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2H-decoupling-accelerated 1H spin diffusion in dynamic nuclear polarization with photoexcited triplet electrons

Negoro, M., et al., [sup 2]H-decoupling-accelerated [sup 1]H spin diffusion in dynamic nuclear polarization with photoexcited triplet electrons. J. Chem. Phys., 2010. 133(15): p. 154504-6

http://link.aip.org/link/?JCP/133/154504/1

In dynamic nuclear polarization (DNP) experiments applied to organic solids for creating nonequilibrium, high 1H spin polarization, an efficient buildup of 1H polarization is attained by partially deuterating the material of interest with an appropriate 1H concentration. In such a dilute 1H spin system, it is shown that the 1H spin diffusion rate and thereby the buildup efficiency of 1H polarization can further be enhanced by continually applying radiofrequency irradiation for deuterium decoupling during the DNP process. As experimentally confirmed in this work, the electron spin polarization of the photoexcited triplet state is mainly transferred only to those 1H spins, which are in the vicinity of the electron spins, and 1H spin diffusion transports the localized 1H polarization over the whole sample volume. The 1H spin diffusion coefficients are estimated from DNP repetition interval dependence of the initial buildup rate of 1H polarization, and the result indicates that the spin diffusion coefficient is enhanced by a factor of 2 compared to that without 2H decoupling.

Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics

Hu, K.-N., et al., Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics. J. Chem. Phys., 2011. 134(12): p. 125105-19

http://link.aip.org/link/?JCP/134/125105/1

Microwave driven dynamic nuclear polarization (DNP) is a process in which the large polarization present in an electron spin reservoir is transferred to nuclei, thereby enhancing NMR signal intensities. In solid dielectrics there are three mechanisms that mediate this transfer—the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Historically these mechanisms have been discussed theoretically using thermodynamic parameters and average spin interactions. However, the SE and the CE can also be modeled quantum mechanically with a system consisting of a small number of spins and the results provide a foundation for the calculations involving TM. In the case of the SE, a single electron–nuclear spin pair is sufficient to explain the polarization mechanism, while the CE requires participation of two electrons and a nuclear spin, and can be used to understand the improved DNP enhancements observed using biradical polarizing agents. Calculations establish the relations among the electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) frequencies and the microwave irradiation frequency that must be satisfied for polarization transfer via the SE or the CE. In particular, if δ, Δ < ω0I, where δ and Δ are the homogeneous linewidth and inhomogeneous breadth of the EPR spectrum, respectively, we verify that the SE occurs when ωM = ω0S ± ω0I, where ωM, ω0S and ω0I are, respectively, the microwave, and the EPR and NMR frequencies. Alternatively, when Δ > ω0I > δ, the CE dominates the polarization transfer. This two-electron process is optimized when ω0S1−ω0S2 = ω0I and ωM ∼ ω0S1 or ω0S2, where ω0S1 and ω0S2 are the EPR Larmor frequencies of the two electrons. Using these matching conditions, we calculate the evolution of the density operator from electron Zeeman order to nuclear Zeeman order for both the SE and the CE. The results provide insights into the influence of the microwave irradiation field, the external magnetic field, and the electron−electron and electron−nuclear interactions on DNP enhancements.

Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics

Hu, K.-N., et al., Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics. J. Chem. Phys., 2011. 134(12): p. 125105-19

http://link.aip.org/link/?JCP/134/125105/1

Microwave driven dynamic nuclear polarization (DNP) is a process in which the large polarization present in an electron spin reservoir is transferred to nuclei, thereby enhancing NMR signal intensities. In solid dielectrics there are three mechanisms that mediate this transfer—the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Historically these mechanisms have been discussed theoretically using thermodynamic parameters and average spin interactions. However, the SE and the CE can also be modeled quantum mechanically with a system consisting of a small number of spins and the results provide a foundation for the calculations involving TM. In the case of the SE, a single electron–nuclear spin pair is sufficient to explain the polarization mechanism, while the CE requires participation of two electrons and a nuclear spin, and can be used to understand the improved DNP enhancements observed using biradical polarizing agents. Calculations establish the relations among the electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) frequencies and the microwave irradiation frequency that must be satisfied for polarization transfer via the SE or the CE. In particular, if δ, Δ < ω0I, where δ and Δ are the homogeneous linewidth and inhomogeneous breadth of the EPR spectrum, respectively, we verify that the SE occurs when ωM = ω0S ± ω0I, where ωM, ω0S and ω0I are, respectively, the microwave, and the EPR and NMR frequencies. Alternatively, when Δ > ω0I > δ, the CE dominates the polarization transfer. This two-electron process is optimized when ω0S1−ω0S2 = ω0I and ωM ∼ ω0S1 or ω0S2, where ω0S1 and ω0S2 are the EPR Larmor frequencies of the two electrons. Using these matching conditions, we calculate the evolution of the density operator from electron Zeeman order to nuclear Zeeman order for both the SE and the CE. The results provide insights into the influence of the microwave irradiation field, the external magnetic field, and the electron−electron and electron−nuclear interactions on DNP enhancements.

Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

Turke, M.-T. and M. Bennati, Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments. Phys. Chem. Chem. Phys., 2011. 13(9): p. 3630-3633

http://dx.doi.org/10.1039/C0CP02126A

We propose the use of the pulse electron double resonance (ELDOR) method to determine the effective saturation factor of nitroxide radicals for dynamic nuclear polarization (DNP) experiments in liquids. The obtained values for the nitroxide radical TEMPONE-D,15N at different concentrations are rationalized in terms of spin relaxation and are shown to fulfil the Overhauser theory.

Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

Turke, M.-T. and M. Bennati, Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments. Phys. Chem. Chem. Phys., 2011. 13(9): p. 3630-3633

http://dx.doi.org/10.1039/C0CP02126A

We propose the use of the pulse electron double resonance (ELDOR) method to determine the effective saturation factor of nitroxide radicals for dynamic nuclear polarization (DNP) experiments in liquids. The obtained values for the nitroxide radical TEMPONE-D,15N at different concentrations are rationalized in terms of spin relaxation and are shown to fulfil the Overhauser theory.

Fast Characterization of Functionalized Silica Materials by Silcion-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization

M. Lelli et al., Fast Characterization of Functionalized Silica Materials by Silcion-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization, J. Am. Chem. Soc., 2010,

http://pubs.acs.org/doi/abs/10.1021/ja110791d

We demonstrate fast characterization of the distribution of surface bonding modes and interactions in a series of functionalized materials via surface-enhanced nuclear magnetic resonance spectroscopy using dynamic nuclear polarization (DNP). Surface-enhanced silicon-29 DNP NMR spectra were obtained by using incipient wetness impregnation of the sample with a solution containing a polarizing radical (TOTAPOL). We identify and compare the bonding topology of functional groups in materials obtained via a sol-gel process and in materials prepared by post-grafting reactions. Furthermore, the remarkable gain in time provided by surface-enhanced silicon-29 DNP NMR spectroscopy (typically on the order of a factor 400) allows the facile acquisition of two-dimensional correlation spectra.

Fast Characterization of Functionalized Silica Materials by Silcion-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization

M. Lelli et al., Fast Characterization of Functionalized Silica Materials by Silcion-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization, J. Am. Chem. Soc., 2010,

http://pubs.acs.org/doi/abs/10.1021/ja110791d

We demonstrate fast characterization of the distribution of surface bonding modes and interactions in a series of functionalized materials via surface-enhanced nuclear magnetic resonance spectroscopy using dynamic nuclear polarization (DNP). Surface-enhanced silicon-29 DNP NMR spectra were obtained by using incipient wetness impregnation of the sample with a solution containing a polarizing radical (TOTAPOL). We identify and compare the bonding topology of functional groups in materials obtained via a sol-gel process and in materials prepared by post-grafting reactions. Furthermore, the remarkable gain in time provided by surface-enhanced silicon-29 DNP NMR spectroscopy (typically on the order of a factor 400) allows the facile acquisition of two-dimensional correlation spectra.

In Situ Detection of PHIP at 48 mT: Demonstration Using a Centrally Controlled Polarizer

K. W. Waddell et al., In Situ Detection of PHIP at 48 mT: Demonstration Using a Centrally Controlled Polarizer, J. Am. Chem. Soc., 2010, 133(1), 97-101

http://dx.doi.org/10.1021/ja108529m

Presented here is a centrally controlled, automated parahydrogen-based polarizer with in situ
detection capability. A 20% polarization, corresponding to a 5 000 000-fold signal enhancement at 48 mT, is demonstrated on 2-hydroxyethyl-1-13C-propionate-d2,3,3 using a double-tuned antenna and pulsed polarization transfer. In situ detection is a refinement of first-generation devices enabling fast calibration of rf pulses and B0, quality assurance of hyperpolarized contrast agents, and stand-alone operation without the necessity of high-field MR spectrometers. These features are essential for biomedical applications of parahydrogen-based hyperpolarization and for clinical translation. We demonstrate the flexibility of the device by recording 13C signal decay due to longitudinal relaxation of a hyperpolarized contrast agent at 48 mT corresponding to 2 MHz proton frequency. This appears to be the longest recorded T1 (101 +/- 7 s) for a 13C hyperpolarized contrast agent in water.

In Situ Detection of PHIP at 48 mT: Demonstration Using a Centrally Controlled Polarizer

K. W. Waddell et al., In Situ Detection of PHIP at 48 mT: Demonstration Using a Centrally Controlled Polarizer, J. Am. Chem. Soc., 2010, 133(1), 97-101

http://dx.doi.org/10.1021/ja108529m

Presented here is a centrally controlled, automated parahydrogen-based polarizer with in situ
detection capability. A 20% polarization, corresponding to a 5 000 000-fold signal enhancement at 48 mT, is demonstrated on 2-hydroxyethyl-1-13C-propionate-d2,3,3 using a double-tuned antenna and pulsed polarization transfer. In situ detection is a refinement of first-generation devices enabling fast calibration of rf pulses and B0, quality assurance of hyperpolarized contrast agents, and stand-alone operation without the necessity of high-field MR spectrometers. These features are essential for biomedical applications of parahydrogen-based hyperpolarization and for clinical translation. We demonstrate the flexibility of the device by recording 13C signal decay due to longitudinal relaxation of a hyperpolarized contrast agent at 48 mT corresponding to 2 MHz proton frequency. This appears to be the longest recorded T1 (101 +/- 7 s) for a 13C hyperpolarized contrast agent in water.

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