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I have a molecule and I need to assign the carbon NMR to each of the carbons.

My molecule is

enter image description here

The peaks I have for the carbon NMR are shown below. I have also used the DEPT to work out if it's a $\ce{CH3}$, $\ce{CH2}$, $\ce{CH}$ or quaternary C.

17.7ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH3}$)
19.5ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH3}$)
25.5ppm $\ce{CH2}$
25.7ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH3}$)
32.1ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
36.7ppm $\ce{CH2}$
39.7ppm $\ce{CH2}$
66.0ppm $\ce{CH2}$
122.1ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
124.4ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
128.2ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
128.3ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
128.6ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
131.5ppm quaternary carbon
136.2ppm quaternary carbon
149.0ppm $\ce{CH3}$ or $\ce{CH}$ (I think this is $\ce{CH}$)
166.4ppm quaternary carbon

I think I can assign the quaternary carbon with the $\ce{C=O}$ in the ester as the 166.4ppm.

Out of the three methyl groups, would the lowest ppm of 17.7ppm be the one at the top of the molecule by itself and then would the other two at the bottom of the molecule on the alkene be 19.5ppm and 25.7ppm? I don't know which one is which, I know it is to do with cis/trans but I'm not sure which way around it is.

I also know that on the benzene ring, only three $\ce{CH}$ peaks show up as the two ortho and the two meta carbons are equivalent.

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  • $\begingroup$ You should be able to narrow down the peaks with chemical shift over 100 ppm pretty easily based on hybridization. $\endgroup$
    – jerepierre
    Oct 23, 2014 at 1:00

2 Answers 2

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If this is a real sample, and not a provided coursework spectrum, then the best recommendation is to run a HSQC and/or HMBC. This will provide complete data to allow this assignment fully, and in much the same time as it takes to run a DEPT135. I don't understand the urge to run DEPTS any more - I haven't run one for years. A phase edited HSQC will give you all of the information that a DEPT can provide, as well as connectivities between proton and carbons. Anyway, off my soapbox.

Below is a simulation of this molecule, which has reasonable alignment with your peaks.

enter image description here

A methyl centre trans to a alkyl chain will be downfield shifted to an equivalent methyl group trans to a proton. So 19 will be downfield shifted from 20, and be fairly similar in chemical shift to 16. Simulations have their limitations, and carbon shifts are solvent dependent to +/- a few ppm at least. To avoid any uncertainty, run HSQC and HMBC. These should be part of your normal procedure for assigning structures.

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Have you tried a C13 NMR predictor?
Highlights according to the predictor:
166.4ppm quaternary carbon - carbonyl
136.2ppm quaternary carbon - benzene ring
131.5ppm quaternary carbon - aliphatic carbon at far end
149.0ppm CH carbon beta to carbonyl
66.0ppm CH2 between ring and ester
The chemical shifts are slightly different (as much as 5 ppm), but I think the order will be correct.

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