Dynamic nuclear polarization surface enhanced NMR spectroscopy: the design of new biradicals enables application to challenging materials

International audienceNMR spectroscopy (often in conjunction with diffraction methods) is the method of choice for characterizing surfaces whenever possible, but the detection limit of NMR is far too low to allow many modern materials to be examined. Because it provides dramatic sensitivity enhancement, solid-state Dynamic Nuclear Polarization (DNP) NMR is currently emerging as a powerful tool to study samples previously inaccessible to NMR. We have recently shown how DNP could be used to selectively enhance the NMR signals from surfaces in a wide range of materials (DNP SENS). While signal enhancements of between 20 and 50 are routinely obtained at 9.4 T and 100 K (for 29Si, 13C, or 27Al nuclei after cross-polarization from protons), these enhancement factors are still far from the predicted maximum values. Much effort is currently devoted to the development of ever more efficient polarizing agents. We will present a series of bTbK derivatives suitable for high-field DNP enhanced solid-state NMR spectroscopy. These biradicals differ by the functional groups at the vicinal position of the two nitroxides. We establish clear relationships between the DNP efficiencies of these new radicals and (i) their molecular weight, (ii) the number of methyl groups and (iii) their electron spin relaxation rates. In particular, a new radical is introduced, that yields proton enhancement of over 200 at 9.4 T and 100 K in bulk solution as well as in mesoporous materials. The temperature and spinning speed dependence of the DNP enhancement factors is also discussed for this new family of polarizing agents. These new polarizing agents are demonstrated through the application of DNP SENS to the characterization of periodic mesoporous organosilicates (POM). The rapid acquisition of high quality natural abundance 1D 13C, 15N, and 29Si, and 2D 1H-13C and 1H-29Si DNP solid-state NMR spectra was essential to distinguish outer and inner layers in these porous materials and to monitor their surface functionalization

Large molecular weight nitroxide biradicals providing efficient dynamic nuclear polarization at temperatures up to 200 K

We acknowledge Dr. Moreno Lelli and Dr. David Gajan for their insightful conversations and advice and Mr. W. Gruning and Dr. M. Conley for preparation of materials.International audienceA 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 (epsilon) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger E. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in E between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on epsilon 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 e may be obtained at higher temperatures

Solid-phase polarization matrixes for dynamic nuclear polarization from homogeneously distributed radicals in mesostructured hybrid silica materials.

International audienceMesoporous hybrid silica-organic materials containing homogeneously distributed stable mono- or dinitroxide radicals covalently bound to the silica surface were developed as polarization matrixes for solid-state dynamic nuclear polarization (DNP) NMR experiments. For TEMPO-containing materials impregnated with water or 1,1,2,2-tetrachloroethane, enhancement factors of up to 36 were obtained at ∼100 K and 9.4 T without the need for a glass-forming additive. We show that the homogeneous radical distribution and the subtle balance between the concentration of radical in the material and the fraction of radicals at a sufficient inter-radical distance to promote the cross-effect are the main determinants for the DNP enhancements we obtain. The material, as well as an analogue containing the poorly soluble biradical bTUrea, is used as a polarizing matrix for DNP NMR experiments of solutions containing alanine and pyruvic acid. The analyte is separated from the polarization matrix by simple filtration