6.3.2 (a) 13C NMR Spectroscopy

(a) analysis of a Carbon-13 NMR spectrum of an organic molecule to make predictions about:

(i) the number of carbon environments in the molecule

(ii) the different types of Carbon environment present, from chemical shift values

(iii) possible structures for the molecule

What does this mean?

Nuclear Magnetic Resonance Spectroscopy uses the spin properties of nuclei to infer the structure of a substance.

Frankly, the reason why only isotopes with odd mass numbers have magnetic fields is Physics and therefore of no earthly interest to normal humans.

But if we place such isotopes into a magnetic field you would expect the field of the nuclei to line up with the field that you have placed it in.

But given a little more energy it will flip into the opposite spin-state which has a higher energy.

It will then resonate between the two states.

So, energy will be absorbed to promote the nucleus to its higher energy state and this energy is in the Radiowave part of the EM spectrum.

Exactly how much energy (and which frequency of radiowaves) will need to be absorbed to make resonance happen depends on the type of nucleus and its immediate environment - neighbouring atoms interfere with the field of the nuclei we are looking at.

And you should know that this only works for atoms with odd Mass Numbers.

THis is why we can only pick up 13C , rather than the much more common 12C

So, since we are looking at 13C NMR we have a reference 13C atom to serve as a baseline.

This is the Carbon atom in Tetramethylsilane (TMS)

Or more precisely, the four equivalent Carbons in TMS.

We place the signal for this substance at 0 ppm (parts per million) and every other Carbon atom has a chemical shift that takes it away from 0.

Above we see that although we are only detecting the 13C atoms generally it is the case that Carbon atoms bonded to other atoms with many electrons tend to be shifted most.

You really don't need to memorise the list - it is given in exams.

It's a good idea to know why TMS was chosen - four identical carbons gives a strong signal and (almost) all chemical shifts from TMS will be positive (same side of 0).

TMS is also inert and can therefore be relied on not to react with the substance being studied.

And it is volatile so easily removed from the substance if you need to use it again.

A solvent is usually needed to dissolve the substance before running the spectrum.

This spectrum has 2 peaks (discounting TMS).

So it has 2 chemical environments for Carbon atoms.

This doesn't necessarily men that there are only 2 Carbon atoms in the molecule because several Carbon atoms can be in the same environment.

One carbon environment appear to be C-O because the chemical shift is around 60 ppm.

The other appears to be in a standard alkyl group because it has a very small chemical shift

If we had been told that the substance was C₂H₆O then we could discount it being dimethyl ether

If only because this molecule would only show one peak at around 50 -80 ppm, since both Carbon atoms are in identical environments.

If we knew nothing else about this molecule it clearly has 4 different chemical environments.

One is C-O = 60 ppm

One is COO- which is always close to 200 ppm.

You wouldn't be expected to identify the other peaks usually other than to say that they are both Carbon atoms in standard alkyl groups.

If you were guessing than the peak at 20 ppm is the CH3 on the left because it is shifted more than the peak at 15ppm which probably represents the CH3 on the righthand side, further from the C=O.

Given the formula C₄H₈O₂ and the above spectrum you could reasonably guess the structure - especially if you were given a few extra hints from other sources, or simply told that it was some sort of ester.

Looking at this spectrum it is clear that there are 5 different Carbon environments because there are 5 peaks.

The peak shifted to 173 ppm only overlaps with C=O on our list.

The peak at 65 ppm could be in a number of environments.

If we assume that we already knew the substannce to be C₆H₁₂O₂ then it could either be C-O or -CH₂- and we couldn't be sure which until we made some more assumptions.

Peaks up to around 30 ppm are usually Carbon atoms in alkyl groups and not immediately next to a functional group.

In our case the tall peak at 20 ppm suggests that there are two C atoms in the same environment - which is how we can have a substance with the formula C₆H₁₂O₂ but only 5 peaks. But "peak heights" can be deceiving.

The easiest (but not only) way to have two carbons in the same environment is to have 2 methyl groups.

At this stage we probably don't have quite enough information to identify the substance unless we have data from IR spectroscopy or 1H NMR or information concerning chemical tests.

But given the displayed formula it is certainly possible to say which peaks represent (most of) the Carbon atoms in the structure.