Pulsed Dynamic Nuclear Polarization Research Articles

Pulsed Dynamic Nuclear Polarization with Trityl Radicals.

Continuous-wave (CW) dynamic nuclear polarization (DNP) is now established as a method of choice to enhance the sensitivity in a variety of NMR experiments. Nevertheless, there remains a need for the development of more efficient methods to transfer polarization from electrons to nuclei. Of particular interest are pulsed DNP methods because they enable a rapid and efficient polarization transfer that, in contrast with CW DNP methods, is not attenuated at high magnetic fields. Here we report nuclear spin orientation via electron spin-locking (NOVEL) experiments using the polarizing agent trityl OX063 in glycerol/water at a temperature of 80 K and a magnetic field of 0.34 T. (1)H NMR signal enhancements up to 430 are observed, and the buildup of the local polarization occurs in a few hundred nanoseconds. Thus, NOVEL can efficiently dynamically polarize (1)H atoms in a system that is of general interest to the solid-state DNP NMR community. This is a first, important step toward the general application of pulsed DNP at higher fields.

Frequency swept microwaves for hyperfine decoupling and time domain dynamic nuclear polarization

Hyperfine decoupling and pulsed dynamic nuclear polarization (DNP) are promising techniques to improve high field DNP NMR. We explore experimental and theoretical considerations to implement them with magic angle spinning (MAS). Microwave field simulations using the high frequency structural simulator (HFSS) software suite are performed to characterize the inhomogeneous phase independent microwave field throughout a 198GHz MAS DNP probe. Our calculations show that a microwave power input of 17W is required to generate an average EPR nutation frequency of 0.84MHz. We also present a detailed calculation of microwave heating from the HFSS parameters and find that 7.1% of the incident microwave power contributes to dielectric sample heating. Voltage tunable gyrotron oscillators are proposed as a class of frequency agile microwave sources to generate microwave frequency sweeps required for the frequency modulated cross effect, electron spin inversions, and hyperfine decoupling. Electron spin inversions of stable organic radicals are simulated with SPINEVOLUTION using the inhomogeneous microwave fields calculated by HFSS. We calculate an electron spin inversion efficiency of 56% at a spinning frequency of 5kHz. Finally, we demonstrate gyrotron acceleration potentials required to generate swept microwave frequency profiles for the frequency modulated cross effect and electron spin inversions.

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.

A peripheral component interconnect express-based scalable and highly integrated pulsed spectrometer for solution state dynamic nuclear polarization

High sensitivity, high data rates, fast pulses, and accurate synchronization all represent challenges for modern nuclear magnetic resonance spectrometers, which make any expansion or adaptation of these devices to new techniques and experiments difficult. Here, we present a Peripheral Component Interconnect Express (PCIe)-based highly integrated distributed digital architecture pulsed spectrometer that is implemented with electron and nucleus double resonances and is scalable specifically for broad dynamic nuclear polarization (DNP) enhancement applications, including DNP-magnetic resonance spectroscopy/imaging (DNP-MRS/MRI). The distributed modularized architecture can implement more transceiver channels flexibly to meet a variety of MRS/MRI instrumentation needs. The proposed PCIe bus with high data rates can significantly improve data transmission efficiency and communication reliability and allow precise control of pulse sequences. An external high speed double data rate memory chip is used to store acquired data and pulse sequence elements, which greatly accelerates the execution of the pulse sequence, reduces the TR (time of repetition) interval, and improves the accuracy of TR in imaging sequences. Using clock phase-shift technology, we can produce digital pulses accurately with high timing resolution of 1 ns and narrow widths of 4 ns to control the microwave pulses required by pulsed DNP and ensure overall system synchronization. The proposed spectrometer is proved to be both feasible and reliable by observation of a maximum signal enhancement factor of approximately -170 for (1)H, and a high quality water image was successfully obtained by DNP-enhanced spin-echo (1)H MRI at 0.35 T.

An apparatus for pulsed ESR and DNP experiments using optically excited triplet states down to liquid helium temperatures

In standard Dynamic Nuclear Polarization (DNP) electron spins are polarized at low temperatures in a strong magnetic field and this polarization is transferred to the nuclear spins by means of a microwave field. To obtain high nuclear polarizations cryogenic equipment reaching temperatures of 1K or below and superconducting magnets delivering several Tesla are required. This equipment strongly limits applications in nuclear and particle physics where beams of particles interact with the polarized nuclei, as well as in neutron scattering science. The problem can be solved using short-lived optically excited triplet states delivering the electron spin. The spin is polarized in the optical excitation process and both the cryogenic equipment and magnet can be simplified significantly. A versatile apparatus is described that allows to perform pulsed dynamic nuclear polarization experiments at X-band using short-lived optically excited triplet sates. The efficient 4He flow cryostat that cools the sample to temperatures between 4K and 300K has an optical access with a coupling stage for a fiber transporting the light from a dedicated laser system. It is further designed to be operated on a neutron beam. A combined pulse ESR/DNP spectrometer has been developed to observe and characterize the triplet states and to perform pulse DNP experiments. The ESR probe is based on a dielectric ring resonator of 7mm inner diameter that can accommodate cubic samples of 5mm length needed for neutron experiments. NMR measurements can be performed during DNP with a coil integrated in the cavity. With the presented apparatus a proton polarization of 0.5 has been achieved at 0.3T.

Theory of the Overhauser effect in the pulsed mode of EPR pumping: exploiting the advantages of coherent electron spin motion

A theoretical approach is proposed to describe Overhauser-type Dynamic Nuclear Polarization (DNP) for pulsed EPR pumping by application of a train of short pulses with a duration on the nanosecond time scale. We obtained an elegant general expression for the NMR enhancement provided by the DNP effect. The expression for the enhancement is similar to that known for cw-pumping except for the saturation factor, which is re-defined as the deviation of the electron spin magnetization from its equilibrium value averaged over the cycle of the pulse sequence. It is shown that one can achieve the maximal theoretically allowed NMR enhancement for pulsed pumping even when the duty cycle of pumping is low. This becomes possible because coherent motion of the electron spins in the B(1)-field is exploited, a key feature of the pulsed DNP experiment also enabling optimization of the achievable NMR enhancement. The dependence of the effect on the duty cycle, pulse duration and electron spin relaxation times has been studied in detail. Once the lines in the EPR spectrum are inhomogeneously broadened, higher DNP effects are expected in the pulsed pumping mode than in the cw-mode for the same total power of microwave irradiation. The theoretical results are in good agreement with experimental data obtained for the pumping frequencies of 300 MHz and 1.4 GHz.

Low-Field, Time-Resolved Dynamic Nuclear Polarization with Field Cycling and High-Resolution NMR Detection

Dynamic nuclear polarization (DNP) effects in aqueous solutions of stable 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) radicals were studied in a pulsed mode of pumping the electron paramagnetic resonance (EPR) transitions. Our fast field cycling setup allowed us to perform the EPR pumping at low magnetic fields and to detect the enhanced nuclear magnetic resonance signals at 7 T with high spectral resolution. Pumping was performed at two different frequencies, 300 MHz and 1.4 GHz, corresponding to magnetic fields around 10 and 48.6 mT, respectively. For both frequencies, the DNP enhancements were close to the limiting theoretical values of −110 (14N TEMPOL) and −165 (15N TEMPOL). Our pulsed experiments exploit coherent motion of the electronic spins in the radio-frequency field as seen by an oscillatory component in the dependence of the DNP effect on the radio-frequency pulse duration. The DNP enhancement was studied in detail as a function of the pulse length, duty cycle, delay between the pulses, and applied power. The analysis of the results shows that pulsed DNP experiments provide an opportunity to achieve enhancements of about −110 with relatively low applied power as compared to the standard continuous-wave DNP experiments. An adequate theoretical approach to the problem under study is suggested.

Cross-Relaxation in Electron−Nuclear Coupled Systems by Pulsed Dynamic Nuclear Polarization at Low Magnetic Fields

A new method for the experimental determination of the cross-relaxation (CR) transition rates in liquid solutions of paramagnetic compounds has been developed utilizing a pulsed dynamic nuclear polarization (DNP) technique. In contrast to NMR relaxation, which is proportional to the sum of relaxation rates, the DNP effect is determined by the ratio of transition rates in the nucleus−electron coupled spin system. By use of pulsed DNP, the NMR relaxation rates and DNP data can be obtained in the same experiment. As a result, a set of independent equations for CR and dipole−dipole (DD) transition rates can be derived. The solution of these equations defines individual cross-relaxation and DD transition rates, as well as molecular−kinetic information, and avoids the necessity of performing complicated variable frequency and temperature measurements. A pulsed DNP relaxometer operating at a proton frequency of 0.5 MHz was constructed. The 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPONE) stable free radical in benzene was chosen for study as a system with strong intermolecular dipole−dipole interactions. The measurement of individual rates of CR and DD transitions gave us the distance (b = 5.0 Å) between spins I and S for the solvated radical of TEMPONE and also the molecular rotational correlation time (τr = 2.5 × 10-10 s). Another system studied was the solvated electron in hexamethylphosphorus amide (HMPA) in which scalar and DD interactions are present at the same time. The basic characteristics of this fundamentally important elementary paramagnetic center have been obtained, such as the lifetime of HMPA molecules in the solvent cage (τh = 2.9 × 10-9 s) and the intermolecular hyperfine constant value (a = 0.08 MHz). Pulsed DNP is shown to be a valuable approach for the study of very weak hyperfine interactions that are not readily detected by other traditional magnetic resonance methods.

Pulsed dynamic nuclear polarization at 5 T

A new dynamic nuclear polarization (DNP) technique utilizing high frequency, pulsed microwaves is described and demonstrated with observation of an Overhauser enhanced, 13C magic angle spinning nuclear magnetic resonance spectrum of a powdered sample of the organic conductor, fluoranthenyl hexafluorophosphate ((FA) 2PF 6). The spectrum was obtained at room temperature in a 5 T magnetic field, where the conduction electron spin resonance frequency is 140 GHz. A simple theory that accounts for the salient aspects of this pulsed DNP technique is presented. In systems where Overhauser mechanisms dominate the DNP, this new pulsed method can produce enhancements which are comparable to or greater than those obtained with conventional cw irradiation techniques, with lower average microwaves powers.

High dynamic nuclear polarization at room temperature

The highly polarized photo-excited triplet state of pentacene in a naphthalene crystal is used for pulsed dynamic nuclear polarization at room temperature. Thus far an enhancement of 5500 of the naphthalene proton polarization has been reached. For this purpose, a newly developed technique, the integrated solid effect, performed while obeying the Hartmann-Hahn condition, is used to transfer the triplet polarization efficiently to the nuclear spin system.