Paramagnetic centers exchange interaction

The NMRD profiles of V0(H20)5 at different temperatures are shown in Fig. 35 (58). As already seen in Section I.C.6, the first dispersion is ascribed to the contact relaxation, and is in accordance with an electron relaxation time of about 5 x 10 ° s, and the second to the dipolar relaxation, in accordance with a reorientational correlation time of about 5 x 10 s. A significant contribution for contact relaxation is actually expected because the unpaired electron occupies a orbital, which has the correct symmetry for directly overlapping the fully occupied water molecular orbitals of a type (87). The analysis was performed considering that the four water molecules in the equatorial plane are strongly coordinated, whereas the fifth axial water is weakly coordinated and exchanges much faster than the former. The fit indicates a distance of 2.6 A from the paramagnetic center for the protons in the equatorial plane, and of 2.9 A for those of the axial water, and a constant of contact interaction for the equatorial water molecules equal to 2.1 MHz. With increasing temperature, the measurements indicate that the electron relaxation time increases, whereas the reorientational time decreases. 

If two paramagnetic centers are related to each other by low symmetry, they are subject to an antisymmetric interaction known as Dzialoshinski-Moriya exchange. The strength of the interaction Dd-m was shown by Moriya to be proportional to the exchange constant as follows ... 

On the other hand, in the case of anionic-coordination polymerization on paramagnetic catalytic centers, the existence of free spins in the active center can lead to the stabilization of the triplet intermediate by involving unpaired electrons of the transition metal in the exchange interaction with those of the intermediate. This should increase the effect of spin exclusion. However, the final result will depend on the competition of two groups of factors stabilizing (pseudosymmetry and exchange interaction) and destabilizing (electric and quantum-chemical fields of the counterion) a locally symmetric triplet intermediate. 

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