Spectroscopic investigation on kinetics, thermodynamics and mechanism for electron transfer reaction of iron(III) complex with sulphur centered radical in stimulated biological system

Article abstract of DOI:10.1016/j.saa.2014.01.011

Electron transfer reactions of biological organic sulphides with several metal ions to generate sulphide radical cations are a great concern in biochemical process. To understand the mechanism, a stimulated biological system having model compounds, iron(III)-bipyridyl complex with thio-diglycolic acid (TDGA) was investigated. Spectroscopic study reveals the kinetics and thermodynamics of the reaction in aqueous perchloric acid medium. The reaction follows first and fractional order of 0.412 with respect to [Fe(bpy) ₃]³⁺ and TDGA, respectively. The oxidation is insensitive to variation in [H⁺] but slightly decreases with increase in ionic strength ([I]). Addition of acrylamide, a radical scavenger has no effect on the rate of the reaction. The high negative value of ΔS^# (-74.3 ± 1.09 J K^-1 mol^-1) indicates the complex formed has a definite orientation higher than the reactants. Based on the above results, a suitable reaction mechanism for this reactionis proposed.

Full text of DOI:10.1016/j.saa.2014.01.011

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 315–321
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigation on kinetics, thermodynamics
and mechanism for electron transfer reaction of iron(III) complex with
sulphur centered radical in stimulated biological system
a
b
c
d
S. Deepalakshmi a, A. Sivalingam a, , T. Kannadasan , P. Subramaniam , P. Sivakumar , S.T. Brahadeesh
⇑
a Department of Chemical Engineering, Coimbatore Institute of Technology, Coimbatore 640 014, India
b Research Department of Chemistry, Aditanar College of Arts and Science, Tirucendur 628 216, India
c Department of Petroleum Engineering, JCT College of Engineering and Technology, Coimbatore 641 105, India
d Department of Petrochemical Engineering, JCT College of Engineering and Technology, Coimbatore 641 105, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Spectral investigation on electron
transfer reactions of organic
sulphides with metal ions were
studied.
Biological system was stimulated
with model compounds, iron(III)-
bipyridyl complex and thio-diglycolic
acid.
N
N
N
H
H
Fe3+
N
O
O
S
O
N
N
ꢀ
OH
OH
ꢀ
ꢀ
Parameters affecting the reaction,
kinetics and thermodynamics were
analyzed.
A model was proposed regarding the
electron transfer reaction.
N
O
N
O
N
Fe2+
N
S
2H+
N
N
OH
OH
O
a r t i c l e i n f o
a b s t r a c t
Article history:
Electron transfer reactions of biological organic sulphides with several metal ions to generate sulphide
radical cations are a great concern in biochemical process. To understand the mechanism, a stimulated
biological system having model compounds, iron(III)-bipyridyl complex with thio-diglycolic acid (TDGA)
was investigated. Spectroscopic study reveals the kinetics and thermodynamics of the reaction in aque-
ous perchloric acid medium. The reaction follows first and fractional order of 0.412 with respect to
[Fe(bpy)3]3+ and TDGA, respectively. The oxidation is insensitive to variation in [H+] but slightly decreases
with increase in ionic strength ([I]). Addition of acrylamide, a radical scavenger has no effect on the rate of
Received 12 November 2013
Received in revised form 19 December 2013
Accepted 5 January 2014
Available online 18 January 2014
Keywords:
Kinetics
Thermodynamics
Mechanism
Iron(III) complex
Electron transfer reactions
the reaction. The high negative value of
DS
# (ꢁ74.3 1.09 J Kꢁ1 molꢁ1) indicates the complex formed has
a definite orientation higher than the reactants. Based on the above results, a suitable reaction mecha-
nism for this reaction is proposed.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
attention has been paid to the chemistry of these species. The elec-
tron transfer reaction of biologically important organic sulphides
Sulphur is an essential component of all living cells. The impor-
tant role of sulphur compounds in sulphur centered radicals in
biological systems are now well established [1,2]. As a result much
with several metal ions to generate sulphide radical cations has
been extensively studied [3–11]. The sulphide radical cation in
the presence of water produces sulphoxide as one of the major
products. The formation of a sulphide radical cation as an interme-
diate during sulphide oxidation to sulphoxide has been recently
identified by flash photolysis [12–16].
⇑
Corresponding author. Tel.: +91 9865 020076.
E-mail address: sivalingam@cit.edu.in (A. Sivalingam).
1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2014.01.011

316
S. Deepalakshmi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 315–321
The electron transfer reaction is a phenomenon of great interest
Preparation of TDGA
in biochemical processes. Their energetics was connected with the
physical nature of the chemical bond and the chemical environ-
ment around the electron donor and acceptor. Among other metal
redox systems, the reduction of iron(III) and iron(II) is one of the
important processes in natural system [17]. Though many reduc-
tants may be responsible for reduction process, one of the impor-
tant classes of reducing agents responsible for the process is
organic sulphur compounds present in the biological system. The
important functional groups present in these compounds are thiols
and thioethers. The reactivity of these functional groups is greatly
influenced by amino and carboxyl groups present close to the thiol
and thioether part. So, kinetic studies on these sulphur compounds
with one-electron oxidants can provide mechanistic details on bio-
logical electron transfers.
In order to understand the mechanism of the redox reactions of
iron in biological systems many model compounds have been syn-
thesized and two important such model compounds often used are
iron(III)-polypyridyl and iron(III)-porphyrin complexes. The litera-
ture shows many iron(III)-polypyridyl complexes undergo efficient
electron transfer reaction with aryl methyl and di-alkyl sulphides,
aryl methyl sulphoxides, aryl thioacetic acid and sulphur contain-
ing amino acids [3,18–20]. They found that the substituents in the
aryl moiety is significantly affect the rate of electron transfer reac-
tions. Further the nature of the ligand in [Fe(NN)3]3+ also affects the
rate of the reaction substantially.
In TDGA, the thio-ether group is flanked between two carboxyl
groups. TDGA was found to possess many biological functions. It
is one of the physiological products of human metabolism. It is
found at low concentration in urine. The determination and
estimation of TDGA in urine help to characterize metabolic imbal-
ances [21]. Besides, TDGA was found to possess lot of industrial
applications like enhancing adhesion in coating processes, as a
raw material for the synthesis of anti inflammatory agents, used
in textile dyeing, etc., Because of the apparent biological
functions and industrial applications the study of TDGA becomes
more important. This has inspired to undertake the present study
of kinetics of electron transfer reaction to investigate the
mechanism of TDGA oxidation by a single electron oxidant,
tris(2,2-bipyridyl)iron(III)-perchlorate as a stimulated biological
system.
A solution of 0.2 M monochloroacetic acid in 30 mL of water
was prepared. To the above solution 0.2 M of sodium bicarbonate
was added in small portions with constant stirring until the solu-
tion become neutral. The mixture was allowed to cool in an ice
bath. To the above mixture 0.12 M of hydrated sodium sulphide
dissolved in 37 mL of water was added slowly with constant stir-
ring by maintaining 25–30 °C. The reaction mixture was cooled
in the ice bath for an hour. To this mixture 15 mL of concentrated
sulphuric acid was added with stirring at room temperature. TDGA
was obtained by extraction and evaporation of the ether layer in a
rotary vacuum evaporator.
TDGA was recrystallized using ethyl acetate and benzene mix-
ture. The purity was obtained from the melting point. The melting
point observed was 129 1 °C at 1 atm was reported by Barkenbus
and Landis as 129–130 °C [22].
Preparation of tris(2,20-bipyridine)iron(III)-perchlorate
2.5 mM ferrous ammonium sulphate was dissolved in 20 mL
of water and 1 mL of 0.01 N sulphuric acid and 7.5 mM of 2,20-
bipyridine was dissolved in 50 mL of hot water containing
2 mL of 1 M perchloric acid. To the hot 2,20-bipyridine solution
ferrous ammonium sulphate solution was added slowly with
stirring. The reaction mixture was allowed to remain in a water
bath for half an hour for digestion. The red colour solution of
tris(2,20-bipyridine)iron(II)-perchlorate was filtered in hot
condition. The absorption spectrum of the filtrate shows
maximum at 524 nm.
a
The prepared solution of tris(2,20-bipyridine)iron(II)-perchlo-
rate was cooled in ice. Then excess of lead dioxide was added.
Oxidation took place and a dark blue solution of tris(2,20-bipyri-
dine)iron(III) complex was formed [23]. The solution was kept for
30 min and then the precipitated lead sulphate and excess lead
dioxide were removed by filtration. The filtrate was added to 1:1
perchloric acid. A blue precipitate of tris(2,2-bipyridine)iron(III)-
perchlorate was formed. The precipitate was filtered, washed with
ether and dried in vacuum. The absorption spectrum of the
complex in 10 N perchloric acid shows a maximum at 618 nm.
The structure of the complex was given in Fig. 1.
The present study discusses kinetics of electron transfer reac-
tion of iron(III)-bipyridyl complex with TDGA and various factors
affecting the electron transfer reaction to understand the mecha-
nism. Finally, a most probable mechanism was proposed based
on the kinetic results of the reaction.
Materials and methods
Deionized water was distilled twice in an all glass-corning ves-
sel. Second distillation was carried out using alkaline potassium
permanganate. Reagents used in the study were prepared using
water. The stock solution of TDGA was prepared by weighing accu-
rate amount dissolved in double distilled water. Iron(III) complexe
standard solution was freshly prepared every time by direct weigh-
ing. Stock solutions of iron(III) complexe were made up in 10 N
HClO4 solutions and kept in refrigerator. Sodium perchlorate
(Himedia, AR) was employed to maintain the required ionic
strength of the medium. Acrylamide (S.D. Fine, Electrophoresis
Grade) was used as such without further purification. All the other
reagents used in the kinetic investigation viz., acetonitrile, per-
chloric acid, etc., were of AR grade and were used as such. ‘A’ Cer-
tified pipettes, burettes and standard measuring flasks were used
throughout the kinetic studies.
Fig. 1. Structure of tris(2,2-bipyridine)iron(III) perchlorate.

S. Deepalakshmi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 315–321
317
Kinetic measurements
D
S# ¼ 4:576 ꢂ ðintercept ꢁ 10:319Þ ꢂ 4:186 JK molꢁ1
The precision of
D D
H# and S# were calculated according to the
All kinetic measurements were performed under pseudo first
order conditions where [TDGA] was always in excess over
[Fe(bpy)3]3+, in perchloric acid medium at a constant temperature
of 30.0 0.05 °C, unless stated. The ionic strength of the medium
was maintained by adding sodium perchlorate solution.
Requisite quantities of TDGA, sodium perchlorate, perchloric
acid and other reagents were thermostated in a reaction vessel.
Tris(2,20-bipyridine)iron(III) separately equilibrated was trans-
ferred to the reaction vessel at zero time and shaken well to initiate
the reaction. The molar extinction coefficients of [Fe(bpy)3]2+ and
[Fe(bpy)3]3+ are 8650 and 320 respectively at the wavelength re-
gions 524 and 618 nm [24,25]. Since the molar extinction coeffi-
cient for the Fe(II) complex is higher than Fe(III) complex, the
rate of formation of [Fe(bpy)3]2+ complex was followed using the
UV–vis spectrophotometer by measuring the increase in the absor-
bance at wavelength 524 nm. The reaction was followed at definite
following equations given by Peterson et al. [26].
2RT0T
The error in
where
D
H# ðdÞ ¼
a
T0 ꢁ T
a
is the maximum fractional error in the overall rate con-
stants, R is the gas constant in Joules and T and T0are the two ex-
treme temperatures at which the reaction is conducted.
ꢀ
ꢁ
1
T
T0 ꢁ T
The error in
D
S#
ð
rÞ ¼ S
þ
2T0T
Results
Dependence of rate on the reactants
The kinetic runs were carried out with a large excess of TDGA
over the oxidant in the ratio 100:1 to ensure pseudo first order con-
ditions. The [Fe(bpy)3]3+ was varied in the range 2.0 ꢂ 10ꢁ4–8.0 ꢂ
10ꢁ4 M dmꢁ3 at fixed [TDGA], [HClO4] and [I]. The non variation
of the pseudo first order rate constant at various concentrations
indicate the order in [Fe(bpy)3]3+ is unity. This was also confirmed
from the linearity of plots of log (A1ꢁAt) versus time (Fig. 2) for up
to 70% completion.
The effect of TDGA on the rate of electron transfer reaction was
studied by varying the concentration from 2.0 ꢂ 10ꢁ2 to 8.0 ꢂ 10ꢁ2
M dmꢁ3 and keeping other factors as constant. A plot of log (A1ꢁAt)
versus time for various concentrations of TDGA is shown in Fig. 3.
Variation of [TDGA] at constant [Fe(bpy)3]3+ and [HClO4] resulted
in variation of the k1 values. The pseudo first order rate constant in-
creases with increase in concentration of TDGA. The observed sec-
ond order rate constant values, calculated using kov = k1/[TDGA]
decreased with increase in TDGA indicating the order in substrate
is different from unity.
The fractional order dependence on TDGA was calculated from
the plot of logk1 vs log [TDGA]. The double logarithmic graph be-
tween [TDGA] and k1 (r = 0.996) is a straight line with the slope
of 0.412 0.02 indicating the order with respect to TDGA is
0.412. Further the overall rate constant, kov calculated by dividing
pseudo first order rate constant by [TDGA]0.412 gave a constant va-
lue. This confirms the order with respect to TDGA is 0.412. The
pseudo first order rate constants and the overall rate constants
calculated using the relation, k1/[TDGA]0.412 at different concentra-
tions of TDGA and [Fe(bpy)3]3+ are summarized in Table 1.
time intervals till maximum absorbance (A ) was reached. The
1
increase in the absorbance of [Fe(bpy)3]2+ with time shows the
progress of the reaction. In some cases duplicate runs were carried
out to ensure the reproducibility.
Evaluation of rate constants
The observed pseudo first order rate constant of each kinetic
run was evaluated from the slope of the linear plot of log (A1ꢁAt)
versus time, by the method of least squares. The success of each fit
is expressed in terms of the correlation coefficient (r) and standard
deviation (s).
According to the first order equation,
2:303
t
Ao ꢁ Ao
Aa ꢁ At
k1 ¼
log10
2:303
k1
Aa ꢁ Ao
Aa ꢁ At
t ¼
log10
where k1 is the pseudo first order rate constant, t is the time in
seconds. A0, At and A denote the absorbance at zero time, time ‘t’
1
and infinite time respectively of tris (2,20-bipyridine)iron(II).
The overall rate constant, kov is evaluated as kov = k1/
[TDGA]0.412. It is expressed in Mꢁ1 sꢁ1. The precision of k values
in all the kinetic runs are given in terms of 95% confidence limit
of students t test.
Calculation of activation parameters
When 1/k is plotted against 1/[TDGA], the plot was linear
ꢁ1
According to Eyring’s equation of transition state theory the
overall rate constant, kov of the reaction is related to the thermody-
namic parameters as.
with finite intercept (r = 0.999) in the rate co-ordinate indicating
the reaction follows the Michaelis–Menten type kinetics with re-
spect to TDGA. This leads to the postulation of complex formation
between the substrate and oxidant before the rate-controlling step
in the mechanism.
D
H#
S#=R
KBT
h
ko
m
¼
eꢁ =RTeD
Dependence on [H+]
where KB is the Boltzmann constant; h is the Planck’s constant and T
is the temperature in Kelvin.
The terms D D
H# and S# are the enthalpy of activation and entropy
The effect of [H+] on the rate of the reaction was studied by
varying the concentration of HClO4. The pseudo first order rate
constants obtained at different [HClO4] are presented in the Table 2.
The perusal of the rate data indicates the reaction is not susceptible
to [H+] variation.
of activation respectively. These activation parameters were calcu-
lated by the method of least squares from a linear plot of log (kov/T)
versus 1/T. On taking logarithm, the Eyring’s equation takes the form:
kov
T
D
S#
D
H#
4:576 4:576T
H# and S# were calculated from the slope and
intercept respectively of the Eyring’s plot. Thus,
log
¼ 10:319 þ
ꢁ
Effect of ionic strength
The values of
D
D
The effect of altering ionic strength on the rate of the reaction
was investigated by adding sodium perchlorate to the reaction
mixture. The rate of the reaction is slightly decreased with increase
D
H# ¼ ꢁ4:576 ꢂ slope ꢂ 4:186 ꢂ 10ꢁ3 kJ molꢁ1

318
S. Deepalakshmi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 315–321
Fig. 2. A plot of log (A1ꢁAt) versus time at different concentrations of [Fe(bpy)3]3+
.
Fig. 3. A plot of log (A1ꢁAt) versus time at different concentrations of TDGA.
Table 2
Effect of varying [H+] on the rate ([TDGA] = 5.0 ꢂ 10ꢁ2 M; T = 30 °C;
[Fe(bpy)3]3+ = 5.0 ꢂ 10ꢁ4 M; [I] = 1.3 M).
Table 1
Pseudo first order and overall rate constants for the oxidation of TDGA by [Fe(bpy)3]3+
[HClO4] (M)
103 k1 (sꢁ1
)
([HClO4] = 0.5 M; [I] = 0.6 M; T = 30 °C).
102[TDGA] (M)
104[Fe(bpy)3]3+ (M)
103 k1 (sꢁ1
)
102 kov (Mꢁ1 sꢁ1
)
0.1
0.3
0.5
0.6
0.8
0.9
1.0
1.2
1.3
0.82 0.02
0.94 0.04
0.88 0.02
1.22 0.04
1.25 0.04
0.92 0.03
0.93 0.02
0.90 0.01
1.12 0.02
2.0
4.0
5.0
6.0
8.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
2.0
4.0
6.0
8.0
0.97 0.04
1.35 0.04
1.47 0.07
1.55 0.04
1.72 0.04
1.51 0.02
1.47 0.03
1.47 0.03
1.60 0.01
4.86 0.02
5.08 0.01
5.05 0.02
4.94 0.01
4.87 0.01
5.19 0.01
5.05 0.01
5.05 0.01
5.05 0.01
Table 3
Effect of [I] on the rate of reaction ([TDGA] = 5.0 ꢂ 10ꢁ2 M; [HClO4] = 0.5 M; [Fe(bpy)
]
3
3+ = 5.0 ꢂ 10ꢁ4 M; T = 30 °C).
I (M)
103k1 (sꢁ1
)
in ionic strength of medium as evidenced by data in Table 3. As one
of the reactants is a neutral species, the decrease in rate with in-
crease in ionic strength may be explained due to the formation
of charge separated transition state in the rate-determining step.
The formation of charge separated transition state will be favored
at low ionic strength conditions.
0.6
0.8
1.0
1.2
1.3
1.47 0.07
1.44 0.04
1.24 0.04
1.05 0.03
0.88 0.02

S. Deepalakshmi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 315–321
319
Table 5
Effect of acrylamide
Effect of composition of the solvent on the reaction rate ([TDGA] = 5.0 ꢂ 10ꢁ2 M;
[I] = 0.6 M; [Fe(bpy)
]
3+ = 5.0 ꢂ 10ꢁ4 M; T = 30 °C; [HClO4] = 0.5 M).
3
Experiments conducted in the presence of a radical scavenger
show that the rate of electron transfer reaction was not affected
with increase in [acrylamide], Table 4. The results show the radical
scavenger and the redox system do not initiate polymerization and
the formation of radical intermediate in the reaction is unlikely.
Solvent composition% (v/v)
D
103k1 (sꢁ1
)
100% H2O
30% CH3OH–70% H2O
30% CH3CN–70% H2O
80
32
37
1.47 0.07
1.17 0.05
0.72 0.02
Effect of solvent
Table 6
Variation of rate constant with temperature ([TDGA] = 5.0 ꢂ 10ꢁ2 M; [HClO4] = 0.5 M;
The influence of solvent polarity on the electron transfer reac-
tion was studied by varying the composition of the medium by
using acetonitrile and methanol. The pseudo first order rate con-
stants at various composition of the medium are recorded in Ta-
ble 5. The reaction rate increases with increasing water content
in the medium. It is, therefore, quite likely that a charge separated
complex is formed in the transition state which is more stabilized
as a result of solvation by an increase in water content as well as
the nucleophilicity of the medium.
[Fe(bpy)
]
3
3+ = 5.0 ꢂ 10ꢁ4 M; [I] = 0.6 M).
Temp. (°C)
103kov (Mꢁ1 sꢁ1
)
D
H# (k J molꢁ1
47.5 0.32
)
D
S# (J Kꢁ1 molꢁ1
ꢁ74.3 1.09
)
25
30
35
40
4.16 0.14
5.05 0.24
7.01 0.38
11.4 0.58
Table 7
Stoichiometric analysis of the electron transfer reaction.
Temperature dependence and activation parameters
102 [TDGA] (M)
Initial
Reacted
½FeðbpyÞ3
½TDGAꢃ
ꢃ
3þReacted
The kinetic runs were carried out at four different temperatures
over the temperature range of 15 °C. The values of the rate con-
stants for the electron transfer reaction are recorded in Table 6.
The rate data fit the Eyring’s equation and the activation parame-
10[Fe(bpy)3]3+ (M)
10[Fe(bpy)3]3+ (M)
5.0
8.0
15.0
5.0
16.0
15.0
1.1
1.59
3.14
2.2
1.99
2.1
ters
D D
H# and S# for the reaction have been evaluated from the
slope and intercept respectively of the linear Eyring’s plot of the
log kov/T against 1/T (r = 0.979). The activation parameters thus cal-
culated are presented in Table 6. The reaction is characterized by a
high negative value of entropy of activation. This implies the com-
plex formed between the substrate and oxidant should have an
exacting specificity of orientation.
Product analysis
Tris(2,20-bipyridine)iron(III) and TDGA were mixed in the ratio
of 2:1 under the experimental conditions of [H+] = 0.5 M at 303 K.
The reaction was followed by measuring the absorbance of
[Fe(bpy)3]2+ formed at 524 nm. When the absorbance reached a
maximum value, showed the completion of the reaction, the reac-
tion mixture was shaken several times with chloroform. The prod-
uct was separated out from the concentrated chloroform extract.
The IR spectrum of the product was recorded and it was found to
be identical with of the corresponding sulphoxide, having S = 0
stretching frequency at 1054 cmꢁ1. There was no evidence for
the formation of sulphone as the product under experimental
condition.
Stoichiometric studies
The stoichiometry of the reaction was studied by taking number
of reaction mixtures containing large excess of [Fe(bpy)3]3+ over
TDGA. The reaction mixtures were allowed to stand in the presence
of 0.5 M HClO4 at 30 °C for completion. After completion of the
reaction, the concentration of the product, [Fe(bpy)3]2+ formed
was calculated spectrometrically from its absorbance maxima
and the known value of molar extinction coefficient. During the
reaction, the amount of [Fe(bpy)3]2+ formed was equal to the
[Fe(bpy)3]3+ consumed. So, [Fe(bpy)3]3+ consumed could be esti-
mated from the amount of product, [Fe(bpy)3]2+ formed. The re-
sults obtained are cited in Table 7. The results indicate one mole
of TDGA is consumed per two moles of oxidant. The overall reac-
tion may be represented by the following equation:
Discussion
The results from kinetic and product analysis study presented
above clearly indicate the iron(III)-bipyridyl complex is capable
of transforming TDGA to their sulphoxide via single electron trans-
fer. The kinetic data point to Michaelis–Menten behavior i.e., a
complex formation between the substrate and the oxidant before
the rate determining step and the reaction is first order in the oxi-
dant and fractional order with respect to TDGA. The spectral stud-
ies reveals that the complex formed does not have any absorption,
as there is no new absorption peak in the spectrum. The decrease
in rate with increase in ionic strength and increase in rate with in-
crease in water content of the medium showed more charge sepa-
ration takes place in the rate determining step. On the basis of
above observations the steps shown in the following scheme has
been proposed for the electron transfer reaction of TDGA with
tris(2,20-bipyridine)iron(III) complex (see Scheme 1).
SðCH2COOHÞ þ 2½FeðbpyÞ ꢃ3þ þ H2O ! O
2
3
2þ
¼ SðCH2COOHÞ þ 2½FeðbpyÞ ꢃ þ 2Hþ
2
3
Table 4
Effect of added acrylamide ([TDGA] = 5.0 ꢂ 10ꢁ2 M; [I] = 0.6 M; [Fe(bpy)3]3+ = 5.0 -
ꢂ 10ꢁ4 M; T = 30 °C; [HClO4] = 0.5 M).
103 [Acrylamide] (M)
103k1 (sꢁ1
)
0.0
0.8
1.0
5.0
10.0
15.0
1.47 0.07
1.23 0.03
1.21 0.03
1.25 0.06
1.10 0.03
1.19 0.04
The first step in the mechanism is the formation of oxidant-sub-
strate complex. The complex formation is proved from the linear
Michaelis–Menten plot. On the basis of the Michaelis–Menten for-
mulation the observed rate constant is given by the following
equation.

320
S. Deepalakshmi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 315–321
Scheme 1. Mechanism for electron transfer reaction of tris(2,20-bipyridine) iron(III) complex with TDGA.
k½substrateꢃ
i.e., high reactivity in aqueous medium and decreased rate in
k1 ¼ kobs
¼
methanol and acetonitrile (Table 5), is also in accordance with
the proposed mechanism. It is pertinent here to mention that the
pseudo first order rate constant in 30% methanol with a low dielec-
tric constant is about two times larger than that observed in 30%
aqueous acetonitrile having a higher dielectric constant. This
shows the importance of protic solvent in the present mechanism.
The ionic strength effect observed has also been explained using
the mechanism. There is a charge separation in the slow rate-
determining step. Since charge separated species are more easily
formed at high ionic strength, an enhancement in rate is observed
with ionic strength in the present investigation.
Km þ ½substrateꢃ
The above equation on rearrangement leads to the equation
1
1
Km
¼
þ
k1
k
k½substrateꢃ
where Km is the Michaelis–Menten constant and k is the rate con-
stant for the formation of the product. These constants are obtained
from the plot of 1/k1 versus 1/[TDGA]. The values of the k and Km
obtained from the slope and intercept values for the present study
are k = 2.26 ꢂ 10ꢁ3 sꢁ1 and Km = 2.67 ꢂ 10ꢁ2 M. Though a complex
is proposed in the mechanism with the present experimental data
it is not possible to definitely state the nature and structure of the
complex.
Finally the sulphur cation radical undergo fast reactions with
water and another molecule of [Fe(bpy)3]3+ to form the product,
sulphoxide of TDGA.
Electron transfer may take place within the complex in a slow
rate-controlling step to yield sulphur cation radical. Rajagopal
et al. and Gallo et al. have shown that the electron transfer reac-
tions of many organic sulphur compounds viz, sulphides, sulphox-
ides, thioacetic acid with [M(NN)3]3+ (NN = 2,20-bipyridine and
1,10-phenanthroline) take place via sulphur radical cation in the
rate determining step [3,18,19,27]. They confirmed the formation
of transient, sulphide radical cation, using flash photolysis tech-
nique. In light of these observations, the formation of sulphur cat-
ion radical is proposed for the electron transfer reaction. The
formed sulphide cation radical may undergo degradation under
different pathways, viz., fragmentation at CAS bond, oxidation,
reaction with the solvent H2O, dimerisation etc. The formation of
sulphoxide in the present study clearly indicates major portion of
sulphide radical cation reacts with the solvent, water.
Conclusion
Detailed kinetic studies have been carried out on the electron
transfer reaction of TDGA with tris(2,20-bipyridine)iron(III) per-
chlorate under pseudo first order conditions. The rate of reaction
was followed by monitoring the increase in concentration of
[Fe(bpy)3]2+ spectrophotometrically at 524 nm. The reaction is first
order with respect to [Fe(bpy)3]3+ and the observed rate constant
remains constant with increase in initial concentration of
[Fe(bpy)3]3+. The reaction is of fractional order with respect to
TDGA and the order is 0.412. A linear Michaelis–Menten plot with
a finite intercept is obtained for the electron transfer reaction
showing the formation of a complex in the reaction, Km is calcu-
lated and the value is 2.67 ꢂ 10ꢁ2 M. The reaction is characterized
by high negative entropy indicating the complex formed has a def-
inite orientation higher than the reactant molecules. Stoichiometry
of the reaction was found to be in the ratio 2:1 for [Fe(bpy)3]3+ and
Thus the presence of protic solvent is essential for the reaction.
The absence of any reaction in 100% acetonitrile in the present
electron transfer reaction of TDGA with [Fe(bpy)3]3+ clearly proves
the proposed mechanism without ambiguity. The effect of solvent,

S. Deepalakshmi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 315–321
321
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.01.011.
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