Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: CrimpJiggler on December 20, 2011, 05:54:17 PM
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I noticed these on my NMR:
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they both have a small integration ratio indicating that they're produced by a single proton. Is the one on the left a triplet of doublets? If so, how is that splitting pattern produced? As for the one on the right, I don't know what to make of that.
Heres the compound:
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I know that bicyclic compounds like this are fairly rigid so there is probably a lot of geminal coupling going on there. I'm still a beginner when it comes to HNMR interpretation so trying to interpret this compounds spectrum has me bewildered. Besides looking for a singlet produced by the hydroxyl proton, I don't even know where to start.
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The first one is 1:1:2:2:1:1 so it is truly a triplet of doublets and most likely belong to the 3rd carbon (counting from CH next to the OH group) with a CH, CH2 and C-(CH3)2 groups as neighbors.
The other one is a 1:1:1:1:1:1:1:1 octet so actually a doublet of doublet of doublets. I can't find any one proton group with three different single proton neighbors so atleast one of the splitting is caused by something other that mere J3(H-H) coupling.
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The first one is 1:1:2:2:1:1 so it is truly a triplet of doublets and most likely belong to the 3rd carbon (counting from CH next to the OH group) with a CH, CH2 and C-(CH3)2 groups as neighbors.
The other one is a 1:1:1:1:1:1:1:1 octet so actually a doublet of doublet of doublets. I can't find any one proton group with three different single proton neighbors so atleast one of the splitting is caused by something other that mere J3(H-H) coupling.
Counting clockwise from the carbon with hydroxy group as 1, protons on carbon 2 seems to fit, perhaps, especially with one relatively large coupling and two smaller ones. Note that these protons are not equivalent.
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Don't forget the "W" coupling between the bridgehead protons.
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The first one is 1:1:2:2:1:1 so it is truly a triplet of doublets and most likely belong to the 3rd carbon (counting from CH next to the OH group) with a CH, CH2 and C-(CH3)2 groups as neighbors.
Wouldn't the two -CH3 groups split that into a septet? If I had to guess what causes a triplet of doublets, I would have said a single proton which splits it into a doublet, then another pair protons which splits the doublet into a triplet. Is that correct? As for a doublet of doublet of doublets, thats what I suspected but I couldn't find any proton to fit the criteria for that either.
Don't forget the "W" coupling between the bridgehead protons.
Whats W coupling?
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Look at the shape of the lines between the two bridgehead protons, it is in the form of a "W" lying in a plane. It is around 2-2.8Hz.
Any good NMR book should give you an exact definition.
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Wouldn't the two -CH3 groups split that into a septet?
No they are 5 bonds away so they cause no splitting. It is the plain carbon in C-(CH3)2 group that is the neighborg.
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Counting clockwise from C(CH3) is 1
The first one is truly a triplet of doublets so it is split by 2 H (of CH2 => triplet) and then by a H (of CH => triplet of doublets) so the best position is 4 (--CH-OH).
And the next one is a douplets of doublets of doublets so it may be split by a H and a H (of 2 CH neighbors) and a H of itself (of CH2). Because CH2 is near a asymmetric carbon it has different shifts. And the best position is 5 ( a H of CH2-)
That's what I think. ^^