A lot more about the electron spin

At variance with the nucleus, the electron is associated with an orbital, i.e. a wavefunction which is related to the distribution in space of the electron cloud, and which displays an angular momentum L and a magnetic moment ill- In analogy with the spin operators (see Eqs. (1.7)), the following relations hold 

In a naive and incorrect way we can say that the electron with S = lk senses the orbital magnetic moment. Actually, a charged particle cannot sense the orbital magnetic moment due to its own movement. However, the electron moves in the electric potential of the charged nucleus. If we change the system of reference, 

We now let the molecule interact with an external magnetic field Bo. The interaction energy, as far as the orbital is concerned, is given by the orbital Zeeman operator 

This interaction will tend to disalign L from its internal axes (Fig. 1.14A). As a result, when the molecule rotates with respect to Bo, the interaction energy of Eq. (1.17) is orientation dependent. 

In coordination chemistry, reference is often made to limiting cases in which the orbital contribution tends to zero. In this case, the treatment is equal to the nuclear case and the same Hamiltonian is used (the opposite sign with respect to Eq. (1.14) is justified by the positive ge). 

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