Nuclear spins and magnetic moments

In the course of developing a Hamiltonian for diatomic molecules, we have so far introduced and discussed two nuclear properties. We considered at length the nuclear kinetic energy in chapter 2, and in chapter 3 we took account of the nuclear charge in considering the potential energy arising from the electrostatic interaction between electrons and nuclei. With respect to the electrostatic interaction, however, we have implicitly treated the nucleus as an electric monopole, and this assumption is re-examined in section 4.4. First, however, we consider another important property of many nuclei, namely their spin and the important magnetic interactions within a molecule which arise from the property of nuclear spin. The possibility that a nucleus may have a spin and an associated magnetic moment was first postulated by Pauli, following the observation of unexpected structure in atomic spectra. The first quantitative theory of the interaction between a nuclear magnetic moment and the ‘outer’ electrons of an atom was provided by Fermi, Hargreaves, Breit and Doermann and Fermi and Segrè. In the case of diatomic molecules with closed shell electronic states, the magnetic interaction of the nuclear moment with the magnetic angular momentum vector, an I · J coupling, was treated by a number of authors. The interaction between the nuclear electric quadrupole moment and the electronic charges, an interaction which has nothing to do with nuclear spins or magnetic moments, was treated by Bardeen and Townes.