Exchange coupling orbital

It is well known that Hund s rule is applicable to atoms, but hardly so to the exchange coupling between two singly occupied molecular orbitals (SOMOs) of a diradical with small overlap integrals. Several MO-based approaches were then developed. Diradicals were featured by a pair of non-bonding molecular orbitals (NBMOs), which are occupied by two electrons [65-67]. Within the framework of Hiickel MO approximation, the relationship between the number of NBMOs,... 

Exchange Coupling in Systems Containing Orbitally Degenerate Lanthanides 21 1.5... 

We wish to introduce here some of the problems which are encountered when dealing with exchange coupling in systems containing orbitally degenerate ions. 

As elaborated in detail in Ref. (5) there are two principal intensity mechanisms for dimer excitations. The single-ion mechanism is based on the combined action of spin-orbit coupling and an odd-parity ligand field potential at the Cr center. It is by this mechanism that spin-forbidden transitions obtain their intensity in mononuclear complexes. The pair mechanism, on the other hand, is restricted to exchange-coupled systems. It leads to the selection rules AS = 0,... 

Fig. 29 Hydrogen-bonded chains that organize radicals into FM exchange-coupled piled stack motif among nitronylnitroxide radicals in BlmNN. Inset (right) shows the piled stack spin orbit N-O/C contact that gives rise to FM exchange.

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