Chemically Induced Dynamic Nuclear Polarization and Exchange Broadening in an Electron-Transfer Reaction

The solution photolysis of para, para′-disubstituted benzophenones (1) in the presence of Dabco (2) has been investigated with the help of nmr techniques. During the irradiation, the ketones showed nuclear spin polarization (CIDNP) effects for the ortho (enhanced absorption) and meta protons (net emission). These results are interpreted as evidence for electron transfer from the amine to the excited triplet ketones. Simultaneously, nmr line broadening was observed for 2 and p, p′-dichlorobenzophenone (1e). These phenomena were ascribed to degenerate electron transfer between 2 and its radical cation (4) and between the ketyl radical (3e) and its parent ketone.