A program has been developed for the implementation of
a mathematical treatment which corrects for a
concentration gradient within the stopped-flow observation cell for
reversible second-order reaction kinetics
studied by longitudinal absorbance measurements. This program has
been tested using experimental kinetic
data for three selected electron-transfer cross reactions with
predicted rate constants of 1.1 × 106, 5.8 ×
107,
and 1.2 × 108 M-1
s-1, respectively. A second
gradient-corrected approach has also been applied based on
the steady-state absorbance which exists after the flow tube has been
filled with the new reaction mixture just
prior to the stopping of the flow (a permutation of the continuous-flow
method). As a third comparison, the
same data were also analyzed using a standard reversible second-order
kinetic treatment, without corrections
for the concentration gradient, by applying an appropriate time base
correction. The experimental kinetic
data were obtained using an unmodified commercial stopped-flow
instrument with a 2.0 cm observation cell,
a measured filling time of 3.8 ms, and a total dead time of 4.6 ms.
For reactions with Δε ≥ 104
M-1
cm-1,
all three methods have been shown to be capable of resolving
second-order rate constants up to and exceeding
108 M-1
s-1 under conditions where the initial
half-life is as small as 600 μs (i.e., about one-eighth the
dead
time). When the absorbance change becomes extremely small, the
steady-state approach appears to generate
the most reliable rate constant values. The most surprising
observation is that the standard second-order
treatmentwhich ignores the existence of a concentration
gradientyields rate constant values which are
virtually identical to those obtained when the gradient correction is
taken into account. The implications of
this discovery are discussed. The demonstrated ability of a
standard commercial stopped-flow instrument to
yield accurate second-order rate constants up to 108
M-1 s-1 represents
at least a 10-fold extension in the
previously presumed limits for this method.
Rate coefficients for the consumption of O atoms by their reaction with N2O have been measured, at pressures
from 130 to 500 mbar, using the high-temperature photochemistry technique. These represent the first direct
measurements of k values of the reaction. The ground-state oxygen atoms were produced by laser photolysis
of SO2, or by flash photolysis of either SO2 or O2, and monitored by time-resolved resonance fluorescence.
The results yield k(1075−1140 K) = 3.2 × 10-11 exp(−9686K/T) cm3 molecule-1 s-1 with 2σ precision
limits of ±12% and corresponding 2σ accuracy limits of ±26%. Results from several sources in the literature
indicate a high sensitivity of the O + N2O reaction system to traces of H2O, which increases the rates if
present as a contaminant. For this reason, possible effects of traces of H2O on the results were modeled.
Simulated decay curves with a hypothetical H2O contaminant were used as a test of the experimental data
reduction procedures. Although the concentration of H2O needed to significantly affect the results is small,
the amount that could have been present is even less and is shown to have had negligible effects. The results
are in qualitative agreement with a recent T ≥ 1680 K shock tube study (D. F. Davidson, M. D. DiRosa, A.
Y. Chang, and R. K. Hanson, ref ) in that extrapolation of their results to the present temperatures indicates
rate coefficients much larger than had been previously thought. However, though the results agree within
error limits for such a long extrapolation, the present results are about a factor of 4 smaller. Combined with
the results of the companion paper by Meagher and Anderson (ref , following paper in this issue), in which
the prior literature is critically reevaluated, it is found that the O2 + N2 product channel dominates at the
present temperatures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.