[egghead01]

Hi guys. Exams are over and my NMR skills are still too weak. Hoping I can get some help here. I'm trying to interpret a 1H NMR spectra for this molecule:



I have no problem assigning peaks to the protons. However, am having a problem with the splitting for the H atoms marked by the arrow. The spectra shows that the signal has 10 peaks. The splitting is not a mess.

I'm thinking that the splitting pattern is like this:
doublet for cis- H on alkene
doublet for trans- H on alkene
triplet for CH2 adjacent to N

I must be wrong because the spectra shows 10 peaks. Please help
Thanks!

[discodermolide]

Hi guys. Exams are over and my NMR skills are still too weak. Hoping I can get some help here. I'm trying to interpret a 1H NMR spectra for this molecule:



I have no problem assigning peaks to the protons. However, am having a problem with the splitting for the H atoms marked by the arrow. The spectra shows that the signal has 10 peaks. The splitting is not a mess.

I'm thinking that the splitting pattern is like this:
doublet for cis- H on alkene
doublet for trans- H on alkene
triplet for CH2 adjacent to N

I must be wrong because the spectra shows 10 peaks. Please help
Thanks!

You forgot the N-Methyl and allylic coupling of the N-CH2 to the olefinic CH2.
The molecule is symmetrical.

[egghead01]

Thanks for replying.

The N-methyl hydrogen are 5 bonds away from the arrow-pointed groups though.

I just tried using the predictor here http://www.nmrdb.org/predictor.
The site gave a ddt split for the arrow-pointed H. Is this correct? There are 12 peaks in the predicted spectra.
Does that mean the spectra that I have earlier is wrong or lacking the resolution maybe?

[fledarmus]

Take your splitting one source at a time:

If you look at the leftmost hydrogen marked with the arrow, step one carbon to the right, how many hydrogens are on that carbon? Are they in identical magnetic environments? If they were the only source of splitting, what sort of patter would you see for the arrowed hydrogen?

Now step one carbon to the left. Answer the same questions...

If you will provide the answers to these questions in your next post, we can show you where to go from there

[egghead01]

I would predict a doublet doublet split for the adjacent alkene 2 H since the cis and trans H would each split the signal into doublets.
and a triplet split for the CH2 group adjacent to N

So the split is a doublet + doublet + triplet. Am I right? But how would this split look like?

[fledarmus]

What would you expect for coupling constants?

[egghead01]

What do you mean 
As in the independent coupling constants?
I have very little understanding of j-constants outside simple splits and my textbook doesn't much examples.

[fledarmus]

I was thinking something along these lines:

http://chem.chem.rochester.edu/~nvd/NMRH1ShiftsandCoupling.doc

There is a very limited list of coupling constants here, but sufficient for this exercise. The first three entries will let you start to prepare a splitting diagram. (You can find much more detailed lists and some methods of calculating coupling constants in most NMR texts and all over the web.)

So get yourself a piece of graph paper and pick a point - that is the chemical shift of your proton. Now, your coupling constants (from the table) are 6-8 for the coupling to the two protons to the right, 11-18 from the trans proton, and 6-15 for the cis proton. Let's just pick some numbers that will make the drawing easy - 8 for the CH2, 14 for the trans, and 10 for the cis.

So your CH2 will give you a triplet, with peak heights in the ratio 1:2:1, with each peak about 8 Hz from the next. On your graph paper, that means you would have your center peak at the point you picked as your chemical shift, with a height of 2, and a peak at +8Hz and another at -8Hz, each with a height of 1.

Then each of these is split by the trans proton, ~14Hz. Each peak will be split into two peaks, 14 Hz apart, which means it will give one peak at +7 and one peak at -7 in relation to your previous peaks. So your peak at -8Hz will be split into two peaks of equal height at -15Hz (-8-7) and -1Hz (-8+7), the peak at 0 will be split into two peaks at +7 and -7 (twice the size of the 15 and -1 Hz peak because this peak had a height of two), and the peak at +8 will be split into two equal peaks at +1 and +15.

I'll leave the third split for you - each of these peaks now gets split into a doublet separated by 10 Hz (+5 and -5). What is your final splitting pattern? You should be able to give the distance from the chemical shift and the peak integration (which I've been calling peak height).

[egghead01]

Wow. Thank you. I've never actually learned how to think of splitting this way. I had all the time just memorized that so and so gives such and such splitting.

So I got 10 peaks cause two of the splits just happen to overlap.

Thanks again! I'm surprised I've never learned this from my prof or TAs