Temperature dependence of pentyl nitrate formation from the reaction of pentyl peroxy radicals with NO

Article abstract of DOI:10.1039/b700285h

Alkyl nitrate yields from the reaction of 1-pentyl, 2-pentyl and 2-methyl-2-butyl peroxy radicals with NO have been determined over the temperature range (261-305 K) and at 1 bar pressure from the photo-oxidation of the iodoalkane precursors in air-NO mixtures. Yields were observed to increase with decreasing temperature and, contrary to previous observations, along the series primary < secondary ? tertiary. Our results suggests a significant temperature dependence for the formation of nitrates from the reaction of pentyl peroxy radicals with NO and represent an extension in the temperature range over which this reaction has been studied experimentally in the past. the Owner Societies.

Full text of DOI:10.1039/b700285h

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www.rsc.org/pccp | Physical Chemistry Chemical Physics
Temperature dependence of pentyl nitrate formation from the reaction of
pentyl peroxy radicals with NO
Paola Cassanelli,* David J. Fox and R. Anthony Cox
Received 16th January 2007, Accepted 12th February 2007
First published as an Advance Article on the web 6th March 2007
DOI: 10.1039/b700285h
Alkyl nitrate yields from the reaction of 1-pentyl, 2-pentyl and 2-methyl-2-butyl peroxy radicals
with NO have been determined over the temperature range (261–305 K) and at 1 bar pressure
from the photo-oxidation of the iodoalkane precursors in air–NO mixtures. Yields were observed
to increase with decreasing temperature and, contrary to previous observations, along the series
primary o secondary D tertiary. Our results suggests a significant temperature dependence for
the formation of nitrates from the reaction of pentyl peroxy radicals with NO and represent an
extension in the temperature range over which this reaction has been studied experimentally in the
past.
The details of the chemical mechanism responsible for the
1
. Introduction
formation of organic nitrates are not entirely understood.
Studies on the pressure dependence of nitrate yields from
reaction (1) have shown that, consistent with expectations
2
The reaction of alkyl peroxy radicals (RO ) with NO (reaction 1)
is an important intermediate step in the gas-phase degradation
of volatile organic compounds (VOC) under tropospheric
1
conditions. The reaction is thought to proceed via formation
for an association reaction, the rate constant ratio k1b/k
2,8,25
1
increases with increasing pressure
although complications
26–28
of a vibrationally excited closed shell peroxy nitrite, RO NO*.
2
have emerged in the theoretical modeling of the reaction.
26,27
This intermediate can decompose to RO and NO2 (1a) or
undergo an internal molecular rearrangement whereby an
O–O bond is broken and two new O–N bonds are formed to
The calculations by Barker and co-workers,
for example,
could reproduce experimental data on the formation of
-pentyl nitrate from the reaction of 2-pentyl peroxy with
2
2
yield the alkyl nitrate product (1b):
2
NO only by using physically unrealistic parameters for colli-
sional energy transfer. More recently, it has been proposed by
2
8–30
several groups
that the intermediate peroxy nitrite ROO-
NO species occurs in two distinct non-interconverting con-
formeric structures, which exhibit different chemical
behaviour and reactivity, and that only one of the conformers
will proceed further to nitrate production.
Several studies have been published on RONO
2
2
formation
An accurate knowledge of the branching ratio for reaction
,8,9,16,17,20–22
as a function of temperature.
Yields have been
(
1) is important to model ozone formation in the troposphere
shown to increase with decreasing temperature but the extent
of the reported temperature dependence for the rate constant
ratio varies over a factor of B2. The work of Atkinson and co-
and to predict the chemistry and hence the environmental
impact of atmospheric VOC emissions. The formation of
organic nitrates from the reaction of peroxy radicals with
2
,8
9
2
–25
workers on C –C RO and that of Harris and Kerr on
5 8 2
NO has been investigated by both experimental
and theo-
The relative importance of channel (1a)
and (1b) depends on the size and complexity of the radical,
with k1b/k1a increasing from B2–8% for C –C up to
The available ex-
perimental evidence also suggests that the branching ratio k1b
2
6–28
heptyl peroxy radicals point to an Arrhenius activation energy
ꢀ
retical methods.
1
for the branching ratio k /k of about (13–14) kJ mol but
1
b
1
1
Orlando et al. and Elrod and co-workers
6
17,20
have reported
3
4
2
,5,18
evidence for a much weaker temperature dependence arising
7 8 2
B20–25% for secondary C –C RO .
ꢀ1
from an activation energy barrier of r(7–8) kJ mol for the
/
1
6
16
17
20
1
8
cyclopentyl, cyclohexyl, ethyl and propyl peroxy radi-
cals. The work of Shallcross and co-workers on CH and
was carried out over an extensive temperature range
B200–300 K) but only upper limits to the values for the
branching ratios k1b/k of 0.1 and 0.05, respectively, were
reported from these studies.
k1 increases from tertiary to secondary, which is unexpected
3
O
2
as tertiary and secondary RO are predicted to have compar-
2
2 5 2
C H O
able enthalpies. The majority of previous work on reaction
(
(
1b) has dealt with peroxy radicals derived from n-alkane
series and secondary peroxy radicals have been the most
studied RO to date. Little data is available on the correspond-
1
2
1,22
2
In this work, we have investigated the formation yield of
three C5 pentyl nitrate isomers from the reaction of the
corresponding pentyl peroxy radicals with NO at atmospheric
pressure and over the temperature range (262–305) K with a
view to obtaining isomer specific data on reaction (1b) and to
ing reaction for tertiary peroxy radicals.
Centre for Atmospheric Science, University Chemical Laboratory,
Lensfield Road, Cambridge, UK CB2 1EW. E-mail:
pc254@cam.ac.uk
4
332 | Phys. Chem. Chem. Phys., 2007, 9, 4332–4337
This journal is ꢁc the Owner Societies 2007

extending the corresponding kinetic data base. When com-
bined with the available literature data on the pressure depen-
2
,25
dence of pentyl nitrate formation,
the present work will
provide useful constraints on theoretical models investigating
the mechanism of reaction (1).
2
. Experimental
Experiments were performed at atmospheric pressure in a
photochemical steady-state flow reactor coupled to a gas
chromatograph. The experimental techniques employed have
3
1
been described previously and only the relevant details are
given here. Pentyl peroxy radicals were formed from the
photolysis of the iodopentane precursors in air-NO mixtures:
ꢂ
ꢂ
RI þ hn ! R þ I
ð2Þ
ꢂ
ꢂ
2
R þ O2 ! RO
ð3Þ
Irradiations were carried out with two fluorescent tubular
lamps (Sylvania G40W) mounted adjacent and parallel to
the reactor cell.
Typical initial concentrations for the iodopentane isomers
1
3
ꢀ3
and for NO were (0.5–5.6) ꢃ 10 molecule cm and (0.2–3.1)
1
3
ꢀ3
ꢃ 10 molecule cm , respectively, with the initial concentra-
tion of NO approximately half the concentration of the
iodopentanes in all experiments. These mixtures were pumped
through the reactor at flow rates of (140–4500) sccm, resulting
in residence times under illumination of (3–80) s. Gas samples
of 60–300 ml were collected at a flow rate of 30 sccm onto
stainless steel sampling tubes packed with 200 mg Tenax TA,
which were subsequently heated to 150–200 1C using a com-
Fig. 1 Chromatograms showing the iodopentane isomers and corre-
sponding pentylnitrate products for the three sets of experiments
carried out at 298 K and atmospheric pressure, with detection by
ECD. Fig. 1C was obtained from an experimental run where 1-
pentylnitrate was added to identify its retention time under the
analytical conditions used for the 2-methyl-2-butylperoxy experi-
ments.
TM
mercial desorber (Unity , cold trap U-T1HBL, Markes
International Ltd). Separation was achieved onto a widebore
capillary column (Agilent HP-5, 30 m, 0.32 mm id, 0.25 mm
ꢀ
1
phase thickness) heated from ꢀ20 1C to 80 1C at 2 1C min
ꢀ1
graphs obtained by both ECD and FID detection methods over
the course of an experiment. The GC-FID calibration factors
were taken to be identical to that of 1-pentylnitrate for all
isomers. Small amounts of 2-methyl-2-butylnitrate were ob-
served to form slowly in the dark in the Tedlar reservoir from
and then ramped to 270–300 1C at 50 1C min . The carrier
gas was He in all analyses, at a head pressure of 1.58 bar,
resulting in an initial flow rate of 2.6 sccm.
The amount of reacted iodopentane was calculated from its
initial concentration and from its photolysis rate constant,
which was measured by UV-Vis spectroscopy in separate
experiments. The measured photolysis rate constant was the
same within experimental error for the three iodopentane
2
2 2
-methyl-2-iodobutane–NO–N –O mixtures. A nonlinear least
ꢀ5
squares fit of these data (y = 15 + 36(1 ꢀ exp(4 ꢃ 10 t)),
where t is the time of mixing in the bag) was used to subtract
this background source of nitrates to all measurements ob-
tained from the photo-oxidation of 2-methyl-2-iodobutane.
ꢀ3
ꢀ1
isomers, (1.1 ꢄ 0.2) ꢃ 10
s . Concentrations of nitrate
products were determined by GC-FID in all experiments and
by both GC-FID and GC-ECD for the 1-iodopentane experi-
ment. GC-FID calibrations and retention times for 1-pentyl-
nitrate were obtained using a pure standard with an estimated
error in the GC response factor of 8%. The uncertainty in the
measured 1-pentylnitrate yields, accounting for uncertainties in
the GC calibration factor, the photolysis rate and the initial
concentration of 1-iodopentane, was of the order of 25%. All
uncertainties reported in the remaining part of this paper refer
to the statistical error (2s) from least squares fits to the data.
For the 2-pentylnitrate and 2-methyl-2-butylnitrate, for which
standards were not available, identification was carried out by
assuming a position of elution of the organic nitrate relative to
that of the iodopentane precursor as for the 1-pentylnitrate and
Materials
2-Iodopentane and 2-methyl-2-iodobutane were synthesized
by the reaction of the parent alcohols.
2-Iodopentane. 2-Pentanol (50 mmol), imidazole (55 mmol)
and triphenylphosphine (55 mmol) were dissolved in dry
3
CH
2
Cl
2
(150 cm ) and the stirred solution was cooled to 0 1C
in the dark. Solid I
2
(55 mmol) was added in small portions
over 20 min. The reaction was stirred for an additional 2 h and
3
then extracted with water (150 cm ). The CH Cl solution was
2 2
dried over sodium sulfate and then reduced in vacuo to give an
3
oily solid that was extracted with pentane (100 cm ). The
1
-iodopentane (see Fig. 1) and by comparison of chromato-
pentane solution was reduced in vacuo and oily residue was
Phys. Chem. Chem. Phys., 2007, 9, 4332–4337 | 4333
This journal is ꢁc the Owner Societies 2007

3
2
distilled twice from copper in the dark under nitrogen at
ambient pressure (3.2 g, 32%); d (400 MHz, CDCl ) 4.19
1 H, dqd, J 8.5, 7, 5 Hz, CHI), 1.91 (3 H, d, J 7 Hz, CH CHI),
2
Alkyl peroxy radicals can also self-react and react with NO :
ꢂ
ꢂ
ꢂ
0
0
H
3
RO þ RO ! RO þ R CHO þ R OH þ O2
ð14Þ
ð15Þ
2
2
(
3
ꢂ
1
.88–1.75 (1 H, m, CH ), 1.61–1.31 (3 H, m, CH ) and 0.91
2
RO2 þ NO2 Ð RO2NO2
2
(
3 H, t, J 7 Hz, CH CH ).
2 3
However, under the experimental conditions employed here,
neither reaction (14) nor (15) is expected to compete with
1
reaction (1): reaction (15) is insignificant due to the rapid
2-Iodo-2-methylbutane. 2-Methyl-2-butanol (50 mmol) and
tetra-butylammonium iodide (5 mmol) were dissolved in dry
3
CH
2
Cl
2
(150 cm ) and the stirred solution was cooled to 0 1C
in the dark. Solid sodium iodide (50 mmol) was added and
back-decomposition of the alkyl peroxynitrates while reaction
(14) becomes relevant only in air mixtures with NO concen-
1
1
ꢀ3
then trimethylsilyl chloride (55 mmol) was added dropwise
trations r10 molecule cm . The only significant sink for
the alkyl peroxy radicals from (3) is thus reaction with NO.
The main fates of the alkoxy radicals formed in reaction (1a)
over 10 min. The reaction was stirred for 3 h and then
3
extracted with water (150 cm ). The CH
2
Cl
2
solution was
dried over sodium sulfate and then reduced in vacuo to give
3
an oily residue that was extracted with pentane (100 cm ). The
are decomposition, isomerization and reaction with oxy-
33,34
gen,
which are represented by reaction (4). An additional
pentane solution was reduced in vacuo and the oily residue was
minor channel is reaction (5), which could interfere with the
analysis of the title reaction. FACSIMILE simulations of the
distilled twice from copper in the dark under nitrogen at
ambient pressure (2.7 g, 27%); d
6 H, s, C(CH ), 1.64 (2 H, q, J 7 Hz, CH
Hz, CH CH ).
The products were degassed and stored under vacuum in the
H
(400 MHz, CDCl
3
) 1.92
2
chemical systems studied showed that most of the NO formed
(
3
)
2
2
) and 1.06 (3 H, t, J
under our experimental conditions was removed by reaction
with OH (reaction 12) and that nitrate formation from reac-
tion (5) accounted for less than 1% of the total organic nitrate
formed in all cases. Computer modeling also showed that OH
was efficiently removed by reactions reaction (11) and (12) and
7
2
3
dark at 277 K. Purity checks were carried out by NMR, gas
chromatography and UV-Vis spectroscopy. Maximum
ꢀ
18
2
cm molecule
ꢀ1
absorption cross sections of B1.3 ꢃ 10
that secondary losses of RI and RONO
2
via reaction with OH
(
260 nm, base e) were measured for the iodopentane isomers.
All other chemicals, and their stated purities, were as
were negligible. The chemistry of the iodine atom from reac-
3
5
tion (2) is less well characterized but previous work carried
out under conditions similar to those employed here suggests
that it is not likely to interfere with the chemistry under study.
Assuming steady-state conditions for all radical intermediates,
the following rate equations were derived:
follows: 1-iodopentane (97%), Acros Organics; 1-pentylnitrate
(
(
95%), Fluorochem Ltd; N (GC grade), He (GC grade), O
2 2
99.999%), Air Products; NO (99%) Argo International. All
liquid compounds were used after repeated degassing and
purity was checked by GC-FID.
d½RIꢅ=dt ¼ ꢀJ ½RIꢅ
2
ðIÞ
and
3
. Results and discussion
k1b
d½RONO2ꢅ=dt ¼
J2½RIꢅ
ðIIÞ
The following main reactions are assumed to occur following
k1a þ k1b
and the observed pentylnitrate yields could be used to estimate
photo-dissociation of the iodides at 1 bar in the presence of O
2
and NO:
the rate constant ratio k /(k + k ). Typical plots of
1b
ꢂ
ꢂ
1b
1a
RI þ hn ! R þ I
ð2Þ
ð3Þ
measured pentylnitrate concentrations vs. the amount of
ꢂ
ꢂ
reacted iodopentane are shown in Fig. 2, 3 and 4 for the
R þ O2 ! RO
2
1
-iodopentane, 2-iodopentane and 2-methyl-2-iodobutane ex-
ꢂ
ꢂ
RO þ NO ! RO þ NO2
ð1aÞ
ð1bÞ
ð4Þ
2
periments, respectively. It can be seen that the data from the 2-
methyl-2-butyl peroxy experiments are more scattered, which
may be partly due to the thermal instability of the RI
precursor in the Tedlar reservoir (see Experimental). Branch-
ing ratios for reaction (1) were derived from linear regression
analysis of these plots forced through the origin. The results at
room temperature and 1 bar pressure are reported in Table 1.
Our yields for 2-pentyl peroxy of (0.11 ꢄ 0.01) compare well
with reported values of B0.10–0.12 obtained from investiga-
ꢂ
RO þ NO ð þ MÞ ! RONO2 ðþ MÞ
2
ꢂ
RO ðþ O2Þ ! oxygenated products ðþ HO2Þ
ꢂ
RO þ NO2 ð þ MÞ ! RONO2 ðþ MÞ
ð5Þ
HO2 þ NO ! OH þ NO2
Iꢂ þ NO þ M ! INO þ M
Iꢂ þ INO ! I2 þ NO
ð6Þ
ð7Þ
tions on the OH-initiated photo-oxidation of
2,5,11,25
C
5
al-
ð8Þ
kanes
at ambient temperature and pressure, in which
branching ratios for the individual alkyl nitrates were derived
from the estimated percent yields of the corresponding alkyl
peroxy radicals formed from the reaction of the OH radical
with the parent alkane.
OH þ RI ! products
ð9Þ
OH þ RONO2 ! products
OH þ NO ! HONO
ð10Þ
ð11Þ
ð12Þ
ð13Þ
As for 2-methyl-2-butyl peroxy, we report a yield of (0.11 ꢄ
0
.04), which is equal, within the uncertainty, to that for the
OH þ NO2 ! HNO3
OH þ I2 ! HOI þ Iꢂ
secondary radical. This is in disagreement with the experi-
6
mental data of Atkinson et al. which was obtained from the
,8
4
334 | Phys. Chem. Chem. Phys., 2007, 9, 4332–4337
This journal is ꢁc the Owner Societies 2007

Fig. 2 Selected plots of observed 1-pentylnitrate yields plotted versus
Fig. 4 Selected plots of observed 2-methyl-2-butylnitrate yields
plotted versus the amount of 2-methyl-2-iodobutane reacted from
the amount of 1-iodopentane reacted from the photo-oxidation of
2 2
1-iodopentane–NO–N –O mixtures at 1 bar (analysis by GC-ECD).
2 2
the photo-oxidation of 2-methyl-2-iodobutane–NO–N –O mixtures
at 1 bar.
photo-oxidation of 2-methylbutane at ambient pressure and
k1b
Arrhenius plots for the rate constant ratio k1b/k1a are shown
in Fig. 5. The following Arrhenius equations were derived
from least squares analysis of the data, where errors indicate
the 95% confidence limit:
6
temperature. Atkinson et al. derived branching ratios
,8
k1aþk1b
for 2-methyl-2-butylnitrate and 2-methyl-3-butylnitrate of
(
0.05–0.06) and (0.11–0.15), respectively, which suggests yields
for tertiary alkyl nitrates a factor of B3 lower than those for
the corresponding secondary alkyl nitrate. Similar results were
obtained by the same group from their photo-oxidation
k1b=k1að1 - pentyl peroxyÞ ¼
:9 ꢃ 10 exp½ðð13:3 ꢄ 3:8Þ kJ molꢀ1Þ=RTꢅ
3
2
8
6
studies of 2,2-dimethyl-2-propane, 2-methylpentane, and
8
3
-methylpentane.
k1b=k1að2 - pentyl peroxyÞ ¼
Little data is available in the literature on the yields of
primary alkyl nitrates—and none on 1-pentylnitrate—that can
:5 ꢃ 10 exp½ðð9:4 ꢄ 3:9Þ kJ molꢀ1Þ=RTꢅ
2
3
be compared with our result of (0.08 ꢄ 0.002). An estimate of
k1b
k1aþk1b
0
.06 can calculated for the branching ratio
for a C
5
k1b=k1að2-methyl-2-butyl peroxyÞ
primary alkyl nitrate using an empirical estimation method
3
¼ 9:6 ꢃ 10 exp½ðð12:0 ꢄ 6:5Þ kJ molꢀ1Þ=RTꢅ
2
6,37
for alkyl nitrate production developed by Atkinson
based
6
,8
on their work on alkane photo-oxidation. Although the
two values agree, within the experimental uncertainty, our
result is slightly higher and seems to suggest a smaller varia-
tion in alkyl nitrate isomer production yields than previously
thought.
Table 1 Nitrate yields from the photo-oxidation of iodopentane
isomers at 1 bar in air and in the presence of NO, quantified by either
FID or ECD (see Experimental). Quoted errors represent the statis-
tical uncertainty (2s) from least squares linear fits to the data
(
Fig. 2–4)
T/K
k1b/(k1a + k1b)
1
-Iodopentane (FID)
298
0.08 ꢄ 0.02
0.07 ꢄ 0.01
0.09 ꢄ 0.01
0.11 ꢄ 0.02
0.1 ꢄ 0.03
0.13 ꢄ 0.02
290
283
273
268
263
1
2
2
-Iodopentane (ECD)
298
0.07 ꢄ 0.02
0.12 ꢄ 0.03
0.13 ꢄ 0.03
0.14 ꢄ 0.03
2
2
2
73
67
62
-Iodopentane (FID)
298
0.11 ꢄ 0.02
0.14 ꢄ 0.04
0.14 ꢄ 0.03
0.18 ꢄ 0.04
2
2
2
86
73
63
-Methyl-2-iodobutane (FID)
305
298
0.12 ꢄ 0.05
0.11 ꢄ 0.04
0.14 ꢄ 0.08
0.24 ꢄ 0.11
Fig. 3 Selected plots of observed 2-pentylnitrate yields plotted versus
2
2
73
61
the amount of 2-iodopentane reacted from the photo-oxidation of
2 2
2-iodopentane–NO–N –O mixtures at 1 bar.
This journal is ꢁc the Owner Societies 2007
Phys. Chem. Chem. Phys., 2007, 9, 4332–4337 | 4335

via a three-centre cyclic transition state with a high barrier
to reaction.
The observed strong negative temperature dependence,
however, suggests an isomerization barrier Ea,1b lower than
the dissociation limit hence seems to point to a complex
2
mechanism. Atkinson et al. have proposed the involvement
of thermalized as well as chemically activated RO NO
2
Fig. 5 Arrhenius plots for the rate constant ratios k1b/k1a for
-pentylperoxy, 2-pentylperoxy and 2-methyl-2-butylperoxy at 1 bar.
(Scheme 1) and product-like transition states have been used
2
7,28
in several calculations.
Recent theoretical work on FONO, HOONO and
1
3
9
29
2
9
MeOONO isomerization indicates that the main contribu-
tion to the barrier of both reactions (1a) and (1b) is the
Values for (Ea,1a ꢀ Ea,1b) and A1a/A1b, in the range of
ꢀ
1
3
B(10–13) kJ mol and B(0.3–3.0) ꢃ 10 , respectively were
observed for the three compounds investigated. These para-
meters are given in Table 2 and are in good agreement with
switching between two electronic structures of the NO
2
moiety
with the free electron on the NO moving from the oxygen to
2
the nitrogen. The energy of the transition states for isomeriza-
tion and dissociation would be expected to be of the same
order of magnitude, with reaction (1b) possibly being energe-
tically favoured due to a better interaction between RO and
2
,8
those reported by Atkinson et al. —which were obtained
from the OH-initiated photo-oxidation of n-pentane and
n-heptane over the temperature range (284–341 K) and 740
9
Torr—and with those of Harris and Kerr, derived from the
NO . Higher nitrate yields should thus correspond to larger
2
photo-oxidation of n-heptane at 253–325 K and 730 Torr.
energy gaps between the two transition states.
1
6
Orlando et al. in a study of the Cl-initiated oxidation of
cyclopentane at 700 Torr observed the overall nitrate yield
increasing from 0.12 ꢄ 0.05 at 296 K to 0.21 ꢄ 0.05 at 230 K.
These results might indicate a weaker temperature dependence
of nitrate formation but are broadly consistent with our data,
which would predict an increase in nitrate yield by a factor of
B2–3 over the same temperature range.
In conclusion, the data set presented here provides new
isomer specific data for the formation of pentyl nitrate isomers
from the reaction of the corresponding peroxy radical with
NO as a function of temperature. Measured Arrhenius para-
meters are consistent with previously reported values for
It is of interest to compare our results with the reported
temperature dependence for the analogous reaction of HO2
with NO, which has been investigated recently as a function of
3
8
temperature at a total pressure of 200 Torr of N2. The
branching ratio k1b/k1a was found to increase B5-fold over
the temperature range (223–298 K) and a plot of the data in
ꢀ
1
Arrhenius form yielded a barrier of 11.8 kJ mol
4
and
ꢀ1
A-factor of 6.4ꢃ10 s , similar to the values derived here to
within experimental error.
2
,27
As pointed out previously,
alkyl nitrate formation
involves the cleavage of an O–O bond and the formation
of two N–O new bonds, hence it would be expected to proceed
Scheme 1
Table 2 Arrhenius parameters for the rate constant ratio k1b/k1a—comparison with literature data
ꢀ
1
Precursor compound
T/K
P/Torr
E
1a ꢀ E1b/kJ mol
A
1a/A1b
Ref.
a
3
2
3
3
1
2
2
-Iodopentane
-Iodopentane
-Methyl-2-iodobutane
262–298
263–298
261–305
284–341
284–341
253–325
760
760
760
740
740
730
13.3 ꢄ 3.8
2.9 ꢃ 10
3.5 ꢃ 10
1.8 ꢃ 10
1.7 ꢃ 10
This work
This work
This work
Ref. 2
Ref. 2
Ref. 9
a
9.4 ꢄ 3.9
a
12.0 ꢄ 6.5
b
n-Pentane
n-Heptane
n-Heptane
a
13.4
13.4
2
b
6 ꢃ 10
2
13.5
6.9 ꢃ 10
b
The errors in (E1a ꢀ E1b) reflect the statistical uncertainty (2s) from least squares linear fits to the data in Arrhenius form (Fig. 5). Temperature
2
dependencies for k1b/k1a for n-pentane and n-heptane at pressure below 1 bar are available from the work of Atkinson et al.
4
336 | Phys. Chem. Chem. Phys., 2007, 9, 4332–4337
This journal is ꢁc the Owner Societies 2007

2
peroxy radicals. The yields for primary
,9
secondary C
5
and C
7
17 D. L. Ranschaert, N. J. Schneider and M. J. Elrod, J. Phys. Chem.
A, 2000, 104, 5758–65.
18 J. Arey, S. M. Aschmann, E. S. C. Kwok and R. Atkinson, J. Phys.
Chem., 2001, 105, 1020–7.
and tertiary organic nitrates are larger than those derived from
3
6
currently available empirical estimation methods and reflect
a trend in organic nitrate formation along the series primary o
secondary D tertiary.
1
9 S. M. Aschmann, J. Arey and R. Atkinson, J. Phys. Chem., 2001,
105, 7598–606.
2
0 J. M. Chow, A. M. Miller and M. J. Elrod, J. Phys. Chem. A, 2003,
07, 3040–7.
1
Acknowledgements
21 A. Bacak, M. W. Bardwell, M. T. Raventos, C. J. Percival, G.
Sanchez-Reyna and D. E. Shallcross, J. Phys. Chem. A, 2004, 108,
10681–7.
We acknowledge the NERC and the EU for funding under the
UTLS thematic program and the UTOPIHAN-ACT project,
respectively.
2
2 M. W. Bardwell, A. Bacak, M. T. Raventos, C. J. Percival, G.
Sanchez-Reyna and D. E. Shallcross, Int. J. Chem. Kinet., 2005, 37,
2
53–60.
2
3 C. Espada, J. Grossenbacher, K. Ford, T. Couch and P. B.
Shepson, Int. J. Chem. Kinet., 2005, 37, 675–85.
4 C. Espada and P. B. Shepson, Int. J. Chem. Kinet., 2005, 37,
686–99.
References
2
1
2
R. Atkinson and J. Arey, Chem. Rev., 2003, 103, 4605–38.
R. Atkinson, W. P. L. Carter and A. M. Winer, J. Phys. Chem.,
25 S. M. Aschmann, W. D. Long and R. Atkinson, J. Phys. Chem. A,
2006, 110, 6617–22.
1983, 87, 2012–8.
2
6 L. L. Lohr, J. R. Barker and R. M. Shroll, J. Phys. Chem. A, 2003,
07, 7429–33.
7 J. R. Barker, L. L. Lohr, R. M. Shroll and S. Reading, J. Phys.
Chem. A, 2003, 107, 7434–44.
3
4
5
6
7
8
9
K. R. Darnall, W. P. L. Carter, A. M. Winer, A. C. Lloyd and J. N.
Pitts, Jr, J. Phys. Chem., 1976, 80, 1948–50.
H. Takagi, N. Washida, H. Bandow, H. Akimoto and M. Okuda,
J. Phys. Chem., 1981, 85, 2701–5.
R. Atkinson, S. M. Aschmann, W. P. L. Carter, A. M. Winer and
J. N. Pitts, Jr, J. Phys. Chem., 1982, 86, 4563–9.
R. Atkinson, S. M. Aschmann, W. P. L. Carter, A. M. Winer and
J. N. Pitts, Jr, Int. J. Chem. Kinet., 1984, 16, 1085–101.
P. B. Shepson, E. O. Edney, T. E. Kleindienst, J. H. Pittman, G. R.
Namie and L. T. Cupitt, Environ. Sci. Technol., 1985, 19, 849–54.
R. Atkinson, S. M. Aschmann and A. M. Winer, J. Atmos. Chem.,
1
2
28 J. Zhang, T. Dransfield and N. M. Donahue, J. Phys. Chem. A,
2004, 108, 9082–95.
29 Y. Zhao, K. N. Houk and L. P. Olson, J. Phys. Chem. A, 2004,
1
08, 5864–71.
3
0 A. Lesar, M. Hodoscek, E. Drougas and A. M. Kosmas, J. Phys.
Chem. A, 2006, 110, 7898–903.
31 P. Cassanelli, D. Johnson and R. A. Cox, Phys. Chem. Chem.
Phys., 2005, 7, 3702–10.
32 P. D. Lightfoot, R. A. Cox, J. N. Crowley, M. Destriau, G. D.
Hayman, M. E. Jenkin, G. K. Morotgat and F. Zabel, Atmos.
Environ., Part A, 1992, 26A, 1805–964.
1987, 5, 91–102.
S. J. Harris and J. A. Kerr, Int. J. Chem. Kinet., 1989, 21, 207–18.
1
1
1
1
1
1
1
0 K. Muthuramu, P. B. Shepson and J. M. O’Brien, Environ. Sci.
Technol., 1993, 27, 1117.
3
3 R. Atkinson, Int. J. Chem. Kinet., 1997, 29, 99–111.
1 R. Atkinson, E. S. C. Kwok, J. Arey and S. M. Aschmann,
Faraday Discuss., 1995, 100, 23–37.
2 S. M. Aschmann, A. A. Chew, J. Arey and R. Atkinson, J. Phys.
Chem. A, 1997, 101, 8042–8.
3 J. M. O’Brien, E. Czuba, D. R. Hastie, J. S. Francisco and P. B.
Shepson, J. Phys. Chem. A, 1998, 102, 8903–8.
4 X. Chen, D. Hulbert and P. B. Shepson, J. Geophys. Res., 1998,
34 J. J. Orlando, G. S. Tyndall and T. J. Wallington, Chem. Rev.,
2003, 103, 4657–89.
35 H. G. Libuda, O. Shestakov, J. Theloke and F. Zabel, Phys. Chem.
Chem. Phys., 2002, 4, 2579–86.
3
6 W. P. L. Carter and R. Atkinson, J. Atmos. Chem., 1989, 8,
65–73.
1
103, 25563–8.
37 R. Atkinson, J. Phys. Chem. Ref. Data, 1997, 26, 215–90.
38 N. I. Butkovskaya, A. Kukui, N. Pouvesle and G. LeBras, J. Phys.
Chem. A, 2005, 109, 6509–20.
39 G. B. Ellison, J. M. Herbert, A. B. McCoy, J. F. Stanton and P. G.
Szalay, J. Phys. Chem. A, 2004, 108, 7639–42.
5 J. Platz, J. Sehested, O. J. Nielsen and T. J. Wallington, J. Phys.
Chem. A, 1999, 103, 2688–95.
6 J. J. Orlando, L. T. Iraci and G. S. Tyndall, J. Phys. Chem. A,
2000, 104, 5072–9.
This journal is ꢁc the Owner Societies 2007
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