On the hypergolicity of trimethyl aluminum in air

Journal Pre-proofs On the Hypergolicity of Trimethyl Aluminum in Air Tien V. Pham, Hue M.T. Nguyen, M.C. Lin PII: DOI: Reference:

S2210-271X(19)30364-0 https://doi.org/10.1016/j.comptc.2019.112668 COMPTC 112668

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Computational & Theoretical Chemistry

Received Date: Revised Date: Accepted Date:

26 September 2019 15 November 2019 2 December 2019

Please cite this article as: T.V. Pham, H.M.T. Nguyen, M.C. Lin, On the Hypergolicity of Trimethyl Aluminum in Air, Computational & Theoretical Chemistry (2019), doi: https://doi.org/10.1016/j.comptc.2019.112668

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On the Hypergolicity of Trimethyl Aluminum in Air Tien V. Pham,a,c Hue M. T. Nguyen,d M. C. Lina,b* a Department bCenter

of Appl. Chem., National Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 30010, Taiwan.

for Emergent Functional Matter Science, National Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 30010,

Taiwan. c

School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi 100000, Vietnam

d Faculty

of Chemistry and Center for Computational Science, Hanoi National University of Education, Hanoi 100000,

Vietnam

Abstract –A theoretical study on the mechanisms and kinetics of the reaction of (CH3)3Al (TMA), an important industrial compound, with O2 has been carried out at the CCSD(T)/6311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level in conjunction with the conventional transition state theory (TST) calculations. The potential energy surface (PES) of the reaction indicates that the TMA + O2 system has two pathways leading to different product pairs: the first one passes through a tight transition state with a high energy barrier, 16.9 kcal/mol, producing (CH3)2AlO2 + CH3 and the other one goes via a loose roaming-like transition state with a much lower energy barrier, 6.8 kcal/mol, yielding CH3Al(O)OCH3 + CH3. The barrier predicted for the former was found to be too high for combustion initiation under the ambient condition. The latter, however, may play a key role in initiating the hypergolic reaction of TMA in the air. Rate constants for both channels have been calculated for the temperature range of 300 – 2000 K. The pressure-independent rate constant for the TMA + O2 reaction via the loose transition state is predicted to be k(T) = 3.05  10-21T3.003 exp(3226.1/T) cm3 molecule-1 s-1; this result gives the half-life of TMA in air under the ambient condition to be as short as 1.7 x 10-2 s, which is sufficiently short for the hypergolic combustion initiation without even considering the ensuing rapid radical chain reactions.

Keywords: (CH3)3Al, O2, CH3Al(O)OCH3, Hypergolic phenomenon, Potential energy surface. *Corresponding authors: M. C. Lin, email address: [email protected] 1

Introduction Trialkyl aluminums are known to be pyrophoric, highly reactive toward oxygen and water; they have to be handled with extreme care in laboratory using a glove box filled with an inert gas such as N2. The reaction of a small trialkyl aluminum such as (C2H5)3Al with O2 has been used as a rocket engine igniter, for example [1]. Trimethyl aluminum, which is widely used in industry, has been classified by NFPA (National Fire Protection Agency) as a level 4 flammable chemical; its explosion in air has been termed as “among the most notorious fireballs in all of chemistry” [2]. We have recently attempted to elucidate computationally the mechanism responsible for the pyrophoric or hypergolic property of trialkyl aluminums toward O2 and H2O using trimethyl aluminum (TMA) as a model [3]. Although its reaction with H2O was predicted to occur with a negligible barrier and quite exothermic, its molecular products, CH4 + (CH3)2AlOH, are both very stable and cannot be responsible for the known hypergolic reaction of TMA in air. Its reaction with O2 producing CH3 + (CH3)2AlO2 radical products was predicted to be endothermic (8.2 kcal/mol) at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level of theory [3], the barrier for the reaction was shown to be too high (16.9 kcal/mol) to react at room temperature, however [4]. In the TMA + O2 reaction, the 16.9 kcal/mol activation barrier for the O2-for-CH3 substitution step may be attributed to the forced extension of one of the CH3-Al bonds from 1.965 Ǻ to 2.119 Ǻ at the transition state (TS) by the approaching O2 [3]. In the current study, we have attempted to adopt an alternative approach following the new mechanism, which we discovered some time ago for the isomerization of CH3NO2 to CH3ONO [5] by the concerted rotation of the NO2 group in concomitant with the stretching of the CH3-N bond. The roaming-like loose transition state with 59.2 kcal/mol activation energy, slightly lower than the C-N dissociation energy (59.8 kcal/mol), was found to be several kcal/mol lower than the conventional isomerization barrier, lying 6 kcal/mol above the dissociation limit predicted at the CCSD(T)/cc-PVTZ//CCSD(T)/cc-PVDZ level of theory [6]. Notably, the concurrent stretching2

rotating, roaming-like loose transition state was found to be responsible for the direct hydrolysis of N2O4 via the ONONO2-H2O reaction [7], as well as for the spontaneous ignition of the N2O4RHNNH2 (R=H, CH3) systems via the ONONO2-RHNNH2 reactions [8], following the roaminglike N2O4-ONONO2 isomerization during the bimolecular collision with H2O or RHNNH2 as a spectator. By means of the same approach, we have attempted to manually lengthen one of the CH3-Al bonds stepwise and optimize the geometry at each step allowing the O2 molecule to freely adjust and approach the Al atom, whose electron density is gradually depleted as the CH3 group is pulling away. The Π-electron of the O2 molecule provides the need, forming a loose, cyclic –O-O-Al– transition structure with a considerably lower barrier, 6.8 kcal/mol, which is 10 kcal/mol lower than the direct O2-for-CH3 substitution barrier cited above. The new mechanism giving rise to the CH3 + CH3Al(O)OCH3 radical products with the 6.8 kcal/mol energy barrier may be effectively attributed to the vibrational activation of a CH3-Al bond, unlike that of the direct O2-for-CH3 substitution process via the tight TS cited above. The result of this novel reaction mechanism presented herein together with the predicted rate constant for the TMA + O2 reaction is critical to our understanding of the hypergolicity of TMA in air.

2. Computational methods We have characterized the mechanism for the TMA + O2 initiation reaction by quantumchemical calculations using several methods, including B3LYP [9-11], BHandHLYP [12,13], M062X [14-16], and MP2 [17] with basis sets 6-311++G(3df,2p) [18] and aug-cc-pVTZ [19-21] to optimize geometries of all species involved by means of the Gaussian 16 package [22]. In this paper, we only utilize the geometries optimized by the B3LYP/6-311++G(3df,2p) for establishment of the potential energy surface (PES) and the prediction of its kinetics. The zero-point energy (ZPE) 3

corrections to the relative energies were also carried out at the B3LYP/6-311++G(3df,2p) level with a scaling factor of 0.985 [23]. In order to further improve the relative energies between stationary structures, single-point energy calculations were finally computed using the coupled-cluster level of molecular orbital theory, (U)CCSD(T)/6-311++G(3df,2p) [24,25], incorporating all the single and double excitations plus perturbative corrections for the triple excitations. On account of the shallow wells in the pre-reaction complex, 1.9 kcal/mol, and the post-reaction complex, 3.8 kcal/mol (based on the PES presented in detail below), the effect of pressure on the system is expected to be negligible. The transition state theory [26] is therefore sufficient for a reliable prediction of the rate constant; we employed the code provided the CHEMRATE program [27]. 3. Results and discussion In this system, the B3LYP/6-311++G(3df,2p) optimized geometries of the reactants, intermediates, transition states and products for the initiation process via the loose roaming-like TS1 and the tight TS0 are summarized in Fig. 1. Those optimized structures by other methods are displayed in Fig. S1 of the ESI. For comparison, their energies predicted at different levels are presented in Table 1, and their frequencies are listed in Table S1 while the Cartesian coordinates of the species involved are listed in Table S2 of the ESI. The full PES predicted at the UCCSD(T)/6311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level is presented in Fig. 2, in which we compare the two channels predicted at the same level of theory for the tight [3] and loose transition states TS0 and TS1, respectively. The structure of TS1 was obtained by manually stretching one of the CH3-Al bonds with the concerted approaching of the O2 molecule through full optimization of the geometry at each step as alluded to in the Introduction. In addition, Fig. S2 presents the IRC profile for the loose roaming-like TS1 predicted at the B3LYP/6-311++G(d,p) level. The result reveals that TS1 is the accurate transition state connecting the pre-reaction complex with the intermediate (CH3)2Al(O)OCH3, lying 38.4 kcal/mol below the reactants. 4

To ascertain the reliability of our single referenced results, we have carried out the T1 diagnostic analysis [28] to measure approximately the multireference character in the wave function. The results for the reactants, products, transition states, and pre-/post-complexes of the title reaction are displayed in Table S3 of the ESI section. The values show that the T1 diagnostics of all species do not have significant multireference character because all the values are less than 0.02 (the threshold for T1 diagnostic of a closed-shell species is 0.02 while those of open-shell species is ~0.045) [29], which indicate that the single-reference methods can be reliably employed in the present study. Moreover, it can be seen from Table S3, the spin contamination values for species in the triplet states are ~ 2.009 while those for species in the doublet states are round 0.75, indicating that the spin contamination influence on the estimation of activation barriers and the structures of all species is negligible.

Both (CH3)2AlO2 + CH3 and CH3Al(O)OCH3 + CH3 product pairs are formed by the O2-forCH3 substitution by very different mechanisms as aforementioned. The first product pair is formed by the kinetic attack of O2 at TMA via the tight TS0 to overcome its 16.9 kcal/mol barrier as depicted in Fig. 2. The full description of the computational detail can be found in Ref. 3. The second product pair, however, is formed via the loose TS1, effectively resulting from the vibrational excitation, namely, the stretching of one of the three CH3-Al bonds with 6.8 kcal/mol energy, resulting in as much as 10 kcal/mol lower energy barrier than that of the former process, as shown in Fig. 2. All of the products are in the doublet state while the complex structure, loose transition state and the intermediate formed in this process, are in the triplet state. The CH3 radical thus formed is a potentially reactive species, which may initiate a chain reaction in air. The relative energies of the stationary points predicted are summarized in Table 1. The values obtained by CCSD(T)//B3LYP/6-311++G(3df,2p), CCSD(T)//BHandHLYP/6-311++G(3df,2p), CCSD(T)//M06 -2X/aug-cc-pVTZ, and CCSD(T)//MP2/6-311++G(3df,2p) calculations are qualitatively similar. 5

Figure 2 shows that the pre-reaction complex, (CH3)3Al:O2, is formed before going through the loose transition state. This complex is easily formed through the interaction between Al and O atoms with a small 1.9 kcal/mol binding energy and 3.016 Ǻ Al∙∙∙O bond length. The (CH3)3Al:O2 complex has C1 symmetry; the angle C-Al-C changes very slightly from 120º in TMA to 119.87º in the complex, and the Al-C bond lengths change somewhat differently, two of them increase slightly from 1.965 to 1.967 Ǻ, while the other to 1.966 Ǻ. At TS1, (CH3)2AlO2∙∙∙CH3, when the molecular oxygen moves closer toward TMA as one of the three methyl groups is pulling away at the distance of 3.89 Ǻ, the departing CH3 begins to bond with one of the oxygen atoms as the system goes over the barrier, resulting in the breaking of the O-O bond to give rise to the (CH3)2Al(O)OCH3 intermediate with the 38.4 kcal/mol exothermicity cited above. At the CCSD(T)/6-311++G(3df,2p) + ZPE level of theory, TS1 lies 8.7 kcal/mol above the (CH3)3Al:O2 complex or 6.8 kcal/mol above the reactants as alluded to above. The (CH3)2Al(O)OCH3 intermediate can fragment readily and barrierlessly to give the products CH3Al(O)OCH3 + CH3 with 34.6 kcal/mol overall exotherimicty, significantly different from the tight O2-for-CH3 substitution process producing (CH3)2AlO2 + CH3 with 8.2 kcal/mol overall endothermicity (see Fig. 2). The predicted structures provide useful information on the hypergolic reaction mechanism of TMA in air. The O–O bond and one of the Al–C bonds are elongated by 0.145 Å and 2.125 Å when going from the complex to the loose transition state. The two Al-O bonds are calculated to be about 1.900 and 1.881 Å in the TS1 structure. The distances of Al∙∙∙C and O∙∙∙C are estimated to be about 3.891 and 3.326 Å, respectively, suggesting that this step may be defined as the critical motion of the roaming transition state. The (CH3)2Al(O)OCH3 intermediate obtained is a triplet species, in which the O–O bond is broken and two Al–C bond lengths are 1.966 and 2.340 Å, respectively. This means that the 2.340 Å Al-C bond, is much weaker than the other Al-C bond; the structure of the final product, CH3Al(O)OCH3, are close to those of the corresponding moiety of the intermediate structure. The Al–C bond length is somewhat shortened only by 0.026 Å in 6

CH3Al(O)OCH3, and the distances of Al–O and Al-OCH3, 1.746 and 1.682 Å, respectively, can be compared to 1.763 and 1.702 Å in the intermediate structure; while the Al-O-C angle increases somewhat from 139º in the intermediate to 141º in the final product. It is clear that the CH3 group can be readily expelled from (CH3)2Al(O)OCH3, which is essentially the post-reaction complex of the final products, CH3Al(O)OCH3 + CH3, with 3.8 kcal/mol binding energy.

As mentioned in the Computational Methods section, different methods such as the DFT with different extents of hybridization (B3LYP, BHandHLYP, and M06-2X) and MP2 were also employed to compute the whole PES of the reaction between TMA and O2 using different basis sets, such as 6-311++G(3df,2p) and aug-cc-pVTZ. Notably, the predicted energies by BHandHLYP with the 6-311++G(3df,2p) basis set for the (CH3)2Al(O)OCH3 intermediate, -46.8 kcal/mol, and the CH3Al(O)OCH3 + CH3 products, -44.2 kcal/mol, are somewhat lower than those by the other methods; the energy for TS1 by BHandHLYP is only slightly lower than those by B3LYP but a little higher than values by M06-2X and MP2 (see Table 1). The small difference in the relative energies of TS1 by three methods supports the existence of the TS1 for the TMA + O2 reaction. It should also be noted that the structure of TS1 has been confirmed by IRC analysis with the B3LYP/6-311++G(d,p) method, along the forward direction from the reactants to the products as well as the reverse direction from the products back to the reactants (see Fig. S2). For the higher level UCCSD(T) single point calculations, the geometries were re-optimized with B3LYP/6311++G(3df,2p), BHandHLYP/6-311++G(3df,2p), MP2/6-311++G(3df,2p), and M06-2X/aug-ccpVTZ and employed for the calculations; they gave essentially the same values for TS1 and the overall heat of reaction as shown in Table 1. 4. Rate Constant Calculations The rate constant for the TMA + O2 reaction has been computed on the basis of the PES obtained at the CCSD(T)//B3LYP/6-311++G(3df,2p) level of theory. On grounds that the binding 7

energies of both pre-reaction ((CH3)3Al:O2) and post-reaction ((CH3)2Al(O)OCH3) complexes, 1.9 and 3.8 kcal/mol, respectively, are small as aforementioned (see Fig. 2), the presence of these complexes along the reaction path is inconsequential to the kinetics of the initiation process; i.e., no pressure effect is expected from their presence. The rate constant for the initiation reaction has, therefore, been calculated by the TST [26] using the CHEMRATE program [27] based on the mechanism as follows: TMA + O2 → TS1 → CH3Al(O)OCH3 + CH3. The pressure-independent second-order rate constants for TMA + O2 → CH3Al(O)OCH3 + CH3 computed for the temperature range 300-2000 K are graphically shown in Fig. 3, in comparison with the result predicted for the process occurring via the tight transition state TS0 [3]. It can be seen from Fig. 3 and Table 2 that the results for the initiation reaction via the loose, roaming-like TS1 are many orders of magnitude greater than the values predicted for the tight transition TS0 path [3] throughout the entire temperature range. For example, the rate constants at 300 and 1000 K are 1.83 × 10-27 and 2.13 × 10-18 cm3 molecule-1 s-1 for the TS0 path and 1.68 × 10-18 and 1.31× 10-13 cm3 molecule-1 s-1 for the TS1 path, respectively. A least-squares analysis of the result for the TS1 path covering the temperature range 300 – 2000 K presented with the modified Arrhenius form, can be given by the expression k(T) = 3.05  10-21T3.003 exp(-3226/T) cm3 molecule-1 s-1. On account of the very loose structure of TS1 with a broader transition state peak, we have also examined the potential effect of the saddle-point position change at varying temperatures on the predicted rate constants, based on the maximum Gibbs free energy criterion of the canonical TST (or CVT) [26]. To calculate rate constants by CVT, 10 reaction coordinate points were selected around TS1 along the minimum energy path (MEP) with 5 points on the reactant side and the other 5 points on the product side optimized at the B3LYP/6-311++G(d,p) level. Single point energy evaluations were made with the CCSD(T)/6-311++G(3df,2p) method. The CVT rate constants

8

(kCVT) predicted for 300 – 2000 K are presented in Table S4 and compared with the TST results (kTST) in Fig. S3. The values obtained by both methods agree closely over the whole temperature range except the low and high temperature ends. At 300 K, the former is slightly bigger, kCVT = 1.4 × kTST while at 2000K, the former becomes slightly smaller, kCVT = 0.6 × kTST. The result reflects the fact that the position of the saddle-point at TS1 moves toward the reactant side at low temperature, whereas at high temperature it moves forward to the product side. Based on the calculated rate constants of the initiation reaction via the loose TS1, the hypergolicity of the reaction can be examined. For example, the half-life of TMA in air under the ambient condition (300 K at atmospheric pressure) is estimated to be t1/2(TMA) = 0.693/k(300) [O2] = 1.7  10-2 s, which is sufficiently short for the exothermic initiation process. Inclusion of the ensuing exothermic chain propagation reactions involving very rapid TMA + radicals (CH3OO, HO2, HO, O(3P)) reactions (T. V. Pham, M. C. Lin, unpublished results) would make the initiation process even more drastic. It should also be mentioned that the CH3Al(O)OCH3 radical formed in the initiation process can fragment readily producing more reactive species CH3AlOH + CH2O (N. T. Nguyen, M. C. Lin, unpublished results).

5. Conclusion In this work, a combined quantum-chemical and kinetic study has been carried out for the reaction of TMA, an important industrial reagent, with O2 which plays an important role in the hypergolic combustion of TMA in air. The reactants, intermediate states, transition states, and products of these reactions have been optimized by density functional theory at the B3LYP level with the 6-311++G(3df,2p) basis set. Energies for all the species have been refined at the UCCSD(T)//6-311++G(3df,2p) level. All the quantum calculations were performed by using the

9

Gaussian 16 program package [22]. The pressure-independent rate constant for product formation has also calculated at different temperatures between 300 and 2000 K. For the TMA + O2 initiation reaction, a novel exothermic reaction pathway via a roaming-like transition state with the energy barrier as low as 6.8 kcal/mol producing the reactive product pair, CH3Al(O)OCH3 + CH3, has been discovered. The barrier is lower than that reported previously [3] for the endothermic process occurring via a tight transition state giving (CH3)2AlO2 + CH3 by as much as 10 kcal/mol. The rate constant predicted for the new reaction, k(T) = 3.05  10-21T3.003 exp(-3226/T) cm3 molecule-1 s-1, is many orders of magnitude faster than that by the tight TS. The predicted half-life of TMA in air under the ambient condition, 1.7 x 10-2 s, can convincingly account for the hypergolicity of TMA in air without even invoking the ensuing highly exothermic and very rapid TMA + radical reactions.

10

ACKNOWLEDGMENTS This work is supported by Ministry of Science and Technology, Taiwan (grant No. MOST1073017-F009-003) and Ministry of Education, Taiwan (SPROUT Project-Center for Emergent Functional Matter Science of National Chiao Tung University). The authors would like to thank Dr. T. N. Nguyen for the discussion on the result for fragmentation of the CH3Al(O)OCH3 radical formed in the title reaction. We also acknowledge the National Center for High-Performing Computers for the use of its facility.

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Table 1. Theoretical predication of relative energies ΔE (kcal/mol) for reactants, intermediates, transition states, and products of the TMA + O2 reaction via the loose transition state TS1. TMA + O2

Precomplex

TS1

(CH3)2Al(O)OCH3

CH3Al(O)OCH3 + CH3

0.0

0.0

8.0

-38.0

-34.6

0.0

-1.9

6.8

-38.4

-34.6

0.0

-0.3

7.6

-46.8

-44.2

0.0

-2.0

6.7

-39.8

-35.1

M06-2X/aug-cc-pVTZ

0.0

-2.0

6.3

-42.0

-35.4

CCSD(T)//M06-2X/aug-cc-pVTZ

0.0

-1.5

6.8

-38.8

-34.1

MP2/6-311++G(3df,2p)

0.0

-1.0

6.4

-35.7

-31.6

CCSD(T)//MP2/6-311++G(3df,2p)

0.0

-1.3

6.9

-39.0

-34.2

Species Levels B3LYP/6-311++G(3df,2p) CCSD(T)/6-311++G(3df,2p)// B3LYP/6-311++G(3df,2p) BHandHLYP/6-311++G(3df,2p) CCSD(T)/6-311++G(3df,2p)// BHandHLYP/6-311++G(3df,2p)

Table 2. The calculated TST rate constants of the TMA + O2 reaction via tight and loose transition states, k (cm3 molecule-1 s-1). T (K) 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

k0 via tight TS0 1.83 × 10-27 2.11 × 10-24 1.68 × 10-22 3.41 × 10-21 3.15 × 10-20 1.75 × 10-19 6.90 × 10-19 2.13 × 10-18 5.49 × 10-18 1.23 × 10-17 2.47 × 10-17 4.54 × 10-17 7.78 × 10-17 1.26 × 10-16 1.93 × 10-16 2.85 × 10-16 4.06 × 10-16 5.61 × 10-16

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k1 (TST) via loose TS1 1.68 × 10-18 5.87 × 10-17 5.90 × 10-16 3.08 × 10-15 1.08 × 10-14 2.94 × 10-14 6.64 × 10-14 1.31 × 10-13 2.34 × 10-13 3.87 × 10-13 5.99 × 10-13 8.80 × 10-13 1.24 × 10-12 1.69 × 10-12 2.23 × 10-12 2.87 × 10-12 3.61 × 10-12 4.45 × 10-12

O2

TMA pre-Complex (CH3)3Al:O2

TS1

TS0

(CH3)2Al(O)OCH3

(CH3)3AlO2

CH3

CH3Al(O)OCH3

(CH3)2AlO2

Figure 1. Geometry of the reactants, intermediates, and products involved in the TMA + O2 reaction optimized at the B3LYP/6-311++G(3df,2p) level of theory. Bond angles and bond lengths are in degree (o) and anstroms (Å), respectively.

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16.9a TS0 8.2a (CH3)2AlO2 + CH3

6.8

a

5.8

TS1

(CH3)3AlO2

0.0 (CH3)3Al + O2

-1.9 (CH3)3Al...O2 pre-reaction complex

-34.6 -38.4

CH3Al(O)OCH3 + CH3

(CH3)2Al(O)OCH3 post-reaction complex

Figure 2. The schematic diagram of the potential energy surface for the TMA + O2 reaction computed at the UCCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) + ZPE level (energies are in kcal/mol). a The values in italic are from Ref. 3.

Figure 3. The predicted TST rate constants for the TMA + O2 reaction in the temperature range of 300 – 2000 K.

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Author Contribution Statement M. C. Lin: Visualization, Supervision, Validation, Writing- Reviewing and Editing, Project administration. Hue M. T. Nguyen: Conceptualization, Methodology, Investigation, Resources. Tien V. Pham: Software, Writing- Original draft preparation, Data curation.

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Declaration of interests

✓The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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A graphical Table of Contents

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Highlights 

Trimethyl aluminum, (CH3)3Al (TMA), is known to be pyrophoric, highly reactive toward oxygen and water.



The mechanism of TMA with O2 in the gas phase was studied.



The TMA + O2 reaction proceeds via a roaming-like transition state.



The pressure-independent rate constants of the TMA + O2 reaction were predicted at T = 300 – 2000 K.



The half-life of TMA in air, 1.7 × 10−2 s, is sufficiently short for the exothermic initiation process.

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