Electron Spin Resonance Spectroscopy:

Since the spectra can be obtained at a variety of frequencies, the field for resonance does not provide any unique identification of compounds. However, the proportionality factor defined as can yield more useful information.

For instance, the value of g is 2.00232 for a free electron. For organic radicals, the value is ranging between 1.99 and 2.01. For transition metal compounds, large variations in the range between 1.4 and 3.0 occurring due to spin-orbit coupling and zero-field splitting.

Similar to NMR, the unpaired electrons are also sensitive to the local environments. The nuclei of the atoms have a magnetic moment and produces a magnetic field at the electron, resulting in an interaction between the electron and nuclei, which is called as hyperfine interaction.

Such hyperfine interactions can also be used to provide a great deal of information about the sample including information about the number and identity of nuclei in a complex as well as their distance from the unpaired electron. This interaction expands the previous energy equation to:

Where A is the hyperfine coupling constant and m_I is the nuclear spin quantum number for the neighboring nucleus.