4) Orban, M.; Kijrb, E. K b b , E.; Noyes, R. M. J. Am. Chem. Soc. 1972, 94, Trans. 1992,88, 917. S = 2 k~l / k~6 / &~3 &~ = 9.52 x 10" b = 2kB6kB9[R]/k~lkW[BrOS-] = 0.545 K 3 k~z / k~l = 1.6 6 = k~l / 2 k~6 = 3.33 x r = k~i k~a / k~k~s = 0.101 Q = 2 k~s k~/ k~, ' = 0.0101 0 = kB,/kB6 = 1.667 x lo3 B = k~3 / 2 k~6 = 500 T,,, = kW[Br03-]trea = 3.91t,/s Repistry No. BrO,-, 15541-45-4; gallic acid, 149-91-7; ferroin, 14708-99-7.Timaresolved fluorescmCe detection Of Cl(9j) following 266-nm laser flash photd~is Of Cl~cO/CH$CH~(DMS)/N~ mixtures has been employed to study the kinetics of the title reaction over the temperature and pressure ranges 240-421 K and 3-700 Torr. The reaction is found to be very fast, occumng on essentially every C1(2PJ) + DMS encounter. The reaction rate increases with decreasing temperature and shows a significant pressure dependence. At 297 K, for example, the rate coefficient increases from a low-pressure limit value of approximately 1.8 X cm3 molecule-' s-' to a value of (3.3 f 0.5) X cm3 molecule-' s-l at P = 700 Torr. A few experiments were carried out with CD3SCD3 or C2H5SC2Hs replacing DMS as the sulfide reactant; within experimental uncertainty, no dependence of the rate coefficient on the identity of the sulfide reactant was observed. In a separate study, time-resolved tunable diode laser spectroscopic detection of HCl has been coupled with 248-nm laser flash photolysis of Cl&O/DMS/COz/Nz mixtures to measure the HCl product yield from the title reaction as a function of pressure at T = 297 K. The HCl yield approaches unity as P -0 but decreases with increasing pressure to a value of -0.5 at P = 203 Torr. The yield experiments demonstrate that hydrogen abstraction is the dominant reaction mechanism in the low-pressure limit. With increasing pressure, stabilization of a (CH3)gCI adduct apparently becomes competitive with the hydrogen abstraction pathway. The fate of the stabilized adduct remains highly uncertain, although it clearly does not dissociate to Cl(9J) or HCl on the time scale of our experiments (several milliseconds). The potential role of the title reaction in marine atmospheric chemistry is discussed.
A laser flash photolysis−resonance fluorescence technique has
been employed to study the kinetics of the
reaction of chlorine atoms with methyl iodide as a function of
temperature (218−694 K) and pressure (5−500 Torr) in nitrogen buffer gas. At T ≥ 364 K,
measured rate coefficients are pressure independent and a
significant H/D kinetic isotope effect is observed, suggesting that
hydrogen transfer is the dominant reaction
pathway; the following Arrhenius expression adequately describes all
kinetic data at 364 K ≤ T ≤ 694 K:
k
1a
= 5.44 × 10-11
exp(−1250/T) cm3
molecule-1 s-1.
At T ≤ 250 K, measured rate coefficients are
pressure
dependent and much faster than computed from the above Arrhenius
expression for the H-transfer pathway,
suggesting that the dominant reaction pathway at low temperature is
formation of a stable adduct; at T = 218
K and P = 500 Torr, for example, k
1
= k
1a + k
1b = 3.0
× 10-11 cm3
molecule-1 s-1,
with 99.4% of the
reactivity being attributable to the addition channel 1b. At
temperatures in the range 263−309 K, reversible
addition is observed, thus allowing equilibrium constants for
CH3ICl formation and dissociation to be
determined. Second- and third-law analyses of the equilibrium data
lead to the following thermochemical
parameters for the association reaction 1b:
= − 53.6 ± 3.4 kJ mol-1,
= − 52.2 ± 3.5 kJ mol-1,
and
= − 88 ± 11 J mol-1
K-1. In conjunction with the well-known
heats of formation of Cl and
CH3I, the above ΔH values lead to the
following heats of formation for CH3ICl at 298 and 0
K:
=
82.3 ± 3.5 kJ mol-1 and
ΔH
f,0 = 91.6 ± 3.6 kJ
mol-1. Ab initio calculations using
density functional theory
(DFT) and G2 theory reproduce the experimental bond strength reasonably
well. The DFT calculations predict
a structure (used in the third-law analysis) where the C−I−Cl bond
angle is 85.2° and the methyl group
adopts a staggered orientation with a pronounced tilt toward chlorine.
Bonding in CH3ICl is discussed as
are
the implications of the new kinetic data for atmospheric
chemistry.
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