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Author Topic: Calculating J value of a multiplet in NMR?  (Read 9792 times)

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Violagirl

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Calculating J value of a multiplet in NMR?
« on: October 09, 2011, 06:37:14 PM »

I am not sure how to determine the J value of a multiplet from an NMR spectrum. Would you take ppm of your middle peak and subtract from an outer peak and multiply by your Hz of your NMR (200 for mine) to obtain it? Or would you report the middle peak itself as your value?
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Dan

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Re: Calculating J value of a multiplet in NMR?
« Reply #1 on: October 09, 2011, 09:49:34 PM »

It depends on the splitting pattern - doublets and quartets, for example, have no "middle peak". If you mean "multiplet" in the sense of an unidentified multiplet - i.e. you can't tell what the splitting pattern is because it is too complex - then you cannot calculate a value for J.The value you need will be the difference in Hz between two peaks - so it will be the the (ppm difference of two peaks) x (frequency of spectrometer). In the case of a triplet, the two peaks in question would be the middle peak and one of the outer peaks as you describe.
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fledarmus

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Re: Calculating J value of a multiplet in NMR?
« Reply #2 on: October 10, 2011, 01:00:30 AM »

The center of a multiplet is the chemical shift - either the middle peak (if it is an odd number: singlet, triplet, quintet, etc.) or the average of the two middle peaks (if it is an even number: doublet, quartet, sextet, etc.). It is reported in ppm.

The coupling constant is the distance between any two adjacent peaks in the multiplet - they should all be the same. Coupling constants are reported in Hz.

The reason is that the chemical shift depends on the external magnetic field, while the coupling constant depends only on the magnetic field generated by the individual nuclei. This means that the actual energy of the chemical shift (in Hz) will change relative to the size of the magnet, while the coupling constant will not. As the magnet size gets larger, the center of the multiplets will remain in about the same place, while the peaks of the multiplets will appear to get closer together, because the number of Hz in the coupling constant is fewer ppm.
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Violagirl

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Re: Calculating J value of a multiplet in NMR?
« Reply #3 on: October 10, 2011, 06:18:45 AM »

That helps a lot! So instead of subracting, would you then divide your two middle peaks and then multiply it by the MHz of the NMR spectrum? For mine, it 200 MHz.
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fledarmus

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Re: Calculating J value of a multiplet in NMR?
« Reply #4 on: October 10, 2011, 06:32:48 AM »

Sorry, no -

To find the coupling constant (J) you would subtract any two adjacent peaks in the multiplet and multiply by the MHz of the NMR spectrum.

To find the chemical shift (d), you would simply average the middle two (if there were an even number of peaks in the multiplet). If there were an odd number of peaks in the multiplet, the chemical shift of the multiplet is the chemical shift of the middle peak.

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Dan

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Re: Calculating J value of a multiplet in NMR?
« Reply #5 on: October 10, 2011, 06:49:17 AM »

To find the coupling constant (J) you would subtract any two adjacent peaks in the multiplet and multiply by the MHz of the NMR spectrum.

Just a quick note for the OP: This is true for simple splitting patterns where there is only one J value (e.g. doublet, triplet, quartet.... etc.). For more complicated patterns, where there are multiple J values, it is not only the difference between adjacent peaks that needs to be measured (e.g. double doublet, double triplet... etc.).

I realise this may be beyond your requirements at this stage.

In a double doublet, for example, there are 4 peaks and two J values - let's say their shifts are a, b, c and d Hz. The two J values are (a-b) and (a-c) - note that the pattern is symmetrical so (a-b) = (c-d); (a-c) = (b-d)
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