Iv. Koptyug et al., MAGNETIC-FIELD DEPENDENCE OF THE P-31 CIDNP IN THE PHOTOLYSIS OF A BENZYL PHOSPHITE - EVIDENCE FOR A T---S MECHANISM, Journal of physical chemistry, 100(35), 1996, pp. 14581-14583
P-31 chemically induced dynamic nuclear polarization (CIDNP) studies o
f the photochemistry of p-acetylbenzyl dimethyl phosphite (1) provide
evidence for the operation of the relatively rare T---S intersystem cr
ossing mechanism for a photochemically generated tripler free radical
pair. The mechanism of intersystem crossing of the triplet radical pai
r p-acetylbenzyl (2) and dimethoxyphosphonyl (3), generated from 1, is
influenced by the large P-31 hfcc of 3 and switches from the T---S me
chanism in a relatively low magnetic field (18.8 kG) to the convention
al T-0-S pathway at a higher field (58.8 kG). This change in mechanism
is evidenced by the photolysis of 1 in the presence of radical scaven
gers (halocarbons, thiophenol) which yields the corresponding products
from atom abstraction by 3 that has escaped the initial radical cage.
These escape products are absorptively polarized at high field (T-0-S
) but are emissively polarized at lower field (T---S) in a particularl
y well-defined display of the effect of magnetic field strength on int
ersystem crossing mechanism for a geminate radical pair. Photolysis of
1 in C6D6 solution yields emissively polarized dimethyl p-acetylbenzy
lphosphonate (4) from combination of radical pair [2,3] in the initial
solvent cage and following diffusive formation of [2,3] free pairs. C
age recombination and disproportionation of secondary F pairs, compris
ed of 2 and the phosphorus-substituted cyclohexadienyl radical (5), af
fords several emissively polarized products (6, 7, and 9) via the T-0-
S mechanism at both magnetic fields. The polarization of 4 remains emi
ssive at both magnetic fields regardless of the presence of radical sc
avengers, a finding which is argued to be consistent with the above re
sults.