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Emerging Trends in Solid-State NMR of Materials

Emerging Trends in Solid-State NMR of Materials. Marek Pruski U.S. DOE Ames Laboratory, Ames, Iowa 50011, U.S.A Department of Chemistry, Iowa State University, Ames, Iowa 50011, U.S.A. New Era in Separations Science, Washington, DC, August 22, 2018.

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Emerging Trends in Solid-State NMR of Materials

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  1. Emerging Trends in Solid-State NMR of Materials Marek Pruski U.S. DOE Ames Laboratory, Ames, Iowa 50011, U.S.A Department of Chemistry, Iowa State University, Ames, Iowa 50011, U.S.A. New Era in Separations Science, Washington, DC, August 22, 2018

  2. Structural characterization by solid-state NMR • SS-NMR spectroscopy has an unparalleled ability to provide atomic-level structural characterization of materials: • most elements have NMR-active isotopes • nuclear spins are excellent, site-dependent “reporters” of local structure and dynamic processes • Challenges: • intrinsically low sensitivity of NMR • low resolution; homogeneous and inhomogeneous line broadening in solids • Transformational role of emerging SSNMR methods/technologies: • ultrafast magic-angle spinning (MAS); indirect detection • hyperpolarization of nuclear spins; dynamic nuclear polarization (DNP) • ultrahigh field NMR

  3. Ultrafast MAS NMR probes with ultrafast MAS capabilities: new pulse sequences/theory; improved resolution & sensitivity • MAS rate: 45 kHz • volume: ~9 μl • introduced: ~2005 • MAS rate: 120+ kHz • volume: ~0.3 μl • introduced: ~2012 Transformational role of ultrafast MAS in SSNMR: • improved 1H resolution • (Δν~(νMAS)-1) • indirect detection • sideband-free spectra • better sensitivity/spin • higher RF fields • improved decoupling/recoupling • dipolar truncation Compatible with DNP & high-field magnets

  4. 1H resolution under ultrafast MAS L-histidine HCl H2O 1H MAS at 14.1 T 110 kHz 100 kHz 80 kHz 60 kHz 40 kHz 20 kHz • 1H resolution and SNR are greatly enhanced by fast MAS • CRAMPS-like resolution approached at 100 kHz • Y. Nishiyama, JEOL Resonance.

  5. t2 : t1 : t1 : t2 : X X 1H 1H Indirect detection of low-γ nuclei under fast MAS; 2D HETCOR NMR Traditional SSNMR approach: direct detection of low-γnuclei (e.g. 13C, 15N) 1H Low-γ • 1H homonuclear RF decoupling • low sensitivity Indirect detection of low-γnuclei 1H Low-γ • 1H decoupling by fast MAS • high sensitivity S/N gain: time performance improves by ~103!; convergenceof solution and solid-state NMR For 15N:

  6. 1H-13C HETCOR under MAS at 100 kHz Functionalized mesoporous silica a c b MP Excellent sensitivity (here 10 nmol) and resolution 1H-1H interactions are suppressed during CP; one-bond selectivity (dipolar truncation) 1H-detected 2D spectra possible for challenging nuclei, e.g., 89Y, 103Rh, 109Ag, 183W c b a 7 h • T. Kobayashi, et al., Angew. Chem. Int. Ed., 2013, 52, 14108.

  7. Hyperpolarizing nuclei via polarization transfer; DNP Double resonance: • Conventional NMR: involve a second nuclide; e.g.,1H-X polarization transfer, via cross-polarization (CP) or INEPT; indirect detection • DNP: involve electrons: • Combining DNP with CP; e.g. a scheme: can yield DNP SSNMR spectrometer installed at the Ames Laboratory in 2014 (9.4 T, 263 GHz) Schematic diagram of DNP SSNMR system • See e.g: T. Maly, R.G. Griffin, et al., J. Chem. Phys. 2008, 128, 052211.

  8. Modern DNP SSNMR DNP uses continuous μw irradiation generated by gyrotron and can be combined with most 1D and 2D SSNMR experiments DNP requires MAS at low temperature (~ 100 K) Sapphire MAS rotor 20 mm diam. LT MAS probe head with waveguide and insert/eject of cold samples 263 GHz beam image, near waveguide end

  9. Modern DNP SSNMR • Polarization mechanism - cross-effect (CE); a nucleus must be coupled to 2 electrons (e.g. in biradicals), such that |ωe1-ωe2| = ωn • Biradicals (below, left) are typically dissolved, and impregnated into the material Polarization mechanism Proline with 10 mM AMUPol in H2O: DNP enhancement: ε= 250 microwave 1H-X polarization transfer cross effect Signal enhancement: ε = γe/γH≈ 660 ε = γe/γN≈ 6500 DNP-enhanced Conventional Nitroxo biradical molecule (AMUPol) 1H 1H ••••• 1H X spin diffusion in frozen solvent R = (CH2CH2O)4Me • T. Maly, R.G. Griffin, et al., J. Chem. Phys. 2008, 128, 052211. • C. Sauvee, M. Rosay, G. Casano, F. Aussenac, R.T. Weber, O. Quari, P. Tordo, Angew. Chem., 2013, 125, 1-5.

  10. Our recent applications of DNP Distributions of surface functionalities on silica (29Si-29Si) Coordination geometries on Pd/γ-Al2O3 catalyst (13C-13C, 13C{27Al}) Biomass structure (13C-13C under natural abundance) Host-guest interactions in MOFs (15N, 195Pt) Studies of OH groups under natural abundance (17O) ….

  11. Ultrahigh magnetic fields; quadrupolar nuclei 14.1 T 19.6 T 25 T (NHMFL, resistive) 40 T (NHMFL, hybrid) 27Al MAS spectra of aluminoborate 9Al2O3+2B2O3 Simulated MAS spectra of 23Na in NaC2O4/NaSO4 mixture at 9.4, 14.1 and 32 T Transformational role of ultrahigh field: the second order quadrupolar broadening and shift are diminished; the line width at 40 T on the left is mainly due to field drift Z. Gan et al., JACS, 124, 5634 (2002)

  12. Challenging nuclei: low-𝛾, dilute and/or strongly quadrupolarPeriodic table at 14 T vs 30+ T; DNP Nuclei accessible at 14.1 T Very low-γ spin-1/2 nuclei Quadrupolar nuclei that are very challenging at 14.1 T : DNP Lucky users of a 30+ T system Nuclei whose quadrupolar interaction competes with NMR at 14.1 T Quadrupolar nuclei that may be accessible at 30+ T in particular circumstances

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