Sample: Fill a solid-state rotor tightly with finely powdered KBr. Use spacers if the sample does not spin.

Load standard ¹³C NMR parameters without proton decoupling. As can be seen from the table for quadrupolar nuclides in the Introduction of Chapter 9, the resonance frequency of “Br is very close to that of ¹³C; therefore, you usually do not necd to retune the probe-head if it was previously used for ¹³C. Spin the sample at 4 kHz. You have to set:

td: 2k sw: 125 kHz o1: on resonance of KBr signal p1: 2 μs “Br transmitter pulse d1: 50 ms transmitter power level (choose value typically used for ¹³C work) rg: receiver gain for correct ADC input Record 128 transients, perform the Fourier transformation and set the offset o1 on resonance of the sharp center signal. For better observation of the effect, adjust the transmitter phase relative to the receiver phase so that mainly the left quadrature channel receives the FID signal (cf. Exp. 2.8). Turn the magic angle adjustment in either direction and observe the side-band signals in the FID. Adjust for maximum side- bands at the end of the FID.

5. Processing

For the adjustment no processing is required, since the FID is observed directly.

6. Result

The figure shows in a the FID of KBr with a slightly misadjusted magic angle, ob- tained using a multinuclear solid-state probe-head with a 7 mm rotor on an AM-400 spectrometer with a wide-bore magnet. In b the FID after magic angle adjustment is shown.

7. Comments

Br is a quadrupolar nucleus with / = 3/2. The crystal symmetry of KBr is cubic, and therefore a sharp central transition for m; = –1/2 to m,=+1/2 is observed. The other transitions, however, also contribute to the spectrum. The side-bands generated by the spinning frequency are easily seen and are very sensitive to the exact setting of the magic angle; misadjustment by 0.5° can be clearly observed.