Q. Meng et al., DNP SPECTRUM OBSERVED IN THE PHOTOLYSIS OF BENZOQUINONE AND SOME OF ITS METHYL-DERIVATIVES - THE TRIPLET MECHANISM IN CIDNP, Zeitschrift für physikalische Chemie, 180, 1993, pp. 95-110
Citations number
32
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
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.