ESR Spectra and Hyperfine Interactions

The g-factor for a free electron is = 2.0023. The excitation frequencies in ESR spectra depend on the total magnetic moment. If we obtain a spectrum at known frequency and magnetic field, we can calculate the g-factor for our sample. The energy levels of a bound, unpaired electron differ 

ESR is a sensitive probe for the local environment of a transition metal. Orbital angular momentum can be very large for the d orbitals of transition metals. As an example, if a transition metal is surrounded by six identical ligands bound symmetrically by the d orbitals, a single transition results (i.e., a single line in the spectrum). As the symmetry is lowered either by substitution of one or more ligands or distortion of the symmetry (Jahn-Teller distortion), anisotropy appears in the ESR signal. ESR can be used to determine the oxidation state and coordination of transition metal centers in compounds. 

The interaction between an unpaired electron and nearby nuclei gives rise to additional allowed energy levels this interaction is called the hyperfine interaction or hyperflne conpling. Hyperfine coupling can tell us the identity and number of atoms in a radical or complex as well as their distances from the electron. The interactions with neighboring nuclei can be described by Eqnation 3.20  

Hyperfine interactions follow the same selection rules as NMR. For isotopes with 1 = 0 (atomic number and mass number both even), no EPR or NMR spectra are seen. For isotopes with odd atomic number and even mass number, the value for I will be an integer, such as N with 7=1. For isotopes with odd mass numbers, I values will be fractional, such as H, I = 1/2. 

As in NMR, if there are 2 adjacent protons, each line would be split iuto two additional lines and these would overlap to give a 1 2 1 triplet pattern. Four protous would give a five line spectrum with intensities predicted (from Pascal s triangle) to be 1 4 6 4 1. 

---