MQMAS spectra dependent on static magnetic fields B0 and RF fields B1

  • Summary

NM050001E

Recent developments of high magnetic fields and high-quality probes permit easy measurements of half-integer quadrupolar nuclei,especially in MQMAS experiments.

Sensitivity of MQMAS spectra is governed by
(1) Quadrupolar coupling constant CQ
(2) Static magnetic field strength B0
(3) RF magnetic field strength B1.

In this Note,we treat three compounds,RbNO3, Na4P2O7, and Na2HPO4,having maximum CQ of 1.94, 3.22, and 3.70 MHz, respectively. Since quadrupolar broadenings are proportional to CQ2/γB0, signal sensitivities are estimated as follows:

RbNO3 Na4P2O7 Na2HPO4
21.9T 28.8 10.9 8.26
14.1T 9.62 3.64 2.75
9.4T 3.64 1.32 1

Clearly from the above table, high magnetic fields are very effective in achieving high sensitivity.

The effects of RF fields B1 on signal sensitivities are demonstrated experimentally, using MQMAS probe (B1=150-250 kHz) and standard CPMAS probe (B1=50-150 kHz) at B0=14.1 T, in the following: Strong RF fields excite MQMAS signals efficiently leading to high sensitivity.

*Experiments at B0=21.9 T were conducted with cooperation of Dr. T. Shimizu of National Institute of Materials Science (NIMS).
 

Na2HPO2

The compound has large quadrupolar couplings so that only high magenetic field and strong RF field give high sensitivity MQMAS spectra.

B0=21.9 T(JNM-ECA930)
B1=227 kHz(MQMAS probe)
68 hours
B0=14.1 T(JNM-ECA600)
B1=192 kHz(MQMAS probe)
96 hours
B0=14.1 T(JNM-ECA600)
B1=83 kHz(CPMAS probe)
96hours
B0=9.4 T(JNM-ECX400)
B1=100 kHz(CPMAS probe)
195hours

Na4P2O7

High magnetic field and strong RF field are effective in giving high sensitivity MQMAS spectra.

B0=21.9 T(JNM-ECA930)
B1=227 kHz(MQMAS probe)
17hoursc
B0=14.1 T(JNM-ECA600)
B1=192 kHz(MQMAS probe)
34hours
B0=14.1 T(JNM-ECA600)
B1=83 kHz(CPMAS probe)
34hours
B0=9.4 T(JNM-ECX400)
B1=114 kHz(CPMAS probe)
68hours

RbNO3

Because of small quadrupolar couplings,moderate magnetic field and RF field can give high sensitivity MQMAS spectra.

B0=21.9 T(JNM-ECA930)
B1=156 kHz(MQMAS probe)
2 hours
B0=14.1 T(JNM-ECA600)
B1=167 kHz(MQMAS probe)
2 hours
B0=14.1 T(JNM-ECA600)
B1=68 kHz(CPMAS probe)
2 hours

B0=9.4 T(JNM-ECX400)
B1=109 kHz(CPMAS probe)
2 hours
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