To indicate that there is an effective interaction (anti-ferromagnetic) between two metal centers, what should the value of G be for the measurement of exchange interaction in ESR spectra. In some journals it says that if the G value is less than 4, there is effective interaction present between metal centers, whereas in some others, it says that it should be greater than 4. Which one is correct?
The g-value is not dependent on magnetic coupling, I guess. In some cases (of ferromagnetic coupling) forbidden half field transitions (ΔMS = ±2) are observed at the low field. The g-value is affected by total electronic angular momentum, the orbital angular momentum, and the spin angular momentum of the metal centers. The line pattern observed is temperature dependent... So the nature of magnetic coupling can been determined by temperature dependent measurement.
In the presence of anti-ferromagnetic coupling, the ground state will be the lowest possible value that you can get by combining the spins of the two metal ions. For example, if both spins were S=3/2, the lowest energy spin state of the antiferromagnetically coupled spins would be zero. In this case, at low enough temperatures, the metal complex will not be paramagnetic. The size and sign (ferromagnetic or antiferromagnetic) of the exchange coupling are typically determined by SQUID magnetometry, although in some cases this can be done by EPR. See, for example,
http://pubs.acs.org/doi/abs/10.1021/ja00045a021
Hello, the real g-factor of an ion or even a cluster does not depend in general on magnetic interactions between paramagnetic ions (or atoms). It is true that for some biological systems sometimes signals arising from coupled centers are seen that are assigned an effective g-value, but this is not the real g-value of the cluster (the line position and therefore the observed g-value would change with microwave frecuency). There are several ways of determining exchange couplings by EPR. One of those is performing a temperature variation of the spectra, taking care of several aspects. If the signal follows Curie´s law (I = Cst/T) then there are no interactions, and if they follow a law of the type I = Cst(T-T0) then there could be interactions. Note: there are many other factors which have to be taken into account, one of which is zero-field splitting in one or more of the ions, and you should study bibliography relevant to the type of cluster you have. Olivier Kahn´s book deals with a lot of cases, and it is a very important reference for anyone studying magnetic interactions between paramagnetic centers.
I hope this helps, this was just a very general answer.
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