DNP SPECTRUM OBSERVED IN THE PHOTOLYSIS OF BENZOQUINONE AND SOME OF ITS METHYL-DERIVATIVES - THE TRIPLET MECHANISM IN CIDNP

CIDNP and dynamic nuclear polarization (DNP) are observed in the photo lysis of benzoquinone and its methyl derivatives. In low concentration s of solutions, the signs of the CIDNP of the ring proton of benzoquin one and methyl protons of methyl derivatives of benzoquinone are in ac cord with the prediction from the Kaptein rule based on the radical pa ir theory; however, at high concentration the sign is reversed. From t he signs of DNP the mechanism of the cross relaxation in the free radi cals is determined. In view of the observed correlation among the sign s of DNP, CIDEP, and CIDNP, the anomalous sign of the CIDNP observed a t high concentration is ascribed to generation of an electron spin pol arization by the anisotropy in the intersystem crossing (the triplet m echanism) and subsequent transfer of this polarization to the nuclei b y cross relaxation in the free radicals. Thus the triplet mechanism of CIDNP is well substantiated.