A Computational Study on the Kinetics and Mechanism for the Unimolecular
Decomposition of o-Nitrotoluene
S. C. Chen,†S. C. Xu,‡E. Diau,†and M. C. Lin*,‡
Department of Applied Chemistry, Institute of Molecular Science, National Chiao Tung UniVersity, Hsichu, Taiwan 300, and Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322
ReceiVed: April 17, 2006; In Final Form: June 23, 2006
The kinetics and mechanism for the unimolecular decomposition of o-nitrotoluene (o-CH3C6H4NO2) have
been studied computationally at the G2M(RCC, MP2)//B3LYP/6-311G(d, p) level of theory in conjunction with rate constant predictions with RRKM and TST calculations. The results of the calculations reveal 10 decomposition channels for o-nitrotoluene and its six isomeric intermediates, among them four channels give major products: CH3C6H4+ NO2, C6H4C(H)ON (anthranil) + H2O, CH3C6H4O (o-methyl phenoxy) + NO,
and C6H4C(H2)NO + OH. The predicted rate constants in the 500-2000 K temperature range indicate that
anthranil production, taking place initially by intramolecular H-abstraction from the CH3group by NO2followed
by five-membered ring formation and dehydration, dominates at temperatures below 1000 K, whereas NO2
elimination becomes predominant above 1100 K and CH3C6H4O formation by the nitro-nitrite isomerization/
decomposition process accounts for only 5-11% of the total product yield in the middle temperature range 800-1300 K. The branching ratio for CH2C6H4NO formation by the decomposition process of CH2C6H4
N-(O)OH is negligible. The predicted high-pressure-limit rate constants with the rate expression of 4.10× 1017
exp[-37000/T] s-1for the NO2elimination channel and 9.09× 1012exp[-25800/T] s-1for the H2O elimination
channel generally agree reasonably with available experimental data. The predicted high-pressure-limit rate constants for the NO and OH elimination channels are represented as 1.49× 1014exp[-30000/T] and 1.31
× 1015exp[-38000/T] s-1, respectively. Introduction
The thermal decomposition of o-nitrotoluene (1-nitro, 2-methyl benzene) has received much attention thanks to its relevance to the trinitrotoluene decomposition chemistry. Gonzalez and co-workers1studied the kinetics of the decomposition reaction in
an experiment by high-power infrared laser heating (laser-enhanced homogeneous pyrolysis). In the experiment, they used a Lumonics K103 CO2laser (wavelength ) 10.6µm, duration
) 1 µs, fluence ) 1 J/cm2) at a constant repetition rate (0.2 Hz)
for the irradiation of a small portion of the reaction cell (typically 24%). They reported that the primary decomposition process occurred by the breaking of the C-NO2bond with 70.2 ( 2.5
kcal/mol activation energy at 1100 K. The rate expression for the dissociation process o-CH3C6H4NO2f CH3C6H4+ NO2
at 110 Torr SF6 pressure over the temperature range
1110-1250 K was given by k1) 1015.9(0.5 exp[-(33000 ( 1000)/
T] s-1. Tsang and co-workers2,3also studied the decomposition
reaction in a single-pulse shock tube at 2700-3400 Torr Ar pressure over the temperature range 1000-1180 K. They obtained the rate expressions of k1) 6.4 × 1014exp[-30900/
T] s-1for NO2production and k2) 1.2 × 1013exp[-26000/T]
s-1 for o-CH3C6H4NO2f C6H4C(H)ON (anthranil) + H2O,
respectively.
The photodissociation of nitrotoluene has been studied using a femtosecond laser photolysis/mass spectroscopic technique at 375 nm.4Both NO
2and NO loss channels have been observed
for all three nitrotoluene isomers (o-, m- and p-nitrotoluenes). In the dissociation of o-nitrotoluene, OH is a significant product that can be attributed to the so-called “ortho effect” (i.e., H-transfer from the CH3- to the NO2-group).4-6The NO
frag-ment from the dissociation of o-nitrotoluene between 224 and 238 nm7and 220 and 250 nm8has also been observed.
Up to now, there has been no theoretical study on the decom-position of o-nitrotoluene, to our knowledge. In the present work, the kinetics and mechanism for the isomerization and decomposition of o-nitrotoluene have been computationally studied at the G2M(RCC, MP2) level of theory.9The potential
energy surface (PES) and the rate constants for o-CH3C6H4
-NO2f CH3C6H4 + NO2, C6H4C(H)ON (anthranil) + H2O,
CH3C6H4O + NO, and CH2C6H4NO + OH production channels
have been predicted, and the results are reported herein for high-temperature combustion modeling applications.
Computational Method
The optimized geometries of o-nitrobenzene, its 8 stable iso-mers, 11 transition states, and products for the four dissociation channels have been calculated at the B3LYP/6-311G(d, p) level. To obtain more reliable values of energies for PES and rate constant predictions, we performed a series of single-point energy calculations for each molecule and transition state with the G2M(RCC, MP2) scheme9based on the optimized
geom-etries at the B3LYP/6-311G(d, p) level. The G2M(RCC, MP2)// B3LYP method with larger basis sets is a reliable computational method for decomposition of phenyl compounds such as nitrobenzene10 and nitrosobenzene.11 The G2M(RCC, MP2)
composite scheme is given as follows: * Corresponding author. E-mail: chemmcl@emory.edu.
†National Chiao Tung University. ‡Emory University.
10.1021/jp0623591 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/02/2006
where the higher level corrections (∆HLC) is given by -5.3nβ - 0.19nRin millihartree, where nRand nβare the numbers of R andβ valence electrons, respectively.
Results and Discussion
1. Isomerization and Dissociation of o-Nitrotoluene. The optimized structures of species 1-32 for o-nitrotoluene, its isomeric intermediates, transition states for the isomerization and dissociation reactions, and dissociated products calculated at the B3LYP/6-311G(d, p) level are presented in Figure 1. The energy diagram of the system, including various isomerization and decomposition reactions, computed with the G2M(RCC, MP2) method discussed above is shown in Figure 2. As shown in Figure 1, o-nitrotoluene has six different isomers: CH2C6H4
-t-N(OH)O (6), CH2C6H4-c-N(OH)O (8), CH2C6H4N(O)OH (10),
C6H4C(H2)ONOH (13), CH3C6H4ONO (24), and CH2(OH)C6H4
-NO (27); among them 27 is most stable, lying below the o-nitrotoluene at 4.1 kcal/mol. The geometries of o-nitrotoluene (1), 6, 8, and 10 have near Cssymmetry. Due to the ortho effect, the H atom of the -CH3group of o-nitrotoluene is transferred
to the NO2group of the 6 by an isomerization reaction. The 6
is the conformer of the 8 due to the internal rotation of OH in the HONO group. The 8 isomerizes to the 10 by the H atom transfer (HONO S ONOH). The 8 isomerizes to the 27 by the OH group transfer from HONO to the CH2group. Isomerization
reactions can occur among the isomers by their corresponding transition states. Similar to the nitro isomerization reaction of nitrobenzene,10o-nitrotoluene isomerizes to 24 by the
nitro-nitrite isomerization reaction.
The o-nitrotoluene molecule can dissociate directly to the products CH3C6H4(19) + NO2with the dissociation energy at
0 K of 76.3 kcal/mol and to C6H4NO2 (18) + CH3 with the
high dissociation energy at 0 K of 103.0 kcal/mol. The predicted C-NO2dissociation energy at 1100 K of 75.7 kcal/mol may
be compared with the experimental activation energy measured at 1100 K and 110 Torr SF6 pressure, 70.2 ( 2.5 kcal/mol.1
CH3C6H4NO2can also dissociate directly to give the products
C6H4CH2(4) + HONO with a 63.6 kcal/mol enthalpy change
after overcoming the 104.3 kcal/mol barrier at TS1, and the
products C6H3CH3(22) + HONO with the predicted enthalpy
of reaction, 72.9 kcal/mol, after overcoming the 74.3 kcal/mol barrier at TS8. In these reactions, species ONOH‚‚‚C6H4CH2
(3) and ONOH‚‚‚C6H3CH3 (21) appearing in the PES are
product complexes. Isomer 13, which can be formed by ring-closure from isomers CH2C6H4-c-N(OH)O (8) and CH2C6H4
N-(O)OH (10) via TS6and TS5, respectively, can eliminate H2O
to give the double-ring product C6H4C(H)ON (16, anthranil)
by overcoming the 37.7 kcal/mol barrier at TS7with -6.0 kcal/
mol overall energy change from the reactant. Isomer CH3C6H4
-ONO (24), formed by the nitro-nitrite isomerization reaction from the reactant via TS9(57.9 kcal/mol), can dissociate readily
to the products CH3C6H4O (25) + NO with an endothermicity
of 14.3 kcal/mol or to the less favored products CH3C6H4(19)
+ NO2with an endothermicity of 76.3 kcal/mol as cited above
for the direct dissociation process. Both dissociation reactions occur without intrinsic barriers. Isomer 8 and its isomer through HONO internal rotation, CH2C6H4-t-N(OH)O (6)sthe initial
H-abstraction product from CH3by the nitro-group via TS2(46.3
kcal/mol)scan dissociate to produce CH2C6H4NO (17) + OH
with 84.9 kcal/mol overall endothermicity and to C6H4CH2(30)
+ HONO with 92.2 kcal/mol endothermicity; both occur by
direct dissociation mechanism. Finally, isomer o-hydroxymethyl nitrosobenzene (27), predicted to be the most stable isomer, can dissociate to give the products CH2(OH)C6H4(28) + NO and
C6H4NO (29) + CH2OH with 55.6 and 104.5 kcal/mol overall
endothermicities, respectively, both by direct dissociation processes.
In summary, there are 10 decomposition channels for o-nitro-toluene and its isomers, where the channels giving rise to NO2,
H2O, and NO products are major reactions. In addition, the OH
loss process was also reported in some photodissociation experiments;4-6this process may involve the dissociation
chan-nels CH2C6H4N(O)OH (10) f C6H4C(H2)NO (14) + OH and
CH2C6H4-t-N(OH)O (6) or CH2C6H4-c-N(OH)O (8) f CH2C6H4
-NO (17) + OH with 74.2 and 84.9 kcal/mol overall endother-micities, respectively; among them the first reaction with a much lower dissociation energy should dominate. Products 17 and 14 formed above are the two structural isomers of o-nitroso-benzyl radicals connected by the -NO torsional isomerization transition state.
2. Rate Constants Calculations.
a. Production of NO2. The NO2elimination reaction
is a direct dissociation process, which is similar to C6H5NO2
f C6H5+ NO2.10The predicted dissociation energy, 76.3 kcal/
mol, is close to the analogous dissociation energy, 74.1 kcal/ mol in C6H5NO2. The rate constant for reaction 1, k1, has been
predicted with the Variflex code12based on the G2M(RCC) PES
and the molecular parameters and frequencies computed by B3LYP/6-311G(d, p). For k1evaluation, the minimum energy
path (MEP) representing the direct dissociation process was obtained by calculating the potential curve along the reaction coordinate C-N of o-CH3C6H4NO2f CH3C6H4+ NO2. Along
the path, the C-N bond length was stretched from the equi-librium value 1.483 to 5 Å with the step size of 0.1 Å and each geometry with a fixed C-N bond length was fully optimized at the B3LYP/ 6-311G(d, p) level. The MEP with the separation of 2.3 to 5.0 Å in the variational TST calculation is approxi-mated with the Morse potential, V(r) ) De[1 - exp[-β(R
-R0)]]2, where R is the reaction coordinate, R0is the equilibrium
value, and De is the bond energy without zero-point energy
corrections. The parameters of the Morse potential obtained by fitting the MEP are R0 ) 1.483 Å, β ) 1.31 Å-1, and De)
81.0 kcal/mol, which was scaled slightly to the G2M value without ZPE-correction. The CH3 groups in CH3C6H4 and
o-CH3C6H4NO2were treated as free rotors with the rotational
constants of 5.563 cm-1 for CH3C6H4 and 5.415 cm-1 for
o-CH3C6H4NO2 evaluated with the ChemRate program.13 In
addition, the Lennard-Jones pairwise potential and the aniso-tropic potential (a potential anisotropy form assuming a bonding potential which is cylindrically symmetric with respect to each fragment) are also added to form the final potential for the variational rate constant evaluation. To compare the theory with experiments, we have calculated the rate constants in the temperature range 500-2000 K under different experimental conditions. The Lennard-Jones (L-J) parameters for different bath gases employed are as follows: Ar,σ ) 3.47 Å, /k ) 114 K;14SF
6,σ ) 5.20 Å, /k ) 212 K;14o-CH3C6H4NO2,σ
) 5.6 Å, /k ) 690 K, derived from its critical temperature (769 K) and volume (376 cm3/mol) using the formulas (/k )
0.897 Tc,σ ) 0.785 Vc1/3).14The average step-sizes for
colli-sional deactivation by Ar and SF6were taken as 〈∆E〉down)
400 and 2200 cm-1, respectively, as previously employed for the C6H5NO2decomposition reaction.10
E0[G2M] ) RCCSD(T)/6-311G(d, p) + MP2/6-311+ G(3df, 2p) - MP2/6-311G(d, p) +∆HLC + ZPE
The predicted high-pressure first-order and low-pressure second-order rate constants for the decomposition reaction in the temperature range 500-2000 K can be represented by
for the Ar-bath gas. The high value of the high-pressure A-factor given above is consistent with those obtained for C6H5NO11
and C6H5NO210fragmentation reactions.
The predicted values shown in Figure 3 for the conditions close to those employed experimentally compare reasonably with the experimental results cited in the Introduction. The rate constants measured by Gonzalez et al.1using the SF
6as bath
gas at 110 Torr pressure are evidently higher than those measured by Tsang and co-workers2,3using Ar as bath gas at
2700-3400 Torr. The values predicted for the conditions employed by Gonzalez et al.1, represented by k
1) 1.5 × 1015
exp[-30900/T] s-1, though slightly lower than their experi-mental results given by k1) 7.9 × 1015exp[-(33000 ( 1000)/
T] s-1, are well within the given error bars as shown in the
Figure 1. Optimized geometries of o-nitrotoluene, its isomers, transition states for the isomerization and dissociation reactions, and dissociation
products calculated at the B3LYP/6-311G(d, p) level.
Figure 2. Schematic energy diagram for decomposition and dissociation reactions of o-nitrotoluene calculated at the G2M(RCC, MP2)//B3LYP/
6-311G(d, p) level, where energy is given in kcal/mol.
k1∞) 4.10 × 1017exp[-37000/T] s-1,
figure. However, the rate constants predicted for the conditions employed by Tsang and co-workers,2,3represented by k
1) 1.6
× 1016 exp[-33500/T] s-1, are noticeably higher than their
experimental values given by k1) 6.4 × 1014exp[-30900/T]
s-1. In both experiments cited above, internal kinetic standards were employed; Gonzalez et al. used cyclohexene1while Tsang
and co-workers2employed 1,2-dimethyl cyclohexene. Tsang and
co-workers2also mentioned that the rate constants of
o-nitro-toluene decomposition obtained by Gonzalez et al.1were
approx-imately a factor of 3 higher than the data they obtained. It should be noted that for the decomposition of nitrobenzene our pre-dicted rate constants for NO2production10were found to agree
closely with the results of both Gonzalez et al. in ref 1 and Tsang et al. reported in ref 2 under their conditions employed. b. Production of Anthranil by Dehydration. The dehydration reaction
is a complex isomerization/decomposition process; it can take place by the following two paths:
In reactions 2a and 2b, the geometries and energies for the reactant, intermediates, transition states, and products are shown in Figures 1 and 2, respectively. In reaction 2a, the transition state TS4 with a 52.9 kcal/mol barrier is the primary
rate-controlling step while in reaction 2b, TS6with a 67.6 kcal/mol
barrier controls the rate of the reaction by this path. The predicted rate constants for these multiple-well decomposition processes in the temperature range 500-2000 K computed with the ChemRate program13were found to be weakly
pressure-dependent. The values predicted at 3000 Torr pressure with Ar as bath gas can be represented by the expressions
The total rate constant, k2) k2a+ k2b, can be represented by
the expression
to which k2bis predicted to contribute only 1-2% over the wide
range of temperature studied. The rate expression predicted for the atmospheric pressure is essentially the same with k2) 8.96
× 1012exp[-25800/T) s-1.
As shown in Figure 4, the predicted values of k2given by
9.8× 1012exp[-25900/T] s-1in the experimental temperature
range 1100-1170 K are in excellent agreement with the experimental data,3k
2) 1.2 × 1013exp[-26000/T] s-1.
c. Production of NO. As shown in Figure 2, this decomposi-tion process can take place via TS9and 24:
A similar nitro-nitrite isomerization/decomposition process has been studied in the nitrobenzene decomposition reaction.10
Because the barrier at TS9 is 43.6 kcal/mol higher than the
dissociation energy of CH3C6H4ONO (24) f CH3C6H4O (25)
+ NO with 14.3 kcal/mol, this decomposition reaction can be treated as a one-step dissociation via TS9. The predicted barrier,
57.9 kcal/mol, may be compared with that of the parallel nitrobenzene decomposition reaction, 61.1 kcal/mol. The sta-tistical factor for the decomposition reaction is 2 because there are two optical isomers in TS9.
The predicted rate constant for reaction 3 in the temperature range 500-2000 K calculated by the Variflex code12for the
high-pressure first-order and low-pressure second-order limits
Figure 3. Predicted rate constants for NO2production. Experimental data: O, ref 2; 0, ref 1; 4, ref 3. Predicted values: solid line, the high-pressure limit; dotted curve, rate constants with Ar as bath gas at 3000 Torr; dashed curve, rate constants with 110 Torr SF6 in the
temperature range of 500-2000 K. o-CH3C6H4NO2f C6H4C(H)ON + H2O (2) o-CH3C6H4NO2798 TS2 CH2C6H4-t-N(OH)O (6) 798 TS3 CH2C6H4-c-N(OH)O (8) 798 TS4 CH2C6H4N(O)OH (10) 798 TS5 C6H4C(H2)ONOH (13) 98 TS7 C6H4C(H)ON + H2O (2a) o-CH3C6H4NO2798TS2 CH2C6H4-t-OHNO (6) 798TS3 CH2C6H4-c-OHNO (8) 798 TS6 C6H4C(H2)ONOH (13) 98 TS7 C6H4C(H)ON + H2O (2b)
Figure 4. Predicted rate constants for anthranil formation. Experimental
data: 4, ref 3. Predicted values: solid line, rate constants with Ar as bath gas at 3000 Torr in the temperature range of 500-2000 K.
k2a) 8.79 × 1012exp[-25800/T] s-1, k2b) 3.65 × 1013exp[-34700/T] s-1. k2) 9.09 × 1012exp[-25800 /T] s-1 o-CH3C6H4NO2798 TS9 24 f CH3C6H4Ο + ΝΟ (3)
in Ar can be represented by the expressions,
To our knowledge, there are no experimental rate constants available for the decomposition channel of NO product.
d. Production of OH. As mentioned in the last section,
with 74.2 kcal/mol dissociation energy is the primary channel of the OH product. The isomerization process from o-CH3C6H4
-NO2to CH2C6H4N(O)OH (10) is the same as reaction 2.
The predicted rate constant for reaction 4 in the temperature range 500-2000 K calculated by the ChemRate program13for
the high-pressure first-order and low-pressure second-order limits in Ar can be represented by the expressions
To our knowledge, there are no experimental data available for the decomposition channel producing the OH product.
e. The Branching Ratios for Formation of NO2, H2O, NO,
and OH. The branching ratios of high-pressure rate constants for o-nitrotoluene to produce NO2, H2O, NO, and OH are shown
in Figure 5. The channel for H2O production dominates at
temperatures below 1000 K, whereas that for NO2production
becomes predominant over 1100 K. The channel producing NO accounts for only 5-10% of the total decomposition rates in the temperature range from 800 to 1300 K. The channel pro-ducing OH contributes negligibly to the total decomposition rate. Conclusions
The potential energy surface for the isomerization and decomposition of o-nitrotoluene, a model compound for TNT,
has been calculated at the G2M(RCC, MP2)//B3LYP/6-311G-(d, p) level. There are 10 decomposition channels for o-nitro-toluene and its six isomeric derivatives, where the channels producing CH3C6H4+ NO2, C6H4C(H)ON (anthranil) + H2O,
and CH3C6H4O (o-methyl phenoxy) + NO are primary
pro-cesses. The predicted rate constants for NO2elimination (k1)
and for anthranil formation (k2) are generally in reasonable
agreement with available experimental data. The predicted rate constants for production of NO and OH (giving CH2C6H4NO)
were found to be much smaller. The branching ratios for o-nitrotoluene decomposition to produce NO2, H2O, NO, and
OH in the temperature range 500-2000 K indicate that the dehydration process dominates at temperatures below 1000 K, whereas the NO2 elimination reaction becomes dominant at
temperature over 1100 K; and the NO product accounts for only 5-10% of the total yield in the temperature range from 800 to 1300 K. The channel producing OH is negligible over the entire range of temperature studied.
Acknowledgment. S.C.X. is grateful for the support of the Emerson Center for Scientific Computations at Emory Univer-sity for an Emerson Visiting Fellowship. M.C.L. acknowledges the support from the Taiwan Semiconductor Manufacturing Company for the TSMC Distinguished Professorship and for the National Science Council of Taiwan for the Distinguished Visiting Professorship at National Chiao Tung University in Hsichu, Taiwan.
Supporting Information Available: Table S1 lists the frequencies and moments of inertia Ii of the decomposition
reaction of o-nitrotoluene calculated at the B3LYP/6-311G(d, p) level. Table S2 lists the predicted forward and reverse rate constants of each and final rate constants of reaction 1, 2a, 2b, 3, and 4 at the high-pressure limit condition. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
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Figure 5. Calculated branching ratios for the four channels of o-nitrotoluene decomposition to NO2(solid curve), H2O (dotted curve),
NO (dashed curve), and OH (dash-dotted curve) products in the temperature range 500-2000 K. k3∞) 1.49 × 1014exp[-30000/T] s-1, k30) 6.21 × 1092× T-21.8exp[-36000/T] cm3mol-1s-1. CH2C6H4N(O)OH (10) f CH2C6H4NO (14) + OH (4) k4∞) 1.31 × 1013exp[-39000/T] s-1, k40) 2.67 × 1028× T-3.7exp[-34000/T] cm3mol-1s-1.
