An Exprimental and Computational Study on the Cl Atom Initiated

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An Experimental and Computational Study on the Cl Atom Initiated Photo-Oxidization Reactions of Butenes in the Gas Phase Siripina Vijayakumar, and Balla Rajakumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04783 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 7, 2017

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The Journal of Physical Chemistry

An Experimental and Computational Study on the Cl Atom Initiated Photo-Oxidization Reactions of Butenes in the Gas Phase S. Vijayakumar 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 Temperature dependent rate coefficients for the reactions of Cl atoms with trans-2-butene and isobutene were measured over the temperature range of 263-363K using relative rate technique with reference to 1,3-butadiene, isoprene and 1-pentene. The measured rate coefficients for the reactions of Cl atoms with isobutene and trans-2-butene are = (3.43±0.11)×10-10 cm3

molecule-1 s-1 and = (3.20±0.04)×10-10 cm3 molecule-1 s-1 respectively at 298K and 760

Torr. Measured rate coefficients were used to fit the Arrhenius equations, which are obtained to be . = (4.99±0.42)×10-11exp[(584±26)/T] cm3 molecule-1 s-1 and . = (1.11±0.3)

×10-10exp[(291±88)/T] cm3 molecule-1 s-1 for isobutene and trans-2-butene respectively. To

understand the reaction mechanism, estimate the contribution of each reaction site and to complement our experimental results, computational studies were also performed. Canonical Variational Transition state theory (CVT) with Small Curvature Tunneling (SCT) in combination with MP2/6-31G(d), MP2/6-31G(d,p), MP2/6-31+G(d,p), CCSD(T)/cc-pvdz and QCISD(T)/ccpvdz level of theories were used to calculate the temperature dependent rate coefficients over the temperature range of 200-400K. The effective lifetimes, thermodynamic parameters and atmospheric implications of the test molecules were also estimated. 1. Introduction Extensively released alkenes react with atmospheric oxidants such as OH radicals, Cl atoms, NO3 radicals and O3 molecules and also eventually lead to the formation of Secondary Organic 1 ACS Paragon Plus Environment


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Aerosols (SOAs) in troposphere1. These alkenes are released into the atmosphere from various sources such as automobile emissions, petroleum emissions and combustion of polymers2. Abstraction of hydrogen atoms from alkenes forms alkenyl fragments/radicals. Thus formed radicals produce enones and enols in presence of atmospheric oxygen, which are responsible for the formation of SOAs3. Among these alkenes, C4 stream is an important raw material to produce fuel additives and highly useful for the synthesis of heavy alkylates in petrochemical industry4. By hydrogenation of olefin oligomers, higher hydrocarbons such as tri-isobutene and poly-isobutene can be produced. They are useful for the synthesis of important chemicals including neo-acids. Triisobutene is used as additives for jet fuel, kerosene and as odorless premium solvents without aromatics5. Poly-isobutene has detergent properties, which resists fouling of fuel injectors leading to reduced particulate and hydrocarbons emissions. It is used as a detergent package that is added to diesel fuel and gasoline to resist buildup of deposits and engine knock5. Trans-2-butene generates hydroxyl radicals when it reacts with O3 molecules, which is one of the major chemical species involved in the oxidation of hydrocarbons in the Earth’s atmosphere. Kroll et al.6 have reported that the ozonolysis of alkenes produces hydroxyl radicals. The cycloaddition of ozone with trans-2-butene leads to the formation of primary ozonide. Then primary ozonide decomposes into acetaldehyde and, both the anti and syn forms of acetaldehyde oxide (Criegee intermediate). Both anti and syn acetaldehyde oxides undergoes rearrangement via ring closure and hydrogen shift. The hydrogen shift in anti-acetaldehyde oxide generates hydroxyl radicals (OH). Syn acetaldehyde oxide also produces OH radicals rather via an indirect way6. Trans-2-butene is a petrochemical product which is produced by either the catalytic cracking of crude oil or the dimerization of ethylene. It is used mainly in the production of gasoline and butadiene. 2-butene is also used to produce butanone. Isobutene is one of the raw material for the synthesis of methyl-tert-butyl ether [MTBE, (CH3)3COCH3]. The dissolution of MTBE in underground water, may lead to environmental pollution and can pose potential threat to humans at high doses. Hence, in 2004 California banned the blending of MTBE in gasoline7. Isobutene is used as an intermediate in the synthesis of a variety of compounds such butyl rubber (poly-isobutene), butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA). The largest application of the higher weight poly2 ACS Paragon Plus Environment

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isobutene is in the production of lube oil viscosity improvers. BHT is obtained from p-cresol and isobutene and it is used as a gum inhibitor in gasoline, an antioxidant for plastics and elastomers. Food grade BHT is an antioxidant and is used to preserve the edible oils8. It is obvious that butenes are extensively used and are therefore, released into the Earth’s troposphere. Therefore, it is essential to predict the atmospheric fate of these butenes. Primary information such as their atmospheric lifetimes, global warming potentials at different time horizons are required to be estimated. To estimate these parameters, the rate coefficients for the reactions of trans-2-butene and isobutene with oxidizing species such as OH and NO3 radicals, O3 molecules and Cl atoms are necessary to be known. Several groups9-15 have measured the rate coefficients for the reactions of the test molecules with OH radicals. Ezell et al.16 measured the rate coefficients for the reactions of Cl atoms with test molecules using relative rate experimental technique with reference to n-heptane and reported the rate coefficients to be k = (3.31±0.47) ×10-10 cm3 molecule-1 s-1 and k = (3.40±0.28)×10-10 cm3 molecule-1 s-1 for trans-2-butene and isobutene respectively at 298±3K and 1 atmospheric pressure. Orlando et al.17 reported the rate coefficient for the reaction of Cl atoms with trans-2-butene to be k = 4.0×10-10 cm3 molecule-1 s-1 at 298K and 1 atmospheric pressure with reference to propylene using relative rate technique. Kaiser et al.18 measured the rate coefficient for the reaction of Cl atoms with trans-2-butene and reported as k = (3.27±0.3)×10-10 cm3 molecule-1 s-1 using relative rate technique with reference to propane at 297K and 900 Torr of N2 using Gas Chromatography (GC). In addition they have carried out the measurements using IR spectrometer with reference to propylene at 297K and 10 Torr of N2 and reported the rate coefficient as k = (3.1±0.16)×10-10 cm3 molecule-1 s-1. Although the title reactions were studied and reported by several groups, all those studies were limited to the room temperature (298K) only. To the best of our knowledge, no temperature dependent rate coefficients were reported till date. In the present investigation, we measured the temperature dependent rate coefficients over the temperature range of 269-363K for the reactions of Cl atoms with isobutene and trans-2-butene relative to 1,3-butadiene, isoprene and 1-pentene. To further understand the mechanism of Cl atoms with butenes, computational calculations were also performed for the title reactions using Canonical Variational Transition state theory (CVT) with Small Curvature Tunneling (SCT) in combination with MP2/6-31G(d), MP2/6-31G(d,p),

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MP2/6-31+G(d,p), CCSD(T)/cc-pvdz and QCISD(T)/cc-pvdz level of theories in the temperature range of 200-400K.

2. Methodology 2.1. Experimental Temperature dependent rate coefficients were measured for the reactions of Cl atoms with isobutene and trans-2-butene over the temperature range of 269-363K. 1,3-butadiene, isoprene and 1-pentene were used as reference compounds in the present study. The detailed description of experimental set up was given in our previous papers19-21. Experiments were carried out in a double walled Pyrex chamber equipped with UV fused silica windows at both ends. Temperature was maintained inside the vessel by circulating a heated/cooled fluid through the outer jacket. Inside the reaction cell, the temperature was calibrated with a K-type thermocouple within the uncertainty of 2K. Cl atoms were produced in situ by photolysis of oxalylchloride (COCl)2 using an Excimer laser (Coherent Compex Pro) at 248 nm with an energy of 6 mJ pulse-1. Reaction mixture (sample, reference and precursor) was photolyzed by about 1000, 1200, 1500, 1800 and 2000 pulses. After each photolysis in every experiment, sample mixture was allowed to reach uniform distribution in the reaction vessel. The concentrations of samples were measured by using GC (Agilent Technology, 7890B) coupled with Flame Ionization Detector (FID). A capillary column (HP-PLOT/Q, 30m× 0.320mm×20.0 µm, 19091P-Q04) was used for the separation of the compounds. The GC oven was operated at 150 oC with a constant carrier gas (N2) flow of 2.8 ml min-1. The initial concentrations of samples were maintained in the reaction chamber as (4-6)×1016 molecules cm-3, (4-6)×1016 molecules cm-3 and (4-6)×1017 molecules cm-3 for reactant, references and oxalylchloride respectively. Before photolysis, the stability of the reaction mixture (isobutene, trans-2-butene, 1,3-butadiene/isoprene/1-pentene, (COCl)2 and 4 ACS Paragon Plus Environment

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nitrogen) was tested. It was kept in dark condition for 4 hours which is more than duration of the total reaction time and there was no loss in the concentrations of samples which indicates that there was no dark reaction and as well as wall loss. To check the influence of any secondary reactions, oxygen was added at 298K and at extreme ends of the temperatures (269 and 363K). No difference in the rate coefficients was observed, which provides the evidence for no influence of secondary reactions because of the presence of oxygen. To the best of our knowledge, there are only three temperature dependent rate coefficients (Cl + 1,3-butadiene20, Cl + isoprene22 and Cl + 1-pentene23) available in the literature (vide infra). The rate coefficients of these compounds with Cl atoms are close to the rate coefficients of the title reactions at the temperatures across the study. Therefore, they were used as reference compounds in the entire studied temperature range. Rate coefficients for the title reactions were measured using the below standard relative rate expression.

! "# )*)+, ln = ln { } $#%#$#&'# )*)+, where, [sample]0 and [sample]t are the concentrations of the sample at time ‘0’ and ‘t’ respectively. [reference]0 and [reference]t are the concentrations of the reference compounds (isoprene/1-pentene) at time ‘0’ and ‘t’. Thus, a plot of ln([sample]0/[sample]t) verses ln([reference]0/[reference]t) gives a straight line, which passes through the origin and the slope gives the ratio of ksample/kreference. Chemicals Isoprene (purity 99%, Aldrich), 1,3-butadiene (purity 99.5%, Praxair), 1-pentene (purity ≥98.5%, Aldrich), isobutene (purity 99.5%, Praxair), trans-2-butene (purity ≥99%, Aldrich), nitrogen (99.995%, Bhuruka, India), oxygen (98%, Bhuruka, India). Isoprene, 1-pentene and oxalylchloride were subjected to repeated freeze-pump-thaw cycles prior to use them. 2.2. Computational The geometries of the reactants, pre-reactive complexes, transition states and products which are formed via different addition and abstraction channels were optimized at Moller-Plesset24 second order (MP2) level of theory with Pople basis25 sets such as 6-31G(d), 6-31G(d,p) and 65 ACS Paragon Plus Environment


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31+G(d,p). The optimized structural parameters, energies and vibrational frequencies of all the reactants, pre-reactive complexes (RCs), transition states and products are given in the supporting information (Tables S.I.1.1 to S.I.4.2). Transition states were identified with one imaginary frequency (NIMG=1) and reactants, pre-reactive complexes and products were identified with zero imaginary frequencies (NIMG=0). Intrinsic reaction coordinates (IRCs) calculations were performed using the MP2/6-31G(d,p) level of theory to verify that the transition states connect to the reactant and products. All the electronic structure calculations were performed using the Gaussian 09 program suite26. The normal modes and structures were viewed in Gauss view27. To get the accurate kinetic parameters, single point energy calculations28 were performed at QCISD(T), CCSD(T) level of theories with different basis sets such as cc-pvdz, 6-31G(d), 6-31G(d, p) and 6-31+G(d, p). 2.3. Kinetics Rate coefficients for the reactions of Cl atoms with test molecules were computed over the temperature range of 200-400K using CVT/SCT29-31 ./ 01, 3 = 4

56 / 8 9: 0/,;3 7

0

ф= 0/3

3exp 0

ABCD 03 56 /

3 …………….. (1)

EA/ 013 = F+ ./ 01, 3 = ./ 1, EA/ 013………….. (2) where, ‘kGT’ and ‘kCVT’ are the generalized rate coefficient and CVT rate coefficient respectively. ‘kB’ is Boltzmann’s constant, ‘σ’ is the reaction path degeneracy, ‘h’ is Planck’s constant, T is the temperature in kelvin, QGT and фR are the partition functions of a generalized transition state at ‘s’ and reactants respectively, ‘sCVT’ is the reaction coordinate at which canonical variational transition state dividing surface was found. ‘VMEP(s)’ is the potential energy of generalized TS at ‘s’. By maximizing the free energy of activation with respect to ‘s’, the canonical variational transition state is located. The tunneling corrected rate coefficients (kCVT/SCT) were obtained by multiplying kCVT with temperature dependent transmission coefficient ƘEA//;E/ (T). k EA//;E/ 013 = ƘEA//;E/ 013k EA/ 013………….

(3)

3. Results and discussion 6 ACS Paragon Plus Environment

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3.1. Reaction of Cl atoms with isobutene 3.1.1. Experimental Rate coefficients for the reaction of Cl atoms with isobutene were measured over the temperature range of 269-363K relative to 1,3-butadiene20, isoprene22 and 1-pentene23. Vijayakumar et al.,20 measured the temperature dependent rate coefficients for the reaction of Cl atom with 1,3butadiene using relative rate method. The reported temperature dependent rate coefficient is -10 3 -1 -1 , JK LM#&#NE" = (1.05±0.19)×10 exp[(332±56)/T] cm molecule s and room temperature

(298K) rate coefficient is (3.31±0.48)×10-10 cm3 molecule-1 s-1 and an error associated with the rate coefficient was 14%. Bedjanian et al.22 have measured the temperature dependent rate

coefficient for the reaction of isoprene with Cl atom using discharge flow – mass spectrometric method with Cl + Br2 as the reference reaction over the temperature range of 233-320K. The

reported temperature dependent rate coefficient is MO $#&#NE" = (6.7±2.0)×10-11 exp[(485±85)/T]

cm3 molecule-1 s-1 and room temperature (298K) rate coefficient is (3.41±0.5)×10-10 cm3

molecule-1 s-1 and 15% error was reported. Coquet et al.23 have investigated the temperature dependent rate coefficient for the reaction of 1-pentene with Cl atom using relative rate method with reference to n-hexane in the temperature range of 283-323K and at atmospheric pressure. The reported temperature dependent rate coefficient is #&#&#NE" = (4.0±2.2)×10-11

exp[(733±288)/T] cm3 molecule-1 s-1 and room temperature rate coefficient is (4.69±0.08)×10-10 cm3 molecule-1 s-1 and an error associated with the rate coefficient was 2%. Rate coefficients

derived from these Arrhenius expressions were used in the present investigation, to measure the rate coefficients for the reaction R1 at 263, 285, 298, 310, 330, 350 and 363K. The plot of relative decrease in the concentration of isobutene due to its reaction with Cl atoms relative to 1,3-butadiene, isoprene and 1-pentene is shown in Figure 1. From Figure 1, it is clear that the behavior of the relative decrease of the reactant is linear and is passing through origin, which explains the non-interference of the secondary chemistry on the reaction. To ensure the consistence and reproducibility, experiments were repeated at least 2 to 3 times at every temperature. Slopes were obtained from the linear least-square fittings and are given in Table 1. Here it should be noted that the rate coefficients measured relative to 1,3-butadiene [ $#"

[ $#"

MP# O , JK LM#&#

MP# O MO $#&# =

=

(3.35±0.49)×10-10

cm3

molecule-1

s-1]

and

isoprene

(3.51±0.54)×10-10 cm3 molecule-1 s-1] were observed to be very close to 7 ACS Paragon Plus Environment


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the reported rate coefficient by Ezell et al.16 [ Q#"" #

1

".

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= (3.4±0.28)×10-10 cm3 molecule-1 s-

]. However, the rate coefficients measured relative to 1-pentene [ $#"

MP# O #&#&#

=

(5.11±0.18)×10-10 cm3 molecule-1 s-1] were observed to be overestimated over the studied temperature range (269-363K). Hence, the rate coefficients measured relative to 1,3-butadiene and isoprene were averaged across the studied temperature and given in Table 1. These averaged rate coefficients were used to fit the Arrhenius equation by using linear least squares methods. Thus deduced temperature dependent rate coefficient for the reaction R1 is . =

(4.99±0.42) ×10-11exp [(584±26)/T] cm3 molecule-1 s-1. The Arrhenius plot is shown in Figure 2 along with the computationally calculated rate coefficients, vide infra.

Slight negative

temperature dependence was observed over the studied temperature range. This negative temperature dependence is mainly due to the addition of Cl atom across the double bonds and would lead to the formation of pre reactive complexes, which are more stabilized, vide infra. 3.1.2. Computational

The optimized geometries of the reactants, pre-reactive complexes, transition states and products at MP2/6-31 G (d,p) level of theory are given in Figure 3. As shown in the structure 1, Cl atom addition at terminal carbon is labeled as TS1a and at middle carbon is labeled as TS2a. In isobutene, the two methyl (-CH3) groups (TS3, TS4 and TS5), (TS6, TS7 and TS8), which are connected on one side of the double bond are identical. In each methyl group, two hydrogens are identical. Two olefinic hydrogens (TS1 and TS2), which are attached on the other side of the double bond are identical. Therefore, totally two independent addition channels (TS1a and TS2a) and three independent abstraction channels (TS1, TS3 and TS4) were found for the reaction R1. Cl atom addition on double bond of isobutene at terminal carbon leads to the formation of transition state (TS1a) via a pre-reactive complex (RC1a) and forms chloroalkyl radical (P1a). 8 ACS Paragon Plus Environment

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Similarly, Cl atom addition at middle carbon leads to the formation of transition state (TS2a) via a pre-reactive complex (RC2a) and forms chloroalkyl radical (P2a). Abstraction of olefinic hydrogen atom from isobutene leads to the formation of product P1 via transition state TS1. Hydrogen atom abstraction from methyl group leads to the formation of products P3 and P4 via transition states TS3 and TS4 respectively. In the abstraction of hydrogen atoms from methyl group, the leaving C-H bond length was stretched up to 30% while in case of abstraction of olefinic hydrogen atom, it was 43% when compared with normal C-H bond lengths in the reactant. The rate coefficients for the reactions of R1 were obtained from the sum of the individual rate coefficients (kTotal= kTS1a+ kTS2a+ kTS1 + kTS2 + kTS3+ kTS4+ kTS5+ kTS6+ kTS7+kTS8). The energies of all the stationary points were refined at CCSD(T)/cc-pvdz and QCISD(T)/cc-pvdz using the optimized geometries obtained at MP2/6-31G(d,p) level of theory. Thus, refined energies were used in dual level CVT calculations to compute the rate coefficients for the reaction R1 over the temperature range of 200-400K. The obtained rate coefficients for the reaction of R1 at different level of theories such as MP2/6-31G(d), MP2/6-31G(d,p), MP2/6-31+G(d,p), CCSD(T)/ccpvdz//MP2/6-31G(d,p) and QCISD(T)/cc-pvdz//MP2/6-31G(d,p) are given in Table 2. Rate coefficients obtained at different level of theories are plotted and given in Figure 2. From Figure 2, it is clear that reaction R1 shows negative temperature dependence over the studied temperature range at all level of theories. The negative temperature dependence is mainly because of the formation of pre reactive complexes in the process of addition by Cl atom across the double bond. From Table 2, it is obvious that the computationally obtained rate coefficients at 298K are approximately two to three orders of magnitude higher than our experimentally measured rate coefficient [ . =

(3.43±0.11)×10-10 cm3 molecule-1 s-1] and reported value by Ezell et al.16 [ Q#"" #

". =

(3.4±0.28)×10-10 cm3 molecule-1 s-1]. Also, our computed rate coefficients are higher than

collision limit (~8×10-10 cm3 molecule-1 s-1). This could happen only when ions are involved in a reaction or in case of the radical – radical reactions. Due to the presence of Cl atom/radical in the title reaction, there is a possibility for the reaction to be very fast. Many studies (Alwe et al.32, 2014; Dash et al.33, 2014; Petit et al.34, 2012 and Lauraguais et al.35, 2015) were reported where the theoretically calculated rate coefficients differ with the experimentally measured rate 9 ACS Paragon Plus Environment


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coefficients. Alwe et al.32, calculated the rate coefficient for the reaction of Cl atoms with 2propyn-1-ol at 298K and reported as 2.48×10-08 cm3 molecule-1 s-1. They have reported the experimentally measured one as (3.49±0.66)×10-10 cm3 molecule-1 s-1 at 298K. Obviously, the theoretically calculated rate coefficients are two orders of magnitude higher than the experimentally measured rate coefficients. They have used QCISD(T) and MP2//6-311++G(d,p) level of theory to calculate the rate coefficients and observed negative temperature dependence over the temperature range of 200-400K. They have reported that addition of Cl atom at terminal carbon is the most prominent channel, which is in consistence with the present observations in the title reactions. Dash et al.33, observed in the reaction of Cl atom with limonene that, the theoretically obtained rate coefficient (2.00×10-09 cm3 molecule-1 s-1) is approximately two to three times higher than the experimentally measured rate coefficient ((7.31±1.81)×10-10 cm3 molecule-1 s-1). They have observed negative temperature dependence and the addition channels contributed maximum to the total rate coefficient as observed in the present investigation. The reasons for such a behavior can partly be attributed to the limitations of the level of theory that was used in the calculations. In addition, the reaction is totally governed by the addition of Cl atom across the double bond present in the substrate. Both the reaction channels (TS1a and TS2a) are exothermic. This is further confirmed by optimizing all the stationary points at different level of theories. The relative energies (∆E0‡ in kcal mol-1) obtained for all TSs at different level of theories with different basis sets are summarized and given in supporting information Table S.I.5. From Table S.I.5, we can say that the relative barrier heights for all abstraction transition states (TS1, TS3 and TS4) are much higher than the addition transition states (TS1a and TS2a) at all level of theories which indicate that the contribution of abstraction channels towards the total rate coefficient is very less. To identify the contribution of each reaction channel in the test molecule, branching ratios were calculated using the theoretically obtained rate coefficients. The branching ratio of each reaction channel was calculated using the following equation 5X

R),+ST U* Sℎ 1W = ∑Z

X[\ 5X

× 100…………… (4)

The branching ratios plot is shown in Figure 4 for the reaction R1. It is clear from Figure 4 that these two channels (TS1a and TS2a) are feasible both kinetically and thermodynamically 10 ACS Paragon Plus Environment

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(because of the submerged transition states). As the reaction dominates through the negative transition states (TS1a and TS2a), there is an increase in the rate coefficient with decrease in the temperature. All these would probably favor the reaction at low temperatures to the largest possible extent. The potential energy surface for the reaction is shown in Figure 5. From Figure 5, it is clear that the contribution of submerged channels via transition states TS1a and TS2a is expected to be maximum. Therefore, the corresponding pre-reactive complexes were identified and used in the rate coefficient calculations. The calculated rate coefficient [ 7#O$` = 9.08×10-09 cm3 molecule-1 s-1] at MP2/6-31G(d,p)

level of theory is nearer to the measured rate coefficients when compared with other level of theories. The deduced temperature dependent Arrhenius expression using the rate coefficients a = (3.90± 0.38)×10-12 exp[(2318±28)/T] cm3 molecule-1 s-1. In obtained at this theory is 7#O$`

case of reaction R1, the computed activation energy is nearly four times (Ea is 4.59 kcal mol-1) higher than the experimentally measured activation energy (Ea is 1.16 kcal mol-1) and the

computed pre-exponential factor is (3.90±0.38)×10-12 nearly an order of magnitude lower than the estimated pre-exponential factor using the experimental data ((4.99±0.42)×10-11).

Pre-

exponential factors mainly depends on how best the partition functions of both the reactants and transition states are estimated which in turn depends on the vibrational frequencies obtained using the selected theory for these calculations. Thermodynamic parameters such as standard Gibb’s free energy (∆Go), standard enthalpy (∆Ho) and standard entropies (∆So) were obtained at MP2/6-31G(d,p) level of theory and given in Table 3. From this thermodynamic data, addition of Cl atom at terminal carbon is the most exothermic and therefore, more favorable channel. Hydrogen abstraction (TS1) from olefinic carbon is more endothermic channel and hence less favorable than all other reaction channels. This is in consistent with the observation made with the computed rate coefficients. 3.2. Reaction of Cl atoms with trans-2-butene 3.2.1. Experimental Rate coefficients for the reaction of Cl atoms with trans-2-butene were measured over the temperature range of 269- 363K relative to 1,3-butadiene, isoprene and 1-pentene. The plot of



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relative decrease in the concentration of trans-2-butene due to its reaction with Cl atoms relative to 1,3-butadiene, isoprene and 1-pentene is shown in Figure 6. The data was observed to be linear and was passing through the origin, ascertains the non-influence of the secondary reactions on the measured rate coefficients. Rate coefficients for the reaction R2 were measured at 263, 285, 298, 310, 330, 350 and 363K. Each experiment was repeated two to three times and the slopes along with the errors which were obtained from linear least-square fitting of the data are given in Table 4. The measured rate coefficients for the reaction of R2 relative to 1,3-butadiene [ $#"

[ $#"

MP# O , JK LM#&#

MP# O MO $#&# =

=

(3.21±0.47)×10-10

cm3

molecule-1

s-1]

and

isoprene

(3.18±0.8)×10-10 cm3 molecule-1 s-1] were observed to be closer to the

reported rate coefficients. Whereas the obtained rate coefficients relative to 1-pentene [ $#"

MP# O #&#&# =

(4.58±0.48)×10-10 cm3 molecule-1 s-1] was observed to be higher when

compared with the reported rate coefficients in the literature16-18. The reported experimentally measured rate coefficients by Ezell et al.,16 Orlando et al.17 and Kaiser et al.18 are Q#"" #

". =

(3.31±0.47)×10-10 cm3 molecule-1 s-1, b$"

molecule-1 s-1,

M#$ # ". =

&LO # ". =

(4.0±0.1)×10-10 cm3

(3.27±0.4)×10-10 cm3 molecule-1 s-1 respectively. Therefore, the

rate coefficients measured relative to1,3-butadiene and isoprene were used to fit the Arrhenius equation. The deduced temperature dependent rate coefficient for the reaction R2 is . =

(1.11±0.31) ×10-10 exp [(291±88)/T] cm3molecule-1s-1. Slight negative temperature dependence

was observed over the studied temperature range. The Arrhenius plot for the reaction R2 is shown in Figure 7 along with the calculated rate coefficients, vide infra. 3.2.2. Computational


Page 13 of 34

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The optimized geometries of reactants, pre-reactive complexes, transition states and products at MP2/6-31 G (d,p) level of theory are given in Figure 8. As shown in the structure 2, in trans-2butene two addition channels (TS1a and TS2a) and eight abstraction channels (TS1 to TS8) were found and addition channels were represented by suffix ‘a’. In trans-2-butene, the two addition channels (TS1a and TS2a) are identical. Two olefinic hydrogens (TS4 and TS8) and two methyl (CH3) groups which are connected to the both sides of the double bond are identical. Out of three hydrogens in each methyl group, two of them are identical. Therefore, totally one independent addition channel (TS1a) and three independent abstraction channels (TS1, TS2 and TS4) were found. To account the presence of two identical methyl groups and olefinic hydrogens, symmetry number σ = 2 was used in the rate coefficient calculations. The Cl atom addition on double bond of trans-2-butene leads to the formation of chloroalkyl radical (P1a) via transition state TS1a with a relative barrier height of -4.4 kcal mol-1 which is the lowest barrier reaction channel. This addition transition state is formed via a pre-reactive complex (RC1a). Abstraction of hydrogen atom reaction from methyl group leads to the formation of product P1 and P2 via transition states TS1 (6.6 kcal mol-1) and TS2 (10.9 kcal mol1

) respectively. The olefinic hydrogen abstraction reaction leads to the formation of product P4

via transition state TS4 with a relative barrier height of 14.2 kcal mol-1, which is the highest barrier among all the possible channels in the test molecule. In the case of abstraction reactions, the leaving C-H bond length was elongated up to 42 and 14% in olefinic and methyl groups respectively when compared with respective normal C-H bond lengths in trans-2-butene. The energies of all the stationary points were refined at CCSD(T)/cc-pvdz and QCISD(T)/ccpvdz using the optimized geometries obtained at MP2/6-31G(d,p) level of theory. Temperature dependent rate coefficients were calculated for the reaction R2 using CVT/SCT approach with the structural and energy parameters obtained at MP2/6-31G(d), MP2/6-31G(d,p), MP2/631+G(d,p), CCSD(T)/cc-pvdz//MP2/6-31G(d,p) and QCISD(T)/cc-pvdz//MP2/6-31G(d,p) level of theories. All the obtained rate coefficients are tabulated in Table 5. The rate coefficients for the reactions R2 were obtained from the sum of the individual rate coefficients (kTotal = kTS1a+ k TS1+

kTS2+kTS4). In case of reaction R2, the obtained rate coefficient [ 7#O$` = 2.80×10-09 cm3

molecule-1 s-1] at MP2/6-31G(d,p) level of theory are observed to be closer to the experimentally measured rate coefficient [ . = (3.20±0.04)×10-10 cm3 molecule-1 s-1] and reported rate



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Page 14 of 34

coefficients in the literature16-18. The Arrhenius plot is shown in Figure 7 along with the experimentally measured rate coefficients in the present study. Linear least squares method was a used to deduce the temperature dependent Arrhenius expression, 7#O$` = (1.52±0.13) ×10-12

exp[(2464±120)/T] cm3 molecule-1 s-1. The calculated branching ratios (using equation 4) of each reaction channel for the reaction R2 is shown in Figure 9. It is clear from Figure 9 that the Cl atom addition across the double bond is kinetically highly favorable. The potential energy surface for the reaction of R2 is given Figure 10. From this figure, it is clear that the transition state TS1a is submerged. Therefore, it is obvious that the reaction would be predominated by this channel. Standard Gibb’s free energy (∆Go), standard enthalpy (∆Ho) and standard entropies (∆So) for the reaction R2 were obtained at MP2/6-31G(d,p) level of theory and are given in Table 6. Addition channel is more exothermic when compared to the other channels. Therefore, the addition channel is thermodynamically more favorable in addition to the kinetic favorability. Olefinic hydrogen abstraction channel (TS4) is more endothermic than all other reaction path ways. 3.3. Reaction pathway The branching ratios of individual reaction channels for the reactions of R1 and R2 are given in Tables 7 and 8 respectively. In case of reaction R1, the Cl atom addition at terminal carbon is the major contributor towards its total rate coefficient whereas, addition at second carbon is the minor contributor and abstraction channels contribution is negligible over the studied temperature range. This is in consistence with the observation made by Walavalkar et al.36 from the product analysis of reactions of Cl atoms with a series of 1-alkenes, which indicates the preferential addition of Cl atom at terminal carbon. This is probably due to the addition of Cl atom at terminal carbon generates tertiary radical, which is more stable when compared with the primary radical generated by Cl addition at the second carbon. To understand the mechanism of Cl atom reactions with alkenes, obtained rate coefficients in the present investigation are compared with analogous molecules in the literature and tabulated in Table 9. The rate coefficients for the reactions of Cl atoms at 298K with ethene23, propene23, isobutene, trans-2-butene, cis-2-butene16 and 2-methyl-2-butene16 are (0.93±0.6)×10-10 cm3 molecule-1 s-1, (2.76±0.6)×10-10 cm3 molecule-1 s-1, (3.43±0.11)×10-10 cm3 molecule-1 s-1, 14 ACS Paragon Plus Environment

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(3.20±0.04)×10-10 cm3 molecule-1 s-1, (3.76±0.84)×10-10 cm3 molecule-1 s-1 and (3.95±0.32)×10-10 cm3 molecule-1 s-1 respectively. An increment in the rate coefficient by factor of 1.8 was observed with methyl group substitution from ethene to propene. Similarly, from propene to isobutene, the increase is by a factor of 0.7 and by factor of 0.5 in case of isobutene to 2-methyl-2-butene. There is an increment in the rate coefficient by methyl substitution from ethene to 2-methyl-2butene, which indicates that in the Cl atom addition reactions, addition followed by elimination of HCl reactions and hydrogen abstraction reactions contribute to the global rate coefficients. 3.4. Atmospheric implications To understand the fate of test molecules in the atmosphere, cumulative lifetimes were calculated with respect to their reactions with OH radicals, Cl atoms, NO3 radicals and O3 molecules using the following equation c c c c c = + + + deff dgh djk dgl dmgl where, τeff is the cumulative life time of the chemical species, τOH, τCl, τO3 and τNO3 are rate coefficients for the reactions of test molecules with OH radicals, Cl atoms, O3 molecules and NO3 radicals respectively. The global average concentrations used in the present investigation are 1×106 radical cm-3, 2.5×108 radical cm-3, 7×1011 molecule cm-3, 1.3×105 atom cm-3 for OH and NO3 radicals37, O3 molecules38 and Cl atoms39 respectively. It should be noted here that, the concentration of Cl atom is higher at marine boundary layer (MBL) compared with the other regions and therefore we have used the concentration at MBL only. The obtained rate coefficients and lifetimes are given in Table 10. From Table 10, it is clear that OH and NO3 radicals are playing major role to degrade the test molecules in the atmosphere. Here it should be noted that the Cl atom reactions are much faster (in the order of 10-10 to 10-11 cm3 molecule-1 s1

) than the corresponding OH radicals37 (in the order of 10-10 to 10-13 cm3 molecule-1 s-1) and NO3

radicals37 (in the order of 10-11 to 10-18 cm3 molecule-1 s-1) reactions. The cumulative lifetimes of the test molecules are in few minutes and these compounds will be lost via oxidation process as soon as they released into the atmosphere.



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Page 16 of 34

3.5. Ozone formation potentials Degradation of butenes generates ozone molecules in the troposphere. As these butenes are released into the Earth’s atmosphere in large quantities, it is necessary to estimate its ozone formation potentials. The methodology which was developed by Bufailini40 was used to calculate the ozone formation potentials. The average ozone production during the reaction of OH radicals with isobutene and trans-2-butene were calculated to be 4 and 4 ppm respectively using the following equation +, 0no3 1 1 − .a× / 0no3 n = × 0 − 3 4.62.7 × 10 t − 0no3 0no3 2.7 × 10 t vw

where, n’ is the maximum possible ozone molecules, which can be produced from one molecule of butene, ka is the rate coefficient of OH radicals with butene and (OH) is the concentration of OH radicals (1×106 radical cm-3)37. The degradation of butenes would lead to a significant amount of ozone formation in the troposphere. 4. Conclusions In the present study, the kinetics of Cl atoms reactions with isobutene and trans-2-butene were investigated. Both experimentally and computationally, negative temperature dependence was observed for the title reactions over the studied temperature range. Title reactions are completely governed by addition of Cl atom across the double bond and contribution of abstraction reactions towards their total rate coefficients are negligible. The cumulative atmospheric lifetimes for the test molecules were calculated with respect to Cl atoms, OH radicals, O3 molecules and NO3 radicals. The molecules are lost in the atmosphere within few minutes after they are released, which suggest their inconsiderable effect on the global warming of the Earth. Supporting information The optimized structural parameters, energies and vibrational frequencies of all the reactants, pre-reactive complexes (RCs), transition states and products are given in the supporting information (Tables S.I.1.1 to S.I.4.2).


Page 17 of 34

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Acknowledgement BR thank the Department of Science and Technology (DST), Government of India for research funding. Both the authors thank Mr. V. Ravichandran for the constant technical support at HPCE of IIT Madras. References (1) Forstner, H. J. L.; Flagan, R. C.; Seinfeld, J. H. Molecular speciation of secondary aerosol from photo oxidation of the higher alkenes: 1-octene and 1-decene. Atmos. Environ. 1997, 31, 1953-1964. (2) Walavalkar, M.; Sharma, A.; Alwe, H. D.; Pushpa, K. K.; Dhanya, S.; Naik, P. D.; Bajaj, P. N. Cl atom initiated oxidation of 1-alkenes under atmospheric conditions. Atmos. Environ. 2013, 67, 93-100. (3) Preston, T. J.; Dunning, G. T.; Orr-Ewing, A. J. Direct and indirect hydrogen abstraction in Cl + alkenes reactions. J. Phys. Chem. A 2014, 118, 5595–5607. (4) Yoon, J. W.; Jhung, S. H.; Chang, J. Trimerization of isobutene over solid acid catalysts: comparison between cation-exchange resin and zeolite catalysts. Bull. Korean Chem. Soc. 2008, 29, 339-341. (5) Park, D. H.; Kim, S.; Pinnavaia, T. J.; Tzompantzi, F.; Prince, J.; Valente, J. S. Selective isobutene oligomerization by mesoporous MSU-SBEA catalysts. J. Phys. Chem. C 2011, 13, 5809-5816. (6) Kroll, J. H.; Clarke, J. S.; Donahue, N. M.; Anderson J. G.; Demerjian, K. L. Mechanism of HOx formation in the gas-phase ozone-alkenes reaction. J. Phys. Chem. A 2001, 105, 15541560. (7) Burnes, E.; Wichelns, D.; Hagen, J. W. Economic and policy implications of public support for ethanol production in California’s San Joaquin Valley. Energy Policy 2005, 33, 1155-1157. (8) Kent and Riegel’s hand book of Industrial Chemistry and Biotechnology, 2007. (9) Atkinson, R.; Pitts, J. N. Jr. Rate constants for the reaction of OH radicals with propylene and the butenes over the temperature range 297-425K. J. Chem. Phys. 1975, 63, 3591-3595. (10) Atkinson, R. Kinetics and mechanisms of the gas-phase reactions of the hydroxyl radical with organic compounds under atmospheric conditions. Chem. Rev. 1986, 86, 69-201.



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(11) Ohta, T. Rate constants for the reactions of OH radicals with alkyl substituted olefins. Int. J. Chem. Kinet. 1984, 16, 879-886. (12) Grosjean, D.; Williams, E. L. Environmental persistence of organic compounds estimated from structure-reactivity and linear free-energy relationships: unsaturated aliphatics. Atmos. Environ. 1992, 26, 1395-1405. (13) Smith, G. P. Laser pyrolysis studies of OH reaction rates with several butenes at 1200 K. Int. J. Chem. Kinet. 1987, 19, 269-276. (14) Vasu, S. S.; Lam, K. H.; Davidson, D. F.; Hanson, R. K.; Golden, D. M. Reactions of OH with butene isomers: measurements of the overall rates and a theoretical study. J. Phys. Chem. A 2011, 115, 2549-2556. (15) Sims, I. R.; Smith, I. W. M.; Bocherel, P.; Defrance, A.; Travers, D.; Rowe, B. R. Ultra-low temperature kinetics of neutral-neutral reactions: rate constants for the reactions of OH radicals with butenes between 295 and 23 K. J. Chem. Soc. Faraday Trans. 1994, 90, 1473-1478. (16) Ezell, M. J.; Wang, W.; Ezell, A. A.; Soskin, G.; Finlayson-Pitts, B. J. Kinetics of reactions of chlorine atoms with a series of alkenes at 1 atm and 298 K: structure and reactivity. J. Phys Chem. Chem. Phys. 2002, 4, 5813–5820. (17) Orlando, J. J.; Tyndall, G.S.; Apel, E.C.; Riemer, D.D.; Paulson, S.E. Mechanisms of the reaction of Cl atoms with a series of unsaturated hydrocarbons under atmospheric conditions. Int. J. Chem. Kinet. 2003, 35, 334–353. (18) Kaiser, E. W.; Wallington, T. J. Pressure dependence of the reaction Cl+C3H6. J. Phys. Chem. 1996, 100, 9788-9793. (19) Vijayakumar, S.; Rajakumar, B. Kinetic investigation of chlorine atom initiated photo oxidation reactions of 2,3-dimethyl-1,3-butadiene in the gas phase: an experimental and theoretical study. RSC Adv., 2016, 6, 67739-67750. (20) Vijayakumar, S.; Rajakumar, B. Experimental and theoretical investigations on the reaction of 1,3-butadiene with Cl atom in the gas phase. J. Phys. Chem. A 2017, 121, 1976-1984. (21) Srinivasulu, G.; Rajakumar, B. Gas phase kinetics of 2,2,2-trifluoroethylbutyrate with Cl atom: an experimental and theoretical study. J. Phys. Chem. A 2015, 119, 9294-9306. (22) Bedjanian,Y.; Laverdet, G; Bras, G. L. Low pressure study of the reaction of Cl atoms with isoprene. J. Phys. Chem. A 1998, 102, 953-959.


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(23) Coquet, S.; Ariya, P.A. Kinetics of the gas-phase reactions of Cl atom with selected C2-C5 unsaturated hydrocarbons at 283 < T < 323 K. Int. J. Chem. Kinet. 2000, 32, 478-484. (24) Moller, C.; Plesset, M. S. Note on an approximation treatment for many-electron systems, Phys. Rev. 1934, 46, 618−622. (25) Frisch, M. J.; Pople, J. A. Binkley, J. S. Self-consistent molecular orbital methods 25. Supplementary functions for Gaussian basis sets. J. Chem. Phys. 1984, 80, 3265-3269. (26) 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 B.01.; Gaussian, Inc.: Wallingford, CT, 2010. (27) Dennington, I. I. R.; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.; Gilliland, R. GaussView, Version 3.09; Semichem, Inc.: Shawnee Mission, KS, 2003. (28) Zheng, J.; Bao J. L.; Meana-Pañeda, R.; Zhang, S.; Lynch, B. J.; Corchado, J. C.; Chuang, Y. -Y.; Fast, P. L.; Hu, W. -P. ; Liu, Y. -P.; et al. Polyrate–version 2016-2A; University of Minnesota, Minneapolis, 2016. (29) Gonzalez-Lafont, A.; Truong, T. N.; Truhlar, D. G. Interpolated variational transition state theory: practical methods for estimating variational transition state properties and tunneling contributions to chemical reaction rates from electronic structure calculations. J. Chem. Phys. 1991, 95, 8875−8894. (30) Lu, D. H.; Truong, T. N.; Melissas, V. S.; Lynch, G. C.; Liu, Y. P.; Garrett, B. C.; Steckler, R.; Isaacson, A. D.; Rai, S. N.; Hancock, G.C. POLYRATE 4: Comput. Phys. Commun. 1992, 71, 235-262. (31) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K; Vitaly, R. Pople, J. A. Gaussian-3 theory using reduced Moller-Plesset order, J. Chem. Phys. 1999, 110, 10, 4703-4709. (32) Alwe, H. D.; Sharma, A.; Walavalkar, M. P.; Dhanya, S.; Naik, P. D. Reactivity of Cl atom with triple-bonded molecules. An experimental and theoretical study with alcohols. J. Phys. Chem. A 2014, 118, 7695−7706 . (33) Dash, M. R.; Rajakumar, B. Reaction kinetics of Cl atoms with limonene: An experimental and theoretical study. Atmos. Environ. 2014, 99, 183-195. (34) Petita, A. S.; Harvey, J. N. Atmospheric hydrocarbon activation by the hydroxyl radical: a simple yet accurate computational protocol for calculating rate coefficients. Phys. Chem. Chem. Phys., 2012, 14, 184–191. 19 ACS Paragon Plus Environment


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(35) Lauraguais, A.; Bejan, I.; Barnes, I.; Wiesen, P.; Coeur, C. Rate coefficients for the gasphase reactions of hydroxyl radicals with a series of methoxylated aromatic compounds. J. Phys. Chem. A 2015, 119, 6179-6187. (36) Walavalkar, M. P.; Vijayakumar, S.; Sharma, A.; Rajakuma,r B.; Dhanya, S. Is H atom abstraction important in the reaction of Cl with 1-alkenes? J. Phys. Chem. A 2016, 120, 40964107. (37) Atkinson, R. Atmospheric chemistry of VOCs and NOx. Atmos. Environ. 2000, 34, 20632101. (38) Singh, H. B.; Thakur, A. N.; Chen, Y. E.; Kanakidou, M. Tetrachloroethylene as an indicator of low Cl atom in the troposphere. Geophys. Res. Lett. 1996, 23, 1529−1532. (39) Spicer, C. W.; Chapman, E. G.; Finlayson-Pitts, B. J.; Plastridge, R. A.; Hubbe, J. M.; Fast, J. D.; Berkowitz, C. M. Unexpectedly high concentrations of molecular chlorine in coastal air. Nature 1998, 394, 353−356. (40) Bufalini, J. J.; Walter, T. A.; Bufalini, M. M. Ozone formation potential of organic compounds. Environ. Sci. Technol. 1976, 10, 908-912.


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Table 1: Relative rate measurements for the reaction of Cl atoms with isobutene over the temperature range of 269-363K at 760 Torr in N2 with reference to 1,3-butadiene, isoprene and 1-pentene.

T (K)

Reference compound

1,3-butadiene

269±2

Isoprene

1-pentene 1,3-butadiene 285±2

Isoprene 1-pentene

1,3-butadiene

298±2

Isoprene


Isoprene 1-pentene 1,3-butadiene

330±2


350±2

Isoprene 1-pentene

363±2

1,3-butadiene

(ksample/ kreference)±2σ 1.06±0.04 1.05±0.02 1.02±0.01 1.08±0.02 1.07±0.01 1.08±0.02 1.02±0.02 1.11±0.03 1.38±0.02 0.98±0.03 0.99±0.05 1.00±0.03 1.11±0.03 1.13±0.01 1.04±0.01 1.25±0.03 1.01±0.04 1.00±0.02 1.03±0.05 1.02±0.02 1.01±0.01 1.06±0.04 1.11±0.03 1.08±0.01 1.08±0.01 1.02±0.04 1.04±0.03 1.03±0.01 1.07±0.01 1.15±0.02 1.14±0.02 0.99±0.02 0.89±0.01 1.00±0.02 1.01±0.04 1.00±0.04 1.16±0.02 0.98±0.02 1.00±0.05 0.99±0.03 0.98±0.04 1.11±0.01 1.19±0.02 0.96±0.02

(ksample/ kreference)Average±2σ

(k±2σ)×10-10 (cm3molecule-1s-1)

1.04±0.02

3.70±0.02

(k±2σ)Average×10-10 (cm3molecule-1s-1)

Lite.k×10-10 (cm molecule-1s-1) at 298K 3

4.04±0.47 1.07±0.01

4.38±0.01

1.17±0.37

7.16±0.37

0.98±0.01

3.40±0.01

1.05±0.15

3.88±0.15

1.14±0.12

5.98±0.12

1.01±0.01

3.35±0.49

7.16±0.37

3.63±0.33

5.98±0.12

3.43±0.11 1.03±0.05

(3.40 ± 0.28)

3.51±0.54

[Ezell et al.] 1.09±0.03

5.11±0.18

5.11±0.18

1.03±0.01

3.20±0.01

1.05±0.05

3.36±0.05

1.15±0.01

4.89±0.01

0.94±0.07

2.87±0.07

1.05±0.01

2.93±0.01

1.08±0.08

3.99±0.08

0.99±0.01

2.61±0.01

0.98±0.01

2.63±0.01

1.15±0.11

3.74±0.11

3.74±0.11

0.97±0.01

2.57±0.01

2.53±0.10

3.28±0.11

4.89±0.01 2.89±0.08

3.99±0.08

2.62±0.03



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Isoprene

1-pentene

0.98±0.05 1.00±0.06 0.98±0.01 1.01±0.02 0.95±0.01 1.13±0.03 1.38±0.02 0.99±0.03

0.98±0.06

2.50±0.06

1.17±0.36

3.52±0.36

Page 22 of 34

3.52±0.36

Table 2: Calculated total CVT/SCT rate coefficients (cm3 molecule-1 s-1) for the reaction of Cl atoms with isobutene obtained at different level of theories.

1.66×10-06

QCISD(T)/ccpvdz//MP2/631G(d,p) 8.65×10-05

CCSD(T)/ccpvdz//MP2/631G(d,p) 4.93×10-07

1.17×10-07

3.76×10-07

1.27×10-05

1.28×10-07

7.00×10-08

4.06×10-08

1.17×10-07

2.77×10-06

4.42×10-08

275

2.85×10-08

1.73×10-08

4.54×10-08

8.10×10-07

1.88×10-08

298

1.43×10-08

9.08×10-09

2.21×10-08

3.16×10-07

9.77×10-09

325

7.33×10-09

4.82×10-09

1.09×10-08

1.25×10-07

9.71×10-09

350

4.36×10-09

2.95×10-09

6.32×10-09

6.10×10-08

3.15×10-09

375

2.79×10-09

1.94×10-09

3.96×10-09

3.29×10-08

2.06×10-09

400

1.90×10-09

1.35×10-09

2.65×10-09

1.93×10-08

1.43×10-09

T(K)

MP2/631G(d)

MP2/631G(d,p)

MP2/631+G(d,p)

200

8.78×10-07

4.45×10-07

225

2.14×10-07

250

a

k at 298K

(3.43±0.11)×10-10 [a] (3.40±0.28)×10-10 [b]

This work (Experimental) and bEzell et al.[16]

Table 3: Barrier heights [∆E0‡, kcal mol−1], heat of reaction [∆H0 (298K), kcal mol−1], Gibbs free energy [∆G0 (298K), kcal mol−1] and entropy of reaction [∆S0 (298K), cal mol−1 K−1] for the reaction of Cl atoms with isobutene obtained at MP2/6-31 G (d,p) level of theory. TSs

∆E0‡,kcal mol−1

∆H0, kcal mol−1

∆G0, kcal mol−1

∆S0, calmol−1 K−1

TS1a TS2a TS1 TS2 TS3

-5.2 -3.3 18.4 11.8 7.2

-17.4 -17.1 21.1 10.3 -2.1

-9.9 -8.4 18.9 8.3 -3.2

-25.2 -29.3 7.3 6.9 3.8


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Table 4: Relative rate measurements for the reaction of trans-2-butene with Cl atoms over the temperature range of 269-363K at 760 Torr in N2 with reference to 1,3-butadiene, isoprene and 1-pentene. T (K)

Reference compound 1,3-butadiene

269±2

Isoprene



298±2

Isoprene



330±2


350±2

Isoprene 1-pentene

(ksample/ kreference)±2σ 1.02±0.03 0.95±0.05 0.91±0.02 0.72±0.02 0.88±0.01 0.78±0.02 1.01±0.02 0.96±0.03 0.98±0.02 0.96±0.04 1.04±0.07 0.79±0.03 0.86±0.03 0.94±0.01 1.00±0.03 0.94±0.04 0.96±0.01 1.01±0.06 0.81±0.02 0.85±0.01 0.99±0.04 1.02±0.03 0.99±0.01 0.92±0.01 1.00±0.05 0.98±0.02 0.94±0.01 0.93±0.01 0.89±0.02 0.96±0.02 0.93±0.01 0.99±0.04 1.02±0.07 0.93±0.02 1.00±0.04 0.96±0.04 0.85±0.02 0.98±0.01 0.99±0.03 0.95±0.07 0.92±0.03 1.01±0.04 0.89±0.01 1.08±0.02 1.07±0.01

(ksample/ kreference)Average±2σ

(k±2σ)×10-10 (cm3molecule-1s-1)

0.96±0.11

3.41±0.11

(k±2σ)Average×10-10 (cm3molecule-1s-1)

Lite.k×10-10 (cm3molecule1 -1 s ) at 298K

3.32±0.25 0.79±0.16

3.23±0.16

0.98±0.05

6.02±0.05

1.00±0.05

3.45±0.05

0.82±0.09

3.03±0.09

0.97±0.08

5.09±0.08

0.97±0.03

3.21±0.47

6.02±0.05

3.24±0.59

5.09±0.08

3.20±0.04 0.88±0.18

3.18±0.81 3.31±0.47 [Ezell et al.]

0.97±0.10

4.58±0.48

0.99±0.01

3.08±0.01

0.93±0.01

2.99±0.01

0.92±0.07

3.97±0.07

1.00±0.02

3.07±0.02

0.96±0.09

2.81±0.09

0.93±0.14

3.44±0.14

0.97±0.02

2.56±0.02

0.92±0.12

2.46±0.12

1.01±0.21

3.29±0.21

4.58±0.48

4.0±0.1 [Orlando et al.] 3.27±0.4 [Kaiser et al.]

3.04±0.11

3.97±0.07

2.94±0.35

3.44±0.14

2.57±0.03

3.29±0.21



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

0.89±0.05 0.94±0.03 0.99±0.06 0.98±0.01 0.95±0.02 0.95±0.01 0.96±0.03 0.97±0.02 0.95±0.03

1,3-butadiene

363±2

Isoprene

1-pentene

0.94±0.1

Page 24 of 34

2.47±0.1 2.46±0.03

0.96±0.03

2.45±0.03

0.96±0.01

2.89±0.01

2.89±0.01

Table 5: Calculated total CVT/SCT rate coefficients (cm3 molecule-1 s-1) for the reaction of Cl atoms with trans-2-butene obtained at different level of theories.

4.38×10-07

QCISD(T)/ccpvdz//MP2/631G(d,p) 4.84×10-05

CCSD(T)/ccpvdz//MP2/631G(d,p) 3.86×10-05

2.64×10-08

1.13×10-07

7.39×10-06

6.04×10-06

1.99×10-08

1.04×10-08

3.86×10-08

1.67×10-06

1.39×10-06

275

8.88×10-09

4.94×10-09

1.62×10-08

4.97×10-07

4.22×10-07

298

4.81×10-09

2.80×10-09

8.40×10-09

1.97×10-07

1.70×10-07

325

2.64×10-09

1.61×10-09

4.40×10-09

7.96×10-08

6.92×10-08

350

1.66×10-09

1.05×10-09

2.66×10-09

3.92×10-08

3.44×10-08

375

1.11×10

-09

7.25×10

-10

1.73×10

-09

1.89×10

-08

1.89×10

-08

7.92×10

-10

5.29×10

-10

1.20×10

-09

1.26×10

-08

1.13×10-08

T(K)

MP2/631G(d)

MP2/631G(d,p)

MP2/631+G(d,p)

200

1.91×10-07

8.53×10-08

225

5.39×10-08

250

400 a

k at 298K

(3.2±0.04)×10-10 [a] (3.31±0.47)

×10-10 [b]

(4.0±0.1)

×10-10 [c]

(3.27±0.4)

×10-10 [d]

This work (Experimental), b Ezell et al.[16], cOrlando et al.[17] and d Kaiser et al.[18]

Table 6: Barrier heights [∆E0‡, kcal mol−1], heat of reaction [∆H0 (298K), kcal mol−1], Gibbs free energy [∆G0 (298K), kcal mol−1] and entropy of reaction [∆S0 (298K), cal mol−1 K−1] for the reaction of Cl atoms with trans-2-butene obtained at MP2/6-31 G (d,p) level of theory. TSs TS1a TS1 TS2 TS4

∆E0‡,kcal mol−1 ∆H0, kcal mol−1 ∆G0, kcal mol−1 -4.4 6.6 10.9 14.2

-17.5 -3.3 9.7 17.4

-9.5 -5.3 7.8 15.1

∆S0calmol−1 K−1 -26.8 6.6 6.6 7.8


Page 25 of 34

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 7: Percentage contribution of kTS1a, kTS2a and ktotal abstraction to the total rate coefficient for the reaction of isobutene with Cl atoms over the temperature range of 200-400K obtained at MP2/6-31 G (d,p) level of theory. T(K) 200 225 250 275 298 325 350 375 400

TS1a 99.64 99.44 99.21 98.94 98.68 98.35 98.04 97.71 97.39

TS2a 0.36 0.56 0.79 1.06 1.32 1.65 1.96 2.29 2.61

Total abstraction 2.0×10-12 6.1×10-11 9.2×10-10 8.5×10-09 4.7×10-08 2.6×10-07 9.9×10-07 3.2×10-06 8.8×10-06

Table 8: Percentage contribution of kTS1a and ktotal abstraction to the total rate coefficient for the reaction of trans-2-butene with Cl atoms over the temperature range of 200-400K obtained at MP2/6-31 G (d,p) level of theory. T(K)

TS1a

Total abstraction

200

100.0

2.9×10-10

225

100.0

6.5×10-09

250

100.0

7.7×10-08

275

100.0

5.9×10-07

298

100.0

2.8×10-06

325

100.0

1.4×10-05

350

100.0

4.6×10-05

375

99.99

1.4×10-04

400

99.99

3.5×10-04



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 34

Table 9: Comparison of rate coefficients for the reactions of Cl atoms with series of alkenes and their Arrhenius parameters.

Molecule

kCl ×1010 3

A×1011 -1 -1

3

-1 -1

Ea (kcal mol-1)

(cm molecule s )

(cm molecule s )

0.93±0.6a

0.39±0.2

-1.88

2.76±0.6a

1.60±1.8

-1.69

3.43±0.11b

4.99±0.4

-1.16

3.20±0.04b

11.3±3.0

-0.57

3.76±0.84c

-

-

3.95±0.32c

-

-

p ro p e n e

a

Coquet et al [23]., b present study, c Ezell et al. [16]


Page 27 of 34

Table 10: Cumulative atmospheric lifetimes (τ) of the butenes calculated with different atmospheric oxidants. Cl atoms a 10

OH radicals b

Molecule

kCl ×10

isobutene

3.43

τ (hr) 4.9

trans-2-butene

3.20

5.6

[a]

11

NO3 radicals b 13

Cumulative

O3 molecules b

lifetimes

5.14

5.4

3.44

3.2

0.11

τ (hr) 35.1

6.4

4.3

3.90

2.84

19.0

2.1

kOH ×10

τ (hr)

kNO3 ×10

τ (hr)

kO3 ×10

17

with reference to isoprene with reference to 1-pentene with reference to 1,3-butadiene

1.4 1.2 1.0 0.8 0.6 0.4 0.2

Isobutene + Cl

0.0 0.0

0.2

0.4 0.6 0.8 ln([reference]0/[reference]t)

1.0

1.2

Figure 1: Plot of the relative decrease in the concentration of isobutene due to its reaction with Cl atoms relative to1,3-butadiene, isoprene and 1-pentene.


τeff (min)

48.7

83.5

present work, [b] Atkinson et al. [27].

ln ([sample]0 /[sample]t)

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

Isobutene + Cl

-15

3

-1 -1

lnk (cm molecule s )

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 28 of 34

-20

MP2/6-31+G(d,p) MP2/6-31G(d), Ezell et al. Experimental,

-25

2.5

3.0

MP2/6-31G(d,p) QCISD(T)/cc-pvdz//MP2/6-31G(d,p) CCSD(T)/cc-pvdz//MP2/6-31G(d,p)

3.5

4.0

4.5

5.0

1000/T (K)

Figure 2: Arrhenius plot of CVT/SCT rate coefficients obtained at different level of theories between the temperature range of 200 and 400K, and experimentally measured rate coefficients between the temperature range of 269 and 363 K for the reaction of Cl atoms with isobutene.


Page 29 of 34

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: Optimized geometries of the reactants, pre-reactive complexes (RCs), transition states (TSs) and products (Ps) for the reaction of Cl atom with isobutene. The obtained bond lengths (Å) are given on structures obtained at MP2/6-31G (d,p) level of theory. Black represents carbon atoms, blue represents hydrogen atoms and green represents Cl atoms. 29 ACS Paragon Plus Environment


100

Branching ratio (%)

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 30 of 34

80

Total abstraction TS1a TS2a

60 40

isobutene + Cl

20 0 200

250

300 T (K)

350

400

Figure 4: Calculated branching ratios vs temperature for the reaction of Cl atoms with isobutene obtained at MP2/6-31 G (d,p) level of theory.

Figure 5: Potential energy diagram for the reaction of Cl atoms with isobutene obtained at MP2/6-31 G (d,p) level of theory at 298K.


with reference to isoprene with reference to 1-pentene with reference to 1,3-butadiene

0.6

0.4

0.2

Trans-2-butene + Cl 0.0 0.0

0.2

0.4 ln ([reference]0/[reference]t)

0.6

Figure 6: Plot of the relative decrease in the concentration of trans-2-butene due to its reaction with Cl atoms relative to 1,3-butadiene, isoprene and 1-pentene. -10

Trans-2-butene + Cl

-12 -14 -16 -18

3

-1 -1

ln k (cm molecule s )

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


ln ([sam ple] 0 /[sam ple] t )

Page 31 of 34

-20 -22

Experimental MP2/6-31+G(d,p) Kaiser et al., MP2/6-31G(d) Ezell et al., MP2/6-31G(d,p) QCISD(T)/cc-pvdz//MP2/6-31G(d,p) CCSD(T)/cc-pvdz//MP2/6-31G(d,p)

-24 -26

2.5

3.0

3.5

4.0

4.5

5.0

1000/T (K)

Figure 7: Arrhenius plot of CVT/SCT rate coefficients obtained at MP2/6-31 G (d,p) level of theory between the temperature range of 200 and 400K, and experimentally measured rate coefficients between the temperature range of 269 and 363K for the reaction of Cl atoms with trans-2-butene. 31 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 32 of 34

Figure 8: Optimized geometries of the reactants, pre-reactive complexes (RCs), transition states (TSs) and products (Ps) for the reaction of Cl atom with trans-2-butene. The obtained bond lengths (Å) are given on structures obtained at MP2/6-31 G (d,p) level of theory. Black represents carbon atoms, blue represents hydrogen atoms and green represents Cl atoms.


Page 33 of 34

100

Branching ratio (% )

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


Total abstraction TS1a

80 60 40

Trans-2-butene + Cl

20 0 280

300

320

340 T (K)

360

380

400

Figure 9: Calculated branching ratios vs temperature for the reaction of Cl atoms with trans-2butene obtained at MP2/6-31 G (d,p) level of theory.

Figure 10: Potential energy diagram for the reaction of Cl atoms with trans-2-butene obtained at MP2/6-31 G (d,p) level of theory at 298K. 33 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 34 of 34

Graphical abstract

Cl atom initiated photo oxidation of isobutene and trans-2-butene


An Exprimental and Computational Study on the Cl Atom Initiated

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