4. Acquisition

Time requirement: 15 min

Sample: 50% TMS in CDCl₃.

A multinuclear probe-head is required for the measurement of ²⁹Si spectra. After tuning the probe-head to ²⁹Si on the observe channel and to ¹H on the decoupler channel, determine the 90° pulse for ²⁹Si. Load the DEPT pulse sequence (see Exp. 6.9) and record a DEPT spectrum. You have to set:

decoupler attenuation and 90° pulse for CPD
td	64 k
sw	250 ppm
o1	70 ppm below the frequency of the ²⁹Si signal of TMS
o2	middle of ¹H NMR spectrum
p1	90° ¹H decoupler pulse
p2	180° ¹H decoupler pulse
p3	16.8° ¹H decoupler pulse corresponding to the 12 equivalent protons of the sample
p4	90° ²⁹Si transmitter pulse
p5	180° ²⁹Si transmitter pulse
d1	1 s
d2	1/[2J(Si,H)] = 0.07 s, calculated from ²J(²⁹Si,¹H) = 7 Hz
ds	2
ns	32

5. Processing

Use standard 1D processing as described in Experiment 3.2 with exponential multiplication (lb = 3 Hz).

6. Result

The figure shows the 79.44 MHz ²⁹Si NMR spectrum obtained for TMS with an AM-400 spectrometer. Spectrum a is a normal spectrum, which shows in addition to the TMS signal a broad signal at approximately −110 ppm. This signal is due to the glass NMR tube and the quartz insert surrounding the receiver coil. Spectrum b was taken with the DEPT sequence under otherwise identical conditions. Note the improvement in the signal-to-noise ratio. The signal of the glass is suppressed.

7. Comments

To understand the DEPT sequence you can follow the discussion in Experiment 6.9. In the DEPT experiment the optimal polarization transfer is controlled by the angle α of the last pulse. The optimum pulse angle α_opt is independent of J and depends only on