NMR Spectroscopy tips

[SuperSaiyan3]

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can anybody give me some learning tips on how to understand NMR graphs?

What does it mean when you say "further down the field? Like more to the left?

Also, what is the difference between H and C13 NMR? Can somebody take the initiative to teach a noob? It would definitely be a good learning experience for the both of us.

Gracias!

 

[wanderer]

This is something you should look through your textbook, it's kinda hard to teach as a bunch of tidbits but....
The number of peaks in each kind of NMR corresponds to the number of types of Hydrogen in the case of H-NMR and number of Carbons in 13-C NMR. This might seem a bit redundant but draw any Carbon chain containing a quaternary Carbon (or any carbon that has no attached hydrogens) and you'll see where 13-C NMR's usefulness comes in. C-NMR peaks don't split each other, and have very wide ranges so it's generally not as usefull as H-NMR.

Downfield means that the hydrogen is deshielded and is more to the left on a NMR graph. A hydrogen is deshielded because of a highly electronegative atom that is adjacent to it, or if there is a pi system (such as benzene) to which the Hydrogen is attached.

Signal splitting occurs due to adjacent Hydrogens, which are split according to the N+1 rule, that is if there are N hydrogens on the adjacent Carbon, the hydrogen in question will have N+1 peaks.

 

[ThirdEye]

I haven't seen/worked any NMR since I took organic 2 back in 2005 so you don't want me to explain it to you. However, I did find some links on my prfessors website that could be of some use.

http://www.cis.rit.edu/htbooks/nmr/
http://www.ch.ic.ac.uk/local/organic/nmr.html

 

[SuperSaiyan3]

This is something you should look through your textbook, it's kinda hard to teach as a bunch of tidbits but....


Downfield means that the hydrogen is deshielded and is more to the left on a NMR graph. A hydrogen is deshielded because of a highly electronegative atom that is adjacent to it, or if there is a pi system (such as benzene) to which the Hydrogen is attached.

actually, this pretty much summed very well. Thank you.

Just one more question:
what does it mean to be "deshielded"? I guess electron withdrawn by some other atom or molecule eh? But aren't benzenes electron donating groups?

So you always start with and look at the quatenary carbon in the structure and try to go from there eh?

I was doing the Kaplan Practice test #3 not too long ago and there were a LOT of questions about the NMR (like 5 I think). Is there supposed to be a peak at the start (the very right)?

There were 7 small split peaks gathered together (which they magnified) at the far left which was supposed to account for the 6 hydrogens on the 2 adjacent carbons, I guess. But I'm not too sure about this now because you told me that the DESHIELDED electrons are more to the left.

And then there was a BIG PEAK bit down from the initial peak that i told you about. This was showed what seemed to be multiple peaks as well (I counted 2 large peaks in this group). The answer sheet explained it, but i couldn't understand what the yahoo it was talking about.

The answer structure of the molecule was 2-bromopropane.

If anybody can clarify this I would be in your greatest debt.

SS3.

 

[wanderer]

actually, this pretty much summed very well. Thank you.

Just one more question:
what does it mean to be "deshielded"? I guess electron withdrawn by some other atom or molecule eh? But aren't benzenes electron donating groups?

Deshielded means that the hydrogens experience a greater portion of the magnetic field. Normally electrons shield the hydrogens from the magnetic field, but a deshielded hydrogen (like that which is next to an electronegative atom) experiences a relatively greater magnetic field because it is stripped of its protecting electrons.

As for benzene it has to do with the inductive effect. Did you take physics 2 yet? Since benzene is a ring its pi electrons can travel around the ring as if in a circuit, and induce a magnetic field in opposition to the applied magnetic field. Since the hydrogens are outside the ring, the magnetic field is actually reinforced where the hydrogens are, so that the hydrogens experience a greater magnetic field, and are deshielded. (this is probably more than you need to know)
The key to this is that magnetic fields are in the form of closed loops.
Edit: The ring current in benzene is actually due to diamagnetism.

So you always start with and look at the quatenary carbon in the structure and try to go from there eh?

Well it depends. 13 C NMR is generally not that useful by itself, so what you look for depends on what the 1 H NMR tells you.

I was doing the Kaplan Practice test #3 not too long ago and there were a LOT of questions about the NMR (like 5 I think). Is there supposed to be a peak at the start (the very right)?

There were 7 small split peaks gathered together (which they magnified) at the far left which was supposed to account for the 6 hydrogens on the 2 adjacent carbons, I guess. But I'm not too sure about this now because you told me that the DESHIELDED electrons are more to the left.

And then there was a BIG PEAK bit down from the initial peak that i told you about. This was showed what seemed to be multiple peaks as well (I counted 2 large peaks in this group). The answer sheet explained it, but i couldn't understand what the yahoo it was talking about.

The answer structure of the molecule was 2-bromopropane.

If anybody can clarify this I would be in your greatest debt.

SS3.

2-bromopopane is H3C-CHBR-CH3
the two methyl groups and their hydrogens are equivalent (there is a plane of symetry going through the middle carbon, so there are 6 equivalent hydrogens. Therefore the middle hydrogen has 7 peaks (6+1). The middle hydrogen is going to be downfield (to the left) since it is sitting right next to an electronegative Bromine. The other peak will be a doublet (N+1 = 2 in this case).

 

[capn jazz]

As for benzene it has to do with the inductive effect (Lenz's Law). Did you take physics 2 yet? Since benzene is a ring its pi electrons can travel around the ring as if in a circuit, and induce a magnetic field in opposition to the applied magnetic field. Since the hydrogens are outside the ring, the magnetic field is actually reinforced where the hydrogens are, so that the hydrogens experience a greater magnetic field, and are deshielded. (this is probably more than you need to know)
The key to this is that magnetic fields are in the form of closed loops.

You learn something new every day! That's a great way of explaining the inductive effect.

 

[SuperSaiyan3]

Therefore the middle hydrogen has 7 peaks (6+1). The middle hydrogen is going to be downfield (to the left) since it is sitting right next to an electronegative Bromine. The other peak will be a doublet (N+1 = 2 in this case).

alright thanks wanderer. but oneeeeeeeeeee more thing to clarify.

When you refer to "the other peak", you are referring to the middle hydrogen that is sitting right next to the electronegative bromine, right? Thus the reason you are applying the N+1 rule to get 2 because you have 1 hydrogen + 1 = 2 signals.

But if that's so, then why wouldn't the middle hydrogen FURTHER AWAY than that of the 7 peaks comprising of the the 6 hydrogens of symmetry?
Wouldn't the middle hydrogen be MORE deshielded?

In the Kaplan test, the 7 peaks were further to the left than the 2 peaks.

Either way, you are a genius. How did you get so good at explaining stuff?
Thanks!

 

[wanderer]

alright thanks wanderer. but oneeeeeeeeeee more thing to clarify.

When you refer to "the other peak", you are referring to the middle hydrogen that is sitting right next to the electronegative bromine, right? Thus the reason you are applying the N+1 rule to get 2 because you have 1 hydrogen + 1 = 2 signals.

But if that's so, then why wouldn't the middle hydrogen FURTHER AWAY than that of the 7 peaks comprising of the the 6 hydrogens of symmetry?
Wouldn't the middle hydrogen be MORE deshielded?

In the Kaplan test, the 7 peaks were further to the left than the 2 peaks.

Either way, you are a genius. How did you get so good at explaining stuff?
Thanks!

The septet corresponds to the middle hydrogen which is more downfield. The doublet corresponds to the methyl hydrogens which are more upfield.
N+1 corresponds to the number of adjacent equivalent hydrogens, so the middle hydrogen is split into a septet because there are 6 adjacent hydrogens.

By other peak I was referring to the methyl hydrogens.

 

[SuperSaiyan3]

The septet corresponds to the middle hydrogen which is more downfield. The doublet corresponds to the methyl hydrogens which are more upfield.
N+1 corresponds to the number of adjacent equivalent hydrogens, so the middle hydrogen is split into a septet because there are 6 adjacent hydrogens.

By other peak I was referring to the methyl hydrogens.

OHHHHHHHHHHHHHHHHHHHHHH I finally get it now!! So that's what they meant!!

So the N+1 corresponded to the number of ADJACENT equivalent hydrogens.... gotcha.

Thanks!!! I get it 100% now.