2
MORIN ET AL.
temperature value of k1 (roughly a factor of three
spread) and its temperature dependence. For example,
Nielsen et al. [6] and Shallcross et al. [7] have reported
negative (E/R = –716 K) and positive (E/R = 699
K) temperature dependence of k1, respectively, in the
temperature range 298–373 K. One absolute [6] and
three relative rate measurements [6,8–10] are available
for the rate constant of OH + PNT reaction; however,
temperature dependence of the rate constant has been
investigated in only one study and in a rather narrow
temperature range T = 298–368 K [6].
[12–14], was employed at low temperatures (248–
355 K). The second reactor (Fig. 1S, in the Support-
ing Information), which was recently developed for
high-temperature kinetic studies (up to T = 1000 K),
consisted of a Quartz tube with an electrical heater
and water-cooled extremities [15]. Temperature in the
reactor was measured with a K-type thermocouple po-
sitioned in the middle of the reactor in contact with
its outer surface. The axial temperature gradient along
the flow tube measured with a thermocouple inserted
in the reactor through the movable injector was found
to be less than 1% [15].
Abstraction of an α-hydrogen atoms in organic ni-
trates by OH radicals leads to the formation of an α-
substituted alkyl radicals:
OH radicals were generated in the fast reaction of
hydrogen atoms with NO2, H atoms being produced in
a microwave discharge of H2/He mixture:
OH + CH3CH2ONO2 → CH3C•HONO2 + H2O
H + NO2 → OH + NO
(3)
(1a)
NO2 was always used in excess over H atoms. OH
radicals were detected as HOBr+ (m/z = 96/98) after
scavenging by an excess of Br2 ([Br2] ࣈ
5 × 1013
molecule cm−3, added at the end of the reactor, 5 cm
upstream of the sampling cone) via reaction:
OH + CH3CH2ONO2 → CH3CH2C•HONO2 + H2O
(2a)
These radicals are known to be unstable, dissociating
spontaneously without an energetic barrier to form a
carbonyl compound and NO2 [11]:
OH + Br2 → HOBr + Br
(4)
CH3C•HONO2 → CH3CHO + NO2
This method for OH detection was preferred to the
direct detection of these radicals at m/z = 17 (OH+)
due to significant background signal at this mass. Simi-
larly, the chemical conversion of OH to HOBr was used
for the measurements of the absolute concentrations of
the radicals: [OH] = [HOBr] = ꢀ[Br2], i.e. concen-
trations of OH were determined from the consumed
fraction of [Br2]. [Br2] was determined from the mea-
sured flow rate of known Br2/He mixtures. The possi-
ble influence of secondary chemistry on this method of
HOBr detection and their absolute calibration proce-
dure was discussed in details in previous papers from
this group [12,13].
ENT and PNT were introduced into the flow re-
actor from a 10-L flask containing nitrate–He mix-
ture or by passing helium through a thermostated
glass bubbler containing liquid nitrate and were de-
tected by mass spectrometry at their fragment peaks at
m/z = 76 (CH2ONO2+), which were much more in-
tense than the parent ones (m/z = 91 and 105, re-
spectively). All other species were detected at their
parent peaks: m/z = 44 (acetaldehyde, CH3CHO+), 58
(propanal, C2H5CHO+), 160 (Br2+), 96/98 (HOBr+),
46 (NO2+). The absolute concentrations of the ni-
trates as well as of other stable species in the reactor
were calculated from their flow rates obtained from the
measurements of the pressure drop of mixtures of the
species with helium in calibrated volume flasks.
CH3CH2C•HONO2 → CH3CH2CHO + NO2
In this respect, the measurements of the yield of corre-
sponding carbonyl compound provide the information
on the extent of H-atom abstraction from α carbon. In
the present paper, we report the measurements of the
temperature dependence of k1 and k2 in an extended
temperature range (248–500) K and the yields of the
carbonyl compounds (at T = 300 K), acetaldehyde and
propanal, resulting from α-hydrogen atoms abstraction
by OH in ENT and PNT, respectively.
EXPERIMENTAL
The gas-phase reactions of OH radicals with alkyl ni-
trates were studied at 1 Torr total pressure of helium
over the temperature range 248−500 K. Experiments
were carried out in a discharge flow tube under lami-
nar flow conditions. Modulated molecular beam mass
spectrometer was used to monitor the reactants and
reaction products in the gas phase. Depending on the
temperature range, we have used two different flow re-
actors. The first one, thermostated Pyrex tube (45 cm
length and 2.4 cm i.d.) covered with halocarbon wax
International Journal of Chemical Kinetics DOI 10.1002/kin.21037