Electron Spin Resonance Spectroscopy:

Hence, a single nucleus with a spin ½ will split each energy level into two, as shown in Figure 36.04. As a result, two transitions can be observed.

Figure 36.04: Effect of hyperfine interaction and typical splitting in the EPR absorption spectrum.

The energy difference between the two absorptions is equal to the hyperfine coupling constant, A. If a signal is split due to hyperfine interactions, the center of the signal (which is used to determine g) is the center of the splitting pattern as marked in figure. So for a doublet, the center would be half way between the two signals as shown in the figure (right) and for a triplet, the center would be the center of the middle line.