Enhancing 29Si NMR Spectra with DEPT

Measuring ²⁹Si NMR spectra of liquids can be very time consuming as ²⁹Si nuclei tend to have very long T₁ relaxation times and therefore very long delays must be left between scans to allow for relaxation. Also, since ²⁹Si has a negative gyromagnetic ratio, the NOE's with protons diminish the signals and inverse gated decoupling should be used. For molecules where there is a measurable ²⁹Si - ¹H J coupling, one can use a DEPT sequence to enhance the ²⁹Si signal. The advantages are two-fold: first, there is an enhancement on every scan from the DEPT sequence and second, the repetition rate of the experiment depends on the ¹H T₁ rather than the ²⁹Si T₁. The second advantage is very significant as the ¹H T₁'s can be shorter than the ²⁹Si T₁'s by two orders of magnitude. The example below demonstrates the advantages.

The traces on the left and right were both acquired in 1 minute with a 2 second recycle delay. One can see the tremendous signal-to-noise-ratio advantage of using a DEPT sequence. For comparison, the trace in the middle was acquired in 16.4 minutes with inverse gated decoupling and a 60 second recycle delay. (Note that the DEPT signal could have been enhanced even more if a 35 degree rather than a 24 degree flip angle was used for the final ¹H pulse in the DEPT sequence)