J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1197-1204
1197
Investigation into the Kinetics and Mechanismof the Reaction of NO,
with CH, and CH,O at 298 between 0.6 and 8.5 Torr: Is there a
Chain Decomposition Mechanism in Operation?
K
Peter Biggs, Carlos E. Canosa
-Mas, Jean-Marc Fracheboud, Dudley E. Shallcrossand
Richard P. Wayne*
Physical Chemistry Laboratory, South Parks Road, Oxford, UK OX1 302
The reactions CH,
system at 298
was used to detect CH,O and multi
as pair of consecutive reactions; however, a simple analytical treatment was not sufficient to describe them
+
NO,
and at pressures between 0.6 and 8.5 Torr. The laser
pass optical absorption to detect NO,. The chemical systems were studied
+
products (l), and CH,O
+
NO,
products (2),have been studied using a flow
T
=
K
-induced fluorescence (LIF) technique
-
a
because CH302was formed as one of the products in the major channel of reaction (2). This species also reacts
with NO, regenerating CH,O. Use of a numerical model to correct for this regeneration process allowed rate
parameters of k ,
be determined at 2.4 Torr. There is no pressure dependence observed for reaction (1) between
but the possibility of a slight pressure dependence for reaction (2) exists. These pressure effects are examined
using the semi empirical quantum RRK method.
=
(3.5+_ 1.0)
x
lo-'' cm3 molecule
-
'
s-' and k,
=
(2.3
&
0.7)
x
lo-', cm3 molecule
-' s-' to
1
and 2.4 Torr,
-
The importance of the nitrate radical (NO,) as a night-time
oxidant in the troposphere has become apparent in recent
years.' Although the two reactions
from which absolute concentrations of NO, could be assign-
ed. The minimum detectable [NO,] for a signal to-noise
ratio of unity with a 10 integration time was ca. 10" mol-
ecule cm-,. Experiments were performed at 298 and
between 0.6 and 8.5 Torr total pressure, with helium as the
carrier gas. The NO, was maintained in excess over the other
reactants. Initial concentrations of NO, were typically in the
-
s
T
=
K
CH,
+
+
NO, -,products
NO, +products
(1)
CH,O
(2)
would not be expected to be significant sinks for methyl and
methoxyl radicals in the troposphere, where reactions with
molecular oxygen
range (0.5
-
3.5)
x
lo', molecule cm-, and the organic rad-
5) lo',
icals were present initially at concentrations of (0.3
-
x
molecule ern-,. For the investigation of reactions (1) and (2),
methyl radicals were prepared by the reaction of
with CH,
CH,O (see later) in the reaction
F
atoms
CH,
+
0,
+
+
M
+
CH,O,
+
+
M
(3)
.
Methyl radicals reacted rapidly with NO, forming
CH,O
0, -+ HCHO
HO,
(4)
are by far the most important reaction pathways, they are
nevertheless worthy of study. First, they are examples of
radical-radical reactions where the possibility of more than
one product channel exists. Secondly, a knowledge of the
product channels and rate parameters for these reactions is
essential in the interpretation of our laboratory studies of the
interactions of the nitrate radical with CH,O,, which may
itself be involved in atmospheric chemistry (see following
paper)., No kinetic results have been reported previously for
either reaction (1) or reaction (2).
CH, NO, CH,O
+
+
NO,
( 1 4
The methoxyl radicals so produced could further react with
NO, to form products
CH,O
+
NO,
products
(2)
and a consecutive reaction sequence was established in a
similar fashion to that observed when NO, reacts with
methyl radicals., Thus, by monitoring the concentration-
time profile of the methoxyl radical by the LIF technique, the
rate coefficients k , and k , could be determined. The details of
the LIF detection of the methoxyl radical are described else
-
where.,
For calibration purposes, and in some kinetic experiments,
we generated CH,O directly in the sliding injector via the
reaction of fluorine atoms with methyl nitrite'
Experimental
1. It is similar to that
The apparatus is shown in Fig.
described in detail elsewhere., A conventional discharge-flow
apparatus is used with a double sliding-injector arrangement,
coupled to a fluorescence cell. An optical multi-pass absorp-
tion cell (12 passes, base path 10 cm) and a quadrupole mass
spectrometer were incorporated in the flow tube downstream
ofthe LIF cell. Nitrate radicals were prepared by the reaction
of fluorine atoms with dry nitric acid
F
+
CH,ONO
CH,O
+
FNO
(7)
and used this system to study reaction (2). Mass spectro-
metric analysis showed that the signal from the species FNO
at m/e
=49 remained essentially constant while CH,O
decayed, indicating that FNO chemistry does not interfere.
Materials
F
+
HNO,
+
HF
+
NO,
(5)
Nitric acid (BDH, 99.9%) was dehydrated by sulfuric acid
and detected by optical absorption at Iz
tive absorption cross section was determined experimentally
for NO, [o (1.1
titration of NO, with NO
=
662 nm.4 An effec
-
(BDH, 99.5%) in a
at ca. 258 K. Helium (BOC) was passed through two traps
held at 77 containing molecular sieve 4A (BDH) to remove
1 :2 volume-to-volume mixture and held
-
&
=
0.1)
x
lo-'' molecule cm-,] via the
K
water and an OXISORB cartridge (Messer Griesheim) to
remove oxygen. Methane (BOC) and fluorine (5% in He)
NO,
+
NO -+ 2N0,
(6)
Biggs, Peter
