7 Basic Rules for the interpretation of EPR spectra

The following rules are applicable to spectra that contain only one radical, they are not validate for mixtures of two or more radicals.

1. The position of the lines of a spectrum is symmetric with respect to the central point of the spectrum.
2. When a spectrum does not present an intense line in its centre it suggests that there exist an odd number of equivalent nuclei $n_i$ with half integer spin ($I=1/2$, $3/2$, $5/2$, $\ldots$). Nevertheless, the presence of an intense central line does not exclude the existence of an odd number of nuclei, because that line can be make by an accidental relation between the hyperfine splittings $a_i$.
3. The total length of a spectrum (separation in mT between the two most external lines) is given by:
   

   $$L = \sum_{i=1}^{r} 2 n_i I_i a_i$$ (8)

   
   
   being $r$ the number of groups of equivalent nuclei and $n_i$ the number of equivalent nuclei with hyperfine splitting $a_i$ and quantum number of spin $I_i$.
4. The maximum number of lines is given by:
   

   $$N = \prod_i \left ( 2 \cdot n_i \cdot I_i\ +\ 1 \right )$$ (9)

   
   
   where $n_i$ represents the number of nuclei with spin $I_i$.
5. The sum of the relative intensities of all lines (total number of transition) is:
   

   $$S = \prod_i \left ( 2 \cdot I_i\ +\ 1 \right )^{n_i}$$ (10)

   
   

6. The distance between the two first lines of the spectrum correspond to the smaller hyperfine splitting. The second hyperfine splitting is the distance between the first line and the following not identified one.