Rate coefficients and mechanistic analysis for reaction of OH with vinyl chloride between 293 and 730 K Page: 9,437
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Reaction of OH with Vinyl Chloride
is not large, suggesting that the noncollision-dependent rate is
small. These combined experimental and modeling observations
suggest that (1,-1) and (2) are dominant at room temperature
and moderate pressures and that Cl elimination plays a minor
role.
Two possible mechanisms for explaining this behavior were
suggested by Perry et al.7 and Atkinson9 and subsequently
evaluated by Zhu et al." The first is that the OH radical addition
only occurs at the #l-carbon and the 1,2-migration of OH is rate-
determining. The second is that the addition occurs at both the
a and #3-position, but mainly at the fl-position, and the 1,2-
migration is sufficiently slow as to be negligible. Because the
elimination can proceed directly only when the OH adds at the
a-position, the elimination would play only a minor role in the
addition-initiated reactions of OH with vinyl chloride. Transition
states for the two addition reactions have been characterized
by Sekusak et al.,12 who employed spin-projected MP2 theory
with basis sets up to aug-cc-pVTZ and found fi addition to be
more favorable.
The purpose of this manuscript is to provide further verifica-
tion of the reaction mechanism of OH radical reactions with
vinyl chloride in the form of new atmospheric pressure, extended
temperature measurements from 293 to 730 K using the LP-
LIF technique. In addition, a refined QRRK analysis of the OH
addition reaction mechanism is performed by combining the
recent analyses of Zhu et al." with variational transition state
theory calculations of the rate of formation of the OH-vinyl
chloride adducts. An evaluation of the temperature-dependent
rate of H atom abstraction is also performed by utilizing G3
(MP2) ab initio calculations in conjunction with thermochemical
transition state theory.13 The most likely reaction mechanisms
based on the modeling of these reaction systems and their
consistency with the available experimental data are discussed.
Experimental Approach and Data Reduction
The experimental procedures developed for LP-LIF studies
of the reaction of OH radicals with chloroethylenes have been
previously published.14-16 A brief summary is given in the
following paragraphs.
To minimize substrate photolysis at wavelengths below 300
nm, HONO was used as a hydroxyl radical source. Parts per
million concentrations of pure HONO (>99%) were generated
as described by Febo et al.17 and Brust et al.18 HONO dissociates
primarily into NO and OH when exposed to near-UV radiation
of 351 nm. A competing dissociation channel, production of
NO2 and H atoms, has been observed to be negligible under
similar experimental conditions.19 A XeF excimer laser (Lamba
Physik Compex model 102) was used as the photodissociation
source. Initial [OH]o ranged over -(3-9) x 1010 molecules
cm-3, and was determined based on the measured excimer
fluence (9-18 mJ cm-2); the most recent published value of
the absorption cross-section for HONO, 1.54 x 10-19 cm2
molecule-1 at 351 nm;18 a quantum yield of unity;20 and
measured values of [NO2-] taken to represent [HONO]
determined using ion chromatography (-(1-3) x 1013 mol-
ecules cm-3).
Initial substrate concentrations in the reactor, based on
measured flow rates, ranged from 1.3 x 1013 to 6.4 x 1014
molecules cm-3. The absence of adsorption on the injector probe
(coated with boric acid) and reactor walls was verified usingGC/MS analysis. All experiments were conducted at a total
pressure of 740 + 15 Torr with helium as the bath gas. High
purity samples (>99%) of vinyl chloride were obtained from
Fluka Chemicals. GC/MS analyses indicated the absence of any_
No
J. Phys. Chem. A, Vol. 105, No. 41, 2001 9437
.4
-- -- - ------ - -- - - - -- ---- - -- -- -
I"0.01
5000
10000
15000
20000
Time, s
Figure 1. Background-subtracted OH decay profiles at 620 K. [CH2=
CHCl]o = 1.48 x 1013 molecules cm-3.
reactive impurities that would impact the rate measure-
ments.
The rate of disappearance of the OH may be presented as
-d[OH]/dt = k[A0] [OH] + kd[OH]
where k = bimolecular rate constant, AO= substrate concentra-
tion, kd = first-order rate for the reaction of OH with HONO
and also includes diffusion out of the reaction volume.
This relationship holds in the absence of any secondary
reactions that may form or deplete OH. Solution of this equation
yields [OH] = [OH]o exp-(k't), where k' = k + kd . OH decay
measurements were best fit by single exponential decays at all
temperatures (relative standard deviations at the 95% confidence
interval were = 5% below 620 K and = 10% above 620 K)
and were fitted by the following nonlinear expression
[OH] = [OH]o exp -(k't) + y
where y is the constant background signal level and t is the
time delay between the laser pulses. OH measurements at 560
and 620 K did not exhibit biexponential behavior (see Figure 1
for typical decay profile at 620 K) suggesting that the regime
where reformation of OH occurs is a very narrow one for this
reaction system. Because the organic concentration was much
greater than the [OH], pseudo first-order exponential OH decays
were observed and the individual temperature-dependent rate
constants were determined by k' = k[substrate] + kd, where
the bimolecular rate constant, k, is the slope of the least-squares
fit of k' vs the [organic]. OH decays were measured over two
to three decay lifetimes over a time interval of 0.2 to 30.0 ms.
Values of kd measured before and after a rate determination
were observed to be constant within experimental uncertainties,
indicating that the HONO source was stable over the course of
an experiment. Typical values of kd decreased from -250 s-1
at low temperatures to ~100 s-1 at elevated temperatures,
suggesting a reduction in the gas-phase concentration of HONO
with increasing temperature. Numerical modeling indicated that
the dominant source of OH decay in the absence of the substrate
was reaction with HONO (k298 = 4.5 x 10-12 cm3 molecule-1
s-1).16 A plot of k' vs substrate concentration at selected
temperatures is shown in Figure 2.
Experimental Results
Data were obtained from 293 to 730 K, under atmospheric
pressure conditions. OH signals decreased below acceptable
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Yamada, Takahiro; Siraj, Mohammad Masud, 1972-; Taylor, Philip H.; Peng, Jingping; Hu, Xiaohua & Marshall, Paul. Rate coefficients and mechanistic analysis for reaction of OH with vinyl chloride between 293 and 730 K, article, September 21, 2001; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc501412/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.