Shielding Effects in Covalent Halogen Compounds

Abstract

Let us consider a molecule fixed in space and containing a magnetic nucleus. If the molecule is exposed to a uniform static magnetic field ${\bar B_O}$ this will act on the electrons to induce a small additional field $\mathop {\rm{B}}\limits^{\rm{\_}} \prime$. At the position of our magnetic nucleus the local field, ${{\bar B}_{loc}}$, will be

$$ {{\bar B}_{loc}} = {\bar B_O}{\rm{ + }}\mathop {\rm{B}}\limits^{\rm{\_}} \prime $$

(3.1)

Experimental evidence as well as theoretical arguments indicate that for reasonably moderate magnetic fields (cf. discussion by Ramsay [143]) the induced field is proportional to ${\bar B_O}$ but not necessarily aligned parallel to it and one may write

$$ \mathop {\rm{B}}\limits^{\rm{\_}} \prime {\rm{ = - \tilde \sigma }}{\bar B_O} $$

(3.2)

where ${\rm{\tilde \sigma }}$ is a second rank tensor — the nuclear shielding tensor.