were used as received. Spectro grade n-butyl chloride from
Aldrich was tested spectrometrically before use in the experi-
ments. Other chemicals used were analytical reagent grade.
Water from a Millipore milli-Q plus system was used for
study in aqueous solutions.
originate from the ionÈmolecule reaction between n-butyl
chloride radical cation and thiophenol, which cannot only be
explained by the electron transfer (5). One of these rapidly
formed species is assigned as the radical cation of the thiophe-
nol (cf. reaction (5)) with a broad optical absorption spectrum
peaking around 530 nm and exhibiting a half-life of about 200
ns. This is demonstrated by the time proÐle taken at 550 nm
Results and discussion
(inset in Fig. 1(c)). Kinetic calculations done by solving the
Pulse radiolysis of thiophenol in n-butyl chloride solution
corresponding rate equations clearly show, that between 450
and 600 nm the solvent radical cation has to be taken into
account, considering the much shorter lifetime due to reac-
tions (1) and (2). Therefore the observed non-resolved spike is
mainly caused by the residual absorption of BuCl~` (about
one-third of the signal), whereas the resolved part is caused by
ArSH~`. The second species is clearly the thiyl radical ArS~
already mentioned above.
In close analogy to the phenol reaction behavior, we inter-
pret the fast thiyl radical formation to proceed via a mecha-
nism in which the polar ÈSH group of the thiophenols
encounters the n-butyl chloride parent ion, and in a concerted
reaction electron transfer and the deprotonation of the thiol
group molecule part occur (reaction (9)). Such a mechanism
would be supported by the high electronegativity of the chlo-
rine atom, and can take place in cases in which deprotonation
is faster than the charge distribution over the whole molecule.
Reactive transients of the radiolysis of pure n-butyl chloride
are the parent ions BuCl~`, butene radical cations Bu~` and
butyl radicals Bu~, reaction (4).10 Electrons are rapidly con-
verted into non-reactive chlorine anions. Free ions are formed
with a G -value (molecules per 100 eV) of about 0.2, whereas
fi
radicals dominate by far (G B 3È4).
R
BuCl ] [BuCl~` ] e] ] BuCl~` ] Bu~` ] Bu~; HCl, Cl~
(
4)
Butene radical cations (Bu~`) having a gas phase ionization
potential around 1 eV below that of BuCl~` are less reactive
by about one order of magnitude compared to BuCl~` and
should therefore not taken into account for the kinetic con-
siderations.10,11
n-Butyl chloride parent cations (BuCl~`) can be directly
observed as the main ionic products with broad optical
absorptions in the visible range peaking at j \ 550 nm and
showing a half-life of about 100 ns (see Fig. 1(a)). Upon the
addition of thiophenol, the BuCl~` signal is shortened and
BuCl~` ] ArSH ] [ArSHÉ É ÉClBu]~` ] ArS~ ] H`(BuCl) (9)
Therefore, the two reactions (5) and (9) represent parallel reac-
tion channels of the IMR and determine the fate of the posi-
tive charges. The complex [ArSHÉ É ÉClBu~`] should be
understood in terms of molecule-encountering in a distinct
geometry. This encounter may happen in a favored and well-
deÐned geometry, but should also be extended to somewhat
less favorable encounters in the sense of a preferred encounter
sphere. A quantum chemical treatment of the geometry of the
intermediate encounter complex supports this hypothesis and
is described later on.
In addition to these fast, non-time-resolved processes, the
slightly delayed formation of the thiyl radical occurs, which
seems to be in accordance with the decay of the thiol radical
cation (reaction (10)). This process could be explained by
spontaneous deprotonation or neutralization with the high
surplus of chloride anions, with reaction (10a) being favored
because of the dose rate independence of the ArSH~` kinetics.
reduced to a non-resolved spike at C
\ 5 ] 10~3 mol
ArOH
dm~3. Initially treated as normal free electron transfer (5), a
rate constant of k \ 1.5 ] 1010 dm3 mol~1 s~1 was deter-
5
mined from the inÑuence on the parent ion signal.
n-BuCl~` ] ArSH ] ArSH~` ] n-BuCl
(5)
On the product side, however, at Ðrst sight completely inade-
quate transient spectra and kinetics were observed (cf. Fig.
1(b)). Here, a well time-resolved growth e†ect showing micro-
second kinetics appears in addition to the fast processes. This
can be explained by the product superposition of the ionic
reaction (5) with that of the radical H-abstraction reaction
caused by butyl radicals,12 as shown in reaction (6).
n-C H ~ ] ArSH ] n-C H ] ArS~
(6)
4
9
4 10
To separate the products of ionic (5) and radical (6) reaction
paths, oxygen was used as scavenger of alkyl radicals along
with ethanol as ion scavenger; reactions (7) and (8).
ArSH~` ] ArS~ ] H`(BuCl)
ArSH~` ] Cl~ ] ArS~ ] HCl
(10a)
(10b)
n-BuCl~` ] C H OH ] n-BuCl~ ] C H OH `
(7)
2
5
2 5
2
This e†ect can be seen in a more pronounced manner using
the example of the radiolysis of thiocresols described in the
next section.
n-C H ~ ] O ] n-C H OO~; k \ 2 ] 109 dm3 mol~1 s~1
4 9
4
9
2
8
(
8)
Additional tests with well-known cation scavengers con-
Ðrmed our assignment of the products of the ionic reaction
channel via reactions (5) and (9), which is mainly explained by
scavenging the precursor cation BuCl~`. Using a very careful
dosage of the cation scavengers ethanol and triethylamine (in
the millimolar range), apart from the depletion caused by
competition between e.g. (7) and (5) and (9), the reaction with
the thiophenol radical cation according to reactions (11) and
(12) could also be characterized by rate constants.
The resulting e†ects are shown in Figs. 1(c) and 1(d). In the
presence of ethanol (Fig. 1(d)), only the radical reaction (6)
takes place and the product spectra are assigned to the
phenylthiyl radical exclusively. The time proÐles for di†erent
phenol concentrations taken at 450 nm (lower inset of Fig.
1(d)) show exponential formation kinetics, corresponding to a
rate constant k \ 1.4 ] 108 dm3 mol~1 s~1.
6
Oxygen converts the alkyl radical into a peroxyl one (8),
which is much less reactive, and so no radical path contribu-
tion is observable (see Fig. 1(c)). Under these conditions,
taking into account the competition of reactions (6) and (8),
the calculation of the maximum amount of thiyls which could
originate from the radical path (6) amounts to less than 5% of
the former absorption, i.e. *O.D. \ 10~3, which does not
a†ect subsequent analysis. Consequently, only ionic products
are found in oxygen-saturated solution; see Fig. 1(c).
However, when analyzing the spectra and kinetics indicated in
Fig. 1(c), two di†erent transients can be seen, both of which
ArSH~` ] C H OH ] C H OH ` ] ArO~;
2 5
2 5
2
k
\ 4 ] 108 dm3 mol~1 s~1 (11)
1
1
ArSH~` ] (C H ) N ] ArSH ] (C H ) N~`;
2
5 3
2 5 3
k
\ 6.1 ] 109 dm3 mol~1 s~1 (12)
12
Because of the di†erent scavenging mechanismÈdeprotonation
(11) or charge transfer (12)Èthe rate constants, which were
obtained from the decrease of the phenol radical cation life-
1214
Phys. Chem. Chem. Phys., 2000, 2, 1213È1220