 Bohr radius of the donor electron, nuclear spin diffusion is important in transferring the polarization to all the spins. However, the spin-diffusion time is much shorter than the Overhauser time due to the relatively weak silicon hyperfine coupling strength. In a 2.35 T magnetic field at 1.1 K, we observed a DNP enhancement of 244 ± 84 resulting in a silicon polarization of 10.4 ± 3.4% following 2 h of microwave irradiation.

Dynamic nuclear polarization ofZn67andH1spins by means of shallow donors in ZnO nanoparticles

Dynamic nuclear polarization (DNP) effects are observed of $^{67}\text{Zn}$ $(I=5/2)$ nuclear spins in ZnO nanoparticles and of $^{1}\text{H}$ $(I=1/2)$ spins of the $\text{Zn}{(\text{OH})}_{2}$ capping layer. The almost complete polarization of these nuclear spins is achieved by saturating the electron paramagnetic resonance transition of the shallow interstitial Li donor present in the ZnO nanoparticles. The remarkable aspect is that this DNP is caused by an Overhauser mechanism although the phonons mediating the polarization process do not fit into the nanoparticles. An explanation of this DNP process is presented, and it is shown that this allows for a measurement of the distribution of phonon modes in the nanoparticles. The enhancement of the nuclear polarization also opens the possibility to study semiconductor nanostructures with NMR techniques.

Spin-labeled gel for the production of radical-free dynamic nuclear polarization enhanced molecules for NMR spectroscopy and imaging

Dynamic nuclear polarization (DNP) has recently received much attention as a viable approach to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and the contrast of magnetic resonance imaging (MRI), where the significantly higher electron spin polarization of stable radicals is transferred to nuclear spins. In order to apply DNP-enhanced NMR and MRI signal to biological and in vivo systems, it is crucial to obtain highly polarized solution samples at ambient temperatures. As stable radicals are employed as the source for the DNP polarization transfer, it is also crucial that the highly polarized sample lacks residual radical concentration because the polarized molecules will be introduced to a biological system that will be sensitive to the presence of radicals. We developed an agarose-based porous media that is covalently spin-labeled with stable radicals. The loading of solvent accessible radical is sufficiently high and their mobility approximates that in solution, which ensures high efficiency for Overhauser mechanism induced DNP without physically releasing any measurable radical into the solution. Under ambient conditions at 0.35 T magnetic field, we measure the DNP enhancement efficiency of 1H signal of stagnant and continuously flowing water utilizing immobilized stable nitroxide radicals that contain two or three ESR hyperfine splitting lines and compare them to the performance of freely dissolved radicals.

Adiabatic transfer of net electron polarization to nuclear polarization

A new type of spin chemical experiment is suggested which utilizes adiabatic reversion of a low magnetic B 0 field for an efficient transfer of net electron spin polarization, as arising from some CIDEP mechanism, to the nuclear polarization of radicals. After trapping the radicals in diamagnetic products and transferring the sample to a NMR probe, the adiabatic polarization transfer effect can be measured as induced CIDNP, as in the recently established SNP method. The optimum conditions concerning delay time and duration of the B 0 field switching, as well as the trapping rate constant are analysed quantitatively for a radical with one nuclear spin I = 1/2. Owing to their efficiency and some parameters that can be controlled in the experiment, adiabatic polarization transfer effects should have clear advantages over experiments relying on DNP effects due to the Overhauser mechanism.

Magnetic field and polarized light dependence of chemically induced dynamic nuclear polarization (CIDNP) of acridines and quinoxalines in various solvents

Acridine and quinoxalines exhibits nuclear polarizations during irradiation in various solvents. In order to elucidate the origin of these polarizations, we have studied the magnetic field and polarized light dependence of the nuclear polarizations in different solvents. The results are interpreted in terms of two different mechanisms. The effects observed in proton-donor solvents are ascribed to a redical-pair mechanism with contribution from what could be a triplet Overhauser mechanism. This last mechanism explains the polarizations observed in benzene solution.

Magnetic field and concentration dependence of CIDNP in some quinone photolyses: Further evidence for an Overhauser mechanism

A study has been made of the magnetic field dependence of chemically induced nuclear spin polarization (CIDNP) of fluorine nuclei in the photolysis of tetrafluoro-1,4-benzoquinone and tetrafluoro-1,4-hydroquinone in benzene. Also studied was the benzoquinone concentration dependence of proton CIDNP in the photolysis of benzoquinone in CDCl3. These studies are compared with the theory of an Overhauser mechanism of CIDNP in which the photoexcited triplet mechanism produces electron-spin-polarized semiquinone radicals whose polarization is transferred to the nuclear spins by electron–nuclear cross relaxation. The results support the conclusion, previously reached for the tetrafluoro-1,4-benzoquinone case on the basis of other evidence, that this Overhauser mechanism contributes to CIDNP in these systems.

On the overhauser mechanism of chemically induced nuclear polarization as suggested by Adrian

The recently proposed Overhauser mechanism of nuclear polarization for certain types of reaction is compared with the conventional radical pair mechanism. It is concluded that both mechanisms will work in these reactions if the lifetimes of the free radicals are of the same order of magnitude or longer than the nuclear T 1.

A possible overhauser mechanism for 19F nuclear spin polarization in the reaction of fluorobenzyl halides with sodium naphthalene

It is proposed that the high field 19F nuclear spin polarization observed in the product of the reaction of fluorobenzyl halides with sodium naphthalene is not due to the usual radical pair mechanism, but rather to a build up of electron spin polarization in the intermediate benzyl radical combined with electron—nuclear spin cross relaxation.

Contact hyperfine interaction and induced dynamic nuclear spin polarization

Theoretical CIDNP spectra for products formed via radical pair reactions were obtained by using the transverse Overhauser mechanism proposed by Closs and the further constraint that the saturated states have their nuclear spin configurations properly polarized according to the sign of the electron spin densities.