Superoxo radical scavenging action by common analgesic drug paracetamol: A model kinetic study

Article abstract of DOI:10.1016/j.ica.2013.04.031

In acid perchlorate media ([H⁺] = 1.0-3.0 M), each mole of paracetamol, HOC₆H₄NHCOCH₃ (APAP), at ambient temperature quantitatively reduces two mole of the metallo-superoxo complex, μ-superoxo-bis[pentaamminecobalt(III)]⁵⁺, [(NH3)5Co(III)(μ- O₂)Co(III)(NH₃)₅]⁵⁺ (1). Here, complex 1 is reduced to [(NH3)₅Co(OH₂)]³⁺, Co²+, O₂ and NH₄⁺ and APAP itself is oxidised to quinone oxime and acetic acid. With a large excess of APAP over 1, the reduction follows first-order kinetics. The observed first-order rate constant (k_o) increases linearly with increasing [H⁺] as well as with T_APAP (TAP_AP being the analytical concentration of APAP). The protonated form of APAP, viz. APAPH⁺ seems to be the kinetically reactive reductant species. The enrichment of aqueous reaction media with D₂O retards the reaction and thus it appears that the reaction proceeds through an electroprotic mechanism. A relatively small ΔH^≠ (57 ± 2 kJ M ¹) and moderately negative ΔS^≠ (-68±8JK ¹ M ¹) supports a compact transition state.

Full text of DOI:10.1016/j.ica.2013.04.031

Inorganica Chimica Acta 406 (2013) 266–271
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Superoxo radical scavenging action by common analgesic drug
paracetamol: A model kinetic study
b
b
b
Bula Singh a, , Ranendu Sekhar Das , Rupendranath Banerjee , Subrata Mukhopadhyay
⇑
a Department of Chemistry, Visva-Bharati, Santiniketan 731 235, India
b Department of Chemistry, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
In acid perchlorate media ([H+] = 1.0–3.0 M), each mole of paracetamol, HOC6H4NHCOCH3 (APAP), at
ambient temperature quantitatively reduces two mole of the metallo-superoxo complex, -superoxo-
bis[pentaamminecobalt(III)]5+, [(NH3)5Co(III)( -O2)Co(III)(NH3)5]5+ (1). Here, complex 1 is reduced to
Article history:
Received 5 March 2013
Received in revised form 21 April 2013
Accepted 22 April 2013
Available online 2 May 2013
l
l
[(NH3)5Co(OH2)]3+, Co2+, O2 and NH4+ and APAP itself is oxidised to quinone oxime and acetic acid. With
a large excess of APAP over 1, the reduction follows first-order kinetics. The observed first-order rate con-
stant (ko) increases linearly with increasing [H+] as well as with TAPAP (TAPAP being the analytical concen-
tration of APAP). The protonated form of APAP, viz. APAPH+ seems to be the kinetically reactive reductant
species. The enrichment of aqueous reaction media with D2O retards the reaction and thus it appears that
Keywords:
Cobalt(III)
Superoxide
Redox
Kinetics
ROS
the reaction proceeds through an electroprotic mechanism. A relatively small
D
H– (57 2 kJ Mꢀ1) and
moderately negative
D
S– (ꢀ68 8 JKꢀ1 Mꢀ1) supports a compact transition state.
Ó 2013 Elsevier B.V. All rights reserved.
Mechanism
1. Introduction
in all biospheres resulting from the mounting level of pollutions
due to different anthropogenic activities have been a major con-
cern for all [27–31].
Oxygen, as it is the terminal oxidant in respiration cycle in aer-
obic organisms, is one of the key factors to sustain higher forms of
life. The singlet oxygen (3O2), present in air is less reactive than its
different reduced derivatives [1] like superoxide (O2ꢀ), peroxide
(O22ꢀ) and hydroxyl radical (ÅOH). These radicals having indepen-
dent existence with one or more unpaired electrons form a part
of reactive oxygen species (ROS) [2]. The limited production of
ROS in normal physiological and metabolic processes plays crucial
roles in respiratory chain, immune systems and signalling path-
ways [3–7]. In biological systems ROS are also deliberately gener-
ated through oxidative burst by phagocyte NADPH oxidase to
defend against harmful microbial pathogens. But several exoge-
nous and endogenous reasons may cause overproduction of ROS
to tip off the balance between the former and the natural antioxi-
dant defence systems [8] fostering severe damages to all the essen-
tial bio-macromolecules, like, carbohydrates, proteins, lipids and
nucleic acids [9–16] resulting in metabolic and cellular disorders.
This alarming situation, called ‘oxidative stress’ may even lead to
critical diseases like cancer, heart problems, Alzheimer’s or
Parkinson’s disease [17–26]. In present day life the deleterious
effect of ROS and other xenobiotics on almost every living system
In this work we have chosen a metal bound superoxo species
[(NH3)5Co(III)(l
-O2)Co(III)(NH3)5]5+ (1) as a suitable model for
ROS and studied the kinetics of reduction of 1 by acetaminophen
(abbreviated as APAP which is N-acetyl-4-aminophenol) [32,33].
APAP is one of the most common water soluble analgesic drugs
which has been reported to inhibit the formation of ROS rather
than eliminating it [34–36]. However, we have observed that APAP
can effectively reduce and mineralize the superoxo complex 1. The
motivation of the study comes from the fact that it may help to
open up the possibility of administering the common drugs of
the same genre as APAP in efficiently reducing the ROS toxicity.
2. Experimental
2.1. Materials
Cobalt(II) chloride hexahydrate (BIOSOL), ammonia (Merck),
ammonium peroxodisulfate (Merck), orthophosphoric acid
(Merck), perchloric acid (Merck), acetaminophen (Aldrich), sodium
perchlorate (Aldrich), and deuterium oxide, D2O (Aldrich, 99%)
were used as received. Doubly distilled water was used throughout
the experiments. Perchloric acid was standardized against stan-
dard sodium hydroxide solution in the usual way [37].
⇑
Corresponding author. Tel.: +91 94331 63075; fax: +91 33 2414 6223.
E-mail addresses: singhbula@rediffmail.com (B. Singh), ju_subrata@yahoo.co.in,
smukhopadhyay@chemistry.jdvu.ac.in (S. Mukhopadhyay).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.04.031

B. Singh et al. / Inorganica Chimica Acta 406 (2013) 266–271
267
2.2. Preparation and characterization of complex 1
0.9
0.6
0.3
0.0
The metallo-superoxide complex (1) was synthesized following
a literature process [38] with necessary modifications [39]. First, a
1
l-peroxo-cobalt(III) complex is prepared from CoCl2, 6H2O and
then it is oxidized to superoxo complex by ammonium peroxodi-
sulfate. The pentachloride salt of 1 was converted to perchlorate
salt by the following procedure: pentachloride salt was dissolved
in 40% orthophosphoric acid and filtered; HClO4 (70%) was added
drop wise to the filtrate to precipitate the perchlorate salt. Pure
perchlorate salt of 1 was obtained by recrystallization from 10%
HClO4.
25
Complex 1 was characterized by UV–Vis absorption and FTIR
study. Complex 1 shows characteristic UV–Vis absorption peak at
481 nm and 670 nm and the purity of 1 was ascertained from the
400
500
600
wavelength (nm)
700
800
molar extinction coefficient (e) value at 670 nm (e ,
in Mꢀ1 cmꢀ1
found 860; reported 890 [38]). FT-IR spectrum for complex 1
(Fig. 1) shows the main absorption bands at 1626 (s), 1336 (s)
and 1321 cmꢀ1 (s) for das (NH3) and 835 cmꢀ1 (s) for rr (NH3) which
are in close agreement with the literature values for similar di-
bridged metallo superoxide complex (das, asymmetric deformation
vibration; rr rocking vibration) [40]. The presence of the peak at
1086 cmꢀ1 (s) confirms the presence of the bridging superoxo
group [41].
Fig. 2. Observed spectral changes when 1 was reduced with APAP. Spectra were
recorded at an interval of 2 min. Condition: [1] = 1.0 mM; TAPAP = 10.0 mM;
[H+] = 1.5 M; I = 3.0 M; T = 25.0 0.1 °C. Spectrum 1 represents the pure complex 1.
first-order conditions. The reacting solutions were prepared by
the addition of measured amount of acidic solution of 1 and so-
dium perchlorate followed by the addition of freshly prepared
aqueous solution of APAP. The reaction was followed by the
decrease in absorbance at 670 nm (Fig. 3), the characteristic visible
absorption peak of 1, where all other species in the reaction
mixture are transparent. The observed first-order rate constants
(ko) were derived using non-linear standard least-squares fit to
the first-order equation of such decay with time.
2.3. Instrumentation
Absorbance and UV–Vis spectra were recorded using a Shima-
dzu spectrophotometer (UV-1700) with 1.00 cm quartz cuvettes
equipped with electrically controlled thermostat. Fourier Trans-
form Infrared (FTIR) data were collected using a Shimadzu FT-IR
8400S. Mass spectrometry (ESI positive mode) was done by micro
mass Q-TOF micro (LCMS) spectrometer. 13C NMR spectra were re-
corded at 400 MHz Bruker spectrometer using tetramethylsilane as
an internal standard.
2.5. Stoichiometry
Estimation of unreacted APAP using conventional analytical
techniques [42–44] as an aftermath of the reaction when APAP
was used in sufficient excess over 1 was not convincing because
the results were random and inconsistent probably due to interfer-
ence from reaction products. Thus, stoichiometric determinations
were carried out by titrating the metallosuperoxide 1 with differ-
ent amount of APAP. For this purpose, several sets of 1 with varying
concentrations were reacted with deficit amounts of APAP and
when the reactions reached equilibrium, absorbance of the reac-
tion mixtures at 670 nm (Ae) were noted whence unreacted [1]
was determined from the difference between the absorbance of
the pure 1 (A0) and Ae (Table S1, see Supplementary information).
The reduction products from 1 were identified and quantified
from the absorbance of the final spectrum of the reaction mixture
2.4. Kinetics
Both complex 1 and APAP are stable in aqueous acid media at
room temperature and APAP is not hydrolysed under the kinetic
conditions employed [42–44]. But the addition of APAP to 1 in acid
perchlorate media (1.0–3.0 M) gradually decreased the absorbance
of 1. A representative UV–Vis spectral change of 1 with time is
shown in Fig. 2. The kinetics were studied in acid media at
25.0 0.1 °C and at a fixed ionic strength, I = 3.0 M, maintained
with NaClO4, unless mentioned otherwise. A large excess of APAP
over 1 were maintained in all the kinetic runs to ensure pseudo
0.8
0.6
0.4
0.2
0.0
100
80
60
40
4000
0
2000
4000
time (s)
6000
8000
3000
2000
cm -1
1000
Fig. 3. Reduction of 1 by APAP follows first-order kinetics. Condition: [1] = 1.0 mM;
TAPAP = 10.0 mM; [H+] = 1.5 M; I = 3.0 M; T = 25.0 0.1 °C.
Fig. 1. FT-IR spectrum of metallo-superoxo complex 1.

268
B. Singh et al. / Inorganica Chimica Acta 406 (2013) 266–271
(Fig. S1) and the volume of the gas evolved during the reaction was
collected by the downward displacement of water and corrected to
N.T.P (1.0 atm, 273.15 K) as usual. The oxidised products of APAP
were characterized from the LC-MS spectra (Figs. S2 and S3), FTIR
spectra (Figs. S4 and S5) and 13C NMR spectra (Fig. S6) of the prod-
uct mixture extracts.
14
12
10
8
3. Results and discussions
6
k
3.1. Stoichiometry and reaction products
4
From repetitive experiments, it has been found that two moles
of 1 reacts with one mole of APAP. On reduction of 1, along with
other products, both O2 and NH3 could evolve as the gaseous prod-
ucts. However, as NH3 has pKa value ꢁ9.2 [45], at the prevailing
2
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
reaction media acidity (1.0–3.0 M), most of the NH3 produced
[H+] (M)
+
should remain in solution as NH4 The evolved gas due to the
.
Fig. 5. Dependence of ko on [H+]. Condition: [1] = 1.0 mM; TAPAP = 25.0 mM;
I = 3.0 M; T = 25.0 0.1 °C.
reduction reaction can thus mainly be conferred as O2 and from
several stoichiometric runs, the approximate volume of the
evolved gas has been found to be one molar, corrected to all other
factors.
which bears the signature peaks of C@O (ꢁ1654 cmꢀ1), C@N
(ꢁ1630 cmꢀ1) and N–OH (1070 cmꢀ1) [49–55]. Acetic acid was
detected by spot test [56] and in 13C NMR (Fig. S6). The 13C NMR
of the CDCl3 extract of reaction mixture which preferentially
dissolves acetic acid over quinone oxime [57] shows the presence
of characteristic peaks at 176.86 and 20.67 ppm of the same [58].
The stoichiometric equation representing the reduction of 1 by
APAP thus appears to be:
Complex 1 is finally reduced to a mixture of [(NH3)5Co(OH2)]3+
,
Co2+
,
O2 and NH4 [46–48]. The formation of both [(NH3)5
+
Co(OH2)]3+ and Co2+ is evident from the absorption value of 0.13
at 500 nm of the final spectrum of reaction mixture that has
reached equilibrium (condition: [1] = 1.0 mM; TAPAP = 25.0 mM;
[H+] = 1.5 M; I = 3.0 M; T = 25.0 0.1 °C; Fig. S1). At 500 nm, the
corresponding
e
values of the species, [(NH3)5Co(OH2)]3+ and Co2+
are 50.0 and 5.0 Mꢀ1 cmꢀ1 respectively [46]. Here it is worth
mentioning that under the reaction condition, APAP is totally
transparent over the wavelength range of 400–800 nm (Fig. S1).
The observed absorbance value, which is cumulative can be
accounted neither by the formation of only [(NH3)5Co(OH2)]3+
nor by only Co2+ but by the formation of both the species. The
absorbance value can best be explained by the presence of
approximately two moles [(NH3)5Co(OH2)]3+ and two moles of
Co2+ in final reaction mixture (calculated absorbance value 0.110;
found 0.113) which also supports the findings mentioned above
that two moles of complex 1 is reduced by one mole of APAP.
On the other hand APAP is oxidised to quinone oxime and acetic
2½ðNH3Þ5CoðIIIÞð
l
ꢀ O2ÞCoðIIIÞðNH3Þ ꢂ5þ þ HOC6H4NHCOCH3
5
þ 10Hþ þ 2H2O
! 2½ðNH3Þ CoðOH2Þꢂ3þ þ 2Co2þ þ O2 þ 10NHþ
5
4
þ OC6H4NOH þ CH3CO2H
ð1Þ
3.2. Effect of variation TAPAP and [H+] on ko
The reduction of 1 with large excess of APAP follows first-order
kinetics and the observed first-order rate constants (ko) increase
with the increase in TAPAP. The plot of ko versus TAPAP was found
to be linear (Fig. 4) and passes through the origin. The representa-
tive data are given in Table S2.
acid [49–54]. The presence of quinone oxime can be confirmed
ꢀ
from the occurrence of a strong peak at m/z = 145 (Na+C6H4NO2
)
in LC-MS spectra of the product mixture (Figs. S2 and S3)
[49–53]. The formation of quinone oxime was further confirmed
from the FT-IR spectra of the product mixtures (Figs. S4 and S5)
The ko values were also found to increase linearly with the in-
crease in [H+] (Table S3, Fig. 5). APAP is a weak acid. The pKa of
8
6
14
12
10
8
4
k
k
6
4
2
0
2
0
0
5
10
15
20
25
30
35
40
0
20
40
60
80
100
% D2O (v/v)
TAPAP (mM)
Fig. 4. Observed dependence of ko on TAPAP. Condition: [1] = 1.0 mM; [H+] = 0.5 M;
I = 3.0 M; T = 25.0 0.1 °C.
Fig. 6. Variation of ko with the increasing% D2O (v/v). Condition: [1] = 1.0 mM;
TAPAP = 25.0 mM; [H+] = 1.5 M; I = 3.0 M; T = 25.0 0.1 °C.

B. Singh et al. / Inorganica Chimica Acta 406 (2013) 266–271
269
TAPAP = 25.0 mM; [H+] = 1.5 M; I = 3.0 M; T = 25.0 0.1 °C). This
supports an electroprotic mechanism [62–64].
11
10
9
The increase of ko values with increasing [H+] along with the
solvent isotope effect, reckon the possibility that any one of the
reactants must take up proton in any equilibrium step. Previous
studies firmly established that superoxo group (O2ꢀ) cannot take
up protons [65,66]. Thus, the other plausible way is the proton-
ation of APAP. The APAP molecule has two potential site of proton-
ation, viz. the nitrogen site and the oxygen of carbonyl group. There
are plenty of literature evidences supporting that a single proton-
ation can take place at any one of the sites [67–72]. So, without any
prejudice to the particular site of protonation, our kinetic results
suggest that the kinetically reactive reductant is not APAP, but
the singly protonated conjugate acid APAPH+.
k
8
7
6
2.0
2.5
3.0
3.5
(M)
4.0
4.5
KH
APAP þ Hþ ꢀ APAPHþ
ð2Þ
I
Fig. 7. Plot of ko vs. I. Condition: [1]=1.0 mM; TAPAP = 25.0 mM; [H+] = 1.5 M;
T = 25.0 0.1 °C.
3.4. Effect of variation of ionic strength on ko
We observed that with increasing ionic strength of the reaction
media the observed rates increase and a simple plot of ko versus I
was found to be a straight line (Fig. 7, Table S4). It indicates a reac-
tion between two species of similar charges. Since the complex 1
bears positive charges, the other reactant, viz. APAP also should
bear similar positive charges. It is only possible if we presume that
the reactive reductant is not APAP, rather APAPH+. This again rein-
forces the fact anticipated earlier (Section 3.3).
its acid dissociation from –OH group is 9.78 [59] (similar to that of
pKa 9.98 of PhOH [60]). No data is available on the acidity of the –
NH group of APAP, however. Still, acidity of this group is not ex-
pected to be much more than that of the –NH group of p-amino-
phenol, (pKa = 5.50) [61]. Thus, under the acidic condition
employed in the present study ([H+] = 1.0–3.0 M), any kind of
deprotonation from APAP should not be the source of observed
acid dependence in this reaction.
3.5. Probable reaction mechanism
3.3. Effect of D2O variation on ko
The overall process regarding the reduction of the metallo-
superoxo complex, 1 by APAPH+ seems to be a complex reaction.
The redox behavior of 1 is relatively simpler and from the spectro-
scopic evidences and stoichiometric analysis of the present study
(vide supra) and with support from other reported works, it can
When the aqueous reaction media was enriched with D2O, the
observed reaction rates, ko have been found to decrease linearly
(Fig. 6). Thus a solvent isotope kinetic effect has been found
(kH/kD = 2.0 0.7) for the reduction of 1 by APAP ([1] = 1.0 mM;
KH
(3)
H+
HAPAP+
5+
APAP
+
III
6+
(4)
III
[(NH3)5Co
O
(1)
+
Co(NH3)5]
III
III
O
O
Co(NH3)5]
[(NH3)5Co
K1
O
HAPAP
HAPAP+
6+
III
III
III
Co(NH3)5]
O
O
III
5+
+ [APAP]+
k
Co(NH3)5]
[(NH3)5Co
O
(5)
[(NH3)5Co
O
slow
H
HAPAP
(2)
III
III
5+
O
O
Co(NH3)5]
[(NH3)5Co
2 [(NH3)5Co(OH2)]3+
(1)
Fast
9H+
(6)
(7)
+
+
+
OC6H4NOH + CH3COOH
[APAP]+
2H2O
III
Co(NH3)5]
III
5+
+ 10 NH4+ + O2
2 Co2+
O
Fast
[(NH3)5Co
+
O
H
(2)
Scheme 1.

270
B. Singh et al. / Inorganica Chimica Acta 406 (2013) 266–271
be said firmly that complex 1 is reduced to [(NH3)5Co(OH2)]3+
,
adduct makes close proximity of the redox agents and was some-
times proposed [73].
+
Co2+, O2 and NH4 [39,46–48]. Here, it is worth mentioning that
there is a finite possibility that the superoxo bridge will form a
hydrogen bond with APAPH+. This kind of hydrogen-bonded
The plausible mechanistic steps for the reduction of 1 by APAP
are shown in Scheme 1. APAP, which is an overall 2eꢀ, 2H+ redox
donor, 1eꢀ, 1H+ each from two potential sites, viz. O–H and N–H
[74,75], undergoes several steps leading to its oxidised products.
Initially, the active reducing species APAPH+ forms an adduct with
1 and then reduces 1 to its corresponding hydroperoxo complex 2.
The resulting [APAP]+ [75] again reacts with 1 in a fast step and
probably via the formation of 2, produces quinone oxime and ace-
tic acid [49–54] along with [(NH3)5Co(OH2)]3+ [46]. The hydroper-
oxo complex 2 is unstable in acidic media and readily decomposes
[76]. In this study, since the kinetics have been studied through the
change of absorption of metallo-superoxo complex 1, the reduction
mechanism of 1 is more clearly perceptible than the oxidation
mechanism of APAP. Thus, Scheme 1 given below shows the pro-
posed mechanistic steps with more emphasis on the reduction of 1.
Scheme 1 leads to Eq. (7).
Table 1
The observed rate constants at different temperatures.
TAPAP (mM)
104ko, sꢀ1 at different temperature (K)
293
298
303
308
313
20.0
25.0
30.0
35.0
3.8
4.7
5.9
6.7
6.5
6.9
8.9
11.6
8.3
12.6
16.3
18.0
21.4
19.4
24.6
29.4
33.2
10.1
12.3
13.7
35
30
25
20
313
k0 ¼ ðkK1KHTAPAP½HþꢂÞ=ð1 þ KH½HþꢂÞ
Assuming KH[H+] ꢃ 1, we get
ko ¼ kK1KHTAPAP½Hþꢂ
ð7Þ
ð8Þ
308
303
Thus, ko values vary linearly with [H+] (Fig. 5). It also justifies
the inequality KH[H+] ꢃ 1 and demands that the maximum value
of KH should be of the order 10ꢀ2 as the maximum [H+] used in this
study was 3 M.
k
15
10
5
298
293
3.6. Temperature variation of ko
0
The reduction of 1 by APAP was performed at different temper-
atures. The observed rate constant (ko) values at different temper-
atures are shown in Table 1. The slope of the plot of ko versus TAPAP
(Fig. 8) divided by [H+] gives the values of kK1KH at five different
temperatures. The Eyring plot of ln(kK1KHh/KBT) versus 1/T was
0
5
10
15
20
TAPAP (mM)
25
30
35
40
Fig. 8. Plot of ko vs. TAPAP at different temperature (K). Condition: [1] = 1.0 mM;
[H+] = 1.5 M; I = 3.0 M.
found to be linear as expected (Table 2, Fig. 9) and the
D
H– and
D
S– of the reaction is estimated as 57 2 and ꢀ68 8 JKꢀ1 Mꢀ1
.
The negative entropy value further supports the possibility of
H-bonding which leads to the close proximity of the reactants [39].
Table 2
ln(kK1KHh/KBT) values at different temperatures.
103/T (Kꢀ1
)
Slope/[H+]
ln(kKHh/KBT)
4. Conclusions
3.19
3.24
3.3
3.35
3.41
0.64
0.41
0.26
0.21
0.13
ꢀ15.4
–15.8
–16.2
–16.5
–16.9
Paracetamol (APAP) promptly scavenge and mineralize the
metallo-superoxo complex, [(NH3)5Co(III)(l
-O2)Co(III)(NH3)5]5+
(1) in acid perchlorate media ([H+] = 1.0–3.0 M). The reduction of
1 follows pure first-order kinetics and the observed first-order rate
constant (ko) increases linearly with increasing paracetamol con-
centration and [H+]. The protonated form of APAP, viz. APAPH+
forms an adduct with 1 which is supported by the moderately neg-
-29.6
-30.0
-30.4
-30.8
-31.2
-31.6
ative
D
S– value (ꢀ68 8 JKꢀ1 Mꢀ1). In subsequent steps, one mole
of APAP reduces two mole of 1 to [(NH3)5Co(OH2)]3+, Co2+, O2 and
+
NH4 and itself oxidized to quinone oxime and acetic acid. This
reaction might have a potential to open up the possibility of
administering the common drugs of the same genre as APAP in effi-
ciently reducing the ROS toxicity.
K
kK
Acknowledgements
Authors thankfully acknowledge departmental research grants
of Jadavpur University.
Appendix A. Supplementary material
3.15
3.20
3.25
3.30
103/T (K-1)
3.35
3.40
3.45
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2013.04.031.
Fig. 9. Eyring plot of ln(kK1KHh/KBT) vs. 103/T.

B. Singh et al. / Inorganica Chimica Acta 406 (2013) 266–271
271
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