Dynamic nuclear polarization assisted spin diffusion for the solid effect case

The dynamic nuclear polarization (DNP) process in solids depends on the magnitudes of hyperfine interactions between unpaired electrons and their neighboring (core) nuclei, and on the dipole-dipole interactions between all nuclei in the sample. The polarization enhancement of the bulk nuclei has been typically described in terms of a hyperfine-assisted polarization of a core nucleus by microwave irradiation followed by a dipolar-assisted spin diffusion process in the core-bulk nuclear system. This work presents a theoretical approach for the study of this combined process using a density matrix formalism. In particular, solid effect DNP on a single electron coupled to a nuclear spin system is considered, taking into account the interactions between the spins as well as the main relaxation mechanisms introduced via the electron, nuclear, and cross-relaxation rates. The basic principles of the DNP-assisted spin diffusion mechanism, polarizing the bulk nuclei, are presented, and it is shown that the polarization of the core nuclei and the spin diffusion process should not be treated separately. To emphasize this observation the coherent mechanism driving the pure spin diffusion process is also discussed. In order to demonstrate the effects of the interactions and relaxation mechanisms on the enhancement of the nuclear polarization, model systems of up to ten spins are considered and polarization buildup curves are simulated. A linear chain of spins consisting of a single electron coupled to a core nucleus, which in turn is dipolar coupled to a chain of bulk nuclei, is considered. The interaction and relaxation parameters of this model system were chosen in a way to enable a critical analysis of the polarization enhancement of all nuclei, and are not far from the values of (13)C nuclei in frozen (glassy) organic solutions containing radicals, typically used in DNP at high fields. Results from the simulations are shown, demonstrating the complex dependences of the DNP-assisted spin diffusion process on variations of the relevant parameters. In particular, the effect of the spin lattice relaxation times on the polarization buildup times and the resulting end polarization are discussed, and the quenching of the polarizations by the hyperfine interaction is demonstrated.     

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It has been shown that the presence of magnetic nuclei with sufficiently high hfi constants in radicals can affect the CIDNP of the other nuclei so that the spin polarization sign can alter and become opposite to that predicted by the Kaptein rules. This mutual nuclear effect on the CIDNP reduces with increasing external field strength and radical acceptor concentrations.     

Dynamic polarization of solvent nuclei in solutions of diphenylpicrylhydrazyl in organic liquids

Measurements have been made of the dynamic polarization of nuclei of solvent in concentrated solutions of diphenylpicrylhydrazyl (DPPG) in a weak magnetic field of approximately 21 Oe. In proton-containing solutions, a negative dynamic polarization was observed, corresponding to dipole-dipole interaction of electronic and nuclear spins. In a hexafluorobenzene solution a considerable contribution of scalar interaction to the relaxation of the fluorine nucleus was observed. The temperature dependence of the dynamic polarization of the fluorine nucleus has been investigated, and an activation energy of 3.3 kcal/mole has been found for the bond between the molecules of hexafluorobenzene and the solvate shell of DPPH molecules.     

Thermodynamics of adiabatic cross polarization

Using a spin-temperature approach, we describe a scheme of adiabatic cross polarization, based on demagnetization/remagnetization, when the Zeeman order of abundant nuclei in the laboratory frame is first adiabatically converted into the dipolar order, and then, into the Zeeman order of rare nuclei. The scheme, implemented with two low-power frequency-sweeping pulses, is very efficient for static samples and can significantly increase polarization of rare nuclei, compared to the conventional Hartmann-Hahn cross polarization. The experimental examples are presented for a solid, liquid crystal, and small molecules in a liquid-crystalline solvent.     

Preparation of highly polarized nuclear spin systems using brute-force and low-field thermal mixing

Over the years, several strategies have been developed for generating highly polarized nuclear spin systems, including dynamic nuclear polarization, optical pumping, and methods exploiting parahydrogen. Here, we present an alternative strategy, using an enhanced 'brute-force' approach (i.e. exposure to low temperatures and high applied magnetic fields). The main problem with this approach is that it may take an excessively long time for the nuclear polarization to approach thermal equilibrium at low temperatures, since nuclear relaxation becomes exceedingly slow due to the loss of molecular motion. We show that low-field thermal mixing can alleviate the problem by increasing the rate at which slowly-relaxing nuclei reach equilibrium. More specifically, we show that polarization can be transferred from a relatively rapidly relaxing (1)H reservoir to more slowly relaxing (13)C and (31)P nuclei. The effects are particularly dramatic for the (31)P nuclei, which in experiments at a temperature of 4.2 K and a field of 2 T show a 75-fold enhancement in their effective rate of approach to equilibrium, and an even greater (150-fold) enhancement in the presence of a relaxation agent. The mixing step is also very effective in terms of the amount of polarization transferred-70-90% of the maximum theoretical value in the experiments reported here. These findings have important implications for brute-force polarization, for the problem becomes one of how to relax the solvent protons rather than individual more slowly-relaxing nuclei of interest. This should be a much more tractable proposition, and offers the additional attraction that a wide range of nuclear species can be polarized simultaneously. We further show that the (1)H reservoir can be tapped repeatedly through a number of consecutive thermal mixing steps, and that this could provide additional sensitivity enhancement in solid-state NMR.     

Solid effect dynamic nuclear polarization and polarization pathways

Using dynamic nuclear polarization (DNP)/nuclear magnetic resonance instrumentation that utilizes a microwave cavity and a balanced rf circuit, we observe a solid effect DNP enhancement of 94 at 5 T and 80 K using trityl radical as the polarizing agent. Because the buildup rate of the solid effect increases with microwave field strength, we obtain a sensitivity gain of 128. The data suggest that higher microwave field strengths would lead to further improvements in sensitivity. In addition, the observation of microwave field dependent enhancements permits us to draw conclusions about the path that polarization takes during the DNP process. By measuring the time constant for the polarization buildup and enhancement as a function of the microwave field strength, we are able to compare models of polarization transfer, and show that the major contribution to the bulk polarization arises via direct transfer from electrons, rather than transferring first to nearby nuclei and then transferring to bulk nuclei in a slow diffusion step. In addition, the model predicts that nuclei near the electron receive polarization that can relax, decrease the electron polarization, and attenuate the DNP enhancement. The magnitude of this effect depends on the number of near nuclei participating in the polarization transfer, hence the size of the diffusion barrier, their T(1), and the transfer rate. Approaches to optimizing the DNP enhancement are discussed.     

Optical detection of nuclear spin polarization via cross relaxation in A p-dibromobenzene single crystal

Spin polarization of ⁸¹Br (I = $\text{3}\text{2}$) nuclei is achieved via cross relaxation between electronic spins of the excited triplet state of a quinoxaline guest molecule and nuclei on neighbouring molecules in a p-dibromobenzene host crystal. The cross relaxation rate is of the order of 10⁶ s^?1 and is driven by the intermolecular hyperfine interaction. Additionally, NQR transitions have been induced in the single ground state and have been optically detected by means of an optical pumping cycle involving nuclear spin polarization.     

13C PHIP NMR spectra and polarization transfer during the homogeneous hydrogenation of alkynes with parahydrogen

Homogeneously catalyzed hydrogenations of unsaturated substrates with parahydrogen not only lead to strong polarization signals in ¹H NMR spectra, but also can give rise to strong heteronuclear polarization, especially if the hydrogenations are carried out in low magnetic fields. As a typical example, the polarization transfer from protons to carbon nuclei during the hydrogenation of alkynes is outlined for several substrates. In systems containing easily accessible triple bonds, e.g. phenylethyne or 3,3‐dimethyl‐1‐butyne, polarization transfer occurs to all carbon nuclei in the molecule. Accordingly, in NMR spectra recorded in situ all ¹³C resonances can be observed with good to excellent signal‐to‐noise ratios using only a single transient. The qualitative influence of symmetry and electronic aspects of the substrate and its hydrogenation product on the efficiency of the transfer of polarization to the ¹³C‐nuclei are discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Elementary Stages of Photochemical Reactions of Substituted 2,6-Diphenyl-<Emphasis Type="Italic">para</Emphasis>-benzoquinones with Thiols

Using the effects of chemical polarization of nuclei, mechanism and elementary stages of photochemical reduction of 2,6-diphenyl-1,4-benzoquinone with thiols have been elucidated. The studied quinone is reduced with ethanethiol in two stages (transfer of electron followed by proton transfer). The final photolysis product is a derivative of dibenzofuran.     

DNP sensitivity of 19F‐NMR signals in hexafluorobenzene depending on polarizing agent type

Low field dynamic nuclear polarization or low field magnetic double resonance technique enables enhanced nuclear magnetic resonance signals to be detected without increasing the strength of the polarizing field. The study reports that the dynamic nuclear polarization of ¹⁹F nuclei in hexafluorobenzene solutions doped with nitroxide, BDPA, MC800 asphaltene and MC30 asphaltene free radicals at 15 G. The ¹⁹F nuclei in all solutions gave positive DNP enhancements changing between 3.42 and 189.54, corresponding to predominantly scalar interactions with the unpaired electrons in the radicals. DNP sensitivity of ¹⁹F nuclei in hexafluorobenzene was observed to be changed significantly depending on the radical type. Nitroxide was found to have the best DNP performance among the polarizing agents. Copyright © 2016 John Wiley & Sons, Ltd.     

A computational study of 2J(HH)(gem) indirect spin-spin coupling constants in simple hydrides of the second and third periods

Several theoretical methods have been used to compute (2)J(HH) in neutral, anionic and cationic HXH hydrides, X being the 14 nuclei from Li to Cl (28 molecules). Since the calculations also provide (1)J(XH) spin-spin coupling constants (SSCC), these have also been analyzed. The best results were obtained using Second-order polarization propagator approximation (SOPPA)/sadJ. The geminal coupling constants appear to be dependent on the electronegativity of the X-atom.     

Scalar dynamic nuclear polarization of protons in trifluoroacetic acid solutions

Strongly scalar ¹H intermolecular dynamic nuclear polarization (DNP) of solvent ¹H nuclei in solutions of organic free radicals has now been observed at room temperature. Transient protonation of the CF₃COOH/pyridyl imidazoline-1-oxyl radical system accounts for the observed effects.     

Sensitizing solid state nuclear magnetic resonance of dilute nuclei by spin-diffusion assisted polarization transfers

Recent years have witnessed efforts geared at increasing the sensitivity of NMR experiments, by relying on the suitable tailoring and exploitation of relaxation phenomena. These efforts have included the use of paramagnetic agents, enhanced (1)H-(1)H incoherent and coherent transfers processes in 2D liquid state spectroscopy, and homonuclear (13)C-(13)C spin diffusion effects in labeled solids. The present study examines some of the opportunities that could open when exploiting spontaneous (1)H-(1)H spin-diffusion processes, to enhance relaxation and to improve the sensitivity of dilute nuclei in solid state NMR measurements. It is shown that polarization transfer experiments executed under sufficiently fast magic-angle-spinning conditions, enable a selective polarization of the dilute low-γ spins by their immediate neighboring protons. Repolarization of the latter can then occur during the time involved in monitoring the signal emitted by the low-γ nuclei. The basic features involved in the resulting approach, and its potential to improve the effective sensitivity of solid state NMR measurements on dilute nuclei, are analyzed. Experimental tests witness the advantages that could reside from utilizing this kind of approach over conventional cross-polarization processes. These measurements also highlight a number of limitations that will have to be overcome for transforming selective polarization transfers of this kind into analytical methods of choice.     

Two-dimensional (13)C-(13)C correlation spectroscopy with magic angle spinning and dynamic nuclear polarization

The sensitivity of solid-state NMR experiments can be enhanced with dynamic nuclear polarization (DNP), a technique that transfers the high Boltzmann polarization of unpaired electrons to nuclei. Signal enhancements of up to 23 have been obtained for magic angle spinning (MAS) experiments at 5 T and 85-90 K using a custom-designed high-power gyrotron. The extended stability of MAS/DNP experiments at low temperature is demonstrated with (1)H-driven (13)C spin-diffusion experiments on the amino acid proline. These (13)C-(13)C chemical shift correlation spectra are the first two-dimensional MAS/DNP experiments performed at high field (>1.4 T).     

Natural abundance solution 13C NMR studies of a phototropin with photoinduced polarization

Strongly spin-polarized 13C NMR lines have been observed upon photoexcitation of FMN-binding LOV domains from the blue-light receptor phototropin. Their origin can be rationalized in terms of intermediate radical-pair spin chemistry. Due to hyperfine-selective branching into singlet and triplet products of different lifetime, nuclear spin polarization builds up on nuclei that possess high electron-spin density in the radical state. By examining point-mutated LOV domains of phototropin, spin-polarized 13C NMR signals in emission arising from 13C nuclei at natural abundance in the apoprotein can be unambiguously assigned to a tryptophan residue that is located at a distance of about 14 A from the FMN cofactor and that undergoes photoinduced electron transfer to the flavin. This demonstrates the potential of photo-CIDNP in unraveling reactive intermediates in protein function.     

Polarization transfer in CIDNP experiments via scalar paramagnetic exchange mechanisms

A theory is outlined explaining how chemically induced nuclear polarization can be transfered by a diamagnetic-paramagnetic exchange reaction from a group of nuclei to another group not originally polarized. The mechanism depends critically on the scalar nuclear spin-spin coupling in the diamagnetic state. Experiments on the photochemical charge transfer between N,N-diethylaniline and pyrene are given as an example of this polarization transfer.     

氢核磁共振谱自旋耦合裂分现象的量子力学解释

Hydrogen nuclear magnetic resonance spectroscopy (¹H-NMR) is an important content in the university course of instrumental analysis and organic structure analysis. The splitting law of spin coupling between hydrogen nuclei is the focus of teaching and learning. Most textbooks explain that the cause of spin coupling splitting is due to the local magnetic field produced by the different spin orientation of other adjacent nuclei (nuclei magnetic dipole-dipole interaction, direct nuclear spin coupling), and a few monograph on Nuclear Magnetic Resonance refers to electron spin polarization mediated nuclear spin coupling (indirect nuclear spin coupling). Here we introduce quantum mechanics for explanation of the splitting law of spin coupling between hydrogen nuclei.     

Correlation of the EPR properties of perchlorotriphenylmethyl radicals and their efficiency as DNP polarizers

Water soluble perchlorinated trityl (PTM) radicals were found to be effective 95 GHz DNP (dynamic nuclear polarization) polarizers in ex situ (dissolution) (13)C DNP (Gabellieri et al., Angew Chem., Int. Ed. 2010, 49, 3360). The degree of the nuclear polarization obtained was reported to be dependent on the position of the chlorine substituents on the trityl skeleton. In addition, on the basis of the DNP frequency sweeps it was suggested that the (13)C NMR signal enhancement is mediated by the Cl nuclei. To understand the DNP mechanism of the PTM radicals we have explored the 95 GHz EPR characteristics of these radicals that are relevant to their performance as DNP polarizers. The EPR spectra of the radicals revealed axially symmetric g-tensors. A comparison of the spectra with the (13)C DNP frequency sweeps showed that although the solid effect mechanism is operational the DNP frequency sweeps reveal some extra width suggesting that contributions from EPR forbidden transitions involving (35,37)Cl nuclear flips are likely. This was substantiated experimentally by ELDOR (electron-electron double resonance) detected NMR measurements, which map the EPR forbidden transitions, and ELDOR experiments that follow the depolarization of the electron spin upon irradiation of the forbidden EPR transitions. DFT (density functional theory) calculations helped to assign the observed transitions and provided the relevant spin Hamiltonian parameters. These results show that the (35,37)Cl hyperfine and nuclear quadrupolar interactions cause a considerable nuclear state mixing at 95 GHz thus facilitating the polarization of the Cl nuclei upon microwave irradiation. Overlap of Cl nuclear frequencies and the (13)C Larmor frequency further facilitates the polarization of the (13)C nuclei by spin diffusion. Calculation of the (13)C DNP frequency sweep based on the Cl nuclear polarization showed that it does lead to an increase in the width of the spectra, improving the agreement with the experimental sweeps, thus supporting the existence of a new heteronuclear assisted DNP mechanism.     

Observing Nearby Nuclei on Paramagnetic Trityls and MOFs via DNP and Electron Decoupling

Dynamic nuclear polarization (DNP) is an NMR sensitivity enhancement technique that mediates polarization transfer from unpaired electrons to NMR-active nuclei. Despite its success in elucidating important structural information on biological and inorganic materials, the detailed polarization-transfer pathway from the electrons to the nearby and then the bulk solvent nuclei, and finally to the molecules of interest-remains unclear. In particular, the nuclei in the paramagnetic polarizing agent play significant roles in relaying the enhanced NMR polarizations to more remote nuclei. Despite their importance, the direct NMR observation of these nuclei is challenging because of poor sensitivity. Here, we show that a combined DNP and electron decoupling approach can facilitate direct NMR detection of these nuclei. We achieved an ∼80 % improvement in NMR intensity via electron decoupling at 0.35 T and 80 K on trityl radicals. Moreover, we recorded a DNP enhancement factor of ∼90 and ∼11 % higher NMR intensity using electron decoupling on paramagnetic metal-organic framework, magnesium hexaoxytriphenylene (MgHOTP MOF).     

Magnetic field dependence of photo-CIDNP MAS NMR on photosynthetic reaction centers of Rhodobacter sphaeroides WT

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in frozen and quinone depleted photosynthetic reaction centers of the purple bacteria Rhodobacter sphaeroides wild type (WT) by (13)C solid-state NMR at three different magnetic fields. All light-induced signals appear to be emissive at all three fields. At 4.7 T (200 MHz proton frequency), the strongest enhancement of NMR signals is observed, which is more than 10 000 above the Boltzmann polarization. At higher fields, the enhancement factor decreases. At 17.6 T, the enhancement factor is about 60. The field dependence of the enhancement appears to be the same for all nuclei. The observed field dependence is in line with simulations that assume two competing mechanisms of polarization transfer from electrons to nuclei, three-spin mixing (TSM) and differential decay (DD). These simulations indicate a ratio of the electron spin density on the special pair cofactors is 3:2 in favor of the L-BChl during the radical cation state. The good agreement of simulations with the experiments raises expectations that artificial solid reaction centers can be tuned to show photo-CIDNP in the near future.