Introduction to NMR: Carbon-13 (¹³C) NMR

In this lesson, we will learn:

• How NMR is used as evidence for the structure of organic molecules.
• How to read ¹³C NMR spectra and identify different carbon environments.
• How to predict a ¹³C NMR spectra for simple organic molecules.

• Nuclear magnetic resonance (NMR) spectroscopy is a very useful tool to help chemists understand the structure of a molecule.
  In a short sentence, NMR spectroscopy tells chemists how the atoms in a molecule are attached to one another. For atoms like carbon and hydrogen, it will give you strong evidence for particular ‘environments’ in the molecule like -CHO, COOH and other groups/bonds.

The longer explanation of how this works is below:
• Some nuclei have a property called spin – it is not literally spinning but for now you can think of it this way. Hydrogen-1 (¹H) and carbon-13 (¹³C) both have this property - very useful for organic chemists.
• NMR machines have a magnetic field that ‘flips’ the spin of a nucleus - think of it as now spinning the other way (clockwise $\,$→$\,$ anticlockwise). When this ‘flips back’ to its original spin state it releases radio-frequency energy which is measured by the NMR spectrometer.
• Electrons around the nucleus interfere with the machine’s magnetic field; the electrons are shielding the nucleus.
• As a result, there is a difference between the machine’s field and the nucleus’ field it experiences. This chemical shift is seen by the change in frequency (Hz) of energy the nucleus emits.
• Because the electrons cause the chemical shift (symbol $\delta$), NMR just reveals what the electrons are ‘doing’ around a nucleus – different shielding means different chemical shift. What bonds are the electrons making?
• NMR spectrometers run at a variety of frequencies (their field strength) so the units on an NMR spectrum is parts per million (ppm) . Ppm is just a fraction (parts per million!) of the nuclei’s resonance frequency compared to the resonance of a reference sample called tetramethylsilane (TMS) defined as 0 ppm . This TMS reference is just like how the freezing point of water was chosen to be 0°C and we measure everything else compared to that. For ¹H and ¹³C the shifts are positive ppm values, which means the larger the ppm, the less shielding of the nucleus.

Using NMR over time, chemists have built up a large catalogue of chemical shifts that are evidence for a nucleus’s environment – what it is bonded to – so we learn the structure of the molecule around these nuclei. This catalogue or database is called an NMR absorption table and they are widely available online and in most chemistry textbooks. This data is how NMR is used to help find a molecule’s structure.


• NMR can be run for any nucleus that has non-zero spin, so conveniently for organic chemists, NMR works for carbon AND hydrogen because they both have a ‘spin active’ isotope. ¹H is the major isotope of hydrogen and is spin-active, and ¹³C is a minor (1.1%) spin-active isotope of carbon.


• An NMR spectrum shows an absorption peak for every environment, not every atom!
  To find a ‘carbon environment’, tell a story of how the specific carbon atom is connected to the rest of the molecule. If the story is different to any other carbon atom, it is a unique environment and will have a unique NMR peak/absorption. You can have every atom in a unique environment, but it’s not always the case.
• Symmetry is your friend when analysing NMR! If you have a symmetrical molecule (such as tetramethylsilane, TMS) some nuclei will produce the same NMR spectrum because they’re in the same environment as something else. This is how NMR spectra can show ‘less peaks’ than the number of atoms in the molecule being analyzed. The reference TMS is an example of this:
• TMS, (CH₃)₄Si, has four identical methyl groups. Because there is only one carbon environment, there is only one peak in the ¹³C NMR spectrum. It has twelve hydrogens; all of them are in one of four identical methyl groups, so there is only one peak in the whole ¹H spectrum too.
• When you are mapping peaks on an NMR spectrum to a molecule, different peaks will usually be given labels like C_a, C_b or H_a/H_b etc.

See below for this example using TMS. It will show only one signal in a ¹H NMR spectrum and only one signal in a ¹³C NMR spectrum – both are defined as 0 ppm for reference.


• A ¹³C NMR spectrum has absorption ‘regions’ where certain carbon environments are generally found:
• Between 0-100 ppm, saturated carbon signals will be found. This region can be split into two further parts:
• 0-50 ppm: saturated alkanes. From lower to higher ppm, this includes terminal carbon atoms (-CH₃), secondary carbon (-CH₂-) and tertiary carbon. Most of the halogens (R₃C-X) are also found here.
• 50-100 ppm: saturated carbon bonded to oxygen, such as ethers (R₃C-O-R’) and alcohols (R₂C-OH). Alkynes (C$\equiv$C) are also found here.
• Between 100-200 ppm, unsaturated carbon signals will be found. This region can also be split into two parts:
• 100-150 ppm: unsaturated hydrocarbon environments. This includes alkenes (R ₂ C=CR ₂ ) and aromatic carbon.
• 150-200 ppm: unsaturated carbon bonded to oxygen. This includes ketones (R₂C=O) aldehydes (RCHO), esters (RCOOR’), amides (RCONR₂) and carboxylic acids (RCOOH).

These values reflect how ‘deshielded’ the nucleus is; the larger the ppm, the more deshielded a nucleus is. Deshielding is more prevalent in double bonds and bonds to electronegative atoms.


• Worked ¹³C NMR example: pentan-3-one.
  Below is the structure of pentan-3-one, C₅H₁₀O, where we will go through explaining why it has the NMR spectrum that it does.
  How many carbon environments can you see? Remember symmetry!

Start by telling the story, from one end of the carbon chain to the other.
• The first carbon is at the end of a hydrocarbon chain bonded to three hydrogen atoms – it’s a CH₃ carbon. Next to it is a -CH₂- group followed by a carbonyl, another CH₂ and finally a -CH₃ chain end. We can call this C_a.
• The second carbon is part of a CH₂ ‘unit’ bonded to a carbon atom on either side. One is a CH₃ chain end, and one is a central carbonyl carbon. This story is already not the same as the first carbon – they’re two different carbon environments and will have different ¹³C NMR peaks. We can call this C_b.
• The third carbon is in the exact middle of the carbon chain. It has no hydrogen atoms bonded to it. This story is already different from the first two so it is unique. We can call this C_c.
• The fourth carbon is a -CH₂- carbon bonded to a -CH₃ chain end carbon and a carbonyl carbon which is in the middle of the molecule. Beyond that is another CH₂ group and beyond that is the CH₃ chain end. This is the same story as the second carbon. These are identical carbon environments and they will produce the same signal in a ¹³C NMR spectrum. This is C_b.
• The fifth carbon is a -CH₃ chain end carbon. Next to it is a -CH₂- group followed by a carbonyl, another CH₂ and then the CH₃ chain end. This is the same story as the first carbon. They are identical carbon environments - C_a.

If you were to run a ¹³C NMR spectrum for pentan-3-one then, you will see three carbon environments despite the molecule having five carbon atoms.
Where will they be? Look at the point above and think about what type of carbon environments we have:
• The first and fifth carbon atoms are -CH₃ which is a saturated alkane/hydrocarbon, mostly occuring between 0-50 ppm. Remember that higher ppm is caused by deshielding, near electronegative atoms and double bonds. There is no double bond or electronegative atom here nor on the CH₂ next door, so this will likely be a very low ppm, around 0-20 ppm.
• The second and fourth carbon atoms are -CH₂- which are saturated alkane/hydrocarbon as well, but they are one carbon away from a carbonyl carbon which is generally much higher ppm. These will be in the 0-50ppm range, but probably on the higher end such as 30-50ppm.
• The third carbon atom is a carbonyl carbon. These are amongst the most deshielded (highest ppm) absorption peaks, in the 150-200ppm range and probably close to 200.

The ¹³C NMR absorption peaks for pentan-3-one are ~8ppm (C_a), ~35 ppm (C_b) and ~210 ppm (C_c).¹ See the image below:


• Exam questions on NMR tend to give you the NMR spectrum (or the peaks with their ppm values) and the molecular formula.
  Going straight to the NMR spectrum when you don’t know what you’re looking for is going to waste your time in an exam - a molecular formula is a HUGE clue to which peaks to expect and where in the NMR spectrum. Use your general formulae:
• C_nH_2n+2 is an alkane – your peaks will all be around the 0-50ppm and you can almost certainly rule out peaks outside the range.
• C_nH_2n is possibly an alkene – look for peaks corresponding to C=C environments (100-150ppm). It could also be a cyclic alkane.
• C_nH_2nO could be a ketone or aldehyde. Look for an obvious peak around 180-200ppm – aldehydes and ketones are extremely clear in NMR spectra.

Train yourself to spot functional groups from the molecular formula. Here’s an example:
• C₅H₁₀O₂ – which functional groups do we know with oxygen atoms?
• Ketones and aldehydes – we could have two to account for the two oxygens, but that would ‘cost’ four less hydrogen atoms than a normal five-carbon molecule so our molecule cannot have the formula C₅H₁₀O₂, it would have to be C₅H₈O₂. It can’t be a dione or dial.
• Alcohols – an -OH group won’t reduce the number of hydrogens on the chain so you would still expect C_nH_2n+n in the formula. It’s unlikely to be an alcohol.
• Ester – this will have two oxygens and compared to a standard alkane you have lost two hydrogen atoms for the C=O bond. This is possibly an ester – look for ester peaks (150-200ppm).
• Carboxylic acid – this is two oxygen atoms and because of the -OH it only loses two hydrogens compared to an alkane’s C_nH_2n+n formula. This matches – you could have a carboxylic acid and carboxylic acid NMR peaks are very obvious, amongst the highest ppm for ¹³C and ¹H NMR.

Knowing what functional groups are possible from your molecular formula is extremely helpful before looking at the NMR spectrum so you know what you are going to be looking for!


• Worked example: C₃H₆O
  The compound with molecular formula C₃H₆O has three peaks in its ¹³C NMR spectrum: a peak at ~5 ppm, ~37 ppm and ~200ppm. Suggest the structure of this molecule.
  
  Let’s start by working on the molecular formula.
• C₃H₆O matches the formula for a carbonyl group in a hydrocarbon chain – an aldehyde or ketone.
• We have three peaks for only three carbon atoms.
  That means there are no equivalent carbon environments, so the carbonyl can’t be in the middle of the chain otherwise the two chain ends would be the same signal. That makes it a carbonyl on a chain end – an aldehyde.

So without even looking at the actual ppm values we have been given, we would expect:
• A carbonyl carbon signal (~200ppm).
• Two alkyl chain signals: one next to the carbonyl chain (-CH₂CHO) which will probably be closer to 50 ppm and one on the end of the chain (H₃CCH₂CHO) which is closer to 0-20 ppm.

These are our signals! 5ppm is the -CH₃ carbon, 37 ppm our -CH₂ middle of the chain, and -CHO is our carbonyl carbon up at 200ppm.
With this said we have accounted for all three peaks/signals shown and have a molecule – propanal – that accounts for the molecular formula. See the image below:

References:
¹ https://www.chem.wisc.edu/areas/reich/nmr/c13-data/cdata.htm - large database of ¹³C and ¹H NMR data values for organic molecules.