Thermal decomposition and ring expansion in 2,4-dimethylpyrrole. Single pulse shock tube and modeling studies

Article abstract of DOI:10.1021/jp030062m

The thermal decomposition of 2,4-dimethylpyrrole was studied behind reflected shock waves in a pressurized driver single-pulse shock tube over the temperature range 1050-1250 K at overall densities of ～3 × 10^-5 mol/cm³. A plethora of decomposition products, both with and without nitrogen, were found in the post-shock mixtures. They were, among the nitrogen containing products: pyridine, two isomers of methylpyrrole, 2-picoline, 5-picoline, HCN, CH₃CN, C₂H₃CN, C₂H₅CN, and CH≡C-CN. Very small quantities of cis- and trans-CH₃CH=CHCN and CH₂=CHCH₂CN were also found in the post-shock mixtures. Among the products without nitrogen were CH₄, C₂H₄, C₂H₆, C₂H₂, CH₃H≡CH, CH₂=C=CH₂, C₄H₄ and C₄H₂, and very small quantities of other C₄ hydrocarbons and C₅ hydrocarbons. The initiation of a chain mechanism in the decomposition of 2,4-dimethylpyrrole takes place via ejection of hydrogen atoms from sp³ carbons and dissociation of the two methyl groups attached to the ring. The H atoms and the methyl radicals initiate a chain mechanism by abstraction of a hydrogen atom from the methyl group and by dissociative recombination of an H atom and removal of a methyl group from the ring. In addition to the dissociation reactions, there are several unimolecular channels that involve ring cleavage. Ring expansion processes that lead to the production of high yields of pyridine and picoline take place from radical species: CH₃[C₄H₂NH]CH₂ in the production of picoline and [C₄H₃NH]CH₂ in the production of pyridine. In addition to the chain mechanism, there are unimolecular breakdown processes of the pyrrole ring to yield stable products such as HCN, CH₃CN, and others. The total decomposition of 2,4-dimethylpyrrole in terms of a first-order rate constant is given by k_total = 10^16.31 exp(-75.7 × 10³/RT) s^-1. A reaction scheme containing 36 species and 69 elementary reactions was composed and a computer simulation was performed over the temperature range 1050-1250 K at 25 K intervals. The agreement between the experimental results and the model prediction for most of the species is satisfactory.

Full text of DOI:10.1021/jp030062m

J. Phys. Chem. A 2003, 107, 4851-4861
4851
Thermal Decomposition and Ring Expansion in 2,4-Dimethylpyrrole. Single Pulse Shock
Tube and Modeling Studies
Assa Lifshitz,* Aya Suslensky, and Carmen Tamburu
Department of Physical Chemistry, The Hebrew UniVersity, Jerusalem 91904, Israel
ReceiVed: January 16, 2003; In Final Form: April 2, 2003
The thermal decomposition of 2,4-dimethylpyrrole was studied behind reflected shock waves in a pressurized
-
5
driver single-pulse shock tube over the temperature range 1050-1250 K at overall densities of ∼3 × 10
3
mol/cm . A plethora of decomposition products, both with and without nitrogen, were found in the post-
shock mixtures. They were, among the nitrogen containing products: pyridine, two isomers of methylpyrrole,
2
-picoline, 5-picoline, HCN, CH
trans-CH CHdCHCN and CH dCHCH
without nitrogen were CH , C , C
quantities of other C hydrocarbons and C
3
CN, C
2
H
2
3
CN, C
CN were also found in the post-shock mixtures. Among the products
, C , CH CtCH, CH dCdCH , C and C , and very small
hydrocarbons. The initiation of a chain mechanism in the
2 5
H CN, and CHtC-CN. Very small quantities of cis- and
3
2
4
2
H
4
H
2 6
2
H
5
2
3
2
2
H
4 4
4 2
H
4
3
decomposition of 2,4-dimethylpyrrole takes place via ejection of hydrogen atoms from sp carbons and
dissociation of the two methyl groups attached to the ring. The H atoms and the methyl radicals initiate a
chain mechanism by abstraction of a hydrogen atom from the methyl group and by dissociative recombination
of an H atom and removal of a methyl group from the ring. In addition to the dissociation reactions, there are
several unimolecular channels that involve ring cleavage. Ring expansion processes that lead to the production
•
of high yields of pyridine and picoline take place from radical species: CH
3
[C
H
4 2
NH]CH
2
in the production
•
of picoline and [C
unimolecular breakdown processes of the pyrrole ring to yield stable products such as HCN, CH
others. The total decomposition of 2,4-dimethylpyrrole in terms of a first-order rate constant is given by ktotal
4
H
3
NH]CH
2
in the production of pyridine. In addition to the chain mechanism, there are
3
CN, and
16.31
3
-1
)
10
exp(-75.7 × 10 /RT) s . A reaction scheme containing 36 species and 69 elementary reactions
was composed and a computer simulation was performed over the temperature range 1050-1250 K at 25 K
intervals. The agreement between the experimental results and the model prediction for most of the species
is satisfactory
I. Introduction
also fragmentize to yield smaller molecules both with and
without nitrogen.
Thethermalreactionsofpyrrole1 anditsmethylderivatives6-8
have been thoroughly investigated in the past by several groups
of investigators. Experimental results, modeling, and quantum
chemical calculations of potential energy surfaces have been
published. In addition to fragmentation of the molecules when
subjected to shock heating, two major types of reactions take
place. In pyrrole, the isomerizations to cis and trans crotonitrile
and vinyl acetonitrile are major reaction channels, whereas in
the various isomers of methylpyrrole, ring expansion to form
pyridine is a major step. The ring expansion processes in all
methylpyrrole isomers take place via methylene pyrrole radicals
-5
In the present investigation, the thermal reactions of 2,4-
dimethylpyrrole are reported, a mechanism for the production
of the decomposition products is suggested, and a computer
simulation is performed to verify the suggested reaction scheme.
II. Experimental Section
A. Apparatus. The decomposition of 2,4-dimethylpyrrole
was studied behind reflected shock waves in a pressurized driver,
heated, 52 mm i.d. single-pulse shock tube. The tube, made of
electropolished stainless steel tubing, was heated and maintained
at 110 °C with variation of ∼(1 °C. The 4 m long driven section
was divided in the middle by a 52 mm i.d. ball valve. The driver
had a variable length up to a maximum of 2.7 m and could be
varied in small steps in order to obtain the best cooling
conditions. A 36 L dump tank was connected to the driven
section at a 45° angle near the diaphragm holder in order to
prevent reflection of transmitted shocks. The driven section was
separated from the driver by a “Mylar” polyester film of various
thicknesses depending upon the desired shock strength.
7
as intermediates. Except for isomerizations resulting from
migrations of the methyl group, no other isomerizations in
methylpyrrole were reported. Experimental results, modeling,
and quantum chemical calculations have also been published
on the thermal reactions of indole which is pyrrole fused to
In this molecule too, the major reactions are
isomerizations that take place either from indole or from its
benzene.9
-12
3-indolenine tautomer.
As far as we are aware, the thermal reactions of either
-
5
dimethylpyrrole or dimethylindole have never been studied in
the past. 2,4-Dimethylpyrrole is an asymmetrical molecule that
can yield upon ring expansion 2-picoline and 5-picoline. It may
After pumping down the tube to approximately 3 × 10
Torr, the reaction mixture was introduced into the section
between the valve and the end plate and pure argon into the
section between the diaphragm and the valve, including the
dump tank. After running an experiment, samples were trans-
*
To whom correspondence should be addressed.
1
0.1021/jp030062m CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/23/2003

4852 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Lifshitz et al.
ferred from the downstream end of the driven section into a
Hewlett-Packard model 5890 gas chromatograph operating with
two Porapak-N columns using flame ionization (FID) and
nitrogen-phosphor (NPD) detectors. All of the transfer tubes
and the injection system were maintained at 110 °C.
Reflected shock temperatures were calculated from the extent
of decomposition of 1,1,1-trifluoroethane that was added to the
reaction mixture in small quantities and served as an internal
standard. Its decomposition to CH2dCF2 + HF is a first-order
1
3
14.85
unimolecular reaction with a rate constant kfirst ) 10
-74.05 × 10 /RT) s .
exp-
3
-1
(
Reflected shock temperatures were calculated from the
measured extent of reaction, using the relation
1
At
T ) -(E/R)/ ln - ln(1-ø)
(I)
[
{
}
]
where E is the activation energy of the HF elimination, A is its
preexponential factor, t is the reaction dwell time, and ø is the
extent of decomposition defined as
ø ) [CH dCF ] /([CH dCF ] + [CH CF ] )
(II)
2
2 t
2
2 t
3
3 t
The additional reflected shock parameters were calculated from
the measured incident shock velocities using the three conserva-
tion equations and the ideal gas equation of state. Dwell times
of approximately 2 ms were measured with an accuracy of ∼5%.
-
5
Cooling rates were approximately 5 × 10 K/s.
B. Materials and Analysis. Reaction mixtures containing
0
.3% 2,4-dimethylpyrrole and 0.1% 1,1,1-trifluoroethane diluted
Figure 1. Gas chromatograms of a shock sample of 0.3% 2,4-
dimethylpyrrole and 0.1% 1,1,1-trifluoroethane in argon, heated to 1170
K. (a) species without nitrogen, (b) species with nitrogen. The numbers
on the chromatograms are multiplication factors.
in argon were prepared manometrically and stored in 12 L glass
bulbs at 700 Torr. Both the bulbs and the gas manifold were
-
5
pumped down to ∼10 Torr before the preparation of the
mixtures. 2,4-Dimethylpyrrole was obtained from Aldrich
Chemical Co. and was listed as 97% pure. However, chromato-
grams of unshocked samples did not show products that
appeared in the shocked samples. The argon used was Matheson
ultrahigh purity grade, listed as 99.9995%, and the helium was
Matheson pure grade, listed as 99.999%.
is the peak area of a product i in the shocked sample, S(pri) is
its sensitivity relative to 2,4-dimethylpyrrole, and N(pri) is the
number of its carbon atoms. F5/F1 is the compression behind
the reflected shock wave, and T1 is room temperature, 110 °C
in this study.
The gas chromatographic analyses of the pre and post-shock
mixtures were performed on two 2 m Porapak-N columns using
flame ionization and nitrogen-phosphor detectors. The initial
column temperature of 35 °C was gradually elevated to 190 °C
in an analysis that lasted approximately 2.5 h. Typical chro-
matograms of 0.3% 2,4-dimethylpyrrole in argon of a shock
heated mixture to 1170 K taken on a FID and NPD are shown
in Figure 1.
The identification of reaction products in the GC was based
on their retention times and on analyses with a Hewlett-Packard
model 5970 mass selective detector. The sensitivities of the
various products to the FID and NPD were determined relative
to 2,4-dimethylpyrrole from standard mixtures. The areas under
the GC peaks were integrated with a Spectra Physics model
SP4200 computing integrator and were transferred after each
analysis to a PC for data reduction and graphical presentation.
C. Determination of Product Concentrations. The con-
centrations of the reaction products C5(pr)i were calculated from
their GC peak areas from the following relations:
III. Results
To determine the distribution of reaction products, some 30
tests were run with mixtures containing 0.3% 2,4-dimethylpyr-
role and 0.1% 1,1,1-trifluoroethane in argon, covering the
temperature range 1050-1250 K. Trifluoroethane served as an
internal standard for temperature determination. Extents of
pyrolysis starting from a few hundredths of one percent were
determined for many of the products. Details of the experimental
conditions and the distribution of reaction products are given
in Table 1 and are shown graphically in Figures 2 and 3. The
percent of a given product in the table, corresponds to its mole
fraction in the post-shock mixture (not including Ar and H2),
irrespective of the number of its carbon atoms. Products of very
small quantities such as C4 compounds other than C4H4 and
C4H2 and C5 hydrocarbons were not included in the list of
reaction products.
C (pr) ) A(pr ) /S(pr ){C (2,4-dimethylpyrrole) }/
5
i
i t
i
5
0
A(2,4-dimethylpyrrole) (III)
0
C (2,4-dimethylpyrrole) )
5
0
{
p %(2,4-dimethylpyrrole)F /F }/100RT (IV)
1 5 1 1
A(2,4-dimethylpyrrole) ) A(2,4-dimethylpyrrole) +
0
t
1
/6ΣN(pr ) A(pr ) /S(pr ) (V)
i i t i
In these relations that are based on carbon atom conservation,
C5(2,4-dimethylpyrrole)0 is the concentration of 2,4-dimethyl-
pyrrole behind the reflected shock wave prior to decomposition
and A(2,4-dimethylpyrrole)0 is the calculated GC peak area of
2,4-dimethylpyrrole prior to decomposition (eq V) where A(pri)t

Decomposition and Expansion in 2,4-Dimethylpyrrole
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4853
TABLE 1: Experimental Conditions and Product Distribution (in Mole Percent) of Products without and with Nitrogen in the
Decomposition of 2,4-Dimethylpyrrole
Products without Nitrogen
5
no.
T
5
(K)
C
5
× 10
t(ms)
DMP
CH
4
C
H
2 4
C
2
H
6
C
H
2 2
C
3
H
4
C
4
H
4
4 2
C H
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1073
1083
1101
1107
1114
1114
1121
1123
1123
1124
1134
1140
1142
1153
1156
1156
1157
1157
1166
1166
1166
1170
1182
1190
1193
1199
1203
1205
1225
1235
1250
2.65
2.80
2.72
2.85
2.65
2.35
2.51
2.32
2.45
2.26
2.34
2.78
2.59
2.44
2.72
2.55
2.39
2.52
2.67
2.59
2.38
2.55
2.46
2.40
2.47
2.32
2.43
2.40
2.58
2.39
2.55
1.42
1.47
1.27
1.34
1.70
1.84
2.0
98.12
97.10
95.58
94.97
93.17
91.36
88.75
87.41
88.88
87.71
82.89
85.61
80.07
80.21
78.0
78.91
68.14
72.98
71.05
69.71
67.30
68.29
59.78
42.42
50.11
35.45
38.21
30.91
24.21
17.09
12.98
0.675
1.14
1.49
1.71
2.24
3.13
3.70
3.58
3.04
4.54
4.97
4.14
5.33
5.13
5.50
5.91
9.12
6.67
6.95
7.18
6.66
7.43
7.86
10.65
10.43
12.80
11.46
14.45
12.66
13.96
12.0
0.0255
0.0423
0.0566
0.0411
0.0650
0.115
0.177
0.190
0.177
0.234
0.297
0.234
0.323
0.363
0.385
0.409
0.810
0.577
0.562
0.603
0.744
0.643
0.933
1.71
0.173
0.314
0.500
0.616
0.959
1.25
0.0141
0.0226
0.0430
0.0359
0.050
0.0689
0.112
0.118
0.176
0.144
0.176
0.145
0.178
0.224
0.217
0.237
0.454
0.297
0.298
0.326
0.573
0.336
0.547
0.775
0.70
0.0073
0.0113
0.0151
0.0181
0.0274
0.0368
0.0472
0.0514
0.0375
0.0619
0.0742
0.0587
0.0882
0.0907
0.101
0.0135
0.0266
0.0416
0.0683
0.107
0.151
0.211
0.233
0.109
0.0289
0.352
0.263
0.411
0.392
0.464
0.469
0.951
0.706
0.665
0.696
0.512
0.735
0.904
1.72
0.0098
0.0166
0.0160
0.0260
0.0355
0.0592
0.0670
0.0743
0.0824
0.110
0.0848
0.127
0.141
0.163
0.161
0.389
0.266
0.256
0.274
0.361
0.293
0.453
1.07
1.90
2.0
2.01
1.55
2.07
1.99
1.50
2.0
1.50
1.70
1.40
2.0
1.80
1.71
1.60
1.60
1.50
1.60
2.18
1.50
2.0
1.89
1.90
1.75
1.61
1.68
1.48
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
2.60
3.06
2.11
3.28
3.20
3.58
3.78
0.104
6.86
0.198
4.99
0.152
4.77
0.142
4.83
0.149
4.61
0.150
5.10
0.157
6.12
0.214
9.51
0.374
1.39
9.10
0.776
1.47
0.310
1.44
2.26
11.77
10.54
13.46
12.06
13.95
11.50
1.06
0.466
2.14
2.42
1.76
1.02
0.491
2.19
2.78
1.97
1.20
0.538
2.52
3.01
2.28
1.20
0.610
2.53
3.71
2.96
1.48
0.734
3.01
4.18
3.77
1.53
0.773
2.80
Products with Nitrogen
CN CN
no.
T
5
(K)
DMP
HCN
CHCCN
CH
3
C
2
H
3
C
2
H
5
CN
pyridine
MP
2-picoline
5-picoline
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1073
1083
1101
1107
1114
1114
1121
1123
1123
1124
1134
1140
1142
1153
1156
1156
1157
1157
1166
1166
1166
1170
1182
1190
1193
1199
1203
1205
1225
1235
1250
98.12
97.10
95.58
94.97
93.17
91.36
88.75
87.41
88.88
87.71
82.89
85.61
80.07
80.21
78.0
78.91
68.14
72.98
71.05
69.71
67.30
68.29
59.78
42.42
50.11
35.45
38.21
30.91
24.21
17.09
12.98
0.0302
0.0232
0.0457
0.0251
0.0425
0.0407
0.122
0.140
0.221
0.154
0.283
0.264
0.342
0.353
0.336
0.454
0.432
0.386
0.468
0.707
1.22
0.0247
0.0422
0.0795
0.0788
0.117
0.127
0.190
0.194
0.268
0.195
0.298
0.254
0.325
0.40
0.405
0.378
0.616
0.561
0.555
0.582
0.863
0.657
0.924
1.21
0.0031
0.0052
0.0087
0.0057
0.0026
0.0101
0.0283
0.0252
0.0842
0.0274
0.0414
0.0293
0.0379
0.0639
0.0609
0.0536
0.151
0.146
0.242
0.472
0.521
0.823
1.02
0.0918
0.0803
0.154
0.411
0.383
0.273
0.665
0.751
0.210
0.553
1.03
0.436
0.622
1.05
1.09
1.55
1.82
1.95
2.31
2.71
1.94
2.96
2.83
3.68
3.66
4.01
3.74
4.90
4.85
5.17
5.29
6.27
5.53
6.98
8.59
6.78
8.36
7.68
7.59
9.36
9.19
9.01
0.242
0.312
0.444
0.395
0.440
0.538
0.911
1.34
1.13
0.454
1.23
1.31
1.67
1.88
2.13
1.59
1.51
1.73
2.66
2.66
3.34
2.68
3.67
4.87
3.84
4.56
4.48
4.18
5.70
5.08
5.60
0.0024
0.0039
0.0075
0.0121
0.0161
0.0089
0.0173
0.0295
0.0182
0.0290
0.0403
0.0457
0.0385
0.106
1.16
1.56
1.40
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
1.25
2.20
0.00079
0.00089
1.95
0.695
1.09
0.870
1.26
3.01
2.98
3.34
0.0040
0.0172
0.0044
0.00370
0.0034
0.0040
0.0039
0.0082
0.0299
0.0180
0.0537
0.0436
0.0863
0.0796
0.108
3.11
0.654
0.964
1.11
4.39
0.107
0.0859
0.0763
0.0695
0.0860
0.0798
0.159
4.53
0.0944
0.0933
0.313
4.76
1.52
5.19
1.63
5.31
1.68
0.831
1.04
0.109
5.67
1.45
0.254
8.02
2.13
1.49
0.393
0.309
12.34
9.33
2.52
1.55
0.971
1.30
0.254
0.20
2.81
2.30
0.579
0.382
12.45
13.29
11.83
17.45
19.07
22.89
2.60
1.67
1.27
0.602
0.406
2.46
3.13
1.32
0.643
0.408
2.98
3.01
1.59
0.767
0.529
2.96
3.53
1.72
0.927
0.631
2.86
5.22
0.20
2.07
1.49
0.917
3.09
The balance of nitrogen vs carbon among the decomposition
products is shown in Figure 4. The sum of the concentrations
of all of the nitrogen containing products, except for the two
isomers of picoline, are plotted against one-sixth the sum of
the concentrations of all of the products, each multiplied by
the number of its carbon atoms. Including the two picolines,
which have the same N/C ratio as the reactant 2,4-dimethylpyr-
role, in the mass balance evaluation would underestimate the
deviation from a perfect mass balance owing to their high
concentration. The diagonal in the figure represents a perfect
mass balance. As can be seen, mass balance is maintained in
the experiments.

4854 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Lifshitz et al.
Figure 2. Distribution of reaction products without nitrogen as a
Figure 5. Arrhenius plot of a first-order rate constant for the total
function of temperature.
decomposition of 2,4-dimethylpyrrole. The value obtained is ktotal )
16.31
3
-1
1
0
exp(-75.7 × 10 /RT) s . See eq VI.
Figure 3. Distribution of reaction products with nitrogen as a function
of temperature.
Figure 6. Arrhenius plot for the production of various reaction
products. See eq VII and Table 2.
Figure 4. Nitrogen-carbonmass balance among the decomposition
products. The 45° line corresponds to a perfect mass balance. Nitrogen-
carbon mass balance is maintained.
products, calculated from the relation
kproduct
)
[
product]i
Figure 5 shows the rate constant for the overall decomposi-
tions of 2,4-dimethylpyrrole, calculated as a first-order rate
constant from the relation
ktotal
[2,4-dimethyl pyrrole] - [2,4-dimethyl pyrrole]
0
t
(
VII)
ktotal ) -ln{[2,4-dimethylpyrrole]t/
Note that the values calculated from eq VII correspond to the
production rates and not to the depletion rate of the reactant
due to the production of a given product. Values of E obtained
from the slopes of the lines and their corresponding preexpo-
nential factors are summarized in Table 2. Because the produc-
tion of all of the stable products are associated in one way or
[2,4-dimethylpyrrole] }/t (VI)
0
The value obtained is ktotal ) 1016.31 exp(-75.7 × 10 /RT) s ,
where R is expressed in units of cal/(K mol). Figures 6 and 7
show Arrhenius plots of the first-order production rate of several
3
-1

Decomposition and Expansion in 2,4-Dimethylpyrrole
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4855
TABLE 2: First Order Arrhenius Parameters for the
Production Rates of the Various Reaction Products
no.
products
log A (s-1)
E (kcal/mol)
1
2
3
4
5
6
7
8
9
total decomposition
16.31
13.18
19.36
17.31
23.78
17.52
19.73
17.09
20.83
29.66
16.02
21.20
23.61
14.82
13.25
14.34
18.99
75.7
61.0
99.4
83.9
124.6
91.0
101.3
90.6
107.5
165.5
82.0
113.0
127.0
73.7
CH
4
C
2
C
2
C
2
C
3
C
4
C
4
H
4
H
6
H
2
H
4
H
2
H
4
HCN
CHtCCN
CH CN
3
10
11
12
13
14
15
16
17
C
C
2
H
H
3
CN
CN
2
5
methylpyrrole
2-picoline
5-picoline
pyridine
62.3
69.8
92.9
Once the two isomers of methyl pyrrolyl are formed (reactions
and 4), they can either undergo ring expansion to yield
3
pyridine or recombine with hydrogen atoms to yield methylpyr-
role. The latter depends on the H-atom concentration in the
system.
Reactions 3 and 4 are considerably faster than reactions 1-2
having lower activation energies and higher preexponential
factors. The removal of a hydrogen atom from the nitrogen is
a slow process both statistically (a factor of 6 less probable)
and owing to the existence of resonance structures that increase
the N-H bond strength. This bond strength increase is due to
the fact that in some of the resonance structures the N-H bond
has a vinyl type character and is this a stronger bond. This step
was neglected in our discussion and computer simulations. Also,
the ejection of a hydrogen atom from a sp carbon in the ring
is very slow and does not contribute at all to the initial
production of hydrogen atoms.
The H atoms and methyl radicals initiate a chain mechanism
by abstraction of H atoms from the methyl group and by
dissociative recombination of H atoms and removal of methyl
groups from the ring. In view of the existence of two methyl
groups in the molecule, each of the following reactions is two
elementary steps:
Figure 7. Arrhenius plot for the production of various reaction
products. See eq VII and Table 2.
2
another with free radical reactions, their Arrhenius parameters
do not correspond to the parameters of true first-order rate
constants. They do provide, however, a convenient way to
summarize general rates.
IV. Discussion
A. Initial Production of Free Radicals, Abstractions, and
Dissociative Recombinations. The initiation steps that produce
free radicals in the decomposition of 2,4-dimethylpyrrole are
3
ejections of hydrogen atoms from the two methyl groups (sp
carbons) and dissociation of methyl groups from the pyrrole
ring:
In each one of these reactions, in addition to the methyl
radicals and hydrogen atoms that are formed, the remaining
unstable intermediates undergo ring cleavage, ring expansion
and other processes.

4856 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Lifshitz et al.
B. Ring Expansion. a. Introductory Remarks. As can be seen
vicinity of the radical site to the nitrogen atom in the ring. As
can be seen in Figure 3, the yield of 2-picoline is somewhat
higher than that of 5-picoline, indicating that the insertion of
the methylene radical into a C-C bond is somewhat easier that
the insertion into a C-N bond.
in the product distribution (Figure 3), the products of ring
expansion are among the products of the highest concentration.
It has also been shown in the study on the decomposition of
8
N-methylpyrrole that ring expansion toward the formation of
pyridine is one of the major processes in its thermal reactions.
It was assumed that ring expansion takes place from methylene
pyrrole radicals rather than from methyl pyrrole itself. In the
expansion process, the hydropyridyl radical is formed, and by
a fast ejection of a hydrogen atom (∆H ∼ 23 kcal/mol) that
follows, pyridine is obtained. This assumption was later verified
by detailed quantum chemical calculations for ring expansion
We have demonstrated in the past that the ring expansion of
N-methylene pyrrole was much faster than the process in the
- and the 3-methylene pyrrole isomers, having a barrier of only
2
∼
35 kcal/mol, compared to 61 and 74 kcal/mol for the other
7
two. We have also found (yet unpublished) that the isomer-
ization barrier for the isomerization from the 2 position to the
N position is equal to ∼50 kcal/mol. We have therefore
concluded that the ring expansion will always take place from
N-methylene pyrrole, regardless of what is the reacting isomer.
From the 2- and 3-methylene pyrrole, this will take place
following the isomerization process that is the rate determining
step. For this reason, we used an activation energy of 48 kcal/
mol for reactions 33 and 34 that can be considered to some
extent as global reactions.
7
from N-methylene and 2 and 3-methylene pyrrole. On the other
hand, the first transition state on the potential energy surface
of the ring expansion process, when N-methylpyrrole is the
starting molecule, lies about 140 kcal/mol above the energy level
7
of N-methylpyrrole. In the transition state, in addition to N-C
bond breaking, one of the hydrogen atoms originating from the
methyl group is already far removed from its original site on
the carbon atom to a distance of 2.35 Å. This means that the
energy level of the transition state is determined by both H-atom
C. Free Radical Reactions vs Unimolecular Decomposi-
tions. As can be seen in Table 1 and Figure 2, there is a
considerable yield of low molecular weight products, both with
and without nitrogen. However, a reaction scheme based on
free radical reactions alone, where the free radical concentrations
are determined by the initiation steps discussed previously, could
not account for the yields of almost all of the decomposition
products. We thus believe that unimolecular ring cleavage
processes that produce unstable intermediates play also an
important role in the decomposition mechanism. Unimolecular
decompositions can take place from both the reactant itself and
from unstable species that are formed after the reactant has lost
a methyl group or a hydrogen atom (reactions 1-4 and 5-8).
The various unimolecular ring cleavage processes starting from
the reactant require much higher activation energies even just
owing to thermochemical considerations. In fact, sensitivity
analyses show that the reactant dissociation is much less
important than its dissociation after losing a hydrogen atom or
a methyl group. Only the latter was thus considered in the final
kinetic scheme. There are many possible reaction channels for
unimolecular cleavage of five membered heterocyclics. We
introduced into the kinetic scheme steps that are known in
7
ejection and N-C bond cleavage. We have assumed that here
too, the ring expansion process takes place from methylene
methyl pyrrole rather than dimethylpyrrole itself.
There are three possible ring expansion processes in 2,4-
dimethylpyrrole. Two processes produce two isomers of picoline
(methyl pyridine), 2-picoline, and 5-picoline. One process yields
pyridine. All of the three reactions take place from radical
species. The two picoline isomers are formed from methylene
methylpyrrole. This is a species that is formed after removing
a hydrogen atom from a methyl group attached to the ring.
Pyridine is formed from a species that is formed after the
removal of a methyl group from the molecule.
b. Mechanism. The ring expansion mechanism is based on
insertion of a methylene group into a C-C or a C-N bond in
the pyrrole ring. It can be expressed schematically as
1
4-17
similar systems
and several assumed channels. A few of
these reaction steps are listed in Scheme 1.
It should be mentioned, however, that by no means these
reactions should be considered as one concerted step each. They
probably involve a series of consecutive reactions that can only
be established by quantum chemical calculations. For the
purpose of modeling, we have estimated an apparent rate
constant for the entire process which is expressed by a simple
unimolecular reaction.
in methylene methylpyrrole, and as
D. Computer Modeling. a. Reaction Scheme. To model the
observed product distribution, we have constructed the reaction
scheme that is shown in Table 3. The scheme contains 36 species
and 69 elementary reactions. The symbol (R) at the end of a
reaction in the scheme indicates that, after a reaction time of 2
ms (or earlier), the reaction proceeds in the reverse direction.
The rate constants listed in the table are given as k ) A exp-
in methyl pyrrolyl and in methylene pyrrole.
Each methylene group in methylene methylpyrrole can, in
principle, be inserted into two different locations in the ring,
but the final products of the ring expansion are the same.
However, owing to the presence of double bonds in the molecule
that are not in full resonance with the single bonds, the insertion
into C-C or C-N single bonds has a lower barrier than the
insertion into a CdC double bond.
n
(-E/RT) or k ) A′T exp(-E/RT) when the rate constant taken
from the database fits a wide temperature range. The units are
3
-1
-1
cm , s , kcal, and mol . The Arrhenius parameters for the
reactions in the scheme are either estimated or taken from
various literature sources. These sources are specific articles
relevant to the present system and databases, mainly the NIST-
The only difference between the two radicals that are obtained
by removing hydrogen atoms from the methyl groups is in the
18
Kinetic Standard Reference Data Base 17. The parameters for

Decomposition and Expansion in 2,4-Dimethylpyrrole
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4857
TABLE 3: Reaction Scheme for the Decomposition of 2,4-Dimethylpyrrole
E
k
f
k
r
∆S
r
∆H
r
no
reactions
A (s-1)
(kcal/mol)
(1150 K)
(1150 K) (1150 K) (1150 K)
ref
•
•
•
15
14
1
2
3
4
5
6
7
8
9
DMPyrrole f DMPyrrole(R1) + H
3.0 × 10
3.0 × 10
4.0 × 10
4.0 × 10
1.20 × 10
1.20 × 10
1.0 × 10
1.0 × 10
1.0 × 10
1.0 × 10
2.50 × 10
2.50 × 10
8.0 × 10
8.0 × 10
2.50 × 10
2.50 × 10
2.50 × 10
2.50 × 10
2.50 × 10
2.50 × 10
1.0 × 10
1.0 × 10
2.50 × 10
2.50 × 10
2.50 × 10
2.50 × 10
2.50 × 10
2.50 × 10
1.62 × 10
6.0 × 10
1.0 × 10
1.0 × 10
5.0 × 10
5.0 × 10
2.0 × 10
2.0 × 10
1.0 × 10
1.0 × 10
1.0 × 10
1.0 × 10
3.0 × 10
3.0 × 10
5.0 × 10
5.0 × 10
5.0 × 10
1.0 × 10
3.0 × 10
7.96 × 10
5.0 × 10
1.43 × 10
5.0 × 10
5.0 × 10
78
78
80
80
9
4.52
4.52
25.1
25.1
4.64 × 10
4.64 × 10
3.52 × 10
3.52 × 10
30.8
30.8
46.7
46.7
83.0
83.0
86.1
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
est
•
15
16
16
14
14
13
13
14
14
13
13
13
13
13
13
13
13
13
13
14
14
13
13
13
13
13
13
13
13
13
13
13
13
14
14
14
14
14
14
14
14
13
13
13
14
14
13
14
14
14
14
14
12
12
7
DMPyrrole f DMPyrrole(R2) + H
•
•
•
DMPyrrole f MPyrrole(R1) + CH
3
•
DMPyrrole f MPyrrole(R2) + CH
3
•
86.1
•
12
DMPyrrole + H f DMPyrrole(R1) + H
2
2.34 × 10 1.38 × 10
3.34
-23.7
-23.7
-24.8
-24.8
-10.9
-10.9
58.3
58.3
52.4
52.4
48.1
•
•
12
7
DMPyrrole + H f DMPyrrole(R2) + H
2
9
2.34 × 10 1.38 × 10
3.34
-3.40
-3.40
7.23
7.23
40.6
40.6
44.8
44.8
32.1
32.1
34.3
34.3
33.2
33.2
7.54
7.54
44.3
44.3
43.0
43.0
40.5
40.5
21.0
21.0
11.1
11.1
19.4
19.4
5.96
•
•
•
10
6
DMPyrrole + CH
3
f DMPyrrole(R1) + CH
f DMPyrrole(R2) + CH
4
15
15
8
1.41 × 10 1.54 × 10
•
10
6
DMPyrrole + CH
3
4
1.41 × 10 1.54 × 10
•
•
12
8
DMPyrrole + H f 2-MPyrrole + CH
0 DMPyrrole + H f 4-MPyrrole + CH
3
3.02 × 10 6.66 × 10
•
•
12
8
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
3
8
3.02 × 10 6.66 × 10
•
•
•
7
1 DMPyrrole(R1) f CH
2 DMPyrrole(R2) f CH
3 DMPyrrole(R1) f C
4 DMPyrrole(R2) f C
3
CN + C
CN + C
4
H
H
5
68
68
60
60
60
60
62.5
62.5
60
60
7
2.99
2.99
4.48 × 10
4.48 × 10
4.48 × 10
4.48 × 10
1.22 × 10
1.22 × 10
1.43 × 10
1.43 × 10
4.87 × 10
4.87 × 10
•
7
3
4
5
•
•
•
7
3
H
H
4
+ C
+ C
2
H
H
4
CN
CN
317.0
•
7
3
4
2
4
317.0
99.1
99.1
33.2
33.2
99.1
99.1
•
•
9
5 MPyrrole(R1) f HCN + C
4
H
H
5
•
•
9
6 MPyrrole(R2) f HCN + C
4
5
48.1
43.0
43.0
41.9
•
•
7
7 MPyrrole(R1) f C
3
H
H
4
+ CH
2
CN
CN
•
•
•
•
7
8 MPyrrole(R2) f C
9 MPyrrole(R1) f CH
0 MPyrrole(R2) f CH
3
4
+ CH
2
•
7
3
CN + C
CN + C
3
H
H
3
•
7
3
•
3
3
41.9
•
•
•
•
12
8
1 DMPyrrole(R1) + H f MPyrrole(R3) + CH
2 DMPyrrole(R2) + H f MPyrrole(R4) + CH
3
3
4.68 × 10 8.68 × 10
-11.0
-11.0
68.1
68.1
55.1
55.1
62.2
62.2
36.3
36.3
25.5
25.5
39.6
39.6
-7.64 est
-7.64 est
-97.1
-97.1
-82.9
-82.9
-23.7
-23.7
-24.8
-24.8
50.8
-91.9
-107.4
33.7
0.727 29, mod.
-4.39 18
-1.63 est
-8.30 18, mod.
-5.47 24
107.7
1.07 18
-90.7
11.5
-57.0
38.2
37.3
94.0
•
•
•
12
8
7
4.68 × 10 8.68 × 10
•
6
3 MPyrrole(R3) f C
4 MPyrrole(R4) f C
5 MPyrrole(R3) f C
6 MPyrrole(R4) f C
7 MPyrrole(R3) f HCN + C
8 MPyrrole(R4) f HCN + C
2
2
2
2
H
3
H
3
H
2
H
2
+ C
+ C
+ C
+ C
2
2
2
2
H
H
H
H
4
4
3
3
4
4
CN
CN
CN
78
78
65
65
72
72
50.6
47
60
60
48
48
7
0.0376
0.0376
6.21 × 10
6.21 × 10
1.20 × 10
1.20 × 10
4.71 × 10
4.71 × 10
•
6
•
•
•
7
11.1
11.1
0.520
0.520
•
7
CN
•
•
7
H
H
5
•
•
7
5
•
•
•
3
10
12
8
9 DMPyrrole(R1) f 5-Picoline + H
3.89 × 10 7.34 × 10
•
4
0 DMPyrrole(R2) f 2-Picoline + H
7.03 × 10 1.32 × 10
•
•
•
•
•
1 MPyrrole(R1) f Pyridine + H
39.6
39.6
9.67 × 10
9.67 × 10
•
8
2 MPyrrole(R2) f Pyridine + H
•
4
12
12
10
10
3 MPyrrole(R3) f Pyridine + H
3.78 × 10 6.78 × 10
7, mod.
7, mod.
•
4
4 MPyrrole(R4) f Pyridine + H
3.78 × 10 6.78 × 10
•
•
•
12
5 2-Picoline + H f Pyridine + CH
3
3
9.35 × 10 1.65 × 10
•
12
6 5-Picoline + H f Pyridine + CH
7
0
0
0
9.35 × 10 1.65 × 10
5.96
•
•
14
7 MPyrrole(R1) + H f 4-MPyrrole
1.0 × 10
1.0 × 10
1.0 × 10
1.0 × 10
0.157
0.157
1.16
-39.4
-39.4
-31.1
-31.1
3.65
est
est
est
est
est
est
8, mod.
8, mod.
18, mod.
est
18, mod.
18
•
•
14
14
14
8 MPyrrole(R2) + H f 2-MPyrrole
•
•
9 MPyrrole(R3) + H f 2-MPyrrole
•
•
0 MPyrrole(R4) + H f 4-MPyrrole
0
1.16
•
•
12
7
1 2-MPyrrole + H f MPyrrole(R3) + H
2
10
10
8
8
51
0
0
34
9
9.56
7
7
8.29
88.6
12.3
0
13.5
30.8
37.2
39.7
93
7
12
7
0
7
50
9
3.77 × 10 1.87 × 10
•
•
12
7
2 4-MPyrrole + H f MPyrrole(R4) + H
2
3.77 × 10 1.87 × 10
3.65
•
•
•
12
8
3 2-MPyrrole + CH
4 4-MPyrrole + CH
3
f MPyrrole(R3) + CH
f MPyrrole(R4) + CH
4
4
1.51 × 10 1.39 × 10
-3.09
-3.09
29.7
•
12
8
3
1.51 × 10 1.39 × 10
•
•
4
12
-2
5 C
6 C
7 C
8 C
9 C
0 C
1 C
2 C
3 C
4 CH
5 CH
6 CH
7 CH
8 CH
4
4
4
4
4
2
4
3
2
H
H
H
H
H
H
H
H
H
5
5
3
3
4
6
4
4
6
f C
4
•
H
4
+ H
1.02 × 10 1.40 × 10
•
•
•
14
+ H f C
4
H
H
6
1.0 × 10
3.0 × 10
1.60 × 10 -30.4
•
14
-4
+ H f C
4
4
2. ×10
-33.0
24.2
•
7
13
11
9
f C
H
4 2
+ H
2.75 × 10 3.53 × 10
•
•
•
12
+ H f C
4
2
2
H
H
H
3
+ H
+ H
2
9.74 × 10 8.14 × 10
5.57
•
12
+ H f C
5
2
2.18 × 10 5.21 × 10
8.18
6.79
5.82
1.44
34.2
6.74
•
•
13
11
10
9
+ H f C
2
•
+ C
2
H
3
2.34 × 10 3.78 × 10
•
13
+ H f CH
3
+ C
H
2
H
+ CH
2
2.34 × 10 3.32 × 10
•
•
1.64T 4
10
+ CH
3
f C
2
•
5
4
7.64 × 10 3.39 × 10
•
17
18
10
-1
4
4
3
3
3
+ Ar f CH
+ H f CH
3
+ H + Ar(R)
1.50 × 10
2.24
2.10 × 10
11
21
•
•
14
3
+ H
H
H
2
(R)
1.44 × 10
6.75 × 10 3.63 × 10
•
•
•
•
15 -0.64
13
+ CH
+ CH
+ CH
3
3
3
f C
f C
f C
2
6
5
4
1.01 × 10 T
1.11 × 10 4.62 × 10 -40.4
26
26
18
24
26
28
•
•
•
+ H•
+ H
13
10
14
2
2
3.01 × 10
8.14 × 10 1.41 × 10
-4.80
-7.55
24.7
23.7
34.1
7.35
-0.089 -13.7
6.65
-39.5
6.65
25.0
3.68
23.8
15
9
H
2
4.72 × 10
6.59 × 10 4.44
•
•
•
9
1.19
6
13
13
12
9
9 C
0 C
2
H
H
5
f C
f C
2
H
H
4
+ H
4.80 × 10 T
1.79 × 10 1.21 × 10
•
14
6
2
3
2
2
+ H
2.0 × 10
5.71 × 10 4.26 × 10
•
•
15
-3
1 CH
2 CH
3 CH
4 CH
5 CH
6 C
7 C
8 C
3
3
3
3
2
CN f CH
2
CN + H (R)
1.00 × 10
2.12 × 10 5.20 × 10
•
•
14
12
CN + H f CH
3
+ HCN
f CH + CH
+ CH
f C CN
1.0 × 10
4.68 × 10 5.26 × 10
-7.06 27, mod.
•
CN•
12
10
7
CN + CH
3
4
2
•
3.0 × 10
1.57 × 10 4.10 × 10
28
28
27, mod.
28, mod.
28, mod.
•
14
12
9
CN + H f H
2
2
CN
2.0 × 10
9.35 × 10 1.31 × 10
-12.6
-83.7
-16.6
50.2
0.870 28, mod.
48.3 28
•
•
12
12
CN + CH
3
2
H
5
3.0 × 10
3.0 × 10
1.70
•
•
14
13
8
H
2 5
H
2 4
H
2 3
CN + H f C
2
H
4
CN + H
2
5.0 × 10
2.34 × 10 5.67 × 10
•
13
3
13
12
14
CN f C
2
•
H
3
CN + H
2
3.0 × 10
9.45 × 10 1.07 × 10
•
14
12
CN + H f H + CHdCHCN
9 CHdCHCN f CHtCCN + H
5.0 × 10
9.74 × 10 2.24 × 10
•
•
13
5
1.0 × 10
40
2.50 × 10 2.27 × 10

4858 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Lifshitz et al.
TABLE 3 (Continued)
Glossary
SCHEME 1
Figure 8. Calculated and experimental mole percent of 2,4-dimeth-
ylpyrrole left after shock heating as a function of temperature. The
calculations are done at 25 K intervals.
the reactions that were taken from the NIST-Kinetics Data Base
are, in many cases, best fits to a large number of entries. The
thermodynamic properties of the species in the scheme were
Figure 9. Calculated (lines) and experimental yields of methane and
acetylene. The calculations are done at 25 K interval, marked on the
lines as crosses.
19-21
also taken from specific articles and various literature sources.
Some were estimated using NIST-Standard Reference Data
symbols in the figures are the experimental yields in mole
percent, and the lines are the best fits to the calculated points
using the scheme shown in Table 3. The calculations were done
at 25 K intervals in the temperature range 1050-1250 and are
shown as (+) on the lines. The agreement between the model
Base 2522 (Structure and Properties program (SP)).
Figure 8 shows the overall decomposition of 2,4-dimeth-
ylpyrrole and Figures 9-14 show experimental and calculated
yields of sixteen products found in the post-shock mixtures. The

Decomposition and Expansion in 2,4-Dimethylpyrrole
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4859
Figure 10. Calculated (lines) and experimental yields of ethane and
C H (allene and methylacetylene). The calculations are done at 25 K
3 4
interval, marked on the lines as crosses.
Figure 12. Calculated (lines) and experimental yields of pyridine,
hydrogen cyanide, acrilonitrile, and cyanoacetylene. The calculations
are done at 25 K interval, marked on the lines as crosses.
Figure 11. Calculated (lines) and experimental yields of 2-picoline,
Figure 13. Calculated (lines) and experimental yields of methylpyrrole
4
2
-picoline, acetonitrile, and propylnitrile. The calculations are done at
5 K interval, marked on the lines as crosses.
4 4
(two isomers), and C H . The calculations are done at 25 K interval,
marked on the lines as crosses.
prediction and the experimental results is satisfactory for most
of the products but not very good for C2H4, C4H4, C4H2, and
methylpyrrole. The model underestimates the yield of C4H2 and
overestimates somewhat the yield of C4H4. We could not suggest
a route where some of the C4H4 yield is transferred to C4H2
unless a major change in the available rate constant had to be
made.
Note the symbol (R) at the end of reaction 1 in the kinetic
scheme, which indicates that the reaction proceeds in the reverse
direction, whereas reaction 2 proceeds all the way in the forward
direction. Reversal in the direction of reaction 1 begins after
approximately 920 out of a reaction time of 2000 µs. The reason
for this behavior is the different production rates of the two
isomers of picoline via reactions 29 and 30 (Table 3) from the
unstable species that are formed in reactions 1 and 2. As can
be seen in Figure 11, the yields of the two picoline isomers are
high and the yield of 2-picoline is higher that that of 5-picoline.
We have assigned a higher rate constant to reaction 30 that
produces 2-picoline by lowering its activation energy by
approximately 3.5 kcal/mol. The high rate of reaction 30
produces high enough yield of hydrogen atoms to change the
direction of reaction 1 after some 900 µs.
b. SensitiVity Analysis. Table 4 shows the sensitivity of the
products to elimination of specific reactions from the kinetic
scheme, at 1100 and 1200 K, respectively. It gives the percent
change in the yield of a particular product as a result of
eliminating a given reaction from the scheme. The calculations
correspond to dwell times of 2 ms. Reactions that show an effect
of less than 10% both at 1100 and at 1200 K are not included
in the table. As can be seen in Table 3, there are several pairs
of reactions where two similar radical species produce the same
products. Elimination of only one reaction out of the two has
much weaker effect than the eliminating the pair of reactions.
We have run also sensitivity analysis with respect to variations
0
(or rather uncertainties) in the ∆fH of species whose thermo-
dynamic properties were estimated or are not known very
accurately. Incorrect values of the thermodynamic functions
result in erroneous values for the rate constants of the back
reactions for a given value of the forward rate constant. In
several sensitivity tests that were performed on uncertain values
of heat of formation of various species, we found that the results
of the computer simulations were only slightly sensitive to
0
variations of ∼3 kcal/mol in the values of the estimated ∆fH .

4860 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Lifshitz et al.
TABLE 4: Sensitivity Spectrum at 1100/1200 Ka
A. Products with Nitrogen
2-MPyrrole 4-MPyrrole 2-picoline 5-picoline pyridine
-12/- -12/- -/- -20/- -17/-
no.
reactions
HCN
-/-
CHtCCN CH
3
CN
2 3 2 5
C H CN C H CN
•
•
1
2
3
4
5
6
7
8
9
DMPyrrole f DMPyrrole(R1) + H
DMPyrrole f DMPyrrole(R2) + H
-28/-13
-36/-27
-/-
-/13
-48/-43 -11/-
-48/-43 -11/-
-18/-
-15/-
-/-
-/10
-57/-47
-57/-47
-25/-13 -25/-13 -32/-34 -/- -30/-18 -/-
•
•
DMPyrrole f MPyrrole(R1) + CH
3
-/19
-15/-42 -11/-
-11/-
-11/-
-30/-20 -48/-43 -18/-
-30/-20 -48/-43 -18/-
DMPyrrole f MPyrrole(R2) + CH
3
-15/-42 -/19
-11/-
-/-
-37/-45 -/-
-/-
•
DMPyrrole + H f DMPyrrole(R1) + H2
18/13
-/-
-/-
-20/-
18/13
-/-
-36/-36 21/12
-/-
-41/-20 -/-
-45/-21 -/- -15/-
-/-
-/-
-/-
-/-
- /-15
-/-
-35/-16
-30/-
37/12
37/12
-73/-70
-26/-21
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/11
-/11
-/-
-/-
-25/-28 -/-
DMPyrrole + H f DMPyrrole(R2) + H
2
-/-
-/-
•
DMPyrrole + CH
3
f DMPyrrole(R1) + CH
f DMPyrrole(R2) + CH
4
4
-/-
-30/-13 -/-
•
DMPyrrole + CH
3
-20/-
-12/-
-/-
-/-
-/-
-/-
-/-
•
•
DMPyrrole + H f 2-MPyrrole + CH
DMPyrrole + H f 4-MPyrrole + CH
3
-82/-43 33/11
-/-
-82/-43 -/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-23/-14 -/-
-23/-14 -/-
•
10
13
14
15
16
17
18
19
20
29
30
33
34
37
38
43
44
45
56
65
66
67
68
69
3
33/11
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
•
DMPyrrole(R1) f C
DMPyrrole(R2) f C
MPyrrole(R1) f HCN + C
MPyrrole(R2) f HCN + C
3
H
H
4
+ C
+ C
2
H
4
CN
CN
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-49/-46 -/-
-49/-46 -/-
-/-
-/-
-/-
-/-
-75/-65 -/-
-24/-19 -/-
•
3
4
2
H
5
4
•
4
H
H
-/-
-/-
-/-
-/-
-/-
•
4
5
-/-
•
MPyrrole(R1) f C
MPyrrole(R2) f C
3
H
H
4
+ CH
2
CN
CN
-/-
-49/-41
-49/-41
-/-
•
•
•
3
4
+ CH
2
-/-
MPyrrole(R1) f CH
MPyrrole(R2) f CH
3
CN + C
3
H
H
3
-/-
-49/-48 -/-
-49/-48 -/-
3
CN + C
3
3
-/-
-/-
•
•
DMPyrrole(R1) f 5-Picoline + H
-35/-27 -35/-27 -13/-
-50/-34 -50/-34 -99/-99 -15/-
-/-
-/-23
-/-
-99/-98 -32/-30 -/13
-21/47
-23/60
-/-30
-/-30
-/22
-/21
-/26
-/14
-/14
-/15
-/15
-/-
45/138
-/14
DMPyrrole(R2) f 2-Picoline + H
-50/-37 -/16
-45/-51 -/-
-44/-51 -/-
79/191
-/-
-/18
•
MPyrrole(R3) f Pyridine + H
-/-24
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
61/46
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
49/36
-/-
-/-
-/-
-/-
-/-
-/-
•
MPyrrole(R4) f Pyridine + H
-/-
-/-
•
MPyrrole(R1) + H f 4-MPyrrole
-/-45
-/-
-/-
-/-
-42/-42 -/-
-42/-42 -/-
-/-13
167/ 79 -/-25
-/-
-/-
-/-
-/-
-/-
-/18
-/18
-/-
-/17
•
MPyrrole(R2) + H f 2-MPyrrole
-/-45
13/72
-/-21
-/-16
13/-45
-/-
-/22
-/-
-/17
•
2-MPyrrole + CH
4-MPyrrole + CH
3
f MPyrrole(R3) + CH
f MPyrrole(R4) + CH
4
4
-/-21
13/72
-/-16
13/-45
-/-
-/-19
-/-19
-/-23
117/102
-/-
-/-
-99/-99
-/-
-/12
•
3
-/-
-/-
-/12
•
+ H•
C
CH
CH
4
H
3
5
•
f C
4
H
3
4
-/-
-/10
-/-31
-/-
-/-
-/-
-/-
-/-
•
+ CH
CN + CH
f C
2
H
6
48/38
-/-14
-/-14
85/-
•
•
2
3
f C
2
H
5
CN
-/-
-/-
-/-
-/-
-/-
-99/-99
-/18
•
•
C
2
C
2
C
2
H
5
H
4
H
3
CN + H f C
2
H
4
CN + H
2
-/-
-/-
•
CN f C
2
•
H
3
CN + H
-/-
-/-
-99/-99 -/-
•
CN + H f H
2
+ CHdCHCN
-/-
-/-
-100/-100 -/-
-100/-100 -/-
-/-
-/-
-/-
-/-
•
•
CHdCHCN f CHtCCN + H
-/-
-/-
B. Products without Nitrogen
no.
reactions
CH
4
C
H
2 4
C
H
2 6
C
H
2 2
C
H
3 4
C
H
4 6
C
4
H
2
4 4
C H
•
•
1
2
3
4
5
7
8
9
DMPyrrole f DMPyrrole(R1) + H
DMPyrrole f DMPyrrole(R2) + H
-/-
-/-
-/-
-/-
-25/-19 -49/-37 -41/-29 -38/-36 -20/-23 -52/-43
-15/-
-12/-
-/-
-/-
-21/-
-52/-42
-52/-42
16/10
-/-
-/-
-/18
-48/-43
-48/-43
-/-
-/-
-/-
-/-
-/-
-10/-13 -14/-
-27/-11 -10/-
-22/-12
•
•
DMPyrrole f MPyrrole(R1) + CH
3
DMPyrrole f MPyrrole(R2) + CH
3
-25/-19 -49/-37 -41/-29 -38/36
-/10
-20/-23 -52/-43
-20/-19 16/11
•
DMPyrrole + H f DMPyrrole(R1) + H2
-/-
-/-
-/-
•
DMPyrrole + CH
3
f DMPyrrole(R1) + CH
f DMPyrrole(R2) + CH
4
4
-35/-10 21/-
-39/-12 13/-
-10/-13 -13/-
-10/-13 -13/-
-/-
-/-
-/-
-/-
-/-
-/-
-12/-21 -18/-14 -12/-
-19/-27 -26/-17 -18/-
-/-12
-/-11
-/-
16/-
12/-
-13/-
-13/-
-/-
-/-
-/-
-/-
-/-
-18/-
-22/-
29/-
29/-
-29/-
-/-
-23/-
-/-
•
DMPyrrole + CH
3
-/-
-/-
-/
-18/-
27/-
-27/-
-18/-
•
•
DMPyrrole + H f 2-MPyrrole + CH
3
26/-
•
10
13
14
15
16
17
18
29
30
33
34
37
38
43
44
45
46
48
49
50
51
52
53
56
57
59
60
DMPyrrole + H f 4-MPyrrole + CH
3
26/-
•
DMPyrrole(R1) f C
DMPyrrole(R2) f C
MPyrrole(R1) f HCN + C
MPyrrole(R2) f HCN + C
3
H
H
4
+ C
+ C
2
H
4
CN
CN
-/-
-/-
-/-
-/-
-/-
-/-
-57/-29 -/-
-17/-
-/-
-/-
-/-
-/-
•
3
4
2
H
5
4
-/-
•
4
H
H
-25/-39 -/-
-25/-39 -/-
-50/-52
-50/-52
-50/-52
-50/-52
-/-
-49/-46
-49/-46
-/-
•
4
5
•
MPyrrole(R1) f C
MPyrrole(R2) f C
3
H
H
4
+ CH
+ CH
2
CN
CN
-/-
-/-
-12/-29 -/-
-12/-29 -/-
•
3
4
2
-/-
-/-
-/-
•
•
DMPyrrole(R1) f 5-Picoline + H
-31/-14 40/88
-37/-25
-50/-31
-/-21
-39/-23/
-50/-29
-/-26
-/-26
-/31
-/31
-/-14
-/-14
-/30
-/38
-/22
-/22
-/11
-/11
-/13
-/13
DMPyrrole(R2) f 2-Picoline + H
-39/-18 67/120
•
MPyrrole(R3) f Pyridine + H
-/-14
-/-14
-/14
-/14
-/11
-/11
-/-
-/-
-/-
-/-
-/-26
-/-
-/-
-/-61
120/-
-35/-
-97/-98 -/-
-/- -/-
-/-
-/-
-/-
-/-20
-/-20
-/30
-/11
-/11
-/-
•
MPyrrole(R4) f Pyridine + H
-/-21
•
MPyrrole(R1) + H f 4-MPyrrole
-/43
•
MPyrrole(R2) + H f 2-MPyrrole
-/-
-/-
-/30
-/-
-/43
•
2-MPyrrole + CH
3
f MPyrrole(R3) + CH
f MPyrrole(R4) + CH
4
4
-/-32
-/-32
-/-10
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/15
-/15
-/-
-/-
-/-
-/-15
-/-
-/-
-/-12
•
4-MPyrrole + CH
3
-/-15
-/-12
•
•
C
C
C
C
C
C
C
C
4
4
4
4
2
4
3
2
H
H
H
H
H
H
H
H
5
5
3
4
6
4
4
f C
4
•
H
4
+ H
-50/-78 -/-
214/1107
-100/-100 -/-
-/-
-/-
-/-
-100/-100 -100/-99
•
•
+ H f C
4
H
6
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/-
-/22
-/-
-/-
-/-37
-/-
•
f C
4
H
2
+ H
-99/-99
-99/-99
-/-
-/14
-/-
-/-
50/-
-/-
-/-
-/-
•
•
•
+ H f C
4
2
2
H
H
H
3
+ H
+ H
2
-/-
-/-
•
+ H f C
5
2
•
•
+ H f C
2
•
+ C
2
H
3
-/-
-/-
-/-
-48/-73 -/-
-/-
-/-
-/-
•
+ H f CH
3
+ C
5
2
H
2
-49/-24 -/24
•
•
6
•
+ CH
3
f C
2
H
+ CH
4
-/-
-/-
•
CH
CH
3
•
+ CH
3
f C
f C
2
H
H
6
145/123
-/-
-/-
-99/-99 79/12
37/-
44/10
-/-
-/-
•
•
+ H•
3
+ CH
3
2
5
-/-
-/-
-/-
-/-
•
•
•
C
C
2
H
H
5
f C
f C
2
2
H
H
4
+ H
-/-
-/-
-/-
•
2
3
2
+ H
-/-
-24/-41 -/-
-/-
a
Percent change in the yields for elimination of a reaction from the scheme. (A) Products with nitrogen and (B) products without nitrogen.
As expected, not all of the elementary steps affect the product
distribution in the sense that their elimination from the scheme
affects the yield of at least one of the products. The majority of
the steps that compose the scheme do not affect or have only a
small effect on the distribution of the reaction products. Removal
of elementary steps that are the sole producers of a given product

Decomposition and Expansion in 2,4-Dimethylpyrrole
J. Phys. Chem. A, Vol. 107, No. 24, 2003 4861
References and Notes
(
1) Lifshitz, A.; Tamburu, C.; Suslensky, A. J. Phys. Chem. 1989, 93,
802.
2) Mackie, J. C.; Colket, M. B., III.; Nelson, P. F; Esler, M. Int. J.
Chem. Kinet. 1991, 23, 733
3) Martoprawiro, M.; Backsay, G. B; Mackie, J. C. J. Phys. Chem. A
999, 103, 3934.
5
(
(
1
(
(
(
4) Dubnikova, F; Lifshitz, A. J. Phys. Chem. A 1998, 102, 10880.
5) Zhai, L.; Zhou, X and Liu, R. J. Phys. Chem. A 1999, 103, 3917.
6) Doughty, A.; Mackie, J. C; Backsay, G. B. Chem. Phys. Lett. 1994,
2
21, 267.
(
(
7) Dubnikova, F; Lifshitz, A. J. Phys. Chem. A 2000, 104, 530.
8) Lifshitz, A.; Shweky, I; Tamburu, C. J. Phys. Chem. 1993, 97,
4442.
(9) Laskin, A; Lifshitz, A. J. Phys. Chem. A 1997, 101, 7787.
(
(
(
10) Zhou, X; Liu, R. THEOCHEM 1999, 461, 567.
11) Smith, B. J; Liu, R. THEOCHEM 1999, 491, 211.
12) Dubnikova, F; Lifshitz, A. J. Phys. Chem. A 2001, 105, 3605-
3
614.
(
(
(
(
(
(
13) Tsang, W.; Lifshitz, A. Int. J. Chem. Kinet. 1998, 30, 621.
14) Lifshitz, A.; Bidani, M; Bidani, S. J. Phys. Chem. 1986, 90, 3422.
15) Lifshitz, A.; Bidani, M; Bidani, S. J. Phys. Chem. 1986, 90, 5373.
16) Lifshitz, A; Wohlfeiler, D. J. Phys. Chem. 1992, 96, 4505.
17) Lifshitz, A; Wohlfeiler, D. J. Phys. Chem. 1992, 96, 7367.
18) Westly, F.; Herron, J. T.; Cvetanovic, R. J.; Hampson, R. F.;
Mallard, W. G. NIST-Chemical Kinetics Standard Reference database 17,
Ver. 5.0; National Institute of Standards and Technology: Washington, DC.
(
19) Pedley, J. B.; Taylor, R. D; Kirby, S. P. Thermochemical Data of
Organic Compounds; Chapman and Hall: London, 1986.
20) Burcat, A.; McBride, B; Rabinowitz, M. Ideal Gas Thermodynamic
Data for Compounds Used in Combustion T. A. E. 657 report; Technion-
Israel Institute of Technology: Haifa, Israel, 2002.
Figure 14. Calculated (lines) and experimental yields of ethylene and
diacetylene. The calculations are done at 25 K interval, marked on the
lines as crosses. The agreement here is unsatisfactory.
(
(21) Warnatz, J. Rate coefficients in the C/H/O system. In Combustion
or are part of a consecutive chain that is the only route for a
product formation reduces the concentration of that product
almost to zero. It should be mentioned, however, that the
sensitivity analysis is done by removing a single reaction at a
time. When a group of reactions are removed from the scheme,
there can be a strong effect on particular products although the
elimination of one step alone, as is shown in Table 4, might
not have an affect at all. These reactions are left in the kinetic
scheme also for completeness and applicability beyond the
temperature range of the present investigation where they might
be more important.
Chemistry; Gardiner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984.
(
22) Stein, S. E.; Rukkers, J. M; Brown, R. L. NIST-Standard Reference
database 25; National Institute of Standards and Technology: Washington,
DC.
(23) Muller, C.; Warth, V.; Jacquemard, E.; Scacchi, G.; Come, G. M.
A FORTRAN computer code package for the eValuation of thermochemical
data of molecules, free radicals and reactions in the gas phase and A. S.
T. M. CHETAH safety criteria. ENSIC-INPL, University of Nancy I: Nancy,
France, 1996.
(24) Tsang, W.; Hampson, R. F. J. Phys. Chem. Ref. Data 1986, 15,
087.
1
(
(
25) Tsang, W. Combust. Flame 1989, 78, 71.
26) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just,
Th.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J.
Phys. Chem. Ref. Data 1992, 21, 411-429.
(
27) Lifshitz, A.; Tamburu, C.; Carroll, H. F. Int. J. Chem. Kinet. 1997,
Acknowledgment. This work was supported by a grant from
the BSF, US-Israel Binational Science Foundation under Grant
agreement 98-00076. We thank Prof. Hai Wang our American
co-investigator for many fruitful discussions.
2
9, 839-849.
(
28) Lifshitz, A.; Tamburu, C. Int. J. Chem. Kinet. 1998, 30, 341-347.
(29) Lifshitz, A.; Tamburu, C; Shashua, R. J. Phys. Chem. A 1997, 101,
1018.