How to Find the Coupling Constant
time2011/10/16
Start by thinking about how NMR works. Just like electrons, neutrons and protons have a property called spin. A hydrogen nucleus has only one lone proton, so it has two possible spin states, either spin = +1/2 or spin = -1/2. Ordinarily these two states have equal energy. When you place the sample in a magnetic field, however, the hydrogen nucleus behaves somewhat like a tiny magnet in the sense that it can either line up with the field, in which case it will have equal coupling one spin state, or line up against it, in which case it will have the other.
Notice that the electron cloud around a hydrogen atom will move in a way that creates a small magnetic field countering the much larger external magnetic field. Consequently, the more electron-rich the hydrogen atom, the less the difference in energy between the spin states will be. Depending on their environment in the molecule, some hydrogen atoms will be more electron-rich than others. A hydrogen atom attached to equal coupling an oxygen, for example, would be much more electron-poor than one attached to a -CH3 methyl group.
Recall that NMR data is in the form of a graph that uses tetramethylsilane as a reference. In other words, rather than displaying the frequency of the radio waves that were absorbed, the graph displays the difference in Hz between the frequency absorbed by tetramethylsilane and the frequency absorbed by your sample divided by the spectrometer frequency in MHz. The resulting units are ppm or parts per million. The number of ppm increases equal coupling as you go down the graph towards the left, and decreases as you go towards the right. This means that the frequency of radio waves absorbed by the sample increases as you go towards the left.
Notice that the electron cloud around a hydrogen atom will move in a way that creates a small magnetic field countering the much larger external magnetic field. Consequently, the more electron-rich the hydrogen atom, the less the difference in energy between the spin states will be. Depending on their environment in the molecule, some hydrogen atoms will be more electron-rich than others. A hydrogen atom attached to equal coupling an oxygen, for example, would be much more electron-poor than one attached to a -CH3 methyl group.
Recall that NMR data is in the form of a graph that uses tetramethylsilane as a reference. In other words, rather than displaying the frequency of the radio waves that were absorbed, the graph displays the difference in Hz between the frequency absorbed by tetramethylsilane and the frequency absorbed by your sample divided by the spectrometer frequency in MHz. The resulting units are ppm or parts per million. The number of ppm increases equal coupling as you go down the graph towards the left, and decreases as you go towards the right. This means that the frequency of radio waves absorbed by the sample increases as you go towards the left.