Triplet Overhauser Mechanism of CIDNP

Abstract

Soon after the discovery of CIDNP it was proposed that the observed nuclear spin polarizations resulted from a small deviation from equilibrium in the electron spin states of the chemically generated radicals (such as all states equally populated), which was converted by electron-nuclear cross relaxation into a large deviation of the nuclear spin states from thermal equilibrium [1, 2]. This model is often called an Overhauser mechanism, because of its similarity to the Overhauser effect in which microwave pumping of the electron spin states of a paramagnetic species in a magnetic field leads to large nuclear spin polarizations [3]. This model failed to account for many features of CIDNP, however, and it was superseded by the radical pair mechanism [4, 5]. This did not prove the Overhauser mechanism is nonexistent, however. It could be that it is usually much less efficient than the radical pair mechanism, which therefore dominates the polarization process and hides the smaller effects of the Overhauser mechanism. If so, the Overhauser mechanism could be important in cases where the radical pair mechanism is inoperative, or if there is a very large initial electron spin polarization.