An experimental and computational study of the thermal oxidation of C6H5NO by NO2

Article abstract of DOI:10.1021/jp0135353

The kinetics and mechanism for the thermal oxidation of nitrosobenzene by nitrogen dioxide have been studied experimentally by pyrolysis/Fourier transform infrared spectrometry and computationally by hybrid density functional theory calculations. The experimental data measured in the temperature range 373-473 K gave rise to the bimolecular rate constant for the direct O-exchange reaction, C₆H₅NO + NO₂ → C₆H₅NO₂ + NO, k₁ = (9.62 ± 0.35) × 10¹⁰ exp[-(6500 ± 144)/T] cm³ mol^-1 s^-1, which can be satisfactorily accounted for by the transition-state theory with the energy barrier, E₁⁰ = 10.0 ± 0.3 kcal mol^-1 at 0 K.

Full text of DOI:10.1021/jp0135353

J. Phys. Chem. A 2002, 106, 2903-2907
2903
An Experimental and Computational Study of the Thermal Oxidation of C6H5NO by NO2
J. Park, Y. M. Choi, I. V. Dyakov, and M. C. Lin*
Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322
ReceiVed: September 14, 2001; In Final Form: January 7, 2002
The kinetics and mechanism for the thermal oxidation of nitrosobenzene by nitrogen dioxide have been studied
experimentally by pyrolysis/Fourier transform infrared spectrometry and computationally by hybrid density
functional theory calculations. The experimental data measured in the temperature range 373-473 K gave
rise to the bimolecular rate constant for the direct O-exchange reaction, C6H5NO + NO2 f C6H5NO2 + NO,
k1 ) (9.62 ( 0.35) × 1010 exp[-(6500 ( 144)/T] cm3 mol-1 s-1, which can be satisfactorily accounted for
by the transition-state theory with the energy barrier, E01 ) 10.0 ( 0.3 kcal mol-1 at 0 K.
I. Introduction
The possibility of the thermal oxidation of nitrosomethane
by nitrogen dioxide to nitromethane in the gas phase, CH3NO
+ NO2 f CH3NO2 + NO, was first suggested by Phillips and
Shaw.1 Cooper et al.2 confirmed the reaction in their study of
the CH3 + NO2 reaction in the presence of NO in the
temperature range 323-455 K. They detected excess yields of
CH3NO2, which could not be accounted for by the association
of CH3 with NO2 alone. An approximate rate constant for the
reaction was obtained by kinetic modeling, k ≈ 109 exp(-5030/
T) cm3 mol-1 s-1, with about 10 kcal mol-1 activation energy.
In a recent study of the thermal decomposition of C6H5NO
in the presence of NO2, we noted the formation of C6H5NO2 in
a long-standing, highly diluted mixture of C6H5NO and NO2 at
room temperature, attributable to the occurrence of the analogous
redox reaction mentioned above. To confirm such a possibility,
Figure 1. Typical FTIR spectra of pyrolyzed and unpyrolyzed mixtures
of C6H5NO/NO2/Ar.
we have measured the decay of C6H5NO and the formation of
C6H5NO2 and NO by Fourier transform infrared (FTIR)
spectrometry in the temperature range 373-473 K under
atmospheric Ar-pressure conditions.
To further substantiate that nitrobenzene was indeed formed
by the direct oxidation reaction,
C6H5NO + NO2 f C6H5NO2 + NO
(1)
we have performed a quantum chemical calculation using the
hybrid density functional (B3LYP) method. Such a direct
displacement transition state was indeed found, and the predicted
transition-state parameters were employed for calculation of the
bimolecular rate constant, k1, for comparison with experimental
data. Both experimental and theoretical results are presented
herein.
II. Experimental Procedure
The thermal reaction of C6H5NO with NO2 was studied in a
spherical quartz reactor of 270 cm3 in volume enclosed in a
temperature-controlled cylindrical furnace. The temperature was
controlled and held within (0.5 K using an Omega CN-9000
solid-state temperature controller. A K-type thermocouple was
placed at the center of the reactor in a small quartz tube. The
Figure 2. Second-order kinetics plot according to eq 2.
detailed experimental procedure of the pyrolysis/FTIR spectro-
metric technique can be found in our earlier studies.3,4
For kinetic measurements, the reactor was filled to an
appropriate pressure with a mixture of NO2 and Ar, to which
an Ar-diluted mixture of C6H5NO was added up to 760 ((0.5)
Torr in the temperature range 393-473 K. Pyrolyzed or
unpyrolyzed reference samples were expanded into the absorp-
* To whom correspondence should be addressed. E-mail: chemmcl@
emory.edu.
10.1021/jp0135353 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/05/2002

2904 J. Phys. Chem. A, Vol. 106, No. 12, 2002
Park et al.
TABLE 1: Summary of the Experimental Conditions and
Rate Constant for the Reaction of C6H5NO with NO2 at
Different Temperaturesa
T, K
[C6H5NO]0
[NO2]0
kbim
kmod
373
395
418
428
438
443
458
463
473
473
2.78
2.62
2.27
2.22
2.17
2.26
2.08
2.16
2.11
2.16
3.27
3.12
2.62
2.56
2.50
2.48
2.39
2.37
2.32
1.65
0.28 ( 0.05
0.67 ( 0.20
1.33 ( 0.87
2.32 ( 0.70
3.69 ( 0.95
2.73 ( 0.78
5.66 ( 0.95
5.67 ( 0.89
8.99 ( 0.75
10.8 ( 2.30
0.30 ( 0.04
0.67 ( 0.06
1.62 ( 0.33
2.66 ( 0.39
3.39 ( 0.20
2.92 ( 0.50
6.02 ( 0.42
5.39 ( 0.72
7.88 ( 1.12
10.4 ( 0.80
a The concentrations are given in units of 10-8 mol cm-3, and rate
constants are given in units of 104 cm3 mol-1 sec-1. The total pressure
of all experiments was 1 atm with the bath gas of argon.
Figure 4. Arrhenius plot for k1: (4) result by eq 2; (O) result by
kinetic modeling; solid line, predicted rate constant with Ea ) 10.0
kcal mol-1; dashed and dotted lines, predicted rate constants with Ea
) 8.1 kcal mol-1 at the B3LYP/cc-pvDZ level and 11.3 kcal mol-1 at
the B3LYP/aug-cc-PVTZ//B3LYP/cc-PVDZ level, respectively.
Figure 5. Sensitivity analyses for C6H5NO and NO at 473 K.
Figure 3. The concentration profiles of the reactants and products
measured in the pyrolysis of C6H5NO/NO2/Ar mixture as functions of
time: (O) C6H5NO; (0) NO; (4) C6H5NO2. Solid curves represent
modeled results of C6H5NO; dashed curves represent modeled results
of C6H5NO2 and NO.
absorbance peak heights is 5%. The pressures of the samples
after expansion were approximately 320 Torr. To minimize the
effect of pressure broadening, the pressure of calibration
mixtures containing varying amounts of the reactants and
products was maintained at 320 ( 10 Torr. Figure 1 shows a
typical set of FTIR spectra obtained before and after the
pyrolysis of a mixture containing C6H5NO and NO2.
The reaction mixtures were prepared with the following initial
concentrations: 0.081-0.089% of C6H5NO and 0.064-0.090%
of NO2 with argon (99.9995%, Specialty Gases) dilution
separately made in different glass bulbs. The NO2 (Matheson)
and NO (Matheson) were purified by the standard trap-to-trap
tion cell inside the FTIR spectrometer (Mattson Instruments,
Polaris) and converted to absolute concentrations. The absolute
concentrations of the reactants and products were determined
by using standard calibration curves plotted in concentration
vs absorbance peak height measured at 1113.0 cm-1 for C6H5-
NO, 1630.0 cm-1 for NO2, 1905.8 cm-1 for NO, and 1358.0
cm-1 for C6H5NO2. Typical error in the determination of
TABLE 2: Reaction Mechanism and Rate Constantsa Employed for Kinetic Modeling of the C6H5NO + NO2 System
reaction
A
n
Ea
refb
1
2
3
4
5
6
7
8
9
C6H5NO + NO2 f C6H5NO2 + NO
C6H5NO ) C6H5 + NO
9.62 × 1017
1.42 × 1017
2.69 × 1012
1.39 × 1013
5.00 × 1012
4.90 × 1012
5.00 × 1014
1.00 × 1013
1.00 × 1013
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
12 900
55 060
-860
111
4500
-68
45 000
0
this work
C6H5 + NO2 f C6H5NO2
c
C6H5 + C6H5 ) C12H10
C6H5 + C6H5NO f C12H10 + NO
C6H5 + C6H5NO f C12H10NO
C12H10NO f C6H5NO + C6H5
C12H10NO + C6H5 f C12H10N + C6H5O
C12H10N + NO f C12H10NNO
0
a Rate constants, defined by k ) ATn exp(-Ea/(RT)), are given in units of cm3, mol, and s; Ea is in units of cal mol-1
associated kinetics are taken from ref 18. c Assumed to be the same as C6H5 + NO (see ref 18).
.
b The mechanism and

Thermal Oxidation of C6H5NO by NO2
J. Phys. Chem. A, Vol. 106, No. 12, 2002 2905
Figure 6. Optimized structures of the reactants, transition state, intermediates, and products for the C6H5NO + NO2 f C6H5NO2 + NO reaction
at the B3LYP/cc-PVDZ level of theory. Bond lengths are in Å, and bond angles are in deg.
distillation using appropriate slush baths. C6H5NO (Aldrich) was
purified by recrystallization at 273 K using C2H5OH as solvent,
followed by extensive pumping at the same temperature to
remove residual solvent.
performed at different temperatures with varying time intervals.
The rate constant for the bimolecular reaction of C6H5NO with
NO2 can be effectively evaluated with the second-order rate
law and kinetic modeling, which also helps to substantiate the
former and the fact that secondary radical processes contribute
negligibly to the measured kinetic data under the low-temper-
ature conditions employed.
With the assumption that reaction 1 takes place exclusively
by the bimolecular O-exchange process, k1 can be calculated
with the instantaneous concentrations of the reactants, [C6H5-
NO] and [NO2], and those of the products, [C6H5NO2] and [NO],
at time t measured by FTIR spectrometry according to the well-
known rate law:14
III. Computational Methods
The hybrid density functional B3LYP method5,6 with a
correlation-consistent basis set (cc-PVDZ)7,8 has been employed
to optimize all of the structures of the reactants, intermediates,
transition state, and products with the Gaussian 98 program.9
From the equilibrium geometries, moments of inertia and
harmonic vibrational frequencies were calculated at the same
level of theory. For the transition-state search, the synchronous
transit-guided quasi-Newton (STQN) method10,11 with the QST2
option for optimizations has been employed, and the intrinsic
reaction coordinate (IRC) method12 and normal-mode analysis13
have been utilized to validate the connection of the transition
state with the reactants and products.
[C6H5NO]0[NO2]
ln
{
}
[C6H5NO][NO2]0
) k1t
(2)
([NO2]0 - [C6H5NO]0)
All of the stationary points were found by using the tight
option9 to improve accuracy and verified by the number of
imaginary frequencies (Nimg); that is, Nimg ) 1 for transition
states and Nimg ) 0 for stable molecules. In this study, all of
the energies include zero-point energy (ZPE) corrections using
the computed frequencies. Also, single-point energies have been
calculated at B3LYP/aug-cc-PVTZ using the geometry com-
puted at the B3LYP/cc-PVDZ level of theory.
where [X]0 denotes the initial concentration of species X. In eq
2, the values of [C6H5NO] and [NO2] can be stoichiometrically
replaced with those of [C6H5NO2] and [NO], respectively. Figure
2 presents three typical second-order plots; the slopes of the
plots give k1. Table 1 lists the evaluated values of k1 as kbim
and compares them with those evaluated by kinetic modeling,
kmod
.
Kinetic modeling was carried out with the mechanism
summarized in Table 2 using the SENKIN program15 in
conjunction with CHEMKIN thermochemical data.16 Figure 3
shows some typical concentration profiles of both experimental
IV. Results and Discussion
A. Kinetic Data Analysis. Kinetic measurements have been

2906 J. Phys. Chem. A, Vol. 106, No. 12, 2002
Park et al.
TABLE 3: Moments of Inertia (IA, IB, IC) and Vibrational
Frequencies of the Species Involved in the C6H5NO + NO2
) C6H5NO2 + NO Reaction at the B3LYP/cc-PVDZ Level
Ii
molecules
C6H5NO
(10-40 g cm2)
vibrational frequencies (cm-1
)
160.2
513.4
673.6
120, 250, 258, 422, 446, 481, 622, 681,
702, 787, 833, 875, 976, 1013, 1015,
1029, 1035, 1088, 1128, 1171, 1189,
1322, 1379, 1481, 1496, 1600, 1646,
1663, 3178, 3189, 3196, 3206, 3212
NO2
3.5
64.8
68.3
758, 1406, 1722
Figure 7. Schematic energy diagram for the C6H5NO + NO2 f C6H5-
NO2 + NO reaction calculated at the B3LYP/cc-PVDZ level of theory.
C6H5NO2
212.0
655.5
867.5
63, 172, 258, 397, 421, 451, 528, 622
694, 696, 719, 811, 862, 867, 969,
1001, 1017, 1024, 1042, 1091, 1122,
1172, 1185, 1318, 1375, 1397, 1485,
1505, 1625, 1640, 1675, 3184, 3197,
3206, 3238, 3238
calculation with the B3LYP/cc-PVDZ method. The optimized
structures and molecular parameters of the reactants, transition
state, intermediates, and products are presented in Figure 6 and
Table 3, respectively. Additional single-point energies have been
computed at the B3LYP/aug-cc-PVTZ level, using the geometry
optimized at B3LYP/cc-PVDZ, for comparison. The two sets
of relative energies are summarized in Table 4. The schematic
energy diagram of this reaction using the energies predicted at
the B3LYP/cc-PVDZ level is illustrated in Figure 7.
NO
TS
0.0
1994
16.5
16.5
404.1
1063.9
1172.4
239i, 44, 60, 128, 158, 191, 263,
369, 400, 419, 480, 532 621, 645,
695, 775, 814, 830, 854, 951, 995,
1008, 1016, 1018, 1041, 1098, 1138,
1172, 1194, 1326, 1370, 1481, 1499,
1563, 1622, 1639, 1661, 3184, 3196,
3206, 3224, 3225
Mechanistically, the oxidation of C6H5NO by NO2 was found
to take place via a local minimum (LM) of the reactants, the
transition state, and the LM of the products, C6H5NO2 and NO:
TABLE 4: Relative Energies in kcal mol-1 of the Reactants,
Transition State, Intermediates, and Products for the
C6H5NO + NO2 f C6H5NO2 + NO Reaction Calculated at
the B3LYP/cc-PVDZ and B3LYP/aug-cc-PVTZ Levels of
Theory
C6H5NO + NO2 f LM1 f TS‡ f LM2 f C6H5NO2 +
NO
species
ZPE B3LYP/cc-PVDZ B3LYP/aug-cc-PVDZ
where “‡” represents the transition state. The local minimum
of the reactants, LM1, found by IRC analysis, has a binding
energy of 2.4 kcal mol-1. The bond lengths between the O atoms
of NO2 and the N atom of C6H5NO are 2.989 and 3.189 Å,
respectively. At the transition state, the forming N-O bond
becomes 1.890 Å with the departing NO group lying above the
ONCC frame. The barrier for the reaction was predicted to be
8.1 kcal mol-1 at the B3LYP/cc-PVDZ level and 11.3 kcal
mol-1 at the B3LYP/aug-cc-PVTZ//B3LYP/cc-PVDZ level (see
Table 4). After overcoming the reaction barrier, the reaction
first forms the local minimum of the products, LM2, with the
exothermicity of 16.4 kcal mol-1 at B3LYP/cc-PVDZ and 15.1
kcal mol-1 at B3LYP/aug-cc-PVTZ, and the overall exother-
micity of 15.2 and 15.5 kcal mol-1, respectively. The binding
energy of LM2 is negligibly small (∼0) within the reliability
of the approximate B3LYP method.
Rate Constant Calculation. The predicted transition-state
parameters and energies presented above were utilized to
calculate the bimolecular rate constant for the O-exchange
reaction using the conventional transition-state theory.14 As the
binding energy of LM1 is small compared with the reaction
barrier, the effect of LM2 to the kinetics is evidently insignifi-
cant.
The predicted rate constants based on the two values of the
barrier, 8.1 and 11.3 kcal mol-1, are included in Figure 4 for
comparison with experimental data. The rate constant predicted
with the 8.1 kcal mol-1 barrier obtained at the B3LYP//cc-PVDZ
level of theory is higher than the experimental result by a factor
of 10 at the middle temperature studied (428 K), whereas the
larger barrier (11.3 kcal mol-1) underpredicted the rate constant
by a factor of 5 at the same temperature.
To quantitatively account for the experimental result, the
reaction barrier should lie in the vicinity of 10.0 ( 0.3 kcal
mol-1, which is within the uncertainty of the approximate hybrid
density functional theory with a computationally affordable size
of basis set.
C6H5NO + NO2 66.6
C6H5NO‚NO2
TS
C6H5NO2‚NO
C6H5NO2 + NO 67.6
a Total energy in hartrees is -566.551 995. b Total energy in hartrees
is -566.727 985. c The reason the relative energies are higher than those
of reactants and products may be the error of the single-point energy
calculations.
0.0a
-2.4
0.0b
67.2
67.3
67.9
0.2c
8.1
11.3
-16.4
-15.1c
-15.5
-15.2
and modeled results of C6H5NO, C6H5NO2, and NO vs time.
In the modeling, the value of k1 was adjusted to fit the
experimental data at different time intervals and the averaged
value was used in the final calculation. As shown in Table 1,
the modeled value, kmod, at each temperature agrees closely with
kbim calculated by eq 2. Figure 4 summarizes both sets of results
graphically in an Arrhenius plot. A weighted least-squares
analysis17 of the two sets of data gave rise to
k1 ) (9.62 ( 0.35) × 1010 exp[-(6500 (
144)/T] cm3 mol-1 s-1 (3)
The good agreement between the results obtained by eq 2
and those obtained by kinetic modeling including potential
secondary radical reactions as listed in Table 2 clearly suggests
that the secondary reactions do not occur to any significant
extent. At the highest temperature employed, 473 K, the half-
life of C6H5NO, which dissociates more effectively than NO2
does, is calculated to be ∼108 s on the basis of the rate constant
for its unimolecular decomposition reaction recently determined
by FTIR spectrometry in our laboratory.18 The result of a
sensitivity analysis at 473 K for C6H5NO and NO (Figure 5)
clearly shows that only reaction 1 is responsible for the decay
of C6H5NO and the formation of NO.
B. Confirmation of the Mechanism by Quantum Calcula-
tion. The mechanism of the reaction has been confirmed by

Thermal Oxidation of C6H5NO by NO2
J. Phys. Chem. A, Vol. 106, No. 12, 2002 2907
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V. Conclusion
The rate constant for the oxidation of nitrosobenzene by
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C6H5NO + NO2 f C6H5NO2 + NO
(1)
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( 144)/T] cm3 mol-1 s-1. The mechanism of this metathetical
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be accounted for by the transition-state theory based on the
predicted transition-state parameters with the energy barrier E1
) 10.0 ( 0.3 kcal mol-1 at 0 K.
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Acknowledgment. The authors are grateful for the support
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Energy, under Contract No. DE-FG02-97-ER14784.
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