Generation and reactivity of aromatic thioether radical cations in aqueous solution as studied by pulse radiolysis

Article abstract of DOI:10.1021/jp970035u

The radical cations of thioanisole (1), p-methylthioanisole (2), and the benzyl phenyl sulfides 3-5 have been produced by pulse radiolysis in aqueous solutions, using SO₄^?- or Tl²⁺ as oxidizing species. The radical cations 1^?+-5^?+ exhibit very similar UV spectra, with strong absorptions between 300-350 and 500-600 nm. In contrast to aliphatic thioether radical cations, 1^?+-5^?+ do not undergo dimerization (via formation of a three-electron bond with the parent thioethers). In the absence of bases, 1^?+ is a long-lived species with a lifetime >30 ms, whereas 3^?+, 4^?+, and 5^?+ decay rapidly by both C-S bond and C-H bond cleavage with k_C-H = 1.3 × 10³ s^-1 and k_C-S = 1.3 × 10³ s^-1 for 3^?+ and k_C-H = 0.95 × 10³ s^-1 and k_C-S = 2.65 × 10³ s^-1 for 4^?+. In the presence of OH^- or HPO₄^2-, also 1^?+ undergoes a deprotonation process, with a rate larger than those of the benzyl phenyl sulfide radical cations. For example, the rate constant for the OH^--induced deprotonation is 3.4 x 10⁷ M^-1 s^-1 for 1^?+ and 9.5 x 10⁶ M^-1 s^-1 for 3^?+. Thioanisole radical cation 1^?+ was also produced by reduction of thioanisole sulfoxide. Under these conditions, it was possible to study the reaction of 1^?+ with a number of nucleophiles or electron donors. It was found that 1^?+ reacts with I^-, N₃^-, PhS^-, PhSH, Br^-, and SCN^- by an electron transfer mechanism, producing the oxidized form of the nucleophile. This reaction is diffusion controlled with the first four nucleophiles, which are more easily oxidized than 1 (E^o < 1.45 V); slower reactions were observed with SCN^- (E^o = 1.62 V) and with Br^- (E^o = 1.96 V). NO₃^- (E^o = 2.3V) is unreactive (k < 10⁶ M^-1 s^-1).

Full text of DOI:10.1021/jp970035u

J. Phys. Chem. A 1997, 101, 2979-2987
2979
Generation and Reactivity of Aromatic Thioether Radical Cations in Aqueous Solution As
Studied by Pulse Radiolysis
Marcella Ioele and Steen Steenken*
Max-Planck-Institut fu¨r Strahlenchemie, D-45413 Mu¨lheim, Germany
Enrico Baciocchi*
Dipartimento di Chimica, UniVersita` “La Sapienza”, I-00185 Roma, Italy
ReceiVed: December 18, 1996; In Final Form: February 7, 1997X
The radical cations of thioanisole (1), p-methylthioanisole (2), and the benzyl phenyl sulfides 3-5 have been
produced by pulse radiolysis in aqueous solutions, using SO4 or Tl2+ as oxidizing species. The radical
•-
cations 1•+-5•+ exhibit very similar UV spectra, with strong absorptions between 300-350 and 500-600
nm. In contrast to aliphatic thioether radical cations, 1•+-5•+ do not undergo dimerization (via formation of
a three-electron bond with the parent thioethers). In the absence of bases, 1•+ is a long-lived species with a
lifetime >30 ms, whereas 3•+, 4•+, and 5•+ decay rapidly by both C-S bond and C-H bond cleavage with
kC-H ) 1.3 × 103 s-1 and kC-S ) 1.3 × 103 s-1 for 3•+and kC-H ) 0.95 × 103 s-1 and kC-S ) 2.65 × 103
s-1 for 4•+. In the presence of OH- or HPO42-, also 1•+ undergoes a deprotonation process, with a rate
larger than those of the benzyl phenyl sulfide radical cations. For example, the rate constant for the OH--
induced deprotonation is 3.4 × 107 M-1 s-1 for 1•+and 9.5 × 106 M-1 s-1 for 3•+. Thioanisole radical cation
1
•+ was also produced by reduction of thioanisole sulfoxide. Under these conditions, it was possible to study
the reaction of 1•+ with a number of nucleophiles or electron donors. It was found that 1•+ reacts with I-,
N3 , PhS-, PhSH, Br-, and SCN- by an electron transfer mechanism, producing the oxidized form of the
-
nucleophile. This reaction is diffusion controlled with the first four nucleophiles, which are more easily
oxidized than 1 (E° < 1.45 V); slower reactions were observed with SCN- (E° ) 1.62 V) and with Br- (E°
) 1.96 V). NO3 (E° ) 2.3V) is unreactive (k < 106 M-1 s-1).
-
Introduction
reduce the propensity of the radical cation to form dimers, and
this has recently been shown to be the case for diphenyl sulfide
Free radicals and radical ions derived from sulfur-containing
compounds have been the subject of continuous study, as they
are involved in a great variety of chemical processes, extending
from those of industrial importance to the biological ones. In
this research area, considerable attention has been devoted to
the chemistry of sulfur-centered radical cations, due to their
importance as intermediates in many enzymatic oxidations of
organic sulfides.1-3 So far, however, most of the work dealing
with the fundamental properties (electronic structure and
reactivity) of these species has concerned dialkyl sulfide radical
cations, which have mainly been investigated by using the EPR
and the pulse radiolysis techniques.4-7
A special feature of the (monomeric) dialkyl sulfide radical
cations is their reaction with the neutral parent to form relatively
stable “dimer” radical cations where the two sulfur atoms are
held together by a two-center-three-electron bond.5 The
equilibrium between the dimer and the monomer therefore has
to be carefully considered in quantitative studies of the reactivity
of dialkyl sulfide radical cations (which mainly involves
deprotonation of an R-C-H bond that is generally attributed to
the monomeric form).
radical cations.9 Clearly, this information is very important since
if only monomeric radical cations are formed, the interpretation
of reactivity data is much less complicated.
In this paper we report the results of a pulse radiolysis study
of the radical cations formed in aqueous solution from thioani-
sole (1), 4-methylthioanisole (2), and the benzyl phenyl sulfides
3-5. The solubility of 5 turned out to be too low to obtain
sufficiently precise kinetic data on the reactivity of its radical
cation or a complete characterization of its decay products. Thus,
in addition to 5, the benzyl phenyl sulfides 3 and 4 were studied,
where the sulfonate functionality provides for sufficient water
solubility, whereas the oxidation potential of the sulfide as well
as the electronic structure of the radical cation are not expected
to be much different from those of 5.
X
SCH3
X
CH2–S
CH2SO3K
3 X = H
4 X = OMe
1 X = H
2 X = Me
CH2–S
Radical cations from aromatic sulfides have been much less
investigated. Quantitative information on these systems would
be of interest since in this case the reactivity is expected to be
influenced by the degree of spin delocalization in the aromatic
ring, for which the conformation of the radical cation is
important.8 It has also been noted that spin delocalization should
5
Experimental Section
Chemicals. Potassium peroxydisulfate, thallium(I) sulfate,
tert-butyl alcohol, and sodium phosphate were all analytical
grade from Merck. Thioanisole (Aldrich) was distilled prior
to use to a purity of 99.5%; benzyl phenyl sulfide, 4-methyl-
X Abstract published in AdVance ACS Abstracts, March 15, 1997.
S1089-5639(97)00035-2 CCC: $14.00 © 1997 American Chemical Society

2980 J. Phys. Chem. A, Vol. 101, No. 16, 1997
Ioele et al.
H2O
thioanisole, and thiophenol (Aldrich) were used as supplied
(purity >99%). The compounds 3 and 4 were obtained with
purity >99% (HPLC) by the reaction of 4-mercaptobenzene-
sulfonic acid with benzyl or 4-methoxybenzyl chloride, respec-
tively, in the presence of anhydrous potassium carbonate in
refluxing acetone.10 4-Mercaptobenzenesulfonic acid was pre-
pared as follows: 4-nitrobenzyl chloride was sulfonated to
sodium 4-nitrobenzenesulfonate,11 subsequently reduced to
4-aminobenzenesulfonic acid and converted into 4-mercapto-
benzenesulfonic acid according to a literature procedure.12
eaq- + C6H5S(O)CH3 f C6H5S(O•-)CH3
8
-2OH-
(C6H5SCH3)•+ (5)
The carbon-centered radicals (1-H)•, (3-H)•, and (4-H)• were
generated by H atom abstraction from 1, 3, and 4, respectively,
by O•-. The latter was obtained by pulse radiolysis of N2O-
purged aqueous solutions containing 0.5 M KOH, by deproto-
nation of •OH (pKa ) 11.8). Accordingly, at such a high [OH-],
the equilibrium OH. + OH- h H2O + O•- is shifted
completely to the right; thus, G(O•-) ) G(OH•).16 For the
determination of the extinction coefficients of (1-H)•, (3-H)•,
and (4-H)• a G value of 6.6 was taken, which is the sum of
G(OH•) ) 6 and G(H•) ) 0.6 since H• reacts with OH-
Sulfoxides of 1-5 (purity >99.5%) were obtained by
oxidation of the corresponding sulfides with NaIO4 in ethanol-
water13 and were purified by silica gel chromatography (eluent
petroleum ether/ethyl ether ) 4/1).
Pulse Radiolysis. Pulse radiolysis was carried out using a
3 MeV van de Graaff electron accelerator, which delivered 400
ns electron pulses with doses between 3 and 30 Gy, by which
OH•, H+, eaq- are generated (eq a), with concentrations of 2-20
µM. The pulse radiolysis setup and the methods of data
handling have been described elsewhere.14 For the optical
experiments, dosimetry was performed with N2O-saturated 10-2
mol dm-3 aqueous KSCN solutions, for which G[(SCN2)•-] )
6.0 × 10-7 mol J-1 and ꢀ[(SCN2)•-] ) 7600 M-1 cm-1 at 480
nm.15 Experiments were all conducted at room temperature (20
( 1 °C), using water purified with a Millipore Milli-Q system.
The pH was adjusted with NaOH or HClO4, and, when close
to neutrality, the solutions were buffered with 0.1-1 mM
phosphate.
-
producing eaq that is then scavenged by N2O to form another
OH• (eq 3).
Results
Formation of the Radical Cations. On reaction of (1-5)
with SO4•- or Tl2+, the decay of SO4•- (determined at 450 nm)
or of Tl
2+ (determined at 360 nm) was found to be synchronous
with the buildup of two strong absorptions at 300-350 and at
500-600 nm (see Figure 1A in the case of 1 and Figure 1B in
the case of 3, 4, and 5). These bands are assigned to the radical
cations of the sulfides in the monomeric form (see below), which
•-
are produced by electron transfer oxidation by Tl2+ or SO4
(eqs 6 and 7). In agreement with this assignment, the absorption
spectra and the lifetimes of these transients are not affected by
the presence of oxygen. The spectral data for the radical cations
are reported in Table 1 together with those of the products of
their decay, which will be discussed later. It should be noted
that the values of the extinction coefficients of the radical cations
As oxidizing species to convert sulfides into the corresponding
•-
•
radical cations we used SO4 and Tl2+. The use of OH for
the oxidation of the aromatic thioethers was avoided, since •OH
is known16 to react by addition rather than by electron transfer.
•-
SO4 was generated by pulse radiolysis of Ar-saturated
solutions containing 2 mM S2O8 and 0.1-1 M t-BuOH at
pH values between 4 and 12. Under these conditions, the
•-
when obtained by oxidation of the parent sulfides with SO4
2-
are lower by about 10% compared to those measured by
oxidation of the same substrates with Tl2+. This suggests that
eq 6 is the main but not the only reaction occurring between
hydrated electrons (eaq-) react with S2O8 to give SO4 (eq
1), whereas the hydroxyl radicals produced by the radiolytic
decomposition of water are scavenged by the alcohol (eq 2)
with a rate constant of 5 × 108 M-1 s-1. The t-BuOH-derived
radical is of low reactivity and also has a low extinction
coefficient at λ > 200 nm and therefore does not interfere with
the measurement.
2-
•-
•-
the SO4 radical and the substrates (see eq 9 below).
(1-5) + SO4•- f (1•+-5•+) + SO4
(6)
(7)
2-
(1-5) + Tl2+ f (1•+-5•+) + Tl+
H2O
H+ + OH• + eaq
The bimolecular rate constants for reaction of 1-5 with Tl2+
and SO4•- were determined by measuring kobs, the rate of optical
density (∆OD) increase of the radical cation at 530 nm (1•+,
5•+) or at 550 nm (2•+, 3•+, 4•+) as a function of the
concentration of added thioether. In all cases the kobs increased
linearly with [thioether]. From the slopes of the linear plots
the bimolecular rate constants shown in Table 2 were obtained.
In the case of thioanisole, the monomeric nature of the radical
cation was established by generating the radical cation of
thioanisole via a different pathway, i.e., via reduction of
thioanisole sulfoxide by the hydrated electron (eq 5). Under
these conditions, the formation of symmetric dimers is not
possible since no neutral sulfide is present. A spectrum was
obtained (Figure 1; λmax ) 310, 530 nm) identical to that found
when the radical cation was formed by oxidation of thioanisole
–
(a)
eaq– + S2O8
SO42– + SO4
(1)
(2)
2–
• –
•OH + (CH3)3COH
H2O + •CH2C(CH3)2OH
In order to produce Tl2+, N2O-saturated solutions containing
2 mM Tl+ at pH 3.2-3.7 were irradiated. The function of N2O
•
is to scavenge the hydrated electron and lead to a further OH
radical (eq 3). Tl2+ is produced by oxidation of Tl+ by OH
•
(eq 4).17 The pH was maintained below 3.7 in order to avoid
the formation of Tl(OH)+, which is a less powerful oxidant.18
eaq- + N2O + H2O f N2 + •OH + OH-
•OH + Tl+ + H+ f H2O + Tl2+
(3)
(4)
•-
with SO4 or with Tl2+. The same result was obtained when
2
•+ was produced by reduction of 4-methylthioanisole sulfoxide
In some experiments the benzyl sulfide radical cations were
also generated starting from the corresponding sulfoxides9 (eq
5). In this case, Ar-saturated aqueous solutions containing 0.2-
0.5 mM of the sulfoxide and 0.2 M t-BuOH (to scavenge •OH)
were irradiated to generate eaq-, which is the reactive species:
(λmax ) 320, 550 nm; see Table 1). Thus, and also on the basis
of the previous work,9 it is concluded that radical cations of
aromatic sulfides have only a very weak tendency to react with
the neutral sulfide to form a three-electron-bonded dimer, in
contrast to the situation found for aliphatic sulfides.6,7,19

Aromatic Thioether Radical Cations
J. Phys. Chem. A, Vol. 101, No. 16, 1997 2981
Figure 1. (A) Absorption spectra of 1•+. (Full circles): data obtained on radiolysis of an Ar-purged aqueous solution of PhSOCH3 (0.5 mM),
Na2HPO4 (1 mM), t-BuOH (0.2 M), pH 8.5, 2 µs after the pulse. (Open circles): data from radiolysis of an Ar-purged aqueous solution (pH 6.0)
of PhSCH3 (0.2 mM), K2S2O8 (2 mM), t-BuOH (0.1 M), 15 µs after the pulse. (Triangles): data from radiolysis of a N2O-purged aqueous solution
(pH 3.2) of PhSCH3 (0.2 mM), Tl2SO4 (1 mM), 60 µs after the pulse. For the reaction with SO4•- and with eaq- the ꢀ values are based on G ) 3.0
× 10-7 mol J-1. For the reaction with Tl2+ ꢀ values are based on G ) 6.0 × 10-7 mol J-1. (B) Absorption spectra of 3•+ (circles), 4•+ (triangles),
and 5•+ (diamonds). Data obtained on radiolysis of Ar-purged aqueous solutions (pH 6.0) of 3-5 (0.1 mM), K2S2O8 (2 mM), Na2HPO4 (1 mM),
t-BuOH (0.1-1M), 10 µs after the pulse. Due to the very low solubility of 5 in water (saturated solution ) 0.04 mM), a spectrum with a lower
apparent ꢀ was obtained for 5•+. In the figure, in order to compare the absorption spectra of 5•+ with those of 3•+ and 4•+, that of 5•+was enlarged
by the factor 1.5. ꢀ values are based on G(radical) ) G(eaq-) ) G(SO4•-) ) 3.0 × 10-7 mol J-1
.
TABLE 1: Spectral Data for the Radical Cations of 1-5 and for the Radicals Produced in Their Decay
transient
λ
max/nm
ꢀmax/M-1 cm-1
method of generationa
1•+
(PhSCH3)•+
310, 530
320, 550
320, 550
320, 550
310, 530
305, 470, 500
295, 460
330
9200, 5400
1, 2, 3b
2•+
(4-CH3PhSCH3)•+
(PhCH2SPhCH2SO3
11000, 6700
10000, 4000
11000, 6700
6300,c 2100c
12700, 2300, 2400
9000, 1800
3800
8000, 1000
7000, 8800, 1300
1
-
•+
3•+
)
2, 3b
3
-
•+
4•+
(4-CH3OPhCH2SPhCH2SO3
)
5•+
(PhSCH2Ph)•+
1
2
2, 4
5
5
-
6
7
•SPhCH2SO3
PhS•
•
(1-H)•
(3-H)•
(4-H)•
PhSCH2
-
PhCH•SPhCH2SO3
360, 480
310, 360, 490
-
4-CH3OPhCH•SPhCH2SO3
5
-
a Radical cations and radicals were generated by the following methods: (1) Reduction of the sulfoxide by eaq (0.4 mM sulfoxide, 0.2 M
t-BuOH, Ar). G(radical) ) G(eaq-) ) 3.0 × 10-7 mol J-1. (2) Oxidation by Tl+2 (pH 3.2-3.5) (0.1 mM substrate, 1 mM Tl2SO4, N2O). G(radical)
•-
) G(OH) ) 6.0 × 10-7 mol J-1. (3) Oxidation by SO4•- (pH 4-8) (0.1 mM substrate, 2 mM K2S2O8, 0.1 M t-BuOH, Ar). G(radical) ) G(SO4
)
) 3.0 × 10-7 mol J-1. (4) Dehydrogenation of thiophenol by •CH2C(CH3)2OH (or by (CH3)2C•OH) (0.1 mM PhSH, 5% t-BuOH/i-PrOH, pH 4.0,
N2O). G(radical) ) G(OH) ) 6.0 × 10-7 mol J-1. (5) Dehydrogenation of the parent compound by O•- (0.1 mM substrate, 0.5 M KOH, N2O).
G(radical) ) G(OH•) + G(H) ) 6.6 × 10-7 mol J-1
.
b In this case a lower value (5-10%) of ꢀ was obtained; see text. c The lower ꢀ value of 5•+
compared to those of the other radical cations is probably an artifact due to the very low solubility of 5 in water, which results in the primary
radicals SO4•-and Tl+2 not being quantitatively scavenged to yield 5•+
.
4•+). In the absence of a base, the aromatic radical cations decay
predominantly by first order.
TABLE 2: Rate Constants for the Oxidation of 1-5 by
•-
SO4 and Tl2+ to Their Corresponding Radical Cations in
Aqueous Solutions
The measured rates for the decay were found to be dose
dependent, which indicates the presence of second-order
contributions due to radical-radical reactions. To correct for
this effect, rates were measured at different (small) doses and
extrapolated to dose ) 0. The first-order rate constants thus
obtained are shown in Table 3. The decay of 1•+ is so slow
(kdecay less than 30 s-1) that it was not possible to obtain
information on the nature of the radical product formed. 3•+
and 4•+ decayed at a much faster rate (kdecay ca. 103 s-1) leading
to the formation of two transients (Figure 2A,B). The transients
with λmax ) 360 nm can be quenched by oxygen (see insets c
and d), and on this basis they are identified as the carbon-
centered radicals (3-H)• and (4-H)•. This assignment is
confirmed by comparison with the authentic spectra of the
radicals (3-H)• and (4-H)•, shown in Figure 3, respectively,
substrate
oxidant
k/M-1 s-1 a
pH
•-
PhSCH3
PhSCH3
SO4
3.9 × 109
>1.5 × 109
3.3 × 109
2.1 × 109
3.9 × 109
2.4 × 109
3.3 × 109 b
7.5
3.2
7.5
3.5
7.5
3.5
7.5
Tl2+
•-
4-CH3PhSCH3
PhCH2SPhCH2SO3
4-CH3OPhCH2SPhCH2SO3
4-CH3OPhCH2SPhCH2SO3
PhCH2SPh
SO4
-
Tl2+
-
-
•-
SO4
Tl2+
•-
SO4
a Error limits (10%. b Due to the low solubility of 5 in water, this
rate constant is affected by a larger error ((20%).
Decay of the Radical Cations in the Absence of Bases and
Nucleophiles. The decay of the sulfide radical cations was
followed by measuring the rate of optical density decrease of
the radical cations at 530 nm (1•+, 5•+) or at 550 nm (2•+, 3•+,

2982 J. Phys. Chem. A, Vol. 101, No. 16, 1997
Ioele et al.
Figure 2. (A) Absorption spectra recorded on pulse radiolysis (dose ) 5 Gy) of Ar-purged aqueous solution (pH 4.0) of 3 (0.1 mM), K2S2O8 (2
mM), and t-BuOH (0.1 M) at the times after the pulse as indicated by the symbols. Inset a shows the decay of the radical cation 3•+ at 550 nm, inset
b shows the rise of conductance resulting from the decay of the radical cation 3•+ at pH 4.5 (dose ) 4 Gy), and inset c shows the formation and
decay (by radical-radical reaction) of the carbon-centered radical (3-H). at 360 nm, in the absence of O2. From inset d it is evident that radical
(3-H). is scavenged by O2. The decay seen has the same rate as that of the radical cation 3•+, and it is due to its weak absorption at 360 nm. (B)
Absorption spectra recorded on pulse radiolysis (dose ) 5 Gy) of an Ar-purged aqueous solution (pH 4.0) of 4 (0.1 mM), K2S2O8 (2 mM), and
t-BuOH (0.1 M) at the times after the pulse as indicated by the symbols. The insets are analogous to those in (A), with the exception that in inset
b the dose was 0.2 Gy.
with the radical •CH2C(CH3)2OH or (CH3)2C•OH, produced by
H-abstraction from t-BuOH or i-PrOH, respectively, by OH.
TABLE 3: Rate Constants for the Decay in Water (kdecay
)
•
and for the Base-Induced Deprotonation (kdep) of the Radical
Cations 1•+-5•+
The attempt to produce the radical (5-H)• by H-abstraction from
5 with O•- was unsuccessful, which is probably due to the
extremely low solubility of 5 in water (the rate constant for
H-abstraction by O•- appears to be <108 M-1 s-1). Therefore
the assignment of the band at 350 nm to (5-H)• was made by
analogy with the absorption spectrum of (3-H)•.
radical cation
kdecay/s-1 a
<30
base
OH-
HPO4
kdep/M-1 s-1 b
1•+
3.4 × 107 c
2.6 × 106
2.5 × 107 c
9.5 × 105 c
9.5 × 106 c
2.2 × 105
2.0 × 107 c
1.8 × 106
-2 d
3•+
4•+
5•+
2.6 × 103
3.5 × 103
2.5 × 103
OH-
HPO4
OH-
-2 d
-2 d
-2 d
The observations reported above indicate that, in the absence
of bases and nucleophiles, the decay of the benzyl phenyl sulfide
radical cations 3•+ and 4•+ occurs by deprotonation at the ben-
zylic position (Scheme 1, path a) and by cleavage of the C-S
bond (path b). In line with these findings is the observation
HPO4
OH-
HPO4
a The kdecay values have been obtained by extrapolation to zero dose;
see text. The values do not depend on the oxidant (SO4•- or Tl2+) and
do not change in the pH range 3.5-8. b Determined from the slope of
the plot kdecay (550 nm) vs [base]. Error limits (10%; with 5•+ it is
.-
that the steady state oxidation of 3 and 4 by SO4 leads to the
formation of 4-methoxybenzyl alcohol and 4-methoxybenzalde-
hyde in an alcohol:aldehyde ratio of 0.9 and 2,5, respectively.21
The aldehydes arise via deprotonation of the radical cation to
yield (3-H)•/(4-H)• followed by oxidation with S2O82-, addition
of water, and hydrolysis of the resulting R-hydroxysulfide. The
benzyl alcohols derive from the benzyl cation, formed by
heterolytic C-S bond cleavage (reaction b in Scheme 1).
It was found that during the decay of the radical cations the
conductance of the solutions changes. In acidic solution, the
conductance increases whereas in alkaline solution it decreases
(with the same rate). The increase in conductance is thus due
to the production of H+ (Scheme 1). The rate of H+ production
was the same as that of the decay of the radical cation.
However, the amplitude of this decay-related conductance
increase was smaller than that of the increase in conductance
originating from the formation of the radical cation and due to
the “fast reactions” leading to eaq- and from there to SO4•- and
the radical cation (eqs a, 1, and 6). This means that a part of
probably twice as large. The radical cations were produced by oxidation
•-
with SO4
.
c Within the experimental error these rates coincide with
those obtained by measuring the buildup of the transient absorbing at
350 nm (the carbon-centered radical). d pH 9.0.
independently generated by hydrogen atom abstraction from 3
or 4 by O•-.
The other transient absorbs at 310 and 450-510 nm and is
not quenched by oxygen. This species is identified as the sulfur-
•
-
centered radical S-C6H4CH2SO3 (6), on the basis of the
comparison with the authentic spectrum of 6 (shown in Figure
-
4A, independently produced by reaction of p-HS-C6H4CH2SO3
with Tl2+ (eq 8).
-O3SCH2C6H4SH + Tl2+ f -O3SCH2C6H4S• + Tl+ + H+
(6)
(8)
•-
SO4 reacts with the sulfide in a process that is accompanied
The radical cation 5•+ decayed at the same rate as 3•+; thus,
the CH2SO3- group has no noticeable influence on the lifetime
of the radical cation. As 5•+ decays, two transients are formed,
one with λmax ) 360 nm and the other with λmax ) 295 and
465 nm, which are assigned to the carbon-centered radical (5-
H)• and the thiophenoxyl radical •SC6H5 (7), respectively.20 The
authentic spectrum of this radical (see Figure 4B) was obtained
by oxidation of thiophenol with Tl2+ at pH 4 or by reaction
by production of H+ and that occurs concomitantly with the
formation of the radical cation. We suggest that this process
•-
consists of the SO4 radical abstracting a H atom from the
substrate’s side chain (eq 9), in addition to the electron
abstraction from the aromatic part of the molecule (eq 6).
SO4•- + 3/4 f HSO4- (H+ + SO42-) + (3-H)•/(4-H)• (9)

Aromatic Thioether Radical Cations
J. Phys. Chem. A, Vol. 101, No. 16, 1997 2983
Figure 3. Absorption spectra recorded at 8 µs after the pulse on irradiation of N2O-saturated aqueous solutions containing 0.5 M KOH and 0.1
mM 3 (A) or 4 (B).
Figure 4. Absorption spectra recorded at 15 µs on pulse radiolysis of a N2O-saturated solution of 1 mM Tl2SO4, pH 3.5, and containing 0.1 mM
4-mercaptobenzenesulfonic acid (A) or 0.1 mM thiophenol (B).
SCHEME 1
1
-2
From the ratio between the conductance increase in the
formation reaction and that due to the decay of the radical cation
(slow reaction) ∆Λfast/∆Λslow extrapolated to dose ) 0, and
taking the following values for the single-ion equivalent conduc-
tivities in water at 20 °C:22,23 λ0H+ ) 314, /2λ0 SO4 ) 72,
1/2λ0 S2O8-2 ) 77, λ0 3•+(4•+) = λ0C6H5CO2- ) 29 (cm2/(Ω-1
mol-1)),24 it is calculated that the contribution of eq 9 to the
•-
overall reaction of SO4 is e10% for 3 and ca. 13% for 4.

2984 J. Phys. Chem. A, Vol. 101, No. 16, 1997
Ioele et al.
Figure 5. (A) Absorption spectra recorded on pulse radiolysis of an Ar-purged aqueous solution (pH 7.5) of 0.4 mM PhSOCH3 and 0.2 M t-BuOH
and containing 0.02 mM KI. (B) Absorption spectra on pulse radiolysis of an Ar-purged aqueous solution (pH 10) of 0.4 mM PhSOCH3 and 0.2
M t-BuOH and containing 0.03 mM PhSH.
The complication introduced by eq 9 as well as some
overlapping of the absorption spectra of thiophenoxyl and
carbon-centered radicals do not allow a precise dissection of
the decay rate of the radical cation into the contributions by
C-H and C-S bond cleavage, respectively. However, infor-
mation in this respect can be derived from the already mentioned
to be independent of the dose used and of the substrate
concentration (from 0.05 to 0.3 mM).
Decay of Thioanisole Radical Cation in the Presence of
Nucleophiles. The decay of 1•+ was also studied in the presence
of compounds that are weak bases but strong nucleophiles or
electron donors (I-, C6H5S-, C6H5SH, N3-, Br-, SCN-, and
NO3-). For this investigation 1•+ was generated under the
•-
observation that the steady state oxidation of 3 and 4 by SO4
leads to the formation of benzyl alcohol (4-methoxybenzyl
alcohol) and benzaldehyde (4-methoxybenzaldehyde) in an
alcohol:aldehyde ratio of 0.9 and 2.5, respectively. On this
basis, after correction for eq 9, it is possible to estimate the
rate of the C-H and C-S bond cleavage at pH 4.0. The values
obtained (extrapolated to zero dose) are kC-H ) 1.3 × 103 s-1
and kC-S ) 1.3 × 103 s-1 for 3•+ and kC-H ) 0.95 × 103 s-1
and kC-S ) 2.65 × 103 s-1 for 4•+.
reductive conditions of eq 5, to circumvent the oxidation of the
•-
nucleophile by SO4 or Tl2+
.
The rate of decay of 1•+ at 530 nm was found to increase
strongly in the presence of I-, N3-, PhS-, and PhSH. A much
smaller increase was observed in the presence of Br- and SCN-,
whereas there was no effect on the lifetime of 1•+ in the presence
of NO3-. In the case of the quenching with I-, the decay of
1•+ was synchronous with the buildup of a transient with λmax
.-
) 380 nm, which is identified as I2
(see Figure 5A).
Decay of the Radical Cations in the Presence of Bases.
In the presence of a base such as OH- or HPO4-2, an accelerated
decay of thioanisole radical cation, 1•+, was observed. This
base-induced decay leads to a transient with an absorption at
320-360 nm, quenchable with oxygen, which is identified as
the carbon-centered radical (1-H)•, by comparison with the
authentic spectrum of this radical, obtained by H-abstraction
from the methyl group of thioanisole with O•- at pH 13.5. Thus,
the decay of 1•+ is ascribed to base-induced deprotonation from
the methyl group (eq 10):
Analogously, (SCN)2•- (λmax ) 480 nm) and Br2•- (λmax ) 370
nm) were produced by quenching of 1•+ with SCN- and Br-,
respectively. These findings clearly show that the reaction
between 1•+ and the above halide or pseudohalide anions (X-)
involves an electron transfer process leading to the formation
of the neutral thioanisole (1) and the radical X• (eq 11) followed
by reaction of X• with X- to form X2•-. This reaction (eq 12)
occurs in water with a diffusion-controlled rate (k ) 1.2 × 1010
M-1 s-1 25,26
)
for X- ) I-, Br-, SCN-.
(PhSCH3)•+ + X- f PhSCH3 + X•
(11)
(12)
-
-2
OH , HPO4
•
(C6H5SCH3)•+
8 C6H5SCH2 (1-H)• (10)
-
-H2O, -H2PO4
•-
X• + X- f X2
Also the rates of decay of the radical cations 3•+ and 4•+
significantly increase in the presence of OH- and HPO42- such
that in basic medium deprotonation is the dominant reaction of
the radical cation. From the absorption spectra measured under
these conditions, it became clear that the deprotonation process
is accelerated by the presence of the base while the rate of C-S
bond cleavage reaction remains unaffected, so that the latter
becomes negligible at a sufficiently high base concentration.
The second-order rate constants for the reactions of 1•+, 3•+,
4•+, and 5•+ with OH- and with HPO4-2 were determined from
the linear plots of the observed decay rates vs the base
concentrations. The values, collected in Table 3, were found
No absorbing products were observed on quenching of 1•+
.
with N3-, but this is very probably due to the fact that N3 does
•-
not readily form the N6 dimer in aqueous solutions and the
monomer radical absorbs only weakly above λ ) 250 nm.27
We therefore assume that also N3- reacts with 1•+ by an electron
transfer mechanism.
The same mechanism holds for the reactions of 1•+ with PhS-
and PhSH, which were carried out by adding thiophenol (pKa
) 6.6) to the aqueous solution of thioanisole sulfoxide at pH
10 and 4.5, respectively. These reactions lead to thioanisole
and the thiophenoxyl radical C6H5S•, according to eq 13. As

Aromatic Thioether Radical Cations
J. Phys. Chem. A, Vol. 101, No. 16, 1997 2985
TABLE 4: Experimental Rate Constants (kred) for the Reactions of Thioanisole Radical Cation (E° ) 1.45 V)32 with a Number
of Nucleophiles (Nu) and Calculated Rate Constants (kd) for the Diffusion-Controlled Reactions of Thioanisole Radical Cation
with the Same Nucleophiles in Water at 20 °C
Nu
I-
E° a
kred/M-1 s-1 b
kd/M-1 s-1 c
D/cm2 s-1 d
R/Åe
pHf
1.2739
1.3339
0.6920
1.0820
1.6239
1.9626
2.339
1.9 × 1010
1.7 × 1010
8.5 × 109
3.5 × 109
2.4 × 108
8.0 × 105
1.9 × 1010
1.7 × 1010
1.1 × 1010
6.0 × 109
1.79 × 10-5
1.61 × 10-5
0.67 × 10-5
1.54 × 10-5
1.83 × 10-5
1.65 × 10-5
8.7
8.3
8.7
7.5
7.5
10.0
4.5
7.5
7.5
-
N3
PhS-
PhSH
SCN-
Br-
1.65 × 1010
1.85 × 1010
1.7 × 1010
8.5
8.5
8.5
-
NO3
7.5
a Standard reduction potential (vs NHE) of the nucleophile in its oxidized form. b Determined from the linear plot kobs vs [Nu]. c Calculated from
the Debye-Smoluchowsky equation, kd ) RRNR(DA + DB)/1000 (15),36 when Nu is an anion and from the Stokes-Einstein equation, kd ) (2RT/
3000η)(2 + rA/rB + rB/rA) (16)38 for Nu ) PhSH. In eq 15, N is the Avogadro number, DA and DB are the diffusion coefficients of the two ions
(reported in the fifth column of the table), RR is taken as 4π, and R (sixth column of the Table) is calculated as described in footnote d. In eq 16,
η is the viscosity of the medium (0.0102 poise for water at 20 °C), rA and rB are the reactants’ radii, which are assumed to have the same value.
d Diffusion coefficients for ions were calculated from their limiting single-ion equivalent conductivities, λ0, using the formula D ) 2.6 × 10-7λ0
at 20 °C. Λ0 values at 20 °C were obtained from the λ0 at 25 °C, by multiplying by 0.9.22 The λ0 of PhS- was approximated as the λ0 of p-anisate
•+
(26 cm2/(Ω-1 mol-1)), whereas the λ0 of PhSCH3 was approximated as the λ0 of the tetramethylammonium ion (40 cm2/(Ω-1 mol-1)).36 e R )
rc/[exp(rc/r0)-1], where r0 is the encounter distance and rc accounts for Coulombic interactions between the two ions. For singly charged ions of
opposite charge, the value of rc in water at 20 °C is -7.5 Å. The value r0 is approximated as the sum of the ionic radii of the reacting ions. The
contact distance for the radical cation was taken as 1.8 Å.36 f pH at which the reaction was carried out.
an example, in Figure 5B is shown the absorption spectrum
following reaction of 1•+ with PhS-, where it is evident that
1•+ is replaced after 9 µs by PhS•.
ancy between the predictions based on PES experiments and
the behaviors of radical cations in water is, however, only an
apparent one. PES experiments refer to properties in the gas
phase. In solution, localization of the charge on the sulfur atom
in 4•+ may be favored by solvation at this site.30
1
•+ + C6H5S-/C6H5SH f 1 + C6H5S•/C6H5S• + H+ (13)
From the data displayed in Table 3 it is clear that in the
absence of base the decay rate of 1•+ is dramatically smaller
than those of the benzyl phenyl sulfide radical cations 3•+, 4•+,
and 5•+, whose decay rates are very close to one another. Since
with 3•+-5•+ deprotonation is a substantial part of the decay,
the above finding indicates that the latter radical cations are
deprotonated at a rate that is at least 3-4 orders of magnitude
larger than that of 1•+. A reason for this huge difference in
rates is possibly the relatively low acidity of the C-H bonds
of the methyl group in 1•+ (estimated pKa ca. 0.9),31-34 which
is too low to allow the observation, on the ≈1 ms time scale of
the pulse radiolysis experiment, of a deprotonation process
induced by so weak a base as H2O (pKa ) -1.7) . In contrast,
with 3•+-5•+, the benzylic C-H bonds are much more acidic
(a pKa value of about -10.8 is estimated for 5•+),35 due to the
presence of the phenyl group. Thus, with 3•+-5•+, H2O is
apparently a base strong enough to enable deprotonation.
The decay of 1•+ induced by the various nucleophiles
followed first-order kinetics, and the second-order rate constants
were determined by plotting the observed first-order rates versus
the nucleophile concentrations. From the slopes of the linear
plots the rate constants shown in Table 4 were obtained.
Discussion
The rate constants for formation of the radical cations by
•-
reaction with SO4 (Table 2) are almost the same for all
substrates (k around 3 × 109 M-1 s-1). On the basis of this
value, the reactions are classified as diffusion controlled, as
expected for processes that are highly exoergonic. Another
observation is that, different from the aliphatic counterparts,6,7,19
aromatic sulfide radical cations appear to have only a very weak
tendency to react with the neutral parent to form a three-electron-
bonded dimer. A reasonable explanation is that, in the radical
cation, part of the positive charge and/or spin density is
delocalized into the aromatic ring, thus decreasing the strength
of a sulfur-sulfur three-electron bond. A similar delocalization
of the sulfur 3p-type nonbonding electron pair in the neutral
substrate should act in the same direction; however, such a
delocalization, certainly possible for 1 and 2, might become
difficult in 3-5 due to the bulkiness of the benzyl group, which
destabilizes the conformation where the sulfur p-type orbital is
aligned with the aromatic π system.8 This conformational effect
should also be present in the radical cation, where, however, it
could be outweighed by the greater energetic gain due to the
delocalization of the positive charge/spin density.
When in the aqueous solution a base like OH- (pKa(H2O) )
2-
15.4) or HPO4 (pKa(H2PO4-)) 7.2) is added, also 1•+ is
rapidly deprotonated with a (second-order) rate very similar (in
fact, it is slightly larger!) to those of 3•+, 4•+, and 5•+. This
situation is very surprising in view of the great differences in
pKa of the radical cations as mentioned above. A tentative
explanation is that deprotonation of 1•+ (the weakest acid)
exhibits a much higher sensitivity to the strength of the base
than the deprotonation of the more acidic radical cations.
It is also noteworthy that the presence of the strong electron
donor MeO on the benzylic phenyl ring does not exert a
significant influence on the decay of the radical cation. For
example, 3•+ and 4•+ exhibit very similar decay rates both in
the absence and in the presence of base. More significantly,
the rates of C-S bond cleavage are also very similar. This is
in contrast to expectation since, as the radical cations of 3 and
4 are both sulfur centered, as suggested above, one expects a
larger rate of C-S bond cleavage for 4•+ than for 3•+ since a
much more stable carbocation is formed in the former case. It
seems, therefore, that in the transition state of the C-S bond
cleavage very little charge is transferred from sulfur to carbon.
An alternative is that, as the positive charge develops on the
The UV/vis spectra of 1•+-5•+ are very similar, with a λmax
around 550 nm. This suggests a similar electronic structure
for these radical cations and, specifically, that in all cases one
is dealing with sulfur-centered radical cations. This conclusion
is expected for 1•+, 2•+, 3•+, and 5•+, but it is not the one
predicted for 4•+ on the basis of PES experiments,28 which
indicate that the SOMO resides on the methoxy-substituted
aromatic ring and not on the sulfur atom. If this were the case,
however, the radical cation should absorb in the 450-470 nm
region of the spectrum,29 which is not observed. This discrep-

2986 J. Phys. Chem. A, Vol. 101, No. 16, 1997
Ioele et al.
Summary and Conclusions
In the present work radical cations from the aromatic
thioethers 1-5 were produced by pulse radiolysis in aqueous
•-
solutions, using SO4 or Tl2+ as the oxidizing species. The
radical cations 1•+-5•+ are present as monomers; i.e., they do
not form three-electron-bonded dimers as in the case of the
aliphatic sulfides. The radical cations 3•+, 4•+, and 5•+ decay
by two pathways: cleavage of the C-S bond, producing a
thiophenoxyl radical, and deprotonation from the benzylic
position, producing a carbon-centered (benzyl type) radical. The
second process is accelerated in the presence of a base, such as
OH- or HPO42-. In the case of thioanisole, the base-catalyzed
deprotonation is the only pathway of decay of the radical cation.
Thioanisole radical cation can be alternatively produced by
reduction of thioanisole sulfoxide. Under these conditions, it
is possible to study the reaction of the radical cation with
reductants. It was found that thioanisole radical cation is
reduced to neutral thioanisole with diffusion-controlled rates
by I-, N3-, PhS-, and PhSH (E° < 1.45 V), whereas the electron
Figure 6. Values of log k for the electron transfer reduction of
thioanisole radical cation by nucleophiles in water at 20 ( 1 °C, Vs
E°(Nu•/Nu-).
carbon atom, it is immediately neutralized by a water molecule,
e.g., by that solvating the sulfur, so that a carbocation is never
formed along the reaction coordinate.
transfer is rate limiting for Br- and SCN-. No reaction was
Finally, we would like to comment on the data concerning
the electron transfer reactions of 1•+ with a number of strong
nucleophiles or electron donors as reported in Table 4 and Figure
6. The reaction rate increases with the reducing power of the
nucleophile Nu- up to a limiting value characterized by the E°
value of the Nu•/Nu- couple becoming lower than that of the
1•+/1 couple (1.45 V), i.e., by the process becoming exoergonic
(see Figure 6). The limiting rate constants have magnitudes in
the vicinity of 1010 M-1 s-1, the diffusion-controlled value in
water. This was checked by calculating the diffusional rate
constants in water (kd) using the Debye-Smoluchowsky
equation36-38 and the Stokes-Einstein equation38 for the reac-
tions of thioanisole radical cation with anions and with neutral
nucleophiles, respectively.
-
observed with NO3
.
Acknowledgment. M.I. thanks the Max Planck Institut for
their hospitality and the colleagues at the Max Planck Institut
for the helpful assistance and stimulating discussions and
gratefully acknowledges the financial contribution of the
European Union (Contract CEE ERBSC1-CT91-0750). E.B.
acknowledges the financial support of the Italian Ministry for
the University and the Scientific Research (MURST) and of
the National Council of Research (CNR).
References and Notes
(1) Doerge D. R.; Niraanjaala, M. C. Biochemistry, 1991, 30, 8960.
(2) Kobayashi, S.; Nakano, M.; Kimura, T.; Schaap, A. P. Biochemistry
1987, 26, 5019-5022.
(3) Ozaki, S.; Ortiz de Montellano, P. R. J. Am. Chem. Soc. 1995,
117, 7056-7064.
(4) Gilbert, B. C.; Hodgeman, D. K. C.; Norman, R. O. J. Chem. Soc.,
Perkin. Trans. 2 1973, 1748.
(5) Sulfur-Centered ReactiVe Intermediates in Chemistry and Biology;
Chatgilialoglu, C., Asmus, K. D., Eds.; Plenum Press: New York, 1990.
See also: Chaudhri, S. A.; Mohan, H.; Anklam, E.; Asmus, K.-D. J. Chem.
Soc., Perkin Trans. 2, 1996, 383.
The results of these calculations, which are reported in Table
4, show that for I-, N3-, PhS-, and PhSH the experimentally
observed rate constants are very close to those calculated for a
diffusional process. However, with SCN- and Br-, the rate
constants measured are significantly smaller than the diffusional
limit. The data can be fitted by the conventional model36 (eq
14), which involves a diffusional encounter of the radical cation
and the nucleophile Nu, with the diffusion step being rate
limiting for the donors that are more easily oxidizable than
thioanisole (E° > 1.45 V) and the electron transfer step
becoming (at least in part) rate limiting for Br- and SCN- whose
E° is somewhat higher than 1.45 V. In line with this scheme,
(6) Bonifacic, M.; Asmus, K. D. J. Chem. Soc., Perkin Trans. 2 1980,
758.
(7) Mo¨nig, J.; Goslich, R.; Asmus, K. D. J. Phys. Chem. 1986, 90,
115-121.
(8) Glass, R., S.; Broeker, J. L.; Jatcko, M. E. Tetrahedron 1989, 45,
1263-1272.
(9) Engman, L.; Lind, J.; Mere´nyi, G. J. Phys. Chem. 1994, 98, 3174.
(10) Baciocchi, E.; Intini, D.; Piermattei, A.; Ruzziconi, R. Gazz. Chim.
Ital. 1989, 119, 649.
(11) Beringer, F. M.; Falk, R. A. J. Am. Chem. Soc. 1959, 81, 2997.
(12) Kaway, H.; Sakamoto, F.; Taguchi, M.; Kitamura, M.; Sotomura,
-
no reaction was observed with NO3 (E° ) 2.3 V), for which
the electron transfer step is highly endoergonic. It has, however,
been reported that sulfur radical cations react with NO3- to give
sulfoxides.40 Clearly, in the light of the present results this
process must be relatively slow. For the reaction of 1•+ with
NO3-, the rate constant is estimated to be <106 M-1 s-1
.
M.; G., T. Chem. Pharm. Bull. 1991, 39, 1422.
(13) Ruff, F.; Kucsman, A. J. Chem. Soc., Perkin Trans. 2 1985, 683.
(14) Jagannadham, V.; Steenken, S. J. Am. Chem. Soc. 1984, 106, 6542.
(15) Schuler, R. H.; Hartzell, A. L.; Behar, B. J. Phys. Chem. 1981, 85,
ket
98
(PhSCH3)•+ + Nu ykdz (PhSCH3)•+‚‚‚Nu
k-d
192.
Nu•+ + PhSCH3 (14)
(16) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J.
Phys. Chem. Ref. Data 1988, 17, 513.
(17) Asmus, K.-D.; Bonifacic, M.; Toffel, P.; O’Neill, P.; Schulte-
Frohlinde, D.; Steenken, S. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1820.
(18) Bonifacic, M.; Asmus, K. D. J. Chem. Soc., Dalton Trans. 1976,
2074.
(19) Bonifacic, M.; Mo¨ckel, H.; Bahnemann, D.; Asmus, K. D. J. Chem.
Soc., Perkin Trans. 2 1975, 675-685.
(20) Armstrong, D. A.; Sun, Q.; Schuler, R. H. J. Phys. Chem. 1996,
Concerning the nature of the electron transfer step, we are
faced with the usual problem: inner-sphere or outer-sphere
electron transfer? At present, no definitive answer is possible.
However, the formation of loose complexes between sulfide
radical cations and halide ions, particularly iodide and bromide,6
has been reported and more recently, the formation of a short
lived intermediate (Ph2SBr)• between Ph2S•+ and Br-. These
are arguments in favor of an inner-sphere mechanism.
100, 9892-9899.
(21) Baciocchi, E.; Lanzalunga, O.; Malandrucco, S.; Ioele, M.; Steen-
ken, S. J. Am. Chem. Soc. 1996, 118, 8973.

Aromatic Thioether Radical Cations
J. Phys. Chem. A, Vol. 101, No. 16, 1997 2987
(22) The values of λ0 at 20 °C were obtained from the λ0 at 25 °C,
multiplying by 0.9, a factor based on the temperature dependencies of λ0
for other anions and cations.23
(BDE) minus the entropy factor T∆S° (8 kcal mol-1). The values used were
E°(1•+/1) ) 1.45 V,32 E°(H+/H•) ) 2.29 V,33 and BDE(PhSCH2-H) ) 93
kcal mol-1 34
.
(23) Electrolyte Solutions; Robinson, R. A., Stokes, R. H., Eds.;
Butterworths: London, 1959.
(24) Handbook of Chemistry and Physics; Vanysek, P., Ed.; CRC
Press: Boca Raton, FL, 1986-87; Vol. 67.
(25) von Sonntag, C.; Schuchmann, H.-P. In Methods in Enzymology;
Inc, A. P., Ed.; Academic Press: New York, 1994; Vol. 233, pp 3-20.
(26) Mere´nyi, G.; Lind, J. J. Am. Chem. Soc 1994, 116, 7872-7876.
(27) Wayner, D. D. M.; Workentin, M. S.; Wagner, B. D.; Lusztyk, J.
J. Am. Chem. Soc. 1995, 117, 119-126. See also: Alfassi, Z. B.; Pru¨tz,
W. A.; Schuler, R. H. J. Phys. Chem. 1986, 90, 1198 and references therein.
(28) Baciocchi, E.; Cauletti, C.; Rol, C.; Sebastiani, G. V.; Stranges,
M., to be published.
(29) Neill, P. O.; Steenken, S.; Schulte-Frohlinde, D. J. Phys Chem.
1975, 79, 2773.
(30) Asmus, K. D. In ref 4, p 169.
(31) Calculated at 25 °C on the basis of a thermochemical cycle by the
equation pKa ) (1/1.37)[C-H(BDFE) - 23.06E°(H+/H•) - 23.06E°-
(sulfide)], where C-H(BDFE) is the homolytic dissociation free energy of
the C-H bond in the neutral substrate, taken as the bond dissociation energy
(32) Jonsson, M.; Lind, J.; Mere´nyi, G.; Eriksen, T. E. J. Chem. Soc.,
Perkin Trans. 2 1995, 67-70.
(33) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287.
(34) Bordwell, F. G.; Zhang, X.-M.; Alnajjar, M. S. J. Am. Chem. Soc.
1992, 114, 7623.
(35) Calculated as reported in ref 31. The data used were E°(5•+/5) )
1.55 V (taken 0.1 V higher than that of the 1•+/1 couple on the basis of the
difference in Ep values), E°(H+/H•) ) 2.29 V,33 and BDE(PhSCH(-H)-
SPh) ) 84 kcal mol-1 34
similar.
.
The pKa values of 3•+ and 4•+ should be very
(36) McClelland, R. A.; Kanagasabapathy, V. M.; Banait, N. S.;
Steenken, S. J. Am. Chem. Soc. 1991, 113, 1009-1014.
(37) Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 3, 1-19.
(38) Debye, P. Trans. Electrochem. Soc. 1942, 82, 265.
(39) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637.
(40) Shine, H. J.; Silber, J. J.; Bussey, R. J.; Okuyama, T. J. Org. Chem.
1972, 37, 2691. Bosch, E.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 1
1995, 1057.