J coupling

[Maxine450]

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Can anyone explain what the "J coupling constant" is in layman's terms? I've never heard about this in NMR and I'm trying to figure out what it is.

 

[circulus vitios]

From what I remember protons (vicinal and geminal) can have different coupling constants. When this happens you can get complex splitting patterns.

"Hypothetical" shows what you would expect it to look like with identical coupling constants (coupling constant of 1 being split by 2 = 6.6 hz, coupling constant of 2 being split by 3 = 6.6 hz.) The other two show different coupling constants.

I think this is a little beyond the scope of the MCAT?

 

[tn4596]

lol i remember doing this in ochem. i hope this isnt on the mcat lol. i would hate to go thru this again

 

[MD Odyssey]

Can anyone explain what the "J coupling constant" is in layman's terms? I've never heard about this in NMR and I'm trying to figure out what it is.

I'll try to explain without getting too technical. In general, a coupling constant defines the strength of an interaction. In this case, the interaction in question is the interaction between nuclear spins, usually proton spins.

The degree of interaction is expressed by the magnitude of the coupling constant. In the case of H-NMR, adjacent protons interact with each other and cause local changes in the magnetic field that is experienced by each proton. These local changes are seen in the fact that the resonance frequency for that proton can be slightly different than one might expect. In some cases, particularly conjugated or aromatic systems, coupling can occur between protons that may be separated by more than one carbon.

So, the J-values for a particular interaction tell you the strength of that interaction and how much the external magnetic field can be changed by. Since it is only due to the possible combinations of the nuclear spins, the magnitude of the external field is irrelevant. J-values are important because they often allow you to determine the structure present based upon the splitting pattern and the degree of coupling which occurs.

 

[MD Odyssey]

Just a side note. If the concept of spin-spin splitting is totally foreign, you might want to review this stuff. Any NMR problem on the MCAT will absolutely require a knowledge of spin-spin coupling.

 

[Maxine450]

How does this affect the graph? How does more J coupling or less J coupling affect how the graph looks? I really don't know much about NMR so I'm just trying to get a really basic understanding.

BTW I've heard that it has been on the MCAT

 

[MD Odyssey]

How does this affect the graph? How does more J coupling or less J coupling affect how the graph looks? I really don't know much about NMR so I'm just trying to get a really basic understanding.

BTW I've heard that it has been on the MCAT

In simple terms, the splitting of the peaks will increase as the coupling constant increases. So, the separation of the peaks in signal that is split by a proton with a coupling constant of 7 Hz will be larger than it would be if the coupling constant were only 2 Hz.

 

[Maxine450]

Awesome, thanks. I hope that's all I need to know for this test.

 

[MD Odyssey]

Awesome, thanks. I hope that's all I need to know for this test.

Do you understand how the coupling actually works? That's probably more important than knowing J-values.

 

[Maxine450]

Not really. I'm going to read up on it in the BR book