Isothermal pyrolysis of iodomethanes in gases

Article abstract of DOI:10.1134/S1070363209120147

The fact was established that the pyrolysis of gaseous iodomethanes RI yields methane and non traces of recombination products R₂. A pyrolysis mechanism was proposed and rate constants of limiting stages of the pyrolysis of iodomethane, trideuteroiodomethane, and diiodomethane over the range of 500-1500 K were determined. Pleiades Publishing, Ltd., 2009.

Full text of DOI:10.1134/S1070363209120147

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 12, pp. 2641–2651. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © G.A. Skorobogatov, V.K. Khripun, A.G. Rebrova, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 12, pp. 2015–
2025.
Isothermal Pyrolysis of Iodomethanes in Gases
G. A. Skorobogatov, V. K. Khripun, and A. G. Rebrova
St. Petersburg State University, Universitetskii pr. 26, St. Petersburg, 198504 Russia
e-mail: Gera.Skor@pobox.spbu.ru
Received March 5, 2008
Abstract―The fact was established that the pyrolysis of gaseous iodomethanes RI yields methane and non
traces of recombination products R2. A pyrolysis mechanism was proposed and rate constants of limiting stages
of the pyrolysis of iodomethane, trideuteroiodomethane, and diiodomethane over the range of 500–1500 K
were determined.
DOI: 10.1134/S1070363209120147
(d[R])/(dt) = k2[RX] – k–2[R][X] + k3[X][RX]
– k–3[R][X2] – 2k5(P,T)[R]2 – kdiff,P[R],
(d[X])/(dt) = k2[RX] – k–2[R][X] – 2k4[M][X]2
2k–4[M][X2],
The method of isochore isothermal pyrolysis was
applied [1–4] to the study of thermal gas-phase
reactions involving perfluoroalkyliodides (CF3I, C2F5I,
C3F7I, …) and -bromides (CF3Br). In this method a
perfluoroalkylhalide RX is subjected to pyrolysis (1) at
a fixed temperature under mild conditions. The
detailed mechanism of the pyrolysis is presented by
reactions (2)–(5).
(61)
(62)
(63)
(d[R2])/(dt) = k5(P,T)[R]2.
Three Eqs. (61)–(63) are sufficient for the
unambiguous solution of the kinetic problem, because
concentrations [Eq. (7)] of remaining components can
be found from conservation laws.
RX → 1/2R2 + 1/2X2,
RX ↔ R· + X·,
(1)
(2)
(3)
(4)
(5)
[RХ]t = [RX]0 – 2[X2]t,
(Kс,2 = k2 /k−2 = [R]eq[X]eq/[RX]eq),
X· + RX ↔ R· + X2
[X2]t = [R2]t.
(7)
Concentrations of intermediate active species fast
reach almost equilibrium values constant at a specified
temperature, which slowly vary as the initial substance
RХ is consumed. Therefore, bypassing the integration
procedure, we find concentrations of the intermediate
particles in explicit form (8), (9) from Eqs. (61) and
(62) (Bodenstein–Semenov approximation of quasi-
stationary concentrations).
(Kс,3 = k3 /k−3 = Kс,2/Kс,4),
2 X· + (Y) ↔ X2 + (Y),
(Kс,4 = k4 /k−4 = [X]2eq/[X2]eq),
2R· + (Y) → R2 + (Y),
(k5 = –½(d[R]/dt)[R]–2).
Here Y are particles of any third agent, i.e. of Y = RX,
R2, X2, R', X', or an inert diluent (Ar, He,...); [R]eq, [X]eq,
and [RX]eq are equilibrium concentrations of cor-
responding components; ki (k–i) is a rate constant of a
direct (inverse) elementary reaction, and Kс,i is a
constant of the ith equilibrium.
[X]t = √Kc,4[X2]t,
[R]t = Kс,2([RX]0 – 2[X2]t)/(Kс,4[X2])0.5.
(8)
(9)
The necessity for integrating Eq. (63) also disap-
pears, for the rate of increasing r(t) [Eq. (10)] of the
concentrations of a molecular halogen is measured
directly under isochorically-isothermal pyrolysis con-
ditions.
The process rate is limited by irreversible (under
conditions of our experiments) reaction (5) that allows
us to determine the rate constant k5 from the
accumulation rate of the recombination product R2 or,
which is more convenient, from the rate of molecular
halogen X2 accumulation. Kinetic Eqs. (6) correspond
to reactions (2)–(5).
r(t) = (d[X2])/(dt) = (d[R2])/(dt) = k5(P,T)[R]2.
(10)
Substituting functions (8), (9) in Eq. (10), we obtain
explicit expression (11) for k5.
2641

2642
SKOROBOGATOV et al.
(11)
k5(P,T) = α(T)(Kc,4)/(K 2c,2),
The resulting information was not highly reliable,
α(Т) = r(t)[X2]t([RX]0 – 2[X2]t)–2.
(12)
as added hydrogen iodide increased the number of the
elementary reactions from 8 up to 12 that did not allow
unambiguous calculating any rate constant from a
unique kinetic curve of the molecular iodine accumula-
tion. Furthermore, the authors of [7, 8] did not use the
advantage of the Bodenstein-Semenov approximation.
In our experiments we did not add HI to initial iodo-
methane, but it could be generated during the pyro-
lysis, so generally it is necessary to suppose a possibi-
lity of occurring reactions (151)–(156) and (16), (17).
Here [RX]0 is the initial concentration, [X2]t is the
measured concentration of the molecular halogen to an
instant t, and α(T) is kinetic invariant (12) independent
of current concentration of components. The constant
Kc,4(T) is known for all halogens, that is why for the
halides CF3I,C2F5I, C3F7I, and CF3Br, for which Kc,2(T)
values are known, we can find k5(P, T) values from the
measured invariant α(T) by formula (11) [1–4].
We could expect that the iodomethane pyrolysis
would follow simple gross-scheme (1), however even
the first experiments have shown [5, 6] that the volatile
products of the iodomethane pyrolysis are presented
exclusively by methane and contain no ethane at all
(Table 1). Therefore, recombination (5) is completely
eliminated during iodomethane pyrolysis (R = СН3˙),
and the detailed mechanism of this process includes
reactions (2), (4), and (13).
k1m
CH3I + I·
CH3 + I2 – 84.1 kJ mol–1,
·
k–1m
(151)
(152)
(Kc,1m = k1m/k–1m),
k2m
k–2m
CH3I
CH3 + I· – 232.9 kJ mol–1,
·
(Kc,2m = k2m/k–2m),
kH
CH4 + CH2I· + 18 kJ mol–1.
·
СН3 + СН3I
(13)
k3m
k–3m
CH3 + CH3I
CH4 + CH2I· + 18.1 kJ mol–1,
·
Iodomethane thermal reactions were studied even
in the first half of the last century [7, 8], however the
principal attention was paid to obtaining information
on kinetic constants of reactions (3) (R = СН3) and
(14), and for which purpose the iodide was diluted by
hydrogen iodide.
(Kc,3m = k3m/k–3m),
(153)
(154)
k4m
CH3 + CH2I·
CH4 + CHI·· – 1.3 kJ mol–1,
·
k–4m
СН3 + НI ⇄ CH4 + I·.
(14)
·
(Kc,4m = k4m/k–4m),
Table 1. Products of pyrolysis (at 500–800 K) of certain haloalkanes
Supposed radical R and its
Found product of recombinations
of radicals (R2)
Others pyrolysis products
ΔfH0298, kJ mol–1
Haloalkane
·
CF3Iа
C2F5Iа
C3F7Iа
CF3
(–471)
C2F6 (100%)
C4F10 (100%)
C6F14 (98%)
I2
I2
·
C2F5 (–921)
·
C3F7 (–1283)
I2, C3F6, C3F8
Br2
·
CF3Brа
C2H5Iа
CF2HBrb
CF2ClBrb
CH3Ib
CF3
(–471)
C2F6 (100%)
Absent
·
C2H5 (+125)
CF2H· (–243)
CF2Cl· (–280)
I2, C2H4, C2H6, C6H6
C2F4H2 (2%)
C2F4Cl2 (3%)
Absent
Br2, HBr, C3F6H2, C3F5H, C2F5H, C2F4
Br2, HBr, C2F4, C2F4ClBr, C3F6ClBr, C4F8ClBr, C3F6
·
CH3
(+147)
I2, CH4, Csolid
I2, CD4, Csolid
·
CD3Ib
CD3
(+137)
Absent
b
CH2I2
CH2I· (+230)
Absent
Absent
I2, CH4, Csolid
I2, CH4, Csolid
b
·
СНI3
CHI2 (+334)
a Published data [1–4]. b Found in the present work.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

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ISOTHERMAL PYROLYSIS OF IODOMETHANES IN GASES
isothermal pyrolysis makes it possible to measure rate
constants k5;
k5m
CH3I + I·
CH4 + I·
CH2I· + HI – 115.0 kJ mol–1,
k–5m
(155)
(156)
(2) Remaining haloalkanes, except for iodome-
thanes, for which gross-scheme (1) is not obeyed and
kinetic measurements by the isothermal pyrolysis
method are impossible;
(Kc,5m = k5m/k–5m),
k6m
k–6m
·
CH3 + HI – 137.7 kJ mol–1,
(Kc,6m = k6m/k–6m),
(3) Iodomethanes, for which gross-scheme (1) is
not obeyed, but the set of final pyrolysis products is so
simple that there is a chance to discover the
mechanism of their pyrolysis.
where index “m” indicates that the constants refer to a
usual methyl group.
Resulting diradical CHI˙˙ is capable to disappear
fast either in volume due to irreversible dissociative
recombination (16) or in heterogeneous process (17).
In the previous work [6] we have shown by an
example of CF3I that the isochore isothermal pyrolysis
method allows us to measure confidently the rate
constant of radical R recombination (5), but only if two
conditions are obeyed. First, radical R in a gas should
behave as a quasi-atom (i.e. not be split); second, for
the iodide RI under study all thermodynamic
parameters of equilibria (2)–(4) should be known. It is
seen from Table 1 that perfluoroalkyliodides (bro-
mides), but not the iodomethanes CH3I, CH2I2, and
CHI3, meet the first condition. However for the last-
named compounds the set of pyrolysis products is very
simple, and below we shall reveal the mechanism of
their low-temperature pyrolysis.
−1
_
HI + HC
CI + 550 + 30 kJ mol ,
k0
2CHI
−1
_
2 HI + C2
+ 15 + 10 kJ mol , (16)
kdiff
CHI
CHI (surface)
−1
_
(17)
Csolid + HI(gas) + 414 + 5 kJ mol .
The enthalpies of reactions (15)–(17) were calcu-
lated by means of the tables [9]. The aim of the present
study was to detect a limiting stage among reactions
(15)–(17) and to measure its rate constant. A similar
problem was solved also for the diiodomethane
pyrolysis.
Photolytic experiments. The absence of С2Н6 from
the iodomethane pyrolysis products sharply contrasts
with the fact that the pulse photolysis of CH3I in Ar
excess leads exclusively to the formation of ethane
[10]. On the contrary, the stationary CH3I photolysis in
Ar excess (λ 230–300 nm, [CH3I] 8.9×1017 molecule/cm3,
[Ar] 1.97×1019 molecule/cm3, T 343 K, Δt 30 min)
leads only to the formation of СН4, I2, and pyrocarbon
on quartz cell walls.
Thermal experiments. A kinetic pyrolysis scheme
of any substance should be first of all coordinated with
the composition of observed final products. Therefore
we have determined final products of the pyrolysis
gross-process for a series of halides by the methods of
gas chromatography and mass spectrometry. In the
case of gaseous CH3I ethane (as well as diiodoethane
at the pyrolysis of CH2I2), i.e. the product of the СН3˙
(or CH2I˙) radical recombination, was completely
absent. Instead of it in the both cases exclusively
methane CH4 and molecular iodine were present in the
volume, and a carbon polymer, on the cell walls (Table 1).
The schemes of CF2HBr and CF2ClBr pyrolysis also
were not limited by reactions (2)–(5), unlike
perfluoroalkyliodides, for which pyrolysis gross-
scheme (1) with reactions (2)–(5) is fully suitable.
To reveal the contribution of reactions of СН3˙
radicals on a quartz cell surface in comparison with the
processes in a reactor volume, we have studied the
photolysis of gaseous acetone (СН3)2СО under condi-
tions when generated radicals СН3˙ certainly have a
possibility to contact with a quartz wall of a reaction
cell.
It was reliably estimated [11] that the photolysis of
acetone at T 398 K and λ 250–320 nm gives the
acetone quantum yield φ(CO) equal to one.
Thus, according to pyrolysis results, all iodides
(bromides) are broken up into three groups.
hν
·
СН3СОСН3
СО + 2 СН3
C2Н6
(18)
(19)
(1) Perfluoroalkyliodides and –bromides, for which
gross-scheme (1) is obeyed and the method of
k19
·
2 СН3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

2644
SKOROBOGATOV et al.
However, recently it has been shown by the method
molecules, whereas reactions (18), (19) produce three
isotope varieties of ethane molecules (C2H6, C2D6, and
C2H3D3), their amounts being highly accurate defined
[13] by combinatorial formulae (23).
of molecular beams [12] that at λ 193 nm the carbonyl
amide photodissociation yields oxalonitrile with a
probability of 6% by the intramolecular rearrangement
mechanism (20).
[C2H6] = 1/2x02, [C2H3D3] = x0y0, [C2D6] = 1/2y02.
(23)
(94 2)%
COCN + CN ,
hν
Here х0 is a mole fraction of initial radicals СН3˙; y0 =
(1 – x0) is a mole fraction of initial radicals СD3. As a
result the excess of C2H6 and C2D6 molecules above
combinatorial distribution (23) in the photogenerated
ethane is entirely defined by the contribution of
predissociation (21).
CO(CN)2
(20)
(6 2)%
CO + NCCN.
Therefore it is logical to add one more reaction
(predissociation) (21) to stages (18), (19) of the
gaseous acetone photolysis, as it is possible that this
stage was not noticed in ordinary photochemical
experiments.
The isotope composition of the ethane formed after
the photolysis of an acetone and deuteroacetone
mixture is presented in Table 2 together with the
calculated fraction of molecules formed by intra-
molecular mechanism (21). This fraction is inde-
pendent of pressure and temperature of photolyzed
acetone and has value (24).
hν
CH3COCH3
CO + C2H6.
(21)
To measure the quantum yield φpredis of reaction
(21), we have fulfilled experiments with multiple-
labelled acetone (containing isotope molecules in
extremely nonequilibrium concentrations). The deute-
rium analog (CD3)2CO is indistinguishable in
photochemical properties from ordinary acetone. Free
radicals CD3˙ recombine by scheme (19a) with almost
the same rate constant (22) as their protium analogs.
φ21 = 0.26 ± 0.02.
(24)
It follows from the value (24) that with a
probability of 74% acetone photolysis generates free
methyl radicals, which inevitably contact a quartz cell
wall in the conditions of a stationary photolysis.
However, within the limits of measuring errors
(±0.1%) methane was not detected among the
photolysis products, though its formation is energy
more favorable.
k19d
·
·
CD3 + CD3
C2D6,
(19a)
(22)
4 СН3 → 2 C2H6 + 760 kJ mol–1,
4 СН3 → 3 CH4 + Csolid + 821 kJ mol–1,
(25)
(26)
·
k19d = (15/18)1/2 k19 = 0.913k19.
·
Therefore in the case of the photolysis of multiply-
labelled acetone [(СH3)2CO + (CD3)2CO] reaction (21)
should result in the formation of only C2H6 and C2D6
Therefore, the formation of methane at the
iodomethane pyrolysis is defined not by the contact of
Table 2. Fraction of ethane formed as a result of intramolecular rearrangement (21) (induced predissociation) at a stationary
photolysis of multiply-labelled acetone
Contribution of recombi-
Measured isotope com-
position of ethane,
mole fraction
Contribution of
predissociation,
mole fraction
Ethane formed in
an intramolecular
regrouping, %
Initial concentration
of acetone,
nation of CH3· and CD3· ,
Photolysis product
(isotope molecule)
molecule/cm3
mole fraction
[by formulae (23)]
9.7×1017
CH3CH3
CH3CD3
CD3CD3
CH3CH3
CH3CD3
CD3CD3
CH3CH3
CH3CD3
CD3CD3
0.477±0.001
0.337±0.001
0.186±0.001
0.487±0.001
0.340±0.001
0.173±0.001
0.360±0.001
0.357±0.001
0.283±0.001
0.3065
0.3373
0.0928
0.3265
0.3402
0.0886
0.2088
0.3573
0.1528
0.170±0.002
0
(64.4% H,
35.6% D)
3.9×1018
26.3±0.4
24.5±2.4
28.1±0.4
0.093±0.002
0.161±0.015
0
(64.4% H,
35.6% D)
3.9×1018
0.084±0.009
0.151±0.002
0
(53.8% H,
46.2% D)
0.130±0.002
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2645
ISOTHERMAL PYROLYSIS OF IODOMETHANES IN GASES
3
σI2,500/10–16 cm2
CH
T, K
2 400 500
600 700 800
Т2
– 1
I2
2.2
2.0
1.8
1.6
1.4
– 2
– 3
– 4
– 5
Т1
12
10
8
2
1
0
CD3I
CH3I
Т3
n-C3F7I
1
6
2
CF3I
400 500 600
700 800
T, K
10
20
30
40
50
60
Fig. 1. Absorption cross sections (σ) of photons published
and measured in the present work. Тhe left-hand scale: I2
(λ 500 nm) and CН2I2 (λ 251 nm). Тhe right-hand scale:
CН3I (258 nm), CD3I (λ 257 nm), CF3I (267.8 nm), and
C3F7I (270 nm). Data of: (1) [14], (2) [15], (3) [16],
(4) [17], and (5) our measurements.
t, min
Fig. 2. Kinetics of the I2 accumulation in the isochore
isothermal pyrolysis of iodomethane: (1) [CH3I]0 5.7×1019,
(2) [CH3I]0 5.45×1019 molecule/cm3; Т1 588±1, Т2 615±1,
Т3 633±1 K.
methyl radicals with a wall, but by the presence of
iodine-containing molecules (radicals) in the gas.
0.5065 (i.e. is greater than a half) and the atomic
fraction of Н is 0.4935; in the 195-th minute the
atomic fraction of D in methane is 0.5775, and the
fraction of Н, 0.4225 that agrees within the limits of
the errors with the data of the kinetic measurements
(Fig. 3).
Kinetics of iodomethane pyrolysis. Current
concentrations of components in a reaction vessel were
found in the kinetic experiments by a signal of the
current optical density of a mixture under pyrolysis (by
the Bouguer–Lambert–Beer law) from the extinction
of each component depending on temperature (Fig. 1).
We have found a part of the data on the extinctions in
the literature [14–17] and determined a part by
ourselves.
The data of Tables 2 and 3 essentially differ from
each other owing to the fact that only molecules with
the number of deuterium atoms multiple of three are
5
Т3
Т2
Т1
To elucidate the mechanism of iodomethane
pyrolysis, we have recorded kinetic curves [I2]t in the
course of the isochore isothermal pyrolysis of pure
CH3I (Fig. 2) and the CH3I + CD3I (50:50) mixture
based on the mass-spectral analysis of the pyrolysis
products (Fig. 3 and Table 3). To minimize a diffusion
drift of free radicals to a wall, the initial pressure of
iodomethane was set at a level of 2 atm. The
anomalous mutual arrangement of the kinetic curves
has come to light: the molecular iodine is accumulated
faster in the case of the pyrolysis trideuteroiodo-
methane, though the iodide CD3I, being more stable
thermodynamically [9] than CH3I, should be pyrolyzed
more slowly. It is easy to calculate from the data of
Table 3 that in the 25th minute of the pyrolysis the
atomic fraction of D in formed methane is equal to
4
3
CD3I
2
1
CH3I
0
10
20
30
40
50
60
t, min
Fig. 3. Kinetics of the I2 accumulation in the isochore
isothermal pyrolysis of gaseous methyliodide CH3I or
CD3I. Т1 599 and 588 K, respectively; Т2 625, Т3 652 K.
[CH3I]0 5.7×1019; [CD3I]0 2.25×1019 molecule/cm3.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

2646
SKOROBOGATOV et al.
Table 3. Isotope composition (mole fraction, %) of methane obtained on the pyrolysis (708 K) of the 50 mol % CH3I +
50 mol % CD3I mixture and also of residual initial methyliodide
Deviation of the experiment from the calculation,
Experiment
Calculation by combinatorics formulae [18]
Isotope
molecule
rel %
25 min
195 min
16.1±0.2
32.5±0.2
25.5±0.3
18.1±0.2
7.8±0.3
25 min
6.58
195 min
11.11
32.54
35.72
17.43
3.19
25 min
+212±12
+26±1
195 min
+44±3
0±1
CD4
20.5±0.8
32.4±0.1
5.5±0.3
CD3H
CD2H2
СDH3
CH4
25.65
37.49
24.35
5.93
–85±1
–29±1
+4±1
12.4±0.3
29.2±0.1
–49±1
+392±2
+144±10
CD3I
44.1±2.0
1.2±0.3
18.5±1.5
6.3±0.5
12.02
37.00
37.97
12.99
7.54
30.93
42.27
19.26
+267±17
–97±1
145±20
–80±2
+39±6
–14±7
CD2HI
CDH2I
CH3I
13.3±1.5
41.4±2.0
58.7±2.5
16.5±1.5
–65±4
+216±18
present in Table 2, i.e. a nonequilibrium distribution of
deuterium over methyl radicals is retained on the
acetone photolysis. On the contrary, in the case of the
iodomethane pyrolysis the isotope composition of both
final methane and initial iodomethane points to
intensive hydrogen exchange between methyl groups.
It follows from these facts that at least reactions (2),
(4), and (13) occur in the starting phase of the iodo-
methane pyrolysis. It was the consideration of possible
reactions (3) and (14) that has led us to the adoption of
kinetic scheme (15)–(17). Kinetic equations (271)–
(276) correspond to this scheme.
the second equality, on the ground that this is reaction
(153) in scheme (15) generating radicals CH2I which is
the limiting stage of the pyrolysis.
4/3CH3I → CH4 + 2/3I2 + 1/3nCn + 61.8 kJ mol–1. (29)
The previous stages (generating I˙ atoms and CH3˙
radicals) fast come to a quasistationary mode, therefore
the necessity of integrating equations (271), (274), and
(275) disappears, and expressions (30)–(32) are ob-
tained from the right-hand parts of these equations.
(30)
[I]t = √Kc,4[I2]t,
Kc,2m[CH3I] – 3r(t)/k–2m
√Kc,4[I2]
,
d[CH3]/dt = k2m[CH3I] – k–2m[CH3][I] + k1m[CH3I][I]
– k–1m[I2][CH3] – k3m[CH3I][CH3] + k–3m[CH4][CH2I]
– k4m[CH3][CH2I] + k–4m[CH4][CHI]
[CH3]t =
(31)
(271)
(272)
(273)
(274)
(275)
{2/3[I2]Kc,6m√Kc,4[I2] + 2r(t)/k–6m}√Kc,4[I2]
k
–6m[HI][CH3] + k6m[CH4][I],
.
[HI] =
(32)
d[CH2I]/dt = k3m[CH3I][CH3] – k–3m[CH4][CH2I]
– k4m[CH2I][CH3] + k–4m[CH4][CHI],
{Kc,2m[CH3I] – 3r(t)/k–2m
}
Rate constants of reactions (155) are absent from
equations (30)–(32), because the effect of these reac-
tions in a quasiequilibrium is doubled by reactions
(153) and (156), as is seen from the following relation-
ship.
d[CHI]/dt = k4m[CH2I][CH3] – k–4m[CH4][CHI]
– 2k0[CHI]2 – kdiff[CHI],
d[I]/dt = k2m[CH3I] – k–2m[I][CH3]
– k1m[CH3I][I] + k–1m[I2][CH3],
d[HI]/dt = –k–6[HI][CH3] + k6[CH4][I]
+ 2k0[CHI]2 + kdiff[CHI],
Kc,5m = Kc,3mKc,6m
.
d[I2]/dt = 2/3d[CH4]/dt = 2/3k3m[CH3][CH3I]. (276)
Equations (30)–(32) also contain no parameters of
reactions (151), as reactions (152) and (4) lead to the
same effect.
Concentrations of the remaining macrocomponents
can be calculated highly accurate from conservation
laws (28).
Kc,1m = Kc,2mKc,4.
[CH3I]t = [CH3I]0 – 2 [I2]t,
In the kinetic experiment (Figs. 2 and 3) we
measured initial concentration of [CH3I]0 and current
concentration [I2]t, and found directly the process rate
r = d[I2]/dt from these data. As a result, the necessity
[CH4]t = 3/2[I2]t.
(28)
The first equality in equation (276) is written
according to gross-scheme (29) of the pyrolysis, and
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2647
ISOTHERMAL PYROLYSIS OF IODOMETHANES IN GASES
of integrating Eq. (276) also disappears. Substituting
expression (31) in Eq. (276), we obtain working
formula (33).
tion (35) follows from our data (for 567–672 K) and
available published data (for 1050–1500 K).
kH ≡ k3m = 10–11.3±0.3exp[–(8060±700)K/T] cm3/(molecule s),
(500 K ≤ T ≤ 1500 K).
(35)
3√Kc,4
r√[I2]t
.
(33)
k3m
=
2Kc,2m ([CH3I]0 – 2[I2]t)2
In the case of deuterated molecules reaction (36)
proceeds instead of process (152).
We find equilibrium constant (34) on the basis of
the tabular data [9] for the range of 500–700 K.
Kc,2m(T) = 1025.00±0.03exp(–28013K/T) molecule/cm3. (34)
k2d
·
·
CD3I
CD3 + I – 241.4 kJ mol–1,
k–2d
(36)
(Kc,2d = k2d/k–2d).
The data on k–2m and k–6m are taken from the
literature [19, 20]. Now, using formulas (30)–(33), we
can calculate the concentrations of [I˙], [CH3˙], and
[HI] and the rate constant k3m (Table 4) for five
experimental situations presented in Figs. 2 and 3. In
the same table we presented the calculated time of the
half-consumption of CH3˙ radicals in reactions with
CH3I and CH3˙.
We calculate equilibrium constant (37) for reaction
(36) from the thermodynamic tables [9].
Kc,2d(T) = 1025.5±0.1exp(–29040K/T) molecule/cm3. (37)
Similarly in the case of completely deuterated
molecules instead of reaction (153) reaction (38)
proceeds.
It is seen from Table 4 that the rate of consuming
CH3˙radicals in reaction (153) exceeds the rate of their
consuming in the recombination by 2–3 orders, i.e. the
methane generation occurs hundreds times faster than
the ethane generation, which explains the absence of
ethane in the products of the iodomethane pyrolysis
(Table 1). This conclusion does not depend on the
ambiguity of the mechanism of radicals СHI˙
disappearance [in reactions (16) or (17)], because rate
constants k0 or kdiff are absent from formulae (30)–(33),
which contain only a changed rate of process r(t)
independent of the knowledge of the gross-process
(29) detailed mechanism.
kD
CD4 + CD2I· + (18±10) kJ mol–1,
(Kc,3d = k3d/k–3d).
·
CD3 + CD3I
k–D
(38)
Rate constants for direct reaction (38) (Table 4)
were found from the kinetic curves of iodine
accumulation at the trideuteroiodomethane pyrolysis
(Fig. 3). Table 4 shows that the rate of consuming
radicals CD3˙ in reaction (38) exceeds the rate of their
consuming in recombination by 2–3 orders, which
explains the complete absence of C2D6 from the
products of CD3I pyrolysis (Table 1). Analogously to
expression (35) we present rate constant k3d in
analytical form (39) (Fig. 4).
The data of the last column in Table 4 are
graphically presented in Fig. 4. Analytical representa-
Table 4. Quasistationary concentrations of particles Z during the pyrolysis of iodomethane ([CH3I]0×1019.41 molecule/cm3) or
deuteroiodomethane ([CD3I]0×1019.35 molecule/cm3); at 600 K; krec×10–10.1 [cm3/(molecule·s)] for R = CH3 and 10–10.2 for R =
CD3
log {[Z]/(molecule/cm3)}
log k3i/
[cm3/(molecule·s)]
[Eq. (33)]
Half-lifetime, ms
log [I2]/
log r(T)/
(molecule/cm3)
R
T, K
[molecule/(cm3 s)]
I·
R·
HI
7.3
1/k3i[RI]
15
1/krec[R]
940
567
599
619
644
672
599
625
652
15.30
15.72
11.64
12.60
13.12
13.64
14.61
12.62
13.02
13.70
13.0
13.5
14.0
14.4
15.0
13.5
14.1
14.5
10.1
10.7
11.0
11.3
11.5
10.3
10.6
11.1
–17.6±0.3
–17.2±0.3
–17.0±0.3
–16.8±0.3
–16.0±0.3
–16.8±0.2
–16.6±0.2
–16.5±0.2
CH3
8.3
9.4
7
230
140
63
16.19
3.5
2.3
0.4
2.8
1.8
1.4
i = m
16.60
10.4
12.0
17.33
40
15.7±0.10
16.3±0.20
16.6±0.12
790
400
130
CD3
i = d
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

2648
log [k3m/(cm3 molecile–1 s–1)]
SKOROBOGATOV et al.
–11
–12
–13
–14
–15
–16
–17
– 1
– 2
– 3
– 4
– 5
– 6
Т7
Т6
10
1
3
8
6
4
2
2
Т2
Т1
Т5
Т4
Т3
10
0.5
1.0
1.5
2.0
0
5
15
20
25
t, min
1000 K/T
Fig. 4. Тemperature dependence of the rate constant of
reaction (153) and analogous reactions of trideuteroiodo-
methane and diiodomethane: (1) k3m according to Тable 4;
(2) k3m according to the reference [21]; (3) k3d according to
Тable 5; (4) k'3 according to Тable 6; (5, 6) data of [22] for
analogous reaction (153) of the bromo- and chloro-
derivatives, respectively.
Fig. 5. Kinetics of I2 accumulation in the isochoric-iso-
thermal pyrolysis of diiodomethane [CH2I2]0: (1) 2.35×
1018, (2) 4.06×1018, (3) 2.11×1019 molecule/cm3; Т1 587±1,
Т2 610±1, Т3 571±1, Т4 590±1, Т5 615±1, Т6 617±1, Т7
636±1 K.
kD = k3d = 10–11.3±0.3exp[–(7600±400) K/T] cm3/(molecule s),
k3'
CH2I· + CH2I2
CH3I + CHI2 + 28.2 kJ mol–1,
·
(500 K ≤ T ≤ 700 K).
(39)
k–'3
It follows from expressions (39) and (35) that the
activation energy [(63±3) kJ mol–1] of limiting stage
(38) of the CD3I pyrolysis is lower by 4 kJ mol–1 than
the activation energy [(67±6) kJ mol–1] of limiting
stage (153) of the iodomethane pyrolysis.
(K' = k' /k' ).
(403)
c,3
3
–3
The gross-scheme of the pyrolysis looks like (41).
CH2I2 → 1/2CH4 + I2 + 1/2Csol. – 22.0 kJ mol–1. (41)
This implies that the rate equation for [I2]
concentration looks like (42).
Kinetics of the gaseous СН2I2 pyrolysis. The
kinetic curves of the molecular iodine accumulation in
the diiodomethane pyrolysis are presented in Fig. 5.
We conclude from the kinetic experiments that the
limiting stage of the diiodomethane pyrolysis is
reaction (403).
r = d[I2]/dt = 2d[CH4]/dt = 2k3'[CH2I][CH2I2].
(42)
The remaining equations are analogous to Eqs. (27),
so, by-passing the integration operation, we obtain
Eqs. (31*) and (33*) for the calculation of the required
parameters in the quasistationary approximation.
k1'
CH2I2 + I·
CH2I· + I2 – 57.6 kJ mol–1,
K' [CH2I2]
c,2
k–'1
[CH2I]t =
(31*)
(401)
√Kc,4[I2]
(K' = k' /k' ),
c,1
1
–1
r√Kc,4[I2]
2K' ([CH2I2]0 – [I2])2
(33*)
k3' =
k2'
c,2
CH2I2
CH2I· + I· – 206.4 kJ mol–1,
k–'2
Using thermodynamic tables [9], we calculate
constant (34*) of equilibrium (402).
(K' = k' /k' ),
(402)
c,2
2
–2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

2649
ISOTHERMAL PYROLYSIS OF IODOMETHANES IN GASES
K
c,2m(T) = 1025.00±0.03exp(–28013K/T) molecule/cm3, (34*)
lest the course of the is constrained in stage (154).
Analogously dissociative recombination (16) should
proceed with a great (almost gas-kinetic) cross section.
We note that in the previous work [5] it was reaction
(16) {reaction (55) in the notations of [5]} which was
accepted as a limiting stage, and therefore the kinetic
constant k55 found there is underestimated by almost an
order as compared to the true (almost gas-kinetic) k0
value.
The calculated data given in Table 5 allow us to
present analytical expression (35*) for the kinetic
constant k' (Fig. 4).
kH ≡ k3m = 10–11.3±0.3exp[–(8060±700)K/T] cm3/(molecule s),
3
(500 K ≤ T ≤ 1500 K).
(35*)
It follows from Table 5 that the [CH3˙] concentra-
tion is lower by 1–3 orders than the concentrations of
other intermediate particles, which suppress the forma-
tion of ethane, but do not hinder the formation of
methane from iodomethane formed in reaction (403).
Reaction (16) of dissociative recombination is
analogous to recombination (43) yielding the trifluro-
ethane molecule, which is so high-energy [23] that in
the very first vibration splits out one or two HF
molecules in reactions (44).
In our experiments we added neither hydrogen
iodide nor other reagents to initial iodomethane, so the
kinetic scheme of the pyrolysis was limited to three
pairs of independent reactions (151), (152), and (153).
The reverse reaction in the last pair has appeared
unessential, and in the first two pairs of reactions a
quasistationary regime was established. Therefore,
only one (required) rate constant k3m and two
equilibrium constants (Kc,2m and Kc,4), which both can
be calculated from the thermodynamic tables [9], enter
into working formula (33). In such situation it is
possible to find unambiguously the rate constant k3m
from the unique kinetic curve of I2 accumulation, as it
is demonstrated in Table 4.
·
·
СH3 + СF3 → (CH3CF3)* + 424 kJ mol–1,
(43)
·
·
СH3 + СF3 → CH2=CF2 + HF(v=0) + 285 kJ mol–1,
→ CH2=CF2 + HF(v=1) + 235 kJ mol–1,
→ CH2=CF2 + HF(v=2) + 186 kJ mol–1,
·
·
СH3 + СF3 → CH≡CF + 2HF(v=0) + 192 kJ mol–1,
→ CH≡CF + 2HF(v=1) + 93 kJ mol–1. (44)
Basically all the elementary processes are rever-
sible, but reaction (16) appears practically irreversible
as С2 molecules are condensed fast and irreversibly (at
T 800 K) immediately in a gas phase [24].
С2 + С2 → С4 + 620 kJ mol–1,
С2 + СHI· · → С3 + HI + 394 kJ mol–1,
So, it is possible to measure rate constants of
limiting elementary reactions in the isochore iso-
thermal pyrolysis of not only perfluoroalkyliodides [1–
4, 6], but also iodomethanes. Everything that occurs
after a limiting stage, is a “black box” for a researcher,
who limits himself by kinetic measurements of
macrocomponents. However, something can be said
about these reactions. So, in the pyrolysis of iodo-
methanes, reactions (154) and (16), in which radicals
CH2I˙ are “utilized”, should be sufficiently fast as
otherwise they would be limiting stages instead of
reaction (153). The rate constant k4m should be greater
than the kinetic constant k3m by a factor of millions,
С2 + С3 → С5 + 613 kJ mol–1,
С4 + СHI· · → С5 + HI + 390 kJ mol–1,
. . . . . . . . . . . . . . . . . . . . . . etc.
(45)
If C2HI molecules are formed in recombination (16)
they also do not come back in scheme of reac-
tions (15), being released from HI molecules either in a
gas phase [see the bottom line in scheme (16)] or on
reactor walls according to scheme (46).
C2HI (on a surface)
→ 2 Сsolid + HI↑ + 280±30 kJ mol–1.
(46)
Table 5. Parameters of the isochore isothermal pyrolysis of diiodomethane
log {[Z]/(molecule/cm3)}
Half-life time, ms
Log {k3' /
[CH2I2]0,
log {[I2]/
log [r(T)/
T, K
[cm3/(molecule·s)]}
[Eq. (33*)]
–17.2±0.2
molecule/cm3
(molecule/cm3)} (molecule/cm3 s)]
I·
CH2I·
1/(k3'[CH2I2]) 1/(krec[CH2I·])
1018.37
587
610
571
590
615
16.2±0.1
16.9±0.1
16.1±0.1
16.5±0.1
16.9±0.1
13.93
14.15
13.46
13.73
14.18
13.6
14.2
13.4
13.9
14.3
12.4
12.5
12.4
12.6
12.8
65
50
40
30
20
400
350
400
300
200
–17.1±0.2
1018.61
–17.8±0.2
–17.7±0.2
–17.5±0.2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

2650
SKOROBOGATOV et al.
diture of macrocomponents by formula (11). Recently
7
[6] we have applied such method to the iodide CF3I
that has allowed us to measure the rate constant k5 in a
transition region of pressures, and in the present work
we have applied the isochore isothermal pyrolysis
method to the compounds CH3I and CH2I2.
6
9
3
10
1
3
2
2
5
4
8
11
EXPERIMENTAL
Fig. 6. Scheme of a kinetic experimental installation.
(1) DDS-30 or DVS-25 lamp; (2) quartz lenses; (3) quartz
windows-stoppers; (4) metal thermostat-heater (accuracy of
±1 K); (5) diaphragms; (6) quartz reaction vessel (diameter
d 30 mm, length l 100-150 mm); (7) PP-63 potentiometer;
(8) switch of monochromator working frequency;
(9) MDR-3 monochromator; (10) FEU-18A photomulti-
plier; (11) self-recorder.
The scheme of an experimental installation for
kinetic studies by the isochore isothermal pyrolysis
method is presented in Fig. 6. The iodides and
bromides under study were purified from air oxygen
by triple vacuum distillation, and from water admix-
tures by a 24-hour exposure above zeolite 4–5 Å,
beforehand degassed in vacuum at 400°С. Solid
triiodomethane was not dried, but only degassed. A
reaction vessel (Fig. 6, position 6) was filled with an
iodide under study on an OM-2 under a control of a
standard manometer on a vacuum installation allowing
vacuum from 10–2 up to 10–4 torr to be created, then
was sealed off from the installation and placed in a
thermostatic furnace (Fig. 6, position 4). Temperature
in the furnace was maintained during pyrolysis
accurate to ±1 K, and the temperature gradient along a
tube axis did not exceed 1 K on the length of 250 mm.
The initial mixture of CH3I (50 mol %) + CD3I
(50 mol %) in the isotope experiment was prepared by
a gravimetric technique, then dried and degassed as
described above.
Biradicals СI2˙˙ formed next after reaction (403) in
process (47) also do not come back in scheme of
reactions (40), as they generate a cascade of energy
favorable (and consequently irreversible at T 800 K)
reactions (48).
CH2I· + CHI2 → CH3I + CI2·· + 80 kJ mol–1,
CI2· + С2 → С3 + I2 + 285 kJ mol–1,
. . . . . . . . . . . . . . . . . . . . . . . . . . .
С2 + С2 → С4 + 620 kJ mol–1,
(47)
·
. . . . . . . . . . . . . . . . . . . . . . . . etc.
(48)
Carbon clusters (45) formed in reactions (45) and
(48) diffuse to reaction vessel walls, on which they are
deposited in the form of a pyrocarbon layer.
In some experiments a cell was cooled by liquid
nitrogen on the pyrolysis termination and then shaken,
and a I2 content was measured by thiosulfate titration
that allowed us to check the reliability of published
data on the gaseous iodine extinction with an accuracy
of ±3%. The iodide extinction was found from its op-
tical density on a required wave length, if its
concentration was measured beforehand (on an OM-2
manometer). The used extinction values are presented
in Fig. 1.
The isochore isothermal pyrolysis method is not
direct unlike pulse methods (pulse photolysis and
pressure-jump methods [25]), so the rate constant to be
determined definitely depends on the correctness of a
postulated gross-process mechanism. However, direct
methods have their imperfections. So, in a direct
pressure-jump method (in a shock tube) temperature in
a shock wave is calculated by a Hugoniot adiabat or by
a deformation of spectral lines, the temperature and
gas density in a shock wave being fast changed. On the
contrary, in the isochore isothermal pyrolysis method,
though intermediate particles are not directly recorded,
the temperature and density of a pyrolyzed gas are
strictly constant and are reliably measured by a
thermometer and a manometer. Therefore, if stages of
mechanism (2)–(5) and gross-scheme (1) of the
pyrolysis correspond to a reality, it is possible to
calculate unambiguously required kinetic constant k5
from the kinetic curves of the accumulation-expen-
The optical scheme of the kinetic installation allows
us (alternately, with a resolution of ±1 nm) to record
optical density of a reaction vessel content either on an
absorption wave length of a consumable iodide or on
an absorption wave length of arising molecular iodine.
Usually the kinetic experiment at a higher temperature
was carried out in the same quartz cell and with the
same mixture, which was studied at a lower
temperature.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 12 2009

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ISOTHERMAL PYROLYSIS OF IODOMETHANES IN GASES
13. Skorobogatov, G.A., Meilakhs, A.G., Pogosyan, J.I.,
and Khripun, V.K., Zh. Obshch. Khim., 2002, vol. 72,
no. 8, p. 1354.
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