Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K

Abstract

A series of seven functionalized nitroxide biradicals (the bTbK biradical and six derivatives) are investigated as exogenous polarization sources for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and with ca. 100 K sample temperatures. The impact of electron relaxation times on the DNP enhancement (ε) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger ε. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in ε between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on ε is investigated for several radicals each characterized by a range of electron relaxation times. In particular, TEKPol, a bulky derivative of bTbK with a molecular weight of 905 g·mol(-1), is presented. Its high-saturation factor makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of 33 at 180 K and 12 at 200 K, suggesting that with the continued improvement of radicals large ε may be obtained at higher temperatures.

Mesoporous Silica-Supported Amidozirconium-Catalyzed Carbonyl Hydroboration

“…We thus resorted to the newly developed DNP method, which enhances the sensitivity of SSNMR of surface species by ∼2 orders magnitude via excitation of the exogenously introduced biradicals (here TEKPol dissolved in 1,1,2,2-tetrachloroethane) at their ESR resonance frequency, followed by transfer of magnetization to the nuclear spins. 50,51 A high-quality DNP-enhanced 15 N CPMAS spectrum of Zr(NMe 2 ) n @MSN was indeed acquired under natural abundance in just over 2 h ( Figure 4A, top spectrum) H nuclei at a distance of about 2 Å, which is consistent with protons in NMe 2 groups being the source. Interestingly, the local maxima in the curve (which were reproducibly measured several times, and evaluated using eq 20c in ref 49), suggest that a small fraction of the 1 H− 15 N pairs reside at a distance of ∼1.0 Å, as would be expected in zirconium amine species.…”

Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3

“…4 The BZY samples for the DNP experiments were prepared by wet impregnation using d 6 This mixture was quickly stirred, centrifuged into a sapphire rotor, capped with a polyfluoroethylene plug and a zirconia drive tip, and inserted into the DNP NMR probe. 4 One sample of BZY30 was prepared similarly using 30 µL of 10 mM of bCTbK 5,6 biradical solution in 1,1,2,2-tetrabromoethane.…”

Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy

“…9 Replacing methyl groups adjacent to the nitroxide group with cyclohexyl or phenyl groups increases electronic relaxation times, facilitating more complete saturation of the EPR. 10 Today, the nitroxide biradicals AMUPol (Figure 1). 12 These radicals perform better at high-field because the narrow isotropic EPR of the carbon-centered radical is more completely saturated and strong magnetic exchange (J) interactions increase coupling between the two electron spins.…”