EPR Parameters Experimental and Calculated

EPR spectra of organic radicals recorded in fluid solution present two principal parameters The g factor and the isotropic hyperfine coupling constant, hfc. The g factor characterizes the center of the EPR signal and spin orbit coupling leads to specific deviations from the value of the free electron, 2.0023. 

The hfc represents the orientation-independent interaction between the free electron and a magnetic nucleus (e.g., H, C, F, etc.). It reflects the spin density and the spin population at the magnetic nucleus and the orbital character of the spincarrying atom (Fermi contact). 

In most cases, carbon-centered radicals carry the unpaired electron in p(7t)-type orbitals. Since C isotopes are present at a very low percentage (natural abundance, 1.11%), the spin distribution is monitored by adjacent H nuclei. The spin from the p-type orbital of the C atom is transferred to the adjacent H atom (H ) via n- y spin polarization (Fig. 7.2a) however, spin transfer to more distant protons follows the model of a hyperconjugation mechanism illustrated in Fig. 7.2b. The closer the Cp-Hp is oriented toward the z-axis of the C p orbital, (0 = 0°) the bigger becomes the Hp hfc. When 0 is equal to 90°, the Hp hfc reaches its minimum value. This behavior is described by an empirical formula  

A third arrangement for a rather efficient long-range spin transfer is a W-plane-like arrangement between the z-axis of the C p orbital and a Cy-H bond (Fig. 7.2c). 

Obviously, the interactions sketched in Fig. 7.2 are very helpful, yet rough, empirical models for efficient spin transfer. Substantially more precise hfc values 

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