Oxidation of phenyl vinyl sulphide and phenyl vinyl sulphoxide in aqueous solution: A pulse radiolysis and theoretical study

Article abstract of DOI:10.1016/j.cplett.2009.07.057

Pulse radiolysis and quantum chemical methods were employed to study the oxidation of phenyl vinyl sulphide (PVS) and phenyl vinyl sulphoxide (PVSO) by ^{{radical dot}}OH and SO₄^{{radical dot}-} radicals. The ^{{radical dot}}

Full text of DOI:10.1016/j.cplett.2009.07.057

Chemical Physics Letters 478 (2009) 155–160
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Oxidation of phenyl vinyl sulphide and phenyl vinyl sulphoxide in aqueous
solution: A pulse radiolysis and theoretical study
M. Shirdhonkar a, H. Mohan b, D.K. Maity c, B.S.M. Rao a,
*
a National Centre for Free Radical Research, Department of Chemistry, University of Pune, Pune 411007, India
b Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
c Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pulse radiolysis and quantum chemical methods were employed to study the oxidation of phenyl vinyl
ꢀ
ꢀꢀ
ꢀ
Received 13 May 2009
In final form 17 July 2009
Available online 23 July 2009
sulphide (PVS) and phenyl vinyl sulphoxide (PVSO) by OH and SO4 radicals. The OH radical reacts
by addition (k ꢁ (1–2) ꢂ 109 dm3 molꢀ1 sꢀ1) and the PVS–OH (kmax = 310 nm) adduct was entirely con-
verted into PVSꢀ+ (kmax = 310 and 550 nm) at pH 1. DFT calculations revealed that the vinylic OH adducts
are in general more stable than the ring addition products and the odd electron spin in both radical cat-
ions is located over S and C atoms.
Dedicated to the memory of Dr. Hari Mohan,
a co-author of this Letter and a source of
inspiration to all of us, who suddenly died
during the course of this work.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
therefore, undertaken to examine the possible reaction pathways
of the OH radical induced oxidation of phenyl vinyl sulphide and
ꢀ
Radiation chemical oxidation studies on aliphatic sulphides and
sulphoxides have been well studied [1–4] but the corresponding
work on aromatic compounds is limited. Only a few reports exist
in the literature on the generation of radical cations of aromatic
thioethers by photochemical [5,6] or radiolytical [7–11] methods.
Highly acidic aqueous medium has been shown to be effective in
generating and stabilising radical cations of aromatic systems
its sulphoxide in aqueous solution using pulse radiolysis and quan-
tum chemical methods. The structures of the compounds used in
this study are shown in Schemes 1 and 2.
2. Experimental
ꢀ
PVS and PVSO obtained from Aldrich Chemicals were of high
purity and used without any further purification. Solutions were
[12–17]. The OH radicals react with aromatic sulphides by the
adduct formation and the subsequent water/OHꢀ elimination pro-
cess leads to the formation of the radical cation. One of the widely
studied systems is phenyl methyl sulphide (thioanisole) where it
was shown that the radiation induced oxidation finally leads to
the formation of the sulphur centred radical cation. Our recent
experimental results and theoretical calculations on its derivative
– phenyl trifluromethyl sulphide revealed very high acid concen-
tration is required to stabilise the radical cation and the charge is
distributed over sulphur and the aromatic ring [18].
prepared in water (conductivity = 18.2 MX cm), obtained from a
Millipore water purification system and freshly prepared solutions
were used for each experiment. The pH of the solution was ad-
justed using HClO4, phosphate buffer or NaOH. High purity N2O
or N2 gas was used for purging the solutions. All other chemicals
used were also of high purity. The absorption spectra of PVS and
PVSO were recorded on Shimadzu spectrophotometer model 1601.
2.1. Pulse radiolysis experiments
Other interesting models in this class of systems are phenyl vi-
nyl sulphide and the corresponding sulphoxide. The OH radical
ꢀ
Pulse radiolysis experiments were carried out with high energy
electron pulses (7 MeV, 50 ns) obtained from a linear electron
accelerator at National Centre for Free Radical Research of Univer-
sity of Pune which was supplied by AS&E USA. All other details of
the machine are described elsewhere [28]. The pulse radiolysis
experiments were carried out in suprasil cuvettes with a cross-sec-
addition to the vinyl carbon is an important reaction channel in
the former as such an attack in the case of cinnamate derivatives
was shown [19,20] by us to be significant. Sulphoxide functional
group is important in organic synthesis [21,22] as well as due to
its involvement in the biological activity of sulphides [23–25]. Be-
cause of its high oxidation potential, its electron transfer reactions
have not been studied extensively [26,27]. The present work is,
ꢀ
tional area of 1 cm2 at 25 °C. The OH reaction was studied in N2O
saturated solutions of PVS and PVSO and due to their limited solu-
bility in water (<10ꢀ3 mol dmꢀ3), solutions were stirred for about
30 min at 35 °C.
* Corresponding author. Fax: +91 20 25691728.
E-mail address: bsmr@chem.unipune.ernet.in (B.S.M. Rao).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.07.057

156
M. Shirdhonkar et al. / Chemical Physics Letters 478 (2009) 155–160
.
.
+
.
CH
S
CH CH
S
CH
OH
+
2
+
1
_
pH 7
OH
/
2
_
OH ,
1
9
=2 x 10
s 1
3
k
dm mol
3
(20%)
2
(80%)
.
_
+
.
_
S
CH CH
2
pH 1
+ OH
k
OH
/
/
2
S
CH CH
1
2
9
-
-1
3mol s
= 1.9 x 10 dm
3
1
_
_
.
SO42
3
1
-
_
+ SO4
/
9dm3 mol
1
-
k
=1.1 x 10
s
Scheme 1. Oxidation reaction mechanism of phenyl vinyl sulphide.
Scheme 2. Oxidation of reaction pathways of PVSO.
The one-electron oxidants, Clꢀ2ꢀ Br2ꢀꢀ and SO4ꢀꢀ were generated
3. Results and discussion
using appropriate solution conditions. Their reactions were carried
out under conditions such that the one-electron oxidant and not
ꢀOH radicals react with the solute. The dose delivered per pulse
was determined using N2O saturated aqueous solution of KSCN
(1 ꢂ 10ꢀ2 mol dmꢀ3) by monitoring the transient ðSCNÞꢀ2ꢀ species
[29] at 480 nm.
The UV–VIS absorption spectra of PVS and PVSO were recorded
in the region 200–800 nm at pH 7 in the aqueous medium and
their spectra exhibited absorption maxima at 245 and 235 nm,
respectively. The wavelength region 250–750 nm was monitored
in pulse radiolysis studies to avoid any interference of the parent
absorption <250 nm.
2.2. Computational methods
ꢀ
3.1. Reactions of OH radical with PVS
Full geometry optimisations on the ground state of PVS and
PVSO, respective OH adducts, radical cations have been carried
out following density functional theory (DFT) adopting a 6-
31 + G(d,p) set of basis functions. Becke’s half-and-half (BHH) non-
local exchange and Lee–Yang–Parr (LYP) nonlocal correlation func-
tionals (BHHLYP) have been reported [30,31] to perform well to
describe a three electron bonded open shell doublet system. The
gas phase optimised molecular geometries are reoptimised fully
including solvent effect following the Onsager reaction field model
with the cavity radius based on gas phase optimised geometry. Cal-
culations have been performed to find out a few low lying elec-
tronic transitions at TDDFT (BHHLYP) level of theory, choosing
the same set of basis functions both in the gas and solution phases,
to determine kmax values of the transient species. Polarizable Con-
tinuum Model (PCM) is also applied to include the solvent effect in
the calculation of ground state geometry as well as excited state
electronic transitions. All the geometry optimisations and TDDFT
calculations were performed applying the GAUSSIAN 03 program
[32]. Visualisations of the geometries were carried out adopting
the MOLDEN [33] program system.
3.1.1. Acidic solutions
The reaction of the hydroxyl radical was studied out in N2O sat-
urated
aqueous
solutions
of
phenyl
vinyl
sulphide
(1 ꢂ 10ꢀ3 mol dmꢀ3) at pH 1. The transient absorption spectrum
of the species produced in this reaction exhibited two peaks at
310 and 550 nm (Fig. 1A) with a shoulder at 380 nm. The rate of
the reaction of the ꢀOH radical with PVS at pH 1 was measured from
the build up of the transient species at the absorption maximum in
the concentration range (0.2–1) ꢂ 10ꢀ3 mol dmꢀ3 and the value of
the rate constant for reaction 2, Scheme 1 was found to be
1.9 0.1 ꢂ 109 dm3 molꢀ1 sꢀ1 from the linear fit (inset Fig. 1A).
The transient bands at 310 and 550 nm decayed with nearly the
same rate as can be seen from the time resolved spectra recorded
at 5 and 40 ls (Fig. 1A). This suggests that only a single transient
species was formed and in order to identify its nature, measure-
ments were carried out with SOꢀ4ꢀ (E0 = 2.45 V), a specific one-elec-
tron oxidant. Fig. 2 depicts the spectrum obtained in this reaction,
which has peaks at 310 and 550 nm. The value of the second order
rate constant obtained from the plot of kobs versus [PVS] is

M. Shirdhonkar et al. / Chemical Physics Letters 478 (2009) 155–160
157
ꢀ
in agreement with that obtained in the reaction of OH (Fig. 1A).
Thus, these two bands are attributed to the solute radical cation.
The shoulder at 380 nm (Fig. 1A) may arise from the reaction of
0.020
0.016
0.012
0.008
0.004
0.000
10
8
ꢀH with PVS as an independent measurement of H atom reaction
with PVS exhibited weak absorption in this wavelength region.
In PVS, the electron density is located on the benzene ring, –
ꢀ
6
4
C@C– due to
p electrons and on sulphur due to the lone pairs.
2
Being an electrophile, the ꢀOH radical can add to all the above pos-
sible sites and the subsequent hydroxide ion elimination leads to
the formation of the corresponding radical cation. In the earlier
work [34] on phenyl methyl sulphide, it has been shown that
0.4 0.5 0.6 0.7 0.8
[PVS] / 10-3 mol dm-3
ꢀ
though the addition of OH can occur initially at both the ring
and sulphur; eventually it leads to the formation of the stable sul-
phur centred radical cation. Assuming complete formation of the
sulphur radical cation in the ꢀOH radical reaction at pH 1 in confor-
mity with the finding [34] in phenyl methyl sulphide and
G(ꢀOH) = 2.8 in acidic solutions, the molar absorptivities for
310 = 4900 and
550 = 1900 dm3 molꢀ1 cmꢀ1. These values are lower than those
found for phenyl methyl sulphide 310 = 9200 and
550 = 5400 dm3 molꢀ1 cmꢀ1). Such lowering in
was reported [7]
for the radical cation having an unsaturated substituent e.g.
310 = 6300 and
(PhSCH2Ph)ꢀ+ where 550 = 2100 dm3 molꢀ1 cmꢀ1
A comparison of the intensities observed in the spectra recorded
300
400
500
λ/nm
600
700
(PhSCH@CH2)ꢀ+ were calculated to be
e
e
0.016
0.012
0.008
0.004
0.000
b
a
0.003
310 nm
(e
550 nm
0.012
e
e
e
e
.
0
0
ꢀꢀ
ꢀ
in the SO4 and the OH radical reaction at pH 1 revealed that
75% of the latter reacted to produce the radical cation (reaction
3, Scheme 1) and the rest possibly reacted by abstraction/addition
to the vinylic group.
0
200
400
0
1000
Time/μs
3.1.2. Neutral solutions
At pH 7, the transient spectrum obtained in the reaction of the
ꢀOH radical with phenyl vinyl sulphide has an absorption maxi-
mum at 310 and a weak peak around 550 nm (Fig. 1B), the ratio
of their intensities being 4.8. Thus, the peak at 310 nm has the con-
tribution from two absorbing transient species namely the OH ad-
duct and the solute radical cation. From molar absorptivities of the
radical cation determined in this work, its contribution was esti-
mated to be only about 20% in neutral solutions. The value of the
200
300
400
500
λ/nm
600
700
800
Fig. 1. The transient absorption spectra obtained in the reaction of ꢀOH radical with
PVS (1 ꢂ 10ꢀ3 mol dmꢀ3) at (A) pH 1 and (B) pH 7 at (j) 5 and (s) 40
ls after the
ꢀ
second order rate constant obtained for the reaction OH radical
pulse. Insets: 1A: plot of kobs versus [PVS] and 1B: decay traces at (a) 310 and (b)
is 2.0 0.1 ꢂ 109 dm3 molꢀ1 sꢀ1. The absorption traces depicting
the decay at 310 (trace a) and 550 nm (trace b) are shown in
Fig. 1B. The relevant reaction sequence for the reactions of ꢀOH rad-
550 nm. Dose/pulse: 12–14 Gy.
ꢀꢀ
ical with PVS at pH 7 (reaction 1) and 1 (reaction 2) and SO4
0.012
(reaction 3) are shown in Scheme 1.
10
ꢀ
8
3.2. Reactions of OH radical with PVSO
6
ꢀ
0.008
The reaction of the OH radical with phenyl vinyl sulphoxide
(1 ꢂ 10ꢀ3 mol dmꢀ3) was carried out in N2O saturated aqueous
4
solutions at pH 1, 7 and 9. The second order rate constants mea-
0.2 0.4 0.6 0.8
are 7.8 0.5 ꢂ 109 and 7.9 0.3 ꢂ
[PVS] / 10-3 mol dm-3
sured at pH
1
and
7
109 dm3 molꢀ1 sꢀ1, respectively (reaction 1, Scheme 2). The time
resolved transient absorption spectra recorded in this reaction at
pH 7 exhibited a prominent peak at 330 nm (Fig. 3). However,
the spectrum measured at pH 1 has low intensities due to poor sig-
nal to noise ratios and hence was not considered further.
In contrast, the transient absorption spectrum obtained in the
reaction of SO4 with PVSO exhibited two bands at 330 and
650 nm (Fig. 4), which is attributed to the monomer radical cation
(structure 4, Scheme 2). This assignment is in accord with the ear-
lier work of Baciocchi et al. [27] on aryl sulphoxide radical cations.
0.004
0.000
ꢀꢀ
300
400
500
600
700
λ/nm
Fig. 2. The transient absorption spectra obtained in the reaction of SOꢀ4ꢀ with phenyl
vinyl sulphide (1 ꢂ 10ꢀ3 mol dmꢀ3) at 10
l
s after the pulse at pH 7. Inset: plot of
The transient absorption was fully developed within 1 ls (inset
Fig. 4) and the rate constant value calculated from the growth of
kobs as a function of [PVS]. Dose/pulse = 13.5 Gy.
the transient band at 330 nm (reaction 2, Scheme 2) is about
ꢀ
k = 1.1 0.1 ꢂ 109 dm3 molꢀ1 sꢀ1 (inset, Fig. 2). The ratio of the
2.7 ꢂ 109 dm3 molꢀ1 sꢀ1. The OH radical reacts by addition and
intensities of absorption at 310 and 550 nm is about 2.7, which is
the molar absorptivity of the OH adduct taking GOH = 5.6, was esti-

158
M. Shirdhonkar et al. / Chemical Physics Letters 478 (2009) 155–160
were observed in PVS and PVSO geometries when the gas phase
0.012
0.008
0.004
0.000
0.012
0.008
0.004
0
geometry was reoptimised by including the solvent effect applying
both the Onsager’s reaction field and PCM models. The atomic
charge (Mulliken) distributions have shown a significant difference
at the two different levels of theory. The fully optimised structures
of PVS and PVSO calculated using the dipole and PCM models are
shown in Fig. 5A and B along with Mulliken charges. The analysis
shows that the hydroxyl radical may add to both ring and vinyl car-
bons as well as sulphur atoms, though with different probabilities.
Both the gas and solution phase calculations were also carried
out on the radical cations of PVS and PVSO. The gas phase opti-
mised geometry of PVS cation has shown that the molecule loses
its symmetry when one electron is removed from the system.
The angle between ipso carbon–sulphur–vinyl carbons was in-
creased by 6.1°. In the PVSO radical cation, the \CSC increase is
13°. The fully optimised geometries of PVS and PVSO radical cat-
ions in the solvent phase are shown in Fig. 5C and D along with
the Mulliken spin densities applying dipole and polarizable contin-
uum models for the solvent effect.
Our calculations revealed that in both PVS (Fig. 5C) and PVSO
(Fig. 5D) radical cations the odd electron spin is over S and C atoms.
This is in contrast to the earlier finding on the phenyl methyl sul-
phide radical cation [34] where the entire charge has been shown
to be located on S atom, but is in good agreement with the work of
Baciocchi et al. [27] on the corresponding phenyl methyl sulphox-
ide radical cation which was attributed to the conjugative interac-
tion between MeSO and the ring. Such delocalisation of charge/
spin is also possible in the PVS system due to the vinylic group.
A comparison of spin densities in PVS system shows that it is
A
330 nm
0
800
Time/μs
300
400
500
600
λ/nm
Fig. 3. The transient absorption spectra obtained in the reaction of ꢀOH with phenyl
vinyl sulphoxide (1 ꢂ 10ꢀ3 mol dmꢀ3) at pH 7: (j) 0.5 and (d) 40
ls after the pulse.
Inset: Decay at 330 nm. Dose/ pulse = 12.8 Gy.
330 nm
0.02
0.00
0.016
0.012
0.008
0.004
0.000
0
2
4
Time/μs
-0.138
-0.104
+0.177
+0.201
+0.039
+0.012
+0.058
+0.048
-0.224
-0.230
-0.773
-0.812
+0.805
+0.829
+0.426
+0.407
-0.242
-0.388
-0.239
-0.221
-0.373
200
300
400
500
λ/nm
600
700
800
-0.219
+0.043
+0.033
+0.694
+0.683
-0.368
-0.352
+0.045
+0.038
-0.002
-0.021
Fig. 4. Transient absorption spectra obtained in the reaction of SOꢀ4ꢀ with phenyl
+0.007
+0.006
vinyl sulphoxide (1 ꢂ 10ꢀ3 mol dmꢀ3, pH 4) at 1
l
s after the pulse. Inset: (a)
+0.032
+0.037
+0.047
+0.039
Absorption build up at 330 nm. Dose/pulse = 12.2 Gy.
mated to be 1940 dm3 molꢀ1 cmꢀ1. The OH adduct decays bimo-
lecularly as can be seen from the trace shown in inset Fig. 1B.
The SOꢀ4ꢀ radical reacts by electron transfer, but no reactivity with
other one-electron oxidants (Clꢀ2ꢀ or Br2ꢀꢀ) with both PVS and PVSO
was noticed.
+0.224
+0.208
-0.214
-0.221
-0.101
-0.098
+0.441
+0.422
+0.590
+0.599
+0.375
+0.365
+0.420
+0.418
-0.073
-0.069
3.3. DFT calculations
-0.044
-0.031
+0.201
+0.123
+0.127
3.3.1. Radical cations of PVS and PVSO
+0.210
+0.134
+0.147
+0.037
+0.034
DFT calculations were performed with GAUSSIAN 03 program to
optimise the geometry of phenyl vinyl sulphide, phenyl vinyl
sulphoxide, their radical cations and different OH addition prod-
ucts using BHHLYP/6-31 + G(d,p) level of theory. The gas phase
optimised geometry of PVS has shown that the benzene ring and
the vinyl group are in two different planes. The angle between
the ipso carbon–sulphur–vinyl carbon is (\CSC = 103.0°). The opti-
mised geometry revealed that the molecule is symmetric about the
plane passing through S atom-ipso and para carbons. On the other
hand, the gas phase optimised geometry of PVSO indicated that the
molecule is not symmetric. The calculated angle between ipso car-
bon–sulphur–vinyl carbon is \CSC = 97°. No significant changes
-0.148
-0.154
-0.068
-0.074
-0.140
-0.088
-0.145
-0.095
+0.270
+0.282
+0.125
+0.139
Fig. 5. Optimised structures of (A) PVS and (B) PVSO showing Mulliken charges on
C, S and O atoms, (C) PVS and (D) PVSO radical cations showing Mulliken spin
population on non-hydrogen atoms. Values were obtained applying dipole (upper
lines) and polarizable continuum (lower lines) models for the solvent effect. Yellow
ball refers S, red ball to O atom and dark and light grey balls to C and H atoms,
respectively. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)

M. Shirdhonkar et al. / Chemical Physics Letters 478 (2009) 155–160
159
located not only on S and ring carbons (ꢁ35% each) but also on one
of the vinylic carbon (25%). On going to PVSO, the distribution is
changed with S and O atoms carrying about 35% each and the rest
equally located on ring and vinylic carbons.
The different (PVS–OH) adducts numbered C1 to C8 and S1 are de-
picted in Fig. 6. In the case of PVSO, no stable structure was ob-
tained by addition of OH to S and the optimised geometries of
(PVSO–OH) adducts at C1, C7 and C8 are shown in Fig. 7 while
the spin densities and binding energies for the rest are tabulated
in Table 1.
3.3.2. OH adducts of PVS and PVSO
The hydroxyl radical can add to eight carbon atoms (six of the
ring i.e. two each of ortho and meta, one each of para and ipso
(C1 to C6) and two vinyl carbon atoms (C7 and C8) and also to sul-
phur atom (S1) to form the corresponding OH adducts of PVS and
PVSO. We have calculated the spin densities and binding energies
of the products obtained by adding the OH to all the above posi-
tions and the optimised geometries in both gas and solution
phases, but the values in the two cases remained nearly the same.
The addition product contains an unpaired electron and its
localisation features can be understood in terms of the spin density
(Sd) in the concerned system. Whenever the addition occurred at a
ring carbon atom, high Sd values are located at the carbon atoms
ortho and para with respect to the addition site. For example, in
(PVS–C1OH) adduct, high Sd values are located on ortho: C2
(0.60) and C6 (0.60) and para: C4 (0.78) positions (Fig. 6). Similarly,
in the (PVSO–C5)OH adduct, high Sd values are seen on ortho C6
ꢀ
Fig. 6. Optimised geometries of (C1–C8)–OH) and (S1–OH) adducts of phenyl vinyl sulphide with binding energies and spin densities above 0.2.
Fig. 7. Optimised geometries of (PVSO–OH) adducts at C1, C7 and C8 with binding energies and spin densities above 0.2.

160
M. Shirdhonkar et al. / Chemical Physics Letters 478 (2009) 155–160
Table 1
singly occupied molecular orbital) transition. It should be noted
that in both systems several electronic transitions contribute to
the predicted absorption band in the UV region (300 nm).
Binding energies Eb (kcal/mol) in solution phase and atoms with spin density (Sd)
values (above 0.2) of various OH addition products of PVSO.
ꢀ
System
Eb (kcal/mol)
Sd
C2
C3
C4
C5
C6
C7
C8
18.5
11.4
27.2
12.6
15.6
29.2
27.2
C3(0.779), C5(0.651), C1(0.573)
C6(0.807), C4(0.684), C2(0.600)
C1(0.725), C5(0.649), C3(0.647)
C2(0.800), C4(0.681), C6(0.559)
C3(0.791), C5(0.628), C1(0.514)
C8(1.154)
4. Conclusions
The ꢀOH radical reacts by addition to phenyl vinyl sulphide
forming PVS–OH adduct and its dehydration was found to be pH
dependent with complete conversion to the (PVS)ꢀ+ at pH 1
C7(1.1)
(kmax = 310, 550 nm, and e310 = 4900, e
550 = 1900 dm3 molꢀ1 cmꢀ1).
In contrast, no formation of the radical cation was seen in PVSO.
Pulse radiolysis data complemented by theoretical calculations
have proven useful in understanding the radiation induced oxida-
tion mechanism of PVS and PVSO.
(0.60) and para C2 (0.80) and C4 (O.68) sites (Table 1). On the other
hand, when the addition is to the vinyl carbon atoms (C7 or C8),
the localisation of the unpaired electron is clearly seen on the other
vinyl carbon atom. For instance, in the PVS system, when it adds to
C7, Sd = 1.15 is observed on C8 and vice versa on addition with Sd
(C7) = 0.97. Very similar features were also observed in the
PVSO–OH addition products.
Our calculations in the solution phase on binding energies pre-
dict that the (C8–OH) and (C7–OH) adducts are relatively more sta-
ble than the ring carbon addition products. For example, the most
stable product is the (C8–OH) adduct followed by the (C7–OH) ad-
duct with binding energies of 29.7 and 26.6 kcal/mol whereas the
(S1–OH) adduct is the least stable (2.1 kcal/mol) with the stabili-
ties of the ring carbon addition products lying in between. This
suggests that the OH radical preferentially adds to the vinylic car-
bons and its addition products to the ring carbons is much lower
and is not selective with nearly equal binding energies of the
(C1–C6)–OH adducts in the range 11.1–13.7 kcal/mol. Such prefer-
ential addition of hydroxyl radicals to the side chain carbon atoms
over the ring carbon atoms was also reported [35] by us in our
work on cinnamic acid derivatives.
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