Energies allowed, unpaired electron

The real power of ESR spectroscopy for identification of radical structure is based on the interaction of the unpaired electron spin with nuclear spins. This interaction splits the energy levels and often allows determination of the atomic or molecular structure of species containing unpaired electrons. The more complete Hamiltonian is given in Equation (6) for a species containing one unpaired electron, where the summations are over all the nuclei, n, interacting with the electron spin. 

Figure 1.2 Energy levels of an unpaired electron in a magnetic field interacting with a spin-1/2 nucleus. The arrows show two allowed transitions.

These two qualitative concepts are not opposed to each other. For example, criticizing the first approach one could mention that the electrons in any molecule are socialized, while it is impossible to establish the exact sites of localization of the charge and unpaired electron. It is not necessary to suppose that a certain trigger mechanism except accumulation of a certain amount of energy in a certain chemical bond is required to start fragmentation. Anyway both theories are qualitative and their joint usage allows successful interpretation of the spectra of various organic compounds. 

In the case of a mixed-valence salt containing neutral TCNQ there are more TCNQ molecules than there are unpaired electrons and, therefore, electrostatic repulsion of charge carriers Is kept at a minimum by allowing conduction electrons to occupy the empty molecular orbitals of TCNQ . This Is a lower energy pathway compared to putting more than one electron on the seime TCNQ site and It may explain how mixed-valence semiconducting salts like CS2 (TCNQ )s and the "switched" form of Cu-TCNQ can exhibit greater conductivity than similar salts with 1 1 stoichiometry. 

Describe the bonding in [Mn(CN)g]3-, using both crystal field theory and valence bond theory. Include the appropriate crystal field d orbital energy-level diagram and the valence bond orbital diagram. Which model allows you to predict the number of unpaired electrons How many do you expect ... 

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