Hyperfine interactions in NMR. The nuclear magnetic moment µI (spin I) interacts with the time average of the electron magnetic moment 〈µS〉(spin 〈S〉) due to unpaired d electrons present on neighboring paramagnetic transition metals. 〈µS〉 (or 〈S〉) arises from the unequal population of the different electron spin energy levels in the presence of B0 and fast transitions between these levels on the timescale of NMR. (a) The Fermi contact interaction occurs through bonds and results in delocalization of unpaired electron spin density from the TM d orbitals to s orbitals on the nucleus A under observation. (b) The electron–nuclear spin dipolar interaction occurs through space and depends on the orientation of the vector connecting the nuclear and electron magnetic moments with respect to the external B0 field, as well as the electron-nuclear distance R.