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Radiation Physics and Chemistry 74 (2005) 220–226

www.elsevier.com/locate/radphyschem

Effect of substituents on different channels of dOH radicalreaction with substituted organic sulfides$

Hari Mohan�, Jai P. Mittal

Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

Abstract

Pulse radiolysis technique has been employed to study the nature ofdOH radical reaction in aqueous solutions of

substituted organic sulfides. The transient absorption band at 345 nm observed on reaction ofdOH radicals in neutral

aqueous solution of 3,30-thiodipropionitrile is assigned to OH-adduct at sulfur. OH-adduct is observed to have high

reactivity with oxygen (k ¼ 8.8� 108 dm3mol�1 s�1). The reaction ofdOH radicals in neutral aqueous solution of

methyl propyl sulfide has shown the formation of sulfur-centered dimer radical cation with a small fraction (�10%)

ofa-(alkylthio)alkyl radicals. The reaction of dOH radicals with thiodiglycolic acid showed an absorption band at

285 nm, which is assigned to a -(alkylthio)alkyl radicals. The reaction of dOH radicals with dimethyl 2,20-thiodiethanoicacid has been assigned to OH-adduct at sulfur, whereas the transient absorption band at 390 observed with 3,30-

thiodipropionic acid is assigned to intra-molecular radical cation formed on p-orbital overlap of oxidized sulfur with

oxygen. In acidic solutions, sulfur-centered dimer radical cation is the only transient species observed with substituted

alkyl sulfides. The concentration of acid required to observe the formation of dimer radical cation is found to depend on

the electron-withdrawing power of the substituted group. The reaction ofdOH radicals in neutral aqueous solution of

substituted aryl sulfides has shown the formation of monomer radical cation and OH-adduct at benzene ring. Sulfur-

centered dimer radical cations are not observed even in acidic conditions.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Pulse radiolysis; Aqueous solution; Substituted organic sulfides

1. Introduction

The oxidation mechanism of sulfur-containing or-

ganic compounds has been the subject of recent

radiation and photochemical investigations (Bobrowski

et al., 1992; Sonntag, 1987; Marciniak et al., 1995;

Mizuno and Otsuji, 1994; Lewis, 1988). Studies on the

ee front matter r 2005 Elsevier Ltd. All rights reserv

dphyschem.2005.04.016

is dedicated in honor of Prof. Robert Schiller,

rch Institute for Physics, Atomic Energy

ute, Budapest, Hungary.

ing author.

ess: harim@apsara.barc.ernet.in (H. Mohan).

radical and radical ions of organic sulfides have gained

importance as sulfur-centered radical species are

the possible key intermediates in biological systems

with sulfur-containing compounds (Chatgilialoglu and

Asmus, 1990; Gobl et al., 1984; Asmus, 1979; Bobrowski

and Schoneich, 1993; Schoneich and Bobrowski, 1993;

Chatgilialoglu et al., 1999; Urbanski and Wine, 1999).

The knowledge of chemistry involved in the reactions of

radicals and radical ions of organic sulfur compounds is

important in understanding electron transfer reactions

and redox properties. Hydroxyl radicals are known to

bring about one-electron oxidation of dialkyl sulfides

(R2S) leading to the formation of sulfur-centered radical

ed.

ARTICLE IN PRESSH. Mohan, J.P. Mittal / Radiation Physics and Chemistry 74 (2005) 220–226 221

cations R2Sd+ (Gilbert et al., 1973; Bonifacic and

Asmus, 1980). Sulfur-centered monomer radical cations

have a high tendency to stabilize by co-ordination with a

free p-electron pair of another sulfur or a heteroatom

(O, N, P, halogen) both by inter- and intra- molecular

association (Bobrowski and Holcman, 1989; Anklam et

al., 1988; Hungerbuhler et al., 1991; Asmus et al., 1985).

These interactions are represented by a two-center three-

electron (2c–3e) bond containing two bonding selectrons and one antibonding s� electron. Two-centerthree-electron bonding (2s21s�) has been a subject ofboth experimental (by ESR and time-resolved techni-

ques such as pulse radiolysis and flash photolysis )and

theoretical investigations (Gilbert et al., 1973; Marciniak

et al., 1998; Bonifacic et al., 1975; Monig et al., 1985;

Gobl and Asmus, 1984; Clark, 1988; McKee, 2003;

Maity, 2002; Varmenot et al., 2004). The formation of

sulfur-centered three-electron bonded species have been

inferred to take place via a complex sequence of

reactions involving a-(alkylthio)alkyl radicals, OH-adduct and monomer radical cations (Bonifacic et al.,

1975; Monig et al., 1985; Monig and Asmus, 1984).

Experimental evidence for the formation of these

intermediates could be obtained only in functionalized

dialkyl sulfides (Bobrowski et al., 1993, 1997; Maity

et al., 1994; Gawandi et al., 1999). The kinetic, spectro-

scopic and redox properties of the transient species

formed on reaction ofdOH radical with dialkyl sulfides

have been reasonably well understood and mainly one

channel (sulfur site) is observed in most of the cases. On

the other hand, in aryl-substituted sulfur compounds,

the benzene ring would act as an additional site for

reaction withdOH radical and spin delocalization would

reduce the tendency of the solute radical cation to form

dimer radical cation (Ioele et al., 1997; Engman et al.,

1994; Korzeniowska et al., 2002; Mohan and Mittal,

2002). It has been shown in this manuscript that the

nature of the functional group, chain length from sulfur,

pH and presence of aryl group play an important role in

the final stabilization of the oxidized sulfur.

2. Experimental

The organic sulfur compounds obtained from Aldrich

Chemicals were used without further purification. All

other chemicals used were also of high purity. The

solutions were prepared in de-ionized ‘‘nanopure’’ water,

and freshly prepared solutions were used for each

experiment. The pH of the solution was adjusted with

NaOH/HClO4 in Na2HPO4/KH2PO4 phosphate buffers.

All other experimental details are described elsewhwere

(Mohan and Mittal, 2002). The pulse radiolysis experi-

ments were carried out with high-energy electron pulses

(7MeV, 50ns) obtained from a linear electron accelerator,

whose details are given elsewhere (Guha et al. 1987).

Aerated aqueous solution of KSCN (1� 10�2mol dm�3)

was used for determining the dose delivered per pulse using

G�500 ¼ 21 520dm3 mol�1 cm�1per 100 eV for the transi-

ent (SCN)2d� species. G denotes the number of species per

100 eV of absorbed energy (G ¼ 1 corresponds to

0.1036mmol J�1) and e is the molar absorptivity of the(SCN)2

d� species at 500nm. The dose per pulse was close

to 15Gy (1Gy ¼ 1 Jkg�1) except for kinetic experiments,

which were carried out at a lower dose of about 10Gy.

The transient species formed on pulse radiolysis were

detected by an optical absorption method using a 450W

pulsed xenon arc lamp and a Kratos (GM-252)

monocromator. The photomultiplier output was digi-

tized with a 100MHz storage oscilloscope interfaced to

a computer for kinetic analysis. The bimolecular rate

constants were determined from the linear regression

plots of kobs versus solute concentration for at least three

independent experiments and the variation was within

15%.

Radiolysis of N2-saturated neutral aqueous solution

leads to the formation of three highly reactive species

(dH,

dOH, eaq

� ) in addition to the less reactive or inert

molecular products (H2, H2O2, and H3O+) (reaction

(1)). The reaction withdOH radical in neutral aqueous

solutions was carried out in N2O-saturated solutions,

where eaq� is quantitatively converted to

dOH radicals

anddOH radical is the main species to react with the

solute (reaction (2)). In acidic solutions, the reaction

withdOH radical was carried out in aerated conditions

where eaq� and

dH are converted to HO2

dradicals

(reactions (3) and (4)).

H2O! dH; dOH; eaq�; H2; H2O2; and H3Oþ; (1)

N2Oþ eaq� ! dOHþOH� þN2; (2)

eaq� þHþ ! Hd þH2O; (3)

Hd þO2 ! HO2d: (4)

3. Results and discussion

3.1. Reaction ofd

OH radicals in neutral solutions

Fig. 1 shows the transient optical absorption obtained

on pulse radiolysis of N2O-saturated neutral aqueous

solution of 3,30-thiodipropionitrile (TDPN, 3.2� 10�3

mol dm�3), 2ms after the pulse. The transient absorption(Fig. 1) was not seen in the presence of t-butyl alcohol

(0.5mol dm�3), an efficientdOH radical scavenger,

showing that the spectrum is mainly due to the reaction

ofdOH radicals with TDPN and the contribution of H

d

atom reaction is negligible. The contribution of Hdatom

reaction with TDPN was independently determined at

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0.01

0.02

0.03

0.04

∆ O

.D.

∆ O

.D.

λ / nm

0.00

0.01

0.02

0.03

c

b

a

Time / µs0 10 20 30

Fig. 1. Transient absorption spectrum obtained on pulse

radiolysis of N2O-saturated aqueous solution of TDPN

(3.2� 10�3mol dm�3, pH ¼ 7) 2 ms after the pulse. Inset showsabsorption-time profiles at 350 nm in N2O (a), N2O/O2 with O2concentrations of 0.8� 10�4 (b) and 2.4� 10�4mol dm�3 (c).

SCH2CH2CN

CH2CH2CN

CH2CH2CN

CH2CH2CN+ OH

.SHO . .

.

(TDPN)

Scheme 1.

H. Mohan, J.P. Mittal / Radiation Physics and Chemistry 74 (2005) 220–226222

pH 1, where its yield is high. Time-resolved studies have

not shown the formation of any other transient

absorption band on decay of the 345 nm band. The

bimolecular rate constant for the reaction ofdOH

radicals with TDPN, determined by formation kinetic

studies at 345 nm, gave a value of 7.1� 109 dm3mol�1

s�1. The bimolecular rate constant was also determined

by competition kinetic studies using 2-propanol as the

standard solute. The value was 6.3� 109 dm3mol�1 s�1,

close to that determined by formation kinetics. The

decay and the absorbance of the transient band at

345 nm remained independent of solute concentra-

tion (0.5–5)� 10�3mol dm�3, suggesting the formation

of a monomeric species. Under these experimental

conditions, the molar absorption coefficient of the

transient band at 345 nm is determined to be 3.8� 103

dm3mol�1 cm�1.

The transient band at 345 nm was observed to decay

by first-order kinetics with k ¼ 9:5� 104 s�1 (inset a ofFig. 1). In the presence of oxygen, the transient

absorption band was observed to decay faster (inset b

and c of Fig. 1), suggesting high reactivity with oxygen.

The pseudo-first-order rate constant (kobs) increased

linearly with oxygen concentration. From the slope of

the linear plot of kobs vs. [O2], the bimolecular rate

constant was determined to be 8.8� 108 dm3mol�1 s�1.

High reactivity of the transient absorption band with

oxygen suggests that the band may not be due to solute

radical cation, which normally has no reactivity with

oxygen.

In dialkyl sulfides, the electron density at sulfur is very

high due to two lone pairs of electrons and the presence

of electron-releasing alkyl groups.dOH radicals, being

strongly electrophilic in nature, can easily take an

electron to form a solute radical cation and the

intermediate OH-adduct would be highly unstable. The

electron-withdrawing power of the CN group is very

high and the effective electron density at sulfur is

reasonably reduced to enabledOH radicals to stabilize

at sulfur to form an OH-adduct in neutral solutions

(Taft, 1957). Thus it is reasonable to assume that with

suitable electron density at sulfur, the stabilization of

OH-adduct is possible. Based on these studies and the

data available in the literature, the transient absorption

band at 345 nm is assigned to OH-adduct (Scheme 1).

It has been reported in the literature that the reaction

ofdOH radicals with dialkyl sulfides results in the

formation of dimer radical cation absorbing in the

region of 450–550 nm region with a small shoulder in the

280–300 nm region (Bonifacic et al., 1975; Gobl and

Asmus, 1984; Bobrowski and Holcman, 1989; Asmus et

al., 1979; Glass, 1999). Based on pulsed conductivity and

optical absorption studies, �90% ofdOH radicals are

observed to undergo one-electron oxidation to form a

dimer radical cation and �10% by H-abstraction to

form a-(alkylthio)alkyl radicals. A very short-lived

species (OH-adduct) is reported to absorb at 340 nm

(Bonifacic et al., 1975). The reaction ofdOH radicals in

neutral aqueous solution of methyl propyl sulfides

(CH3SC3H7) showed the formation of transient absorp-

tion band at 470 nm with a shoulder in the 270–310 nm

region (Fig. 2a). In analogy with the studies reported in

the literature, 470 nm band is assigned to a dimer radical

cation and the band in 270–310 nm region is due toa-(alkylthio)alkyl radicals (Table 1). The reaction of

dOH

radicals with thiodiglycolic acid (2,20-thiodiethanoic

acid, TDGA) showed the formation of a transient

absorption band at 285 nm (Fig. 2b). This should be due

to the presence of COOH group with high electron-

withdrawing power (+2.94) (Taft, 1957). The reaction

ofdOH radicals with 2,20-thiodiethanol has also shown

the formation of a transient absorption band at

285 nm.The OH group has slightly lower electron-

withdrawing power (+1.55) (Taft, 1957). The reaction

ofdOH radicals with 3,30-thiodipropanol showed an

absorption band at 420 nm with a small shoulder in the

280–300 nm region (Fig. 3a). The transient absorption

band at 420 nm is assigned to an intra-molecular radical

cation formed on p-orbital overlap of oxidized sulfur

with oxygen (Mohan and Mittal, 1992). On comparison

ARTICLE IN PRESSH. Mohan, J.P. Mittal / Radiation Physics and Chemistry 74 (2005) 220–226 223

of results between 2,20-thiodiethanol and 3,30-thiodipro-

panol, it is clear that the increase in the chain length

between S and OH by an additional CH2 group has

changed the nature of thedOH radical reaction.

Similarly, the reaction ofdOH radicals with 3,30-

thiodipropionic acid has shown the formation of a

transient absorption band at 390 nm with a shoulder in

the 280–300 nm region (Mohan, 1990). These bands are

assigned to intra-molecular radical cation formed on p-

orbital overlap of oxidized sulfur with oxygen anda-(alkylthio)alkyl radical. If the COOH group is replaced

with COOCH3, which contains an electron-releasing

group (CH3), the reaction ofdOH radicals with dimethyl

Table 1

Fraction (given in %) of transients formed in the respective organic s

Substituted alkyl sulfide a-(alkylthio) alkylradical (%)

OH-adduct

CH3SC3H7 �10 —

S(CH2COOH)2 100 —

S(CH2CH2COOH)2 �10 —

S(CH2COOCH3)2 — 100

S(CH2CH2OH)2 100 —

S(CH2CH2CH2OH)2 �10 —

S(CH2CH2COOCH3)2 — 100

CH3S(CH2)4OH — �30

CH3S(CH2)3OH — —

CH3S(CH2)2OH 100 —

S(CH2CH2CN)2 100

300 400 500 6000.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

b

a

λ / nm

∆ O

.D.

Fig. 2. Transient optical absorption spectra obtained on

pulse radiolysis of N2O-saturated neutral aqueous solution of

CH3SC3H7 (1� 10�3mol dm�3) (a) and TDGA (1� 10�3

mol dm�3) (b).

2,20-thiodiethanoic acid showed the formation of a

transient absorption band at 340 nm (Fig. 3b), which is

assigned to OH-adduct (Maity et al., 1994). The reaction

ofdOH radicals with dimethyl 3,30-thiodipropionate has

also shown the formation of OH-adduct (Table 1).

The initial step for the reaction ofdOH radicals with

organic sulfur compounds is expected to be the

formation of OH-adduct. Depending on the electron

density at sulfur, the lifetime of OH-adduct may be

reasonably high to be observed under pulse radiolysis

conditions (�1 ms). If the OH-adduct is not stable, itmay be converted to other species. Chain length is

observed to affect the nature ofdOH radical reaction

ulfide as a result of thedOH-induced oxidation

(%) Dimer radical cation

(%)

Intra-molecular

radical cation (%)

�90 —

— —

— �90

— —

— —

— �90

— —

�70 —

— 100

— —

300 350 400 450 500 550 600 6500.000

0.005

0.010

0.015

0.020

0.025

0.030

b

a

λ / nm

∆ O

.D.

Fig. 3. Transient optical absorption spectra obtained on pulse

radiolysis of N2O-saturated neutral aqueous solution of S(CH2CH2CH2OH)2 (1� 10

�4mol dm�3) (a) and S(CH2COOCH3)2(1� 10�3mol dm�3) (b).

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0.01

0.02

0.03

0.04

d

c

b

a

λ / nm

∆ O

.D.

Fig. 4. Transient optical absorption spectra obtained on pulse

radiolysis of N2O-saturated neutral aqueous solution

(1� 10�3mol dm�3) of CH3S(CH2)2OH (a) CH3S(CH2)3OH

(b) and CH3S(CH2)4OH 1ms (c) and 4ms (d).after the pulse.

300 400 500 600 7000.00

0.01

0.02

λ / nm

∆ O

.D.

Fig. 5. Transient absorption spectrum obtained on pulse

radiolysis of an aerated acidic (HClO4 ¼ 4.9mol dm�3) aqu-

eous solution of TDPN (4.8� 10�3mol dm�3) 1.5 ms after thepulse.

H. Mohan, J.P. Mittal / Radiation Physics and Chemistry 74 (2005) 220–226224

with (methylthio)alkanols. The reaction ofdOH radicals

with CH3S(CH2)2OH produced a transient absorption

band at 295 nm (Fig. 4a) and is assigned toa-(alkylthio)alkyl radicals. When the chain length is

increased with a CH2 group,dOH radicals showed the

formation of a transient absorption band at 410 nm

(Fig. 4b), which is assigned to a favorable five-

membered ring configuration formed between oxidized

sulfur and oxygen. With the presence of an additional

CH2 group,dOH radicals are observed to react with the

formation of OH-adduct and sulfur-centered dimer

radical cation (Fig. 4c). The transformation of OH-

adduct to dimer radical cation could also be observed

(Fig. 4d).

3.2. Reaction ofd

OH radicals in acidic solutions

Fig. 5 shows the transient absorption spectrum

obtained on pulse radiolysis of an aerated acidic

(HClO4 ¼ 4.9mol dm�3) aqueous solution of TDPN

(4.8� 10�3mol dm�3). The transient band at 520 nm

was observed to decay by first-order kinetics with

k ¼ 2:7� 105 s�1. The absorbance at 520 nm was

observed to increase with [HClO4] and saturation value

could not be observed even at 10mol dm�3 [HClO4]. The

absorbance at 520 nm was also observed to increase with

[TDPN]. Based on these studies, the transient absorp-

tion band at 520 nm is assigned to sulfur-centered dimer

radical cation. The increase in the absorbance and

lifetime of the transient absorption band with solute

concentration suggests the existence of an equilibrium

between monomer and dimer radical cation. In acidic

solutions, OH-adduct undergoes acid-catalyzed dehy-

dration to form sulfur-centered dimer radical cation.

Monomer radical cations could not be observed as they

are known to have a very short lifetime and have a high

tendency to stabilize on co-ordination with another

sulfur atom. In neutral aqueous solutions of substituted

alkyl sulfides, the reaction ofdOH radicals is observed

to depend on the nature of the functional group and the

chain length between sulfur and the functional group.

However in acidic solutions, dimer radical cation is the

main transient species (Gawandi et al., 2000; Maity

et al., 1994; Mohan and Mittal, 1992). The only

difference is in the concentration of acid required for

the formation of dimer radical cation, which is observed

to increase with the electron-withdrawing power of the

substituted group (Fig. 6). A very high concentration of

HClO4 (46mol dm�3) is required to observe the

formation of dimer radical cation of S(CH2COOH)2(Fig. 6a). The dimer radical cation of CH3SCH2COOH

could be seen when [HClO4] was more than 2mol dm�3

(Fig. 6b) whereas lower concentration of HClO4(0.5mol dm�3) is required for S(CH2COOCH3)2 (Fig.

6c). The dimer radical cation of S(CH2CH2OH)2 is

observed even at pH 3 (Fig. 6d).

The reaction ofdOH radicals with substituted alkyl

sulfides results in the formation of OH-adduct at sulfur,

sulfur-centered radical cation ora-(alkylthio)alkyl radi-cals. The presence of a C6H5 group can offer additional

channels due to the formation of OH-adduct at benzene

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0 4 6 8 100.00

0.02

0.04

0.06

b

c

a

[HClO4] / M

86420 100.00

0.01

0.02

d

pH

2

∆ O

.D.

Fig. 6. Variation of transient absorbance of the dimer radical

cation of substituted alkyl sulfides as a function of HClO4concentration. S(CH2COOH)2 (a), CH3SCH2COOH (b),

S(CH2COOCH3)2 (c) and S(CH2CH2OH)2 (d).

H. Mohan, J.P. Mittal / Radiation Physics and Chemistry 74 (2005) 220–226 225

ring and monomer radical cation due to spin delocaliza-

tion. In acidic solutions, the reaction ofdOH radicals

with substituted aryl sulfides results in the formation of

monomer radical cation. Sulfur-centered dimer radical

cations were not observed. The concentration of acid

required to observe the formation of solute radical

cation depends on the nature of the functional group.

4. Conclusions

The results presented here clearly demonstrate that

the functional group plays an important role in the

nature ofdOH radical reaction with substituted dialkyl

sulfides. The internal hydrogen bonding between hydro-

xyl hydrogen and oxygen located with the functional

group is not essential for the formation of OH-adduct

via internal hydrogen bonding in substituted alkyl

sulfides. Even if hydrogen bonding takes place between

hydroxyl hydrogen and nitrogen of the CN group, it

would be much less and a seven-membered ring

configuration is likely to be unstable. The Hammett

parameter (s�) for CH3 group is 0 anddOH radicals

react with dialkyl sulfides mainly to form dimer radical

cation (Taft, 1957). The nature ofdOH radical reaction

with dialkyl sulfides remains independent of pH.

However in the presence of electron-withdrawing

groups, the nature ofdOH radical reaction depends

strongly not only on the nature of the functional group

but also on the chain length between sulfur and the

functional group. The presence of aryl group has also

been observed to affect the nature ofdOH radical

reaction in neutral aqueous solutions. The concentration

of acid required was observed to increase with the

presence of functional groups having high electron-

withdrawing power. Only one channel is observed for

the reaction ofdOH radicals with substituted sulfides in

acidic solutions.

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