Spin delocalization and electronic structure

An excellent introduction to this topic is provided by La Mar. (40) This review clearly distinguishes between the three possible spin-transfer mechanisms, namely ligand (L) - metal (M) a-spin transfer, L - M 7t-spin transfer, and M L 7r-spin transfer. In addition to these direct spin delocalization mechanisms, where a fraction of unpaired electron density originally associated with the metal is actually placed into an MO that includes the ligand to some extent, there are indirect spin delocalization or spin polarization mechanisms. These involve essentially a redistribution of electron spin in an MO otherwise 

More recently, Marcellus et al. (47) have questioned the usefulness of dividing spin density distributions into direct spin delocalization and spin polarization mechanisms. Both are non-observables and not uniquely defined in most calculations. The authors show that, if both unrestricted and restricted HF INDO calculations are performed, the spin densities due to spin delocalization Pain) and spin polarization Pp(n) may be precisely defined respectively as  

Delocalization (Z ) and polarization (P) contributions to the isotropic -proton hyperfine coupling constant for the ethyl radical 

This major section of the review is subdivided in a manner similar to that of the previous review in the series, (1) the main difference being that Overhauser phenomena have not been included since they are regularly reviewed elsewhere. (48) The section dealing with biological systems is very much more extensive than in previous reviews and is a reflection of the enormous growth of interest in this area. 

Variation of the hyperflne coupling constant for the / -proton in the ethyl radical as a function of the angle 0. The points are extracted from INDO calculations the solid curve (/, is a cos h fit. (47) 

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