Friday, August 26, 2016

On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid-State and Dissolution 13 C NMR Spectroscopy #DNPNMR

Bretschneider, C.O., et al., On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid-State and Dissolution 13 C NMR Spectroscopy. ChemPhysChem, 2016: p. n/a-n/a.

Dynamic nuclear polarization (DNP) is a versatile option to improve the sensitivity of NMR and MRI. This versatility has elicited interest for overcoming potential limitations of these techniques, including the achievement of solid-state polarization enhancement at ambient conditions, and the maximization of 13 C signal lifetimes for performing in vivo MRI scans. This study explores whether diamond's 13 C behavior in nano- and micro-particles could be used to achieve these ends. The characteristics of diamond's DNP enhancement were analyzed for different magnetic fields, grain sizes, and sample environments ranging from cryogenic to ambient temperatures, in both solution and solid-state experiments. It was found that 13 C NMR signals could be boosted by orders of magnitude in either low- or room-temperature solid-state DNP experiments by utilizing naturally occurring paramagnetic P1 substitutional nitrogen defects. We attribute this behavior to the unusually long electronic/nuclear spin-lattice relaxation times characteristic of diamond, coupled with a time-independent cross-effect-like polarization transfer mechanism facilitated by a matching of the nitrogen-related hyperfine coupling and the 13 C Zeeman splitting. The efficiency of this solid-state polarization process, however, is harder to exploit in dissolution DNP-enhanced MRI contexts. The prospects for utilizing polarized diamond approaching nanoscale dimensions for both solid and solution applications are briefly discussed.

Wednesday, August 24, 2016

Dynamics of 4-oxo-TEMPO-d16-(15)N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295K

Goslar, J., S.K. Hoffmann, and S. Lijewski, Dynamics of 4-oxo-TEMPO-d16-(15)N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295K. J Magn Reson, 2016. 269: p. 162-75.

ESR spectra and electron spin relaxation of nitroxide radical in 4-oxo-TEMPO-d16-(15)N in propylene glycol were studied at X-band in the temperature range 10-295K. The spin-lattice relaxation in the liquid viscous state determined from the resonance line shape is governed by three mechanisms occurring during isotropic molecular reorientations. In the glassy state below 200K the spin-lattice relaxation, phase relaxation and electron spin echo envelope modulations (ESEEM) were studied by pulse spin echo technique using 2-pulse and 3-pulse induced signals. Electron spin-lattice relaxation is governed by a single non-phonon relaxation process produced by localized oscillators of energy 76cm(-1). Electron spin dephasing is dominated by a molecular motion producing a resonance-type peak in the temperature dependence of the dephasing rate around 120K. The origin of the peak is discussed and a simple method for the peak shape analysis is proposed, which gives the activation energy of a thermally activated motion Ea=7.8kJ/mol and correlation time tau0=10(-8)s. The spin echo amplitude is strongly modulated and FT spectrum contains a doublet of lines centered around the (2)D nuclei Zeeman frequency. The splitting into the doublet is discussed as due to a weak hyperfine coupling of nitroxide unpaired electron with deuterium of reorienting CD3 groups.

Monday, August 22, 2016

Dynamic nuclear polarization at 40 kHz magic angle spinning #DNPNMR

Chaudhari, S.R., et al., Dynamic nuclear polarization at 40 kHz magic angle spinning. Phys Chem Chem Phys, 2016. 18(15): p. 10616-22.

DNP-enhanced solid-state NMR spectroscopy under magic angle spinning (MAS) is rapidly developing into a powerful analytical tool to investigate the structure of a wide range of solid materials, because it provides unsurpassed sensitivity gains. Most developments and applications of DNP MAS NMR were so far reported at moderate spinning frequencies (up to 14 kHz using 3.2 mm rotors). Here, using a 1.3 mm MAS DNP probe operating at 18.8 T and approximately 100 K, we show that signal amplification factors can be increased by up to a factor two when using smaller volume rotors as compared to 3.2 mm rotors, and report enhancements of around 60 over a range of sample spinning rates from 10 to 40 kHz. Spinning at 40 kHz is also shown to increase (29)Si coherence lifetimes by a factor three as compared to 10 kHz, substantially increasing sensitivity in CPMG type experiments. The contribution of quenching effects to the overall sensitivity gain at very fast MAS is evaluated, and applications are reported on a functionalised mesostructured organic-inorganic material.

Friday, August 19, 2016

Solvent responsive catalyst improves NMR sensitivity via efficient magnetisation transfer #DNPNMR

Ruddlesden, A.J. and S.B. Duckett, Solvent responsive catalyst improves NMR sensitivity via efficient magnetisation transfer. Chem Commun (Camb), 2016. 52(54): p. 8467-70.

A bidentate iridium carbene complex, Ir(kappaC,O-L1)(COD), has been synthesised which contains a strongly electron donating carbene ligand that is functionalised by a cis-spanning phenolate group. This complex acts as a precursor to effective magnetisation transfer catalysts which form after reaction with H2 and a suitable two electron donor. In solvents such as benzene, containing pyridine, they are exemplified by neutral, chiral Ir(H)2(kappaC,O-L1)(py)2 with inequivalent hydride ligands and Ir-O bond retention, whilst in methanol, Ir-O bond cleavage leads to zwitterionic [Ir(H)2(kappaC,O(-)-L1)(py)3](+), with chemically equivalent hydride ligands. The active catalyst's form is therefore solvent dependent. Both these complexes break the magnetic symmetry of the hydride ligands and are active in the catalytic transfer of polarisation from parahydrogen to a loosely bound ligand. Test results on pyridine, nicotinaldehyde and nicotine reveal up to approximately 1.2% single spin proton polarisation levels in their (1)H NMR signals which compare to the normal 0.003% level at 9.4 Tesla. These results exemplify how rational catalyst design yields a solvent dependent catalyst with good SABRE activity.

Wednesday, August 17, 2016

Silica materials with wall-embedded nitroxides provide efficient polarization matrices for dynamic nuclear polarization NMR #DNPNMR

Besson, E., et al., Silica materials with wall-embedded nitroxides provide efficient polarization matrices for dynamic nuclear polarization NMR. Chem Commun (Camb), 2016. 52(32): p. 5531-3.

Hybrid mesoporous silica materials with wall-embedded nitroxides are shown to efficiently polarize impregnated substrates in high-field dynamic nuclear polarization magic-angle spinning solid-state NMR experiments.

Monday, August 15, 2016

Single spin magnetic resonance

Wrachtrup, J. and A. Finkler, Single spin magnetic resonance. J Magn Reson, 2016. 269: p. 225-36.

Different approaches have improved the sensitivity of either electron or nuclear magnetic resonance to the single spin level. For optical detection it has essentially become routine to observe a single electron spin or nuclear spin. Typically, the systems in use are carefully designed to allow for single spin detection and manipulation, and of those systems, diamond spin defects rank very high, being so robust that they can be addressed, read out and coherently controlled even under ambient conditions and in a versatile set of nanostructures. This renders them as a new type of sensor, which has been shown to detect single electron and nuclear spins among other quantities like force, pressure and temperature. Adapting pulse sequences from classic NMR and EPR, and combined with high resolution optical microscopy, proximity to the target sample and nanoscale size, the diamond sensors have the potential to constitute a new class of magnetic resonance detectors with single spin sensitivity. As diamond sensors can be operated under ambient conditions, they offer potential application across a multitude of disciplines. Here we review the different existing techniques for magnetic resonance, with a focus on diamond defect spin sensors, showing their potential as versatile sensors for ultra-sensitive magnetic resonance with nanoscale spatial resolution.

Friday, August 12, 2016

Low-field thermal mixing in [1-(13)C] pyruvic acid for brute-force hyperpolarization

Peat, D.T., et al., Low-field thermal mixing in [1-(13)C] pyruvic acid for brute-force hyperpolarization. Phys Chem Chem Phys, 2016. 18(28): p. 19173-82.

We detail the process of low-field thermal mixing (LFTM) between (1)H and (13)C nuclei in neat [1-(13)C] pyruvic acid at cryogenic temperatures (4-15 K). Using fast-field-cycling NMR, (1)H nuclei in the molecule were polarized at modest high field (2 T) and then equilibrated with (13)C nuclei by fast cycling ( approximately 300-400 ms) to a low field (0-300 G) that activates thermal mixing. The (13)C NMR spectrum was recorded after fast cycling back to 2 T. The (13)C signal derives from (1)H polarization via LFTM, in which the polarized ('cold') proton bath contacts the unpolarised ('hot') (13)C bath at a field so low that Zeeman and dipolar interactions are similar-sized and fluctuations in the latter drive (1)H-(13)C equilibration. By varying mixing time (tmix) and field (Bmix), we determined field-dependent rates of polarization transfer (1/tau) and decay (1/T1m) during mixing. This defines conditions for effective mixing, as utilized in 'brute-force' hyperpolarization of low-gamma nuclei like (13)C using Boltzmann polarization from nearby protons. For neat pyruvic acid, near-optimum mixing occurs for tmix approximately 100-300 ms and Bmix approximately 30-60 G. Three forms of frozen neat pyruvic acid were tested: two glassy samples, (one well-deoxygenated, the other O2-exposed) and one sample pre-treated by annealing (also well-deoxygenated). Both annealing and the presence of O2 are known to dramatically alter high-field longitudinal relaxation (T1) of (1)H and (13)C (up to 10(2)-10(3)-fold effects). Here, we found smaller, but still critical factors of approximately (2-5)x on both tau and T1m. Annealed, well-deoxygenated samples exhibit the longest time constants, e.g., tau approximately 30-70 ms and T1m approximately 1-20 s, each growing vs. Bmix. Mixing 'turns off' for Bmix > approximately 100 G. That T1m>>tau is consistent with earlier success with polarization transfer from (1)H to (13)C by LFTM.