[Eratosthenes]

Hi all,

I just wanted to see if someone might be able to point me in teh right direction as to how to correctly find the equivalent nuclei in a molecule when you put it in an NMR machine.

I'm not quite certain how to go about finding out what are the actual equivalent protons after I guess at them – so I’m really curious about the kinks (which are equivalent, why are they equivalent, why aren’t these equivalent, etc) . I attached a wretched first guess work attempt at labeling some below. Any help I’d gobble!


Also I don't know where to start with C13 signals.

Extra:
I don't want to overwhelm anyone but I really am curious.


Why did anyone ever search to find out  which protons are equivalent in an NMR to begin with? - was it really because when someone stuck a molecule in NMR for the first time and noticed wiggles that they knew some of the hydrogens had to show up under the same wiggle, and thus the search for 'equivalent nuclei' was created?
I know people must work with NMR machines every day, why are they looking for equivalent nuclei and labeling them? And how did they find out which ones are equivalent on NMR and ones that aren't?
Per Carbon 13, How did people find out C13 looked different from C12 in NMR, and why did people start trying to even figure out what were the equivalent carbons in molecules?

Anyone who tries to tackle that you are awesome!

[Babcock_Hall]

Hi all,

I just wanted to see if someone might be able to point me in teh right direction as to how to correctly find the equivalent nuclei in a molecule when you put it in an NMR machine.
SNIP
I know people must work with NMR machines every day, why are they looking for equivalent nuclei and labeling them? And how did they find out which ones are equivalent on NMR and ones that aren't?
Per Carbon 13, How did people find out C13 looked different from C12 in NMR, and why did people start trying to even figure out what were the equivalent carbons in molecules?

Anyone who tries to tackle that you are awesome!
1.  Carbon-12 has no NMR signal, but C-13 does.
2.  There are (at least two) kinds of equivalency in NMR.  There is equivalent in chemical shift, meaning having the same chemical shift.  There is also such a thing as magnetic equivalency.  Even two nuclei may be have the same chemical shift, they may not be megnetically equivalent.  This latter definition is important when trying to understand spectra that do not follow simple rules for splitting patters.  I suspect that you are more interested in the first definition, but perhaps you can clarify.

[fledarmus]

Wow, you have a lot of questions tucked in there. It's hard to figure out where to start. But let's start at the top.

To begin with, two nuclei are equivalent if they are related by symmetry. This means that if you can draw a mirror plane or a rotational axis of symmetry through your molecule that will interchange the two nuclei and leave the molecule indistinguishable, the two nuclei are identical. In the first structure on your page, the cis-1,2-dimethylcyclohexane, there is a mirror plane running through the middle of the molecule - each nuclei above the mirror plane is equivalent to one below the mirror plane. Since they are equivalent, they will have exactly the same magnetic properties and identical peaks on the NMR spectra.

Even if two nuclei are not related by symmetry, they may appear to be equivalent in the NMR spectrum if they are rapidly interchanged by rotation or by other molecular movements which occur rapidly at room temperature. For unsubstituted cyclohexane, the axial hydrogens are not, strictly speaking, identical to the equitorial protons on the same carbon. However, on the NMR timescale, the unsubstituted cyclohexane ring interchanges axial and equitorial protons rapidly by ring-flipping, and for all practical purposes they are equivalent in the NMR spectra. The same with the three hydrogens of a methyl group, in (for example) your o-chlorotoluene. If you draw the hydrogens in place, you would expect them to be in different magnetic environments based on whether they are in the plane of the benzene ring, or whether they are close to the chloro group or further away from the chloro group. However, the methyl group is rotating so quickly at room temperature that the three hydrogens appear to be equivalent and their chemical shifts average out. (Note - there are ways of slowing down some of these processes, and at very low temperatures you can sometimes get distinct chemical shifts for nuclei which at room temperature appear equivalent).

Looking at your labeled examples, many of them are wrong. You are being asked to label equivalent protons, not equivalent carbons - the first thing you will need to do is to draw in all your protons. Then you can see which ones are related by symmetry or by rapidly interchanging. Making models will help. Your first compound, the dimethyl cyclohexane - there are two protons on each carbon that you have marked "c" and "d" which are not equivalent to each other. Can you tell why? In your chlorotoluene, all of the phenyl protons are different - can you tell why?

As for why we are doing all this stuff, looking for equivalent nuclei and labeling them, it is critical for identifying our molecules. When you synthesize a compound in the lab, you have some idea of what you expect the structure of your product to be. You run an NMR, and if the NMR is not consistent with the structure that you thought you were making, you have a problem. If I was trying to add a chloro group to toluene in the ortho position, I would want to see four different phenyl protons (plus one peak for all three of the methyl protons) in my NMR spectrum. If I only saw two distinct phenyl signals, but each was twice as large as I expected, that would mean I had added it to the para position instead. (Can you tell why?)

And finally, for 13C and 12C - well, 12C doesn't show up in an NMR at all. Why? Because the basis of NMR is that atomic nuclei spin. If the nuclear charge is evenly distributed in the nuclei, nothing interesting happens, but if the charge is not evenly distributed, the spinning nuclei generates a tiny little magnetic field. If you apply a really large external magnetic field, you can force some alignment among all these tiny little magnetic fields. The atoms will try to spin in parallel. It takes slightly more energy to spin in a direction opposed to the magnetic field than to spin in a direction supported by the magnetic field. You can shoot that energy in, in the form of radio waves, and give some of the nuclei enough energy to spin in the direction opposed to the magnetic field, but then they will eventually get tired of fighting it, give off the energy again, and go back to spinning with the field. This is the energy that you are measuring as a chemical shift. The chemical environment that each nucleus is in will have a tiny effect on the magnetic field strength that it sees, and thus a tiny little effect on the chemical shift, on the order of parts per million. In fact, the majority of proton resonances all fall within 10 parts per million of each other - thus, the ppm scale that we use for measuring chemical shifts. (13C resonances are spread much further, across more than 200 ppm - but still, a very small space if you think about it).

How do you know if the charge is evenly distributed in an atomic nucleus? The number of protons and the number of neutrons are the same. If that happens, you cannot get an NMR signal from that atom. Deuterium, Oxygen-16, Carbon-12, Nitrogen-14, Helium-4, none of these will resonate in a magnetic field. Hydrogen-1, Carbon-13, Nitrogen-15, Fluorine-19, all of those will.

i know, probably way too much information. Sorry, I was in a talkative mood.