* and
,* Triplets.Reprinted from the Journal of the American Chemical Society, 118, 12339-12349 (1996).
© American Chemical Society, 1996.
* and
,* Triplets.William J. Leigh,* Edward C. Lathioor, and Michael J. St. Pierre
Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: Nanosecond laser flash photolysis studies have been carried out of the kinetics
of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone,
and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the
geometric requirements for abstraction by carbonyl n,* and
,* triplets is derived from
the results for a series of ketones which contain a para-phenolic moiety attached via a
para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect
on the triplet lifetime in acetonitrile solution indicates that triplet decay is determined by the
rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding
phenoxyl-hemipinacol biradical. The biradicals have also been detected, and are about an order of
magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the
intramolecular process follow the same trend as that observed in the rates of bimolecular quenching
of the parent methoxy-substituted ketones by para-cresol. Deviation from this trend is observed for
the alkoxyindanone derivative, where an in-plane approach of the phenolic hydrogen to the carbonyl
n-orbital is not possible. The trends in the rate constants for bimolecular quenching of a series of
substituted benzophenones by para-cresol indicate that for n,* triplet
abstractions, the quenching mechanism is different for electron donor- and electron acceptor-substituted
ketones. For ,*
triplets and donor-substituted (n,* triplet) benzophenones, abstraction is proposed to occur by
a mechanism involving the intermediacy of a hydrogen-bonded exciplex, which yields the
corresponding radicals by sequential electron- and proton-transfer. The rate constant for
quenching by this mechanism thus depends mainly on the basicity of the ketone triplet state
and the oxidation potential of the phenol.