Background to C-13 NMR and simple interpretation of spectra背景C-13 NMR光谱和简单的解释.docxVIP

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2018-03-26 发布于江西
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Background to C-13 NMR and simple interpretation of spectra背景C-13 NMR光谱和简单的解释.docx

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Background to C-13 NMR and simple interpretation of spectra背景C-13 NMR光谱和简单的解释.docx

The background to C-13 NMR spectroscopyNuclear magnetic resonance is concerned with the magnetic properties of certain nuclei. We will currently focus on the magnetic behavior of carbon-13 nuclei.Carbon-13 nuclei as little magnetsAbout 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 ? nuclei (for reasons which dont really need to concern us at this stage). The effect of this is that a C-13 nucleus can behave as a little magnet. C-12 nuclei dont have this property.If you have a compass needle, it normally lines up with the Earths magnetic field with the north-seeking end pointing north. Provided it isnt 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 Earths magnetic field.It is very unstable opposed to the Earths 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.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. (The Hawaiian Music Station can be found at 105.1 MHz!) Its 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 reson