([0.01 mol dm~3), competition between the phenol and the
radiolytically primarily formed H atoms in the solvent, gener-
ates additional phenoxyl radicals (cf. reactions (9a) and (9b)).13
phenol radical cations (Tables 1 and 2). This can be attributed
to the fact that since the heteroatom sulfur has a higher elec-
tronegativity than oxygen, it causes more localization of
charge leading to faster deprotonation.2
H~ ] c-C H ] H ] c-C H ~;
12 11
k
k
\ 106 s~1 (9a)
\ 106 s~1 (9b)
Analysing the e†ect of the type of solvent on the stability of
the phenol radical cations, it should be stated that, for experi-
mental reasons, we only had non-polar solvents at our dis-
posal with a maximum relative permittivity of about 8.0.
Within this limitation, we found that the solvent type rarely
inÑuences the decay kinetics of the phenols, and never more
than to a very minor extent. In this respect, phenol radical
cations of 4-Cl-ArOH have nearly identical lifetimes in all the
solvents. This also holds for the radical cations of 4-MeO-
ArSH and 4-MeO-ArOH in the various solvents except cyclo-
hexane. The unsubstituted phenol (ArOH) shows identical life-
times in the alkyl chlorides and apparently higher values in
the solvents of alkane type. A genuine exception seems to exist
for the phenol radical cations of DtBMeP, where the decay
times vary between 270 and 610 ns (Table 2), which we are
unable to interpret.
6
2
6
9a
H~ ] ArOH ] H ] ArO~;
2
9b
Relation between phenol structure and the stability of ArOH~‘
In contrast to the electron-transfer phenomenon (8a, reactions
and (8b), the decay kinetics of the phenol radical cations cer-
tainly depends on the electronic e†ects. The fate of the phenol
radical cation can be inferred from the time proÐles. The time
proÐle of the radical cation formed under identical conditions
of dose and concentration was compared for the di†erent
phenols. Thus it can be seen that the radical cation of 4-CN-
ArOH undergoes the most rapid decay (110 ns, cf. also Table
1). The electronic and steric e†ects can be judged by looking
at the lifetime proÐles of the di†erently structured phenol
radical cations. It is evident that the radical cations of phenols
In general, however, within a relatively large error, it can be
stated that the radical cations of the phenols show nearly
identical decay kinetics in all the solvents used. This error
which is reÑected in the lifetimes of the phenol radical cations,
is less (O5%) in a deÐned solvent. The variation in this error
from solvent to solvent, however, does not inÑuence our cal-
culations. Additionally, it was found that the decay does not
depend on the concentration of the phenol radical cation
(ArOH~`). All this supports the hypothesis that the main
decay channel for the phenol radical cations is deprotonation,
a unimolecular reaction which ought to be largely indepen-
dent of the non-polar surroundings.
Concerning the error limit, the inÑuence of solvent primary
processes such as the early fragmentation of excited parent
ions19 may generate species of additional optical absorptions
(oleÐn radical cations) which are not markedly involved in the
reaction mechanism we analysed. However, such e†ects, and
also that of added carbon tetrachloride as electron scavenger
in the alkanes, only modiÐes the transient absorptions
observed in di†erent solvents to a small extent.
with electron-donating groups (4-NMe -ArOH) decay more
slowly than those of phenols with electron-withdrawing
2
groups (4-ClÈArOH). This is because electron-donating
groups such as -methoxy, -amino and -N,N-dimethylamino
stabilize the radical cation while the electron-withdrawing
groups like -nitro, -cyano and -chloro destabilize it. By con-
trast, radical cations of phenols with sterically hindering
groups (DtBMeP and IRGANOX - 1076}) showed little dif-
ference in the rate of decay. This is reÑected by the values of
their lifetimes (Table 1). Generally, the large di†erence in the
lifetimes indicates the strong electronic e†ect of the substit-
uents. In sharp contrast to this, there is only a small steric
e†ect. This may also be understood in terms of the inductive
e†ect of the alkyl groups.
The deprotonation of the phenol radical cations in the non-
polar surroundings seems to be mostly governed by intramol-
ecular e†ects and represents an irreversible monomolecular
decay reaction (3a) rather than the acidÈbase equilibria, reac-
tion (10), reported to exist in protic and polar solvents.15
A dependence on phenol structure can also be seen in the
case of phenoxyl radicals (Table 3). In aqueous solutions they
were found to decay by second-order kinetics (Table 3) with
2k in the range (1.0È6.0) ] 108 dm3 mol~1 s~1. As expected,
the thiophenoxyl radical of 4-MeO-ArSH decays much faster
than the phenoxyls.
ArO~ ] H O` ¢ ArOH~` ] H O
(10)
3
2
Considering the electronic e†ects of the various para-
substituents, a convincing correlation between the experimen-
tal lifetimes and the quantum chemical calculated spin density
at the phenolic oxygen has been found for the mono-
molecular deprotonation of ArOH~` in non-polar media dis-
cussed herein.34 In this manner, the spin density S(O) at the
oxygen atom and the charge di†erence between the singlet
ground state and the radical cation *q(OH) at the hydroxy
group of the radical cation were calculated and compared
with the experimentally determined lifetimes q (cf. Table 4).
As expected and already mentioned, phenols with electron-
donating groups which stabilize the radical cation have long
lifetimes. Based on the earlier discussion, the quantum chemi-
Conclusions
The pulse radiolytic study of phenols in non-polar solvents
involved the analysis of two features viz. the free electron
transfer from phenols to solvent radical cations and the stabil-
ity of the phenol radical cations.
The results ascertained the importance of the role of
geometry of encounter in the electron transfer from the solute
phenol to the solvent radical cation. This was based on the
evidence for the observation of synchronously formed phenol
radical cations and phenoxyl radicals. This phenomenon was
observed for phenols of very di†erent electronic structure and
was found to be hardly a†ected by the structure of the phenols
or the non-polar solvents used. Although such a universality
implies that the local electron transfer is a general e†ect of this
type of ionÈmolecule reaction, it was only identiÐed in the
case of molecules able to form both metastable radical cations
(electron transfer involving the aromatic moiety) and radical
cations with extremely low stability (electron transfer from the
phenolic OH group). To the best of our knowledge, this is the
Ðrst time that such local, encounter-geometry-controlled elec-
tron transfer in relatively small molecules has been shown to
be a general e†ect. This provides a basis for the general theo-
cal calculations show that these phenols (4-NMe -ArOH, 3-
2
NMe -ArOH, 4- NH -ArOH and 4-Me-ArOH) have low
2
2
values of S(O) and *q(OH). In contrast, high values of S(O)
and *q(OH) are obtained for radical cations of phenols with
electron-withdrawing groups (4-NO -ArOH, 4-Cl-ArOH,
2
4-CN-ArOH) with short lifetimes. Thus good correlation was
obtained between S(O) and *q(OH), and the lifetime of the
phenol radical cations. This corroborates well with the experi-
mental discussion mentioned earlier.
Radical cations of aromatic thiols2 have been found to
exhibit similar behaviour to the phenol cations. This is
observed in this study using the example of 4-MeO-ArSH
where the synchronous formation of thiol radical cations and
thiyl radicals is also demonstrated. However, the radical
cations of the aromatic thiols decay much faster than the
4954
Phys. Chem. Chem. Phys., 2000, 2, 4947È4955
Ganapathi
