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### Topic: J Coupling Constants in Proton NMR  (Read 49332 times)

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#### CopperSmurf

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##### J Coupling Constants in Proton NMR
« on: March 07, 2009, 04:30:28 PM »
I'm kind of lost about how J coupling constants work. I think I know how to calculate it using my simplified formula:
J = (difference of chem shifts in ppm)*(field strength in MHz)
I've worked out a few alkene examples but I'm not sure how it would work for aromatic compounds. For example, if I had a para-substituted benzene ring:
_
R-/ \ -NR2'
\ /
-

or more simply:  R-Ph-NR2'
I understand that there are two sets of 2 equivilant H's on the aromatic benzene ring and I think I got J = 8, which I got from measuring the chemical shifts from the two chemically equivalent H on the aromatic ring closest to R. But that value of 8 implies ortho, but I thought it should be J = 3 to indicate meta between those two equivilent H's. Or does that J = 8 that I calculated properly mean that it is coupled to the H ortho to it on the ring?
« Last Edit: March 07, 2009, 04:59:26 PM by CopperSmurf »

#### sjb

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##### Re: J Coupling Constants in Proton NMR
« Reply #1 on: March 08, 2009, 08:25:56 AM »
Unless the R or R' groups are preventing free rotation around the bonds to the aromatic ring, or contain an overall chiral substituent, what you're seeing is that H1a and H1b, and H2a and H2b are identical in terms of the NMR.

The coupling you're seeing is J3 coupling betwen H1 and H2. I'm not 100% sure where you're going with the "because J is 8 then this, rather than J is 3 so this" arguement, bear in mind J coupling constants are typically a range, so cis alkene protons usually have a larger J-value than the corresponding trans alkene.

#### CopperSmurf

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##### Re: J Coupling Constants in Proton NMR
« Reply #2 on: March 08, 2009, 10:51:02 AM »
Thanks for clearing up the J coupling I was seeing. I had a proton NMR and was trying to figure out what was that J value I measured and calculated from 2 peaks that were really close to each other in the 7.4 ppm chemical shift area, which was 8, but I was expecting a different number, 3.

And according to my references, 3J = 8 refers to ortho-substituted benzene ring and 3J = 3 refers to a meta-substituted benzene ring, but results are results. Now that I know which one is coupled to what, I'm almost good to go.

Is it possible to "see" longer ranged couplings on a proton NMR? Maybe over 4 or 5 bonds on an aromatic ring?
Like, would I see coupling between H1b and H1a ?
And what about coupling between H1b and  H2a ?

#### sjb

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##### Re: J Coupling Constants in Proton NMR
« Reply #3 on: March 08, 2009, 11:30:57 AM »
It's possible that you may see longer range couplings, but as you've found they do tend to tail off quite fast - I've seen meta couplings before, but rarely para ones.

In the simple example you've given it's unlikely you'll see coupling between H1b and H1a because they are identical, just as you typically do not see coupling between the methylenes in e.g EtOAc.

#### Zartek

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##### Re: J Coupling Constants in Proton NMR
« Reply #4 on: March 22, 2009, 08:54:56 AM »
Could I get some clarification on this.  When you are calculating the coupling constant are you multiplying the chemical shift (as in the shift the peak has undergone by  the electron shielding effect of its neighborhood) or what exactly by the value in MHz of the NMR?

Edit: Also what is the standard for writing the coupling constants in a report.  For example where would I add the value in this δ = 8.5 (d, 2H, N=C-H).

Thanks

#### Med90

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##### Re: J Coupling Constants in Proton NMR
« Reply #5 on: March 22, 2009, 10:39:34 AM »