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Monday, December 10, 2007

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)

6 comments:

Egon Willighagen said...

Hi UONMRF blogger,

great blog! Items are so informative that I always think they are too short!

BTW, may I request use of <sup> in the text for things like 29Si? Unfortunately, it is not allowed in comments :(

Glenn Facey said...

Egon,

Suggestion noted.

Glenn

PASUPATHY said...

DEPT 135 of cyclohexyl derivative (contains only CH2 groups, no CH and CH3) showed positive signals.
We recorded DEPT 90 out of curiosity. It gave same type of spectrum as DEPT 135.
Is it something to do with the cyclohexyl flipping (CH2 appears like CH). Is it a well known phenomenon. Your expert comments are appreciated.

pashu

Glenn Facey said...

Dear Pasupathy,

Thank you for the comment. First of all would not a cyclohexyl derivative have one CH and 5 CH2's?

In any event, a 13C DEPT 135 spectrum of a system containing all methylenes should of course have all peaks phased negatively, however the automatic phasing routine on your spectrometer does not know anything about absolute phases so it may phase all of the peaks positively. Just adjust the phase by 180 degrees so that the absolute phase is correct.

A DEPT-90 is very sensitive to proper pulse calibration and probe tuning. I suspect that one of these issues is the problem with your DEPT 90. It is very difficult to obtain a DEPT 90 spectrum completely free of artifacts.

I hope this helps.

Glenn

Unknown said...

Hi Glenn

When running a DEPT pulse sequence for 29Si detection, is it necessary to use the same pulse length on the I and S nuclei pulse sequences when the pulses coincide?

Killian

Glenn Facey said...

Killan,
No. It is not necessary that the simultaneous pulses are of the same duration.
Glenn