Soliton Based Dynamic Nuclear Polarization: An Overhauser Effect in Cyclic Polyacetylene at High Field and Room Temperature.

Polyacetylene, a versatile material with an electrical conductivity that can span 7 orders of magnitude, is the prototypical conductive polymer. In this letter, we report the observation of a significant Overhauser effect at the high magnetic field of 14.1 T that operates at 100 K and room temperature in both linear and cyclic polyacetylene. Significant NMR signal enhancements ranging from 24 to 45 are obtained. The increased sensitivity enabled the characterization of the polymer chain defects at natural abundance. The absence of end methyl group carbon-13 signals provides proof of the closed-loop molecular structure of cyclic polyacetylene. The remarkable efficiency of the soliton based Overhauser effect DNP mechanism at high temperature and high field holds promise for applications and extension to other conductive polymer systems.

Shimon Vega in the eyes of his students and postdocs.

Professor Shimon Vega (1943-2021) of the Weizmann Institute of Science passed away on the 16-th of November. Shimon Vega established theoretical frameworks to develop and explain solid-state nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP) techniques and methodologies. His departure left a profound mark on his many students, postdocs, and colleagues. Shortly after his passing, we all assembled spontaneously for an international online meeting to share our reflections and memories of our experiences in Shimon's lab and how they affected us deeply during that period of timeand throughout our scientific careers. These thoughts and feelings were put here into writing.

Overhauser effects in non-conducting solids at 1.2 K.

Recently, it was observed that protons in non-conducting solids doped with 1,3-bisdiphenylene-2-phenylallyl (BDPA) or its sulfonated derivative (SA-BDPA) can be polarized through Overhauser effects via resonant microwave irradiation. These effects were present under magic angle spinning conditions in magnetic fields between 5 and 18.8 T and at temperatures near 100 K. This communication reports similar effects in static samples at 6.7 T and, more importantly, at temperatures as low as 1.2 K, in a different dynamic regime than in the previous study. Our results provide new information towards understanding the mechanism of the Overhauser effect in non-conducting solids. We discuss possible origins of the fluctuations that can give rise to an Overhauser effect at such low temperatures.

Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning.

We report a strategy to push the limits of solid-state NMR sensitivity far beyond its current state-of-the-art. The approach relies on the use of dynamic nuclear polarization and demonstrates unprecedented DNP enhancement factors for experiments performed at sample temperatures much lower than 100 K, and can translate into 6 orders of magnitude of experimental time-savings. This leap-forward was made possible thanks to the employment of cryogenic helium as the gas to power magic angle sample spinning (MAS) for dynamic nuclear polarization (DNP) enhanced NMR experiments. These experimental conditions far exceed what is currently possible and allows currently reaching sample temperatures down to 30 K while conducting experiments with improved resolution (thanks to faster spinning frequencies, up to 25 kHz) and highly polarized nuclear spins. The impressive associated gains were used to hyperpolarize the surface of an industrial catalyst as well as to hyperpolarize organic nano-assemblies (self-assembling peptides in our case), for whom structures cannot be solved using diffraction techniques. Sustainable cryogenic helium sample spinning significantly enlarges the realm and possibilities of the MAS-DNP technique and is the route to transform NMR into a versatile but also sensitive atomic-level characterization tool.

The (13)C solid DNP mechanisms with perchlorotriphenylmethyl radicals--the role of (35,37)Cl.

The microwave frequency swept DNP enhancement, referred to as the DNP spectrum, is strongly dependent on the EPR spectrum of the polarizing radical and it reveals the underlying DNP mechanisms. Here we focus on two chlorinated trityl radicals that feature axially symmetric powder patterns at 95 GHz, the width of which are narrower than those of TEMPOL or TOTAPOL but broader than that of the trityl derivative OX63. The static DNP lineshapes of these commonly used radicals in DNP, have been recently analyzed in terms of a superposition of basic Solid Effect (SE) and Cross Effect (CE)-DNP lineshapes, with their relative contributions as a fit parameter. To substantiate the generality of this approach and further investigate an earlier suggestion that a (35,37)Cl-(13)C polarization transfer pathway, termed "hetero-nuclear assisted DNP", may be in effect in the chlorinated radicals (C. Gabellieri et al., Angew. Chem., Int. Ed., 2010, 49, 3360-3362), we measured the static (13)C-glycerol DNP spectra of solutions of ca. ∼10 mM of the two chlorinated trityl radicals as a function of temperature (10-50 K) and microwave power. Analysis of the DNP lineshapes was first done in terms of the SE/CE superposition model calculated assuming a direct e-(13)C polarization transfer. The CE was found to prevail at the high temperature range (40-50 K), whereas at the low temperature end (10-20 K) the SE dominates, as was observed earlier for (13)C DNP with OX63 and (1)H DNP with TEMPOL and TOTAPOL, thus indicating that this is rather general behavior. Furthermore, it was found that at low temperatures it is possible to suppress the SE, and increase the CE by merely lowering the microwave power. While this analysis gave a good agreement between experimental and calculated lineshapes when the CE dominates, some significant discrepancies were observed at low temperatures, where the SE dominates. We show that by explicitly taking into account the presence of (35/37)Cl nuclei through a e-(35,37)Cl-(13)C polarization pathway in the SE-DNP lineshape calculations, as proposed earlier, we can improve the fit significantly, thus supporting the existence of the "hetero-nuclear assisted DNP" pathway.

Overhauser effects in insulating solids.

We report magic angle spinning, dynamic nuclear polarization (DNP) experiments at magnetic fields of 9.4 T, 14.1 T, and 18.8 T using the narrow line polarizing agents 1,3-bisdiphenylene-2-phenylallyl (BDPA) dispersed in polystyrene, and sulfonated-BDPA (SA-BDPA) and trityl OX063 in glassy glycerol/water matrices. The (1)H DNP enhancement field profiles of the BDPA radicals exhibit a significant DNP Overhauser effect (OE) as well as a solid effect (SE) despite the fact that these samples are insulating solids. In contrast, trityl exhibits only a SE enhancement. Data suggest that the appearance of the OE is due to rather strong electron-nuclear hyperfine couplings present in BDPA and SA-BDPA, which are absent in trityl and perdeuterated BDPA (d21-BDPA). In addition, and in contrast to other DNP mechanisms such as the solid effect or cross effect, the experimental data suggest that the OE in non-conducting solids scales favorably with magnetic field, increasing in magnitude in going from 5 T, to 9.4 T, to 14.1 T, and to 18.8 T. Simulations using a model two spin system consisting of an electron hyperfine coupled to a (1)H reproduce the essential features of the field profiles and indicate that the OE in these samples originates from the zero and double quantum cross relaxation induced by fluctuating hyperfine interactions between the intramolecular delocalized unpaired electrons and their neighboring nuclei, and that the size of these hyperfine couplings is crucial to the magnitude of the enhancements. Microwave power dependent studies show that the OE saturates at considerably lower power levels than the solid effect in the same samples. Our results provide new insights into the mechanism of the Overhauser effect, and also provide a new approach to perform DNP experiments in chemical, biophysical, and physical systems at high magnetic fields.

Increasing sensitivity of pulse EPR experiments using echo train detection schemes.

Modern pulse EPR experiments are routinely used to study the structural features of paramagnetic centers. They are usually performed at low temperatures, where relaxation times are long and polarization is high, to achieve a sufficient Signal/Noise Ratio (SNR). However, when working with samples whose amount and/or concentration are limited, sensitivity becomes an issue and therefore measurements may require a significant accumulation time, up to 12h or more. As the detection scheme of practically all pulse EPR sequences is based on the integration of a spin echo--either primary, stimulated or refocused--a considerable increase in SNR can be obtained by replacing the single echo detection scheme by a train of echoes. All these echoes, generated by Carr-Purcell type sequences, are integrated and summed together to improve the SNR. This scheme is commonly used in NMR and here we demonstrate its applicability to a number of frequently used pulse EPR experiments: Echo-Detected EPR, Davies and Mims ENDOR (Electron-Nuclear Double Resonance), DEER (Electron-Electron Double Resonance|) and EDNMR (Electron-Electron Double Resonance (ELDOR)-Detected NMR), which were combined with a Carr-Purcell-Meiboom-Gill (CPMG) type detection scheme at W-band. By collecting the transient signal and integrating a number of refocused echoes, this detection scheme yielded a 1.6-5 folds SNR improvement, depending on the paramagnetic center and the pulse sequence applied. This improvement is achieved while keeping the experimental time constant and it does not introduce signal distortion.

Origin of the decoherence of the extended electron spin state in Ti-doped ß-Ga2O3.

The mechanism of decoherence of the electron spin of Ti(3+) in ß-Ga2O3 was investigated by pulsed electron paramagnetic resonance. At 4.2 K, both instantaneous and spectral diffusion contribute to the decoherence. For electron spin concentrations ≈10(25) m(-3) in the studied samples, calculations indicate that electron-electron couplings and electron couplings with (69)Ga and (71)Ga nuclei yield similar contributions to the spectral diffusion, but that electron-nuclei interactions could become the dominant cause of spectral diffusion for only slightly lower spin concentrations. Above 20 K, an additional contribution to the decoherence as well as to the spin-lattice relaxation arises from a two-optical-phonon Raman process, which becomes the leading decoherence mechanism for T > 39 K. Rabi oscillations with a damping time of about 79 ns at 4.2 K could also be observed. The damping of the Rabi oscillations, independent of the oscillation frequency, is suspected to arise from electron-nuclei interactions.