NMR Interpretation

1. ¹H NMR:

Before we start, I suggest you to read "NMR Spectrum" paragraph in theory. By doing so you will understand how to interpret the spectrum.

1.1 Chemical Shifts:

The common chemical shifts for ¹H NMR are listed in the table below. As you can see the chemical shifts (δ) ranges from 0-15 ppm, which is much smaller than what is seen in ¹³C NMR. This is due to the fact that for hydrogen, the chemical shifts are influenced by the electrons in S orbital which has only 2 electrons orbiting in it, rather than the 6 electrons that are orbiting in P orbital, for the ¹³C.

1.2 Resonance & Electronegativity:

As you can see from the table above, the more electronegative atoms, attached to the carbon, bonded to the proton of the interest, will de-shield the hydrogen and therefore the more downfield the peak will be. One of the most important chemical shifts to be noted, is the one for the aromatic compounds such as benzene. The chemical shifts for these compounds range between 6-7 ppm but lets take closer a look at what is causing this effect.
In aromatic compounds, the electrons are delocalised below and above the ring, due to the resonance structures. This will result in production of a magnetic field, which will reinforces the strength of the external magnetic field applied to the analyte.

1.3 Proton Exchange:

The peaks produced for O-H, N-H and S-H are usually "broad". This is because of the fact that these compounds can exchange proton with the NMR solvent or water, and as the exchange process is rapid and continuous, we usually see only ONE peak for these compounds and won't see any couplings with neighbour protons.

1.5 Spin-Spin Coupling:

You might expect, analysing a proton will result in a single signal in the NMR spectrum, however this is not always the case for ¹H NMR. If there are other protons adjacent, that singlet peak, will turn into to multiplet. This is because those other protons also produce a magnetic field which will affect the proton of the interest You might think that things are getting complicated but the splitting pattern is absolutely crucial to find the structure of the compounds, as you will find out soon.
Nothing is better than an example, to teach you how coupling works. So lets take a look at the NMR spectrum of dichloroethane below:

we are going to start with the chemical shifts first. As expected, the de-shielded hydrogen has a larger chemical shift (5 ppm) than ones that are not (2 ppm). There are two peaks indicating two different chemical environments and lastly, the peak intensity at 2 ppm is three times larger than 5 ppm, this is because there are three protons in the same chemical and magnetic environment at 2 ppm, and only one at 5 ppm. Feel free to look up these in Theory if you think you have forgotten some concepts.
Now lets focus on the splittings in the spectrum and learn the rationale behind it. The reason for this coupling is the fact that protons can orient against or with the field, meaning two possibilities for each proton. The general equation for simple couplings is:

The Spin Quantom Number for ¹H and ¹³C is (I = 1/2), and as a result they have 2 orientations. If we represent these orientation with arrows, one pointing up and one pointing down, it would be much easier to explain how splitting occurs:

The diagram above represents the splittings for hydrogen in environment "a".
As you can see there are four orientation possibilities in total for the neighbouring hydrogens, as shown by arrows pointing in different direction. It is important to note that Hbs are attached by sigma bond to carbon and therefore they can rotate. For this reason the order of arrows being up and down does not matter [↑↑↓ = ↑↓↑] and just the number of protons with the specific orientation matter.

1.6 Coupling Constant (J):

The space between the splittings are called J constants and measured in Hz. It gives us some important information about the chemical shifts.
In general if the J constant between splittings of two multiplets are same, there is a chance that those two environments are neighbours. Nonetheless J constant is a great help in determination of complex splittings in aromatic compounds and also alkenes.

1.7 Complex Splittings:

In some chemical environments, signals may split with different J constants. This is because the coupling constant of the neighbouring groups are different, affecting the splitting of the proton of the interest. This happens when the neighbouring groups are TWO different environments. It is best explained by the examples in the diagram below:

As you can see Hb and Hc have different environments, with different J constants which will lead to quartet rather than triplet. Now lets imagine the J constant for both splittings are equal. What would happen then is that the middle signals will overlap leading to one signal and consequently a triplet for that proton, like the diagram on the left hand side.
The general formula for these types splittings is:

Now lets take a look at the intensities of splittings. The intensities within each splitting, is proportional to the probability in which the proton, will experience such a coupling. As an example, in the triplet, the middle signal is twice as much as the others, meaning the proton will experience that coupling twice as much as the others.

1.8 Benzene Splittings:

Benzene has an aromatic ring and as explained before, the protons are highly de-shielded and the signals are shown at around 6-7 ppm. Benzene usually shows a multiplet signal, and the reason for that is that the protons within the benzene ring couple with each other.
The general couplings can be explained with the help of the benzene diagram below, with the strength of coupling decreasing from Ortho > Meta > Para.

Most aromatic compounds such as drugs, have a substituted benzene ring with electron withdrawing and donating groups, therefore all these factors lead to a very complex NMR but also important for drug discovery and analysis. As a result, it is suggested to read more about the aromatic's NMR in textbooks, because they have got much more examples which will help you to learn how to interpret all those splittings.

2. ¹³C NMR

The ¹³C NMR is a bit different from ¹H NMR in two aspects:
1) They are usually "Decoupled" therefore no splitting is seen in them.
2) They range from 0-250 ppm as explained in theory section.
The ¹³C NMR is mainly used in combination with ¹H NMR for structural determination.

2.1 ¹³C Chemical Shifts:

The chemical shifts happen for the same reasons as ¹H NMR with regards to polarisation and all the other factors. However the only difference are the values that can be seen and compared in the table below:

This tutorial ends here, and hope you learnt the basics of NMR.