Spin angular momentum Quenching

This is known as the spin-only formula. Many ions which in the free state have orbital angular momentum L 0), lose it, completely or partially, when incorporated into a complex. This phenomenon is called orbital angular momentum quenching. It can be shown that for A states the quenching is complete and that for T and E states it is incomplete. Because of this, the spin-only formula applies to more situations than might have been expected. 

Because of the spin-space geminal-space isomorphism found in the simple model, consideration of the behavior of the energies and the wave functions runs along completely parallel tracks in the two cases. This includes the identification of conditions under which the lowest two states are degenerate, of particular interest for photochemistry. Phenomena such as "sudden polarization" and "orbital angular momentum quenching" all fit into a single picture. 

The result of this lower symmetry of the environment is the quenching of the orbital angular momentum of the electron the orbital angular momentum is no longer a good quantum number and has an average value of zero. Some contribution of the orbital motion however remains, and this causes a positive or negative deviation of the g-factor from the free spin value of 2.0023. 

There is an odd electron partially distributed between the dyz and dzx orbitals, which still retain their degeneracy and their rotational properties with respect to the 2-axis. The orbital angular momentum is thus not quenched by the ligand-field, and values of the magnetic moment as high as 4.1 B.M. are possible. The moment should be temperature dependent, and at low temperatures should drop rapidly towards zero, as the lowest state due to spin-orbit coupling is in fact nonmagnetic (74, 75). 

The magnetic moments for solid silver(II) N-heterocyclic complexes are generally close to 1.8 BM suggesting quenched orbital angular momentum and spin-only paramagnetism (Table... 

In connection with the Jahn-Teller effect the role of A may be well defined. It has been shown that if a dynamic Jahn-Teller effect is operative, very substantial quenching of orbital angular momentum may take place — the Ham effect. In the case of T ground terms, if the dynamic Jahn-Teller frequencies bear the right relationship to spin-orbit coupling and temperature, A may be reduced to values well below 0.5.113119... 

In this case the planar complex is diamagnetic and possesses the usual narrow line, high-resolution diamagnetic spectrum. The tetrahedral complex in Td symmetry would possess a 3T ground state. In approximately tetrahedral nickel(II) complexes the orbital angular momentum is incompletely quenched the result is a very short electron spin relaxation time and an NMR spectrum with relatively narrow, paramagnetic shifted resonances. 

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