C13+NMR+Spectroscopy

=NMR Spectroscopy= NMR spectroscopy looks at how particles with spin behave in a magnetic field. Spin quantum number of electron can have a value of +1/2 or -1/2

Examples of nuclei with spin of
 * **+ 1/2 ** || 1**H** || 13**C** || 15**N** || 19**F** || 31**P** ||
 * +1 || 2**H** || 14**N** ||  ||   ||   ||
 * +3/2 || 11**B** || 23**Na** || 35**Cl** ||  ||   ||
 * **+5/2** || 170 ||  ||   ||   ||   ||

What is spin? Spin is a fundamental property of nature, like electrical charge or mass. Spin comes in multiples of 1/2 and can be + or -. Protons, electrons, and neutrons possess spin. Individual unpaired electrons, protons, and neutrons each possesses a spin of 1/2. To be NMR active, i.e., able to generate NMR signals, spin has to be nonzero. Some nuclei not detectable in NMR, e.g., 12C and 16O
 * Background **

Fortunately, spin = ½ for 1H and 13C, i.e., it allows NMR use of these most abundant nuclei for structure determination of organic compounds.

About 1% of all carbon atoms are the C-13 isotope; the rest (apart from tiny amounts of the radioactive C-14) is C-12. C-13 NMR relies on the magnetic properties of the C-13 nuclei. Carbon-13 nuclei fall into a class known as "spin ½" nuclei for reasons which don't really need to concern us at the introductory level this page is aimed at (UK A level and its equivalents). The effect of this is that a C-13 nucleus can behave as a little magnet. C-12 nuclei don't have this property. If you have a compass needle, it normally lines up with the Earth's magnetic field with the north-seeking end pointing north. Provided it isn't sealed in some sort of container, you could twist the needle around with your fingers so that it pointed south - lining it up opposed to the Earth's magnetic field. It is very unstable opposed to the Earth's field, and as soon as you let it go again, it will flip back to its more stable state. Because a C-13 nucleus behaves like a little magnet, it means that it can also be aligned with an external magnetic field or opposed to it. Again, the alignment where it is opposed to the field is less stable (at a higher energy). It is possible to make it flip from the more stable alignment to the less stable one by supplying exactly the right amount of energy.
 * Carbon-13 nuclei as little magnets **



The energy needed to make this flip depends on the strength of the external magnetic field used, but is usually in the range of energies found in radio waves - at frequencies of about 25 - 100 MHz. (BBC Radio 4 is found between 92 - 95 MHz!) If you have also looked at proton-NMR, the frequency is about a quarter of that used to flip a hydrogen nucleus for a given magnetic field strength. It's possible to detect this interaction between the radio waves of just the right frequency and the carbon-13 nucleus as it flips from one orientation to the other as a peak on a graph. This flipping of the carbon-13 nucleus from one magnetic alignment to the other by the radio waves is known as the **//resonance condition.//**

What we've said so far would apply to an isolated carbon-13 nucleus, but real carbon atoms in real bonds have other things around them - especially electrons. The effect of the electrons is to cut down the size of the external magnetic field felt by the carbon-13 nucleus.
 * The importance of the carbon's environment **



Suppose you were using a radio frequency of 25 MHz, and you adjusted the size of the magnetic field so that an isolated carbon-13 atom was in the resonance condition. If you replaced the isolated carbon with the more realistic case of it being surrounded by bonding electrons, it wouldn't be feeling the full effect of the external field any more and so would stop resonating (flipping from one magnetic alignment to the other). The resonance condition depends on having exactly the right combination of external magnetic field and radio frequency. How would you bring it back into the resonance condition again? You would have to increase the external magnetic field slightly to compensate for the shielding effect of the electrons. Now suppose that you attached the carbon to something more electronegative. The electrons in the bond would be further away from the carbon nucleus, and so would have less of a lowering effect on the magnetic field around the carbon nucleus.



The external magnetic field needed to bring the carbon into resonance will be smaller if it is attached to a more electronegative element, because the C-13 nucleus feels more of the field. Even small differences in the electronegativities of the attached atoms will make a difference to the magnetic field needed to achieve resonance.

**Summary ** For a given radio frequency (say, 25 MHz) each carbon-13 atom will need a slightly different magnetic field applied to it to bring it into the resonance condition depending on what exactly it is attached to - in other words the magnetic field needed is a useful guide to the carbon atom's environment in the molecule.

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**The C-13 NMR spectrum for ethanol** This is a simple example of a C-13 NMR spectrum. Don't worry about the scale for now - we'll look at that in a minute.
 * Features of a C-13 NMR spectrum **



<span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">There are two peaks because there are two different environments for the carbons. <span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">The carbon in the CH3 group is attached to 3 hydrogens and a carbon. The carbon in the CH2 group is attached to 2 hydrogens, a carbon and an oxygen. <span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">The two lines are in different places in the NMR spectrum because they need different external magnetic fields to bring them in to resonance at a particular radio frequency.

**<span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">The C-13 NMR spectrum for a more complicated compound ** <span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">This is the C-13 NMR spectrum for 1-methylethyl propanoate (also known as isopropyl propanoate or isopropyl propionate).



<span style="background-color: #ffffcc; font-family: Helvetica,Arial; font-size: medium;">This time there are 5 lines in the spectrum. That means that there must be 5 different environments for the carbon atoms in the compound. Is that reasonable from the structure? <span style="background-color: #ffffcc; font-family: Helvetica,Arial; font-size: medium;">

<span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">You might reasonably ask why the carbon in the CH3 on the left isn't also in the same environment. Just like the ones on the right, the carbon is attached to 3 hydrogens and another carbon. But the similarity isn't //exact// - you have to chase the similarity along the rest of the molecule as well to be sure. <span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">The carbon in the left-hand CH3 group is attached to a carbon atom which in turn is attached to a carbon with two oxygens on it - and so on down the molecule. <span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">That's not //exactly// the same environment as the carbons in the right-hand CH3 groups. They are attached to a carbon which is attached to a single oxygen - and so on down the molecule. <span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">We'll look at this spectrum again in detail on the next page - and look at some more similar examples as well. This all gets easier the more examples you look at. <span style="font-family: Helvetica,sans-serif; font-size: 13.5pt;">For now, all you need to realise is that each line in a C-13 NMR spectrum recognises a carbon atom in one particular environment in the compound. If two (or more) carbon atoms in a compound have //exactly// the same environment, they will be represented by a single line.