The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged. Proton nuclei can be modeled as a spinning positive charge, which means they have a magnetic moment (according to the right-hand rule) that is perpendicular to that rotation, and this is noted as either spin “up” or “down”. When a nuclei is placed in a magnetic field (B_o), the nuclei’s magnetic moment can either align or oppose the applied magnetic field, so there is a low-energy state (α), and a high energy state (β), respectively, which can exist. If the appropriate energy is input (in NMR, radio wave frequencies), the α state can flip to the β state, and if this occurs, the proton is said to be “in resonance” with the external magnetic field, hence the name nuclear magnetic resonance spectroscopy. When the spin relaxes back down to the α state, each proton’s resonance energy is recorded by the NMR spectrometer as a frequency and a signal is plotted on the spectrum. The α-to-β energy difference is related to the frequency of the resonance transition by the equation: E = hv where h is Planck’s constant and v is the frequency, so B is directly proportional to E and v (see the figure below).

Spectral analysis is explained in more detail below:

1. The position of the signal can tell you about the electronic structure directly around the proton.
2. The size/integrated area under each signal can tell you about equivalent protons.
3. The multiplicity can tell you about nearby protons (spin-spin splitting).

The Position of the Signal
Electrons near a proton nucleus (from other atoms, like C, O, N) create their own induced magnetic field in the opposite direction of B_o, which decreases the magnitude of the effective magnetic field (B_eff) actually experienced by the proton, B_eff < B_o. This is an effect called shielding. The magnetic field, B, is directly proportional to the frequency, v, so if the magnetic field decreases, so does the frequency of that proton’s resonance. Since NMR signals are plotted as a function of this frequency (relative to a standard), changing the frequency of resonance changes the position, or chemical shift (ppm), of the proton signals in the final spectrum. Relatively shielded protons are bound to C (not very electronegative) and their signals have lower frequencies (upfield).

The opposite effect can also occur: deshielded protons, which may be bound to a carbon which is also bound to an electronegative atom (i.e. Cl, O), have chemical shifts at higher frequencies on the spectrum. The more deshielded (i.e. the more distorted away from the atom the electron cloud is), the further downfield (the to the left) in an NMR spectrum it will appear. Most protons in organic compounds have chemical shift values between 0-12 ppm. Note that the position of protons directly bound to heteroatoms (as in OH or NH₂ functional groups) may appear at a wide range of chemical shift values depending on the sample solvent and H-bonding interactions.

Equivalent Protons
Protons in different environments give different signals in an NMR spectrum, but if there are protons in the same environment, they are considered equivalent and there will only be one signal for all equivalent protons. Integrating the area under a signal in the NMR spectrum can tell you how many equivalent protons that signal corresponds to. To determine if two protons are equivalent, it is important to consider symmetry and chirality. Generally, if protons are equivalent, you should be able to substitute X at any of their positions and end up with the same compound in each case.

Spin-Spin Splitting
The spin-spin splitting phenomenon occurs when nonequivalent hydrogens interact with each other. The signals in an NMR spectrum can be split due to interactions with magnetic fields produced by neighboring protons (these are protons on neighboring atoms, or 3 bonds away from a proton). The degree of splitting depends on the number of adjacent hydrogens, and a signal will be split into n+1 peaks (with n=number of neighboring/interacting protons), and the peak height ratios generally follow the pyramid scheme.

For the ethanol example below, the OH proton will interact via H-bonding with other ethanol and water molecules in the sample solution, so practically speaking it will show up as a broadened singlet and not be split by or cause splitting to neighboring protons (although in theory it should, this is often an effect seen for OH or NH groups). However, the three equivalent H_a protons will be split by the H_b protons (n+1=2+1) and therefore show up as a triplet, and the H_b protons will be split by the H_a protons (n+1=3+1) and therefore show up as a quartet.