Spin-Spin Coupling:
Under low resolution i.e., at low fields we generally get as many signals in the NMR spectrum as are the different types of protons in the molecule. The ratio of intensities of the NMR signals as measured by the areas under the peaks is a measure of the number of protons in each group. Therefore, at high resolution, spectral bands of a molecule containing nonequivalent protons are split into two or more components depending on the nature of neighbouring groups. The splitting of the signals is due to the interaction of nuclear spins of adjacent nuclei and the phenomenon causing such splitting of the signals is called spin-spin coupling. The interaction or the coupling of the spins is through the bonding electrons. This effect is generally not observed if the coupled protons are more than three σ bonds away. The splitting pattern of different signals can provide very important structural information. Now let us learn about it through taking the example of ethyl chloride.
Under the conditions of low resolution, ethyl chloride shows two signals in its NMR spectrum one each corresponding to -CH3, and -CH2 protons. The peak areas are in the ratio of 3:2. While the similar spectrum is recorded with a high resolution spectrometer, CH3 absorption band splits into three lines (called triplet) and CH2 band splits into four lines (a quartet). This splitting can be explained in terms of the changes in magnetic field experienced by the protons of one group due to the possible spin arrangements of the protons in the adjacent groups.