Kinetics of the brominated alkyl radical (CHBr2, CH 3CHBr) reactions with NO2 in the temperature range 250-480 K

Article abstract of DOI:10.1002/kin.20725

The gas-phase kinetics of CHBr₂ + NO₂ and CH ₃CHBr + NO₂ reactions have been studied in direct time resolved measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals were generated by pulsed laser photolysis of bromoform and 1,1-dibromoethane at 248 nm. The subsequent decays of the radical concentrations were monitored as a function of [NO₂] under pseudo-first-order conditions. The rate coefficients of both reactions are independent of bath gas (He) pressure and display negative temperature dependence under the conditions of 2-6 Torr pressure (He) and 250-480 K. The obtained bimolecular rate coefficients are k(CHBr₂ + NO₂) = (9.8 ± 0.4) × 10^-12 (T/300 K) ^{-1.65 ± 0.18} cm³ s^-1 (288-483 K) and k(CH₃CHBr + NO₂) = (2.27 ± 0.06) × 10 ^-11 (T/300 K)^{-1.28 ± 0.11} cm³ s ^-1 (250-483 K), with the uncertainties given as one standard error. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are ±25%. The reaction products identified were CBr ₂O for the CHBr₂ + NO₂ reaction and CHBrO and CH₃CHO with minor amounts of CH₃ for the CH ₃CHBr + NO₂ reaction, respectively. 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 767-777, 2012 Copyright

Full text of DOI:10.1002/kin.20725

Kinetics of the Brominated
Alkyl Radical (CHBr2,
CH CHBr) Reactions with
3
NO in the Temperature
2
Range 250–480 K
MATTI P. RISSANEN, ARKKE J. ESKOLA, RAIMO S. TIMONEN
Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, FIN-00014 Helsinki, Finland
Received 22 September 2011; revised 24 January 2012; accepted 2 February 2012
DOI 10.1002/kin.20725
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The gas-phase kinetics of CHBr2 + NO2 and CH3CHBr + NO2 reactions have been
studied in direct time resolved measurements using a tubular flow reactor coupled to a pho-
toionization mass spectrometer. The radicals were generated by pulsed laser photolysis of
bromoform and 1,1-dibromoethane at 248 nm. The subsequent decays of the radical concen-
trations were monitored as a function of [NO2] under pseudo–first-order conditions. The rate
coefficients of both reactions are independent of bath gas (He) pressure and display nega-
tive temperature dependence under the conditions of 2–6 Torr pressure (He) and 250–480 K.
−
12
The obtained bimolecular rate coefficients are k(CHBr2 + NO2) = (9.8 ± 0.4) × 10
−1.65± 0.18
3
−1
−11
(
(
T /300 K)
cm s (288–483 K) and k(CH3CHBr + NO2) = (2.27 ± 0.06) × 10
−1.28± 0.11
3
−1
T /300 K)
cm s (250–483 K), with the uncertainties given as one standard error.
Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are
25%. The reaction products identified were CBr2O for the CHBr2 + NO2 reaction and CHBrO
and CH3CHO with minor amounts of CH3 for the CH3CHBr + NO2 reaction, respectively.
2012 Wiley Periodicals, Inc. Int J Chem Kinet 1–11, 2012
±
ꢀ
C
INTRODUCTION
compounds together [1,10]. In contrast to its abun-
dance, the amount of direct kinetic studies performed
with bromoform’s primary photolysis product, CHBr2
radical, are very sparse, constituting only two pa-
pers describing the kinetics of the reactions with O2
and NO [11] and with HBr [12]. The aim of the
present paper is to broaden this data set and also to
continue our attempts to extract the factors affecting
the reactivity between carbon-centered free radicals
and NO2.
Polybromomethanes are found in considerable quanti-
ties over marine environments and coastal areas. They
mainly originate from algal production, with substan-
tial additional amounts attributable to human activities
[1–9]. At ground level, bromoform (CHBr3) frequently
carries more bromine than all the other organobromine
Correspondence to: Raimo S. Timonen; e-mail: raimo
To date there have been several direct kinetic mea-
surements of halogen-substituted, carbon-centered free
.
timonen@helsinki.fi.
ꢀ
c 2012 Wiley Periodicals, Inc.

2
RISSANEN, ESKOLA, AND TIMONEN
radical reactions with NO2. Eskola and coworkers
13–16] previously measured the rate coefficients of
with an 8-mm inner diameter and was coated with halo-
carbon wax (HW) or polydimethylsiloxane (PDMS).
The reaction gas mixture was continuously sampled
through a small hole (0.4 mm i.d.) in the wall of the
reactor and formed into a beam by a conical skim-
mer in the entrance of the vacuum chamber contain-
ing the photoionization mass spectrometer. As the gas
beam traversed the ion source, a portion of molecules
was photoionized with a resonance lamp and the ions
formed were mass selected in a quadropole mass spec-
trometer (Extrel, C-50/150-QC/19 mm rods). Mass-
selected ion signals were recorded with a multichannel
scaler (EG&G Ortec MCS plus) from 10 ms before the
laser pulse to up to 80 ms after it. Data from 3000 to
25,000 laser pulses were accumulated, and the nonlin-
ear least-squares method was used to fit an exponential
[
selected R + NO2 reactions (R = CH2Cl, CH2I,
CH2Br, CHCl2, CHBrCl, CCl2, CCl3, CH3CHCl, and
CH3CCl2). All of these are exothermic, generally fast
reactions that show no pressure dependence in the few
Torr buffer gas pressure range, display negative temper-
ature dependence and thus appear to proceed from re-
actants to products without an activation barrier. Other
halogenated alkyl radical reactions with NO2 subject
to direct studies include the relatively widely explored
CF3 + NO2 reaction, most recently studied by Bre-
heny et al. [17] with IR spectrometry (see [17] and
references there in) and CF2Cl + NO2 reaction in-
vestigated by Slagle and Gutman [18] using photoion-
ization mass spectrometry. Temperature dependencies
have been obtained for the reactions measured by Es-
kola et al. [13–16] and for the mentioned CF3 + NO2
reaction [17].
ꢁ
function [R]t = [R]0 × exp (−k t) to the acquired data.
Parameters [R]0 and [R]t are the signals proportional to
the radical concentration promptly after the laser pulse
ꢁ
In the present study, we describe the first direct mea-
surements of the rate coefficients of two brominated
radical reactions:
and at time t, and k is the first-order rate coefficient.
All experiments were conducted under conditions
where the molecular reactant was in large excess so that
only two significant processes consumed the radicals
in the reactor: reaction with NO2 and the first-order
heterogeneous loss on the walls of the reactor. This
ensured that the results obtained showed pseudo–first-
order kinetics, and the bimolecular rate coefficients for
reactions (1) and (2) could be obtained from the slopes
CHBr2 + NO2 → Products
CH3CHBr + NO2 → Products
(1)
(2)
Possible reaction pathways based on observed prod-
ucts are discussed, and the reactivity of the radicals is
briefly compared with other previously studied R +
NO2 reactions.
ꢁ
of the plots of k against [NO2]. A typical example of
this procedure is shown in Fig. 1 for the CHBr2 + NO2
reaction. The insets in Fig. 1 display an example of
a CHBr2 radical decay observed in the measurements
together with the corresponding formation profile of
CBr2O product. Figure 2 displays a similar plot for the
CH3CHBr + NO2 reaction.
EXPERIMENTAL
Details of the experimental apparatus and procedures
have been described previously [19,20], so only a sum-
mary is given here. The gas mixture flowing through a
reactor contained the radical precursor (<0.02%), NO2
in varying amounts and an inert carrier gas (He) in
large excess (>99.7%). The brominated radicals were
homogeneously generated along the tubular flow re-
actor by a pulsed, unfocused KrF (248 nm) exciplex
laser (ELI-76) photolysis of bromoform (CHBr3) and
At the beginning and at the end of each experiment,
the wall loss rate (kw) was determined in the absence
ꢁ
of NO2 (i.e., k when [NO2] = 0). Radical precursor
concentrations were reduced until the observed wall
loss rate did not depend on the concentration and an
exponential fit to the radical signal showed no devia-
tion from the first-order decay. Once this was accom-
plished, it was presumed that all radical–radical and
radical–atom reactions occurring in our system had
negligible rates compared with the first-order reactions
under study.
1,1-dibromoethane (CH3CHBr2). Output fluence of the
photolysing pulse was measured (Gentec ED-200) in
each experiment and was typically between 10 and 25
The CHBr2 radicals were generated in the photoly-
sis of CHBr3, where the possible dissociation pathways
following single photon absorption at 248 nm are [21]
−2
mJ cm . The laser was operated with a 5-Hz repeti-
tion rate, and the gas flow velocity through the tubular
temperature controlled region of the reactor was held
−1
at about 5 ms . This ensured that the gas flowing
through the reaction volume was always completely
replaced between laser pulses. The reactor tube used
in measurements was made of seamless stainless steel
CHBr3 + hv (248 nm) → CHBr2 + Br (3a)
→
CHBr + Br2 (3b)
→ CBr2 + HBr (3c)
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

KINETICS OF THE BROMINATED ALKYL RADICAL REACTIONS WITH NO2
3
2
2
40
00
CHBrCH3
CHBr2
250
–
10
0
20 40
60 70
–
10
0
20
40
60 70
Time / ms
Time / ms
200
150
1
60
20
1
CH CHO
′
3
k
′
k
80
1
00
–
10
0
20 40
Time / ms
60 70
40
CBr O
2
CHBrO
0
20
40
60
80
50
Time / ms
0
–
10
0
20 40
20 40
60 70
60 70
–
10
0
0
5
10
15
20
Time / ms
12
-3
0
[
NO ] / 10 cm
2
0
2
4
2
6
8
10
1
2
-3
Figure 1 A typical plot of the measured pseudo–first-order
rate coefficients k as a function of [NO2] for a set of exper-
[
NO ] / 10 cm
ꢁ
iments conducted to determine the bimolecular rate coeffi-
cient of the CHBr2 + NO2 reaction at 298 K and 5.2 Torr
He. The upper inset is an ion signal profile observed for the
CHBr2 radical decay, from which k was obtained, and the
lower inset shows the concomitant product formation profile
of CBr2O. The ion signals were recorded under the conditions
Figure 2 A plot of the pseudo–first-order rate coefficients
ꢁ
(k ) against [NO ] of the CH CHBr + NO reaction at 298 K
2
3
2
and 6.0 Torr He. The ion signal profiles shown in insets
were recorded under the conditions of the solid square in
ꢁ
1
2
−3
17
the plot: [NO ] = 3.3 × 10 cm , [He] = 1.9 × 10
2
−3
cm . The lines drawn through the data in the insets are
first-order exponential fits, with decay and formation coeffi-
1
3
−3
,
of the solid square in the plot: [NO2] = 1.1 × 10 cm
17
−3
ꢁ
−1
ꢁ
[
He] = 1.7 × 10 cm . The first-order exponential fits
cients: k (CH CHBr, m/z = 107) = 131 ± 8 s , k (CHBrO,
3
d
f
ꢁ
to the signals gave decay and formation coefficients: k
−1
ꢁ
d
m/z = 108) = 114 ± 16 s , and k (CH3CHO, m/z = 44) =
f
−1
ꢁ
(
1
CHBr2, m/z = 173) = 125 ± 7 s , k (CBr2O, m/z =
−1
.
f
88 ± 18 s
−1
86) = 107 ± 8 s , where uncertainties are one standard
deviation (1σ).
we estimated the initial CH3CHBr radical concentra-
tions from the precursor decomposition signal; the de-
pletion of the CH3CHBr2 signal due to the laser pulse
photolysing the gas mixture. Initial radical concentra-
The channel (3a) has been described as the almost
exclusive reaction pathway of photolysis of CHBr3 at
248 nm [21].
11
tions were determined to be between 0.8 × 10 and
The photodissociation of CH3CHBr2, was used to
produce the CH3CHBr radicals [22]:
1
1
−3
4
× 10 cm in all measurements. NO2 concentra-
tions were measured by a pressure change in a cali-
brated volume method and were corrected for dimer-
ization, as described below.
CH3CHBr2 + hv (248 nm) → CH3CHBr + Br (4a)
→
Other products (4b)
The resonance lamps used in the study to
seek and detect radicals and products were a Cl-
lamp (8.9–9.1 eV) for CHBr2, CH3CHBr, and
HCO; a H lamp (10.2 eV) for CHBr2, CH3CHBr,
CH3CHBrO, CH3CHBrNO2, CHBr2NO2, CBr2NO,
CHBr2O, CHBrO, CBr2, CBr2O, CHBr, BrNO2,
BrNO, BrO, CH3CHO, CH3CH, CH3, HCO, HNO2,
Initial CHBr2 radical concentrations in the experiments
were estimated from the known absorption cross sec-
tion of bromoform [21], laser intensity, and concentra-
tion of the precursor in the gas flow. Because the ab-
sorption cross section for CH3CHBr2 is not available,
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

4
RISSANEN, ESKOLA, AND TIMONEN
Table I Results and Conditions of the Experiments Used to Measure Rate Coefficients of Reactions R + NO2 →
a
Products (R = CHBr2 and CH3CHBr)
[He]/(1017 cm
−3
)
[NO2]/(1012 cm
−3
)
k/(10
−12
cm s
3
−1 b
)
Reactor Coating
−1
kwall /(s )
T /(K)
R = CHBr2, (CHBr2 + NO2 → Products)
−12
−1.65± 0.18
3
−1
k(CHBr2 + NO2) = (9.8 ± 0.4) × 10
(T /300 K)
cm s
288
288
288
298
338
363
383
433
483
0.80
1.61
1.98
1.62
1.60
1.57
1.59
1.61
1.46
2.0–19.6
1.9–18.4
2.0–14.6
4.9–18.5
2.2–22.1
4.2–26.5
6.4–32.0
6.9–33.6
5.8–38.5
10.4 ± 0.80
10.9 ± 0.65
10.1 ± 0.12
10.5 ± 0.33
8.76 ± 0.60
5.94 ± 0.45
6.30 ± 0.44
6.04 ± 0.44
4.38 ± 0.34
HW
12
19
19
19
8
10
52
57
57
HW
HW
HW
HW
HW
PDMS
PDMS
PDMS
c,d
c
e
c,d
R = CH3CHBr, (CH3CHBr + NO2 → Products)
−11
−1.28± 0.11
3
−1
k(CH3CHBr + NO2) = (2.27 ± 0.06) × 10
(T /300 K)
cm s
250
267
298
298
298
298
336
393
433
483
1.85
1.74
0.78
1.62
1.69
1.94
1.58
1.45
1.38
1.36
2.8–8.5
2.2–5.1
1.8–8.7
2.0–13.5
4.6–11.8
2.8–8.5
2.5–9.8
2.0–8.4
2.0–10.2
2.3–13.3
26.6 ± 2.89
29.4 ± 1.46
24.5 ± 3.27
21.7 ± 2.21
22.3 ± 2.03
21.6 ± 0.89
19.3 ± 1.24
17.4 ± 1.11
14.6 ± 0.39
11.5 ± 1.70
HW
HW
HW
HW
HW
58
74
33
60
50
46
47
70
72
63
e
c,d
c,d
e
HW
HW
PDMS
PDMS
PDMS
Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are ± 25%. Abbreviations: HW, halocarbon wax;
PDMS, polydimethylsiloxane.
a
12
−3
13
−3
Range of precursor concentrations used: (3–10) × 10 cm for CHBr
3
and (1.6–4.4) × 10 cm for CH
3 2
CHBr .
b
c
Statistical uncertainties shown are one standard error.
11
−3
Estimated initial radical concentration was under 1 × 10 cm . In all the other experiments, the estimated initial radical concentration
1
1
−3
was under 4 × 10 cm
.
d
≈ 6–10 mJ cm−2) and low precursor concentration ([CHBr
12
−3
Experiments performed with low laser power (E
p
3
] ≈ (3–5) × 10 cm
,
1
3
−3
[
CH
3
e
CHBr
2
] ≈ (1.6–2.2) × 10 cm ).
≈ 20–25 mJ cm−2) and high precursor concentration ([CHBr
] ≈ 10 cm , [CH
13
−3
Experiments performed with high laser power (E
p
3
3 2
CHBr ]
1
3
−3
≈
(3.4–4) ×10 cm ).
HNO, and NO; and a Ne-lamp (16.7 and 16.9 eV) for
CHBr2NO2, CHBr2NO, CBr2O, CHBrOCH3 CHBrO,
CH3CHOBr2, BrO, BrNO2, BrNO, HBr, Br, HNO,
HNO2, and CO. All the kinetics were measured
with a chlorine lamp. However, a few radical de-
cay profiles were obtained using a hydrogen lamp
for comparison. No differences, other than change
in the signal intensity and background level were
observed.
RESULTS AND DISCUSSION
The results and conditions of the rate coefficient mea-
surements are presented in Table I. The stated un-
certainties are statistical one standard error obtained
from fits to the experimental data. The estimated over-
all uncertainty in the measured bimolecular rate co-
efficients is about ± 25%, which was estimated from
uncertainties in determined NO concentrations and
2
Radical precursors, CH3CHBr2 (Acros Organics;
purity 99%) and CHBr3 (Merck; purity 96%) were
degassed by several freeze–pump–thaw cycles before
use. The NO2 gas (Merck; purity 98%) was diluted in
helium to form a 6%–12% mixture and was stored
in and used from a blackened Pyrex bulb. Helium
uncertainties in the measured pseudo–first-order rate
coefficients. The NO concentrations were determined
2
with the pressure increase in a known volume method
and thus contain uncertainties from the calibration of
the known volume, measurements of the time the pres-
sure increased in this volume, the uncertainties of the
measured main gas flow velocities and pressures, and in
addition, the uncertainty of the purity of the prepared
(Messer-Griesheim; purity 99.9996%) was employed
as supplied.
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

KINETICS OF THE BROMINATED ALKYL RADICAL REACTIONS WITH NO2
5
NO2 gas mixture. The uncertainties of the fitting of
the pseudo–first-order rate coefficients to the decay-
ing radical signals mainly resulted from the random
noise in the measured temporal signals (see Figs. 1
and 2).
tolytic precursors (CHBr3 and CH3CHBr2) and NO2,
which are all the time in great excess over radical con-
centrations. This was verified by performing simula-
tions of the current reaction system. The simulations
showed that, for example, the lifetime of O-atoms in
the system is in the order of half a millisecond or
less by considering the fast reactions of CHBr3 + O
and NO2 + O only, which are all the time present
in the CHBr2 + NO2 experiments (k(CHBr3 + O)=
In calculating the reactant concentrations, attention
has to be paid to the dimerization of NO2 to N2O4 in the
storage bulb and its decomposition back to NO2 in the
reactant flow. The unimolecular decomposition [23,24]
−1
−10
3
−1
rate of N2O4 is about 1000 s under the conditions
under which the reactant flow rates were measured
6.6 × 10
cm s and k(NO2 + O) = 1.04 ×
−11
3
−1
10 cm s ) [23].
(
about 8 Torr and 298 K). Owing to the rapid conver-
The usable temperature range in the experiments
was extended by applying two different reactor tube
coating materials. When PDMS was used as a reactor
coating, the observed first-order wall loss rates were
higher than when using HW coating (Table I). Other-
wise, the obtained results are consistent with each other
and the determined bimolecular rate coefficients fitted
to the same temperature dependency for both used coat-
ing materials (see Fig. 3). Measurements of the CHBr2
+ NO2 reaction rate coefficients were not continued be-
low 288 K due to the instability of the observed CHBr2
radical wall loss rates, that is kw(CHBr2) became much
higher and irregular at temperatures lower than T =
288 K and the experiments were discontinued (accord-
ing to the NIST recommendation [26] Tfus(CHBr3) =
279 ± 10 K with six different determinations; see orig-
inal references from [26]).
Tests were also performed to investigate the rate co-
efficients dependence on helium bath gas density. The
same rate measurements were carried out at nearly
three times higher bath gas density without changing
the concentration of the reactants. The obtained rate
coefficients were the same within statistical uncertain-
ties, suggesting that the title reaction rate coefficients
are pressure independent within the experimental range
(2–6 Torr).
CBr2O (m/z = 186) was observed as a product of
the CHBr2 + NO2 reaction (Fig. 1). In addition, a
weak formation signal attributed to CHBrO (m/z =
108) was detected. An implication that the measured
CBr2O (m/z = 186) signal originates from the studied
reaction is that it has the same first-order formation
rate as the CHBr2 (m/z = 173) radical decay obtained
from sequential measurements (see Fig. 1). Other po-
tential products that were sought but not detected for
the CHBr2 + NO2 reaction included CHBr2NO2 (m/z
= 217, 219), CHBr2NO (m/z = 201), CBr2NO (m/z =
200), CHBr2O (m/z = 189, 191), CHBr (m/z = 92),
BrNO2 (m/z = 125), BrNO (m/z = 109), BrO (m/z =
95), CHO, CO, HNO2, and HNO.
sion of dimers to monomers compared with the rather
long residence time in a measuring volume (flow from
a high p source bulb to a low p measuring volume),
the conversion from concentrated to dilute gas happens
fast. According to equilibrium thermodynamics [25],
about 31% (depending on the used concentration) of
NO2 is in the form of N2O4 in the source bulb. This
fraction decreases to about 1% in the measuring vol-
ume and is below 0.1%, at the reactor inlet. Such a
small amount of N2O4 in the reaction mixture is of no
concern, even though N2O4 has a larger absorption co-
efficient than NO2 at 248 nm [23]. Photolysis of N2O4
at this wavelength produces only NO2 molecules in
various excited states, which are quenched within the
first millisecond after the photolysing laser pulse. The
first millisecond of the radical decay signal is always
omitted from the analysis of the first-order exponential
decay rate to ensure that only kinetics of thermalized
species are observed. Hence only the fraction of NO2
that is in the form of N2O4 in the source bulb, in ad-
dition to the total p(NO2, N2O4), is needed for the
reactant concentration calculations and is calculated
using equilibrium thermodynamics [25].
Photodissociation of NO2 at 248 nm [23], which
produces ground state oxygen atoms and NO in the
system, could be a source of problems in the rate
determinations. In this case, the possible problems
could include (i) production of the measured radicals in
chain reactions with O-atoms and precursor molecules,
effectively decreasing the observed first-order rate
coefficients; (ii) enhancement of the determined rate
coefficients when NO and O are reacting with the mea-
sured radicals; or (iii) lowering of the concentration of
available NO2 molecules resulting in decreased rate co-
efficients. Typical initial oxygen atom and NO concen-
10
10
−3
trations were between 1 × 10 and 2 × 10 cm and
10
−3
never above 3.5 × 10 cm and were estimated from
the measured laser intensities and absorption cross sec-
tion of NO2 [23]. This small amount of O-atoms/NO-
molecules cannot cause any of the problems (i)–(iii)
presented above, as the low O-atom and NO-molecule
concentrations are effectively scavenged by the pho-
According to previous studies of analogous carbon-
centered free radical reactions with NO2 [27–29],
the expected products of the CHBr2 + NO2 reaction
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

6
RISSANEN, ESKOLA, AND TIMONEN
the total energy of the parent CHBr2O radical gained
6
−1
CH CH
from the reaction exceeds about 45 kcal mol . If the
product CHBr2O is formed and it is formed with con-
siderable amount of internal excitation (denoted by an
asterisk) then the oxygenated products could be ex-
plained through the following reaction scheme:
3
2
4
CH CHCl
3
CH CHBr
∗
3
CHBr + NO → CHBr O + NO
(1a)
(1b)
(5a)
(5b)
(5c)
2
2
2
2
→
Other products
CHCl2
∗
CHBr2O → CBr2O + H
→
→
CHBrO + Br
Other products.
1
k
CHBrCl
0
0
.8
−
1
However, 45 kcal mol is a fairly high excitation and
could be produced only in a very exothermic reaction.
We examined the thermochemistry of reaction (1a) us-
ing the literature [12,32–34] (ꢀfH(CHBr2) = 47.6 kcal
.6
0
.4
CHBr2
−1
−1
mol [12], ꢀfH(CHBr2O) = 21.3 kcal mol [32–
−1
3
=
4], ꢀfH(NO2) = 7.9 kcal mol [33], and ꢀfH(NO)
2
00
300
400 500
−1
21.6 kcal mol [33]) and calculated the exother-
−1
micity of the channel (1a) to be about 13 kcal mol
;
T / K
clearly too low to explain the observations. Also, it
should be pointed out that the reaction thermochem-
istry calculation is very unlikely uncertain by as much
Figure 3 Double-logarithmic plot of the measured bi-
molecular reaction rate coefficients as a function of T to-
gether with selected previously published results. The cur-
rent results are indicated by filled symbols. The data for
−1
as 30 kcal mol
.
−
12
The assignment of the observed CBr O as a product
2
CHCl2 + NO2 reaction [k = (8.90 ± 0.16) × 10
(T /300
−
1.48± 0.13
of the CHBr + NO reaction could be thought of be-
K)
CHBrCl + NO2 reaction [k = (8.81 ± 0.28) × 10
] (open triangles) was obtained from [13], for
2
2
−
12
(T /300
] (stars) from [14], for CH3CHCl + NO2 re-
ing problematic because it could have been postulated
−
1.55± 0.34
K)
action [k(CH3CHCl + NO2) = (2.38 ± 0.10) × 10
to form in a CBr2 + NO2 reaction, if CBr2 radical was
−11
produced in the CHBr photolysis (3c). Zou et al. [21]
3
−1.27± 0.26
(
T /300 K)
] (squares), and CH3CH2 + NO2 reac-
11
have studied the photolysis of bromoform at 248 nm
−
tion [k(CH3CH2 + NO2) = (4.33 ± 0.13) × 10
(T /300
0.34± 0.22
] (circles) from [16]. Units of the rate coeffi-
using vacuum ultraviolet (VUV) ionization photofrag-
ment translational spectroscopy. They could not find
evidence of significant CBr2 production, considering
also the secondary dissociation processes of CHBr2.
Xu et al. [35] have studied the photodissociation of bro-
moform at 234 and 267 nm by ion velocity imaging and
have not observed anything suggesting the formation of
the CBr2 radical. Furthermore, we have not been able
to observe any signal that could have been assigned
to the CBr2. This was already thoroughly investigated
and discussed by Seetula and Eskola in their CHBr2 +
HBr study [12], where the production of CBr2 could
have had more serious consequences.
−
K)
3
−1
cients are cm s . [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
are CHBr2ONO and CHBr2NO2. From these, the ni-
trite product (CHBr2ONO) preferably decomposes to
CHBr2O and NO [27–29] and the formed alkoxy rad-
ical (CHBr2O) is expected to promptly dissociate fur-
ther to CHBrO and Br [11,30–32]. Bayes et al. [11]
have postulated that any substituted methoxy radical
containing merely one bromine atom will dissociate
rapidly, even at atmospheric pressures.
Drougas and Kosmas [32] have investigated the
CHBr2O radical computationally and have calculated
the Rice–Ramsperger–Kassel–Marcus rate coefficients
for the different unimolecular decomposition chan-
nels. They found that although the C Br bond scis-
sion process (5b) is the energetically most favorable
pathway, the C H bond scission (5a) and the forma-
tion of CBr2O prevails in the high-energy region when
There also exists indirect evidence that the observed
CBr2O does not come from the CBr2 + NO2 reaction.
Eskola et al. [15] measured the rate coefficients of the
CCl2 + NO2 reaction resulting in a room temperature
−13
3
−1
value of k300 K = 1.76 × 10
cm s . In line with this
(see below) and previous studies, the rate coefficients
of halogenated carbon-centered free radical reactions
with NO2 are not affected much by substituting Cl
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

KINETICS OF THE BROMINATED ALKYL RADICAL REACTIONS WITH NO2
7
with Br atoms. Taking this into account, the observed
CBr2O formation kinetics are much too fast to be pro-
duced in a CBr2 + NO2 reaction, that is, k300 K(CHBr2
(m/z = 109), BrO (m/z = 95), CHO, CHCH3, HNO2,
and HNO.
Drougas and Kosmas [38] and Orlando and Tyn-
dall [39] have investigated the unimolecular decom-
position of the CH3CHBrO radical and have found
that unlike the HCl elimination from the analogous
CH3CHClO radical [39], the C Br bond scission and
formation of CH3CHO and Br is much more probable.
We could not detect CH3CHBrO in the product search,
but this cannot be taken as a proof of its absence since
the alkoxy species are known to be difficult to detect
with photoionization using resonance gas lamps in the
millisecond time scale [27]. Expected alkoxy radicals
were sought in all of the previous R + NO2 studies
[13–16,27,40], but the attempts were always unsuc-
cessful. Most likely their detection in the millisecond
time window is not possible due to their rapid dissoci-
ation caused by internal excitation from the reaction.
Another possibility for their absence is photodissocia-
tion while attempting ionization, and their successful
detection may require a tunable UV source [41] for
ionization near the threshold.
−12
+
NO2)/k300 K(CCl2 + NO2) ≈ 9.8 × 10 /1.76 ×
13
−
10
≈ 55.7. Finally, it should be noted that as all
of the title reaction (1) kinetics were measured with
a chlorine lamp, the CBr2 ion signal cannot interfere
with the measurements, because CBr2 has an ioniza-
tion energy (IE) above the energy of the chlorine lamp
(
IE(CBr2) = 10.11 eV [36]).
The mechanism of the CHBr2 + NO2 reaction re-
mains unclear. Considering the previous studies and the
reasoning presented above, it seems that the observed
CBr2O is very unlikely formed through the CHBr2O
alkoxy radical (reactions (1a) and (5a)). Another pos-
sible pathway to product formation is via a four-center
transition state; first postulated for the NO2 reaction
by Sugawara et al. [37] in a CF3 + NO2 reaction and
later by us to explain the CH3CHO formation in the
CH3CHCl + NO2 reaction [16]. The transition state
for the CHBr2 + NO2 reaction (1c) could be composed
of a ring with C, O, N, and H atoms:
If the primary product is CH3CHBrO and it is
formed with substantial internal excitation, then the
observed products CH3CHO, CHBrO, and CH3 could
be formed in a reaction sequence:
(1c)
which dissociates to CBr2O and HNO fragments. The
HNO formation, despite multiple attempts with dif-
ferent ionization energies, could not be verified in the
measurements, probably due to the low detection sen-
sitivity of the apparatus to HNO.
Also another similar type of transition state (1d)
can be proposed, with C, O, N, and Br forming the
four-center ring structure:
∗
CH3CHBr + NO2 → CH3CHBrO + NO (2a)
→
Other products
(2b)
∗
CH CHBrO → CH CHO + Br
3
3
(6a)
(6b)
(6c)
→
→
CHBrO + CH3
Other products.
The theoretical study by Drougas and Kosmas [38]
showed that dissociation routes (6a) and (6b) are the
most prominent with associated energy barriers of
1.1 and 11.5 kcal mol , respectively. With the lit-
erature [12,33,38,42], we were able to calculate the
exothermicity of the reaction (2a) (ꢀfH(CH3CHBr)
(
1d)
−1
It would be interesting to compare the energetics
between alkoxy (reactions (1a) and (5a)) and the sug-
gested four-center elimination pathways, but unfortu-
nately energetics for the latter are not available.
Products observed for the CH3CHBr + NO2 reac-
tion were CHBrO (m/z = 108) and CH3CHO (Fig. 2)
with minor amounts of CH3. The measured ion signal
for CH3 was weak and noisy, and the quality of the sig-
nal did not allow a proper fitting to obtain its formation
kinetics. Nevertheless, formation of a product with a
mass of 15 Da was evident when a H lamp (10.2 eV)
was used for ionization. Other potential products that
were sought, but not detected included CH3CHBrNO2
−
1
= 31.9 kcal mol [12], ꢀfH(CH3CHBrO) = −3.3
−1
−1
kcal mol [33,38,42], ꢀfH(NO2) = 7.9 kcal mol
−1
[33], and ꢀfH(NO) = 21.6 kcal mol [33]), result-
−1
ing in 21 kcal mol . Based on the previous stud-
ies and on the observed products (CHBrO, CH3CHO,
and CH3), an internally excited CH3CHBrONO is ex-
pected to be produced in the CH3CHBr + NO2 reac-
tion. The formed CH3CHBrONO dissociates rapidly
into CH3CHBrO and NO, and the alkoxy radical
(CH3CHBrO) decomposes further to the observed
fragments via reactions (6a) and (6b).
(
m/z = 153), CH3CHBrO (m/z = 123), BrNO
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

8
RISSANEN, ESKOLA, AND TIMONEN
It should be noted, however, that to our knowledge
tained in this study. The room temperature rate coef-
ficients obtained in these previous experiments were
nothing excludes the four-center transition state path-
way to be feasible for this reaction too. As mentioned
above, we observed CH3CHO as a primary product in
a previous CH3CHCl + NO2 investigation, in contrast
to the expected CH3CO and HCl products from the
alkoxy decomposition channel. If the transition state
is similar to the one proposed for the CH3CHCl +
NO2 reaction [16] (and the CHBr2 + NO2 reaction)
then the expected products would be CH3CHO and
BrNO. From these, the latter will most likely disso-
ciate due to internal excitation from the reaction or
during photoionization [16]. However, the presence of
CH3 (which can be formed through reaction (6b) and
not easily in a ring process) might point to at least a
minor involvement of the alkoxy intermediate.
3
−1
(in units of cm s ) k300 K(CHCl2 + NO2) = (8.90 ±
0.16) × 10 , k300 K(CHBrCl + NO2) = (8.81 ± 0.28)
−
12
−
11
12
× 10 , and k(CH3CHCl + NO2) = (2.38 ± 0.10) ×
−
10 . It is interesting to notice that there is only a
very little difference in the absolute values of the rate
coefficients and in the temperature dependencies of the
reactions of CHBr2 (k300 K(CHBr2 + NO2) = (9.8 ±
−12
3
−1
0.4) × 10
cm s ) and the chlorinated analogue
CHCl2, as well as the CHBrCl containing both Cl and
Br atoms (see Fig. 3). The corresponding temperature
dependencies (as noted with the parameter n in the ex-
n
pression k300 K(T /300 K) ) are −1.48 ± 0.13, −1.55
± 0.34, and −1.65 ± 0.18 for the CHCl2, CHBrCl,
and CHBr2 reactions with NO2, respectively. In sub-
stituted ethyl radicals, the same behavior is observed
already at the monosubstitution level; the bimolecu-
lar rate coefficients and the temperature dependencies
obtained for the CH3CHCl + NO2 and CH3CHBr +
NO2 reactions are the same (k300 K(CH3CHBr + NO2)
NO was also observed in both of the reactions,
but assigning its production to reactions (1a) and (2a)
is problematic. Photodissociation of NO2 produces
prompt NO and oxygen atoms to the system. The oxy-
gen atoms will most likely react with present NO2 to
form NO + O2. Also it is possible that some labile prod-
ucts as well as photolysis side-products can react with
NO2, forming more NO in the reaction mixture. Hence,
identification of NO as a reaction product is severely
hindered by these background signals and cannot be
unambiguously shown to be produced in the studied
R + NO2 reactions.
−
11
−1.28± 0.11
3
−1
= (2.27 ± 0.06) × 10
(T /300 K)
cm s
−11
and (k(CH3CHCl + NO2) = (2.38 ± 0.10) × 10
−1.27± 0.26
3
−1
(T /300 K)
certainties.
cm s ) within experimental un-
Other halogenated alkyl radical reactions with NO2
relevant to this study include the relatively widely ex-
plored CF3 + NO2 reaction (see [17] and references
there in) and CF2Cl + NO2 reaction. The CF3 + NO2
reaction has been studied by Breheny et al. [17] over a
large pressure range (1.5–110 Torr), but in a relatively
narrow temperature range (251–295 K). They used IR
spectrometry to detect CF2O and FNO product forma-
tions, which enabled them to extract the kinetics of CF3
removal by NO2. No variation of the rate coefficients
with buffer gas pressure was observed, which is in
accordance with the present study, where no pressure
dependencies are seen. They reported a bimolecular
reaction rate coefficient k295K(CF3 + NO2) = (1.75 ±
The results from current experiments together with
selected previously measured rate coefficients are pre-
sented in Fig. 3. The solid lines in the figure present lin-
ear least-squares fits of an exponential function k = A
n
×
(T /300 K) to the current experimental data. In this
equation, T is temperature in Kelvin and A and n are
empirical parameters.
The bimolecular rate coefficients of the reactions (1)
and (2) have not been measured previously, but they
share the common characteristics that have previously
been observed for the carbon-centered free radical re-
actions with NO2: exothermicity, rapidness, negative
temperature dependence, and pressure independence at
a few Torr pressure range. It has also been established
that halogen substitution at the radical center reduces
the reactivity of substituted alkyl radical reactions, be-
cause of the electron-withdrawing inductive effect of
the electronegative halogen atoms [13,14,16]. Elec-
tropositive alkyl substituents have the opposite effect,
and they enhance the observed reaction rates. These
trends are again observed in the current results (Figs. 3,
−
11
3
−1
0.26) × 10
cm s with no apparent temperature
dependence in the 251–295 K range. Slagle and Gut-
man [18] investigated the CF2Cl + NO2 reaction at 295
K and 1–2 Torr pressure range using photoionization
mass spectrometry. They employed infrared multiple-
photon-induced decomposition of CF2Cl2 as a source
of Cl-atoms, which also allowed them to study the
CF2Cl + NO2 reaction rate coefficients, resulting in a
room temperature value: k295K(CF2Cl + NO2) = (9.6
−
12
3
−1
± 1.9) × 10
cm s .
4
a, and 4b).
Correlations between reactivity of radicals and dif-
ferent radical properties have been sought to gain
understanding of the factors affecting their chemical
reactivity. An example of such a procedure is the lin-
ear relationship of the electron affinity (EA) of the
In previous experiments [13,14,16], we have mea-
sured the bimolecular rate coefficients of the CHCl2 +
NO2, CHBrCl + NO2, and CH3CHCl + NO2 reactions,
which are relevant for comparison with the results ob-
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

KINETICS OF THE BROMINATED ALKYL RADICAL REACTIONS WITH NO2
9
(
a)
(b)
6
0
8
6
0
0
CH CH
3
2
CH CH2
3
s-C H
n-C H
40
4
9
i-C H
C H
3
7
CH CHBr
3
3
7
2
3
4
2
1
0
C H
t-C H
4 9
t-C H
3
5
4
9
i-C H
CH CHCl
3
7
CH CO
CH3
3
3
CH2I
CH3
CH Br
CH Cl
20
2
2
CH2I
CF3
CH CN
0
CH CHCl
2
3
CH Cl
2
CF3
CH Br
2
CH CHBr
CF3
3
CF Cl
2
3
CH CCl2
CF Cl
CF Cl
2
0
8
CH CCl
CHBr2
3
2
2
1
8
0
CHBr2
CHBrCl
CHCl2
CHCl2
CHBrCl
6
6
k
4
k
CCl3
4
CCl3
2
0.0
1.0
2.0
–
1
0
1
2
3 4
5
6
EA(R) / eV
Δ
Electronegativity
Figure 4 (a) Bimolecular room temperature reaction rate coefficients of current and selected R + NO2 reactions plotted against
the electron affinities of the radicals, EA(R). The rate coefficient of the reaction, displayed on a logarithmic scale, serves as a
measure of the radicals’ reactivity toward NO2. Results from this study are presented with circles. The line in the picture is a fit
−
11
−0.33EA(R)
. The selected rate coefficients are from
to the data and can be presented by an equation: k300 K = (3.66 × 10 ) × 10
CH2Cl, CHCl2, and CCl3 [13]; CH2Br, CH2I, and CHBrCl [14]; CH3CH2, CH3CHCl, and CH3CCl2 [16]; CF3 [17]; CF2Cl
and CH3CO [18]; CH2CN [27];n-C3H7, i-C3H7, s-C4H9, and t-C4H9 [40]; CH3 [51]; C2H3 [52]; and C3H [53]. Electron
5
affinities were taken from [26] except for CH2I [54], and for CHBrCl [55]. In the case of substituted ethyl radicals, the EA(R)
was unavailable and was estimated as EA(CH3CHBr) = EA(CH3CH2) + (EA(CH2Br) − EA(CH3). (b) A semilogarithmic plot
of the room temperature rate coefficients plotted against the ꢀElectronegativities of the radicals in the current and selected R +
NO2 reactions. (ꢀElectronegativity = ꢁXS − XH , where XH is the hydrogen atom electronegativity and XS is the substituent
atom or group electronegativity). Electronegativities were taken from Pauling [45,46], and an electronegativity value of 1.82
was assigned to a methyl group substituent in accordance with [47]. The line in the picture is a fit to the data omitting the values
for fluorinated radicals. The fluorinated radicals share the same relationship when an effective electronegativity value of 2.8 is
given to the fluorine atom as a substituent; as was discussed previously along the studies of R + Cl2 reactions [48,49] and is
depicted with horizontal arrows in the picture. The selected rate coefficients were taken from CH2Cl, CHCl2, and CCl3 [13];
CH2Br, CH2I, and CHBrCl [14]; CH3CH2, CH3CHCl, and CH3CCl2 [16]; CF3 [17]; CF2Cl [18]; i-C3H7 and t-C4H9 [40]; and
CH3 [51]. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
molecular reagent subtracted from the ionization po-
tential of the radical plotted against the logarithm of
the room temperature rate coefficient, log(k300 K) ver-
sus (IP(R) − EA(reactant)), used by Paltenghi et al.
Seetula and Gutman [47] to explain the observed differ-
ences in the rate coefficients of methyl and halogenated
methyl radical reactions with HI. The larger alkyl rad-
icals could be included to the same linear relationship
when a group electronegativity value of 1.82 was as-
signed to the methyl substituent. A similar linear rela-
tionship was shown to exist also for the rate coefficients
of the R + Cl2 [48,49] and R + Br2 [50] reactions.
As noticed by Eskola et al. [13], log(k300 K) ver-
sus (IP(R) − EA(reactant)) does not produce a lin-
ear correlation among NO2 reactions with chlorinated
alkyl radicals. Instead it has been observed that the
logarithms of the reaction rates correlate with EA of
[
43] to correlate the reactivity of alkyl radicals in
their reactions with molecular oxygen and ozone. An-
other example is the ꢀElectronegativity scale; an ar-
bitrary scale proposed by Thomas [44] for estimating
the electron-withdrawing inductive effects of halogen
substituents to carbon 1s IPs calculated by simple sum-
mations of the Pauling electronegativities [45,46] of
the substituent atoms/groups. To radical kinetics, this
semiempirical scale of inductive effects was adapted by
International Journal of Chemical Kinetics DOI 10.1002/kin.20725

1
0
RISSANEN, ESKOLA, AND TIMONEN
the radical species; EA(R). log(k300 K) versus EA(R)
and log(k300 K) versus ꢀElectronegativity plots were
reproduced here, including also the reactions of the
current study. The plots are shown in Figs. 4a and 4b
together with selected previously published data. As
noted previously [16], polar effects are likely to be
important in determining the reactivity differences be-
tween the studied R + NO2 reactions. The lower the
charge density in the radical center, that is, the higher
the polarity of the radical, (CH3CHBr > C2H5 and
CHBr2 > CH2Br > CH3) the lower is the reactivity
toward NO2.
der conditions of low pressure (2–6 Torr) and variable
temperature (250–480 K). In the CHBr2 + NO2 reac-
tion, an unexpected product CBr2O was detected and a
mechanism consisting of a four-center transition state
was postulated to explain its formation. This pathway
is in contrast to the common “alkoxy channel” of R
+ NO2 reaction, in which an internally excited nitrite
intermediate is formed from R + NO2 association that
breaks into alkoxy radical and nitric oxide. The same
kind of mechanism cannot be ruled out from occurring
in the CH3CHBr + NO2 reaction, but the previous stud-
ies and the observed products suggest the formation of
an alkoxy radical (CH3CHBrO) in the reaction. Reac-
tivity of radicals toward NO2 was briefly discussed and
reasoned to be caused by charge density differences at
the radical centers.
Briefly summarizing what has been observed: (i)
Electronegative halogen substitution at the radical cen-
ter decreases the observed R + NO2 reaction rate co-
efficients. (ii) Electropositive alkyl substitution has an
opposite effect and enhances the rate coefficients. (iii)
Br and Cl substitutions in the radical center, in dis-
ubstituted methyl, and monosubstituted ethyl radicals
have almost the same influence on reaction rates, that
is, k300 K(CHBr2 + NO2) ≈ k300 K(CHBrCl + NO2) ≈
k300 K(CHCl2 + NO2) and k300 K(CH3CHCl + NO2) ≈
k300 K(CH3CHBr + NO2). (iv) The level of substitution
affects the temperature dependence; The more substi-
tuted the radical, the stronger is the observed tempera-
ture dependence. This is best demonstrated among the
measured chlorinated radical reactions, where the n pa-
rameter describing T dependence decreases (becomes
more negative) as C2H5 > CH3CHCl > CH3CCl2 [16]
and CH2Cl > CHCl2 > CCl3 [13]. (v) Branching of
the radical from the α-carbon in comparison with the
substituents carbon chain length, that is, i-C3H7 ver-
sus n-C3H7 and t-C4H9 versus s-C4H9, seems to have a
smaller effect on the reactivity of the radical, in contrast
what has been observed for R + Cl2 reactions [48,49],
but the results are still inconclusive and to verify them,
more data from isomeric radical reactions are needed.
Considering the CH3CHBr radical as an example, the
described effects will work on the observed reactivity
in the following way: Reactivity is reduced compared
with the C2H5 radical through halogen substitution at
the radical site and enhanced compared with the CH2Br
upon methyl substitution. The reactivity with NO2 is
rather identical in the CH3CHCl an CH3CHBr radi-
cals (changing bromine to chlorine substitution) and
reduced in comparison with the C3H7 and C4H9 radi-
cals as CH3CHBr contains only one alkyl substituent
that has the shortest possible carbon chain length.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20725