Adjust the Magic Angle with KBr 

By Shaoxiong Wu, Emory University November 2, 2002

 Background:

Magic Angle Spinning (MAS) is used routinely in the most of solid state NMR experiments, where its primary task is to remove the effects of chemical shift anisotropy and to assist in the remove of hereronuclear dipolar-coupling effects. It also used to narrow lines from quadrupolar nuclei and its increasingly the method of choice for removing the effects of homonuclear dipolar coupling. It requires very high spin rate.

The solid sample rotor is spun rapidly about a spinning axis oriented at the magic angle 54.74⁰ with respect to the applied magnetic filed axis. This angle must be set accurately. Most CPMAS probe has an adjustable knob that allows us to fine tune the magic angle mechanically.

Why use ⁷⁹Br?  It is a quadrupolar nucleus with I = 3/2. The crystal symmetry of KBr is cubic and therefore a sharp central line could be observed and easy to recognize the spin sidebands for adjusting. Another advantage is the resonance frequency of ⁷⁹Br is very close to that of ¹³C, so a little effort  to tune the probe. 

Hardware and Connections:

Use X tune and X Match to tune the probe for ⁷⁹Br. Two cables from interface to the probe 1H and X, The 1H is connected to the number 4 preamplifier with 1000 Watts input, The X is connected to the number 6 preamplifier with 1000 Watts input.  Two BP filters are used: 1H BP ( 0 – 31P) for proton channel and 0-31P, 19F LP(1H) for X channel. A 125 – 205 MHz filter is installed on number 6 for X-channel.

Sample and Preparation:

KBr (Potassium Bromide). Fill a 4mm ZrO2 solid state NMR rotor tightly and evenly with finely powered KBr. Use spin test station, make sure it is able to spin at 5 – 8KHz. Keep the sample in a dry box  when it is not in use since KBr is very hygroscopic.

Parameters:

TD: 2k                                                                         Pulprog: zg

SW: 125k                                                                    NS: 64

O1: on the resonance of KBr signal                         AQ: 8.2ms

P1: 10us                                                                       PL1: 20DB

D1: 500ms                                                                   RG: 256

Spin Rate: 5KHz                                                          SFO1: 150.365 MHz

Pulse Program:

zg

 Operation Procedure:

1. Insert KBr sample
2. Load the HXY-KBr parameters and type ii
3. Type WOBB to tune the probe for ⁷⁹Br.
4. Set spin rate to 5kHz
5. Check all parameters
6. NS set to 1 and zg to run a scan to check.
7. Use GS to tune and Magic Angle.

 Result and Spectrum:

Magic Angle is properly tuned. 

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Last updated: February 05, 2003.