Reaction of limonene with F2: Rate coefficient and products

Article abstract of DOI:10.1021/jp509498a

The kinetics of the reaction of limonene (C₁₀H₁₆) with F₂ has been studied using a low pressure (P = 1 Torr) and a high pressure turbulent (P = 100 Torr) flow reactor coupled with an electron impact ionization and chemical ionization mass spectrometers, respectively: F₂ + Limonene → products (1). The rate constant of the title reaction was determined under pseudo-first-order conditions by monitoring either limonene or F₂ decay in excess of F₂ or C₁₀H₁₆, respectively. The reaction rate constant, k₁ = (1.15 ± 0.25) × 10^-12 exp(160 ± 70)/T) was determined over the temperature range 278-360 K, independent of pressure between 1 (He) and 100 (N₂) Torr. F atom and HF were found to be formed in reaction 1, with the yields of 0.60 ± 0.13 and 0.39 ± 0.09, respectively, independent of temperature in the range 296-355 K. (Graph Presented).

Full text of DOI:10.1021/jp509498a

Article
pubs.acs.org/JPCA
Reaction of Limonene with F2: Rate Coefficient and Products
Yuri Bedjanian,* Manolis N. Romanias, and Julien Morin
Institut de Combustion, Aer
Cedex 2, France
́ ́ ́ ́ ́ ́
othermique, Reactivite et Environnement (ICARE), CNRS and Universite d’Orleans, 45071 Orleans
ABSTRACT: The kinetics of the reaction of limonene (C10H16) with F2 has been studied
using a low pressure (P = 1 Torr) and a high pressure turbulent (P = 100 Torr) flow
reactor coupled with an electron impact ionization and chemical ionization mass
spectrometers, respectively: F2 + Limonene → products (1). The rate constant of the title
reaction was determined under pseudo-first-order conditions by monitoring either
limonene or F2 decay in excess of F2 or C10H16, respectively. The reaction rate constant, k1
= (1.15 0.25) × 10−12 exp(160 70)/T) was determined over the temperature range
278−360 K, independent of pressure between 1 (He) and 100 (N2) Torr. F atom and HF
were found to be formed in reaction 1, with the yields of 0.60 0.13 and 0.39 0.09,
respectively, independent of temperature in the range 296−355 K.
In addition to general interest, the information on the
reactions between closed-shell molecules is of practical interest
for the flow tube and smog chamber kinetic and mechanistic
investigations of the gas phase reactions, where the reactions
between closed-shell molecules can act as unexpected and
surprisingly rapid side processes. For example, we have
observed the reaction between F2 and limonene (the title
reaction) upon experimental study of the reaction of limonene
with OH radicals, where reaction F + H2O was used as a source
of OH.9
In the present work we report the results of the kinetic and
mechanistic study of the reaction of F2 with limonene (C10H16),
a monoterpene, containing a cyclohexene ring and ethylene
tailing group:
1. INTRODUCTION
Compared with reactions involving free radicals, the kinetic and
mechanistic data on the elementary reactions between closed-
shell molecules are very limited. Generally, these reactions
exhibit significant energy thresholds and, consequently, are very
slow at moderate temperatures. In this respect, the F2 molecule,
manifesting relatively high reactivity with respect to some stable
molecules, seems to be an exception of the rule. For example,
crossed molecular beam experiments supported by ab initio
calculations have demonstrated that reactions of molecular
fluorine with organosulfur compounds, DMS (CH2SCH3) and
DMDS (CH3SSCH3), are barrierless reactions.1−3 An un-
expectedly high value, 1.6 × 10−11 cm3 molecule−1 s−1 at 298 K,
was reported for the rate constant of the reaction of F2 with
DMS.4
F + C10H16 → products
(1)
2
Reactions of F2 with alkenes represent another interesting
class of reactions of molecular fluorine, which were shown to
proceed with a relatively low barrier. Experimental crossed
beam data give a threshold in collision energy of 5.55 and 2.4
The rate constant of the reaction as well as the reactive
pathways were determined over the temperature range 278−
360 K.
6
kcal mol−1 for F2 reactions with C2H4 and C3H6, respectively,
2. EXPERIMENTAL SECTION
while F2 reaction with double methyl-substituted ethylene is
expected to be barrierless according to the theoretical
predictions.7 Concerning the products of the reactions of F2
with C2H4, C3H6, and C4H8, in recent crossed molecular beam
studies only the F atom formation channel was observed.5−7 It
should be noted that despite recent interesting experimental
(basically cross beam data) and theoretical findings, there are
virtually no quantitative kinetic data (in particular, on the
temperature dependence of the rate constants) on the reactions
of F2 with closed-shell molecules. More studies are needed in
order to eliminate some existing discrepancies between
experiment and theory,8 and to better understand the nature
of the specificity of F2 molecule regarding its reactivity toward
stable molecules.
Two experimental setups were used: low pressure discharge
flow reactor combined with an electron impact ionization mass
spectrometer (EIMS)10−13 and high pressure turbulent flow
reactor with chemical ionization mass spectrometry (CIMS) as
a detection method.14
2.1. Low Pressure Flow Tube/EIMS. The low pressure
flow tube, shown in Figure 1 along with the movable injector of
the reactants, consisted of a Pyrex tube (45 cm length and 2.4
cm i.d.) with a jacket for the thermostated liquid circulation
(water or ethanol). The walls of the reactor as well as of the
Received: September 19, 2014
Revised: October 16, 2014
Published: October 20, 2014
© 2014 American Chemical Society
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Figure 1. Diagram of the flow reactor.
injector were coated with halocarbon wax in order to minimize
the heterogeneous loss of active species.
reactor was operated at T = 302 K and pressure of 100 Torr of
N2 with a flow velocity of 2080 cm s−1 (Reynolds number Re ≈
4100). For the introduction of the reactants into the reactor the
configuration similar to that described above for the low
pressure flow system was used.
The limonene vapor was introduced into the flow reactor by
passing helium through a thermostated glass bubbler containing
liquid limonene or from a 10 L flask with known limonene/He
mixture. In the low pressure experiments, all species were
detected by mass spectrometry at their parent peaks m/z = 136
The reactive mixture, sampled from the TFR through a
Teflon cone, was ionized in the ion−molecule reactor (stainless
steel, 20 cm length and 4 cm i.d., 0.9 Torr total pressure of Ar).
Primary Ar+ ions were produced by electron impact ionization,
the electrons being emitted by a heated filament (thoriated
irridium) and accelerated with 20 V potential. The primary
(C10H16+), 38 (F2 ), 20 (HF+), 98/100 (FBr+), 160 (Br2 ). The
concentrations of the stable species (including limonene) 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. For limonene, similar
(within 10%) results were obtained when absolute calibration
of the mass spectrometer was performed by injecting known
amounts (a few μL) of liquid limonene inside the flow tube and
integrating the area of the mass spectrometric signals of C10H16
at m/z = 136.
+
+
−
negative ions, SF6 , were formed via electron attachment to
−
SF6. F2 was detected in a negative mode as F2 (m/z = 38)
−
formed by electron transfer from SF6 . Limonene molecules
were detected in a positive mode as C10H16+ ions (m/z = 136),
formed via electron impact ionization.
Nitrogen, used as the bath gas in CIMS experiments, was
obtained by evaporation of liquid nitrogen (Air Liquide). The
purities of other gases used were as follows: He >99.9995%
(Alphagaz), argon >99.9995% (Alphagaz), passed through
liquid nitrogen traps; limonene >98% (Fluka), degassed before
use; Br2 >99.99% (Aldrich); F2, 5% in helium (Alphagaz).
F atoms and HF were observed as the products of the title
reaction. Fluorine atoms were detected at m/z = 98/100 (FBr+)
after being scavenged with Br2:
F + Br2 → Br + FBr
(2)
This reaction was also used for the determination of the
absolute concentrations of FBr via conversion of F atoms to
FBr in reaction 2 with an excess Br2: [FBr] = Δ[Br2], i.e.,
concentration of FBr was determined from the consumed
fraction of [Br2]. In the calibration experiments, F atoms were
formed in the microwave discharge of F2/He mixtures. In order
to reduce F atom reactions with glass surface inside the
microwave cavity, a ceramic (Al2O3) tube was inserted in this
part of the injector. For the absolute calibration of HF signals,
the reaction of F atoms with hydrogen was applied:
3. RESULTS
3.1. Reaction Rate Constant. 3.1.1. EIMS Measurements.
The measurements of the rate constant of reaction 1 were
carried out at 1 Torr total pressure of helium under pseudo-
first-order conditions in excess of fluorine over limonene. The
initial concentration of limonene was (0.5−0.8) × 1012
molecules cm−3. The range of the concentrations of F2 as
well as the flow velocities in the reactor are presented in Table
1. The concentrations of both limonene and F2 were
simultaneously measured as a function of reaction time.
Consumption of F2 was negligible as a result of its sufficient
excess over limonene.
F + H2 → H + HF
(3)
Two different approaches were used. In the first one, the
absolute concentration of HF was determined via complete
conversion of H2 to HF with an excess of F atoms: [HF] =
[H2]0. In another approach, F atoms were successively
converted to HF and HBr in reactions with excess H2 and
Br2, respectively:
Table 1. Experimental Conditions and Results of the
Measurements of the Rate Constant of the Reaction F2 +
Limonene at 1 Torr Total Pressure of Helium
a
b
c
d
No./exp.
T, K
flow velocity
[F2]
k1
F + H2 → H + HF
(3)
6
278
298
323
350
360
2440−3120
1280−1660
1370
0.38−5.05
0.12−1.65
0.04−0.65
0.11−1.58
0.16−2.00
2.03
1.99
1.79
1.82
1.83
11
6
H + Br2 → HBr + Br
(4)
In this case, [F]0 = [HF] = Δ[Br2], i.e., concentration of HF,
could be determined from the consumed fraction of [Br2]. Two
methods of HF calibration gave similar (within 10%) results.
2.2. CIMS Apparatus. A turbulent flow reactor (TFR)
coupled to a quadrupole mass spectrometer with chemical
ionization was described earlier.14 In the present study, the
7
1980−2410
2000−2160
6
a
b
c
Number of kinetic runs. Units of cm s−1. Units of 1014 molecules
d
cm−3. Units of 10−12 cm3 molecule−1 s−1, estimated uncertainty on k1
is nearly 15%.
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As could be expected for the bimolecular reaction carried out
under pseudo-first order conditions in excess of molecular
fluorine, the exponential decays of limonene were observed
(Figure 2).
respective temperatures. All the results obtained for k1 within
the described approach and at different temperatures are shown
in Table 1. At lower temperatures in the reactor, we have
observed an abnormal increase of the reaction rate constant,
which might be related to the surface adsorbed limonene. The
possible impact of the wall reaction on the results obtained in
this section is discussed below, based on the data observed in a
high pressure turbulent flow reactor.
As it will be shown below, F atoms and HF were found to be
the primary products of reaction 1
F + C10H16 → F + C10H16F
(1a)
(1b)
2
F + C10H16 → HF + C10H15F
2
Additional F atoms could be produced in reactions of F2 with
fluorinated organic radicals formed in reaction 1a. In this
respect, the possible impact on the measurements of k1 of the
secondary reactions of F atoms with limonene
F + C10H16 → products
(5)
was explored in separate series of experiments at T = 298 K,
where the rate constant of reaction 1 was measured with fixed
concentration of F2 (4.0 × 1013 molecules cm−3) and
concentration of limonene varied in the range (0.4−22.3) ×
1012 molecules cm−3. The results of these measurements are
shown in Figure 4.
Figure 2. Examples of the limonene consumption kinetics in reaction
1 observed with different concentrations of F2: T = 360 K, P = 1 Torr.
Figure 3 shows the pseudo-first-order rate constant, k1′ =
k1[F2], as a function of the concentration of fluorine. All the
Figure 4. Dependence of the rate coefficient of reaction 1 on the initial
concentration of limonene: T = 298 K, [F2] = 4.0 × 1013 molecules
cm−3. Error bars correspond to estimated 15% uncertainty on the
measurements of k1.
Figure 3. Example of pseudo-first-order plots obtained from limonene
decay kinetics in excess of F2 (T = 298 and 360 K).
At low concentrations of limonene (≤2 × 1012 molecules
cm−3), the measured rate constant (k1 = 1.87 × 10−12 cm3
molecule−1 s−1, dotted line in Figure 3) was found to be
independent (within a few %) of the initial concentration of
limonene, indicating the negligible contribution of the
secondary chemistry to the limonene loss under the
experimental conditions used in the measurements of k1:
[Limonene]0 = (0.5−0.8) × 1012 molecules cm−3. A trend
toward an increase with increasing [C10H16]0 is observed for k1
at higher initial concentrations of limonene. Unfortunately,
these data do not allow for estimation of the rate constant of
measured values of k1′ were corrected for axial and radial
diffusion15 of limonene. For the diffusion coefficient of
limonene in He we have used that of benzene, D0 = 460
Torr cm2 s−1 at T = 423 K16 with T3/2-dependence on
temperature. Corrections were generally less than 5%, being
higher (up to 12%) in a few kinetic runs only. The straight lines
in Figure 3 represent linear through origin (no consumption of
limonene was observed in the absence of F2 in the reactor) fits
to the experimental data. Their slopes give the values of k1 at
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the reaction 5, since neither F atom sources (reactions of F2
with radicals produced in reaction 1a) nor loss processes (wall
loss, reactions with fluoro organic radicalsproducts of
reaction 1 are well characterized.
heterogeneous complications, when F2 is in excess, and that
the rate constant of reaction 1 is pressure independent in the
range 1−100 Torr.
3.1.3. Temperature Dependence of k1. All the results
obtained for k1 at different temperatures are shown in Figure 6.
3.1.2. CIMS Measurements. All the values of k1 measured in
a low pressure flow tube (Table 1) were obtained from the
kinetics of limonene consumption in excess of F2. The attempts
to measure k1 monitoring decays of F2 in excess of limonene led
to systematically higher values of the rate constant. This made
us think of possible contribution of the heterogeneous reaction
of F2 with surface adsorbed limonene. In order to verify if the
low pressure measurements are affected by heterogeneous
chemistry we have carried out the measurements of the rate
constant of reaction 1 in a high pressure turbulent flow reactor,
which can be considered as a wall-free reactor due to limited
mass exchange between turbulent core (central portion of the
flow) and laminar sublayer (the region near the wall).17
Two series of experiments were conducted: k1 was
determined from both limonene ([Limonene]0 ∼5 × 1011
molecules cm−3) and F2 ([F2]0 ≤ 1012 molecules cm−3) decays
monitored in excess of F2 ([F2] = (0.07−1.20) × 1014
molecules cm−3) and limonene ([C10H16] = (0.14−1.21) ×
1014 molecules cm−3), respectively. The measurements were
carried out at near room temperature and 100 Torr total
pressure of N2 in the reactor, the flow velocity in the reactor
was 2080 cm s−1. Figure 5 shows the experimental data
observed for the pseudo-first-order rate constants k1′.
Figure 6. Temperature dependence of the rate constant of the reaction
F2 + Limonene: EIMS, electron impact ionization mass spectrometry,
P = 1 Torr; CIMS, chemical ionization mass spectrometry, P = 100
Torr.
The combined uncertainty on the measurements of the rate
constant was estimated to be in the range 15−20%, including
statistical error and those on the measurements of the flows,
pressure, temperature, and the absolute concentrations of the
relevant species. The unweighted exponential fit to the present
data for k1 yields the following Arrhenius expression:
k1 = (1.15 0.25) × 10−12exp(160 70)/T)
cm3 molecule−1 s−1
where the cited uncertainties are 1σ statistical ones. The
experimental results for k1 can also be well represented with
temperature independent value (dotted line in Figure 6) of
k1 = (1.92 0.30) × 10−12 cm3 molecule−1 s−1
at T = 278−360 K
Figure 5. Pseudo-first-order plots obtained in turbulent flow reactor
(P = 100 Torr of N2) from limonene decay kinetics in excess of F2 (T
= 302 K, filled symbols) and from F2 decays in excess of limonene (T
= 303 K, open symbols).
3.2. Reaction Products. Experiments on the identification
and quantification of the reaction products were carried out in a
low pressure flow tube/EIMS system. Significant increase in the
peak intensity at m/z = 20 was observed upon reaction of
limonene and F2, that was attributed to the formation of HF as
a reaction product. In addition, we have observed (but not
quantified) the appearance of the signal at m/z = 154, which
could correspond to C10H15F, coproduct of HF in reaction 1b.
In order to check for the formation of another possible reaction
product, F atom, Br2 was added in the reactive system: if F
atoms are formed in reaction 1 they could be scavenged in a
rapid reaction with Br2 forming FBr molecules. Indeed, the
formation of FBr at m/z = 98/100 (FBr+) was observed upon
addition of Br2, showing the presence of F atom in the reaction
products. Another observation was that HF signal decreased
The slopes of the linear through origin dependencies in
Figure 5 provide the following values for the rate constant: k1 =
2.12 × 10−12 at T = 302 K and 1.86 × 10−12 cm3 molecule−1 s−1
at T = 303 K, measured in excess of F2 and limonene,
respectively. These results are in good agreement (within
estimated 15% uncertainty of the measurements) with each
other, as well as with the value of k1 = 1.99 × 10−12 cm3
molecule−1 s−1 measured with EIMS at T = 298 K and 1 Torr
total pressure in the reactor. These results indicate that the low
pressure measurements were not affected by possible
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upon addition of Br2, indicating that in Br2-free system, HF was
partly formed in secondary reactions of F atoms.
Table 2. F Atom and HF Yields in the Reaction of F2 with
Limonene As a Function of Temperature
Typical experiments on the determination of the yields of the
reaction products, F and HF, consisted of the measurements of
the concentration of limonene consumed and concentrations
formed of the products in excess of F2 ([F2] = (2.5−5.0) × 1013
molecules cm−3) and in the presence of relatively high
concentrations of Br2 ([Br2] = (1.5−4.3) × 1014 molecules
cm−3) in the reactor. Reaction time was approximately 0.03 s in
all the experiments and initial concentration of limonene was
varied in the range (0.76−8.30) × 1012 molecules cm−3. Under
these experimental conditions, more than 80% of limonene was
consumed in the reaction with F2. The concentrations formed
of the reaction products, HF and F, were detected at m/z = 20
(HF+) and 98 (FBr+) as a function of the concentration
consumed of limonene. Example of the experimental data
observed at T = 323 K is shown in Figure 7.
a
b
b
No./exp.
T (K)
k1a/k1
0.568
k1b/k1
0.428
5
5
7
5
6
5
5
296
315
323
333
338
343
355
0.607
0.601
0.627
0.611
0.585
0.631
0.411
0.385
0.420
0.354
0.351
0.403
c
mean:
0.604 0.022
0.393 0.031
a
b
Number of experimental runs. All data were obtained in excess of F2
over limonene, except T = 338 K, where excess of limonene over F2
c
was used. Uncertainty is 1σ statistical one.
secondary reactions of F2 with fluoroorganic radicals produced
in reaction 1.
F + C10H16F → F + C10H16F2
(6)
2
It can be assumed that in the reactive system used, C10H16F
radicals react preferentially with Br2, considering high reactivity
19,20
of hydrocarbon radicals toward Br2
(although halogen
substitution somewhat reduces the reactivity)21 and excess of
Br2 over F2 in the present experiments:
Br2 + C10H16F → Br + C10H16FBr
(7)
This interpretation seems to be supported by the experimental
observation of the invariance of the branching ratio data,
obtained with [Br2]/[F2] ratio varied from 4 to 16.
Finally, we have carried out another series of measurements,
this time, in excess of limonene over F2. The concentrations of
the reaction products were monitored as a function of the
consumed concentration of F2. Initial concentration of fluorine
was varied in the range (0.7−5.0) × 1012 molecules cm−3.
Reaction time and concentration of limonene being ∼0.04 s
and ∼1013 molecules cm−3, respectively, nearly half of F2 was
consumed in reaction 1 under these experimental conditions. It
was observed that addition of Br2 ((2−5) × 1014 molecules
cm−3) into the reactive limonene/F2 mixture led to significant
decrease of the limonene concentration, most probably due to
possible secondary (chain) reactions of limonene with Br atoms
initially formed in reactions of Br2 with F atoms and organic
radicals - products of reaction 1:
Figure 7. Concentrations of F atoms and HF formed in reaction 1 as a
function of the consumed concentration of limonene: T = 323 K.
Error bars correspond to 15% uncertainties on the determination of
the absolute concentrations of the relevant species.
The slope of the straight line in Figure 7 provides the
branching ratios for the fluorine atoms and HF forming
channels of reaction 1: 0.601 and 0.385, respectively. All the
results obtained in this way for k1a/k1 and k1b/k1 at different
temperatures are shown in Table 2.
Br + C10H16 → HBr + C10H15
(8)
Br + C10H16(+M) → C10H16Br(+M)
(9)
Br2 + C10H15 → Br + C10H15Br
(10)
The reactive system limonene/F2/Br2 being rather complex,
the possible impact of the secondary and side reactions on the
results of the measurements of the product yields should be
discussed. First, it should be noted that Br2 did not react with
either limonene or F2, despite the high concentrations of
bromine used. This was verified in separate experiments with
Br2/limonene and Br2/F2 mixtures. F atoms, formed in reaction
1, were scavenged with Br2, but they could also participate in
secondary reactions with limonene and products of reaction 1.
However, the possible impact of these reactions can be
neglected, considering the very high value of the rate constant
of the reaction F+Br2, k2 = (2.2 1.1) × 10−10 cm3 molecule−1
s−1 at T = 299 K,18 and high [Br2] to [limonene]0 ratio (≥20)
used in these experiments. Another issue, which should be
considered, is possible formation of additional F atoms in
Br2 + C10H16Br → Br + C10H16Br2
(11)
In the context of the measurements of k1a/k1 and k1b/k1 ratios,
it is important to note that this secondary chemistry does not
impact the concentration of F2 and those of the reaction
products, F (detected as FBr) and HF. Results of these
experiments are shown in Figure 8.
The yields of F atoms and HF, defined by the slopes of the
linear dependencies in Figure 8, are 0.611 and 0.354,
respectively. These data are very close to those obtained in
excess of F2 over limonene. Similarity of the results obtained
under different experimental conditions is an indication of the
limited impact of the uncontrolled side chemistry on the
measurements of the product yields. In particular, it indicates
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F + C10H16 → F + C10H16F (ring)
2
ΔH = −15.8 kcal mol−1
ΔG = −14.3 kcal mol−1
F + C10H16 → F + C10H16F (tail)
2
ΔH = −14.8 kcal mol−1
ΔG = −14.6 kcal mol−1
F + C10H16 → C10H16F2 (ring)
2
ΔH = −123 kcal mol−1
ΔG = −111 kcal mol−1
ΔG = −109 kcal mol−1
F + C10H16 → C10H16F2 (tail)
2
ΔH = −121 kcal mol−1
The first channel concerns the formation of HF and the
corresponding ring or tail monofluoro-limonene, with ΔH ≈
−108 and ≈ −104 kcal mol−1, respectively. The second
pathway corresponds to the attack of F2 to one of the double
bonds of limonene leading to the formation of F atoms and the
corresponding ring or tail fluorinated radical with ΔH ≈ −15.8
and ≈ −14.8 kcal mol−1, respectively. Finally, the last channel
(not observed in our experiments) represents the addition of F2
to limonene double bonds forming the ring or tail difluoro-
limonene with ΔH ≈ −123 and ≈ −121 kcal mol−1,
respectively. These calculations show that the reaction channels
leading to the formation of the observed products, F and HF,
are exothermic, as one could expect considering the absence of
the reaction barrier, indicated by the observed slightly negative
temperature dependence of the total rate constant of reaction 1.
Besides, the observed negative temperature dependence of the
rate constant seems to point to the addition/elimination
mechanism. It should be noted that to get accurate energies,
single point calculations should be performed at higher levels of
theory, which demonstrate small deviations in comparison with
experimental values of energies and enthalpies of forma-
tion.23,24 However, in the case of heavy molecules (e.g.,
limonene) single point calculations are time-consuming, require
high computer power, and were beyond the scope of this work.
To our knowledge, the present study is the first one
reporting the experimental measurements of the rate constant
for reaction of F2 with unsaturated organic compound. The
previous crossed molecular beam studies of F2 reactions with
alkenes5−7 provided the information on the reaction threshold,
but not on the reaction rate constant.
In the present study, the limonene fluorination channel (HF
forming channel) was found to proceed with the branching
ratio of nearly 40%. This strongly differs from the results
observed previously for reactions of F2 with alkenes: HF was
not observed in crossed molecular beam studies of the reactions
of F2 reactions with C2H4, C3H6, and isomers of C4H8.5−7 At
this stage, it is difficult to assign the reason for the difference in
the mechanisms of the interaction of F2 with double bonds in
limonene and simple alkenes. More detailed experimental
(kinetic and mechanistic) and theoretical studies are needed to
better understand the dynamics of the reactions of F2 with
unsaturated organic compounds.
Figure 8. Concentrations of F atoms and HF formed in reaction 1 as a
function of the consumed concentration of F2: T = 338 K. Error bars
correspond to 15% uncertainties on the determination of the absolute
concentrations of the relevant species.
that in excess of F2, the possible secondary reaction of F2 with
the coproduct of HF, C10H15F, which is a closed shell molecule
with two double bonds, does not affect the results of the
measurements of F and HF yields in reaction 1.
Finally, the k1a/k1 and k1b/k1 ratios (Table 2) were found to
be independent of temperature between 296 and 355 K, and
the mean values
k1a/k1 = 0.60 0.13
k1b/k1 = 0.39 0.09
can be recommended from this study, with estimated
uncertainty of nearly 20%.
4. DISCUSSION
F atoms and HF molecules were observed as products of
reaction 1 with the sum of their yields corresponding to nearly
100% of the fluorine mass balance. In order to verify the
thermochemistry of the reaction pathways leading to the
formation of these products, we have conducted density
functional theory (DFT) calculations. All of the corresponding
conformer structures were obtained by performing geometry
optimization calculations at the B3LYP/6-31+G(d) level of
theory, with the Gaussian 09 program suite.22 Thermochemical
parameters were obtained from the calculations of vibrational
frequencies, at the same level of theory. Three different
pathways, all resulting from the initial interaction of F2 with one
of the double bond of limonene, were considered in the
calculations:
F + C10H16 → HF + C10H15F (ring)
5. CONCLUSION
2
ΔH = −108 kcal mol−1
ΔG = −107 kcal mol−1
In this work, we investigated the kinetics and products of the
reaction between two closed-shell molecules, F2 and limonene
(C10H16), using two experimental setups: a low pressure (P = 1
Torr) and a high pressure turbulent (P = 100 Torr) flow
reactor coupled with an electron impact ionization and
F + C10H16 → HF + C10H15F (tail)
2
ΔH = −104 kcal mol−1
ΔG = −103 kcal mol−1
10238
dx.doi.org/10.1021/jp509498a | J. Phys. Chem. A 2014, 118, 10233−10239

The Journal of Physical Chemistry A
Article
(11) Bedjanian, Y.; Laverdet, G.; Le Bras, G. Low-Pressure Study of
the Reaction of Cl Atoms with Isoprene. J. Phys. Chem. A 1998, 102,
953−959.
(12) Bedjanian, Y.; Poulet, G.; Bras, G. L. Low-Pressure Study of the
Reactions of Br Atoms with Alkenes. 1. Reaction with Propene. J. Phys.
Chem. A 1998, 102, 5867−5875.
(13) Bedjanian, Y.; Riffault, V.; Le Bras, G.; Poulet, G. Kinetic Study
of the Reactions of OH and OD with HBr and DBr. J. Photochem.
Photobiol., A 1999, 128, 15−25.
chemical ionization mass spectrometers, respectively. The
reaction rate constant determined either from limonene or
from F2 loss kinetics was found to be independent of pressure
between 1 and 100 Torr and to slightly decrease with increase
of temperature in the range 278−360 K. Two major reaction
pathways leading to production of F atom and HF were
identified and the corresponding branching ratios were
measured to be 0.60
0.13 and 0.39
0.09, respectively,
independent of temperature in the range 296−355 K. Density
functional theory (DFT) calculations showed that the reaction
channels leading to the formation of the observed products, F
and HF, are exothermic, as one could expect considering the
absence of the reaction barrier, indicated by the observed
slightly negative temperature dependence of the total rate
constant. Similar experimental studies of the reactions of F2
with smaller alkenes in combination with high level calculations
would be very useful to better understand the dynamics of the
reactions between these closed-shell molecules.
(14) Kukui, A.; Borissenko, D.; Laverdet, G.; Le Bras, G. Gas-Phase
Reactions of OH Radicals with Dimethyl Sulfoxide and Methane
Sulfinic Acid Using Turbulent Flow Reactor and Chemical Ionization
Mass Spectrometry. J. Phys. Chem. A 2003, 107, 5732−5742.
(15) Kaufman, F. Kinetics of Elementary Radical Reactions in the
Gas Phase. J. Phys. Chem. 1984, 88, 4909−4917.
(16) Poling, B.; Prausnitz, J.; Connell, J. O. The Properties of Gases
and Liquids; McGraw-Hill Education, 2000.
(17) Seeley, J. V.; Jayne, J. T.; Molina, M. J. High Pressure Fast-Flow
Technique for Gas Phase Kinetics Studies. Int. J. Chem. Kinet. 1993,
25, 571−594.
(18) Bemand, P. P.; Clyne, M. A. A. Kinetic Spectroscopy in the Far
Vacuum Ultraviolet. Part 2.-Fluorine Atom Resonance Spectrometry
AUTHOR INFORMATION
■
2
and the Measurement of F P Atom Concentrations. J. Chem. Soc.,
Corresponding Author
Faraday Trans. 2 1976, 72, 191−206.
*Tel.: +33 238255474, Fax: +33 238696004, e-mail: yuri.
bedjanian@cnrs-orleans.fr.
(19) Timonen, R. S.; Seetula, J. A.; Gutman, D. Kinetics of the
Reactions of Unsaturated Hydrocarbon Free Radicals (Vinyl,
Propargyl, and Allyl) with Molecular Bromine. J. Phys. Chem. 1993,
97, 8217−8221.
Notes
The authors declare no competing financial interest.
(20) Timonen, R. S.; Seetula, J. A.; Gutman, D. Kinetics of the
Reactions of Alkyl Radicals (CH3, C2H5, i-C3H7, and t-C4H9) with
Molecular Bromine. J. Phys. Chem. 1990, 94, 3005−3008.
(21) Timonen, R. S.; Seetula, J. A.; Niiranen, J.; Gutman, D. Kinetics
of the Reactions of Halogenated Methyl Radicals with Molecular
Bromine. J. Phys. Chem. 1991, 95, 4009−4014.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A. et al. Gaussian 09, Gaussian, Inc.: Wallingford, CT,
USA, 2009.
(23) Baboul, A. G.; Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.
Gaussian-3 Theory Using Density Functional Geometries and Zero-
Point Energies. J. Chem. Phys. 1999, 110, 7650−7657.
(24) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.;
Pople, J. A. Gaussian-3 (G3) Theory for Molecules Containing First
and Second-Row Atoms. J. Chem. Phys. 1998, 109, 7764−7776.
ACKNOWLEDGMENTS
■
J. M. is very grateful for his PhD grant from CAPRYSSES
project (ANR-11-LABX-006-01) funded by the French Na-
tional Research Agency (ANR) through the PIA (Programme
d’Investissement d’Avenir).
REFERENCES
■
(1) Shao, H.-C.; Xie, T.; Lu, Y.-J.; Chang, C.-H.; Pan, J.-W.; Lin, J. J.
Barrierless Reactions between Two Closed-Shell Molecules. II.
Dynamics of F2+CH3SSCH3 Reaction. J. Chem. Phys. 2009, 130,
014301.
(2) Lu, Y.-J.; Lee, L.; Pan, J.-W.; Xie, T.; Witek, H. A.; Lin, J. J.
Barrierless Reactions between Two Closed-Shell Molecules. I.
Dynamics of F2+CH3SCH3 Reaction. J. Chem. Phys. 2008, 128,
104317.
(3) Lu, Y.-J.; Lee, L.; Pan, J.-W.; Witek, H. A.; Lin, J. J. Dynamics of
the F2+CH3SCH3 Reaction: A Molecule-Molecule Reaction without
Entrance Barrier. J. Chem. Phys. 2007, 127, 101101.
(4) Turnipseed, A. A.; Birks, J. W. Kinetics of the Reaction of
Molecular Fluorine with Dimethyl Sulfide. J. Phys. Chem. 1991, 95,
6569−6574.
(5) Lu, Y.-J.; Xie, T.; Fang, J.-W.; Shao, H.-C.; Lin, J. J. Dynamics of
the F2 Reaction with the Simplest π-Bonding Molecule. J. Chem. Phys.
2008, 128, 184302.
(6) Fang, J.-W.; Xie, T.; Chen, H.-Y.; Lu, Y.-J.; Lee, Y. T.; Lin, J. J.
Dynamics of the F2 Reaction with Propene: The Effect of Methyl
Substitution. J. Phys. Chem. A 2009, 113, 4381−4386.
(7) Li, Z.-Q.; Tsai, C.-H.; F Chen, A.; Lu, Y.-J.; Lin, J. J. Double
Methyl Substitution in Simple Alkenes Leads to Low-Barrier Reactions
towards Molecular Fluorine. Chem. Phys. Lett. 2011, 510, 42−47.
(8) Lin, J. J. Dynamics of Reactions between Two Closed-Shell
Molecules. Phys. Chem. Chem. Phys. 2011, 13, 19206−19213.
(9) Braure, T.; Bedjanian, Y.; Romanias, M. N.; Morin, J.; Riffault, V.;
Tomas, A.; Coddeville, P. Experimental Study of the Reactions of
Limonene with OH and OD Radicals: Kinetics and Products. J. Phys.
Chem. A 2014, 118, 9482−9490.
(10) Bedjanian, Y.; Le Bras, G.; Poulet, G. Kinetics and Mechanism
of the IO + ClO Reaction. J. Phys. Chem. A 1997, 101, 4088−4096.
10239
dx.doi.org/10.1021/jp509498a | J. Phys. Chem. A 2014, 118, 10233−10239