IONIZATION OF NATURAL PHENOLS BY ELECTRON TRANSFER
673
For the phenoxyl radicals, a comparison of its decay
rates (2k7) shows that TOꢀ decays more slowly than SOꢀ
and COꢀ. Overall, in the FET, the phenols SOH, COH and
TOH (which stand for other naturally occurring phenols)
exhibit relatively higher yields of radical cations [reac-
tion (4a)] than the phenoxyl radicals [reaction (4b)].
Furthermore, the stability of the radical cations derived
from the natural phenols was found to be considerably
higher than those of other phenols.7,27 This can be
derived from the deprotonation rates (lifetimes) given
in Table 2.
Scheme 2
phenoxyl radical [Eqn (4b)]. It has been shown earlier
that log(1/ꢄ) of the solute radical cation of various
phenols increase with the spin density S(O) and the
difference of Mulliken charges at the OH group between
the cation radical and the singlet ground state Áq(OH).23
The same trend between log(1/ꢄ) (or ꢄ) and S(O) or
Áq(OH) is observed for the radical cations of SOH, TOH
and COH (Table 2). This consolidates the assumed
reaction mechanism and rate constants. The data show
that the methylendioxy substituent of sesamol and ꢃ-
delocalization of curcumin impart more stability to their
cation radicals as compared with even naphthols.23
The rate constants obtained for the free electron
transfer (2) from SOH, COH and TOH to BuClꢀþ
(Table 1) are of the order found also for other phenol
derivatives, i.e. they are diffusion controlled.8,12 This
holds also for the electron transfer quenching of ArOHꢀþ
by TEA. Considering the single steps of the FET
(Scheme 2) it should be stated that diffusion is the slowest
one whereas the electron jump itself is a completely
unhindered and therefore very rapid process. Hence, in
each encounter of the reactants the electron jump happens
at the first approach (collision). This seems to be the real
reason for the reflection of the rotation conditioned
conformers observed experimentally; see reaction chan-
nels of the FET (4).
In this paper, it was not intended to analyse in depth the
influence of distinct modes of molecule oscillations on
the electron transfer mechanism, which has been reported
previously.7 This product ratio of FET (4) involving the
actual natural phenols studied here, with 57–62% of
ArOHꢀþ, only a slightly higher yield compared with the
50% ArOHꢀþ contribution for the ‘simple’ phenols7 has
been found. This could mean that on the one hand the
alkoxyl groups in ortho- and para-positions stabilize the
cations by the well-known electron-donating effect. On
the other hand, and more convincing, the rotating bond
(C—OH) exhibits in these cases a higher rotation barrier.
This is indicated by the quantum chemical calculations at
least for COH and TOH.
CONCLUSIONS
Sesamol, curcumin and trolox react with solvent cation
radicals, like other phenols, to produce simultaneously
phenol radical cations and phenoxyl radicals. This is
followed by a delayed deprotonation of the metastable
phenol radical cation resulting in a delayed formation of
the phenoxyl radical. The peculiarities of the kinetics and
mechanism of the ionization of these compounds in non-
aqueous systems depends, to a large extent, on the sub-
stituents, i.e. the p- and o-alkoxyl groups are very effective
stabilizing factors. A good correlation is observed be-
tween the experimatal data (lifetime, ArOHꢀþ/ArOꢀ ratio)
and the theoretically calculated data S(O), Áq(OH) and
activation energies for the OH group rotation.
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