Direct measurements of C3F7I dissociation rate constants using a shock tube ARAS technique

Article abstract of DOI:10.1002/kin.21244

This work presents the first direct experimental study on the thermal unimolecular decomposition of n-C₃F₇I. Experiments were performed behind incident and reflected shock waves using the atomic resonance absorption spectroscopy (ARAS) technique on a resonant line of atomic iodine at 183.04?nm. The reaction C₃F₇I?+?Ar?→?C₃F₇?+?I?+?Ar?(1) was studied at specific temperature (800–1200?K) and pressure (0.6–8.3?bar) ranges. Under experimental conditions, the obtained values of the rate constant at temperatures below 950?K are close to the high-pressure limit; however, considering theoretical calculations, the influence of pressure on the rate constant at elevated temperatures remains noticeable. The resulting value of the experimental rate constant of reaction 1 is presented in the following Arrhenius form: 1st(±30%) = 1.05×1014 exp(?200.4kJ?mol^?1∕)(s^?1). Experimental data were found to correlate with the results of the Rice–Ramsperger–Kassel–Marcus –master equation analysis based on quantum-chemical calculations. The following low- and high-pressure limiting rate coefficients were obtained over the temperature range T?=?300–3000?K: (Formula presented.). with the center broadening factor F_c?=?0.119.

Full text of DOI:10.1002/kin.21244

Received: 31 August 2018
DOI: 10.1002/kin.21244
Revised: 5 December 2018
Accepted: 7 December 2018
A R T I C L E
Direct measurements of C3F7I dissociation rate constants using a
shock tube ARAS technique
Nikita Bystrov1,2
Alexander Emelianov1
Alexander Eremin1
Boris Loukhovitski3
Alexander Sharipov3
Pavel Yatsenko1,2
1Joint Institute for High Temperatures of
the Russian Academy of Sciences, Moscow,
Russia
Abstract
This work presents the first direct experimental study on the thermal unimolec-
ular decomposition of n-C3F7I. Experiments were performed behind incident
and reflected shock waves using the atomic resonance absorption spectroscopy
(ARAS) technique on a resonant line of atomic iodine at 183.04 nm. The reaction
C3F7I + Ar → C3F7 + I + Ar (1) was studied at specific temperature (800–1200 K)
and pressure (0.6–8.3 bar) ranges. Under experimental conditions, the obtained val-
ues of the rate constant at temperatures below 950 K are close to the high-pressure
limit; however, considering theoretical calculations, the influence of pressure on the
rate constant at elevated temperatures remains noticeable. The resulting value of the
experimental rate constant of reaction 1 is presented in the following Arrhenius form:
2Bauman Moscow State Technical University,
Moscow, Russia
3Central Institute of Aviation Motors,
Moscow, Russia
Correspondence
Alexander Eremin, Joint InstituteforHigh
Temperatures oftheRussian Academy of
Sciences, Izhorskaya13 Bldg. 2, Moscow
125412, Russia.
Email:eremin@ihed.ras.ru
Fundinginformation
RussianScience Foundation, Grant/Award
ꢀ1st( 30%) = 1.05 × 1014 exp(−200.4 kJ ⋅ mol−1∕ꢁꢂ ) (s−1
)
Number:14-19-00025P
Experimental data were found to correlate with the results of the Rice–Ramsperger–
Kassel–Marcus –master equation analysis based on quantum-chemical calculations.
The following low- and high-pressure limiting rate coefficients were obtained over the
temperature range T = 300–3000 K:
(
)
ꢀ0 ꢃ−C3F7I + Ar = 8.849 × 1019ꢂ −1.444 exp (−11, 454.4∕ꢂ ) (cm3mol−1 −1
s
) and
(
)
ꢀ∞ ꢃ−C3F7I = 9.676 × 1016ꢂ −0.378 exp (−26, 956.8∕ꢂ ) (s−1
)
with the center broadening factor F = 0.119.
c
K E Y W O R D S
atomic resonance absorption spectroscopy, chemical kinetics, density functional theory, dissociation rate
constant, perfluoropropyl iodide, Rice–Ramsperger–Kassel–Marcus/master equation, shock tube
used for effective fire protection.5,6 Note that according to
the Montreal Convention, the use of ozone-depleting com-
1
INTRODUCTION
This work is a continuation of a number of works1–4 on
the study of the kinetics of dissociation of various halocar-
bons using the precision method of atomic or molecular res-
onance absorption spectrometry. Scientific interest in these
substances is high due to the broad range of their technical
and industrial applications. In particular, different freons are
pounds, in particular, the most effective bromine-containing
freons, such as 13B1 (CF3Br), 114B2 (C2F4Br2), and 12B1
(CF2ClBr), nowadays is forbidden. Therefore, the search
for the new chemically active additives to combustible
mixtures, well-inhibiting ignition, and detonation processes
in different hydrocarbon–air mixtures,6,7 is very topical.
Int J Chem Kinet. 2018;1–9.
wileyonlinelibrary.com/journal/kin
© 2018 Wiley Periodicals, Inc.
1

2
BYSTROV ET AL.
Iodine-containing halogenated hydrocarbons (e.g., CF3I,
C2F5I, C3F7I), because of their complete ozone-friendly
and perfectly suitable physical properties in spite of their
high cost, can be a prospective substitute for brominated-
containing freons. Therefore, the perfluoropropyl iodide
n-C3F7I was chosen for a detailed study of its kinetic
properties.
carried out in this work under the corresponding experimental
conditions. In our recent work,1 direct ARAS measurements
of the dissociation rate constant of this molecule on the res-
onance line of atomic iodine at 183.04 nm were carried out.
This paper1 contains a detailed analysis of all previously pub-
lished data and summarizes the results obtained.
The kinetics of C2F5I dissociation9,20,21 was studied by the
same groups of coauthors as the C3F7I8–10 molecule. To deter-
mine the rate constant, Zaslonko et al.21 detected the iodine
molecule in a wide spectral range of 500–800 nm behind
shock waves at a temperature of 600–1400 K. Dobychin
et al.9 and Skorobogatov et al.20 used the stationary pyrolysis
method; therefore, their kinetic data are only available in the
low-temperature regions (380–480 and 610–790 K, respec-
tively).
To the best our knowledge, there are no works in which
the unimolecular decomposition rate constant of C3F7I has
been directly determined. There are only three works8–10
that investigate the dissociation kinetics of n-C3F7I or i-
C3F7I. All previous studies were carried out using an isother-
mal pyrolysis method. The temperature range investigated by
Dobychin et al.9 was 380–480 K, and in other works8,10 it
was 560–700 K. The pressure was 0.59 bar in two studies,8,10
and it was varied from 0.04 to 0.4 bar in another study.9
All authors also indicated that the high-pressure limit for
rate constants was experimentally reached. According to
these data, the primary decomposition proceeds through the
reaction
In addition, the same papers present the dissociation rate
constants at low temperatures for such molecules as C4F9I 9,20
and C6F13I.10
Data on other fluoroalkyl iodides could not be found in
the existing literature. Therefore, in the future, the study of
the kinetic properties of larger iodine-containing halogenated
hydrocarbons should be continued. The specific goal of the
present study was to obtain the direct kinetic data on reaction
(1) at high temperatures (T > 800 K) and at a wide range of
pressures using the highly sensitive ARAS technique and to
compare these data with the results of the RRKM-based mas-
ter equation (RRKM/ME) analysis.
C3F7I + (M) → C3F7 + I + (M)
(1)
because the C–I bond is by far the weakest bond in the
molecule. However, the method applied in works8–10 does not
directly measure the reaction rate constant (1). The observa-
tion was carried out for molecular I2, which was formed in
various secondary reactions at large initial concentrations of
C3F7I (from 3% C3F7I in Ar to no diluted C3F7I). In this case,
the correct consideration of all reactions is quite difficult and
could lead to inaccuracies.
2
EXPERIMENTAL INSTALLATION
It is also worth noting that the deficit of experimental
kinetic data on the alkyl iodides dissociation is characteris-
tic of the whole range of homologues of CnF2n+1I, except for
the simplest trifluorodiodomethane.
All experiments were carried out in a high-vacuum kinetic
shock tube with an internal diameter of 108 mm and made
of stainless steel. The driven and driver sections of the
shock tube were 6 and 2 m long, respectively. The high
sensitivity of the ARAS measurements requires high purity
of substances. Therefore, the driven section was pumped out
successively by scroll oil-free and turbo-molecular pumps up
The CF3I molecule has been studied quite well by various
methods in a wide temperature range (from 400 to 3000 K)
at fairly low pressures (from 0.04 to 11 bar). After the pub-
lication of the first work of Modica and Sillers,11 a series
of studies12–14 by stationary pyrolysis in the low-temperature
region followed. The first direct measurements of the dissoci-
ation of CF3I by detecting the CF3 radical were carried out by
Saito et al.15 at a wavelength of 290 1.5 nm. Brouwer and
Troe16 and Zaslonko et al.17 determined the primary disso-
ciation rate constant of trifluorodiodomethane directly by the
change in the concentration of CF3I itself in the range of 230–
350 and 316 nm, respectively. Kumaran et al.18 studied a mix-
ture of CF3I with Kr and used the atomic resonance absorp-
tion spectroscopy (ARAS) method to observe atomic iodine
in the spectral range of 165–185 nm. The high-temperature
region was experimentally studied in the work of Kiefer and
Sathyanarayana19 using the schlieren method. A theoreti-
cal calculation of the CF3I dissociation rate constant by the
Rice–Ramsperger–Kassel–Marcus theory (RRKM) was also
−7
to a pressure of 4 × 10 mbar. The leakage did not exceed
−6
10 mbar min−1. Only pure gas components were used at
the composition of the gas mixtures studied: Ar (Linde Gas
Rus): 99.9999%, CF3I: 99%, and n-C3F7I: 96% (both P&M
Invest). The i-C3F7I isomer is the main impurity to n-C3F7I.
All other impurities are less than 1%. No further purification
was carried out. The experimental setup is described in more
detail elsewhere.1 Iodine atom concentration–time profiles
were measured in the vacuum ultraviolet (VUV) range of
the spectrum at a wavelength 183.04 nm, which corresponds
to the transition I (4P5/2–2P3/2).22 As a source of resonance
radiation of iodine atoms, a microwave discharge lamp with
the flow of 0.4% CF3I in He at 6 mbar of pressure and excited
by a 60-W microwave discharge was used. The radiation from
the microwave discharge lamp was recorded using an Acton

BYSTROV ET AL.
3
applicable for interpreting the measured absorption. The func-
tional relationship between the absorber concentration and
fractional absorption A can be fitted to a modified Lambert–
Beer equation:
[
]
ꢅ = 1 − exp − (ꢆꢇ[I]ꢃ)
(2)
where A is the measured absorption, l is the optical length
in cm, [I] is the iodine atoms concentration in cm−3, and ꢆ
and n are the fitting parameters providing the best correla-
tion of expression (2) with the measurements. The best agree-
ment with the experimental data is observed for ꢆ = 8.64 ×
−12
10
cm1.25 and n = 0.75.
The lowest detected concentration of the iodine atoms
F I G U R E 1 Calibration curve showing the dependence of
absorption on ꢄ = 183.04 nm on the concentration of iodine atoms.
Square: experimental data, line: modified Lambert–Beer equation
at a given arrangement of ARAS measurements was about
1012 cm
(Figure 1). The lower and the upper (about
−3
1014 cm−3) detection limits are due to noise of the detection
system and to saturation of the optical transition, respectively.
VM-502 vacuum monochromator and a PMT-181 (MELF)
detector.
4
RESULTS AND DISCUSSION
The basic experiments measuring the time profiles of the
iodine concentrations during the dissociation of C3F7I were
carried out in mixtures of 0.13–10 ppm C3F7I in Ar at tem-
peratures of 800–1200 K and pressures of 0.6–8.3 bar. The
absorption was registered both behind the incident and behind
the reflected shock wave.
3
CALIBRATION MEASUREMENTS
To determine the absolute dependence of concentration of
atomic iodine on its absorption, a series of calibration exper-
iments were performed in our previous work.1 Calibration
measurements were carried out in mixtures with varying ini-
tial concentrations of CF3I in Ar from 1 to 4 ppm, temper-
atures above 1300 K, and pressures behind the incident and
reflected shock waves from 0.27 to 2.7 bar. At such temper-
atures, the iodine atom is known to be in a free state as a
result of the complete dissociation of the CF3I molecule.1 The
spectral resolution of the monochromator was 0.4 nm and was
determined by the presence of nearby lines of a given atom or
bands of the CF3 and CF2 radicals. Within the VUV lamp,
self-absorption at the resonance frequency occurs causing a
self-reversal of the emission profile. Moreover, the emission
line can be broader then the absorption line of the detected
I atoms. Therefore, the Lambert–Beer law is not directly
Typical absorption profiles at different pressures and sim-
ilar temperatures are shown in Figure 2. Figure 2 shows an
increase in the resonant absorption of atomic iodine followed
by relatively steady values. It is noteworthy that the rate of
the absorption growth is approximately the same at differ-
ent pressures. Using a calibration curve given in Figure 1,
the atomic iodine concentration profiles were obtained for all
experiments (see example in Figure 3). Taking into account
the extremely low concentration of C3F7I, secondary reac-
tions could be ignored, and a steady level of iodine concentra-
tion at late reaction times indicates the complete decomposi-
tion of C3F7I.
F I G U R E 2 Typical examples of an absorption profile at wavelength 183.04 nm. Experimental conditions: (a) 2.5 ppm C3F7I in Ar,
T = 972 K, P = 2.53 bar. b) 1 ppm C3F7I in Ar, T = 976 K, P = 7.85 bar
5
5
5
5

4
BYSTROV ET AL.
values of the rate constants. The upper temperatures of reli-
able measurements were about 1200 K since at temperatures
above 1200 K the dissociation time of C3F7I is too fast. At
such reaction rates, the limited time resolution of the exper-
imental setup (about 12 ꢊs) did not allow us to accurately
measure the derivative d[I]/dt and, correspondingly, the rate
constant. The inaccuracy of the experimentally obtained val-
ues of the rate constant did not exceed 30%. The majority of
the error was due to the uncertainty of the calibration curve
and by the approximation of the absorption profile (up to 15%
and 8%, respectively). Figure 3 shows the calculation of the
atomic iodine concentration profile using the experimentally
obtained rate constant in the same conditions. Figure 3 also
shows the concentration profiles, corresponding to the differ-
F I G U R E 3 Iodine atom concentration–time profile:
T = 1092 K, P = 2.70 bar at 4 ppm C3F7I in Ar. Solid red line:
5
5
ence of k by 30% from the best value at the experimental
1st
approximation of concentration profile by Equation 4 using the best k
1st
rate constant. As can be seen, these calculations lie within the
estimated error of the dissociation rate constant. The differ-
ence between the steady-state level of the experimental and
the simulated concentration profile (up to 20%) in later times
could be regarded solely to the error of the calibration curve.
In Figures 4 and 5, the obtained experimental data of the
rate constants of reaction (1) in Arrhenius coordinates in the
second-order and first-order reaction approximations at differ-
ent pressures are summarized. As one can see from Figure 4,
in the second-order reaction approximation (Equation 3), the
data at different pressures form three straight lines. In Fig-
ure 5, the same data processed under the assumption of a
first-order reaction, according to Equation 4, are well approx-
imated by a single relationship described by the equation
( 30% estimated uncertainty in k1st):
value. Upper and lower dash-dotted lines: 1.3 × k and 0.7 × k
1st
1st
respectively. Dashed line: initial slope [Color figure can be viewed at
wileyonlinelibrary.com]
As mentioned above, under conditions of such a strong
dilution, the only significant channel of the formation of
iodine at a characteristic observation time is the reaction of
C3F7I dissociation (1). In these conditions, the time profile
of the iodine concentration can be described by a differential
equation:
[
]
ꢈ [I] ∕ꢈꢉ = ꢀ2nd C3F7I [Ar]
(3)
or under the assumption of a first-order reaction:
[
]
ꢈ [I] ∕ꢈꢉ = ꢀ1st C3F7I .
(4)
(
)
ꢀ1st = 1.05 × 1014exp −200.4 kJ ⋅ mol−1∕ꢁꢂ .
(5)
Table 1 provides a list of the experimental conditions as
well as the values of the experimental rate constants obtained
from approximation of experimental iodine atom concentra-
tion profiles by Equations (3) and (4). For all experiments, the
determination of the derivative d[I]/dt in Equations 3 and 4
was carried out using an initial slope method, where the angle
between the concentration profile and the t-axis at the initial
time is used (see the dashed line in Figure 3). The concentra-
tion [C3F7I] at an initial time is practically equal to the known
initial concentration [C3F7I]0; therefore, in expressions (3)
and (4), the concentration [C3F7I] is approximately equal to
[C3F7I]0. In addition to estimating the possible inaccuracy of
this method, in some experiments the first-order rate constant
Thus, the coincidence of the rate constant values obtained
at different pressures in the first-order reaction approxima-
tion supposedly indicates that under the investigated condi-
tions, the dissociation of n-C3F7I proceeds close to the high-
pressure limit.
Figure 5 also shows the data of previous works8–10
that were obtained at lower temperatures (below 700 K).
The authors carried out studies at various pressures (0.04–
0.40 bar), and they claimed that the high-pressure-limit rate
constants were reached. It is worth noting that the data in the
temperature range 800–1200 K were obtained in the present
work for the first time, and extrapolation of the literature data
to the high-temperature region indicates a qualitatively good
agreement of all the results.
k
1st was directly derived from ln[ln(It/I∞)/ln(I /I∞)] = −k t,
where I , I , I∞ are intensities at time t = 0, t,0and ∞, respec-
1st
0
t
tively. Alternative analysis was only used for those experi-
ments where the absorption value did not exceed 50%, since
the present equation assumes following the Lambert–Beer
law. Despite the fact that we used the modified Lambert–
Beer law to describe the whole range of experimental points,
this condition is well satisfied for low absorption values.1 It
is notable that both methods resulted in practically the same
However, it would be interesting to compare the obtained
results with the theoretical RRKM estimates and to imple-
ment RRKM extrapolation of the rate constant toward a wide
range of thermodynamic variables. For this purpose, first of
all, the energy barrier of the process (1) must be determined.
As far as the activation energy for this dissociation reaction

BYSTROV ET AL.
5
TA B L E 1 Summary of the experimental results
[C3F7I]
(ppm)
4
P2
T2
(K)
–
P5
T5
[Ar]
(× 1019 cm−3
1.7
[C3F7I]
(× 1013 cm−3
6.8
k1st
(s−1
k2nd (× 10−19 cm3
(bar)
(bar)
2.724
2.700
2.684
2.709
2.737
2.722
2.745
2.684
2.669
2.686
2.662
2.668
2.668
2.531
2.616
2.592
2.736
2.669
8.516
8.250
7.946
8.233
7.966
7.878
7.853
8.116
8.319
–
(K)
1,183
1,092
934
1,056
1,060
923
879
873
798
838
812
853
969
972
972
875
1,021
910
851
948
846
1,126
1,047
880
976
1,087
1,030
–
)
)
)
(molecule⋅s)−1
17,906
11,890
503
)
–
30,441
21,402
1,057
13,000
23,184
730
4
–
–
1.8
7.2
4
–
–
2.1
8.4
2
–
–
1.9
3.8
6,842
12,202
332
2
–
–
1.9
3.8
2
–
–
2.2
4.4
2
–
–
2.3
4.6
102
44.4
1
–
–
2.3
2.3
102
44.4
10
10
10
10
2.5
2.5
2.5
2.5
2.5
3
–
–
2.5
25
4.48
1.79
–
–
2.4
24
41.6
17.3
–
–
2.4
24
11.3
4.71
–
–
2.3
23
59.6
25.9
–
–
2.0
5.0
1,654
2,273
2,100
80.7
827
–
–
1.9
4.8
1,196
1,050
36.7
–
–
2.0
5.0
–
–
2.2
5.5
–
–
2.0
3.4
9,020
285
4,510
130
–
–
2.2
6.6
0.125
0.125
3
–
–
7.3
0.91
0.80
21
164
22.5
–
–
6.4
692
108
–
–
6.9
60.4
8.75
3
–
–
5.4
16
13,734
7,700
91.3
2,543
1,375
13.8
1
–
–
5.6
5.6
1
–
–
6.6
6.6
1
–
–
5.9
5.9
3114
12,727
7,627
135
528
0.2
0.2
10
10
10
10
10
10
–
–
5.5
1.1
2,314
1,293
237
–
–
5.9
1.2
0.666
0.692
0.617
0.642
0.653
0.667
855
1012
981
905
875
868
0.57
0.43
0.46
0.52
0.55
0.56
5.7
–
–
4.3
1,864
2,410
239
4,335
5,239
460
–
–
4.6
–
–
5.2
–
–
5.5
301
547
–
–
5.6
118
211
is expected to be equal to its enthalpy (or, equivalently, the
reverse reaction has no activation barrier), special attention
should be paid to the accurate determination of its dissociation
energy D.
DFT energies were augmented with an empirical dispersion
correction27 that can be essential to reach the so-called chem-
ical accuracy for relatively large atomic systems.28
Conventional harmonic frequency analysis was conducted
at the same level of theory for the PES minima to provide
the vibrational frequencies required for the subsequent mod-
eling and to assess the zero-point energy contribution to the
total energy of the considered species. The heights of torsion
barriers required for calculations were also determined using
the relaxed PES scan technique at the B3LYP/cc-pVTZ-PP
level of treatment. All quantum-chemical calculations were
conducted using the Firefly QC program package,29 which is
partially based on the GAMESS (US) source code.30
In the present work, density functional theory (DFT) cal-
culations were conducted to explore the regions of potential
energy surface (PES) relevant to n-C3F7I dissociation. Fol-
lowing Fortin et al.,23 the geometries of stationary points on
the PES were optimized with the B3LYP DFT functional24
in conjunction with the correlation consistent polarized triple
zeta basis set cc-pVTZ-PP of Peterson et al. with the
pseudopotential for the core electrons of heavy atoms.25,26
When applying gradient optimization of the structure, the

6
BYSTROV ET AL.
Note that the average value of C–I dissociation energy
in different molecules of homologues CnF2n+1I is about
D = 210 kJ mol−1, and it decreases as follows: CF3I,20
C2F5I,20 C3F7I,8 C4F9I,20 and C6F13I10 (228, 219, 212, 206,
and 201 kJ mol−1, respectively). An analogous regularity is
presented by Skorobogatov et al.,10 indicating a decrease in
the activation energy with an increase in molecular weight,
not only for iodine compounds but also for fluorine, chlorine,
and bromine compounds.
P
P
P
Note also that the only available theoretical study of the
dissociation energy in the C3F7I molecule33 gives a slightly
lower value D (from 201 to 213 kJ mol−1, depending on
the calculation method) than was obtained at the B3LYP/cc-
pVTZ-PP level of theory in the present work; however, these
results are relevant to another (i-C3F7I) isomeric form of
iodoperfluoropropane.
F I G U R E 4 Arrhenius plot of the measured second-order rate
constant for C3F7I (+M) = C3F7 + I (+M) [Color figure can be viewed
at wileyonlinelibrary.com]
To estimate the appropriate pressure-dependent rate con-
stants, RRKM/ME analysis was performed with the use of
the ChemRate program package.34,35 Energy-dependent rate
constants for the C–I bond fission were obtained using the
procedure for loose transition state dating back to Ben-
son's restricted rotor concept34 and implemented in Chemrate
program.35 In doing so, the exponential-down model of colli-
sional energy transfer was applied, and the parameter 훼, which
governs the magnitude of average energy transferred dur-
ing collisional deactivation, was specified in a temperature-
dependent form 훼(T) = 훼 + 훼 T (cm−1).
et al.
et al.
et al.
et al.
et al.
0
1
The collisional diameter ꢆ and the well depth of the
Lennard–Jones potential 휀 for CF3I and C3F7I species
required for RRKM/ME analysis were estimated using the
methodology36 on the basis of computed spatial structures
and electric properties (dipole moment and polarizability).
The same quantities for Ar were adopted from the work of
Cambi et al.37 These values are also summarized in Table 2.
To treat the pressure-dependent dissociation of n-C3F7I
in Ar, the following parameters of the simple exponential-
down model for collisional energy transfer were applied:
훼 = 50 cm−1, 훼 = 0.2 cm−1 K−1. Just such 훼(T) dependence
F I G U R E 5 Arrhenius plot of the measured first-order rate
constant for C3F7I (+M) = C3F7 + I (+M) and data of previous
works.8–10 The solid dark line is a linear least-squares fit of the present
data [Color figure can be viewed at wileyonlinelibrary.com]
0
1
TA B L E 2 Parameters used for RRKM/ME modeling
governing the collisional deactivation of vibrationally excited
n-C3F7I* complex in Ar ensures (together with the collision
parameters given in Table 2) the most accurate RRKM/ME
description of the CF3I decay in the same bath gas registered
by Bystrov et al.1
˚
Molecule D(0 K) (kJ/mol) D(298 K) (kJ/mol) ꢀ (A) ꢁ (K)
CF3I
n-C3F7I
Ar
224.6
212.5
–
227.3
214.8
–
5.07
6.03
3.33
125
130
135
In the course of RRKM/ME analysis, the mole fraction of
n-C3F7I in a bath gas was specified to be ∼10−4. The cal-
culations of the effective dissociation rate constant and the
subsequent fitting to the Troe form38,39 were carried out in
the following range of thermodynamic parameters: T = 300–
3000 K, P = 10−4–102 bar. As a result, the limiting low- and
high-pressure rate coefficients were obtained:
The obtained values of dissociation energy of the singlet
n-C3F7I molecule into doublet C3F7 and I(2P) are presented
in Table 2. To ensure that the reliable values of D(0 K) and
D(298 K) are achieved in this case, the respective values for
the CF3I molecule are also given here. One may observe that
the obtained D(298 K) magnitude for CF3I coincides well
−1
with the available thermochemical values (225.631 kJ mol
(
)
ꢀ0 ꢃ−C3F7I + Ar = 8.849 × 1019ꢂ −1.444
and 228.632 kJ mol−1) that support the reasonability of apply-
ing the current computational scheme.
exp (−11, 454.4∕ꢂ ) (cm3mol−1 −1
s
) and

BYSTROV ET AL.
7
calculation matches well with the work of Tedeev et al.8 in
the low-temperature region.
Finally, it would be interesting to compare present results
with the rate constants of dissociation for similar molecules.
One of the most similar molecules in structure and properties
with the C3F7I is the molecule C3H7I. Note that the kinetics
involving this molecule is noticeably lacking from the litera-
ture. Only some data on the rate constant of dissociation have
been presented in old works40,41 for a very narrow temper-
ature range (533–573 and 703–767 K, respectively). Owing
to the absence of high-temperature kinetic data, we did not
directly compare the present data with the C3H7I molecule.
The results of our work were matched with the data in a similar
temperature range of all available homologues of CnH2n+1
I
and some other substances with the C–I bond, including a
number of smaller CnF2n+1I molecules. The results compari-
son are shown in Figure 7. Larger molecules such as C4F9I and
C6F13I are also not included in Figure 7, because like C3H7I,
they have only been studied in a narrow range of low temper-
atures.
F I G U R E 6 Measured and calculated rate constant for
C3F7I (+M) = C3F7 + I (+M) and its dependence on the total
concentration. Numbers 1, 2, 3, and 4 are RRKM/ME-based
approximations at the temperatures of T = 1050, 970, 850, and 720 K,
respectively. Dotted lines are low- and high-pressure limiting rate
constants for the same temperatures. Symbols represent experimental
data: the triangle, T = 1050 20 K; the square, T = 970 10 K; the
circle, T = 850 25 K; the rhombus, data from Tedeev et al.8 at
T = 720 K [Color figure can be viewed at wileyonlinelibrary.com]
As can be seen from the plot, the rate constants of the
primary dissociation for all the presented molecules of the
CnH2n+1I and CnF2n+1I series are located in a close range
of values. The results of Kumaran et al.,42 with the activa-
tion energy of the C2H5I molecule 132.2 kJ mol−1, differ
significantly from the general values, that can be explained
by the deviation of the rate constant from the high-pressure
limit in the high-temperature region. The new data obtained
in our work fall well within the general range of literature
(
)
ꢀ∞ ꢃ−C3F7I = 9.676 × 1016ꢂ −0.378
exp (−26, 956.8∕ꢂ ) ( s−1
)
with the center broadening factor F = 0.119.
values. Although at thermal decomposition of the CnH2n+1
I
c
Figure 6 shows the theoretically calculated pressure depen-
dence of the n-C3F7I dissociation rate constants at different
temperatures. In this figure, the experimental values of the
rate constant at the corresponding close temperatures are also
given. Despite the fact that, within the experimental error,
it was impossible to detect conclusively the experimental
dependence of the rate constant on the pressure value, the
RRKM/ME results under appropriate conditions still exhibit a
noticeable pressure dependence for temperatures above 900–
950 K. This fact does not allow us to conclude unequivocally
that the high-pressure limit has been reached for the whole
range of our experimental conditions. In this case, the fitting
of the experimental data by one Arrhenius equation can give
a slightly underestimated value of the activation energy in
comparison with the value of the dissociation energy at the
C–I bond fission, and their values cannot be directly corre-
lated with each other. As shown in Table 2, the theoretical
value of the dissociation energy is in good agreement with
the available experimental data8–10 on the activation energy
at the high-pressure limit.
molecules, the parallel reaction CnH2n+1I → CnH2n–1 + HI
plays an important role, it is clear that this process does
not significantly affect the rate of the C–I bond fission in
comparison with the CnF2n+1I series. Therefore, in future
investigations, comparing the kinetic properties of C3H7I
with the C3F7I molecules, we can expect that the values of
their rate constants will also be similar. The only thing that
should be noted is that the density of energy states in the
homologous series CnH2n+1I is lower than in the CnF2n+1
I
series. Thus, the falloff curve is shifted to a region of lower
pressures in case of CnH2n+1I. Accordingly, to approach
the limiting rate constant of high pressure, it will be nec-
essary to perform experiments at pressures higher than for
C3F7I.
As for the iodobenzene molecules, the value of the dissoci-
ation rate constant of this cyclic substance noticeably differs
from the constants of the above-mentioned molecules with a
linear structure. Despite the fact that during dissociation in
the same way as in the molecules CnH2n+1I and C F2n+1I, the
primary formation of the iodine atom occurs, thenmechanism
of this process seems to be different. Therefore, it is incorrect
to make comparisons with other cyclic molecules and assume
similar kinetic properties.
A comparison of the absolute values of the experimen-
tal and theoretical rate constants of reaction (1) presented
in Figure 6 shows a good match. Therefore, the theoretical

8
BYSTROV ET AL.
(this work)
(this work)
F I G U R E 7 Arrhenius dependence of the
first-order dissociation rate constant for some
iodine-containing halogenated
hydrocarbons1,15,17,20,21,43–50. The superscript a indicates
the h-p limit (= high-pressure limit rate constant).
[Color figure can be viewed at wileyonlinelibrary.com]
3. Bystrov NS, Emelianov AV, Eremin AV, Yatsenko PI. ARAS
monitoring of various halogen atoms formation in pyrolysis reac-
tions behind shock waves. IOP Conf Ser: J Phys: Conf Ser.
2018;946:012069.
5
CONCLUSION
Using the precise ARAS technique on the atomic iodine
line at 183.04 nm, the rate constant of dissociation of C3F7I
at different temperatures (800–1200 K) and pressures (0.6–
8.3 bar) was experimentally measured. Compared with pre-
vious studies, the ranges of pressures and temperatures were
significantly expanded. Theoretical RRKM/ME calculations
in a wide range of thermodynamic parameters were car-
ried out, and values of the limiting low- and high-pressure
rate coefficients were obtained. Comparison of the performed
RRKM/ME calculations with the present experimental data
and data of previous works confirmed the plausibility of the
obtained values. By analyzing all available results on the rate
constant of reaction C3F7I (+M) = C3F7 + I (+M), it was
concluded that the process of initial dissociation of perfluo-
ropropyl iodide under our experimental pressure range at ele-
vated temperatures still does not reach the high-pressure limit.
4. Emelianov AV, Eremin AV, Yatsenko PI. The study of C2F4Br2 dis-
sociation kinetics using methods of atomic and molecular resonance
absorption spectroscopy behind shock waves. IOP Conf Ser: J Phys:
Conf Ser. 2018;946:012070.
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1973;77A:733–754.
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fluorinated hydrocarbons on the combustion of gaseous mixtures in
a closed vessel. Combust Flame. 2000;121:542–547.
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ACKNOWLEDGEMENTS
The support of this study by the Russian Science Foundation
grant no. 14-19-00025P is gratefully acknowledged.
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ORCID
Alexander Eremin
https://orcid.org/0000-0002-6835-7933
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How to cite this article: Bystrov N, Emelianov A,
Eremin A, Loukhovitski B, Sharipov A, Yatsenko P.
Direct measurements of C3F7I dissociation rate con-
stants using a shock tube ARAS technique. Int J Chem
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