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

Summary
Information

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 C_Q，
(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 C_Q of 1.94, 3.22, and 3.70 MHz, respectively. Since quadrupolar broadenings are proportional to C_Q²/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).