U. Kolczak et al., REACTION-MECHANISM OF MONOACYLPHOSPHINE AND BISACYLPHOSPHINE OXIDE PHOTOINITIATORS STUDIED BY P-31-CIDNP, C-13-CIDNP, AND H-1-CIDNP AND ESR, Journal of the American Chemical Society, 118(27), 1996, pp. 6477-6489
P-31-NMR-CIDNP (CIDNP = chemically induced dynamic nuclear polarizatio
n) spectroscopy was applied for the first time to investigate the form
ation and reaction of phosphorus-centered radicals obtained from phosp
horus-containing photoinitiators. C-13-NMR- and H-1-NMR-CIDNP and ESR
spectroscopies were used as complementary experimental techniques for
the elucidation of the photochemistry of these compounds. The large hy
perfine coupling constants of the P-31-nucleus results in a violation
of Kaptein's rules, which is the only observation of this kind in C-13
-NMR-CIDNP spectra reported so far. Interpretation of the CIDNP spectr
a, using a modification of Kaptein's rules for the C-13-NMR- and H-1-N
MR-CIDNP, consistently shows that all compounds investigated undergo a
photoinduced cleavage of the carbonyl-phosphinoyl bond from a triplet
state. The fate of the primary radicals is discussed, and it is unamb
iguously shown by trapping experiments that the novel bisacylphosphine
oxide photoinitiators give four radicals in a stepwise process.