Nuclei with an number of protons and neutrons possess a property called spin. Nuclear spin can be thought as either up or down with the spin states normally possessing energy levels while generating equal but opposite When a molecule is placed in an NMR, a large field is applied making the spin states become of unequal energy, with the spin that generates a magnetic field in the same direction of the applied magnetic field being in energy. The NMR then uses to 'flip the spin' of each nuclei, wherein each nuclei will require a specific energy level of radiation that corresponds to its difference in energy of spin states, this is often called the nuclei's frequency, which is measured by the NMR and translated to teh final spectra.