benandjerry
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Does anyone know what those brackets with dashes through them above the peaks mean?
Also, there are what looks like 7 peaks. How is this possible? The molecule is a hydrocarbon. I do not want to know what the structure is, just shine some light on the above questions. Thanks.
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charco
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(Original post by benandjerry)
Does anyone know what those brackets with dashes through them above the peaks mean?
Also, there are what looks like 7 peaks. How is this possible? The molecule is a hydrocarbon. I do not want to know what the structure is, just shine some light on the above questions. Thanks.
It looks like they are the coupling constants ...
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TheBlueBiro
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(Original post by benandjerry)
Does anyone know what those brackets with dashes through them above the peaks mean?
Also, there are what looks like 7 peaks. How is this possible? The molecule is a hydrocarbon. I do not want to know what the structure is, just shine some light on the above questions. Thanks.
Ok I'm also learning this now so I might be wrong but just bare with me, I might be of some help.
This is High Resolution proton NMR (HR-NMR) because it contains clusters of peaks and not defined peaks. From what I understand each cluster of peaks shows the number of protons on the adjacent Carbon atom.
The number of peaks - 1= The number of protons on adjacent Carbon atom(s).
You have 2 clusters
A triplet between 1-2ppm
And a Quintet between 4-5ppm (if I remember your picture right)
The triplet shows the adjacent C contains 2H's. Because 3-1=2.
Similarly, the quintet shows the adjacent C(s) contain 4H's. The position on the chemical shifts will show the carbon environment and the number of clusters show the number of different C environments.

I'm guessing the small arrows between the peaks show the distance in chemical shift or something but idk.
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username913907
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(Original post by benandjerry)
Does anyone know what those brackets with dashes through them above the peaks mean?
Also, there are what looks like 7 peaks. How is this possible? The molecule is a hydrocarbon. I do not want to know what the structure is, just shine some light on the above questions. Thanks.

(Original post by charco)
It looks like they are the coupling constants ...
Looks like the J's to me too.
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benandjerry
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(Original post by charco)
It looks like they are the coupling constants ...
why do you think this?
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charco
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(Original post by benandjerry)
why do you think this?
It's to show that the two signals are splitting each other. They all have the same value.
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benandjerry
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(Original post by charco)
It's to show that the two signals are splitting each other. They all have the same value.
Okay so each area of splitting on the nmr table has been produced by 2 different proton environments each that have split eachothers signals?
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charco
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(Original post by benandjerry)
Okay so each area of splitting on the nmr table has been produced by 2 different proton environments each that have split eachothers signals?
Correct

The triplet is caused by being adjacent to a CH2 group while the quintet would be caused by being adjacent to two equivalent CH2 groups (or four equivalent CH groups).

This does not give an simple hydrocarbon. (It would be much simpler if it were a triplet and a quartet).

However the shift of the quintet suggests that it is adjacent to an electronegative unit (not possible in a hydrocarbon I know)

It also looks like the integral is written underneath suggesting that there are three times as many hydrogen atoms in the triplet than in the quartet (quintet).
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benandjerry
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(Original post by charco)
Correct

The triplet is caused by being adjacent to a CH2 group while the quintet is caused by being adjacent to two equivalent CH2 groups (or four equivalent CH groups).

This does not give an simple hydrocarbon. (It would be much simpler if it were a triplet and a quartet).

However the shift of the quintet suggests that it is adjacent to an electronegative unit (not possible in a hydrocarbon I know)
Okay thanks for the help. Also, the molecule has some C=C bonds(the molecular formula is c5h8). How much of a difference does this make to splitting?
Edit: I think I've got it. Would this work?

CH2CHCH2CHCH2

The red Hs split eachothers signal to give quintet.
Blue H couples with the other blue H to give triplet.
What do you think?


actually no that wont work as there are meant to be 6Hs that are R-CH hydrogens. Dont worry
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charco
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(Original post by benandjerry)
Okay thanks for the help. Also, the molecule has some C=C bonds(the molecular formula is c5h8). How much of a difference does this make to splitting?
Would have been helpful to know in the first place ...

This is starting to look like a quintet and triplet in a alicyclic molecule.

I suggest that it is cyclopentene, with the two vinyllic protons either hidden or off the scale.
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benandjerry
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(Original post by charco)
Would have been helpful to know in the first place ...

This is starting to look like a quintet and triplet in a alicyclic molecule.

I suggest that it is cyclopentene, with the two vinyllic protons either hidden or off the scale.
Sorry...
What do you mean by hidden or off the scale? Just that it doesnt produce splitting?
You'll have to bare with me because I am currently only an A level student.

I can see how there could be coupling between one vinylic proton and 2H 2 carbons down so that would give a triplet. However, how does the quintet arise?
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charco
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(Original post by benandjerry)
Sorry...
What do you mean by hidden or off the scale? Just that it doesnt produce splitting?
You'll have to bare with me because I am currently only an A level student.

I can see how there could be coupling between one vinylic proton and 2H 2 carbons down so that would give a triplet. However, how does the quintet arise?
The quintet arises from the CH2 in between two equivalent CH2 groups (this makes four equivalent protons)

The triplet is due to the two equivalent CH2 groups being split by the intermediate CH2 .

There is little coupling between the vinylic protons and the protons in the CH2 groups.

The vinylic protons themselves probably appear off the scale shown by the spectrum, i.e. above ppm delta 6.

No need for any apologies - it is not an easy question.
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TheBlueBiro
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OP is this question from a past exam paper? If so, which exam board? I'm currently taking A2 OCR Chemistry A and have a test on this NMR stuff soon.

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benandjerry
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(Original post by charco)
The quintet arises from the CH2 in between two equivalent CH2 groups (this makes four equivalent protons)

The triplet is due to the two equivalent CH2 groups being split by the intermediate CH2 .

There is little coupling between the vinylic protons and the protons in the CH2 groups.

The vinylic protons themselves probably appear off the scale shown by the spectrum, i.e. above ppm delta 6.

No need for any apologies - it is not an easy question.
Okay thanks for the help!
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benandjerry
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(Original post by TheBlueBiro)
OP is this question from a past exam paper? If so, which exam board? I'm currently taking A2 OCR Chemistry A and have a test on this NMR stuff soon.

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No it isn't a PP question we wont get stuff like this in the exam dw
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TheBlueBiro
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(Original post by benandjerry)
No it isn't a PP question we wont get stuff like this in the exam dw
Phew! Haha I was starting to panic.


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benandjerry
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(Original post by charco)
The quintet arises from the CH2 in between two equivalent CH2 groups (this makes four equivalent protons)

The triplet is due to the two equivalent CH2 groups being split by the intermediate CH2 .

There is little coupling between the vinylic protons and the protons in the CH2 groups.

The vinylic protons themselves probably appear off the scale shown by the spectrum, i.e. above ppm delta 6.

No need for any apologies - it is not an easy question.
wait, is there any reason why this (3-methylbutan-1,2-diene) couldn't fit the bill?
Also I think that from looking in my text book the nmr is intended to be septet even though I did pass the tiny peaks off as not relevant at first.
It's C5H8
Has 6H atoms in proton environment of R-CH
Has 2H atoms in proton environment of C=CH (though I'm not fully sure the blue Hs are in the same proton environment)
I've tried to indicate the splitting in this picture-
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charco
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(Original post by benandjerry)
wait, is there any reason why this (3-methylbutan-1,2-diene) couldn't fit the bill?
Also I think that from looking in my text book the nmr is intended to be septet even though I did pass the tiny peaks off as not relevant at first.
It's C5H8
Has 6H atoms in proton environment of R-CH
Has 2H atoms in proton environment of C=CH (though I'm not fully sure the blue Hs are in the same proton environment)
I've tried to indicate the splitting in this picture-
3-methylbutan-1,2-diene has two CH3 groups attached to a carbon without protons - there would be no splitting. You would get two singlets.
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TheBlueBiro
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I hope you don't mind me asking questions here OP but I'm really struggling with a past paper question regarding and dunno how to start a new thread.

The spectrum only has 3 peaks and the molecular formula is C5H10O2 (an ester) but there are no integration values. Any help on how I'd work those out?

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charco
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(Original post by TheBlueBiro)
I hope you don't mind me asking questions here OP but I'm really struggling with a past paper question regarding and dunno how to start a new thread.

The spectrum only has 3 peaks and the molecular formula is C5H10O2 (an ester) but there are no integration values. Any help on how I'd work those out?

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More information needed. The number of peaks (splitting pattern) and the chemical shifts at least.
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