Thermal Decomposition of Nitromethane and Reaction between CH3

Subscriber access provided by RYERSON UNIVERSITY

Article

3

2

Thermal Decomposition of Nitromethane and Reaction between CH and NO Akira Matsugi, and Hiroumi Shiina J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Thermal Decomposition of Nitromethane and Reaction between CH3 and NO2 Akira Matsugi,* and Hiroumi Shiina National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan. *Corresponding Author. E-mail: [email protected]

ABSTRACT The thermal decomposition of gaseous nitromethane and the subsequent bimolecular reaction between CH3 and NO2 have been experimentally studied using time-resolved cavity-enhanced absorption spectroscopy behind reflected shock waves in the temperature range 1336–1827 K and at pressure of 100 kPa. Temporal evolution of NO2 was observed following the pyrolysis of nitromethane (diluted to 80–140 ppm in argon) by monitoring the absorption around 400 nm. The primary objectives of the current work were to evaluate the rate constant for the CH3 + NO2 reaction (k2) and to examine the contribution of the roaming isomerization pathway in nitromethane decomposition. The resultant rate constant can be expressed as k2 = (9.3 ± 1.8) × 10−10 (T/K)−0.5 cm3 molecule−1 s−1, which is in reasonable agreement with available literature data. The decomposition of nitromethane was found to predominantly proceed with the C–N bond fission process with the branching fraction of 0.97 ± 0.06. Therefore, the upper limit of the branching fraction for the roaming pathway was evaluated to be 0.09.

1 ACS Paragon Plus Environment


1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

INTRODUCTION Nitromethane (CH3NO2) is the simplest organic nitro compound and a representative energetic material that can be used as an explosive or a monopropellant.1 A number of kinetic studies have been reported for the thermal decomposition of nitromethane behind shock waves2–9 over wide temperature and pressure ranges. Most of these studies presumed or implied that the decomposition exclusively proceeds via direct C–N bond fission to form CH3 and NO2. On the other hand, CH3O and NO were also detected as direct reaction products from infrared multiphoton dissociation (IRMPD) of nitromethane in a collision-free environment of a molecular beam.10 They were tentatively attributed as products generated through isomerization of nitromethane to methyl nitrate (CH3ONO) followed by dissociation to

CH3O

+ NO.

This

mechanism

was

later supported

by

multiconfiguration

self-consistent-field calculations performed by Saxon and Yoshimine,11 who identified a transition state for the nitro-nitrite rearrangement which was characterized as a loose combination of CH3 and NO2. Such mechanisms are currently known as roaming reactions,12– 15

and were recently found to be rather ubiquitous pathways not only in molecular

photodissociation processes but also in thermal decomposition reactions.16,17 Two computational studies have been reported on the kinetics of the roaming reaction in the thermal decomposition of nitromethane. Zhu et al.18,19 performed statistical rate theory (variational RRKM and master equation) calculations based on the potential energy surface characterized using the UCCSD(T)/CBS//UB3LYP and CASPT3//CASSCF methods. Their results indicated that roaming could be the dominant channel under the conditions of the IRMPD experiments but should not contribute to the thermal decomposition process at the pressures of shock tube experiments. Their predicted branching fractions for the roaming channel at temperatures higher than 1000 K were approximately 0.2, 0.07, and less than 0.05 at pressures of 10, 100, and 760 Torr, respectively. Conversely, the master equation analysis 2 ACS Paragon Plus Environment

Page 3 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


based on the rigid-body trajectory (RBT) simulation conducted by Annesley et al.2 suggested a more moderate pressure-dependence in the branching fraction. They predicted that contributions of the roaming channel are 15–30% at pressures of 10–100 Torr, and still ~10% at 760 Torr. There exists only one experimental report on the branching fraction in the thermal decomposition of nitromethane. Annesley et al.2 explored variation in the density gradients for nitromethane decomposition following the passage of incident shock waves at pressures of 30–120 Torr. With a complementary theoretical analysis and kinetic simulation, they obtained the branching fraction of 0.10–0.19 for the roaming channel, which were in reasonable agreement with the two computational predictions. However, since the direct C–N bond fission channel and roaming channel (followed by the subsequent methoxy dissociation to CH2O + H) were found to have similar effective enthalpies of reaction, the density gradient profiles were also consistent with a no-roaming scenario. Moreover, their analysis is further complicated by the existence of the reaction of CH3 + NO2 because the density gradient profiles have a marked sensitivity to the rate constant of this reaction. Accurate kinetic data for the CH3 + NO2 reaction are required to interpret the experimental data of the nitromethane decomposition. This reaction is also known to be important in understanding the influence of nitrogen oxides on the oxidation of hydrocarbons.7,20,21 Nevertheless, the literature on its kinetics is sparse and the reported high-temperature rate constants have large uncertainty and scatter. The recombination of CH3 and NO2 firstly produces chemically activated adducts, CH3NO2* or CH3ONO*. The latter activated adduct has a low energy dissociation channel producing CH3O + NO fragments,2,22–24 while the former adduct can only dissociate back to the reactants or stabilize to CH3NO2. Since the stabilization is effective only under low-temperature conditions,24,25 the CH3 + NO2 reaction is expected to exclusively produce CH3O + NO at high temperatures. There is a possibility of 3 ACS Paragon Plus Environment


1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

forming CH2O + HNO products from dissociation of CH3ONO*, but its contribution is considered minor as discussed in ref 2. Glänzer and Troe3 monitored concentrations of nitromethane and NO2 behind shock waves following the pyrolysis of nitromethane diluted in Ar, and analyzed their temporal profiles to determine a rate constant for the CH3 + NO2 reaction to be 2.16 × 10−11 cm3 molecule−1 s−1 at temperatures of 1200–1400 K and the buffer gas density of 1.5 × 10−6 < [Ar] < 3.5 × 10–4 mol cm–3. This value was in good agreement with the reported room temperature values.22–24 However, because the kinetic scheme used in this analysis was found to be too simplified, Glarborg et al.7 reanalyzed the concentration profiles of Glänzer and Troe using a detailed kinetic model that incorporated possible side reactions, and obtained the slightly smaller value of 1.66 × 10−11 cm3 molecule−1 s−1 at 1180 K with an uncertainty as high as a factor of two. Srinivasan et al.25 conducted shock tube experiments at higher temperatures to measure the rate constant for CH3 + NO2. They employed the multipass resonance absorption technique to detect OH radicals produced by the secondary reactions. Their rate constant was, on average, 2.4 × 10−11 cm3 molecule−1 s−1 at 1360–1695 K and 36–60 kPa, but had relatively large scatter ranging from 1.4 × 10−11 to 3.5 × 10−11 cm3 molecule−1 s−1. In the present study, the thermal decomposition reaction of nitromethane and the subsequent reaction between CH3 and NO2 were investigated by observing the temporal profile of NO2 during the pyrolysis of nitromethane behind shock waves: CH3NO2 → CH3 + NO2

∆H°298 = 255 kJ mol−1

(R1a)

CH3NO2 → CH3O + NO

∆H°298 = 187 kJ mol−1

(R1b)

CH3 + NO2 → CH3O + NO

∆H°298 = −68 kJ mol−1

(R2)

A time-resolved cavity-enhanced absorption method enabled sensitive detection of NO2 and the rate constants could be determined with a high degree of accuracy. The upper limit of the branching fraction for the roaming pathway R1 was also determined.


Page 5 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL METHOD The experiments were performed behind reflected shock waves in a diaphragmless shock tube. NO2 produced from the thermal decomposition of nitromethane was monitored using the time-resolved broadband cavity-enhanced absorption spectroscopy (BBCEAS). The shock tube apparatus and the time-resolved BBCEAS method have been fully described elsewhere,26 and only specific details are presented here. The sample gases used were mixtures of 80–140 ppm nitromethane (>98% purity; Tokyo Chemical Industry; degassed by repetitive freeze-pump-thaw procedures before use) diluted in argon (>99.9999% purity; Taiyo Nippon Sanso). The mixtures were prepared from pressure measurements using capacitance manometers in a glass vacuum line, stored in glass vessels, and allowed to homogenize for at least 12 hours prior to the experiments. NO2 was detected by its absorption over the wavelength range 390–410 nm and the wavelength-averaged absorbance is reported here because there was almost no wavelength dependence in the absorption cross section at high-temperatures. The effective absorption path length in the BBCEAS measurement was calibrated using the absorption of NO2 at room temperature26; it was 160–230 cm in the studied wavelength region. The absorption cross section of NO2, σ, at high temperature was directly measured by shock-heating mixtures of 100–500 ppm NO2 diluted in argon.

RESULTS AND DISCUSSION Absorption Cross Section of NO2.

The high-temperature absorption cross section of

NO2 was measured behind reflected shock waves at temperatures ranging from



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

approximately 1000 to 2000 K and pressure of 100 kPa. Typical temporal profiles of the absorption coefficients (averaged over 390–410 nm), α, are shown in Fig. S1 (Supporting Information). At temperatures below 1500 K, the absorption coefficient of the heated NO2 remains nearly constant because NO2 does not decompose, and the absorption cross section can be directly determined from the post-reflected-shock absorption coefficients. At higher temperatures, the absorptions gradually decrease due to the decomposition of NO2. Here two reactions are contributing to the profiles under the present experimental conditions27: NO2 + M → NO + O + M

∆H°298 = 306 kJ mol−1

NO2 + O → NO + O2

∆H°298 = −192 kJ mol−1 (R4)

(R3)

and

Kinetic analysis was performed employing the literature rate constant for R4, k4 = 6.51 × 10−12 exp(−120 K / T) cm3 molecule−1 s−1,28 to determine the rate constant for R3, k3, and the absorption cross section of NO2. The resultant k3 is in excellent agreement with the literature rate constant27 as shown in Fig. S2 (Supporting Information). Figure 1 shows the obtained absorption cross section of NO2 together with the literature data taken at a wavelength of 405 nm.29 The present results are consistent with the literature and can be represented as

σ = (6.55 ± 0.35) × 10−19 exp(−T/3380 K) cm2 molecule−1

(1)

where the stated uncertainty corresponds to the 95% confidential interval (CI) estimated from the statistical deviation, 1.5% uncertainty in the reference NO2 absorption cross section at room temperature (used to determine the effective path length), and 1% uncertainty in the initial reagent concentration. This value was used to quantify the concentration of NO2 produced from nitromethane.


Page 7 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Absorption cross sections of NO2 at high temperature. The circle and square symbols are the present and literature experimental data points, respectively. The solid and dashed lines denote the representation given by eq. 1 and its 95% CI, respectively.

NO2 Time Profiles in Nitromethane Pyrolysis.

The experiments for the thermal

decomposition of nitromethane were carried out at post-reflected-shock temperatures of 1336–1827 K and pressure of 100 (±2) kPa. Figure 2 shows example NO2 time profiles obtained at 1370 and 1783 K. At the low temperature, the formation and consumption rates of NO2 are competing, resulting in a rise-decay profile. In contrast, the formation of NO2 is almost promptly completed at the higher temperature due to the rapid decomposition of nitromethane. The initial rise rate of NO2 depends on the rate constant for R1, whereas the decay part is mainly controlled by that of R2.



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Time profiles of NO2 concentration observed during the decomposition of nitromethane behind reflected shock waves. The dashed lines are the fitted profiles.

Besides the reactions R1 and R2, there are a number of side reactions that contribute to the NO2 profiles. Therefore, numerical simulation of the NO2 profile was conducted using a reaction mechanism involving possible secondary reactions. The mechanism was constructed based on that developed by Annesley et al.2 with some modification and addition of reactions. The updated reactions are listed in Table S1 in Supporting Information. The updates were primarily related to the decomposition reaction of NO2 to NO + O, which has moderate sensitivity in the NO2 profiles at high temperature, and the subsequent reactions of O atoms. All reactions are treated as reversible. Thermochemical data for species included in the mechanism were taken from the literature.2,30 The simulations were performed using the Cantera program31 and the total rate constants for R1 (k1) and R2 (k2) and the branching fraction for R1a in R1 (φ1a) were determined by nonlinear least-squares fits of the experimental NO2 profiles to the simulated ones. The simulated profiles with the best-fit parameters are shown in Fig. 2 as the dashed lines. 8 ACS Paragon Plus Environment

Page 8 of 29

Page 9 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 shows the sensitivity coefficients for the NO2 concentration for the experimental conditions shown in Fig. 2. The sensitivity coefficients are defined as the fractional change in the NO2 concentration relative to the fractional change in the rate constant for individual reactions. The formation of NO2 is primarily sensitive to R1a, while R1b has only low sensitivity because the CH3 + NO2 channel was found to be dominant in the decomposition of nitromethane. Nevertheless, the nascent concentration of NO2 reflects the branching ratio between the R1a and R1b channels, and the branching fraction could be determined from the fits. At temperatures higher than 1500 K, the experimental profiles cannot resolve the formation rate of NO2 and the early profile is sensitive only to the branching ratio but not to the total rate constant of R1. Therefore, the total rate constant of R1 was determined only for temperatures lower than 1500 K. The rate constant extrapolated from the lower temperature data was used in the kinetic simulation at higher temperatures. The consumption of NO2 is not solely caused by its reaction with CH3, but also by its reaction with H atoms H + NO2 → OH + NO

∆H°298 = −124 kJ mol−1 (R5)

where the H atoms are mainly generated from the decomposition of the methoxy radical CH3O + M → CH2O + H + M

∆H°298 = 87 kJ mol−1

(R6)

though this reaction is too fast to be sensitive for the NO2 profile. The reaction between CH2O and OH CH2O + OH → HCO + H2O

∆H°298 = −128 kJ mol−1 (R7)

also exhibits negative sensitivity coefficients because the HCO radical formed in this reaction almost instantly decomposes to yield another H atom, which contributes to R5. The recombination reaction between two CH3 radicals CH3 + CH3 → C2H6

∆H°298 = −377 kJ mol−1 (R−8)

contributes to the consumption of CH3 radicals and therefore shows positive sensitivity under



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

low temperature conditions. This reaction is not effective at higher temperatures where the reverse decomposition reaction (R8) becomes dominant. Under high temperature conditions, the decomposition of NO2 (R3) also takes place and slightly affects the latter region of the NO2 profile.

Figure 3. Sensitivity for NO2 concentration under the experimental conditions shown in Fig. 2.

Rate Constants and Branching Fraction. The experimental conditions and derived rate constants and branching fractions are summarized in Table 1. Systematic uncertainty limits on the derived values were evaluated by combining the uncertainty contributions from the various error sources. Table 2 shows results of the uncertainty analyses for the experimental conditions shown in Fig. 2. The contributions considered are from uncertainties


Page 10 of 29

Page 11 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in time zero (estimated to be ±1.5 µs based on the temporal response of the pressure transducer placed at the same axial position as the optical ports), absorption cross section of NO2 (±5.3%), the reagent concentration (±2%), and the rate constants for secondary reactions employed in the simulation. The rate constants for the sensitive reactions, R3, R5, R7, and R8, are all well-known27,32–34 and have small uncertainties. For R8, only the rate constant for the reverse recombination reaction (R–8) was sensitive to the NO2 profiles. This rate constant was evaluated from that for the forward decomposition reaction34 and its equilibrium constant calculated from the thermodynamic properties employed.2,30 Therefore, the uncertainty in the rate constant for the recombination reaction was evaluated as the root-sum-squares of those in the forward rate constant (±20%34) and the equilibrium constant (assumed to be ±30% based on a comparison of different data sources2,30,35). The contributions from the individual sources were determined by performing the kinetic analysis using the source parameter perturbed to the estimated bound. The total uncertainty was evaluated as the root-sum-squares of the uncertainties for each source. The dominant uncertainty source for k1 was the position of time zero, whereas the accuracy of the rate constants for the secondary reactions is important for determining k2. The branching fraction was most sensitive to the absorption cross section and was not severely affected by the secondary reactions.

Table 1. Experimental Conditions, Rate Constants for R1 and R2, and Branching Fraction for R1a in R1 temperature

pressure

nitromethane

(K)

(kPa)

(ppm)

1336

100

80

1366 1370 1389 1409

99 99 99 99

k2 (cm3

–1

k1 (s )

100 80 140 80

molecule−1 s−1)

φ1a

5.27 × 104

2.51 × 10−11

0.958

6.78 × 10

4

2.66 × 10

−11

0.993

7.16 × 10

4

2.58 × 10

−11

0.979

9.03 × 10

4

2.57 × 10

−11

0.975

1.25 × 10

5

2.30 × 10

−11

0.962



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1413

99

1459

99

1513

99

1553 1575

0.963

2.25 × 10

−11

0.982

2.28 × 10

−11

0.985

2.52 × 10

−11

1.000

2.46 × 10

−11

0.967

2.18 × 10

−11

0.947

2.02 × 10

−11

0.954

2.23 × 10

−11

0.987

2.16 × 10

−11

0.964

2.34 × 10

−11

0.958

2.32 × 10

−11

0.968

2.36 × 10

−11

0.981

1.93 × 10

−11

0.942

2.11 × 10

80

101

1827

0.991

−11

100

101

1786

2.39 × 10

140

100

1783

1.000

−11

80

101

1772

2.31 × 10

140

101

1749

0.965

−11

100

100

1747

2.37 × 10

140

99

1700

0.968

−11

100

99

1680

2.42 × 10

1.63 × 10

80

98

1647

100

−11

140

99

1636

0.979

5

80

98

1611

2.56 × 10−11

100

99

1607

1.12 × 105

80

99

1580

100 100

99

140

100

Page 12 of 29

140

Table 2. Uncertainty Analysis for the Rate Constants and Branching Fraction source

source uncertainty

uncertainty contributions k1

k2

φ1a

80 ppm nitromethane/Ar, 1370 K, 99 kPa time zero

7.1%

1.5%

0.8%

NO2 absorption cross section

± 1.5 µs ± 5.3%

0.2%

2.2%

4.2%

reagent concentration

± 2%

0.4%

2.5%

1.5%

± 18%

0.1%

4.6%

0.0%

± 25%

3.1%

4.3%

1.0%

5.8%

13.7%

2.2%

0.0%

0.0%

0.0%

9.7%

15.5%

5.1%

-

1.0%

1.6%

-

1.2%

3.8%

a

H + NO2 = OH + NO

CH2O + OH = HCO + H2O CH3 + CH3 = C2H6

b

c

± 36% d

NO2 + M = NO + O + M

c

± 15%

total

80 ppm nitromethane/Ar, 1783 K, 101 kPa time zero NO2 absorption cross section

± 1.5 µs ± 5.3%


Page 13 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reagent concentration

± 2%

-

3.7%

1.6%

H + NO2 = OH + NO

± 18%

-

10.0%

0.8%

CH2O + OH = HCO + H2O

± 25%

-

2.3%

0.4%

CH3 + CH3 = C2H6

± 36%

-

0.5%

0.0%

NO2 + M = NO + O + M

± 15%

-

4.2%

0.3%

-

11.8%

4.5%

total a

ref 32.

b

ref 33.

c

based on ref 34 (see text).

d

ref 27.

Figure 4 shows values of the total rate constant for R1 obtained from the present study and the literature. For the sake of simplicity, only the data from Zaslonko et al.6 are plotted here; the other literature sources3,8,9 reported consistent rate constants for Ar bath gas as summarized in ref 2. The lines drawn are the result of the unimolecular master equation36 calculation conducted to extrapolate the rate constant to be used in the kinetic simulation at the higher temperature. Since the C–N bond fission channel was found to be dominant in the decomposition of nitromethane, the master equation was solved only for the single-channel system. This pragmatic approach suffices for the purpose of the present extrapolation since the NO2 profiles at high temperature are insensitive to the value of k1 used in the simulation. The calculation was performed using the SSUMES program.37 The microscopic rate constant for dissociation was estimated by inverse Laplace transform method38 using the high-pressure limiting rate constant of k1,∞ = 1.78 × 1016 exp(− 29440 K / T) s−1.3 The density of states for nitromethane was calculated based on the rovibrational properties computed at the ωB97X-D/6-311++G(d,p) level of theory39 using the Gaussian 09 program.40 Rigid-rotor and harmonic oscillator approximations (with a frequency scaling factor41,42 of 0.950) were adopted except for the internal rotation, which was treated as a free rotor. The collision frequencies were calculated using Lennard-Jones collision parameters of σ = 3.89 Å and ε =



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

197 cm−1 for nitromethane colliding with Ar bath gas,2 and the collisional energy transfer probability was estimated by the exponential down model.36 An average downward energy transfer, 〈∆Edown〉, was optimized to be 510 cm−1 to reproduce the present rate constant. With this value, the literature rate constants at a variety of pressures were also well reproduced as shown in Fig. 4. The rate constant at a pressure of 100 kPa can be parametrized as k1 = 4.33×1058 (T/K)−13.41 exp(− 37005 K / T) s−1

(2)

for the temperature range 600–2000 K.

Figure. 4. Arrhenius plot of the present and literature6 rate constants for the thermal decomposition of nitromethane. The lines represent the result of the master-equation calculation at pressures of 3500, 410, 150, 100, 52, and 27 kPa from top to bottom.

The rate constants for R2 are plotted in Fig. 5 together with the previously reported values.2,7,22,23,25 The present k2 are in excellent agreement with the average value of the rate constants determined by Srinivasan et al.,25 but the scatter in the data has been significantly reduced. Glarborg et al.7 reported a rate constant of 1.66 × 10−11 cm3 molecule−1 s−1 at 1180 K


Page 14 of 29

Page 15 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by reanalyzing the species concentration profiles following the pyrolysis of nitromethane observed by Glänzer and Troe.3 Although Glarborg et al. analyzed the profile using a reaction scheme that was similar to that used in the present study, their rate constant was about 40% smaller than the present value. Given that the absolute concentration of the reactant in their profile was more than an order of magnitude higher than that in the present experiment, this discrepancy is possibly caused by the influence of secondary reactions. Nevertheless, the rate constants agree with each other within the stated uncertainty limits; Glarborg et al. noted that there was likely an uncertainty of ±30% in their rate constant arising from errors in the kinetic model they employed, and that the overall uncertainty may be as high as a factor of two when other experimental uncertainties are taken into account (both of their uncertainty limits are drawn as error bars in Fig. 5). The present k2 values are also close to the room temperature values determined by Yamada et al.23 and Biggs et al.,22 indicating that the rate constant does not depend strongly on temperature. In contrast, the rate constant computationally predicted in the work of Annesley et al.2 suggested a clear negative temperature dependence, as shown by the dashed line in Fig. 5. The prediction was made by the RBT simulation on a six-dimensional analytic potential energy surface. It has an approximate T−0.5 dependence and the high-temperature value was only slightly (~10%) higher than that of the present results. However, the predicted rate constant near room temperature was about a factor of two larger than the experiments.22,23 It is worthwhile to note that the present rate constants also exhibit slightly negative temperature dependence similar to the predicted T−0.5 dependence. Assuming the same dependence, the present data can be represented as k2 = (9.3 ± 1.8) × 10−10 (T/K)−0.5 cm3 molecule−1 s−1 (3) at 1336–1827 K, where the uncertainty limit is evaluated as the root-sum-squares of the statistical uncertainty (±11% at the 95% CI as estimated from the scatter of the data) and the



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

systematic error (~16% as shown in Table 2). The rate constant determined in the present study is a factor of 1.4–1.7 larger than that employed in the work of Annesley et al.2 to simulate their density gradient profiles. Since this reaction is exothermic, employing a larger rate constant for R2 decreases the density gradient during the period for which the density gradient profile is sensitive to this reaction. They noted that simulations with k2 from their RBT simulation, which is close to the present value, made the density gradient too negative almost from the start of the reaction.2 Nevertheless, the disagreement is not severe considering the uncertainty in the present rate constant and given that the density gradient has complex dependence on rate constants and enthalpies of reaction for secondary reactions which potentially contribute to uncertainty in the simulated profiles.

Figure 5. The present and literature2,7,22,23,25 rate constants for R2.

The branching fraction for the R1a channel in the decomposition of nitromethane was


Page 16 of 29

Page 17 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


found to be close to unity over the studied temperature range as shown in Table 1. Simply averaging the obtained values yields the fraction φ1a = 0.97, which might indicate a subtle but potential contribution of the roaming channel. However, only the upper limit on the roaming fraction could be determined because of the uncertainty in φ1a; the scatter in the measured φ1a gives a statistical uncertainty of ±3% (at the 95% CI) and φ1a has an additional ~5% systematic uncertainty (see Table 2). The overall uncertainty in φ1a was estimated as ±6% by the root-sum-squares, suggesting an upper limit of 0.09 for the branching fraction for the roaming pathway at a pressure of 100 kPa. This upper limit does not conflict with the reported computational2,19 and experimental2 values. Zhu et al.19 predicted the branching fraction for the roaming isomerization pathway to be ~0.03 under conditions similar to the present experiment (deduced from their Fig. 2). This value coincides with the average value obtained in the present study, 1 − φ1a = 0.03. Although the RBT/master equation simulation2 suggested a somewhat higher probability (~10%) for the roaming branching under the same conditions, their prediction is not inconsistent with the present result given their stated uncertainty of a factor of 1.6. The present upper limit is clearly smaller than the previous experimental evaluation of Annesley et al.2 They deduced the roaming fraction of 0.10–0.19 based on the density gradients measurements at pressures of 30–120 Torr (4–16 kPa). This difference is still within reasonable bounds considering the expected pressure dependence2,19 and the difficulty in estimating the roaming fraction from the density gradient profiles.2

CONCLUSION Quantitatively following the temporal behavior of NO2 during the pyrolysis of dilute nitromethane, the rate constants for the thermal decomposition of nitromethane (R1) and the CH3 + NO2 → CH3O + NO reaction (2) and the upper limit for the roaming fraction in R1



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

were determined over the temperature range 1336–1827 K at pressure of 100 kPa. The total rate constant for nitromethane decomposition was in reasonable agreement with previously reported values and could be readily reproduced by the master equation calculation. The rate constant for R2 was determined as k2 = (9.3 ± 1.8) × 10−10 (T/K)−0.5 cm3 molecule−1 s−1. This result was also consistent with available literature data at high temperatures, whereas the uncertainty in the present determination is substantially lower than those in the previous studies. The C–N bond fission channel was found to be the dominant reaction pathway in the decomposition of nitromethane at pressure of 100 kPa with the branching fraction of φ1a = 0.97 ± 0.06.

ASSOCIATED CONTENT Supporting Information Time profiles of the absorption coefficients of the shock-heated NO2 (Fig. S1), Arrhenius plot of the low-pressure limiting rate constants for the NO2 + M → NO + O + M reaction (Fig. S2), and the updated reaction mechanism used in the kinetic analysis (Table S1).

ACKNOWLEDGMENT The authors would like to thank Robert S. Tranter for helpful discussions. This work was supported in part by JSPS KAKENHI Grant Number 15K17991.


Page 18 of 29

Page 19 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Boyer, E.; Kuo, K. K. Characteristics of Nitromethane for Propulsion Applications. 44th AIAA Aerospace Sciences Meeting and Exhibit. Reno, Nevada. 2006, DOI: 10.2514/6.2006-361 (2) Annesley, C. J.; Randazzo, J. B.; Klippenstein, S. J.; Harding, L. B.; Jasper, A. W.; Georgievskii, Y.; Ruscic, B.; Tranter, R. S. Thermal Dissociation and Roaming Isomerization of Nitromethane: Experiment and Theory. J. Phys. Chem. A 2015, 119, 7872–7893. (3) Glänzer, K.; Troe, J. Thermische Zerfallsreaktionen Von Nitroverbindungen, I. Dissoziation Von Nitromethan. Helv. Chim. Acta 1972, 55, 2884–2893. (4) Hsu, D. S. Y.; Lin, M. C. Laser Probing and Kinetic Modeling of NO and CO Production in Shock-Wave Decomposition of Nitromethane under Highly Diluted Conditions. Journal of Energetic Materials 1985, 3, 95–127. (5) Zhang, Y-X.; Bauer, S. H. Modeling the Decomposition of Nitromethane, Induced by Shock Heating. J. Phys. Chem. B 1997, 101, 8717–8726. (6) Zaslonko, I. S.; Petrov, Y. P.; Smirnov, V. N. Thermal Decomposition of Nitromethane in Shock Waves: the Effect of Pressure and Collision Partners. Kinet. Catal. 1997, 38, 321– 324. (7) Glarborg, P.; Bendtsen, A. B.; Miller, J. A. Nitromethane Dissociation: Implications for the CH3 + NO2 Reaction. Int. J. Chem. Kinet. 1999, 31, 591–602. (8) Petrov, Y.; Karasevich, Y.; Turetskii, S. Decomposition of Nitromethane in Shock Waves: the Primary Stage and the Kinetics of Decomposition at Pressures of about 40 atm. Russ. J. Phys. Chem. B 2010, 4, 566–573. (9) Kuznetsov, N.; Petrov, Y.; Turetskii, S. Kinetics of NO2 Formation upon the Decomposition of Nitromethane behind Shock Waves and the Possibility of Nitromethane Isomerization in the Course of the Reaction. Kinet. Catal. 2012, 53, 1–12. (10) Wodtke, A. M.; Hintsa, E. J.; Lee, Y. T. Infrared Multiphoton Dissociation of Three Nitroalkanes. J. Phys. Chem. 1986, 90, 3549–3558. (11) Saxon, R. P.; Yoshimine, M. Theoretical Study of Nitro-Nitrite Rearrangement of CH3NO2. Can. J. Chem. 1992, 70, 572–579.



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12) Townsend, D.; Lahankar, S. A.; Lee, S. K.; Chambreau, S. D.; Suits, A. G.; Zhang, X.; Rheinecker, J.; Harding, L. B.; Bowman, J. M. The Roaming Atom: Straying from the Reaction Path in Formaldehyde Decomposition. Science 2004, 306, 1158–1161. (13) Suits, A. G. Roaming Atoms and Radicals: a New Mechanism in Molecular Dissociation. Acc. Chem. Res. 2008, 41, 873–881. (14) Homayoon, Z.; Bowman, J. M. Quasiclassical Trajectory Study of CH3NO2 Decomposition via Roaming Mediated Isomerization Using a Global Potential Energy Surface. J. Phys. Chem. A 2013, 117, 11665–11672. (15) Dey, A.; Fernando, R.; Abeysekera, C.; Homayoon, Z.; Bowman, J. M.; Suits, A. G. Photodissociation Dynamics of Nitromethane and Methyl Nitrite by Infrared Multiphoton Dissociation Imaging with Quasiclassical Trajectory Calculations: Signatures of the Roaming Pathway. J. Chem. Phys. 2014, 140, 054305. (16) Sivaramakrishnan, R.; Michael, J. V.; Klippenstein, S. J. Direct Observation of Roaming Radicals in the Thermal Decomposition of Acetaldehyde. J. Phys. Chem. A 2010, 114, 755–764. (17) Sivaramakrishnan, R.; Michael, J. V.; Harding, L. B.; Klippenstein, S. J. Shock Tube Explorations of Roaming Radical Mechanisms: the Decompositions of Isobutane and Neopentane. J. Phys. Chem. A 2012, 116, 5981–5989. (18) Zhu, R. S.; Lin, M. C. CH3NO2 Decomposition/Isomerization Mechanism and Product Branching Ratios: an Ab Initio Chemical Kinetic Study. Chem. Phys. Lett. 2009, 478, 11– 16. (19) Zhu, R. S.; Raghunath, P.; Lin, M. C. Effect of Roaming Transition States Upon Product Branching in the Thermal Decomposition of CH3NO2. J. Phys. Chem. A 2013, 117, 7308– 7313. (20) Bendtsen, A. B.; Glarborg, P.; Dam-Johansen, K. Low Temperature Oxidation of Methane: the Influence of Nitrogen Oxides. Combust. Sci. Tech. 2000, 151, 31–71. (21) Faravelli, T.; Frassoldati, A.; Ranzi, E. Kinetic Modeling of the Interactions between NO and Hydrocarbons in the Oxidation of Hydrocarbons at Low Temperatures. Combust. Flame 2003, 132, 188–207. (22) Biggs, P.; Canosa-Mas, C. E.; Fracheboud, J-M.; Parr, A. D.; Shallcross, D. E.; Wayne, R. P.; Caralp, F. Investigation into the Pressure Dependence between 1 and 10 Torr of the Reactions of NO2 with CH3 and CH3O. J. Chem. Soc., Faraday Trans. 1993, 89, 4163– 4169.


Page 20 of 29

Page 21 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Yamada, F.; Slagle, I. R.; Gutman, D. Kinetics of the Reaction of Methyl Radicals with Nitrogen Dioxide. Chem. Phys. Lett. 1981, 83, 409–412. (24) Wollenhaupt, M.; Crowley, J. N. Kinetic Studies of the Reactions CH3 + NO2 → Products, CH3O + NO2 → Products, and OH + CH3C(O)CH3 → CH3C(O)OH + CH3, over a Range of Temperature and Pressure. J. Phys. Chem. A 2000, 104, 6429–6438. (25) Srinivasan, N. K.; Su, M.-C.; Sutherland, J. W.; Michael, J. V. Reflected Shock Tube Studies of High-Temperature Rate Constants for OH + CH4 → CH3 + H2O and CH3 + NO2 → CH3O + NO. J. Phys. Chem. A 2005, 109, 1857–1863. (26) Matsugi, A.; Shiina, H.; Oguchi, T.; Takahashi, K. Time-Resolved Broadband Cavity-Enhanced Absorption Spectroscopy behind Shock Waves. J. Phys. Chem. A 2016, 120, 2070–2077. (27) Röhrig, M.; Petersen, E. L.; Davidson, D. F.; Hanson, R. K. A Shock Tube Study of the Pyrolysis of NO2. Int. J. Chem. Kinet. 1997, 29, 483–493. (28) Tsang, W.; Herron, J.T. Chemical Kinetic Data Base for Propellant Combustion. I. Reactions Involving NO, NO2, HNO, HNO2, HCN and N2O. J. Phys. Chem. Ref. Data

1991, 20, 609–663. (29) Huffman, R. E.; Davidson, N. Shock Waves in Chemical Kinetics: the Thermal Decomposition of NO2. J. Am. Chem. Soc. 1959, 81, 2311–2316. (30) Goos, E.; Burcat, A.; Ruscic B. Ideal Gas Thermochemical Database with Updates from Active Thermochemical Tables. http://burcat.technion.ac.il/dir/ (accessed Dec 19, 2016). (31) Goodwin, D. G.; Moffat, H. K.; Speth, R. L. Cantera: An object oriented software toolkit for chemical kinetics, thermodynamics, and transport processes, version 2.1.1; Caltech: Pasadena, CA, 2014. (32) Su, M.-C.; Kumaran, S. S.; Lim, K. P.; Michael, J. V.; Wagner, A. F.; Harding, L. B.; Fang, D.-C. Rate Constants, 1100 ≤ T ≤ 2000 K, for H + NO2 → OH + NO Using Two Shock Tube Techniques: Comparison of Theory to Experiment. J. Phys. Chem. A 2002, 106, 8261–8270. (33) Vasudevan, V.; Davidson, D. F.; Hanson, R. K. Direct Measurements of the Reaction OH + CH2O → HCO + H2O at High Temperatures. Int. J. Chem. Kinet. 2005, 37, 98–109. (34) Oehlschlaeger, M. A.; Davidson, D. F.; Hanson, R. K. High-Temperature Ethane and Propane Decomposition. Proc. Combust. Inst. 2005, 30, 1119–1127.



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(35) Lockhart, J. P. A.; Goldsmith, C. F.; Randazzo, J. B.; Ruscic, B.; Tranter, R. S. An Experimental and Theoretical Study of the Thermal Decomposition of C4H6 Isomers. J. Phys. Chem. A 2017, DOI: 10.1021/acs.jpca.7b01186. (36) Gilbert, R. G.; Smith, S. C. Theory of Unimolecular and Recombination Reactions; Blackwell: Oxford, U.K., 1990. (37) Miyoshi, A. SSUMES, revision 2010.05.23m2; The University of Tokyo: Tokyo, Japan, 2010. (38) Forst, W. Unimolecular Reactions: A Concise Introduction; Cambridge University Press: Cambridge, UK, 2003. (39) Chai, J-D.; Head-Gordon, M. Long-Range Corrected Hybrid Density Functionals with Damped Atom-Atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615– 6620. (40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2010. (41) Alecu, I. M.; Zheng, J.; Zhao, Y.; Truhlar, D. G. Computational Thermochemistry: Scale Factor Databases and Scale Factors for Vibrational Frequencies Obtained from Electronic Model Chemistries. J. Chem. Theory Comput. 2010, 6, 2872–2887. (42) Matsugi, A.; Shiina, H. Kinetics of Hydrogen Abstraction Reactions from Fluoromethanes and Fluoroethanes. Bull. Chem. Soc. Jpn. 2014, 87, 890–901.


Page 22 of 29

Page 23 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Image



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Absorption cross sections of NO2 at high temperature. The circle and square symbols are the present and literature experimental data points, respectively. The solid and dashed lines denote the representation given by eq. 1 and its 95% CI, respectively. 70x67mm (600 x 600 DPI)

ACS Paragon Plus Environment

Page 24 of 29

Page 25 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Time profiles of NO2 concentration observed during the decomposition of nitromethane behind reflected shock waves. The dashed lines are the fitted profiles. 82x89mm (600 x 600 DPI)



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Sensitivity for NO2 concentration under the experimental conditions shown in Fig. 2. 88x106mm (600 x 600 DPI)


Page 26 of 29

Page 27 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure. 4. Arrhenius plot of the present and literature6 rate constants for the thermal decomposition of nitromethane. The lines represent the result of the master-equation calculation at pressures of 3500, 410, 150, 100, 52, and 27 kPa from top to bottom. 69x65mm (600 x 600 DPI)



1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The present and literature2,7,22,23,25 rate constants for R2. 70x65mm (600 x 600 DPI)


Page 28 of 29

Page 29 of 29

1 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 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Image 43x26mm (600 x 600 DPI)


Thermal Decomposition of Nitromethane and Reaction between CH3

Recommend Documents