What are the coupling constants (J)?

1 Answer
Truong-Son N.
Sep 25, 2015

The coupling constant J is pretty much the peak-to-peak distance, usually reported in Hz. Matching it up with other nearly-identical coupling constants elsewhere in the spectrum usually tells you which protons are near which others.

For example:

![sdbs.db.aist.go.jp)

Peak Data:
Hz ppm Intensity

Proton A:
371.56 4.149 24
365.38 4.080 56
359.25 4.012 72
353.13 3.943 60
347.06 3.876 28

Proton B:
193.00 2.155 335

Proton C:
110.44 1.234 1000
108.19 1.209 27
104.31 1.165 939
102.06 1.140 25

What is shown here for proton A is that 4.008 ppm is the average chemical shift of the 1-4-6-4-1 pattern (according to Pascal's triangle) from the rightmost peak to the leftmost peak, and the entire signal there has multiple peaks. The peaks split like so:

1 → 1-1 → 1-2-1 → 1-3-3-1 → 1-4-6-4-1

It is not visible in this zoom, but the distance between each peak is roughly identical. This distance is the numerical equivalent of the coupling constant J in Hz. For proton A:

4.149−4.080=0.069 ppm
4.080−4.012=0.068 ppm
4.012−3.943=0.069 ppm
3.943−3.876=0.067 ppm

Interestingly enough, if you look at protons C at the averaged 1.200 ppm, you would also see that that doublet has the same J value (ideally that doublet should have both peaks at identical intensities too, but the shimming was not perfect, so they are a bit off). For proton C:

1.234−1.165=0.068 ppm

From the identical (or nearly-identical) coupling constant, you can determine which protons are "communicating" with each other and thus which protons they neighbor.

If you take this number and multiply it by the MHz of your NMR, you get the coupling constant in Hz. So, if your NMR's magnetic field frequency is 89.56 MHz (like for this particular spectrum), then:

0.068 ppm⋅89.56 MHz

=0.068HzMHz⋅89.56 MHz

=6.09 Hz

Indeed, for proton C, 110.44−104.31=6.13 Hz≈6.09 Hz.

Therefore, without seeing the structure of the analyzed molecule, you can still figure out that proton A and protons C are coupling/"communicating" with each other.

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