[macman104]

Hi,

I've been working on some spectrum that in addition to MS, IR, 1HNMR, also contain both C13 decoupled and off-resonance coupled spectra. 

Two spectra:

Spectrum 1) - Suspected Formula C₇H₁₃NO₂ - Run in CDCl₃
In the decoupled spectrum, there is a peak at ~-1 ppm.  In the coupled spectra, it's a little hard to tell, due to the noise level, but I'm pretty sure that the peak corresponds to a quartet or less likely a doublet.  If it's a quartet, I assume it's TMS

Spectrum 2) - Suspected Formula C₇H₁₄O₃ - Run Neat

This one is much clearer, it has a peak ~0.5ppm, and in the coupled spectrum it is a clear neat quartet.  Is it safe to assume this is TMS?

Three Questions:
a) Is it common for TMS to show up like this, and has it always shown up like that, and I've just missed it for some reason?

b)If it is common of TMS, for future reference, how do I discern a TMS peak, from a non-TMS peak that is actually part of the spectrum?

c)  If one of the peaks are not TMS, what kind of compound should I be looking for to generate a peak that is so upfield?

I will try and upload a scan of the spectra, as I know it's difficult to understand completely what I am describing.

[Custos]

Yes I'm pretty sure it is TMS. Carbon spectra of "normal" organic compounds have resonances that generally fall from 10 ppm to 220 ppm downfield from TMS. That's why TMS is used as a reference (arbitrarily designated as 0 ppm) -- it's out of the way of the resonances of most organic compounds.

It is possible for some nuclei, both carbon and proton, to be so highly shielded that they resonate upfield of TMS, but this is uncommon. You usually only see this when, for example, a methyl group is sticking into the face of a benzene ring such that the pi clouds of the benzene are shielding the methyl group.

[macman104]

Thanks for the info.  I sort of figured it was TMS, but I could have sworn on some spectra I didn't see it.  I was getting a little worried that no one was responding, so I'm very glad you took the time.  Thank you!

[organoman]

Sorry to intervene as I am not an expert in NMR. But does TMS give a quartet??

[Custos]

TMS will give a singlet on a decoupled spectrum, but for fully coupled and off resonance decoupled spectra you see the coupling between the protons on the methyl group and the carbon, hence a quartet. All -CH₃ groups in a coupled carbon spectrum will appear as a J-1 quartet. You may also see two bond and three bond coupling (J-2 and J-3, respectively) in fully coupled spectra.

[macman104]

Thanks again for everyone's help.  I finally got a chance to scan the files into the computer to send to a friend for help, but I thought I would post them here as well.  You can see the work I've done so far, but the splitting is so strange (especially on the first one, but still on the second), I've no idea how to go about interpreting it.  And some of the C13 is a little puzzling.  Anyway, any thoughts/ideas are appreciated.

EDIT:  Also, hopefully these will allow you to see the peaks near 0ppm in the CNMR, and be able to understand my confusion (and maybe even have an answer! )

[Custos]

Looks perfectly reasonable to me. Off resonance decoupled spectra are run so you can work out how many hydrogens are attached to each carbon. A fully decoupled spectrum has J-1, J-2, and J-3 coupling, hence unless it's a compound with few hydrogens (heteroaromatics for example) the spectrum becomes unusably complex. Off-res decoupled just shows the J-1 coupling so a CH₃ shows up as a quartet, a CH₂ as a triplet, a CH as a doublet and a carbon without hydrogens as a singlet. TMS is obviously going to be a quartet, but it's out of the way so that doesn't matter. By the way, CDCl₃ continues to show up as a triplet - do you know why?

[macman104]

Looks perfectly reasonable to me. Off resonance decoupled spectra are run so you can work out how many hydrogens are attached to each carbon. A fully decoupled spectrum has J-1, J-2, and J-3 coupling, hence unless it's a compound with few hydrogens (heteroaromatics for example) the spectrum becomes unusably complex. Off-res decoupled just shows the J-1 coupling so a CH₃ shows up as a quartet, a CH₂ as a triplet, a CH as a doublet and a carbon without hydrogens as a singlet. TMS is obviously going to be a quartet, but it's out of the way so that doesn't matter. By the way, CDCl₃ continues to show up as a triplet - do you know why?

Sorry, maybe I hadn't made my knowledge on the spectra clear.  I understand clearly why the carbons have different peaks, and differences between fully decoupled and off-resonance decoupled spectra, etc.  I realize that CDCl₃ gives a triplet of equal heights (if I remember correctly, it has something to do with the ²H having a spin of 1 instead of a 1/2 like ¹H).  I was hoping people could offer their insight/opinions on some aspects of the spectra.  In order to narrow the scope of my "Help me interpret these spectra question" (which I've read is something to aim for), let me put a little list of some of the aspects that are giving me the most difficulty.

For the first spectra:

1H NMR
-The splitting is so incredibly confusing.  I've looked at the blowup to the left of the peaks, and there is clearly some sort of ____ of doublets, or something causing the strange splitting pattern, but I've no clue as to what.

C13NMR
-There are two messy peaks in the off-resonance spectrum.  Are those 2 separate carbons that are just very similar?  Or is it one, and it's just...messy.
-Off resonance again, down by 0ppm, is that the two middle peaks of the TMS quartet?

-Decoupled spectrum, there are two peaks, one at 85, the other just below 100, ignore those?  Are they part of my compound?


For the second spectrum

1H NMR
-I know that irradiating a particular hydrogen isolates from coupling with any other hydrogens.  This allows us to more closely look at the various coupling and helps figure out what hydrogens are coupled to eachother.  However, I'm struggling to see the difference, if there is any for most the irradiation inserts.  The only one that really jumps out is the circled one, on the bottom left.  It's pattern is different compared to the other irradiations on that side.  The irradiation in the middle, of the peak at 4.2, shows no (?) change.  I suppose one could argue that it appears the irradiation at the top (of 1.2) has a lower coupling constant that the middle irradiation (of 3.55), but I'm not sure.

C13 NMR
-I assume the peak at 0ppm is TMS.  At 65 ppm, I would imagine that is 2 carbons, ya?


Hopefully, that'll help narrow the spectrum of things to discuss, and allows people to focus on what parts I'm truly confused about.  Basically, I fully understand what the different spectra are, and how to use them, and for the most part, how to interpret them.  I've got many more in this packet, that I did not need to post about, because I was able to handle them, these are the 2 that I'm stuck on.

Thanks again for any insights!

-Mac

[macman104]

I was just wondering, if there was something wrong with the attachment, or if my question didn't make sense, or if people simply did not know how to answer/what the answer was/didn't have time to answer.  I'm glad I got some response initially, because I know there are threads that go unanswered (though viewed many times).  Anyway, I'll try and veil my *Ignore me, I am impatient* not as a nagging, "Someone answer me!", but more as a respectful nudging in hopes of someone helping me understand the issues I'm having (Did it work?  )

Either way, thanks for the insight so far,

-Mac