Understanding 13 C NMR spectroscopy

Nuclear magnetic resonance is concerned with the magnetic properties of certain nuclei. In this course we are concerned with the magnetic behaviour of 13 C nuclei. About 1% of all carbon atoms are the 13 C isotope; the rest (apart from tiny amounts of the radioactive 14 C) is 12 C. Carbon-13 nuclei fall into a class known as "spin ½" nuclei. The effect of this is that a 13 C nucleus can behave as a little magnet. 12 C nuclei don't have this property. 13 C NMR relies on the magnetic properties of the 13 C nuclei. Why carbon–13?

Since only 1% of carbon atoms are 13 C, only a fraction of the molecules in a sample will contain any 13 C. However, even a single drop of a compound will contain billions of molecules. Among all of those, there will be plenty of 13 C atoms in every possible position within the molecule.

Protons and neutrons in nuclei can be regarded as spinning about their axis. In many atoms these spins are paired against each other and so the nucleus has no overall spin (e.g. 12 C, 16 O). In other atoms, including 13 C and 1 H, the nucleus has an overall spin. A nucleus that spins (a moving electric charge) generates a magnetic field. The direction of the magnetic field depends on which way the nucleus spins. Nuclear spin

Normally the two possible spin states of the nucleus have the same amount of energy. However, in a magnetic field, the two spin states have different energies. Energy Applied magnetic field Spin state aligned with applied field Spin state opposed to applied field Energy gap corresponds to frequency of radio waves

The nucleus of a 13 C atom has a very weak magnetic spin (represented as a tiny magnet ). If a molecule containing 13 C is placed in a strong magnetic field, the magnetic 13 C nucleus can line up with the field or line up against it. Which of these is the higher energy orientation? High energy Low energy S N N S

When the spin falls back into line with the magnetic field it releases energy. We detect this energy and it provides information on the environment of the carbon atom in the molecule. Add energy NS S N Aligned = Low energy Energy released Excited state = High energy N S NS NS Back to low energy ground state S N

In physics, resonance occurs when a system is able to easily oscillate between two different energy conditions. The flipping of 13 C nuclei from one spin state to the other is nuclear magnetic resonance. The energies involved, which depend on the exact conditions around each carbon nucleus, are those corresponding to radio waves – about 25–100 MHz. When a nucleus in a particular magnetic field has exactly the right amount of energy to flip, it has reached the resonance condition. In NMR spectroscopy we normally supply a constant amount of energy (eg 40 MHz) and change the magnetic field.

So far we've been considering 13 C nuclei in isolation, but these nuclei are normally surrounded by electrons. These electrons help to shield the nucleus from the magnetic field. You need a stronger field to flip a 13 C nucleus inside an atom than you would if that nucleus had been stripped of all its electrons. Most 13 C nuclei aren't found in discrete atoms though, they're found in molecules whose atoms are bonded together by covalent bonds. The amount of shielding provided the electrons around each nucleus depends on which atoms it is bonded to, and which atoms they are bonded to. Electron shielding

What happens if a 13 C atom is bonded to an oxygen atom? Oxygen is more electronegative than carbon, causing the shared electrons to spend more time with the oxygen atom and less with the carbon. This reduces the amount of shielding around the 13 C nucleus, allowing the nucleus to resonate in a weaker magnetic field than if it were bonded to another carbon atom.

Tetramethylsilane Tetramethylsilane, usually called TMS, is used as a calibration standard in NMR spectroscopy. Silicon has a lower electronegativity than carbon, (carbon's is 2.5 and silicon 1.8) causing the carbon nuclei to have slightly more electron shielding than they would have when bonded to most other atoms. Thus the carbon nuclei in TMS (and all four nuclei are in the same environment) require a stronger magnetic field to reach the resonance condition than those in other compounds. The field required for TMS is set to zero, with all other carbon atoms compared to it.

What we're measuring in NMR spectroscopy is the difference in magnetic field required to bring each 13 C nucleus into resonance. This difference is called the chemical shift, symbol δ. Even in the powerful magnetic fields used in NMR spectroscopy, the differences in field required to flip a 13 C nucleus in TMS, and one bonded to a C or O atom is very, very small – so much so that we need to multiply this difference by a million to get a useful number! This equation produces a ratio, which has no units. Because we've multiplied by a million though, the unit for chemical shift is given as ppm (parts per million). Scale and units

Chemical shift measured the difference in the magnetic field required to reach resonance, compared to TMS. But virtually every carbon atom we study will have a smaller field than required for TMS. The scale for chemical shift, like that for IR spectroscopy, runs backwards. The peak for TMS – if shown – will appear on the right hand edge. A peak at a chemical shift of 120 is said to be downfield of TMS.

CDCl 3 is often used as a solvent in 13 C NMR. Chloroform is a useful solvent in organic chemistry, but the 1 H atom in normal chloroform, (CHCl 3 ) has a magnetic spin like 13 C, so it has been replaced by deuterium, 2 H, which has no magnetic spin. The peak from CDCl 3 appears at a chemical shift of 77 downfield of TMS. Like the TMS peak, it has been deleted from the spectra in the Spectral Database of Organic Compounds. Deuterated chloroform – CDCl 3

This is the 13 C NMR spectrum for ethanol. It has two peaks, because there are two carbon atoms. The peak at 60 ppm is from the right-hand C atom.

CH 3 —C—CH 2 —CH 3 CH 3 | OH 2-methylbutan-2-ol has 5 carbons, but there are only 4 peaks on this 13 C NMR spectrum. Why? Two of the carbon atoms are in exactly the same environments, so their chemical shifts will be the same.

CH 3 —C—CH 2 —CH 3 CH 3 | OH On this spectrum, we have one tall peak and three shorter peaks. It is reasonable to suggest that the tall peak corresponds to those two identical CH 3 groups. However, because of the random distribution of 13 C atoms through the sample, you cannot be certain that any extra- tall peak is caused by multiple carbons in the same environment. That particular sample may simply have had a lot of molecules containing 13 C in the same position. Peak height

Recognising equivalent carbon environments is a critical step in interpreting 13 C NMR spectra. CH 3 —C—CH 2 —CH 3 CH 3 | OH Can you see why 2-methylbutan-2-ol has 4 carbon environments and not 3? Why is the final CH 3 group different from the other two? It is attached to a CH 2, whereas they are attached to a carbon bonded to an oxygen. Who the neighbours are – and who their neighbours are – changes the environment. Carbon environments

Experienced chemists use tables of chemical shifts to identify every peak on a 13 C spectrum. All you will be required to do is to use a 13 C NMR spectrum to distinguish between small numbers of possible compounds, mostly by matching the spectrum to the correct compound by its number of carbon environments.