Introduction to Solid State NMR (2)
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INTRODUCTION TO SOLID STATE NMR (2) PSE797MR Weiguo Hu October 2013
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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). - PowerPoint PPT Presentation
Transcript of Introduction to Solid State NMR (2)
INTRODUCTION TO SOLID STATE NMR(2)
PSE797MRWeiguo HuOctober 2013
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TECHNIQUES THAT DEAL WITH CHALLENGES IN THE SOLID STATE
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)
3
MAS 31P SPECTRUM OF DOPC LIPID
Blue: non-spinningRed: MAS at 1090 Hz
* : spinning sidebandsSideband position = N * spinning speed (Hz)
*
**
*
***
centerband
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13C MAGIC ANGLE SPINNING (MAS) NMR
8/28/2013
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
DP
CP
NMR Pulse Sequences
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13C CP/MAS SPECTRUM OF A POLY(URETHANE UREA) SAMPLE
Spin speed = 5000 Hz
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CP VS. DP 13C NMR
8/28/2013
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
O
A
MDP
CPx10
CP
SSNMR of UHMWPE Fibers,Hu & Schmidt-Rohr, Polymer 2000
29SI NMR
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
EXPERIMENT SETUP - 1
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 - 2 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
