Comparative study of the physicochemical properties of ortho-substituted aromatic nitroso compounds

Article abstract of DOI:10.1021/je400016c

C-Nitroso compounds represent a class of chemical species with radical spin trapping capacity. They are widely employed to scavenge nitric oxide (NO) in biological and chemical systems and prospectively could be applied to reduce and control NO formation from important chemical processes. In this study, we examined the physicochemical properties of four ortho-substituted aromatic nitroso compounds, 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS), 4-nitrosobenzenesulfonate (NBS), 3,5-dimethyl-4-nitrosobenzenesulfonate (DMNBS), and 3,5-dichloro-4-nitrosobenzenesulfonate (DCNBS). These compounds, like most C-nitroso spin traps, exist in a monomer-dimer equilibrium, and only the monomeric form is active as a free radical spin scavenger. The influence of the aromatic ring substituent(s) on the dissociation of the dimer to give the monomer was investigated using UV-vis spectrophotometry. We describe the thermodynamic and kinetic parameters associated with the monomer-dimer equilibria, which allow us to provide a mechanistic understanding of NO trapping by C-nitroso compounds.

Full text of DOI:10.1021/je400016c

Article
pubs.acs.org/jced
Comparative Study of the Physicochemical Properties of Ortho-
Substituted Aromatic Nitroso Compounds
Wendy K. P. F. Venpin,† Eric M. Kennedy,*,† John C. Mackie,†,‡ and Bogdan Z. Dlugogorski†
†Process Safety and Environment Protection Research Group, School of Engineering, The University of Newcastle, Callaghan, New
South Wales 2308, Australia
‡School of Chemistry, The University of Sydney, New South Wales 2006, Australia
S
* Supporting Information
ABSTRACT: C-Nitroso compounds represent a class of
chemical species with radical spin trapping capacity. They
are widely employed to scavenge nitric oxide (NO) in
biological and chemical systems and prospectively could be
applied to reduce and control NO formation from important
chemical processes. In this study, we examined the
physicochemical properties of four ortho-substituted aromatic
nitroso compounds, 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS), 4-nitrosobenzenesulfonate (NBS), 3,5-dimethyl-4-
nitrosobenzenesulfonate (DMNBS), and 3,5-dichloro-4-nitrosobenzenesulfonate (DCNBS). These compounds, like most C-
nitroso spin traps, exist in a monomer−dimer equilibrium, and only the monomeric form is active as a free radical spin scavenger.
The influence of the aromatic ring substituent(s) on the dissociation of the dimer to give the monomer was investigated using
UV−vis spectrophotometry. We describe the thermodynamic and kinetic parameters associated with the monomer−dimer
equilibria, which allow us to provide a mechanistic understanding of NO trapping by C-nitroso compounds.
between the electron-donating substituent at the para position
and the nitroso group enhances the stability of the monomeric
form of the nitrosobenzene with respect to its dimer.4
Conversely, stabilization of the dimer form relative to
monomeric derivative is caused by ortho substitution and is
most pronounced when both ortho positions are occupied by
substituent groups other than hydrogen.5 In the case of
DBNBS, the dimer configuration is favored because of the
presence of bromo moieties at both ortho positions. We note
that in order to develop efficient nitroso traps, which by
necessity exist primarily in monomeric form, the nitro-
sobenzene should have a less electron-withdrawing substituent
at the para position and less bulky groups at the ortho position.
On the basis of these considerations, we investigated a range
of nitroso compounds, namely, 4-nitrosobenzenesulfonate
(NBS), 3,5-dichloro-4-nitrosobenzenesulfonate (DCNBS),
and 3,5-dimethyl-4-nitrosobenzenesulfonate (DMNBS). These
compounds are congeners of DBNBS, and all have the
common feature of a sulfonate group attached to the benzene
ring para to the nitroso group, which enhances their water
solubility. They are distinct, however, in that they have different
substituents at the ortho positions of the aromatic ring, namely,
bromine in DBNBS, chlorine in DCNBS, methyl in DMNBS,
and hydrogen in NBS.
INTRODUCTION
■
The release of nitric oxide formed during the sensitization of
ammonium nitrate emulsion explosives can affect the health of
underground miners unless they are trained to avoid excessive
acidification of the sensitized emulsion or are required to
evacuate during the gassing process.1 Thus, there is a practical
need to develop new technologies that minimize the risk of
exposure of workers to NO. We recently described the
application of an aromatic nitroso compound, 3,5-dibromo-4-
nitrosobenzenesulfonate (DBNBS), under conditions simulat-
ing the chemical gassing process of emulsion explosives to
effectively reduce the net amount of nitric oxide released from
the reaction mixture.2 The NO moiety in the nitroso
compound is generally believed to be responsible for trapping
the nitric oxide radical, and thus, any nitroso trap that is soluble
under aqueous conditions and does not affect the pH of the
chemical gassing reaction potentially could act as a nitric oxide
scavenger. As we established in our previous research, the rate
of DBNBS monomer formation greatly influences the overall
rate of reaction of DBNBS with NO. The pace of monomer
formation is the rate-determining step in the overall reaction.3
Consequently, if the nitroso compound exists primarily in a
monomeric form, then the reaction will occur at or close to its
maximum rate.
Thermodynamic properties were determined by means of
UV−vis spectrophotometry at temperatures ranging from 25
In the determination of the equilibrium constant for the
DBNBS dimer−monomer equilibrium (KC), it was established
that the majority of dissolved DBNBS was in the dimeric form
and only a relatively small proportion of DBNBS existed as
monomers suitable for participating in the trapping reaction.3
Fortunately, in aromatic nitroso compounds, conjugation
Received: January 5, 2013
Accepted: February 19, 2013
Published: March 14, 2013
© 2013 American Chemical Society
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°C to 60 °C. Absorption UV−vis spectrophotometry is an ideal
analytical tool for quantitative analysis of nitroso compounds as
a function of concentration, as nitroso compounds in solution
have a characteristic monomer absorption band centered in the
range from 630 nm to 790 nm due to an n−π* transition of
nonbonded electrons localized on nitrogen.6
quartz cell (1 cm optical length). A single-cell Peltier-
thermostatted cell holder (Varian, Inc.) was used to control
the temperature of the quartz cell ( 0.5 °C).
Dimer−Monomer Equilibrium Constant. The dimer−
monomer equilibrium is shown in reaction R1, and the
corresponding equilibrium constant KC is given by eq 2:
Knowledge of the equilibrium constant KC is essential in
order to calculate the concentration of the monomer capable of
trapping NO and is important during the analysis of kinetic
data obtained from the trapping reaction with nitroso spin traps
in chemical systems. We also studied the kinetics of the
dissociation of the nitrosobenzenesulfonate dimers into
monomers at different temperatures in order to calculate the
enthalpy of activation (ΔH⧧), entropy of activation (ΔS⧧), and
Gibbs free energy of activation (ΔG⧧).
k1
nitroso dimer XooY 2(nitroso monomer)
k−1
(R1)
[monomer]2
[dimer]
KC =
(2)
Under the assumption that the light absorption at or near the
wavelength of maximum absorption is due to the monomer
only, Holmes et al.12 used eq 2 and Beer’s law to obtain eq 3
(for the derivation, see ref 3):
EXPERIMENTAL SECTION
■
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎠
D
CtL
D2
2
Chemicals. DBNBS, DCNBS, DMNBS, and NBS were
synthesized using published methods.7−9 Table 1 lists relevant
information about these chemicals.
= −
+ ε
C L2
K ε
C
(3)
t
where D is the optical density (also called absorbance, D =
ε·[monomer]·L), Ct is the initial concentration of equivalent
monomer (mol·kg−1), ε is the molal extinction coefficient
(kg·mol−1·cm−1), and L is the optical path length (cm). The
equilibrium constant KC is taken as a dimensionless number,
and all of the concentration values are referenced to the
standard state. Equation 3 indicates that a plot of D/CtL against
D2/CtL2 should be linear; the intercept allows an estimation of
the extinction coefficient, which can be combined with the
slope to obtain KC.
Table 1. Data for Nitroso Compounds Used in This Work
purification
final mole
analysis
method
1H NMR
compound
NBS
source
method
fraction purity
synthesis vacuum
distillation
0.85
DMNBS
DCNBS
DBNBS
synthesis recrystallization
synthesis recrystallization
synthesis recrystallization
0.99
0.99
0.99
13C NMR
13C NMR
13C NMR
Thermodynamic Parameters. The standard enthalpy
change (ΔH°) and standard entropy change (ΔS°) for reaction
R1 were determined by applying the van’t Hoff equation using
the KC values obtained at different temperatures.
Calculated Chemical Structures of the Nitrosobenze-
nesulfonate Monomers. All of the calculations were carried
out using the Gaussian 09 software package.10 The geometries
of the nitrosobenzenesulfonate compounds were optimized at
the B3LYP/6-311+G(d,p) level of theory. Enthalpies of
solvation were determined at the same level of theory using
the polarizable continuum model (PCM)11 with water as the
solvent, as implemented in Gaussian 09. The C−N bond
dissociation energies (BDEs) of the nitrosobenzenesulfonate
compounds were calculated using eq 1:6
Kinetics of Dimer−Monomer Interchange. The rapid
dilution of a known initial concentration of nitrosobenzenesul-
fonate with distilled deionized water was studied by stopped-
flow UV−vis spectrophotometry using an RX-2000 rapid
mixing accessory from Applied Photophysics coupled to a
Varian Cary 50 UV−vis spectrophotometer. Rapid dilution
allows the dissociation of nitroso dimer and shifts the
equilibrium to favor monomer formation. Different (initial)
concentrations of the nitrosobenzenesulfonates [(3.75 to
10)·10−3 mol·kg−1] were examined. The absorbance at the
wavelength of maximum absorption was measured in order to
determine the concentration of monomer using the extinction
coefficient for each nitrosobenzenesulfonate species. The effect
of temperature on the monomer/dimer equilibrium distribu-
tion from 25 °C to 60 °C was also investigated to determine
the enthalpy and entropy of activation for the reaction of each
aromatic nitroso compound. A circulation water bath was
employed to control the temperature in the stopped-flow
module and the cell compartment.
BDE(R−NO) = ΔH°(R·) + ΔH°(NO) − ΔH°(R−NO)
(1)
where the standard state (denoted by °) is a hypothetical 1
mol·kg−1 ideal solution at 298.15 K.
Reaction R1 was incorporated into the software package
DYNAFIT,13 and the rate constants for the formation of
monomer (k1) and dimer (k−1) were obtained from global
nonlinear least-squares fitting of the monomer concentration
profile. The effect of temperature on the rate of the dimer−
monomer interchange reaction was studied using the Arrhenius
equation.
Thermodynamic Analysis of the Dimer−Monomer
Equilibrium. Samples for UV−vis spectral analysis were
prepared by dissolving synthesized DBNBS, NBS, DCNBS,
or DMNBS [equivalent monomer concentrations of (0.05 to
8)·10−3 mol·kg−1] in distilled, deionized water. For each
solution, the visible spectra (400 nm to 900 nm) at 25 °C, 30
°C, 40 °C, 50 °C, and 60 °C were recorded on a Varian Cary
50 Scan UV−vis spectrophotometer coupled with a single-cell
Peltier thermostatted cell holder (Varian, Inc.) using a sealed
Determination of the Activation Parameters ΔH⧧,
ΔS⧧, and ΔG⧧. An alternative to Arrhenius theory is transition-
state theory, which was introduced by Eyring in 193514,15 (see
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eq 4). In this approach, the activated complex is viewed a
distinct chemical species that exists at the transition state, where
the potential energy is the highest along the reaction pathway;
the activated complex is considered to be in equilibrium with
the reactants. The rate of the reaction is then given by the
product of the concentration of the activated complex and the
frequency with which these species pass to the product state.
The theory indicates that the activation parameters ΔH⧧ and
ΔS⧧ can be obtained from the temperature dependence of the
rate constant on the basis of the following equation:
ΔH⧧
RT
ΔS⧧
R
kB
h
k
T
ln
= −
+ ln
+
(4)
where k is the rate constant, T is the absolute temperature, h is
Planck’s constant, kB is Boltzmann’s constant, and R is the gas
constant. Thus, a plot of ln k/T versus 1/T (an Eyring plot)
should result in a straight line with a slope of −ΔH⧧/R, from
which the enthalpy of activation can be derived, and an
intercept of ln kB/h + ΔS⧧/R, from which the entropy of
activation can be obtained. The free energy of activation, ΔG⧧,
can then be evaluated using eq 5:
Figure 1. Plot of D/CtL vs D2/CtL2 at 25 °C for nitrosobenzenesul-
fonate compounds: , NBS; , DMNBS; ×, DCNBS; , DBNBS. D
is the optical density, Ct is the initial concentration of equivalent
monomer, and L is the optical path length.
□
○
△
ΔG⧧ = ΔH⧧ − TΔS⧧
(5)
Table 3. Nitroso Dimer−Monomer Equilibrium Constants
(KC) in Aqueous Solution at 25 °C
RESULTS AND DISCUSSION
■
a
b
compound
ε
KC
Structure and Behavior of Ortho-Substituted Nitro-
sobenzenesulfonates. The structural data estimated for the
nitrosobenzenesulfonate compounds are summarized in Table
2. All four molecules were found to be planar, in good
DBNBS
NBS
34.4
35.0
20.2
31.3
−
1.29·10−3
5.11·10−3
3.46·10−3
1.97·10−3
5.7·10−3
DMNBS
DCNBS
c
2,6-dichloro-4-bromonitrosobenzene
Table 2. Structural Parameters of the
Nitrosobenzenesulfonate Compounds
c
2,4,6-trichloronitrosobenzene
−
4.9·10−3
a
Molal extinction coefficient in units of kg·mol−1·cm−1. Standard
C−N BDE
kJ·mol−1
dC−N
dN−O
∠CNO
b
uncertainty: u(ε) = 0.5 kg·mol−1·cm−1. Standard uncertainty: u(KC) =
c
0.0003. In benzene solution at 25 °C.12
compound
Å
Å
deg
NBS
230
215
200
191
1.42
1.41
1.42
1.32
1.22
1.23
1.22
1.19
116
117
118
133
DMNBS
DCNBS
DBNBS
nitroso compounds investigated were within the range from 30
kg·mol−1·cm−1 to 35 kg·mol−1·cm−1, except for DMNBS,
whose value was around 20.2 kg·mol−1·cm−1. Theoretically,
the relatively low value of the extinction coefficient in the
visible region for nitroso compounds indicates a strong
tendency to dimerize for aromatic compounds in which the
resonance interaction between the benzene ring and the NO
group is weak.18 A lack of resonance between the NO group
and the benzene ring is normally a result of charge polarization
(i.e., the influence of strongly electron-withdrawing substitu-
ents) or steric crowding induced by ortho substitution. In
contrast, the methyl group at the ortho position in DMNBS is
neither a strongly electron-withdrawing substituent nor steri-
cally hindering in comparison with bromine and chlorine
atoms. The monomer−dimer equilibria of a series of 2,6-
dichloro-4-substituted-nitrosobenzenes have been studied in
benzene solvent, and electron-donating substituents have been
found to favor dissociation of the dimer to the monomer.12
Similarly, the fact that methyl is an electron-donating group
would qualitatively account for the large value of KC for
DMNBS, even though it has a relatively low extinction
coefficient.
agreement with the results of an experimental study on
nitrosobenzene carried out by Hanyu and Boggs.16 The
presence of an electron-withdrawing substituent at the ortho
position in the aromatic ring leads to a weakening of the C−N
bond relative to the unsubstituted parent compound, NBS.
However, we note that for all of the nitrosobenzenesulfonate
compounds, regardless of substitution, the C−N BDE was
estimated to be in excess of 191 kJ·mol−1, and thus, these
compounds are considered to be thermally stable. The C−N−
O angle increases with increasing molecular weight of the
nitroso compound. This discloses the presence of conjugation
between the nitroso group and the phenyl ring, since greater
delocalization tends to result in an enhanced C−N−O bond
angle. The latter may be demonstrated by the angles found for
DCNBS and DBNBS compared with the one obtained for
NBS.
Monomer−Dimer Equilibration. From the data pre-
sented in Figure 1, the extinction coefficient of DBNBS was
determined to be 34.4 kg·mol−1·cm−1, in good agreement with
the value of 34 kg·mol−1·cm−1 reported by Ide et al.17 Table 3
summarizes the equilibrium data for nitrosobenzene com-
pounds at 25 °C. We note that the extinction coefficients of the
Accordingly, the values of KC for the nitroso compounds
were determined to decrease in the following order: NBS >
DMNBS > DCNBS > DBNBS. This result is in good
qualitative agreement with the supposition that ortho groups
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other than hydrogen favor the dimerization of aromatic nitroso
compounds.19 For instance, it is well-known that 2,6-
dichloronitrosobenzene is strongly dimerized even in dilute
solution, while dimerization is much less favored in p- and m-
chloronitrosobenzene and 3,5-dichloronitrosobenzene.5 We
also observed an effect attributable to steric inhibition in
DBNBS as a result of the relatively large kinetic diameter of
bromine relative to the ortho substituents in the other
compounds. Thus, our data are consistent with the premise
that steric effects enhance dimerization due to the size of the
ortho subtituent in nitrosobenzene compounds.
In Table 3, we also included KC values for 2,6-dichloro-4-
bromonitrosobenzene and 2,4,6-trichloronitrosobenzene that
were evaluated by Holmes et al.12 for comparison with the
value for DCNBS. Despite the fact that halogen groups are
generally known to have electron-withdrawing character by
induction, the observation that their presence at the para
position in these two compounds favors dissociation to the
monomer suggests that halogens may also act as electron-
donating substituents, as reported by Holmes et al.12 The
presence of the sulfonate group in DCNBS, which has strongly
electron-withdrawing character, favors dimer formation; the KC
value for DCNBS is 2.5 to 3 times smaller than those obtained
for 2,6-dichloro-4-bromonitrosobenzene and 2,4,6-trichloroni-
trosobenzene, respectively. We can nevertheless note that the
data obtained by Holmes et al.12 and those reported in the
present contribution are within the same order of magnitude,
even though the compounds have slightly different chemical
structures.
DBNBS exhibiting the highest value. Likewise, the entropy and
free energy changes for the reactions were essentially equivalent
for all of the nitrosobenzenesulfonates investigated in the
present study. Nevertheless, a small stepwise increase can be
observed in these thermodynamic parameters as the size of the
ortho substituent increases, suggesting that bulkier substituent
groups favor formation of the dimer.
Kinetic Analysis of the Nitrosobenzenesulfonate
Dimer−Monomer Interchange. The rates of nitrosobenze-
nesulfonate dimer dissociation to give monomers were
investigated by UV−vis spectrophotometry. The results
obtained for k1 and k−1 by fitting the experimental
concentration profiles for the four nitrosobenzenesulfonate
compounds for the dissociation of the dimer are tabulated in
the Supporting Information. Figure 2 shows the measurements
of NBS monomer concentration with time employed to
determine the rate of dissociation of NBS dimer to monomers
at 25 °C.
Temperature Dependence of KC. The effect of temper-
ature on KC for the four nitroso compounds was also
investigated. The equilibrium data and the extinction
coefficients of the aromatic nitroso compounds over the
temperature range from 25 °C to 60 °C are provided in the
Supporting Information. The extinction coefficient for the
nitroso compounds remained essentially constant over the
temperature range studied, while an increase in the equilibrium
constant with increasing temperature was observed for the four
nitroso compounds under investigation. The increase in KC as a
function of temperature provides evidence that formation of the
nitroso monomer is favored upon heating.
Figure 2. Measurement of NBS monomer concentrations (in
mol·kg−1) for determining the rate of dimer dissociation to monomers
−1
−1
□
△
at 25 °C: +, Ct = 5 mmol·kg ; , Ct = 7.5 mmol·kg ; , Ct = 10
mmol·kg−1. Solid lines represent model predictions.
Activation Parameters. We summarize in Table 5 the
Thermodynamic Analysis of the Nitrosobenzenesul-
fonate Dimer−Monomer Equilibrium. The standard
enthalpy change ΔH° and standard entropy change ΔS° for
reaction R1 were evaluated on the basis of the van’t Hoff
equation using the KC values obtained over the temperature
range from 25 °C to 60 °C. The results are listed in Table 4.
Relatively little variance in ΔH° is observed across the series, as
the values range from 47.6 kJ·mol−1 to 53.1 kJ·mol−1, with
activation parameters for the dissociation of dimers to
Table 5. Activation Parameters for the Dissociation of
a
Nitrosobenzenesulfonate Dimers
b
Ea
ΔH⧧
ΔS⧧
ΔG⧧
compound kJ·mol−1 kJ·mol−1 J·mol−1·K−1 kJ·mol−1 Arrhenius factor
NBS
59.2
62.9
71.4
79.7
−
56.6
60.3
68.9
77.0
84.5
−73.2
−63.7
−45.6
−15.8
−6
78.4
79.2
82.6
81.8
86
2.7·109
8.4·109
6.9·1010
2.7·1012
−
DMNBS
DCNBS
DBNBS
Table 4. Standard Enthalpy, Entropy, and Free Energy
Changes for the Dimer−Monomer Equilibrium (Reaction
a
c
R1) at 298.15 K
DMNB
a
Standard uncertainties: u(Ea) = 0.3 kJ·mol−1, u(ΔH⧧) = 0.3 kJ·mol−1,
λ
ΔH°
ΔS°
ΔG°
b
u(ΔS⧧) = 1 J·mol−1·K−1, u(ΔG⧧) = 1 kJ·mol−1. Calculated at 298.15
kJ·mol−1
J·K−1·mol−1
kJ·mol−1
c
K. Measurement performed in acetonitrile.26
compound
nm
NBS
755
770
765
760
47.6
49.0
50.1
53.1
116
13.1
14.1
15.2
15.3
DMNBS
DCNBS
DBNBS
117
monomers that were obtained from the Arrhenius and Eyring
plots. Arrhenius plots for the forward and backward reactions of
the dimer−monomer interchange of the nitroso compounds, as
well as the associated Eyring plots, are provided in the
Supporting Information. The nitrosobenzenesulfonate dimers
119
123.0
a
Standard uncertainties: u(λ) = 0.05 nm; u(ΔH°) = 0.2 kJ·mol−1;
u(ΔS°) = 1 J·K−1·mol−1; u(ΔG°) = 1 kJ·mol−1.
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undergo dissociation to give monomers in a first-order process
with activation energies ranging from 59 kJ·mol−1 to 79
kJ·mol−1. The activation barriers for dimerization range from
11.5 kJ·mol−1 to 27.9 kJ·mol−1. The activation energies for the
dissociation of dimers obtained for the aromatic nitroso
compounds are lower than the reported values for aliphatic
nitroso compounds, which range from 83.7 kJ·mol−1 to 150.7
kJ·mol−1.20−23 Likewise, we note that the ΔH⧧ values are lower
than those reported for aliphatic nitroso compounds.24 This
variation is doubtlessly a distinctive characteristic between
aromatic and aliphatic nitroso compounds, as noted by Azoulay
and Wettermark when they compared o,o′-azodioxytoluene and
azodioxycyclohexane in acetonitrile.25 The possible existence of
conjugation between the NN bond and the aromatic ring
could lower the barrier to rotation about the NN bond.
Thus, the rate of dissociation of aromatic nitroso dimers is
considerably higher than that of aliphatic nitroso dimers.
While it is desirable to compare the activation parameters
obtained in the current study with reported literature values for
other nitroso compounds, most aromatic compounds are
insoluble in water and were therefore investigated in solvents
such as acetonitrile and dichloromethane. However, com-
pounds that are dissociated in a polar solvent such as methanol
or acetone have lower entropies of activation than under
circumstances where the compound is dissolved in a nonpolar
solvent such as benzene or toluene. This indicates that the
solvent effectively associates with the bond that is about to
break in the dissociation.27 For instance, Batt and Gowenlock20
and Gowenlock and Kay28 studied the dissociation of 3-methyl-
3-nitrosobutanone in various solvents and reported a decrease
in entropy of activation with increasing dielectric constant.
They reported that lower values of the entropy change are
evident with polar solvents such as ethanol or acetonitrile. In
view of this, it is anticipated that the nitrosobenzenesulfonates
will have lower entropies of activation when dissolved in water
(ε = 80) than in acetonitrile (ε = 36.6). For this reason, the
entropy of activation for DMNBS in water is lower than that for
2,6-dimethylnitrosobenzene (DMNB) in acetonitrile.
In general, the variance in the enthalpy of activation for the
nitroso compounds studied increased slightly with increasing
ortho substituent size. In comparison, the entropy of activation
highlighted significant differences in reaction rates. The value of
ΔS⧧ for a reaction provides an estimate of the change in the
order of the system as it proceeds from the reactants to the
activated complex. The relatively large and negative activation
entropy in NBS suggests that the activated complex is more
ordered than the dimer. While ΔH⧧ and ΔS⧧ provide
important information with respect to the reaction mechanism,
ΔG⧧ determines the rate of the reaction and is particularly
useful in understanding and comparing groups of compounds.
On the basis of the present results, we observe a relatively small
difference in activation free energy, and accordingly, the rate of
dimer dissociation to give monomers was the highest for NBS
among the four compounds studied. Overall, the results
disclose that variations in the electronic and steric character-
istics as a result of minor changes in the structure may
significantly change the relative amount of monomer present in
the system.
compounds to scavenge NO. On the basis of the C−N BDEs
of the nitrosobenzenesulfonates, we established that these
compounds are thermally stable. Since it is necessary for the
aromatic nitroso compounds to favor the monomeric form to
be effective in trapping NO, an investigation of the kinetic and
thermodynamic parameters of the dimer−monomer inter-
change was undertaken. From these data it is evident that
aromatic ring substituents strongly influence the reactivity. In
particular, substituents at the ortho position can greatly
influence the rate of nitric oxide scavenging. Increasing the
size of the ortho substituent favors the stability of the dimer
form of these compounds, making such ortho-substituted
species less reactive for scavenging NO.
ASSOCIATED CONTENT
■
S
* Supporting Information
Table S.1 showing the temperature dependence of KC, Table
S.2 listing kinetic data for dimer−monomer interchange, Figure
S.1 showing Arrhenius plots, and Figure S.2 showing Eyring
plots. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +61 (0)2 4985 4422. Fax: +61 (0)2 4921 6893. E-mail:
Eric.Kennedy@newcastle.edu.au.
Funding
Authors acknowledge the Australian Research Council and
Dyno Nobel Asia Pacific Pty Ltd. for funding this research.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Mohammednoor Altarawneh for his valuable
advice concerning the performance of the computational
studies. The authors also acknowledge the generous provision
of supercomputing time on the platforms of Intersect Limited
(Sydney, Australia).
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The physicochemical properties of four substituted nitro-
sobenzenesulfonate compounds were studied in order to
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