13C -NMR: Cracking Those Carbons
A little background:
Protons, neutrons, and electrons all have something called “spin.” This doesn’t
mean that they’re actually spinning around in tight circles like Olympian ice skaters, but
they’re moving nonetheless and this movement creates a magnetic field around each
particle. But for simplicity (and because Chem 14C doesn’t require anything more
complicated) we’ll just picture these subatomic particles like miniature planets that are
spinning in space around a central axis.
When nothing around the atom is generating an external magnetic field these axes
point in random directions, but when an external magnetic field is applied, like it is in
NMR, they will align with each other. This doesn’t mean that they will all point in the
same direction, however. If we stick with the planet metaphor we can picture all the little
planets of our molecule now having north and south poles that point along the same line,
but some of them have the north pole facing up (in the same direction as the external
field, B0) and some have the north pole facing down (in the opposite direction of B ). I0
chemistry terms, the particles that have axes pointing in the same direction are called
parallel, and those with axes pointing in opposite directions are called antiparallel.
In NMR, energy from photons causes nuclei to change their spin. Generally they
have a “ground state” spin of +1/2, and excitation causes the poles to swap and the spin
changes to the higher energy state of -1/2. It is the amount of energy required for this spin
flip (or alternatively the amount of energy released as a nucleus relaxes to its original
state) that we measure in NMR.
What it is: C-NMR is a technique very similar to H-NMR, in which we study the
nuclear spin flips of various atoms and use a lot of math and brainpower to put the pieces
together and try to figure out the structure of a molecule.
What does it do: C -NMR gives us the carbon backbone of a molecule, which is
incredibly useful when we’re trying to figure out the structures of organic substances.
What do we use: NMR works for any atom whose nucleus has a spin quantum number
that does NOT equal zero. Since l ≠ 0 for any atom that has an odd number of protons
and/or neutrons, we use the C isotope for C -NMR.
Now that we know why we’re putting ourselves through the hassle of understanding all
of this, let’s get started with the analysis process.
First, we need to understand what the C -NMR spectrum is showing us.
1. The number of signals tells us if there are equivalent carbons.
a. If the number of signals is less than the total number of carbons in the
molecule, there is at least one pair of equivalent carbons. This also
means that there is some symmetry to the molecule; if you’re confused
about this, build a model. 2. The chemical shift tells us if there are highly electronegative atoms or pi
electron clouds in t1e molecule. Both of these things influence chemical shift
just like the do in H -NMR, the only thing that’s different is that the
numerical value of the shifts is higher in C -NMR. See table 1 below.
3. Integration gives us the ratios of equivalent carbons. Remember that this is
NOT always the exact number of carbons, it is a ratio!
4. The coupling patterns tell us the number of neighbors each carbon has. A
piece of good news: this is actually much more simple to figure out than in H
-NMR! Here’s why:
a. Only nuclei with l ≠ 0 can couple with each other, so C isn’t a valid
b. When you look at a list of all the atoms C could couple with ( H, H, 1 2
5B, C, F, etc.), the most common one by far is H. The relative
abundance of C is only 1.1%, so the likelihood of having two C
atoms right next to each other is only 0.012%. That’s reallyyyyy
unlikely. In fact it’s SO unlikely that for our purposes it’s completely
insignificant and we can just ignore it! So basically all we do is look at
the hydrogen atoms bonded to each carbon.
c. C -NMR splitting is limited to nuclei separated by just one sigma
bond. This means that all we have to care about are hydrogens directly
attached to carbons.
d. There are only 5 splitting patterns in C -NMR:
4 H’s = pentet 3 H’s = quartet 2 H’s = triplet 1 H = doublet 0 H’s = s