Thermochemistry and Kinetic Studies on the Autoignition of 2

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Thermochemistry and Kinetic Studies on the Autoignition of 2-Butanone: A Computational Study Saravanan Kuzhanthaivelan, and Balla Rajakumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05167 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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The Journal of Physical Chemistry

Thermochemistry and kinetic studies on the autoignition of 2-butanone: A computational study S. Kuzhanthaivelan and B. Rajakumar* Department of Chemistry, Indian Institute of Technology, Madras, Chennai-600036, India. *Address for correspondence: [email protected] http://chem.iitm.ac.in/faculty/rajakumar/ http://www.profrajakumar.com

ABSTRACT Unimolecular

reactions

of

alkylperoxy(ROO•),

hydroperoxyalkyl(•QOOH)

and

hydroperoxyalkylperoxy(•OOQOOH) radicals of 2-butanone, which is a potential biofuel molecule, have been studied computationally. These radicals are responsible for the chain branching at low temperature oxidation and play a significant role in modeling the autoignition. The composite CBS-QB3 method was used to study the thermochemistry and energetics of all the species involved. Intrinsic reaction coordinate (IRC) calculations were carried out for all the transition states along various reaction pathways. All the possible reactions like H-migration, •

OH elimination and HO•2 elimination reactions were studied for these radicals. It was found

that, the isomerization of •OOQOOH to HOOQOO• is the most favourable channel, which involves 8- and 9-membered cyclic transition states. However, the decomposition pathway involves the H-migration from carbon to oxygen. The mechanism for the decomposition of all •

OOQOOH radicals with their potential energy level diagrams are reported. The temperature

dependent rate coefficients were also studied using Canonical Variational Transition state theory (CVT) with small curvature tunneling (SCT) in the temperature range of 400 – 1500 K, which is relevant to the combustion. Thermodynamic parameters for all the reactions involved were 1 ACS Paragon Plus Environment

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calculated. The high barrier (1,3 H-migration) reactions were found to be exothermic and spontaneous, which is unexpected.

1. INTRODUCTION Ketones play a significant role in both atmospheric and combustion chemistry.1 The increasing demand for the traditional fossil fuels forces the transportation sector to search for alternatives. At the same time, CO2 emissions resulting from the combustion of fossil fuels have to be reduced to limit the global warming. Biofuels have the advantage of closing the carbon cycle.1 However, well selected biofuels have the tendency to reduce the CO2 emission by both direct (reduced CO2 in a well-to-wheel evaluation) and indirect (improvement in engine efficiency) ways.1 Investigations have been carried out by researchers globally to identify and develop the biofuels, which exhibit properties for engine applications. New biofuels are being developed and investigated in the Cluster of Excellence ‘‘Tailor-Made Fuels from Biomass” (TMFB) at RWTH Aachen University. One such biofuel molecule is methyl ethyl ketone (MEK) or 2-butanone.2 Hoppe et al.1,2 had studied the potential of 2-butanone in direct injection spark ignition engine and reported a significant reduction of particle emissions compared to conventional gasoline. Moreover, Badra et al.3 and Serinyel et al.4 measured the ignition delay times of 2butanone by shock tube studies over the temperature range of 1100 to 1850 K. Badra et al.3 also studied the kinetics of 2-butanone with •OH radical over the temperature range from 950 to 1400 K. In addition to this, Arzamendi et al.5 studied the combustion of 2-butanone in air over Pt/Al2O3 catalyst. Also, Mendes et al.6 studied the hydrogen abstraction reactions of 2-butanone and other ketones with HO•2 radical using theoretical methods. Elena Jimenez et al.7 experimentally studied the tropospheric reaction of OH radical with 2-butanone and other 2 ACS Paragon Plus Environment

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selected linear ketones over a temperature range of 228 to 405 K. Hudzik and Bozelli8 studied the thermochemistry and bond dissociation energies of 2-butanone and other ketones for their importance in combustion and atmospheric chemistry. Lam et al.9 measured the overall rate constants for the reactions of OH radicals with 2butanone and other ketones behind reflected shock waves over the temperature range of 8701360 K. Zhou et al.10 also studied the reaction of 2-butanone and other ketones with OH radicals by theoretical methods in the temperature range between 500 to 2000 K. Furthermore, Kopp et al.11 predicted the kinetics for H-atom abstraction from 2-butanone by H atom and •CH3 radical and the subsequent unimolecular reactions of 2-butanoyl radical using ab initio methods. In addition to this, Lam et al.12 investigated the high temperature pyrolysis of 2-butanone behind reflected shock waves using multi-species time-history measurements. Hemken et al.13 investigated the combustion-related intermediates formed through 2-butanone oxidation in laminar flame. Burke et al.14 measured the ignition delay times of 2-butanone by shock tube and developed a kinetic model which includes both experimental and computational data. The model includes different classes of reaction in both high and low temperature combustion. Thion et al.15 studied the oxidation of 2-butanone in a jet-stirred reactor and developed a model based on theoretical calculations for the fuel decomposition, H abstraction, radicals decomposition and oxidation. Hemken et al.16 investigated the oxidation of 2-butanone in three different experimental methods (flow reactor, rapid compression machine and a shock tube using laser absorption techniques). Autoignition is a process which occurs spontaneously when a fuel-air (oxidizer) mixture undergo chemical reactions (radical chain reactions) to sustain and stimulate oxidation. These chain reactions lead to increase in radical concentration and hence the reaction rate and 3 ACS Paragon Plus Environment


temperature. The rates of these reactions strongly depend on temperature and pressure. The precise measurement of autoignition times and their dependence on pressure, temperature and composition is significant for advanced engine technologies, where the ignition event is governed by chemical kinetics. Autoignition is also susceptible to chain branching and termination of initial reactions, which depends on the chemical structure of the fuel. The autoignition study of alkanes are relatively better when compared to the oxygenates (e.g ketones, esters, alcohols, ethers, etc.,), which are typical components of biofuels and can show different behavior. Zador et al.17 elaborated on the key reactions involved in autoignition and low temperature combustion which involves two critical class of intermediates namely hydroperoxyalkyl radicals (•QOOH) and hydroperoxyalkylperoxy radicals (•OOQOOH). The chemistry of these radicals play an important role in modeling the autoignition of any fuel in the low temperature regime. The authors also listed all the experimental and theoretical chemical kinetic studies of ROO•, •QOOH and •OOQOOH radicals, which are usually formed by the reactions of different alkyl, cycloalkyl, unsaturated/aromatic and oxygenated radicals with molecular oxygen. Savee et al.18 reported the first direct observation and kinetic measurements of •QOOH intermediate in the oxidation of 1,3-cycloheptadiene. Zador et al.19 directly produced a •QOOH radical (2-hydroperoxy-2-methylprop-1-yl) and experimentally determined the rate coefficients for its unimolecular decomposition and its association reaction with O2. 2-butanone being such an important compound, lack of such studies has prompted us to investigate the kinetics of ROO•, •QOOH and •OOQOOH radicals, which are formed during its combustion. Sebbar et al.20-22 studied the thermochemistry and kinetics for the reactions of three

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carbon centered radicals of 2-butanone (2-butanone-1-, 3- and 4-yl radicals) with oxygen by G3MP2B3, G3 and B3LYP/6-311G(d,p) methods. In the present work, we report the details of the autoignition mechanism of 2-butanone. Our main focus was to study the unimolecular reactions of hydroperoxyalkylperoxy(•OOQOOH) radicals and their subsequent decomposition mechanism for the production of •OH radicals, which are critical in autoignition. Electronic structure calculations were used to study the energetics, thermochemistry of the reactions of alkylperoxy(ROO•), hydroperoxyalkyl(•QOOH) and hydroperoxyalkylperoxy(•OOQOOH) radicals. We report the kinetic parameters, which were calculated using Canonical Variational Transition State Theory (CVT) with small curvature tunneling (SCT) in combination with CBS-QB3 method. The •OOQOOH radical is formed from the addition of O2 to •QOOH. Therefore, we studied the chemistry of •QOOH and ROO• radicals too which were studied previously by Sebbar et al.20-22 using different methodology.

2. COMPUTATIONAL METHODS The electronic structure calculations for all the species involved in our study were performed using the Gaussian 0923 program suite. The composite method CBS-QB324,25 was the method of choice, as it has been widely used in the study of peroxy radical chemistry26-30 for its cost efficiency and accuracy. This method uses the density functional theory (B3LYP/6311G(2d,d,p)) for the geometry optimization and vibrational frequencies. Then the single point energies were calculated using CCSD(T)/6-31+G(d′) and MP4SDQ/CBSB4 level of theories. In this method, the B3LYP/6-311G(2d,d,p) zero-point energy (ZPE) and vibrational frequencies were scaled by a factor of 0.99 and 0.97 respectively. All the optimized geometries of the reactants and products were identified with zero imaginary frequency (NIMG=0) and all the transition states were identified with one imaginary frequency (NIMG=1). The intrinsic reaction 5 ACS Paragon Plus Environment


coordinate (IRC) calculations were carried out using B3LYP/6-311G(2d,d,p) level of theory to verify if the transition state connects with the respective reactants and products. T1 diagnostic test was performed to verify the multireference character in CCSD(T) wave function and the values were less than 0.045, which is the threshold value for the open-shell systems.31-33 We examined spin contamination before and after annihilation for the species involved and the < S2> values for the doublet were from 0.75 to 0.77 before annihilation. This is close to the expected value of the pure doublet state 0.750 after annihilation.33 Therefore, the spin contamination is negligible. The T1 diagnostic values and values are tabulated in Table S-IV of the supporting information. Canonical Variational Transition state theory34-36 (CVT) with small-curvature tunneling3738

(SCT) was used to calculate the temperature dependent rate coefficients using the following

equations. POLYRATE 2008 program39 and GAUSSRATE 2009A40 developed by Truhlar’s group were used to obtain the kinetic parameters.

= min , = ,

,

, −!"#$ = ℎ ∅

where kGT (T, s) and kCVT (T) are the rate coefficients of generalized and canonical variational transition state theories respectively, σ is reaction path degeneracy, kB is Boltzmann’s constant, h is Planck’s constant, T is temperature in Kelvin, sCVT is the reaction coordinate (s) at which canonical variational transition state dividing surface was found. QGT and ΦR are the partition functions of a generalized transition state at ‘s’ and reactants respectively. VMEP(s) is the classical


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potential energy of generalized transition state at ‘s’. The canonical variational transition state is located by maximizing the free energy of activation with respect to ‘s’. The nomenclature for the alkylperoxy (ROO•) radicals is given based on the position of the carbon atoms in 2-butanone. The hydroperoxyalkyl (•QOOH) radicals are named as per their formation from ROO•. For example, •Q1OOH15 represents the hydroperoxyalkyl radical formed by the 1,5 H-migration reaction of R1OO• radical. The hydroperoxyalkylperoxy (•OOQOOH) radicals are named from their parent ROO• radical.

3. RESULTS AND DISCUSSION 3.1 Reactions of alkylperoxy (ROO•) and hydroperoxyalkyl (•QOOH) radicals: The general schematic mechanism for the autoignition of any hydrocarbon or a biofuel is given in Figure 1.17 The reaction of 2-butanone with either •OH, •CH3, H atom or HO•2 radical results in the formation of three carbon centered radicals, via hydrogen atom abstraction. These reactions were earlier studied using both experimental as well as theoretical methods.3,4,6,7 These radicals react with O2 to form alkylperoxy(ROO•) radicals which in turn undergoes isomerization or H-migration reactions to give hydroperoxyalkyl(•QOOH) radicals. The schematic diagram of these reactions is given in Figure 2 in which the formation of favorable •QOOH radicals from their respective alkylperoxy radicals are shown in blue color and circled. The chemistry of these two radicals was reported by Sebbar et al.20-22 using G3MP2B3, G3 and B3LYP/6-311G(d,p) methods. In Table 1 and Table 2, the computed CBS-QB3 energy barriers and reaction energies of the unimolecular reactions of ROO• and •QOOH radicals of this study are compared with the G3MP2B3 studies reported by Sebbar et al.

20-22

Note that, the energy barriers and reaction

energies listed in Table 1 and 2 are relative to the ROO• radicals and •QOOH radicals respectively. 7 ACS Paragon Plus Environment


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RH H2O

alkyl radical

OH

R O2

alkylperoxy radical

direct HO2 elimination

ROO

HO2 + alkene

internal H abstraction hydroperoxyalkyl radical

QOOH O2

second O2 addition hydroperoxyalkylperoxy radical

chain propagation

OH + O-heterocycle

OOQOOH

internal H abstraction

HOOQ-HOOH

ketohydroperoxide

HOOQ-HO + OH

chain branching

OQ-HO + 2 OH Figure 1: Schematic mechanism for oxidation and autoignition chemistry for the hydrocarbon and other biofuels.


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Figure 2: Formation of alkyl and alkylperoxy radicals of 2-butanone along with the different reactions of alkylperoxy radicals. The favorable •QOOH radical formed from each ROO• radical are shown in blue color and circled. These radicals are used for further study.



CBS-QB3 energies for the formation of ROO• radicals is less exothermic by ~1 kcal mol1

compared to the G3MP2B3 energies. For the H-migration reactions of ROO•, CBS-QB3 energy

barriers are similar to the barriers obtained by G3MP2B3 method. For the two H-migration reactions (1,4 and 1,6 H-migrations) of R3OO•, Sebbar et al.22 calculated the barriers using B3LYP/6-311G(d,p) method. For the HO•2 elimination from R2OO•, CBS-QB3 energy barrier is 0.7 kcal mol-1 less than the G3MP2B3 method. The G3MP2B3 value for the HO•2 elimination from R3OO• radical was not reported in the literature so far, to the best of our knowledge. For the R1OO• and R2OO• radicals, 1,5 H-migration is the favorable reaction channel which involves a six membered transition state. 1, 6 H-migration is the dominant reaction channel for the R3OO• radical having 7-membered transition state structure.


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Table 1: Energies of formation (at 0 K, in kcal mol-1) for R-OO• radicals, activation barriers ∆E‡ (at 0 K, in kcal mol-1) and reaction energies ∆E (at 0 K, in kcal mol-1) of H-migration and HO•2 elimination reactions of RmOO• at CBS-QB3 and G3MP2B3 method. The values are related to their respective ROO• radicals. Products formed

Reactions

CBS-QB3 ∆E‡

∆E

G3MP2B3 ∆E‡

∆E

Formation of R1OO• radical

-25.3

-26.6


-24.4

-25.5


-33.6

-34.9

R1OO• reactions 1,3 H-migration

Carbonyl + OH

38.5

-24.1

41.9

-22.3

1,5 H-migration

•

22.7

2.67

22.6

2.3

1,6 H-migration

•

Q1OOH-16

25.6

14.0

29.5

12.8

Carbonyl + OH

36.1

-28.0

38.3

-25.8

Q1OOH-15

R2OO• reactions 1,3 H-migration 1,4 H-migration

•

35.5

14.4

36.7

17.8

1,5 H-migration

•

Q2OOH-15

26.3

7.8

26.6

9.1

Olefin + HO•2

27.4

19.2

28.1

19.6

Carbonyl + OH

41.5

-24.8

45.5

-22.7

HO•2 elimination

Q2OOH-14

R3OO• reactions 1,3 H-migration 1,4 H-migration

•

33.7

6.7

26.6a

6.5

1,6 H-migration

•

Q3OOH-16

22.1

10.0

26.6a

10.6

Olefin + HO•2

26.3

17.6

-

-

HO•2 elimination a

Q3OOH-14

indicates the barrier calculated from B3LYP/6-311G(d,p) level of theory. 11 ACS Paragon Plus Environment


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Table 2: Activation barriers ∆E‡ (at 0 K, in kcal mol-1) and reaction energies ∆E (at 0 K, in kcal mol-1) of •OH and HO•2 elimination reactions of •QOOHs at CBS-QB3 and G3MP2B3 level of theories. The values are related to their respective •QOOH radicals.

CBS-QB3 Reactions

G3MP2B3

∆E‡

∆E

∆E‡

∆E

26.6

-10.1

31.1

-6.5

•

OH elimination

13.0

-35.8

17.8

-32.7

HO•2 elimination

49.0

6.2

•

OH elimination

13.9

-16.0

12.3

-18.3

HO•2 elimination

16.4

4.5

14

1.8

•

OH elimination

27.5

-10.9

21.3a

-9.7

HO•2 elimination

61.6

35.5

-

-

•

OH elimination

17.9

-9.9

-

-

HO•2 elimination

17.7

10.5

-

-

•

OH elimination

20.8

-29.2

-

-

HO•2 elimination

51.9

12.8

-

-

•

Q1OOH-15 •

OH elimination

•

Q1OOH-16

•

Q2OOH-14

•

Q2OOH-15

•

Q3OOH-14

•

Q3OOH-16

a

indicates the barrier calculated from B3LYP/6-311G(d,p) level of theory.


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For all the 1,3 H-migration reactions of ROO• radicals, CBS-QB3 reaction energies are more exothermic than the G3MP2B3 method by 1.8-2.2 kcal mol-1. For the other H-migration reactions of R1OO• radicals, CBS-QB3 energies are more endothermic than G3MP2B3 energies; whereas, it is less endothermic in case of R2OO• radicals. In the chemistry of •QOOH radicals, we studied only two important chain branching reactions: one is the •OH elimination and the other one is HO•2 elimination reaction. Between these two reactions, •OH elimination reaction (to form cyclic ether or lactone) is the dominant one (except in case of •Q3OOH-14 radical, where the HO•2 elimination reaction dominates by a small energy barrier difference of 0.2 kcal mol-1). It is clear from Table 2 that, the •OH radical elimination reactions are exothermic, whereas the HO•2 elimination reactions are endothermic. In this study, the reported HO•2 elimination reactions are single step reactions with the formation of cyclic ketones (which were not studied by Sebbar et al.) and vinylic ketones. The formation of cyclic ketones with HO•2 elimination has very high barriers because of the formation of 3- and 4membered cyclic structures. However, the formation of methyl vinyl ketone via HO2 elimination is dominant, as the product formed is resonance stabilized.

3.2 Chemistry of •OOQOOH radicals: The •QOOH radical (•Q1OOH15, •Q2OOH15 and •Q3OOH16) formed (most favorable product) from the H-migration reaction of each ROO• radical undergoes further oxidation (second O2 addition) to give three different •OOQOOH radicals. All the possible reaction channels of these radicals (Figures 3-6) and their kinetic studies have been discussed in detail in the following sections. The energy barriers and reaction energies for various reactions of •

OOQOOH radicals are listed in Table 3.



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Table 3: Activation barriers ∆E‡ (at 0 K, in kcal mol-1) and reaction enthalpies ∆E (at 0 K, in kcal mol-1) for the reactions of four •OOQOOH radicals derived from 2-butanone at CBS-QB3 level of theory. •

∆E‡

OOQOOH

∆E

•

OOQ1OOH

1,3 H-migration

38.6

0.8

1,4 H-migration

35.6

15.8

1,5 H-migration

24.8

-0.1

1,7 H-migrationa

11.9

0.3

HO•2 elimination

25.9

18.4

1,3 H-migration

41.5

-0.4

1,5 H-migration

20.7

0.6

1,6 H-migration

28.3

15.5

a

14.2

-0.3

1,3 H-migration

40.8

-2.8

1,5 H-migration

21.7

6.0

1,6 H-migration

15.2

-29.9

1,8 H-migrationa

12.2

-0.7

1,3 H-migration

41.4

-29.3

1,4 H-migration

34.2

6.7

1,6 H-migration

17.2

-2.1

a

12.9

0.7

HO•2 elimination

27.9

14.6

•

OOQ2OOH

1,7 H-migration •

OOQ3OOH

•

OOQ1'OOH

1,8 H-migration a

indicates the peroxy radical interconversion reaction.


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3.2.1 Potential Energy studies: An important reaction for these radicals is the isomerization of •OOQOOH to HOOQOO• involving the H-migration from oxygen atom at one end to the oxygen atom at the other end. In the reactions of •OOQ1OOH radical, 1,7 H-migration reaction to form •OOQ2OOH has the lowest energy barrier of 11.9 kcal mol-1 among all the other possible H-atom migration pathways. The reverse isomerization of •OOQ2OOH to •OOQ1OOH has an energy barrier of 14.2 kcal mol-1, which is the lowest barrier pathway among all the reactions of •OOQ2OOH. This reaction involves an 8-membered ring transition state structure. The next feasible reaction channel of both •

OOQ1OOH and •OOQ2OOH radicals is 1,5 H-migration reaction with the corresponding barriers

of 24.8 and 20.7 kcal mol-1 respectively, which involve 6-membered transition states. α-•QOOH species (having two –OOH groups) are the products of 1,5 H-migration reactions, which will decompose via two ways: (i) •OH elimination with oxetane formation via a 4-membered transition state, and (ii) can be dissociated into •OH radical and a dicarbonyl compound via a barrierless reaction. These products further dissociate to give a second •OH radical and an alkoxy radical (by the second –OOH group). The formation of two •OH radicals in this reaction sequence causes the radical chain branching, which significantly contributes to autoignition. Since the 1,5 H-migration of •OOQ2OOH radical has a low barrier (20.5 kcal mol-1) when compared to the 1,5 H-migration of •OOQ1OOH (24.8 kcal mol-1) by 4.1 kcal mol-1. Therefore, it is expected that the removal of both these radicals happen via 1,5 H-migration of •OOQ2OOH, as these two radicals are in isomerization with each other. The next favorable reaction of •OOQ1OOH is the HO•2 elimination with the formation of a compound with C=C bond, which involves a 5-membered transition state. The energy barrier for this reaction is very close to that of 1,5 H-migration and therefore, the HO•2 elimination 15 ACS Paragon Plus Environment


reaction can compete with the 1,5 H-migration in •OOQ1OOH. Other reactions of •OOQ1OOH are the 1,4 and 1,3 H-migrations, which involves very high energy barriers of 35.6 and 38.6 kcal mol-1 respectively. The products of both these H-migration reactions also undergoes decomposition to give •OH radicals, which is a radical chain propagation reaction. The other reactions of •OOQ2OOH are 1,6 and 1,3 H-migrations, whose transition state structures involve 7- and 4-membered rings. The products of these reactions are same as that of the products of 1,4 and 1,5 H-migration reactions of •OOQ1OOH, and follows the same decomposition pathways. The potential energy level diagrams for the reaction sequences of •OOQ1OOH and •OOQ2OOH are shown in the Figure 3 and Figure 4 respectively along with their schemes in which their decomposition pathways are shown in blue arrows. For •OOQ3OOH, the lowest energy barrier path (12.2 kcal mol-1) is the peroxy radical inter-conversion reaction which involves 1,8 H-migration to give a new hydroperoxyalkylperoxy radical which we named as •OOQ1'OOH radical, since it has the hydroperoxy group (-OOH) on the first carbon. The •OOQ1'OOH radical can be formed from the addition of O2 to Q1OOH16 radical. The next feasible reaction for •OOQ3OOH is 1,6 H-migration reaction whose barrier is 15.2 kcal mol-1. The product of this reaction having a carbon radical with hydroperoxy group (•

C-OOH), which is less stable and hence dissociates to a stable dicarbonyl compound and a •OH

radical. This dicarbonyl compound dissociates without any barrier to give the second •OH radical and an alkoxy radical. 1,5 and 1,3 H-migrations are the other reactions of •OOQ3OOH, whose barrier heights are 21.7 and 40.8 kcal mol-1 respectively. 1,3 H-migration reaction becomes insignificant because of its high barrier. The product of 1,5 H-migration reaction of •OOQ3OOH is α-•QOOH species in which the first •OH elimination can be formed by two ways


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Figure 3: CBS-QB3 potential energy profiles and scheme for the reactions of •OOQ1OOH radical. Red color shows the structural changes in the reactions and blue arrow indicates the decomposition channel. All the energies given in the units of kcal mol-1. 17 ACS Paragon Plus Environment


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O

O O OOH

OOH

1,3 H-mig 41.5 OO

-0.4

1,7 H-mig

OO

peroxy radical OOQ2OOH interconversion 14.2

OOH

OO

0.6

1,6 H-mig 1,5 H-mig 28.3

O

O

20.7

OOH OOH

OOH OOH

0.5

15.5 21.3

O

O

OOH

OH

OH

O

-13.2

HOO

-27.8

O

O

O

OH O

(unstable)

O

O O

20.8

OH

O O

-1.8 O

Figure 4: CBS-QB3 potential energy profiles and scheme for the reactions of •OOQ2OOH radical. Red color shows the structural changes in the reactions and blue arrow indicates the decomposition channel. All the energies given in the units of kcal mol-1 18 ACS Paragon Plus Environment

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viz. either with the formation of an oxetane or by the formation of cyclic ether. They can further dissociate and give a second •OH radical and an alkoxy radical. For the newly formed •OOQ1'OOH radical, the reverse reaction to form •OOQ3OOH has the lowest barrier of 12.9 kcal mol-1. The second most favorable reaction is the 1,6 H-migration reaction with a barrier height of 17.2 kcal mol-1, which is 2 kcal mol-1 higher than that of the 1,6 H-migration reaction of •OOQ3OOH. Therefore, the removal pathway of both the •OOQ3OOH and •OOQ1'OOH radicals is via the 1,6 H-migration reaction of •OOQ3OOH. The decomposition pathways from the product of 1,6 H-migration reaction of •OOQ1'OOH to form the •OH radicals involves the formation of substituted furan and dicarbonyls. The other reactions of •OOQ1'OOH are HO•2 elimination (which is similar to that of •OOQ1OOH), 1,4 and 1,3 H-migrations, which are less significant due to high barrier heights. The potential energy level diagrams for the reaction sequences of •OOQ3OOH and •OOQ1'OOH are shown in the Figures 5 and 6 respectively along with their reaction schemes in which their decomposition pathways are shown in blue arrows. All the transition state structures involved in the reactions of •OOQOOH radicals are shown in Figure 7.



Page 20 of 39

O

O

HOO

HOO

1,8 H-mig

OOH

OO OOQ3OOH peroxy radical

1,3 H-mig 40.8

-2.8

O OO

OOH

interconversion

12.2 15.2 1,6 H-mig

O

-0.7

21.7 1,5 H-mig

O OOH

OOH HOO

OOQ1'OOH

HOO

unstable

6.0

O OOH 2 O

OH

-29.9

O O

OH O

17.9

Figure 5: CBS-QB3 potential energy profiles and scheme for the reactions of •OOQ3OOH radical. Red color shows the structural changes in the reactions and blue arrow indicates the decomposition channel. All the energies given in the units of kcal mol-1 20 ACS Paragon Plus Environment

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O

O

O OOH 1,8

OOH HOO

OO

-29.3

1,3 H-mig 41.4

1,4 H-mig 34.2

O

H-mig

OOQ1'OOH peroxy radical

OO

HOO

0.7

interconversion

12.9 HO2-elim 27.9 17.2 1,6 H-mig

O OOH

OOH O

HO2

14.6

HOO

6.7

OOH HOO

-2.1

O

O

HOO

OH

OH HOO

-25.2

OH2 O

-30.7 O

O

OH

OH

O O

19.2 OH O

7.5

Figure 6: CBS-QB3 potential energy profiles and scheme for the reactions of •OOQ1'OOH radical. Red color shows the structural changes in the reactions and blue arrow indicates the decomposition channel. All the energies given in the units of kcal mol-1 21 ACS Paragon Plus Environment



Page 22 of 39

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Figure 7: Transition state structures (bond length in Å) optimized at the CBS-QB3 level of theory for the reactions of all the four •OOQOOH radicals of 2-butanone. Black color indicates carbon, red color indicates oxygen and the blue color indicates hydrogen atoms respectively.

3.2.2 Kinetic studies: The rate coefficients for all the reactions of •OOQOOH radicals were calculated using the CVT/SCT method. The Arrhenius plots of the computed rate coefficients for all the reactions in the temperature range of 400-1500 K are shown in Figure 8. The calculated rate coefficients were fit by linear least squares method and the Arrhenius fit parameters are given in Table 4. The rate constants of all the studied reactions have a positive temperature dependence over the temperature range, which is consistent with the positive activation barriers. In case of •OOQ1OOH, the peroxy radical inter-conversion (1,7 H-migration) is the dominant reaction pathway because of lower barrier and hence higher rate coefficients in the studied temperature range. HO•2 elimination is the next dominant channel. However, 1,5 Hmigration is the favorable path due to low energy barrier as discussed above. Therefore, the 1,5 H-migration, which is energetically feasible, is not kinetically favorable when compared to the HO•2 elimination. 1,4 H-migration reaction is insignificant due to its very low rate coefficients over the studied temperature range.


Page 24 of 39

Page 25 of 39

1500

1000

800 700

600

500

30

400 .

OOQ1OOH

20

lnk

10

0

1,4 H-migration 1,5 H-migration 1,7 H-migration HO2-elimination

-10

1.0

1.5

2.0

2.5

500

400

-1

1000/T (K )

1500

1000

800

700

600

.

25

OOQ2OOH

20 15 lnk

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


10 5 0

1,5 H-migration 1,6 H-migration 1,7 H-migration

-5 1.0

1.5

2.0 -1

1000/T (K )


2.5


1500

1000

800

700

600

500

400

25 .

OOQ3OOH

lnk

20

15

1,5 H-migration 1,6 H-migration 1,8 H-migration

10

5 1.0

1.5

2.0

2.5

500

400

-1

1000/T (K )

1500

1000

800

700

600

25 .

OOQ1'OOH

20 15

lnk

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 26 of 39

10 5 0

1,4 H-migration 1,6 H-migration 1,8 H-migration HO2-elimination

-5

1.0

1.5

2.0

2.5

-1

1000/T (K )

Figure 8: Arrhenius plots for the reactions of •OOQOOHs derived from 2-butanone. 26 ACS Paragon Plus Environment

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The kinetics of •OOQ2OOH follow the same pattern as that of the activation barriers; where the 1,7 H-migration to form •OOQ1OOH is the dominant channel followed by the 1,5 Hmigration. 1,6 H-migration is less favorable owing to its high barrier and low rate constants. The reactions of •OOQ3OOH radical show two different trends over the studied temperature range. At temperatures 400 to 1000 K, the isomerization of peroxy radical (1,8 Hmigration) is the dominant pathway when compared to other H-migration reactions. At temperatures 1100 to 1500 K, the 1,6 H-migration dictates the kinetics, which is the removal pathway for both the •OOQ3OOH and •OOQ1'OOH radicals as discussed in the previous section. Similarly, •OOQ1'OOH follows the same kinetic trend as that of •OOQ3OOH. The peroxy radical inter-conversion (1,8 H-migration) controls the kinetics from 400 to 1000 K and then the 1,6 H-migration reaction dominates. The HO•2 elimination reaction crossover the 1,8 Hmigration at 1300 K. At 1500 K, both 1,6 H-migration and HO•2 elimination reactions have almost similar rate coefficients. The high barrier 1,3 H-migration reactions of all the •OOQOOH radicals of 2-butanone are not considered in the kinetic studies, as it has very low rate coefficients over the studied temperature range.



Page 28 of 39

Table 4: Fitted Arrhenius parameters for the reactions of •OOQOOH radicals derived from 2-

butanone. •

A (s-1)

Ea (cal mol-1)

1,4 H-migration

2.83×1011

33378

1,5 H-migration

3.19×1011

19866

1,7 H-migrationa

2.94×1011

1891

HO•2 elimination

1.78×1013

18455

1,5 H-migration

3.73×1012

17774

1,6 H-migration

4.75×1012

26570

1,7 H-migrationa

9.47×1011

3896

1,5 H-migration

8.70×1011

17717

1,6 H-migration

5.18×1011

11256

1,8 H-migrationa

4.06×1009

999

1,4 H-migration

5.05×1011

27446

1,6 H-migration

4.09×1011

11287

1,8 H-migrationa

4.14×1009

1457

HO•2 elimination

2.87×1013

23123

OOQOOH

•

OOQ1OOH

•

OOQ2OOH

•

OOQ3OOH

•

OOQ1'OOH

a



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3.2.3 Thermochemistry of the reactions of •OOQOOH radicals: The thermodynamic parameters (∆H°, ∆G°, ∆S°) for all the reactions of the different •

OOQOOH radicals are given in the Table 5. It is obvious from this Table that, the 1,3 H-

migration reactions of all the •OOQOOH radicals except •OOQ1OOH are exothermic and spontaneous which is unexpected due to their high energy barriers involved in the formation of highly strained 4-membered ring transition states. The probable reason could be the formation of the stable products from these reactions. In case of •OOQ1'OOH, 1,3 H-migration reaction appears to be more spontaneous (∆G°=-36.4 kcal mol-1) and exothermic (∆H°=-28.7 kcal mol-1), since it gives a stable hydroperoxy substituted dicarbonyl compound and a •OH radical. The same products are formed via 1,6 H-migration reaction of •OOQ3OOH radical, which was stated above as the removal pathway for both the •OOQ3OOH and •OOQ1'OOH radicals. The isomerization reactions of peroxy radicals (energetically feasible) are not thermodynamically favored, when compared to the other H-migration reactions. This is due to the stability of radicals (formed as products via other H-migration reactions) either by resonance with the nearby ketonic group or by cleavage of –•C-OOH moiety to oxo group and •OH radical, which are not possible in the case of peroxy radical inter-conversion reactions. The 1,5 Hmigration of •OOQ2OOH, which is the removal pathway for both •OOQ1OOH and •OOQ2OOH radicals, are less spontaneous (by 1.1 kcal mol-1) than the 1,5 H-migration of •OOQ1OOH. The 1,4 H-migration reactions (•OOQ1OOH and

•

OOQ1'OOH), 1,5 H-migration

(•OOQ3OOH) and 1,6 H-migration (•OOQ2OOH) are neither favored kinetically nor thermodynamically. The HO•2 elimination reactions are possible only for the •OOQ1OOH and •

OOQ1'OOH radicals of 2-butanone which are favorable entropically.



Page 30 of 39

Table 5: Thermodynamic parameters for all the reactions of •OOQOOH radicals of 2-butanone

calculated by CBS-QB3 method. •

∆E‡(kcal mol-1)

OOQOOH

∆H°(kcal mol-1)

∆G°(kcal mol-1)

∆S° (cal/mol-K)

•

OOQ1OOH

1,3 H-migration

38.6

0.8

1.4

-1.9

1,4 H-migration

35.6

15.7

16.4

-2.3

1,5 H-migration

24.8

0.1

-0.5

2.1

1,7 H-migrationa

11.9

0.2

0.7

-2.0

HO•2 elimination

25.9

19.1

7.6

38.6

1,3 H-migration

41.5

-0.02

-1.2

4.1

1,5 H-migration

20.7

0.7

0.6

0.1

1,6 H-migration

28.3

15.6

15.7

-0.3

a

14.2

-0.1

-0.7

2.0

1,3 H-migration

40.8

-2.5

-2.4

-0.5

1,5 H-migration

21.7

6.6

5.5

3.5

1,6 H-migration

15.2

-29.3

-36.9

25.5

1,8 H-migrationa

12.2

-0.6

-0.5

-0.4

1,3 H-migration

41.4

-28.7

-36.4

25.9

1,4 H-migration

34.2

7.2

6.0

3.9

1,6 H-migration

17.2

-1.9

-1.9

-0.1

1,8 H-migrationa

12.9

0.6

0.5

0.4

HO•2 elimination

27.9

15.2

4.5

36.1

•

OOQ2OOH

1,7 H-migration •

OOQ3OOH

•

OOQ1'OOH

a



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4. CONCLUSIONS In this study, the oxidative decomposition mechanism of 2-butanone (which is a potential biofuel) is reported. The formation of cyclic ether with •OH elimination is more dominant than the HO•2 elimination reaction (by almost 30 kcal mol-1) for the reactions of •QOOH radicals except for the •Q2OOH-14 and •Q3OOH-14 radicals, where both the reactions are competing with each other. This is also supported by the exothermic nature of the •OH elimination reaction. We explored the chemistry of •OOQOOH radicals derived from 2-butanone which were not studied before. It was found that, the peroxy radical inter-conversion reactions (H-migration between two peroxy groups) are the dominant pathways for all the •OOQOOH radicals over the studied temperature range of 400-1500 K except for the •OOQ3OOH and •OOQ1'OOH radicals, where, it is dominant only up to 1000 K. 1,6 H-migration is the dominant pathway for the •

OOQ3OOH radical from 1100 to 1500 K, which is the removal pathway of both •OOQ3OOH

and OOQ1'OOH radicals. Similarly 1,6 H-migration of •OOQ1'OOH are dominant from 1100 to 1500 K. However, HO•2 elimination reactions (possible only for •OOQ1OOH and •OOQ1'OOH radicals) are not feasible thermodynamically. The removal of •OOQ1OOH and •OOQ2OOH radicals happen via the 1,5 H-migration of •OOQ2OOH radical. The high barrier reactions of 1,3 H-migration are exothermic and spontaneous (due to the formation of stable products) which is unexpected. The energetic and kinetic studies were performed for the key elementary chain branching reactions of autoignition and low temperature combustion, which involves the two radical intermediates namely hydroperoxyalkyl radicals (•QOOH) and hydroperoxyalkylperoxy radicals (•OOQOOH). These studies are useful in modeling the autoignition of 2-butanone.



Supporting Information Optimized geometries (Cartesian coordinates) and Vibrational frequencies (cm-1) of all the species involved in our study are given in the Table S-I and S-II respectively. Rate constants (s-1) for the reactions of alkylperoxy (ROO•) and hydroperoxyalkylperoxy (•OOQOOH) radicals for the temperature range from 400-1500K by CVT/SCT method are given in Table S-III. T1 diagnostics results and the values for all the transition state structures involved are reported in Table S-IV. Energy profile diagrams for all the transition states involved in the studied reactions were obtained with IRC calculations and are shown in Figure S-I.

Acknowledgment We thank Professor Donald G. Truhlar for providing the POLYRATE 2008 and GUASSRATE 2009A programs. The authors thank the High Performance Computing centre and Mr V. Ravichandran for providing computer resources at IIT Madras. B.R. thank Defense Research and Development Organization (DRDO), Government of India for funding. S.K. is very grateful to University Grants Commission (UGC) for providing research fellowship.

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Thermochemistry and Kinetic Studies on the Autoignition of 2

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