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Topic: Why is Si-29 NMR spectroscopy not used more routinely? Why is Si2(Mes)4 yellow?  (Read 4206 times)

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Offline cheese (MSW)

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(a) Why is Si2(Mes)4 yellow?
(b) Why is Si-29 NMR spectroscopy not used more routinely?
« Last Edit: May 30, 2012, 12:55:32 AM by Mitch »

Offline Mitch

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Re: Two Questions: Si Chem
« Reply #1 on: May 30, 2012, 12:54:51 AM »
(b) has to do in part with the low isotopic abundance of Si-29 (~5%).
« Last Edit: May 30, 2012, 01:48:28 AM by Mitch »
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Offline fledarmus

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Another issue is that the silicon in the NMR tube and in the probe give a large background signal that has to be removed.

a summary here:

http://chem.ch.huji.ac.il/nmr/techniques/1d/row3/si.html

Offline cheese (MSW)

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(a) Mes2C=CMes2 is I believe colorless.  Mes = mesityl: -C6H2(2,4,6-Me)3

(b)  Receptivity = a × γ^3
a = natural abundance; γ = gyromagnetic  ratio
When you plug the numbers in for C-13 and Si-29, the Si-29 nucleus (spin ½) is about twice as receptive as C-13.  And of course C-13 NMR spectroscopy is routine.
A typical NMR tube has a broad peak from -80 to -300 ppm.  This does not usually affect the spectra of organo-Si cmpds that have shifts in the range +100 to -100 ppm.  Of interest is the reference is TMS (SiMe4) that as you know is δ = 0.0 for H-1 and C-13.  (But my compliments for thinking outside the box!)

The answers to these questions have considerable pedagogical value so thank you for your posts.

Offline cheese (MSW)

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More hints:  (a)  π → π* and overlaps
(b) The gyriomagnetic ratio of Si-29 has the opposite sign to H-1!

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