Oxygen-initiated chain mechanism for hydride transfer between NADH and NAD+ models. Reaction of 1-benzyl-3-cyanoquinolinium ion with N -methyl-9,10-dihydroacridine in acetonitrile

Article abstract of DOI:10.1021/jo301874q

A reinvestigation of the formal hydride transfer reaction of 1-benzyl-3-cyanoquinolinium ion (BQCN⁺) with N-methyl-9,10- dihydroacridine (MAH) in acetonitrile (AN) confirmed that the reaction takes place in more than one step and revealed a new mechanism that had not previously been considered. These facts are unequivocally established on the basis of conventional pseudo-first-order kinetics. It was observed that even residual oxygen under glovebox conditions initiates a chain process leading to the same products and under some conditions is accompanied by a large increase in the apparent rate constant for product formation with time. The efficiency of the latter process, when reactions are carried out in AN with rigorous attempts to remove air, is low but appears to be much more pronounced when MAH is the reactant in large excess. On the other hand, the intentional presence of air in AN ([air] = half-saturated) leads to a much greater proportion of the chain pathway, which is still favored by high concentrations of MAH. The latter observation suggests that a reaction intermediate reacts with oxygen to initiate the chain process in which MAH participates. Kinetic studies at short times show that there is no kinetic isotope effect on the initial step in the reaction, which is the same for the two competing processes. Our observation of the chain pathway of an NADH model compound under aerobic conditions is likely to be of importance in similar biological processes where air is always present.

Full text of DOI:10.1021/jo301874q

Article
pubs.acs.org/joc
Oxygen-Initiated Chain Mechanism for Hydride Transfer Between
NADH and NAD+ Models. Reaction of 1‑Benzyl-3-Cyanoquinolinium
Ion with N‑Methyl-9,10-Dihydroacridine in Acetonitrile
Weifang Hao and Vernon D. Parker*
Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, United States
S
* Supporting Information
ABSTRACT: A reinvestigation of the formal hydride transfer reaction of 1-
benzyl-3-cyanoquinolinium ion (BQCN+) with N-methyl-9,10-dihydroacri-
dine (MAH) in acetonitrile (AN) confirmed that the reaction takes place in
more than one step and revealed a new mechanism that had not previously
been considered. These facts are unequivocally established on the basis of
conventional pseudo-first-order kinetics. It was observed that even residual
oxygen under glovebox conditions initiates a chain process leading to the
same products and under some conditions is accompanied by a large increase
in the apparent rate constant for product formation with time. The efficiency
of the latter process, when reactions are carried out in AN with rigorous
attempts to remove air, is low but appears to be much more pronounced
when MAH is the reactant in large excess. On the other hand, the intentional
presence of air in AN ([air] = half-saturated) leads to a much greater
proportion of the chain pathway, which is still favored by high concentrations
of MAH. The latter observation suggests that a reaction intermediate reacts with oxygen to initiate the chain process in which
MAH participates. Kinetic studies at short times show that there is no kinetic isotope effect on the initial step in the reaction,
which is the same for the two competing processes. Our observation of the chain pathway of an NADH model compound under
aerobic conditions is likely to be of importance in similar biological processes where air is always present.
based solely upon whether or not the experimental data are
consistent with the pseudo-first-order relationship, i.e., the
linearity of −ln(1 − ER)−time arrays, where ER is the extent of
reaction. Application of this methodology has resulted in
conclusive evidence that neither the formation of the 1,1-
dimethoxy Meisenheimer complex derived from the reaction of
2,4,6-trinitroanisole with methoxide ion27 nor the proton
transfer reactions of simple nitroalkanes with hydroxide ion
in water28 take place by the mechanisms previously believed to
be well-established.
In this paper, we show that the reaction of MAH with
BQCN+ in AN is further complicated by a multistep competing
chain process under most circumstances. The chain process
must be initiated by the reaction of oxygen with an
intermediate, presumably a reactant complex, since the reaction
of O2 with MAH is very slow, but it is efficient even in the
presence of residual oxygen under glovebox conditions as long
as BQCN+ is present in the AN solution. We also attempt to
clarify the mechanism at short times, where the interference
from the chain process is minimized.
INTRODUCTION
■
The N-methyl-9,10-dihydroacridine (MAH)/1-benzyl-3-cyano-
quinolinium ion (BQCN+) reaction in acetonitrile (AN) is of
particular importance, since it has been studied extensively and
is considered to be a genuine example of a direct one-step
hydride transfer reaction, mainly due to the work of Kreevoy
and co-workers.1−6 The outer-sphere electron transfer (ET)
mechanism can be considered to be unlikely due to the large
standard potential difference (>1.2 V in AN) between that for
the reduction of BQCN+ and the oxidation of MAH.7
A
review,8 as well as a number of recent studies9−11 of the
mechanisms for the reactions of various cations with NADH
model compounds, illustrates that interest in these systems
remains high.
In 2003, we proposed a three-step preassociation mechanism
(Scheme 1) for the hydride transfer reaction between MAH
and BQCN+ in AN.12 In 2008, Perrin13 advanced the opinion
that our data was not conclusive and on the basis of a
nonkinetic 1HNMR study suggested that the one-step
mechanism is correct. Prior to 2008 we had found
evidence14−26 for similar complex behavior in a number of
other organic reactions, and doubt was expressed13 about our
conclusions in these papers as well.
The present work is a part of our general effort to determine
the mechanisms of fundamental organic reactions. In recent
years, we have adopted methodology for mechanism analysis
Received: August 30, 2012
Published: October 1, 2012
© 2012 American Chemical Society
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a
Scheme 1. Proposed Three-Step Mechanism for the Reaction of MAH with BQCN+ in AN
a
This mechanism was proposed in 2003.12
Figure 1. UV−vis absorption spectra for the reaction of BQCN+ (0.04 M) with MAH (0.01 M) in AN at 298 K. (a) Half-life is 1000 s. (b) Time
between spectra equals 25 min. (Inset) Absorbance−time decay curve recorded at 525 nm under the same conditions.
Table 1. Changes in the Apparent Pseudo-First-Order Rate Constants (kapp) as a Function of the Degree of Reaction Analyzed
for the Reactions of MAH with BQCN+ in AN at 298 K under Various Conditions at 430 nm
a
b
c
d
[BQCN+]
[BQCN+]/air
[MAH]
[MAH]/air
no. of half-lives analyzed 104 × kapp/s−1 103 × intercept 104 × kapp/s−1 103 × intercept 104 × kapp/s−1 103 × intercept 104 × kapp/s−1 103 × intercept
0.5
1.0
2.0
3.0
4.0
8.27
8.58
9.04
9.45
9.81
−2.30
−8.17
−26.2
−52.0
−82.5
11.8
13.2
15.3
17.3
19.2
−6.58
−24.4
−73.7
−144
−227
6.64
6.64
6.73
6.91
7.16
−0.284
−0.496
−5.64
−20.6
−49.5
6.22
6.19
6.32
6.59
6.94
1.82
2.26
−5.29
−29.9
−71.4
a
b
[BQCN+] = 0.5 mM, [MAH] = 40 mM in AN containing only residual oxygen. [BQCN+] = 0.5 mM, [MAH] = 40 mM in AN half-saturated with
c
d
air. [BQCN+] = 40 mM, [MAH] = 0.5 mM in AN containing only residual oxygen. [BQCN+] = 40 mM, [MAH] = 0.5 mM in AN half-saturated
with air.
order kinetics. Since that time our non-steady-state kinetic
methods have continually developed and been improved
significantly.27−29 Due to the negative critique of our
mechanism13 and the incomplete nature of our experimental
study,12 the data that this paper is based on were obtained using
our most recently developed kinetic techniques.27−29
The spectra illustrated in Figure 1 were obtained during the
reaction of BQCN+ (0.04 M) with MAH (0.01 M) in AN at
298 K. The absorbance due to the product MA+ is indicated by
the upward arrow in Figure 1a. The peak with maximum
RESULTS
■
Our previous work12 on this reaction was carried out using a
conventional UV−vis spectrophotometer and the kinetic
conclusions were based on data recorded in the ER−time
range from 0.05 to 0.5. The reason for this restriction was that
we considered that the data at ER < 0.05 was uncertain due to
the fact that manual mixing of the reactants was used. Under
these conditions, we found that, at reaction temperatures below
308 K, we could not observe any deviations from simple first-
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Figure 2. Pseudo-first-order plots for the reactions of BQCN+ (0.50 mM) with MAH (40 mM) (a and b) and BQCN+ (40 mM) with MAH (0.5
mM) (c and d) in AN half-saturated with air at 298 K at 430 nm.
absorbance at 525 nm, observed immediately after mixing, was
observed to decay with time and disappear as the reaction
approached completion, as shown in Figure 1b (insert). A close
examination of the wavelength region around 481−482 nm
revealed that several of the spectra intersect at about 481.6 nm
but others do not, indicating that there is no true isosbestic
point in that wavelength region. The species giving rise to the
broad absorbance band at 525 nm is a charge-transfer complex.
Similar spectra have been reported for MAH and related
donors with a number of quinones as acceptor molecules.30
The answer to the question of whether or not the reaction of
BQCN+ with MAH in AN follows a one-step mechanism, even
at temperatures below 308 K, can readily be obtained using a
modification of the conventional −ln(1 − ER)−time profiles.
All of kinetic work here is based upon the conversion of
absorbance−time profiles to −ln(1 − ER)−time profiles for the
evolution of MA+.
Thus, whether or not the reactions conform to simple
mechanism kinetics will be assessed by the fit of the least-
squares correlation lines to the experimental data. In addition
to the visual inspection of pseudo-first-order plots, the latter is
readily accomplished by the quantitative comparison of slopes
and intercepts as a function of the interval in the data analyzed
with the expected result for a one-step mechanism. The latter is
demonstrated by the data in Table 1.
The data in Table 1 allow for a close examination of
deviations from pseudo-first-order behavior of the reactions of
BQCN+ with MAH in AN at 298 K and 430 nm. The data in
column 2 show the kapp values for the reaction of excess MAH
(40 mM) with BQCN+ (0.5 mM) in AN under an attempt to
exclude air. The kapp measured over the first 0.5 half-lives (HL)
was observed to be equal to 0.000 827 s−1 and to vary uniformly
to 0.000 981 s−1 as the data segment was increased to 4.0 HL.
The intercepts of the corresponding first-order plots varied
uniformly over a range of 36, as shown in column 3. Under
comparable conditions, except that the reaction solution was
half-saturated with air, a similar pattern of kapp as a function of
the degree of reaction encompassed in the analysis was
observed, but in this case (column 4), the variations were
greater, 0.001 18−0.001 92 s−1, and the corresponding
intercepts (column 5) varied over a range of 34 over the
same set of degree of reactions. A very similar trend was
observed for the case (columns 6 and 7) when BQCN+ was the
excess reagent (40 mM) reacting with MAH (0.5 mM), in AN
under conditions where rigorous attempts were made to
exclude air. In this case, kapp varied between 0.000 664 and
0.000 716 s−1 in the same range as the other analyses. The
values of kapp (column 8) changed from 0.000 622 s−1 at 0.5 HL
to 0.000 694 s−1 at 4.0 HL with a large change in the intercept
(column 9) when the reaction solution was half-saturated with
air and BQCN+ was the excess reagent (40 mM).
A more detailed procedure used is to divide the data arrays
into 24 data segments on which the analysis is carried out. The
times recorded for the data segments correspond to the center
point of the segment. This procedure has been described in
detail.27 The successive series of rate constants (kapp) obtained
can then be plotted vs the times at the midpoints of each
segment. This procedure gives a clear indication of any changes
in the values of the apparent pseudo-first-order rate constant
(kapp) as the reaction proceeds. An advantage of the procedure
is that standard deviations (SD) in data from multiple
absorbance−time profiles are provided. The results obtained
by this method are illustrated in the following sections for
reactions carried out under the same four sets of conditions as
illustrated in Table 1.
Conventional Pseudo-First-Order Analysis of the
Reactions of MAH and BQCN+ in Which either Reactant
Acts as the Limiting or the Excess Reagent. The pseudo-
first-order analyses of the reactions in which either BQCN+ or
MAH is the excess reactant are shown in the Supporting
Information (Figure S-1). The comparison of the effect of
which reactant is the excess reagent is illustrated by the pseudo-
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Table 2. Changes in kapp and Standard Deviations (SD) as a Function of the Degree of Reaction Analyzed for the Reactions of
MAH/MAH-d2 (40 mM) with BQCN+ (0.5 mM) in AN at 298 K in the Presence of Residual Oxygen at 430 nm
a
MAH
a
data set 1
104 × kapp/s−1
data set 2
104 × kapp/s−1
MAH-d2
104 × kapp/s−1
time
2.47
104 × SD
104 × SD
104 × SD
KIEapp(1)
KIEapp(2)
segment
7.10
6.95
6.87
6.83
6.88
6.88
6.86
6.87
6.87
6.88
6.89
6.89
6.91
6.93
6.96
7.02
7.09
7.16
7.24
7.33
7.42
7.52
7.62
7.72
0.18
0.23
0.10
0.07
0.08
0.07
0.07
0.07
0.08
0.09
0.11
0.13
0.14
0.16
0.17
0.18
0.18
0.18
0.18
0.18
0.17
0.17
0.17
0.16
8.42
8.77
8.80
8.87
8.85
8.80
8.84
8.93
9.00
9.07
9.12
9.18
9.24
9.29
9.34
9.39
9.44
9.50
9.56
9.63
9.69
9.75
9.82
9.89
0.34
0.26
0.11
0.10
0.08
0.05
0.03
0.03
0.03
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.05
0.05
0.05
0.05
0.06
0.06
0.07
0.08
2.14
1.80
1.68
1.59
1.54
1.35
1.32
1.30
1.29
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
1.28
0.41
0.28
0.28
0.23
0.13
0.12
0.04
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
3.32
3.86
4.09
4.30
4.47
5.10
5.20
5.28
5.33
5.38
5.38
5.38
5.40
5.41
5.44
5.48
5.54
5.59
5.66
5.73
5.80
5.88
5.95
6.03
3.93
4.87
5.24
5.58
5.75
6.52
6.70
6.87
6.98
7.09
7.13
7.17
7.22
7.26
7.30
7.34
7.38
7.42
7.47
7.52
7.57
7.62
7.67
7.73
1
2
4.72
6.97
3
9.22
4
11.47
22.72
45.22
67.72
90.22
112.7
135.2
157.7
180.2
202.7
225.2
247.7
270.2
292.7
315.2
337.7
360.2
382.7
405.2
427.7
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
Average of 10 stopped-flow repetitions.
first-order plots in which figures on the left (Supporting
Information, Figure S-1a−c) are for the reaction in which
[MAH] is in excess and those on the right (Supporting
Information, Figure S-1a′−c′) are when [BQCN+] is in excess.
It will become obvious later that the reactant in excess has the
greatest influence on the data. The reactions were followed by
monitoring the evolution of product at 430 nm.
was carried out over 4 HL (Figure 2b), when [MAH] is in large
excess. When BQCN+ is the reactant in excess, deviation from
the correlation is barely visible during the first HL (Figure 2c)
and not nearly so severe at 4 HL (Figure 2d) as was observed in
the previous case.
Reactions in the Presence of Residual Air. Successive
Correlation Analysis of the Reactions of BQCN+ with MAH/
MAH-d2 in AN at 298 K. The application of the successive
correlation method (24-point procedure) to data obtained
under the four different sets of reactant concentrations is
illustrated in Tables 2, 3, 5, and 6 for data gathered over 1 HL
while monitoring product formation in the range from 430 to
450 nm. The data for those reactions studied in the presence of
residual oxygen are summarized in Tables 2 and 3.
The most obvious result of a visual comparison of the
pseudo-first-order plots on the left and the right of Figure S-1
(Supporting Information) is that deviations from the
experimental data from the least-squares correlation lines are
significantly greater when [MAH] is in excess than when
[BQCN+] is the excess reactant. A close examination of the
plots reveals that there are deviations in all cases but that in the
case of the plots constructed for data over less than 1 half-life,
the deviations appear to be so small that it might be considered
to be “justifiable” to attribute them to experimental error.
The effect of the presence of oxygen from air dissolved (half-
saturated) in AN on the kinetic plots is illustrated in Figure 2
for the reactions of BQCN+ (0.5 mM) with MAH (40 mM)
and BQCN+ (40 mM) with MAH (0.5 mM) over either 0.5 HL
(Figure 2a,c) or 4 HL (Figure 2b,d). Acetonitrile half-saturated
with air was used for a practical reason, i.e. the BQCN+ solution
saturated with air was mixed with the MAH solution that was
protected from air in order to ensure that no reaction with
oxygen takes place before mixing. The [O2] in the half-
saturated solution was estimated to be equal to 0.85 mM from
the solubility of O2 in AN reported.31 The deviations of the
correlation line from experimental data are obvious very early in
the reaction in Figure 2a and is extreme when the correlation
The data in Table 2 for the reaction of BQCN+ (0.5 mM)
with MAH (40 mM) at 298 K in the presence of residual
oxygen show very little variation in kapp during the early stages
of the reaction but continuously increase as the first HL is
approached. The other new information available from the data
in Table 2 is the magnitude of SD of data from repetitive
stopped-flow shots. In this case, the SD is on the order of 3−
5% of the kapp values. The three data sets in Table 2 were
obtained over a time period of about 6 months and were
completely independent of one another, and the duplicate data
sets for MAH result in significant differences in KIEapp(1) and
KIEapp(2). The reason for this may be related to differences in
residual oxygen concentrations.
In the presence of residual air the kapp obtained from the
reactions of MAH/MAH-d2 (0.5 mM) with BQCN+ (40 mM)
in AN were observed (Table 3) to be nearly independent of
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Table 3. Changes in kapp and Standard Deviations (SD) as a Function of the Degree of Reaction Analyzed for the Reactions of
MAH/MAH-d2 (0.5 mM) with BQCN+ (40 mM) in AN at 298 K in the Presence of Residual Oxygen at 430 nm
a
MAH
a
data set 1
104 × kapp/s−1
data set 2
104 × kapp/s−1
MAH-d2
104 × kapp/s−1
104 × SD
104 × SD
104 × SD
KIEapp(1)
KIEapp(2)
segment
5.95
6.06
6.01
5.99
5.98
5.99
6.03
6.06
6.07
6.07
6.07
6.06
6.05
6.05
6.04
6.04
6.03
6.03
6.03
6.02
6.02
6.02
6.02
6.01
0.54
0.47
0.38
0.31
0.24
0.12
0.07
0.05
0.04
0.03
0.02
0.02
0.02
0.02
0.03
0.03
0.04
0.04
0.04
0.04
0.04
0.05
0.05
0.05
6.25
6.36
6.34
6.32
6.31
6.32
6.33
6.31
6.29
6.28
6.27
6.26
6.25
6.25
6.24
6.23
6.21
6.21
6.20
6.19
6.18
6.17
6.16
6.15
0.36
0.23
0.15
0.12
0.08
0.04
0.04
0.03
0.03
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
1.15
1.23
1.10
1.06
1.07
1.12
1.14
1.13
1.14
1.14
1.13
1.13
1.13
1.13
1.13
1.13
1.13
1.13
1.13
1.13
1.13
1.13
1.13
1.13
0.34
0.13
0.08
0.08
0.09
0.05
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
5.17
4.93
5.46
5.65
5.59
5.35
5.29
5.36
5.32
5.32
5.37
5.36
5.35
5.35
5.35
5.35
5.34
5.34
5.34
5.33
5.33
5.33
5.33
5.32
5.43
5.17
5.76
5.96
5.90
5.64
5.55
5.58
5.52
5.51
5.55
5.54
5.53
5.53
5.52
5.51
5.50
5.50
5.49
5.48
5.47
5.46
5.45
5.44
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
Average of 10 stopped-flow repetitions.
Table 4. Apparent Rate Constants and Apparent Kinetic Isotope Effects at Short Times for the Reactions of MAH/MAH-d2 (10
mM) with BQCN+ (40 mM) in AN at 298 K in the Presence of Residual Oxygen at 440 nm (over the first 0.3% reaction)
a
a
a
data set 1
data set 2
data set 3
time/s 104 × kappH1/s−1 104 × kappD1/s−1 KIEapp 104 × kappH2/s−1 104 × kappD2/s−1 KIEapp 104 × kappH3/s−1 104 × kappD3/s−1 KIEapp segment
1
2
3
0.019
0.031
0.044
0.056
0.069
0.13
0.26
0.38
0.51
0.63
0.76
0.88
1.01
1.13
1.26
1.38
1.51
1.63
1.76
1.88
2.01
2.13
2.26
2.38
13.1
10.2
10.2
3.36
1.28
3.04
2.22
2.01
2.54
3.52
4.99
6.31
6.49
6.22
5.88
5.68
5.58
5.44
5.35
5.30
5.26
5.21
5.16
5.17
5.13
5.13
5.14
5.14
12.3
10.2
5.93
5.17
3.14
3.21
3.40
1.81
0.76
0.68
0.74
0.81
0.88
0.93
0.97
0.99
1.01
1.02
1.03
1.03
1.04
1.05
1.05
1.06
1.06
1.06
2.08
1.97
2.88
2.68
2.53
3.41
6.86
7.44
6.91
6.35
5.88
5.59
5.37
5.27
5.19
5.15
5.10
5.10
5.06
5.01
5.01
4.96
4.96
4.96
11.2
7.75
11.6
6.83
0.97
1.13
1.60
1.61
1.60
2.18
4.76
6.00
6.06
5.77
5.59
5.35
5.26
5.17
5.14
5.10
5.05
5.01
5.01
4.96
4.97
4.93
4.93
4.93
1
2
8.78
3.96
4.23
3.23
1.76
1.11
0.87
0.85
0.89
0.95
0.99
1.01
1.04
1.06
1.07
1.08
1.09
1.10
1.10
1.11
1.11
1.11
1.11
9.04
8.41
8.07
7.58
5.77
5.14
4.98
5.03
5.08
5.14
5.19
5.21
5.22
5.24
5.25
5.25
5.26
5.26
5.26
5.27
5.27
5.27
5.27
5.25
5.01
4.75
2.65
1.08
0.83
0.83
0.88
0.92
0.97
0.99
1.01
1.02
1.03
1.04
1.05
1.05
1.06
1.06
1.07
1.07
1.07
3
8.52
8.22
6.20
5.54
5.49
5.52
5.54
5.59
5.62
5.64
5.66
5.67
5.67
5.68
5.68
5.68
5.69
5.69
5.69
5.70
5.71
8.61
8.60
6.18
5.21
5.06
5.11
5.14
5.17
5.20
5.21
5.22
5.24
5.25
5.25
5.25
5.26
5.26
5.26
5.26
5.26
5.26
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
Average of 20 stopped-flow repetitions.
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Figure 3. Apparent instantaneous rate constants (kIRC) vs time plots for the reactions of BQCN+ (40 mM) with MAH (a)/(MAH-d2) (b) (10 mM)
in AN containing residual air at 298 K at 440 nm.
Table 5. Changes in kapp and Standard Deviations (SD) as a Function of the Degree of Reaction Analyzed for the Reactions of
MAH/MAH-d2 (40 mM) with BQCN+ (0.5 mM) in AN at 298 K Half-Saturated with Air at 430 nm
a
MAH
a
data set 1
104 × kapp/s−1
data set 2
104 × kapp/s−1
MAH-d2
104 × kapp/s−1
104 × SD
104 × SD
104 × SD
KIEapp(1)
KIEapp(2)
segment
8.23
8.59
8.59
8.53
8.48
8.35
8.27
8.25
8.25
8.31
8.49
8.75
9.06
9.39
9.74
10.1
10.5
10.9
11.3
11.7
12.1
12.5
12.9
13.4
0.28
0.08
0.11
0.13
0.11
0.08
0.07
0.05
0.04
0.01
0.03
0.06
0.08
0.09
0.11
0.13
0.14
0.16
0.18
0.20
0.22
0.24
0.26
0.27
13.6
13.9
13.9
14.1
14.1
14.3
14.7
15.2
15.6
16.0
16.4
16.7
17.1
17.5
17.9
18.3
18.7
19.2
19.6
20.2
20.7
21.3
22.0
22.8
0.37
0.12
0.14
0.09
0.09
0.08
0.10
0.09
0.10
0.09
0.09
0.09
0.09
0.09
0.08
0.09
0.08
0.09
0.10
0.10
0.11
0.11
0.12
0.13
2.71
2.59
2.57
2.60
2.63
2.70
2.72
2.70
2.69
2.68
2.67
2.65
2.64
2.62
2.61
2.61
2.60
2.59
2.59
2.59
2.59
2.60
2.60
2.61
0.23
0.09
0.06
0.04
0.04
0.04
0.05
0.04
0.05
0.05
0.04
0.04
0.04
0.04
0.04
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
3.04
3.32
3.34
3.28
3.22
3.09
3.04
3.06
3.07
3.10
3.18
3.30
3.43
3.58
3.73
3.87
4.04
4.21
4.36
4.52
4.67
4.81
4.96
5.13
5.02
5.37
5.41
5.42
5.36
5.30
5.40
5.63
5.80
5.97
6.14
6.30
6.48
6.68
6.86
7.01
7.19
7.41
7.57
7.80
7.99
8.19
8.46
8.74
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
Average of 10 stopped-flow repetitions.
segment number. Earlier12 using lower [MAH/MAH-d2] we
found no deviations in reactions of MAH or MAH-d2 over the
first HL at temperatures lower than 308 K. The latter
observation is confirmed by the data in Table 3. As before,
the three data sets in Table 3 were collected over a several
month period and the differences in the duplicate MAH data
sets are reasonably small, as reflected by the values of KIEapp(1)
and KIEapp(2).
followed, which essentially eliminates significant error for this
reason. The tabulated data for these experiments are shown in
Table 4. The purpose of the three data sets was to derive KIEapp
at short times. The three data sets were conducted
independently of each other over a period of months. A
comparison of the three KIEapp time profiles shows that they
start at low values and end at steady-state values.
Instantaneous rate constant (IRC) analysis26,29 of the
reactions between MAH/MAH-d2 and BQCN+ in AN is
illustrated by the plots in Figure 3. An important feature of the
plots in Figure 3 is that the experimental data for these figures
was the increase in absorbance at 440 nm. Neither the reactants
nor the CT complex has large enough extinction coefficients at
this wavelength to have significant absorbance at the low
concentration of these species. In the absence of another
Under these conditions, the [MA+] which was monitored at
430 nm is very low at short times, so the concentration of the
limiting reagents (MAH/MAH-d2) was increased to 10 mM
from 0.5 mM. This increase in [substrate] resulted in
absorbance changes that could be monitored with a higher
degree of accuracy. Although these conditions are not strictly
pseudo-first-order, only the first 0.3% of the reaction was
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intermediate at this wavelength, the expected response for MA+
evolution is a plot that approaches zero at very short times and
evolves to the steady-state value with increasing times.
Obviously, the latter is not the case and the decreasing values
of kIRC in both plots in Figure 3 correspond to the decrease in
the concentration of an intermediate on the way toward the
steady state. The same conclusion can be arrived at from the
decreasing values of kapp and the increasing values of KIEapp in
Table 4. The very low values of KIEapp at short times led to the
same conclusion.
Reactions in the Presence of Intentionally Included Air.
Data for reactions in AN half-saturated with air for reactions of
BQCN+ (0.5 mM) with MAH and MAH-d2 (40 mM) at 298 K
are summarized in Table 5. The apparent rate constants for the
reaction of MAH vary considerably, from 0.000 823 in segment
1 to 0.001 34 s−1 in segment 24 (data set 1) and from 0.000 136
to 0.000 228 (data set 2). The increase in kapp with segment
number can be attributed to a chain reaction initiated by
oxygen in which oxygen intercepts an intermediate in the
reaction, resulting in the increase in the rate of product
formation. On the other hand, kapp for the MAH-d2 reaction
was essentially invariant within experimental error over all
segments. The latter suggests that MAH-d2 is, as expected,
much less prone than MAH to react with O2.
It should be noted that the SD at the top of the columns 2, 4,
and 6 in Tables 2−6 are significant and decrease progressively
to very small values. The reason for this trend is that SD values
decrease directly in proportion to the increase in the number of
points taken into the correlation.
Kinetic Isotope Effects for the Reaction of BQCN+ with
MAH in AN at 298 K at Short Times. The observation of time-
dependent KIE has been shown to be a very effective tool in
mechanism analysis.28 It is particularly important to attempt to
estimate the KIE as zero-time is approached, since an
extrapolated KIE equal to 1 is very strong evidence that there
is no C−H bond breaking in the first step of a multistep
mechanism. The observed value of KIEapp at the shortest time
(0.97) in Table 4 is certainly associated with great enough error
to justify rounding off to 1.0. The time-dependence of KIE is
only observed at times before steady state is reached. This
implies that a plot of KIEapp vs time can be used to estimate the
time necessary to reach steady state. Although the latter is
correct, it should be pointed out that a much more accurate
H
analysis can be carried out using plots of kapp and kappD. This
allows for the fact that the time necessary for steady state to be
reached may differ for the two different substrates.
The determination of the time necessary to reach steady state
for the reactions of BQCN+ (40 mM) with MAH/MAH-d2 (10
mM) in AN at 298 K in the presence of residual air is illustrated
in Figure 3. The time to reach steady state under these
conditions can be estimated to be on the order of 0.5 s for both
isotopic substrates [MAH (Figure 3a) and MAH-d2 (Figure
3b)].
A fact that must be considered when analyzing data obtained
at very low conversions is that absorbance due to products is
very low under these conditions. As a consequence of this,
parameters based on changes in product absorbance are subject
to much greater uncertainty than those observed at higher
conversions. This is demonstrated by a comparison of the data
from three identical sets of experiments in Table 4. The first set
of experiments resulted in KIEapp ranging from 1.28 to 5.14, in
the second set from 2.08 to 4.96, and in the third set the values
ranged from 0.97 to 4.93 (KIEapp vs time plots shown in
Supporting Information, Figure S-2). The differences in the
three ranges are almost entirely due to the large differences in
the initial values. Under the reaction conditions that the data in
Table 4 were gathered, the corresponding half-lives for the
reaction of MAH were about 1000 s and that for MAH-d2 was
close to 5000 s.
The values of kapp in Table 6 are for the reaction of MAH
(0.5 mM) with BQCN+ (40 mM) in AN half-saturated with air.
The point of most interest is the fact that kapp in this case is
only moderately changed in going from segment 1 to segment
24.
Table 6. Changes in kapp and Standard Deviations (SD) as a
Function of the Degree of Reaction Analyzed for the
Reaction of MAH (0.5 mM) with BQCN+ (40 mM) in AN at
a
298 K Half-Saturated with Air at 430 nm
time/s
104 × kapp/s−1
104 × SD
segment
2.47
4.72
6.20
6.20
6.30
6.33
6.34
6.26
6.11
6.03
6.00
5.97
5.95
5.93
5.92
5.91
5.90
5.88
5.87
5.86
5.85
5.84
5.83
5.82
5.80
5.79
0.21
0.17
0.17
0.13
0.13
0.07
0.01
0.02
0.03
0.04
0.04
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.02
0.02
0.02
0.02
0.02
0.01
1
2
6.97
3
9.22
4
11.5
5
22.7
6
45.2
7
67.7
8
90.2
9
The data for the reaction of MAH (10 mM) with BQCN+
(0.5 mM) in AN in the presence of residual oxygen shown in
Table 7 are for three identical sets of reactions over the first
half-life. The data not only show relatively small SD values but
also only small differences in the values of kapp over the entire
range of segments. The reproducibility of the data for the three
experiments under identical conditions is remarkable and
consistent with that observed in other studies using the same
methodology.27,28
The data in Table 8 for the isotopic substrates encompass
conversion in the range from 0 to 0.6%. In connection with the
point we are making concerning reliability of data depending
upon the concentration of product [MA+] analyzed is clearly
illustrated by the columns labeled 104 SD. The SD values in the
first row (segment 1) are on the order of 40% of the pertinent
kapp values, while those in segment 24 are only 0.3% of the
corresponding kapp values. The low value of KIEapp in segment 1
112.7
135.2
157.7
180.2
202.7
225.2
247.7
270.2
292.7
315.2
337.7
360.2
382.7
405.2
427.7
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
Average of 10 stopped-flow repetitions.
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Table 7. Changes in kapp and Standard Deviations (SD) as a
Function of the Degree of Reaction Analyzed for the
Reaction of MAH (10 mM) with BQCN+ (0.5 mM) in AN at
298 K in the Presence of Residual Oxygen at 450 nm over 1
HL
Table 8. Changes in kapp and KIEapp as a Function of the
Degree of Reaction (over the first 0.6% reaction) Analyzed
for the Reactions of MAH/MAH-d2 (10 mM) with BQCN+
(40 mM) in AN at 298 K in the Presence of Residual Oxygen
at 440 nm
a
a
a
a
a
data set 1
data set 2
104 ×
data set 3
104 ×
average
MAH
104 ×
MAH-d2
104 ×
104 ×
104
×
/
104 ×
SD
104 ×
104 × kapp
SD
/
104 × kapp
SD
/
104 × kapp
SD
time/s
kapp
kapp
SD
H
D
kapp
/
KIEapp segment
s−1
s−1
s−1
s−1
segment
0.032
0.057
0.082
0.107
0.132
0.257
0.507
0.757
1.007
1.257
1.507
1.757
2.007
2.257
2.507
2.757
3.007
3.257
3.507
3.757
4.007
4.257
4.507
4.757
10.26
7.99
7.16
6.84
6.60
5.80
5.89
6.07
6.10
6.13
6.15
6.16
6.16
6.17
6.17
6.17
6.16
6.17
6.17
6.17
6.18
6.18
6.18
6.18
4.02
2.93
1.79
1.33
0.92
0.29
0.16
0.08
0.06
0.04
0.03
0.03
0.02
0.02
0.02
0.01
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
8.24
4.59
2.49
1.71
1.34
0.69
0.77
0.90
0.98
1.02
1.06
1.07
1.08
1.08
1.08
1.08
1.09
1.10
1.10
1.11
1.11
1.12
1.12
1.12
2.73
1.97
1.40
1.11
0.89
0.47
0.26
0.14
0.10
0.09
0.08
0.07
0.05
0.04
0.04
0.04
0.04
0.04
0.03
0.03
0.03
0.03
0.03
0.02
1.25
1.74
2.88
4.00
4.93
8.41
7.65
6.74
6.22
6.01
5.80
5.76
5.70
5.71
5.71
5.71
5.65
5.61
5.61
5.56
5.57
5.52
5.52
5.52
1
2
1.69
1.72
1.71
1.71
1.71
1.71
1.71
1.72
1.72
1.73
1.74
1.75
1.77
1.78
1.80
1.81
1.82
1.83
1.84
1.84
1.85
1.85
1.85
1.85
0.08
0.06
0.04
0.03
0.02
0.01
0.02
0.02
0.02
0.02
0.02
0.03
0.03
0.03
0.04
0.04
0.05
0.05
0.06
0.06
0.06
0.07
0.07
0.07
1.75
1.72
1.75
1.77
1.77
1.77
1.77
1.77
1.78
1.79
1.80
1.81
1.83
1.85
1.87
1.88
1.89
1.90
1.91
1.92
1.93
1.93
1.94
1.94
0.07
0.05
0.04
0.03
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.03
0.03
0.04
0.04
0.05
0.05
0.05
0.05
0.06
0.06
0.06
0.06
0.06
1.66
1.66
1.68
1.69
1.71
1.74
1.74
1.74
1.75
1.75
1.76
1.78
1.79
1.81
1.82
1.83
1.84
1.85
1.86
1.87
1.87
1.88
1.88
1.88
0.07
0.03
0.04
0.04
0.04
0.04
0.03
0.04
0.04
0.04
0.04
0.05
0.05
0.06
0.07
0.07
0.08
0.08
0.09
0.09
0.09
0.09
0.10
0.10
1.70
1.70
1.71
1.72
1.73
1.74
1.74
1.74
1.75
1.76
1.77
1.78
1.80
1.81
1.83
1.84
1.85
1.86
1.87
1.88
1.88
1.89
1.89
1.89
0.05
0.03
0.04
0.04
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.05
0.05
1
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
Average of three sets of 20 stopped-flow repetitions.
a
Average of 10 stopped-flow repetitions.
two sets of KIEapp (using a single set of kappD) ranging from 3.04
to 5.13 for set 1 and from 5.02 to 8.74 for set 2. Although the
values of KIE are different in the two sets, most likely due to
differences in [O2], the trends in the two KIEapp sets are similar,
beginning at short times at relatively low values and increasing
by 69 and 74% over time for sets 1 and 2, respectively.
In the absence of intentionally added air with MAH as
limiting reagent (condition c) the data are much more
reproducible from one data set to the next. This is illustrated
in Table 3. Under these conditions the variations in KIEapp are
much smaller, settling down at about 5.33 for set 1 and 5.44 for
set 2, respectively. The larger variations in values from
segments 1−5 are consistent with the larger standard deviations
observed for data in these segments.
The conditions for the reactions described by the data in
Table 4 are quite different from those previously discussed. The
concentration of the limiting reagents (MAH/MAH-d2) was
increased to 10 mM. The purpose of this was to allow the
reactions to be analyzed with very low conversions (0.3%) and
still produce high enough product concentrations so that the
precision in the rate constants would not suffer too greatly.
Standard deviations for the three data sets are appreciably larger
than for the data discussed above, but the overall result, the
time-dependent KIEapp, are reasonably close for the three data
sets, again with the largest variations observed in the early
reaction segments. The ranges of KIEapp were observed to be
1.28−5.14, 1.97−4.96, and 0.97−4.93 for data sets 1, 2, and 3,
surely is indicative that the latter approaches unity as reaction
time approaches zero.
DISCUSSION
■
The conventional kinetic data in Table 1 suffice to answer the
question regarding whether or not the reactions follow a one-
step or a complex mechanism. Under all conditions
studied(a) BQCN+ as limiting reagent in AN with residual
oxygen, (b) BQCN+ as limiting reagent in AN half-saturated
with air, (c) MAH as limiting reagent in AN with residual
oxygen, and (d) MAH as limiting reagent in AN half-saturated
with airthe rate constants derived and the intercepts of linear
correlation plots varied as the degree of reaction of the analyses
increased. This leaves little room for doubt that the reaction
between MAH and BQCN+ in AN follows a multistep
mechanism.
The successive correlation data in Table 2 (condition a) over
about 1 HL show that even in the absence of intentionally
added air at 298 K there are significant changes in kapp with
time. The data also show that under these conditions there are
differences in kapp in the two sets of experiments. These
differences could be a consequence of differences in the trace
oxygen concentration. We have pointed out that in another
comparison of three identical runs (Table 7) the differences
observed in kapp are very small.
The data in Table 5 are under condition b and once again we
observe that the two different sets of kappH differ and give rise to
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Article
Scheme 2. Proposed Multistep Mechanism for the Reaction of MAH with BQCN+ in AN in the Presence of Residual or
a
Intentionally Added Air at 298 K
a
The dashed lines connecting structures in steps d and e are between the propagating species in the two equations.
respectively, and the average value was 1.44 for the initial
values.
precision of the data was achieved (Table 8) under these
conditions during the first 0.6% of the reaction. At this low
degree of conversion, the deviation from pseudo-first-order
kinetics is expected to be minimal. The data in Table 8
strengthen our case that KIEapp for this reaction is time-
dependent and approaches unity as the time approaches 0. In
this case, KIEapp was observed to be equal to 1.25 at short times
and to increase relatively rapidly to a maximum value of 8.41
and then slowly settle down to a value equal to 5.52 in the later
segments of the reaction (Supporting Information, Figure S-2).
Before suggesting a mechanism that describes this massive
amount of experimental data, it is helpful to summarize the
mechanistic evidence. The most pertinent evidence is
summarized below. The first point is very important and
seems difficult to argue with.
The mechanistic significance of KIEapp approaching unity at
near zero time is that it provides very strong evidence that there
is no KIE in the first step of the reaction.28 A further conclusion
which can be arrived at from this observation, when product
evolution is followed, is that hydride (or hydrogen atom or
proton) transfer is not rate-determining under these circum-
stances. This would suggest that the mechanism of the reaction
showing these kinetic characteristics must involve a minimum
of three microscopic steps.
Data for another set of reactions under still different
conditions, 10 mM MAH and 0.50 mM BQCN+, are illustrated
in Table 7. The data of most interest is in column 7, which
shows that the average value of kapp changes smoothly from
1.70 to 1.89 over 1 HL with very small standard deviations. It is
evident that under these conditions there is no problem with
the precision of the measurements or the data, and that even in
the presence of only residual concentrations of air, there is a
definitive dependence of kapp on time.
(1) The conventional pseudo-first-order kinetics are incon-
sistent with a one-step mechanism.32
(2) The rate of the reaction is dependent upon [O2], and the
increase in kapp with time is even observed in the
presence of only residual air in a glovebox under a
nitrogen atmosphere.
(3) KIEapp values under a variety of conditions approach 1.0
at short times before proceeding to a maximum value and
then slowly decreasing.
This leads us once again to the dependence of KIEapp on the
extent or time of reaction and to the data in Table 8. In this
case, the limiting reagents are MAH and MAH-d2 (10 mM) in
the presence of BQCN+ (40 mM). We observed that high
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the CT complex, not dissociation. But what we have argued
before12 and still believe is that the reactant complex is tight by
the virtue of the structural and solvation changes that must take
place to form from the CT complex. The latter process may
have a significant entropic contribution, especially if solvation
changes lag behind bond changes, as is often proposed. A fact
that must always be kept in mind when proposing a mechanism
is that it must be consistent with the experimental evidence.
The mechanism is then generally accepted as correct until new
experimental data rule it out.
A logical question to ask is, why did we not observe
deviations from simple mechanism kinetics at temperatures
below 308 K in our 2003 study? The fact that we used a very
limited range of extent of reaction (0.05−0.50) in the earlier
study is the main factor for the latter but not the only one.
Another factor contributing to that observation is that oxygen
participation is much more prevalent when MAH is the
reactant in excess, conditions that were not examined in our
earlier study.12 The final factor is that our new kinetic
methodology, based upon the traditional first-order analysis,
that involves multiscan stopped-flow spectrophotometry allows
us to detect much smaller deviations from simple mechanism
behavior than the conventional UV−vis spectrophotometry
used in our 2003 study. Also, it should be pointed out that the
reproducibility in measurements when MAH is in large excess is
substantially lower than when BQCN+ is the reactant in large
excess.
(4) The air effect is much more prominent when [MAH] is
in large excess.
The observation of the air effect requires that O2 reacts with
some species, either reactant or intermediate. The obvious
possibility is that O2 abstracts a hydrogen atom from the 10-
position of MAH. This reaction apparently takes place but at a
very low rate (Supporting Information, Figures S-3 and S-4) in
the absence of BQCN+ in AN. It appears that the H atoms at
the 10-position of MAH require activation to make the H-atom
abstraction reaction feasible at rates high enough to be
significant in the product-forming reaction of MAH with
BQCN+. However, it is likely that the reaction does provide
enough radicals to serve as an initiator in a chain process.
Finally, the approach of KIEapp toward unity as the time
approaches zero suggests not only that there is no KIE in the
first step of the reaction but also that the mechanism involves a
minimum of three microscopic reactions. We believe that the
chain mechanism illustrated in Scheme 2 takes into account all
of the experimental data and the points listed above. The
reaction between MAH and oxygen is not included in the
scheme but must be considered to be an initiation step.
We will begin the mechanism discussion by justifying each
step in Scheme 2. In our previous investigation, we found that
the CT complex formed (step a) at a rate too rapid to be follow
by stopped-flow with an equilibrium constant close to 1.0 at
298 K.12 The CT complex could be the intermediate reacting
with oxygen. However, we do not believe that the loosely
bound CT complex would change the reactivity of MAH
sufficiently to enhance the rate of H-atom transfer to O2 to the
degree necessary for this reaction to take part in this
mechanism. We view the formation of the “reactant complex”
(step b) to involve the extrusion of solvent between the two
entities in the CT complex and movement of atoms in the latter
to an arrangement more conducive to the bond changes that
must take place in going to the transition state for hydrogen
atom (or hydride in mechanism 1) transfer (step b). We
propose that the three-dimensional structure of the reactant
complex is similar to that of the CT complex but tighter. Once
MAH radical is formed (step c), the chain mechanism (steps d
and e) takes place. The dashed lines in Scheme 2 connect chain
carriers in the two steps (d and e). The product formation step
(step 3 in Scheme 1) of the polar mechanism takes place
concurrently with the chain mechanism and under some
conditions is expected to dominate, i.e., when the substrate is
MAH-d2 and when the limiting reagent is MAH. The radical
reactions (steps d and e) are expected to take place very rapidly.
During the pre-steady-state period of the overall reaction, steps
a and b dominate, and at times approaching zero, the
mechanism predicts that KIEapp will approach unity, as was
observed experimentally.
Facts about Mechanism 1 and the Importance of the
Ratio, kb/kp. The apparent rate constant according to
mechanism 1, kapp = kpkf/(kp+ kb), can be rewritten as
kf
kapp
=
1 + kb/kp
(4)
to emphasize the importance of kb/kp in the kinetic response to
the mechanism. The experimental value of the kapp at 299 K,
where MAH (1.5 mM) is allowed to react with BQCN (9
mM), is on the order of 0.017 M−1 s−1 and the value of KCT
reported in 2003 is equal to 1 M−1.12 The data in Table 2 of
that paper for kp, kf, and kb are not experimental values but
rather were estimated from the best fit data in the temperature
range from 308 to 325 K. This was accomplished with aid of
k
app at 299 K and extrapolation of the Arrhenius plot of the best
fit data for kp.
Perrin’s13 main criticism of our mechanism is related to our
estimation of kb equal to only 0.004 s−1 at 299 K (for example).
He states that kb has to be very much larger than we report. We
do not have any argument with that statement. What we argue
is that there is no basis for the general conclusion that all of our
experimental data are erroneous and that we are chasing
artifacts.14−26 Although we no longer think the best fit rate
constants12 are applicable, since this work shows that the
mechanism is much more complex than believed in 2003, we
can now show that our best fit rate constants are not unique
and that there are an infinite number of values of kb that fit the
data, depending upon the value of kb/kp. This is shown by the
results of simple calculations, where kf is held constant at 0.019
M−1 s−1 while kb and kp are varied systematically while kb/kp is
kept constant. The value reported for kapp was 0.0165 M−1
s−1.12
Before concluding we would like to return to Perrin’s13
apparently logical argument that the rate constant (kb) that we
reported12 in mechanism 1 is too small for the dissociation of a
weakly bound complex. The complex referred to was the CT
complex, for which the equilibrium constant was reported to be
close to 1.0.12 We realized that the CT complex could not be
the species responsible for the deviations from first-order
kinetics and proposed another intermediate, which we called
the reactant complex, which most likely forms from the CT
complex. We do not know the structure of the reactant complex
and therefore cannot use quantitative terms in describing its
properties, in particular the Gibb’s free energy of conversion to
We see from the data in Table 9 that kapp remains constant
while kb is varied smoothly from 109 to 104 s−1. The
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absorbance−time profiles at 430 or 450 nm. Each experiment was
repeated at least three times. The 2000-point absorbance−time curve
data were collected over either more than 1 HL or more than 4 HL.
Absorbance−time (Abs−t) profiles for product evolution were
analyzed individually by two different procedures. The first step in
both procedures was to convert the Abs−t profiles to (1 − ER)−time
profiles. This was carried out by dividing each absorbance value by the
infinity value obtained from the product extinction coefficient and the
reactant concentration and subtracting the value from 1.0. This
procedure gave (1 − ER)−time profiles that decayed from (1 − ER) =
1 for either 1 or 4 HL, depending on which analysis procedure was
used. For pseudo-first-order kinetic analysis, the (1 − ER)−time
profiles were converted to −ln(1 − ER)−time profiles. For further
processing, the individual 4 HL (1 − ER)−time profiles were first
averaged to give the average profiles. The first kinetic procedure used
was simply a least-squares linear correlation of the −ln(1 − ER)−time
profiles to give the apparent pseudo-first-order rate constants over
either 1, 2, 3, or 4 HL. The second procedure involved recording the
Abs−t profiles over slightly more than the first HL followed by the
sequential 24 linear correlations described in the Results section.
Table 9. Magnitude of kb in Best Fit Data is Variable
Depending on kb/kp
kf/M‑1 s‑1
kp/M‑1 s‑1
kb/s‑1
1 + kb/kp
kapp/M‑1 s‑1
0.019
0.019
0.019
0.019
0.019
0.019
109
108
107
106
105
104
0.15 × 109
0.15 × 108
0.15 × 107
0.15 × 106
0.15 × 105
0.15 × 104
1.15
1.15
1.15
1.15
1.15
1.15
0.0165
0.0165
0.0165
0.0165
0.0165
0.0165
experimental data in our 2003 paper could have been fit with
any of the lines of parameters in Table 9.
Another important aspect of the kb/kp ratio is how the latter
is reflected in KIEapp. This ratio must be in the range from 0.01
to 100 in order to have any observable effect on KIEapp. The
closer to a value of 1 for this ratio, the larger the effect on
KIEapp will be. Outside of this range, the reaction mechanism
will be indistinguishable from that for the simple one-step
reaction. This fact can also be deduced from the denominator
of eq 4.
ASSOCIATED CONTENT
* Supporting Information
Figures of kinetic data under various conditions. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
CONCLUSIONS
■
The most important conclusion for the “big picture” is that the
reaction of MAH with BQCN+ in AN takes place by a multistep
mechanism. Another conclusion of some general importance is
that oxygen takes part in a chain process during this reaction
and is important even under conditions where rigorous
attempts are made to eliminate it by thorough degassing of
the solutions before placing them in a glovebox containing the
stopped-flow instrument under an atmosphere of purified
nitrogen. This leads to the conclusion that the chain pathway of
formal hydride transfer in biological systems in nature may also
involve the participation of oxygen.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vernon.parker@usu.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a grant (ARRA: CHE-0923654)
from the National Science Foundation. This support is
gratefully acknowledged.
Another important conclusion of this work as well as of our
other recent publications27,28 is that these studies verify the
value of the kinetic methods developed in our laboratory over
the past 15 years, and they continue to be developed.
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EXPERIMENTAL SECTION
■
Materials. N-Methylacridinium iodide was prepared from acridine
(Aldrich) and a 3-fold excess of methyl iodide in a minimum amount
of acetone. 10-Methyl-9,10-dihydroacridine was prepared by reduction
of N-methylacridinium iodide using sodium borohydride in dry
methanol, followed by recrystallization from absolute ethanol.33 10-
Methyl-9,10-dihydroacridine 9,9-d2 was prepared as described in the
literature.34 1-Benzyl-3-cyanoquinolinium perchlorate was prepared by
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the reaction of 3-cyanoquinoline with benzyl bromide.35 The bromide
salt was dissolved in dry acetonitrile in the presence of a 50-fold excess
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was washed with water and collected by filtration. The process was
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̈
1
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NMR (300 MHz, CD3CN): δ 6.18 (2H, s), 7.43 (5H, m), 8.10 (1H,
m), 8.32 (1H, m), 8.44 (2H, d), 9.50 (1H, s), 9.55 (1H, s).
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