Thermal decomposition of 4-methylpyrimidine. Experimental results and kinetic modeling

Article abstract of DOI:10.1021/jp0040392

The decomposition of 4-methylpyrimidine was studied behind reflected shock waves in a pressurized driver single-pulse shock tube at 1160-1330 K and overall densities of ～ 3 × 10^-5 mole/cc. A plethora of decomposition products, both with and without nitrogen, (HCN, CH₃CN, C₂H₃CN, CH₄, C₂H₆, C₂H₄, etc.) was found in the post-shock mixtures. The attack of methyl radicals on the methyl group in 4-methylpyrimidine produced CH₄, which was a major product among the species not containing nitrogen, leaving the radical 4-methylene pyrimidyl. The H atoms and methyl radicals initiated a chain mechanism by abstraction of an H atom from the methyl group and by dissociative attachment of an H atom and removal of a methyl group from the ring. The decomposition mechanism was discussed.

Full text of DOI:10.1021/jp0040392

3542
J. Phys. Chem. A 2001, 105, 3542-3554
Thermal Decomposition of 4-Methylpyrimidine. Experimental Results and Kinetic Modeling
Assa Lifshitz,* Aya Suslensky, and Carmen Tamburu
Department of Physical Chemistry, The Hebrew UniVersity, Jerusalem 91904, Israel
ReceiVed: October 31, 2000; In Final Form: January 26, 2001
The decomposition of 4-methylpyrimidine was studied behind reflected shock waves in a pressurized driver
single-pulse shock tube over the temperature range 1160-1330 K at overall densities of ∼3 × 10-5 mol/cm3.
A plethora of decomposition products, both with and without nitrogen was found in the post-shock mixtures.
They were HCN, CH3CN, C2H3CN, pyrimidine, C2H5CN, cis- and trans-CH3CHdCHCN, CH2dCHCH2CN,
CHtC-CN, and C2N2, among the nitrogen containing products, and CH4, C2H2, CH3CtCH, C2H6, CH2d
CdCH2, and C2H4 as products without nitrogen. It is suggested that the decomposition of 4-methylpyrimidine
has two major initiation steps. (1) An ejection of a hydrogen atom from the methyl group that is connected
•
to the ring, C4H3N2-CH3 f C4H3N2-CH2 + H•, and (2) split of the methyl group from the ring, C4H3N2-
•
•
CH3 f C4H3N2 + CH3 . The ejection of H atom from the ring is much slower and does not contribute much
to the total rate. The H atoms and methyl radicals initiate a chain mechanism by abstraction of an H atom
from the methyl group and by dissociative attachment of an H atom and removal of a methyl group from the
•
•
ring. The pyrimidyl (C4H3N2 ) and methylene pyrimidine (C4H3N2-CH2 ) radicals decompose by ring cleavage
followed by breakdown of the open-ring radical. The total decomposition of 4-methylpyrimidine in terms of
a first-order rate constant is given by ktotal ) 1015.37 exp(-83.5 × 103/RT) s-1. A reaction scheme containing
33 species and 91 elementary reactions was constructed to account for the observed product distribution. A
discussion on the decomposition mechanism is presented.
I. Introduction
120 °C with an accuracy of (1 °C. The driven section, was
4 m long and was divided in the middle by a 52 mm i.d. ball
The thermal reactions of pyrimidine and pyrazine have been
studied in the past using shock tubes as the heating device, with
a variety of diagnostic techniques.1-4 It has been suggested that
in both isomers the decomposition begins with an ejection of a
hydrogen atom from the ring followed by opening of the ring
in â positions to the radical site and fragmentation by successive
â-scissions with and without hydrogen atom shifts. Overall
reaction schemes for the decomposition have been composed
and computer modeling was run to support the suggested
mechanisms. The addition of a methyl group to pyrimidine, to
form methyl pyrimidine, introduces two additional bonds:
-CH2-H and C-CH3. These bonds are considerably weaker
than the C-H bonds in pyrimidine even though some of the
radicals that are formed are resonance stabilized by adjacent
nitrogen atoms. Owing to the formation of different radicals in
the process of initiation, the decomposition pathways are
different. Also being a larger molecule with a methyl group,
production of more products is expected.
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 45° angle near the diaphragm holder in
order to prevent reflection of transmitted shocks. The driven
section was separated from the driver by “Mylar” polyester film
of various thickness depending upon the desired shock strength.
After pumping down the tube to approximately 3 × 10-5
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 injected
from the downstream end of the driven section into a Hewlett-
Packard model 5890 gas chromatograph operating with 2
Porapak-N columns using flame ionization (FID) and nitrogen-
phosphor (NPD) detectors. All the transfer tubes and the
injection system were maintained at 120 °C.
Reflected shock temperatures were calculated from the extent
of decomposition of 1,1,1-trifluoroethane which was added to
the reaction mixture and served as an internal standard. Its
decomposition to CH2dCF2 + HF is a first-order unimolecular
reaction with a rate constant5: kfirst ) 1014.85exp(-74.05 × 103/
In the present investigation the thermal reactions of 4-meth-
ylpyrimidine are studied and a detailed mechanism for the
production of the many products that are formed is suggested.
A kinetic scheme is composed and computer modeling is run
to support the suggested mechanism.
RT) s-1
.
Reflected shock temperatures were calculated from the
relation
II. Experimental Section
1. Apparatus. The decomposition of 4-methylpyrimidine was
studied behind reflected shocks in a pressurized driver, heated,
52 mm i.d.. single pulse shock tube. The tube, made of electro-
polished stainless steel tubing, was heated and maintained at
1
At
T ) -(E/R) ln - ln(1 - ø)
(I)
[
{
}]
/
where E is the activation energy of the HF elimination, A is its
10.1021/jp0040392 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/21/2001

Thermal Decomposition of 4-Methylpyrimidine
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3543
their GC peak areas from the following relations:
C5(pr)i ) A(pri)t/S(pri) {C5(4-mpyrim.)0/A(4-mpyrim.) (III)
C5(4-mpyrim.)0 ) {p1× %(4-mpyrim.)F5/F1}/100RT1) (IV)
A(4-mpyrim.) ) A(4-mpyrim.) +1/ N(pr ) ×
∑
5
0
t
i
A(pri)t/S(pri) (V)
In these relations, C5(4-mpyrim.)0 is the concentration of
4-methylpyrimidine behind the reflected shock wave prior to
decomposition and A(4-mpyrim.)0 is the calculated GC peak
area of 4-methylpyrimidine prior to decomposition (eq V) where
A(pri)t is the peak area of a product i in the shocked sample,
S(pri) is its sensitivity relative to 4-methylpyrimidine, 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,
120 °C in this study.
The identification of the reaction products was based on their
GC retention times and was assisted also by a Hewlett-Packard
model 5970 mass selective detector. The sensitivities of the
various products to the FID and NPD were determined relative
to 4-methylpyrimidine 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.
Figure 1. Gas chromatograms of a post-shock mixture of 0.25%
4-methylpyrimidine in argon heated to 1296 K, taken on NPD (top)
and FID (bottom). The numbers on top of the peaks indicate
multiplication factors. The products are (NPD) 1. C2N2 2. HCN 3. CHt
C-CN 4. CH3CN 5. C2H3CN 6. C2H5CN 7. cis-CH3CHdCHCN 8.
CH2dCHCH2CN 9. trans-CH3CHdCHCN 10. pyrimidine 11. pyrazine
12. 4-methylpyrimidine. (FID) 1. CH4. 2. C2H4. 3. C2H6 4. C2H2. 5.
CH2dCdCH2. 6. CH3CtCH. 7. C2N2. 8. HCN. 9. CHtC-CN. 10.
CH3CN. 11. CH2dCHCN. 12. cis-CH3CHdCHCN. 13. CH2dCHCH2-
CN. 14. trans-CH3CHdCHCN. 15. Pyrimidine. 16. Pyrazine. 17.
4-Methylpyrimidine (a) CH2dCF2, (b) CH3CF3.
III. Results
To determine the distribution of reaction products, some 30
tests were run with mixtures containing 0.25% 4-methylpyri-
midine in argon, covering the temperature range 1160-1330
K. Extents of pyrolysis starting from less than one hundredth
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. 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.
preexponential factor, t is the reaction dwell time, and ø is the
extent of decomposition defined as
ø ) [CH2dCF2]t/([CH2dCF2]t+[CH3CF3]t)
(II)
The balance of nitrogen vs carbon in the decomposition
products is shown in Figure 2. One-half the sum of the
concentrations of all the nitrogen containing products each
multiplied by the number of its nitrogen atoms, are plotted
against one-fifth the sum of the concentrations of all the products
each multiplied by the number of its carbon atoms. The diagonal
in the figure represents a perfect mass balance. As can be seen
mass balance is maintained.
The additional reflected shock parameters were calculated from
the measured incident shock velocities using the three conserva-
tion reactions and the ideal gas equation of state. Dwell times
of approximately 2 ms were measured with an accuracy of ∼5%.
Cooling rates were approximately 5 × 10-5 K/s.
2. Materials and Analysis. Reaction mixtures containing
0.25% 4-methylpyrimidine and 0.25% 1,1,1-trifluoroethane
diluted in argon were prepared manometrically and stored in
12 L glass bulbs at 700 Torr. Both the bulbs and the line were
pumped down to ∼10-5 Torr before the preparation of the
mixtures. 4-methylpyrimidine was obtained from Aldrich Chemi-
cal Co. and was listed as 97% pure. However, chromatograms
of unshocked samples did not show any of the 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%.
Figure 3 shows the rate constant for the overall decomposi-
tions of 4-methylpyrimidine, calculated as a first-order rate
constant from the relation:
ktotal ) -ln{[4-methylpyrimidine]t/
[4-methylpyrimidine]0}/t (VI)
The value obtained is ktotal ) 1015.37exp(-83.5 × 103/RT)
s-1, where R is expressed in units of cal/(K mol). Figure 4
shows, as an example, Arrhenius plots of the first-order
production rate of HCN, CH4 and C2N2, calculated from the
relation:
The gas chromatographic analyses of the post-shock mixtures
were performed on two 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 which lasted approximately 1 h. Typical chromatograms
of 0.25% 4-methylpyrimidine in argon of a shock heated mixture
to 1296 K are shown in Figure 1.
[product]i
kproduct
)
ktota (VII)
[4-methypyrim]0 - [4-methypyrim]t
3. Determination of Product Concentration. The concen-
trations of the reaction products C5(pr)i were calculated from
Note that the values calculated from Eq. VII correspond to
the production rates and not to the depletion rate of the reactant

3544 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Lifshitz et al.
propyne
TABLE 1: Product Distribution in Percent
(a) Hydrocarbons
no.
T5 K
C5 × 105 mol/cm3
t (ms)
CH4
C2H4
C2H6
C2H2
allene
1
1160
1167
1172
1173
1174
1197
1200
1202
1203
1205
1207
1213
1238
1247
1249
1252
1252
1260
1266
1266
1278
1281
1284
1296
1301
1304
1321
1337
1341
2.70
2.78
2.78
2.97
2.86
2.77
2.76
2.76
2.83
2.54
2.76
2.74
2.56
2.47
2.57
2.59
2.58
2.27
2.42
2.44
2.31
2.59
2.36
2.36
2.30
2.32
2.19
2.22
2.22
2
0.037
0.033
-
0.022
0.015
0.023
0.022
0.014
0.064
0.067
0.052
0.058
0.062
0.076
0.091
0.126
0.154
0.198
0.204
0.352
0.226
0.378
0.232
0.446
0.505
0.692
0.668
1.04
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
2.11
1.90
1.95
1.90
1.91
2.08
2.08
1.93
1.85
1.94
1.98
2.04
2.10
2
0.0076
-
0.0030
0.0080
0.038
0.020
0.169
0.116
0.103
0.061
0.151
0.0076
0.043
0.025
0.020
0.0067
0.017
0.0033
0.012
0.0029
0.00090
0.0079
0.0074
0.016
0.021
0.019
0.019
0.011
0.037
0.053
0.026
0.070
0.032
0.052
0.077
0.119
0.078
0.121
0.094
0.245
0.462
0.465
0.0084
0.016
0.015
0.012
0.020
0.075
0.024
0.0083
0.024
0.078
0.0043
0.015
0.011
0.015
0.017
0.030
0.018
0.025
0.018
0.042
0.050
0.064
0.063
0.099
0.098
0.237
0.413
0.375
0.013
0.027
0.029
0.034
0.039
0.064
0.048
0.084
0.044
0.091
0.112
0.146
0.148
0.221
0.220
0.545
0.888
0.797
0.135
0.174
0.233
0.135
0.296
0.245
0.474
0.176
0.429
0.495
0.536
0.554
0.908
0.824
1.88
2
2.08
2.09
2.02
2.11
2.04
2.12
2
1.90
2.02
2.07
2.11
1.98
2.03
0.081
0.076
0.104
0.108
0.236
0.671
0.471
1.06
2.56
5.05
4.19
3.02
2.24
(b) Nitrogen-Containing Species
C5 × 105
methyl-
pyrimi-
dine
no.
T5
mol/cm3 t (ms) pyrimidine
C2N2
HCN
CH/CCN CH3CN C2H3CN C2H5CN CH3-CHdCH-CN
1
2
3
4
5
6
7
8
9
1160
1167
1172
1173
1174
1197
1200
1202
1203
2.70
2.78
2.78
2.97
2.86
2.77
2.76
2.76
2.83
2.54
2.76
2.74
2.56
2.47
2.57
2.59
2.58
2.27
2.42
2.44
2.31
2.59
2.36
2.36
2.30
2.32
2.19
2.22
2.22
2
99.82
99.53
99.69
99.75
99.79
99.33
98.83
99.28
98.61
99.33
98.93
99.06
98.16
97.86
97.65
97.59
95.65
96.84
94.96
96.19
93.51
94.40
91.75
90.76
87.75
89.15
73.45
54.53
57.73
0.00091
0.0012
0.0017
0.0011
0.00066
0.0026
0.0033
0.0036
0.0030
0.0026
0.0049
0.0041
0.0067
0.0063
0.0076
0.0091
0.018
0.0083
0.019
0.013
0.024
0.025
0.026
0.035
0.034
0.040
0.059
0.091
0.174
0.112
0.073
0.288
0.381
0.197
0.443
0.369
0.442
0.50
0.872
1.11
1.20
1.36
2.08
1.61
2.23
1.70
3.26
2.86
4.59
4.76
6.75
0.011
0.0067
0.015
0.010
0.0070
0.025
0.026
0.031
0.023
0.024
0.036
0.032
0.046
0.054
0.054
0.065
0.109
0.077
0.102
0.086
0.158
0.169
0.201
0.241
0.285
0.269
0.722
1.27
0.024
0.043
0.038
0.051
0.041
0.075
0.095
0.047
0.182
0.074
0.102
0.138
0.194
0.247
0.197
0.158
0.408
0.364
0.516
0.397
0.636
0.376
0.555
0.831
0.862
0.883
1.86
0.0075
0.014
0.023
0.013
0.014
0.033
0.048
0.044
0.063
0.038
0.067
0.056
0.10
0.136
0.101
0.109
0.214
0.205
0.298
0.220
0.357
0.293
0.369
0.530
0.567
0.580
1.38
0.0052
0.022
0.033
0.028
0.018
0.036
0.027
0.032
0.093
0.034
0.064
0.073
0.090
0.113
0.082
0.060
0.199
0.163
0.301
0.198
0.256
0.220
0.237
0.342
0.323
0.344
0.639
0.996
1.04
0.012
0.092
0.0065
0.141
2.12
1.90
1.95
1.90
1.91
2.08
2.08
1.93
1.85
1.94
1.98
2.04
2.10
2
0.0069
0.038
0.093
0.047
0.142
0.047
0.198
0.143
0.236
10 1205
11 1207
12 1213
13 1238
14 1247
15 1249
16 1252
17 1252
18 1260
19 1266
20 1266
21 1278
22 1281
23 1284
24 1296
25 1301
26 1304
27 1321
28 1337
29 1341
0.055
0.058
0.090
0.108
0.061
0.129
0.129
0.204
0.112
0.350
0.328
0.420
0.358
0.465
0.676
0.729
0.682
1.79
0.071
0.064
0.241
2
2.08
2.09
2.02
2.11
2.04
2.12
2
1.90
2.02
2.07
2.11
1.98
2.03
0.165
0.349
0.251
0.054
0.166
0.242
0.214
0.259
0.439
0.533
0.567
5.39
0.062
0.101
0.108
13.97
23.64
22.50
3.18
3.16
2.47
2.36
2.78
2.87
1.13
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. Since the production
of all the stable products is associated 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
1. Reaction Mechanism. A. Initiation. There are in principle
five initiation steps in the decomposition of 4-methylpyrimidine
that can produce active free radicals and unstable intermediates
whose decomposition can lead to a free radical mechanism.
Three steps involve ejection of hydrogen atoms from three

Thermal Decomposition of 4-Methylpyrimidine
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3545
Figure 2. Nitrogen-Carbon mass balance among the decomposition
products. The 45° line indicates a perfect mass balance. Nitrogen-
carbon mass balance is maintained.
Figure 4. Arrhenius plots for the formation of HCN, CH3CN and C2H4
calculated from the relation kproduct ) ([product]t/[reactant]0 - [reac-
tant]t)ktotal
.
Figure 3. Arrhenius plot of a first-order rate constant for the overall
decomposition of 4-methylpyrimidine. The rate constant is calculated
from the relation: ktotal ) -ln{[4-methylpyrim.]t/[4-methylpyrim.]0}/t.
Figure 5. Profiles of reactive free radicals calculated for 1250 K. Six
free radicals of the highest concentrations were introduced into the
kinetic scheme.
The value obtained is ktotal ) 1015.37exp(-83.5 × 103/RT) s-1
.
TABLE 2: First-Order Arrhenius Parameters for
Formation Rate
locations in the ring:
product
log(A, s-1
)
E (kcal/mol)
methylpyrimidine (total dec.)
15.37
83.5
pyrimidine
HCN
CH3CN
C2H3CN
C2H5CN
CHtCCN
C2N2
17.42
13.05
14.23
11.33
13.43
12.95
16.83
11.97
95.0
73.7
82.0
65.8
79.2
81.7
96.2
67.8
CH3-CHdCHCN
CH4
C2H4
C2H6
C2H2
a-C3H4
p-C3H4
10.40
15.20
17.18
19.24
63.7
86.6
104.4
113.9
Owing to the resonance stabilization by adjacent nitrogen atoms,
out of the three possible H atom ejections from the ring the
4-methyl-2-pyrimidyl radical is considerable more stable than
both the 5- and 6- isomers. Its production is associated with a

3546 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Lifshitz et al.
lower activation energy and is thus much faster. Out of the three
H atom ejections from the ring we considered only the
production and decomposition of 4-methyl-2-pyrimidyl.
The other two steps are ejection of a hydrogen atom from
the methyl group, and removal of the methyl group from the
ring. The activation energy for the H atom ejection from the
Figure 6. Calculated and experimental yields of HCN, CH3CN and
C2H4 plotted against the temperature. The lines are the best fit through
the calculated points obtained at 25 K intervals, shown here as (+).
methyl group is considerably lower than the activation energy
for an ejection of a sp2 hydrogen from the pyrimidine ring, even
in comparison to the production of the 2-pyrimidyl radical. The
removing of a methyl group has also a lower barrier than
removing a hydrogen atom from the ring. We therefore
considered three initiation steps in establishing the free radical
mechanism: formation of 4-methylene pyrimidine, 4-methyl-
2-pyrimidyl, and 4-pyrimidyl.
Following these initiation steps, hydrogen atom and methyl
group attack on the reactant and eventually on the products,
and unimolecular decompositions of the above-mentioned
unstable intermediates are the significant steps in the mechanism
of the decomposition.
B. Decomposition of Unstable Intermediates. a. 4-Methylene
Pyrimidine. The attack of methyl radicals on the methyl group
in 4-methylpyrimidine produces methane which is a major
product among the species not containing nitrogen, leaving the
radical 4-methylene pyrimidyl: The result of H atom attack on
Figure 7. Calculated and experimental yields of C2H3CN and CH3Ct
CH plotted against the temperature. The lines are the best fit through
the calculated points obtained at 25 K intervals, shown here as (+).
4-methylpyrimidine is either an abstraction to produce mainly
C4H3N2-CH2• and molecular hydrogen, or dissociative recom-
bination to yield pyrimidine and a methyl radical.
Figure 8. Calculated and experimental yields of C2H2 and CH2dCd
CH2 plotted against the temperature. The lines are the best fit through
the calculated points obtained at 25 K intervals, shown here as (+).
In comparison to the unstable radical intermediates, pyrimi-
dine is a stable moleculesits only decomposition is ejection of
a hydrogen atom, mainly from the position between the two
nitrogen atoms in the ring. We have included in the present
scheme a subscheme describing the decomposition of pyridine
taken from the work of Kiefer et al. with some modifications
which are relevant to the present investigation.
Most of the nitrogen containing species are obtained by the
•
decomposition of 4-methylene pyrimidine (C4H3N2-CH2 ). The
latter that is iso-electronic to benzyl radical exists in three
resonance structures. It opens to a noncyclic compound,

Thermal Decomposition of 4-Methylpyrimidine
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3547
Figure 11. Calculated and experimental yields of C2N2 and CHtC-
CN plotted against the temperature. The lines are the best fit through
the calculated points obtained at 25 K intervals, shown here as (+).
Figure 9. Calculated and experimental yields of the sum of CH3CHd
CHCN and CH2dCHCH2CN, and pyrimidine plotted against the
temperature. The lines are the best fit through the calculated points
obtained at 25 K intervals, shown here as (+).
Figure 12. Calculated and experimental yields of C2H5CN and the
total decomposition of 4-methylpyrimidine plotted against the temper-
ature. The lines are the best fit through the calculated points obtained
at 25 K intervals, shown here as (+).
Figure 10. Calculated and experimental yields of C2H6 and CH4 plotted
against the temperature. The lines are the best fit through the calculated
points obtained at 25 K intervals, shown here as (+).
•
l-C4H3N2-CH2 , that will decompose to smaller segments by
pathways, the final products are
•
•
l-C4H3N2-CH2 f C2H3 + 2HCN
(12)
(13)
l-C4H3N2-CH2 f CH2CN• + C2H3CN
•
l-C4H3N2-CH2 f CH2CN• + HCN + C2H2 (14)
•
successive â-scissions. From all the possible decomposition
Because information is not available on the precise pathways

3548 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Lifshitz et al.
in the decomposition of this species, we have treated each
pathway as one single global process and inserted these three
reactions to the reaction scheme as single step reactions.
b. 4-Methyl-2-pyrimidyl. This species is obtained by ejection
or abstraction of a H atom from position 2 in the ring, namely,
from the position between the two nitrogen atoms: Similar to
mol-1. 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 issue,
and databases, mainly the NIST-Kinetic Standard Reference
Data Base 17.6 The parameters for 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 involved were also taken from specific articles and
various literature sources.7-9 Some were estimated using NISTs
Standard Reference Data Base 2510 (Structure and Properties
program (SP)).
Figures 6-12 show experimental and calculated yields of
fourteen products found in the post-shock mixtures. The symbols
in the figures are the experimental yields 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 and are
shown as (+) on the lines. Cis and trans crotonitrile and
vinylacetonitrile were treated as one species as they interi-
somerize quite rapidly. Also, pyrazine which was found in very
small quantities was not taken into consideration in the kinetic
scheme. The agreement between the calculations and the
experimental results are generally good. For several products
the agreement at the low-temperature end of the range, where
the yields of the individual products do not exceed a few
hundredths of one percent is not very good. We suspect that
this is caused by residual minute background in the G. C. or in
the transfer tubes that could not be accounted for properly. When
the yields are less than one hundredth of one percent, even a
small background can cause a serious disagreement. Also, the
model calculation overestimates the yield of CHtC-CN. The
reaction to which the yield of CHtC-CN is most sensitive is
the reaction: CHdCHCN• f H• + CHtC-CN. We have used
for the back reaction of this step the rate constant of the reaction
H• + CHtCH f CH2dCH• (slightly reduced). A decrease in
the rate constant of this reaction could lead to a good agreement
between the experimental and the calculated yield of CHtC-
CN.
4-methylene pyrimidine it will open by breaking two C-N
•
bonds in the â-position on the ring to form l- C4H2N2 -CH3
and will further fragmentize to smaller species by successive
â-scissions. From all the possible decomposition pathways the
final products are
l-C4H2N2 -CH3 f CH3-CtCH + HCN + CN• (16)
•
l-C4H2N2 -CH3 f CH3-CtCH + CHNCN• (17)
•
•
l-C4H2N2 -CH3 f C2H2 + CH3CN + CN•
(18)
•
As has been done previously with l-C4H3N2-CH2 , we treated
these three global reactions as a single step reactions.
c. 2- and 4-Pyrimidyl. 2-Pyrimidyl is obtained by H atom
ejection from pyrimidine. The latter is obtained by dissociative
recombination of H atoms with 4-methylpyrimidine (reaction
19) to yield pyrimidine and a methyl radical. It decomposes by
a series of â-scissions. We have adopted the mechanism
suggested by Mackie et al.2 and Kiefer et al.1-3 in their study
on the decomposition of diazines. The decomposition yields
mainly C2H2 and CHNCH•.
4-Pyrimidyl is obtained by rupture of the C-CH3 bond in
4-methylpyrimidine. Its production from the dissociation of
pyrimidine is practically negligible. It has been suggested that
B. SensitiWity Analysis. Table 4 shows the sensitivity of the
products to elimination of specific reactions from the kinetic
scheme at 1150 and 1350 K. 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 1150 K and at 1350 K are not included in the table.
•
this species will decompose directly to HCN + CHdCHCN
and/or will open to •CHdN-CHdCH-CN and then decompose
to HCN + •CHdCHCN1. These two channels were introduced
to the reaction scheme.
C. ConsecutiWe Free Radical Reactions. Except for the three
initiation steps (reactions 1-3 in the kinetic scheme) that are
unimolecular processes, all the reactions that compose the
kinetics scheme involve free radicals. Figure 5 shows profiles
of the free radicals in the system. The profiles were calculated
for 1250 K, during a reaction time of 2 ms. As can be seen,
As expected, not all 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 elementary reactions that compose the scheme do not affect
or have only a small effect on the distribution. It should be
mentioned, however, that the sensitivity analysis is done by
removing a single reaction at a time. When a group of reactions
is eliminated 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.
CH3 , CH2CN• C3H3 , CHdCHCN•, H•, and CN• have the
•
•
highest concentrations. They were introduced into the reaction
•
scheme in abstraction and other reactions. CH3 is involved in
abstractions and recombinations and is responsible for the
production of species such as CH4, C2H4, C2H6, CH3CN, and
others. H atoms play a very important role in the production of
many of the species. C3H3• is responsible for the production of
allene and propyne. CN• contributes, to some extent, to the
formation of cyanogen.
2. 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 33 species
and 91 elementary reactions. The symbol (R) at the end of a
reaction in the scheme indicates that the reverse reaction takes
place. The rate constants listed in the table are given as k ) A
exp(-E/RT), or k ) A′Tn exp(-E/RT) when the rate constant
fits a wide temperature range. Units are cm3, s-1, kcal, and
Removal of elementary steps that are the sole producers of a
given product 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. As has been mentioned before the
formation of all the products are associated with free radical
reactions. The most important initiation reaction is reaction 2
(Table 3) which produces methyl radicals. These radicals

Thermal Decomposition of 4-Methylpyrimidine
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3549
TABLE 3: Reaction Scheme for the Decomposition of 4-Methylpyrimidine

3550 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Lifshitz et al.
TABLE 3: (Continued)

Thermal Decomposition of 4-Methylpyrimidine
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3551
TABLE 3: (Continued)

3552 J. Phys. Chem. A, Vol. 105, No. 14, 2001
Lifshitz et al.
TABLE 4: Sensitivity Spectrum: Percent Change in the Yield for Elimination a Step from the Reaction Scheme at 1170/1320 K
(a) Hydrocarbons
(b) Nitrogen-Containing Species

Thermal Decomposition of 4-Methylpyrimidine
J. Phys. Chem. A, Vol. 105, No. 14, 2001 3553
TABLE 4: (Continued)
(b) Nitrogen-Containing Species (Continued)
participate in the production of almost all the products, with
and without nitrogen. Elimination of reaction 2 from the scheme
reduces the yield of almost all the products almost to zero. This
behavior is more emphasized at low temperatures. At the higher
temperature end the effect slightly decreases owing to other
channels that begin to contribute. Another interesting point is
the effect of eliminating reaction 9 on the yield of cyanogen.
Reaction 9 is the main consumption reaction of cyanogen
radicals. When this reaction is eliminated from the scheme the
concentration of cyanogen increases by orders of magnitude as
recombination of the latter produces cyanogen.
We have also run sensitivity analysis with respect to variations
(or rather uncertainties) in the ∆fH0 of species whose thermo-
dynamic properties were estimated or are not known accurately
enough. An 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
variations of ∼3 kcal/mol in the values of the estimated ∆fH0.
Acknowledgment. This work was supported by a grant from
GIF (German Israel Foundation) for Scientific Research and
Development.
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