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Introduction to Solid State NMR (2)

PSE797MR Weiguo Hu October 2013. Introduction to Solid State NMR (2). To remove orientation dependence of chemical shifts: Magic-angle spinning (MAS) Magic angle: To remove the strong 1H-13C dipolar coupling: High power decoupling Signal intensity improvement: Cross polarization (CP).

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Introduction to Solid State NMR (2)

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  1. PSE797MR Weiguo Hu October 2013 Introduction to Solid State NMR(2)

  2. To remove orientation dependence of chemical shifts: • Magic-angle spinning (MAS) • Magic angle: • To remove the strong 1H-13C dipolar coupling: • High power decoupling • Signal intensity improvement: • Cross polarization (CP) Techniques that deal with challenges in the solid state

  3. MAS 31P spectrum of DOPC Lipid centerband * * * * * * * Blue: non-spinning Red: MAS at 1090 Hz * : spinning sidebands Sideband position = N * spinning speed (Hz)

  4. Samples spins at high speed (>= 3kHz) during data acquisition to achieve high resolution • Direct Polarization (DP) • 1H to 13C Cross Polarization (CP): • Enhancement of signal 13C Magic Angle Spinning (MAS) NMR DP CP NMR Pulse Sequences

  5. 13C CP/MAS spectrum of a poly(urethane Urea) sample Spin speed = 5000 Hz

  6. Crystalline and amorphous signals are distinguished by different conformation and packing • Orthorhombic PE: 32.8 ppm • Monoclinic: 34.2 ppm • Amorphous: ca. 30 ppm • CP over-represents rigid phase • CP relies on strong dipolar coupling • DP over-represents mobile phase • Mobile phase usually has shorter 13C T1 • Quantitative when T1 relaxation is full CP vs. DP 13C NMR O A M DP CPx10 CP SSNMR of UHMWPE Fibers, Hu & Schmidt-Rohr, Polymer 2000

  7. 29Si in silica and zeolites usually has very long T1 • Why? • DP-MAS NMR often requires recycle delay of > 50s • 1H-29Si CP spectrum over-represents surface Si and underrepresent inside Si • Why? • CP pulse time (contact time) needs optimization • It needs time for 1H signal to transfer to 29Si • However, T1r relaxation happens during contact time • 1H-13C CP faces the same problem 29Si NMR

  8. Magnet safety • Will destroy watches and magnetic memories • Will strongly attract iron and steel objects • May harm embryos and stop heart pacemakers • Packing rotors • Stand on mat while packing; don’t drop rotors on the hard floor • Do not use damaged caps • Spin the rotor • Automatic vs manual mode Experiment setup - 1

  9. Create sample file from an existing standard file – dir; new • Tuning the probe – wobb • What if no dip is visible in window • Check and optimize parameters • Recycle delay • Calibration of 90-deg pulse • CP optimization • Pulsing safety • Never misuse pulse length unit: default for pulses are ms; delays are s Experiment setup - 2

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