The Theory of Nuclear Spin-Spin Couplings

One of the reasons for difficulties in explaining indirect nuclear spin-spin coupling constants is that this phenomenon has no analogs in classical physics. The main driving force for inducing nuclear spin-spin couplings in molecules is not electromagnetic interactions but the Pauli’s exclusion principle, operating between electrons with the same spin. It was demonstrated that Fermi correlation, due to the Pauli’s exclusion principle, can be considered to be the mechanism whereby distant atoms communicate with each other [1]. The indirect nuclear spin-spin coupling is described by the form of J _MN I _M · I _N in which I _M and I _N are the nondimensional nuclear spin vectors, and J _MN is called an isotropic nuclear spin-spin coupling constant [2,3]. J _MN has the units of hertz (2π rad/s) Unlike the direct interaction of magnetic dipoles, an energy of this sort of nuclear spin-spin coupling does not average out to zero when the molecules are rotating, so its effect still remains in the spectra of liquids. This fact indicates that the indirect nuclear spin-spin coupling comes from an indirect coupling mechanism via the electrons in the molecule. The indirect coupling mechanism between nuclear spins will be considered in the next section.