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Tuesday, September 18, 2007

Proton NMR of TMS

The proton NMR signal of TMS is a singlet at 0 ppm ..... right? - WRONG!
Naturally occurring TMS is a mixture of isotopomers. Silicon is 92.2% silicon -28 (I = 0), 4.7% silicon-29 (I = 1/2) and 3.1 silicon-30 (I = 0). Carbon is 98.9% carbon-12 (I = 0) and 1.1% carbon-13 (I = 1/2). Each isotopomer has a different NMR spectrum. The protons are coupled to both 13C and 29Si but not 12C and 28Si.The figure below shows a 400 MHz proton NMR spectrum of TMS. The major isotopomers responsible for the lines are labelled.

2 comments:

MD said...

Yes, of course,
I see this same on my spectrum. I see also signals 0.07ppm, small but I see that you have also. I think this signals become from isotop effect Si(CH3)3(CDH3). The signals CH2D should be as two triplets – so the intensity is very small. But the influence on the chemical shift should be larger. The position of signals is this same in spectra recorded on the 400MHz, and 500MHz. The istope effect is 0.07ppm, so it is large. What do you think about this?
MD

Glenn Facey said...

MD,

Thank you for your comment. The natural abundance of deuterium is 0.015%. The occurance of the Si(CH3)3(CH2D) isotopomer is therefore 0.18%. Although the 0.07 difference between the small signal in the spectrum and the large singlet is what one might expect to observe as an isotope effect, I am not so sure that this is what we are seeing. I suspect the small peak is due to an impurity.

One would expect the -CH3 of Si(CH3)3(CH2D)to be largely a 1:2:1 triplet due to the coupling of the 9 equivalent -CH3 protons to the -CH2D protons (there will also be much smaller coupling to the deuteron and 29Si satellites). The -CH2D protons would give a complicated multiplet due to the coupling to the deuteron and the other 9 equivalent -CH3 protons (again there would also be 29Si satellites). The signal we observe looks to be a singlet and have a bit too much intensity to be due to the mono-deutero isotopomer.

Do you agree?

Glenn