Line-broadening mechanisms quadrupole interaction

If the nucleus of the acceptor atom M has a nuclear spin quantum number greater than -j, the nucleus has an electric quadrupole moment as well as a magnetic dipole moment. The quadrupole interacts with any electric field gradient at the nucleus and, in combination with the molecular motion of the complex, this can provide an important mechanism of relaxation of the M nucleus. At relaxation rates that are slow compared with Jp M, spin multiplets are observable in the spectra of M and P. As the relaxation rate increases, the multiplets broaden, but the line separations may still be used to derive 7p Mwith good accuracy. At faster relaxation rates the multiplet components of M and P coalesce into a single broadened line, and at high relaxation rates, the phosphorus resonance becomes sharp and the M resonance may become so broad as to be unobservable. 

All of the heteroatoms possess at least one naturally occurring isotope with a magnetic moment (Table 15). The nuclei 14N, 170 and 33S also possess an electric quadrupole moment which interacts with the electric field gradient at the nucleus, providing a very efficient mechanism for relaxing the nuclear spin. The consequence of this facilitation of relaxation is a broadening of the NMR signals so that line widths may be 50-1000 Hz or even wider. To some extent this problem is offset by the more extensive chemical shifts that are observed. The low natural abundances and/or sensitivities have necessitated the use of accumulation techniques for all of these heteroatoms. The relative availability of 170 and 15N enriched... 

---