Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: mira on February 02, 2012, 12:13:16 PM
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Hi
I have isolated my final product, and now even though I know my compound is fairly pure and the proton NMR supports the suggested structure, however, my carbon NMR spectrum always show all my signals with the exception of one carbon. This carbon is part of an internal alkyne (so one C in the C-C triple bond shows but the other doesn't). Note: I did run the sample overnight so I know that it's not a matter of scan numbers. Any suggestions why? Any thoughts would be appreciated.
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With the quarternary carbons (like an internal alkyne), you often need longer relaxation and delay times in order to excite and relaxe these carbons respectively. What kind of relaxation and delay times are you running?
I sometimes have problems with my carbons vanishing inside the solvent residual carbon peaks (for CD3Cl). It might be worth a quick zoom-in on those peaks just to make sure nothing is hiding in there:D
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Thanks for the reply...
For the delay time, I am setting it to d1=2 seconds, and as I mentioned I did run it overnight. I am also pretty sure that my solvent signals (around 77 ppm) do not overlap with the alkyne carbon signals (which usually appear around 80-100 ppm). I am thinking that is it possible that both alkyne carbon are giving rise to a single peak?
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If it's a symmetrical alkyne they will overlap. Otherwise, they should give different peaks.
What kind of substituents do you use? I usually find that EWG makes the relaxation slower and quarternary carbons are lost in the noise, this after p*ssing all my co-workers off by using up the whole night runs.
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Hi
I have isolated my final product, and now even though I know my compound is fairly pure and the proton NMR supports the suggested structure, however, my carbon NMR spectrum always show all my signals with the exception of one carbon. This carbon is part of an internal alkyne (so one C in the C-C triple bond shows but the other doesn't). Note: I did run the sample overnight so I know that it's not a matter of scan numbers. Any suggestions why? Any thoughts would be appreciated.
Use more sample? Do you have 50 mg? You can also try setting a much longer d1. I use 9 sec when I want to see carbonyls.
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Am I missing something here: have you run the sample in a different solvent (e.g. CDCl3, C6D6, (CD3)2CO)? It would be difficult for your truant signal to keep hiding under a solvent peak. :) Furthermore, accidental degeneracy of chemically-inequivalent C atoms (that happens!) is often removed in a different solvent.
Is there a reason why the alkyne C should have an extremely long T1? For example, it has no H atoms within four bonds (e.g., C≡C-C6X2H3). Need a longer delay time (as previously suggested). I hope it is not bound to a N atom (i.e., C≡C-N)? If this fails get back to me and I will tell you of a trick we used back in the dark ages of C-13 NMR to quicken the relaxation of lazy C atoms.
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I actually managed to get all my carbon signals. What i did is that I ran the sample in CD2Cl2 (instead of my standard solvent CDCl3). So now I am glad I have all my carbon atoms accounted for :)
And if you don't mind, can you tell me what trick you use to get the "lazy" carbons in the spectrum (just for future reference).
Thanks alot
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Alkyne quats, obviously depending on your substitution, usually come near the chloroform peaks. It was probably not lazy, just overlapped.
As for stuff not showing up well (for me, ketones and amides can be a bit reluctant), a more concentrated sample and more scans is probably the way forward. We have a few spectrometers in our department, but I generally only run my carbons on the higher frequency one as the resolution is far better.
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One way to find quaternary carbons is to do a 2D hydrogen-carbon that is set up for long range coupling (two and three bond C-H), not one-bond coupling. HMBC or heteronuclear multiple bond correlation is what this is called. The pulse sequences to do this have various acronyms (like HSQC and HMQC), but the details may be unimportant.