NMR-spectroscopy (Nuclear magnetic resonance s.) observes the magnetic properties of the nucleus of the atoms. This only works with atoms that have an odd number of protons or neutrons. If a nuclide has any configuration other than even-even it is NMR-active.
E.g. 1H: 1 Proton (odd), 0 Neutrons (even) --> works
Organic compounds are mostly made of Carbon, Oxygen, Hydrogen and Nitrogen.
The most important (98,9% commonness) Carbon nuclide is 12C, this has 6 Protons (even) and 6 Neutrons (even) --> this nuclide is NMR-inactive and cannot be measured with NMR-spectroscopy.
Then again 13C with 7 Neutrons (odd) and 6 Protons (even) is NMR-active and is also used p.a.
So let’s work through a nice example of a rather clean NMR-spectrum:
You can see that on the X-Axis there’s a ppm (parts per million) scale from right to left. This is the scale of the chemical shift of the Hydrogen-atom. Dependent on the electronic surroundings of the Hydrogen the nucleus is shielded differently by the electrons and the location of the electrons is dependent of the electronegativity of the surrounding atoms.
This may sound confusing at first but look at it like this: An Hydrogen has EN (electronegativity) of 2.2 and a Carbon has about 2.5 so that is a rather small deviation and the electron of the hydrogen is located in-between the two atoms. With Oxygen-Hydrogen bonds the Oxygen has a much higher EN of 3,5 so the electron of the Hydrogen is located a little bit closer to the Oxygen atom, only poorly shielding the nucleus of the Hydrogen atom. Worse shielded nucleus results in higher chemical shift.
For standardization Tetramethylsilane (Si(CH3)4) is taken as 0ppm chemical shift (All hydrogen atoms in this compound are equally shielded because of the symmetry).
Chemical shift:
The Y-Axis is NOT AT ALL important for interpretation, if a peak is higher than the others that doesn’t mean anything! This is not MS, where peak-height has something to do with intensity, in NMR-spectroscopy the area of the peak defines its intensity. The integral symbols show you the ratio of the signals.
In this example I took the peak at ~4.9ppm as 2 Hydrogen atoms, as you can see there are two other peaks with “2”, three peaks with intensity “1”, and one peak with intensity “3” (this never is 100% accurate even if there are no impurities in the compound like here).
So the ratio of the Hydrogen atoms is 1:1:1:2:2:2:3. However this does not mean there have to be 12 Hydrogen atoms total, there also could be 24, 36, 48 etc. it’s only a ratio. In this example when we count the expected H-Atoms we see that it we are in fact right with 12 atoms.
So if that is totally off, that is already a sign for impurities, for example if I got an extra peak with like 4.5 intensity that would be solvent or byproduct or reagent or something.
Often you will see not integrated little peaks like here at 7.26ppm, this is actually not an impurity, it’s the solvent in which the NMR was taken (in this case with 7.26ppm it is CDCl3). In the case of other solvents (for salts like HCl or Fumarate often DMSO is chose as a solvent because of solubility issues) you can look up the shifts of the solvents online (just google it).
Next we will analyze the appearance of the peaks:
Single peaks that look like a needle or a flat mountain (e.g. the two peaks at >12ppm) are called “Singlets”. Double peaks that are near to each other are doublets (they are both equally high).
With triplets it gets more complicated: The middle one is higher than the ones on each side.
Quartets: two equally high ones in the middle and one equally high peak on each side.
Quintets: You get the idea.
The appearance gives you information about how many Hydrogen-neighbors the Hydrogen-atom you’re looking at has.
Let’s look for example at the aliphatic Hydrogens at the chain O-CH2-CH3:
You can see them in the peaks at ~1.6ppm and ~4.9ppm. Now what do we know about chemical shifts? The methylen-group (CH2) has a higher chemical shift, because it is right to the Imido-Oxygen.
The Methyl-group (CH3) is shielded better because the Methylen-group has doesn’t pull the electrons as much as the Oxygen does. Let’s look at a close capture of those two peaks:
As you can see the CH2-Group is a quartet and the CH3-Group is a triplet: That’s because the number of peaks tells you the number of neighbors those Hydrogens have. It’s always n-1 neighbors (n being the number of peaks). So, Methylen-group has 4 peaks = 3 neighbors (the 3 Hydrogens at the CH3). The Methyl-group has 3 peaks = 2 Neighbors (the 2 Hydrogens from the Methylen-group).
For advanced analysis: Subtract each chemical shift of each peak and calculate the average: If the averages of the CH3 and of the CH2 are equal then those two are neighbors. Because of course it can happen that a molecule has more than one aliphatic chain and every chain has a slightly different so called “coupling value” or “J”.
Now we will take a closer look on the five aromatic Hydrogens:
First, let’s try to predict the appearance of the peaks:
There are two Hydrogens with only 1 direct neighbor (those on position 2, ortho, on the ring). Those two are chemically identical because they have the same neighbors in all the directions. This should be a doublet (1 Neighbor + 1 = 2 peaks). Intensity = “2”
Then there are the two Hydrogens next to them (on position 3 and 5 or meta-position). Those are identical as well but they have 2 Hydrogen neighbors (on the left and on the right), so this will be a triplet. Intensity = “2”
And then there’s para, 4-Hydrogen with 2 neighbors and intensity “1”; also a triplet.
Let’s see if we are right (The peak at 7.27 is solvent as mentioned above):
Aromatic Hydrogens are always somewhere between 6.5 and 8.0ppm chemical shift, I will put a link to a table with all the shifts at the end.
What about the last two peaks left?
Well those are the two Hydrogens located at the Nitrogen-atom. One is from the Hydrochloric acid and one is bound to the Imine. Those are always Singlets as they have only Nitrogen as a neighbor. To be totally honest: I am not so sure why they are shifted so far to the left, if you look Imines, Amines up in NMR-shift-tables they should be around 5ppm I think this has something to do with the Imine being a hydrochloric salt.
