[janapix]

Even with the aid of a great textbook, I've been struggling to assign these spectra for weeks now, mostly due to lack of experience in structural analysis.

I've added images to help explain what I've achieved so far with the H-NMR, C-NMR and 2d spectra of this compound. Mind, This is the first time I've attempted to do 2d spectra. I've been given HMBC and HMQC spectra, and am finding the former horrendous... no good easy guides online either so I'm hoping someone here will have the patience to help me walk the baby steps with 2d spectra and point out what I'm missing with C-NMR and H-NMR.

image link: http://i317.photobucket.com/albums/mm395/MunsterMunster/University%20stuff/spectrumhelp.png


image link: http://i317.photobucket.com/albums/mm395/MunsterMunster/University%20stuff/spechelp2.png

I see 12 C peaks for 12 C-environments, and in the C-NMR have assigned peaks A & B as C1 and C3. I've picked up the shorter peaks D,E & L (knowing they won't be attached to protons) and assigned them as C5, C7 & C13 respectively, based on their distances from electronegative groups. That is all.

With the proton NMR there are 7 peaks for the seven H-environments. I'm not sure where to go from here... are the para- protons (H16, H10) on the benzenes more affected by electron withdrawal than protons closer to such groups (H14, H18, H6)?

If there is more I can eke out of these spectra, please let me know- if not, perhaps you can help them make sense with the 2d spectra I'm about to post.

[janapix]

Here are my 2D spectra. Forgive me if my annotations include/exclude valid cross-peaks.
In addition to wanting to know HOW these are meant to be analysed, I'd also appreciate some information on how these should be reported in a report (A 2-step synthesis of this compound via hippuric acid).

image link: http://i317.photobucket.com/albums/mm395/MunsterMunster/University%20stuff/2dspechelp1.png


image link:http://i317.photobucket.com/albums/mm395/MunsterMunster/University%20stuff/2ddd.png

[djt]

janapix,

Fulling assigning all proton/carbon signals is good practice, even if journals do not require. You will however find that if you write a thesis (in synthetic organic chemistry) examiners will expect you to do so, or at the very least show you have made an attempt.

They tend to get easier with practice. That said it's not always possible to fully assign spectra and the spectra can be extremely complicated.

I've not really got the time to go through the NMR in detail so will just try and point you in the right direction for the time being.

A COSY might also be beneficial, do you have that?

I start off with the basic proton and assign what I can from that. I then use the cosy to further assign the proton signals where I can.

Next I look for at the direct C-H coupling and assign what carbons I can. A DEPT can also be useful at this point (maybe not in this case), this can help distinguish between close peaks in different environments (CH3/CH2/CH).

Lastly and most difficult is interpreting the long range C-H coupling (HMQC). Using the information you have collated so far it usually possible to fill in the gaps. This is quite difficult to explain, so let me come back to you, maybe see how you get on post what you have managed to assign.

Finally, count the number of protons/carbons in your assignment and make sure they match the number in the compound you are assigning. You will be amazed how many typos you can make writing up spectra.

Another thing I should add, never trust a 2D spectra printed straight from the spectrometer automatically, you should always reprocess 2D spectra.  

Good luck
djt

[janapix]

Thanks djt;
 I have checked my counting: there are 12 carbon environments and 13 peaks in the C-NMR, but I assume the one on the far right is solvent. The H-NMR has 8 peaks and 7 H-environments, so I think there's a sovent peak to the far right.

I've haven't run these myself (I'm at the stage where we just do IRs...), but have been given PDF files of H-NMR, C-NMR, HMBC and HMQC.

The problem with my assignments is that they're a bit too hypothetical for comfort. Here's what I have thus far:
C-NMR: peaks D, E & L (lowest intensity) are C5, C13 and C7 respectively. Of these three carbons, C7 was the furthest away from electronegative atoms N & O, and C5 was the closest. Peaks A and B have to be C1 and C3 respectively. Peaks G & H have the second lowest intensity, so I think these should be C16 and C10, though I'm uncertain which is which.
How am I doing so far? Making sense?
I've no idea why there is a solvent signal in the H-NMR because d6-DMSO, the solvent contains D instead of H atoms.

Looking at the HMQC (attached below), I see C5 and C13 producing no cross peaks, which corroborates the argument. C6, which I believe to be C-NMR peak C, has one cross-peak with H-NMR peak C, defining the C--H link at that position. The integral of the H-NMR peak supports this single-proton assignment.


edit: removed HMQC since it was already posted.

[djt]

Lastly and most difficult is interpreting the long range C-H coupling (HMQC).

Sorry, I meant HMBC.

I think you are probably on the right track, but maybe going about things the wrong way.

Write out you spectra in the form of:
8.0 (s, 1H): A singlet at 8.0 ppm, which integrates to 1 proton.
7.32 (d, J = 6.8 Hz, 2H): A doublet at 7.32 with a coupling constant of 6.8 Hz.
And so on. This will help you the more you assign, as you can coupling constants easily in a number form and hence which protons are coupled to which.

This is a difficult NMR, especially for your first attempt at 2D NMR. There is an obvious starting place though? Can you assign proton 6? Once you are able to assign carbon 6 (hint what is it coupled to?), which in turn should allow you to assign carbons 5 & 7 (from the HMBC). From there you should be able to assign the rest of the proton/carbon signals of the same ring (8,9,10,11 & 12).

Once you have fully assigned one ring the second will be easier, which you will become obvious from the splitting patterns.

As for observing residual solvent (DMSO) peak in your NMR, this is one of those things. A solvent can't be completely deuterated and most NMR solvents will be 99.9% deuterated, with 0.1% being non-deuterated. Have a look at the aldrich website for various NMR solvents you will see none are 100%-deuterated.

This link is also useful, if you have access to ACS journals.

http://pubs.acs.org/doi/abs/10.1021/jo971176v?journalCode=joceah&quickLinkVolume=62&quickLinkPage=7512&volume=62

djt

[djt]

Another thing that can also be useful, is to draw out your structure and write out the coupling you would expect for each proton. In this case lets just think about primary coupling, so for example proton 16 would be triplet, as would proton 10.....

[janapix]

Just one question about HMBC spectra (I'm attempting it with fresh eyes!.. again)
The question is: what exactly is meant by long-range couplings? Cross-peaks 1 and 2 on my HMBC belong to C1 and C3, which are connected to no hydrogens.
How far does this coupling extend? How will I know if carbons are coupling with hydrogens 2,3 or 4 bonds away? Do the cross-peaks look larger/smaller?

[janapix]

Lastly and most difficult is interpreting the long range C-H coupling (HMQC).

 Can you assign proton 6? Once you are able to assign carbon 6 (hint what is it coupled to?), which in turn should allow you to assign carbons 5 & 7 (from the HMBC). From there you should be able to assign the rest of the proton/carbon signals of the same ring (8,9,10,11 & 12).

I've assigned C6 to peak C. From the HMQC, it shows me that H-NMR peak C is H6.
The cross-peaks on the HMQC are:
17 - very slightly, previously assigned C7 (peak L)
16 - strongly
18 - very slightly

I am confused because: 
1. there is no long-range coupling to C5 (peak D) on the HMBC which I assigned based on its low peak intensity and deshielding from nearby O & N atoms.
2. If I am wrong in assigning C5 to peak D, then I am completely lost as to what the HMBC is trying to say. Cross-peak 16 can NOT lead to C5 (because this seems to have LR-coupling with yet another H-atom and the structure doesn't justify it).

Am I just plain wrong in grouping Carbon assignments based on peak intensity?
I think peaks A, B & L have no attached hydrogens  , as seen in C5, C7 & C13 (lowest intensities)
and that peaks C, G & H (second lowest intensity) must correspond somehow to the instances where a carbon bonds to one hydrogen, but there is no duplicate environment, as seen in C6, C10 and C16.

[djt]

OK, I'll try and start you off.

7.35 (s, 1H, C₆-H). From the HMQC, it looks like it is coupled to carbon signal at ~ 130.74.

The carbon signal at 130.74 in turn looks like it is coupled to the doublet at ~ 8.3 ppm, thus 8.32 (d, J = xx Hz, 2H, C_8/12-H).

You should now be able to assign the rest of the proton signals for that ring and hopefully the rest of the carbon signals for that ring. This is where the coupling constants will be useful as I mentioned in a previous post.

2. If I am wrong in assigning C5 to peak D, then I am completely lost as to what the HMBC is trying to say. Cross-peak 16 can NOT lead to C5 (because this seems to have LR-coupling with yet another H-atom and the structure doesn't justify it).

To answer your other questions, short range coupling (HMQC) usually refers to single C-H bonds. Longer range coupling (HMBC) is usually 2 -bond coupling, sometimes more. So in your example, proton 10 is coupled to carbon 9 & 11. Proton 11 is coupled to carbon 10 & 12.

Am I just plain wrong in grouping Carbon assignments based on peak intensity?
I think peaks A, B & L have no attached hydrogens  , as seen in C5, C7 & C13 (lowest intensities)
and that peaks C, G & H (second lowest intensity) must correspond somehow to the instances where a carbon bonds to one hydrogen, but there is no duplicate environment, as seen in C6, C10 and C16.

Using the intensity of carbon signals can be a useful guide, but shouldn't be used as a method for assigning your spectra. There are a number of factors that that can affect the intensity of a carbon signal not just it's chemical environment.

See how you get on now.

djt