Theoretical study on the gas phase reaction of methyl chavicol with hydroxyl radical

Accepted Manuscript Theoretical Study on the Gas Phase Reaction of Methyl Chavicol with Hydroxyl Radical R. Bhuvaneswari, K. Senthilkumar PII: DOI: Reference:

S2210-271X(19)30045-3 https://doi.org/10.1016/j.comptc.2019.02.005 COMPTC 12424

To appear in:

Computational & Theoretical Chemistry

Received Date: Revised Date: Accepted Date:

12 December 2018 26 January 2019 7 February 2019

Please cite this article as: R. Bhuvaneswari, K. Senthilkumar, Theoretical Study on the Gas Phase Reaction of Methyl Chavicol with Hydroxyl Radical, Computational & Theoretical Chemistry (2019), doi: https://doi.org/10.1016/ j.comptc.2019.02.005

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Theoretical Study on the Gas Phase Reaction of Methyl Chavicol with Hydroxyl Radical R. Bhuvaneswari a and K. Senthilkumar*, a a

Department of Physics, Bharathiar University, Coimbatore, India-641 046.

*Corresponding author: Fax No: +91-422-2422387, E-Mail: [email protected]

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Abstract Methyl chavicol (MC) is an oxygenated aromatic compound, suspected to be harmful to human health at high concentration. The present study focuses on the gas-phase oxidation mechanism of methyl chavicol initiated by OH radical by employing combined quantum chemical calculations and kinetic modelling. The initiation of the reaction is dominated by the abstraction of methyl chavicol hydrogen atom by OH radical and electrophilic addition of OH radical to the methyl chavicol. Energetically favourable reaction pathways and the stable products were identified by obtaining the reaction heats and potential energy surface. The rate constant for the favourable initial reactions has been calculated over the temperature range of 278-350 K using canonical variational transition state (CVT) theory with small curvature tunnelling (SCT) correction. The calculated thermodynamic and kinetic results reveal that the abstraction of hydrogen atom and addition of OH radical is more likely will occur at allyl group carbon atoms, C7, C8 and C9 of methyl chavicol to form radical intermediate, I4 and MC-OH adducts, I5 and I6. The subsequent secondary reactions for the initially formed intermediates were studied with O2, NO and HO2 radicals. The possible oxidation products identified are 4-methoxybenzaldehyde, (4-Methoxy-phenyl)-acetaldehyde and 4-methoxy toluene. The products identified in the present study are same as that identified experimentally and contribute to secondary organic aerosol loading in the atmosphere.

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Introduction Biogenic Volatile Organic Compounds (BVOCs) includes atmospheric trace gases other than methane, carbondioxide and monoxide [1]. It is a well recognized fact that the atmosphere is loaded with a wide variety of BVOCs, for which the main source is from vegetative emissions [2,3]. The amount of emission of hydrocarbons into the atmosphere by plants as BVOCs far exceeds the level from anthropogenic activity [4]. More than 30,000 different BVOCs, including terpenes, oxygenated aromatics, green leaf volatiles, benzoids, phenylpropanoids and methyl esters are emitted by plants [5]. In recent years, with the evolution of advancements in the analytical instrumentation, a wide suite of BVOCs have been measured in the atmosphere [6,7]. BVOCs plays a crucial role in fuelling the tropospheric air chemistry through the interaction between the biosphere and atmosphere [8]. Among the BVOCs, oxygenated aromatics, methyl chavicol (C10H12O) is contributing to the formation of secondary organic aerosol (SOA) and in predicting the oxidative capacity of the atmosphere [9,10]. It has been shown in earlier studies that the photo oxidation of volatile organic compounds (VOCs) is also responsible for the formation of SOA [11,12]. The present study focuses on the oxidation of methyl chavicol (MC) by reaction with OH radical. Among the oxygenated aromatics, MC was identified as the main floral emission from oil palm plantation, contributing 40% yield to the production of SOA by oxidation [13,14]. Growing interest of this compound in the atmospheric science community is relatively recent, though it has been reported to be a major component of pondesora pine emission, almost 30 years ago [15]. Although methyl chavicol is a C-10 semi volatile organic compound found in the ambient air near many ecosystems, it is not classified as torpedoed compounds because it is produced by the phenylpropanoid pathway rather than terphenoid pathway [15]. Methyl chavicol also known as estragole is synthesized by variety of plants and it is found to be a major essential oil component of many common herbs, such as basil,

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tarragon and fennel [7]. In addition, methyl chavicol is used in various applications, such as perfumes, food seasoning and flavouring, aromatherapy and in medicines [16].

Methyl

chavicol, like many other BVOC is suspected to be harmful to human health at high concentration (EPA 2002) [17]. Literature survey reveals that, only a few experimental studies were performed on oxidation of methyl chavicol with various oxidants. In the atmosphere, methyl chavicol is degraded after the reaction with oxidants, such as hydroxyl radicals (OH), ozone (O3) or nitrate radicals (NO3) as well as photolysis. Photo oxidation and gas phase reaction of methyl chavicol with OH radical was experimentally investigated by Lee et al. [14] Perriera et al. [13,18] and Gai et al. [19]. The room temperature rate constant for the oxidation of methyl chavicol with OH radical was reported as (5.2±0.78) x10-11 cm3molecule-1s-1 by Brown et al. [7]. The rate constant value reported by Gai et al. [19] for OH initiated

photo oxidation of

methyl chavicol is 5.4 x 10 -11

cm3 molecule -1s-1 at 298 K. The experimental rate constant values are comparable with the values posted by Environmental Protection Agency (5.7 x 10-11 cm3 molecule-1s-1) [17]. Experimentally,

4-methoxybenzaldehyde,

(4-Methoxy-phenyl)-acetaldehyde

and

4-

methoxytoluene are identified as the products by Lee et al. [14], Perriera et al. [13,18] and Gai et al. [19] through the photo oxidation of methyl chavicol. However, experimental kinetic study provides only the average rate constant and product formation, and does not provide information about the detailed thermochemistry and reaction pathways involved in the reaction process. Upon now theoretical gas phase atmospheric reaction study on methyl chavicol is scarce and hence we are driven to investigate the atmospheric fate of methyl chavicol by reaction with OH radical through electronic structure calculations. Reaction between methyl chavicol and OH radical is expected to proceed with hydrogen atom abstraction and OH radical addition pathways, both pathways will end up with the loading of SOA into the atmosphere. The reaction mechanism and kinetics of methyl chavicol with OH

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radical are studied through combined quantum chemical and kinetic calculations. In order to determine the energetically favourable reaction pathways, the energy barrier (∆E) and reaction heat (∆H) were calculated for both H-atom abstraction and OH radical addition pathways. Further, the formation of the products through the secondary reactions of the alkyl radical intermediates and MC-OH adducts formed in the most promising reaction paths were studied in detail. Computational Details The geometry optimization and harmonic vibrational frequency calculations of all the stationary points involved in the reaction of methyl chavicol with OH radical were performed using density functional theory (DFT) with a variety of exchange-correlation functionals. The DFT methods used in this work includes, M06-2X [20], B3LYP [21, 22] and dispersioncorrelated ωB97XD [23] in conjunction with 6-311+G(d,p) basis set. In order to validate the accuracy of the structure and energy of the stationary points, the optimization and frequency calculations were performed using all the above mentioned DFT methods. Minima along the reaction potential energy surface of the studied reactions were identified with zero imaginary frequency, and each transition state was identified with one imaginary frequency. Intrinsic reaction coordinate (IRC) [24, 25] calculations were performed to verify the transition state, that connects the respective reactant and product at all the above mentioned level of theories. The enthalpy of the reaction and Gibbs free energy values were calculated by including thermodynamic correction to the energy at 298.5 K and 1 atmospheric pressure. In addition, we have also performed the single point energy calculation at M06-2X/ aug-cc-pVDZ and CCSD(T)/cc-pVDZ level of theory on the geometry optimized at M06-2X/6-311+G(d,p) level of theory. The thermochemical parameters, like enthalpy and Gibbs free energy for all the initial reaction pathways were also calculated using highly accurate composite method,

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ROCBS-QB3 [26,27]. The Gaussian 09 [28] program was used to perform all the electronic structure calculations. The kinetic calculations for the titled reactions was performed on the basis of potential energy surface information obtained from M06-2X/6-311+G(d,p) level of theory. The rate constant for the favourable reactions of methyl chavicol with OH radical was calculated based on canonical variational transition state theory [29-31], including quantum tunnelling effect [30,31]. The kinetic calculations were performed by using the GAUSSRATE 2010A [32] program which is an interface program between the Gaussian 09 [28] and POLYRATE 2010A [33] programs. Results and Discussion Geometry optimization of methyl chavicol (MC) molecule predicts three possible conformers (CI, CII and CIII) with positive eigenvalues, and their structures are shown in Fig. 1. The three conformers differ mainly in the dihedral angle, (C1-C7-C8-C9) with the value of 1200,

Fig. 1: The structures of the three conformers of methyl chavicol. 6

00 and -1200, respectively. In order to explore the minimum energy conformers of methyl chavicol molecule, the potential energy surface scan as a function of dihedral angle, (C1-C7-C8-C9) is performed at M06-2X/6-311+G(d,p) level of theory and is shown in Fig.2. The result of potential energy scan reveals that conformer CII is the most stable conformer. The millimetre-wave spectrum analysis by Peter et al. [34, 35] clearly shows the existence of the conformers, CI and CII. Table S1(see Supplementary material) lists some of the important structural parameters of the identified conformers along with the values obtained by Orawan et al. [35]. The results show that the structure of the titled molecule obtained from the present study is closer to the values reported earlier. Among the three conformers, only the conformers, CI and CII are found to exist both experimentally and

Fig. 2: Relaxed potential energy surface scan along the dihedral angle (C1-C7-C8-C9) at the M06-2X/6-311+G(d,p) level of theory. The relative energy given in y-axis is with respect to the energy of the minimum energy conformer, CII.

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theoretically, with an energy difference of 0.2 kcal/mol. Since the two conformers are closer in energy, the initial hydrogen atom abstraction and OH radical addition reactions were studied for both the conformers, CI and CII and the energetic information are given in Tables S2 and S3 (see Supplementary material). As shown in Scheme 1, the initial reaction of methyl chavicol molecule with OH radical will proceed either via H-atom abstraction from MC by OH radical or OH radical addition mechanism. While comparing the energetics of the two conformers, it is observed that the most favourable reaction and the nature of the reaction mechanism were same for both the conformers. The intermediate complexes formed in the initial reactions of conformer CI is higher in energy than that of the conformer CII by 0.2 kcal/mol only. Hence, in the present investigation, the reaction mechanism and kinetics of methyl chavicol with OH radical is studied in detail for the conformer, CII, and the intermediates formed in the favourable pathways were further subjected to subsequent reactions with O2, NO• and HO2• and the formed intermediates were further subjected to decomposition reactions. In analogy to the oxidation mechanism of aromatic compounds [36-38] initiated by OH radical, the reaction between methyl chavicol and OH radical proceed via an indirect mechanism, in which the reactant and intermediate complexes were formed. In order to choose the best quantum chemical method, the relative energy, enthalpy and free energy of all the pathways (R1-R12) were calculated using DFT and composite methods and are summarized in Table S3(see Supplementary material) along with the single point energy calculated at M06-2X/aug-cc-pVDZ//M06-2X/6-311+G(d,p) and CCSD(T)/cc-pVDZ//M062X/6-311+G(d,p) level of theory. It has been observed that the structural parameters of the stationary points optimized using the DFT functional, M06-2X, ωB97XD and B3LYP methods with 6-311+G(d,p) basis set are similar. For instance, the average root mean square deviation (rmsd) between the internal coordinates obtained from M06-2X and ωB97XD

8

methods for the reactant complex, transition state and intermediate of the favourable hydrogen atom abstraction pathway, R4 is 0.07 Å, 0.08 Å and 0.1 Å, respectively, and for M06-2X and B3LYP methods, the rmsd is 0.1 Å, 0.05 Å and 0.03 Å. The rmsd between the internal coordinates of the reactive species obtained using the M06-2X and ωB97XD methods for the favourable OH radical addition pathway, R12 is 0.2 Å, 0.1 Å and 0.1 Å for the reactant complex, transition state and intermediate complex, respectively, and for M06-2X and B3LYP methods, the rmsd is 0.1 Å, 0.1 Å and 0.2 Å. The similar rmsd values were found for the reactive species involved in other reactions. Note that, the above mentioned rmsd values includes both bonded and non-bonded interacting atoms. As given in Table S3 (Supplementary material), the energetics calculated at different DFT methods are comparable and for few cases the maximum deviation is about 1-3 kcal/mol. The thermochemical parameters, like enthalpy and Gibbs free energy calculated from DFT and ROCBS-QB3 methods agree within the deviation of 2-3 kcal/mol. As noted from

Table

S3

(Supplementary

material),

the

relative

energy

obtained

at

M06-2X/6-311+G(d,p), ωB97XD/6-311+G(d,p) level of theories and single point energy calculation at M06-2X/aug-cc-pVDZ level of theory are in closer agreement with each other, and a slight deviation is noted for the reaction energy obtained at B3LYP level of theory. While comparing the results obtained with the different methods mentioned above, it is observed that the most favourable reaction and the nature of the reaction mechanism remains the same with respect to the methods of calculation. It has been shown in earlier studies that, M06-2X functional with adequate basis set is a suitable method to provide reliable results for thermochemistry and kinetics [39-41] and this method is most widely used to study the chemical reactions involving radicals. Hence, in the present work the geometry and the energetics obtained using M06-2X functional with 6-311+G(d,p) basis set are discussed in detail and are used in further kinetic calculations. The optimized structure of the transition

9

states and other reactive species involved in the primary reaction mechanism is shown in the Figs. S4 and S5 (see Supplementary material). The potential energy surface of the methyl chavicol + OH• reaction obtained at the M06-2X/6-311+G(d,p) level of theory is depicted in Fig. 3. Initial Reactions As shown in Scheme 1, Pathways R1-R6 corresponds to abstraction of hydrogen atom from C10, C5, C6, C7, C8 and C9 carbon atoms of MC, yielding H2O and radical intermediates, I1-I6, and the Pathways, R7-R12 corresponds to the electrophilic addition of OH radical at six different carbon atoms, C4, C5, C6, C1, C8 and C9 of MC to form MC-OH adducts, I7-I12. For numbering of atoms and pathways see Fig. 1 and Scheme 1. The relative energy, enthalpy and Gibbs free energy calculated at M06-2X level of theory for H-atom abstraction and OH radical addition reactions are summarized in Table S3 (Supplementary material). Pathway, R1 corresponds to the hydrogen atom abstraction from methoxy group carbon atom, C10 with an energy barrier of 7.5 kcal/mol. This reaction proceeds through a reactant complex, RC1 and a transition state, TS1. In the transition state, TS1 the C10-H11 bond is elongated by 0.1 Å with respect to the reactant complex, RC1 and the distance between the cleaved H-atom of MC and O atom of OH radical is 1.45 Å which is 0.5 Å longer than the O-H bond length of H2O molecule. The relative enthalpy and Gibbs free energy for the formation of I1+H2O is -19.67 and -21.67 kcal/mol. This pathway is found to be highly exothermic and exoergic. Reaction pathways, R2 and R3 corresponds to the formation of radical intermediates, I2 and I3 by the hydrogen atom abstraction from ortho (C5) and meta (C6) carbon atoms of phenyl ring. These intermediates are formed through the transition states, TS2 and TS3 with an energy barrier of 12.79 and 11.52 kcal/mol. In the structure of the transition states, TS2 and TS3 the C-H bond is elongated by 0.14 Å and 0.12 Å with respect to the reactant

10

complexes, RC2 and RC3, and the distance between the cleaved H-atom and O-atom of the OH radical is 1.27 Å and 1.3 Å, which is 0.31 and 0.34 Å longer than the O-H bond length of newly formed H2O molecule. These reactions are exothermic and exoergic with a relative enthalpy and Gibbs free energy of -5.76 and -6.96 kcal/mol for the formation of I2+ H2O and -5.48, -6.57 kcal/mol for I3+ H2O formation.

Scheme 1: Proposed reaction scheme for the OH addition and hydrogen atom abstraction

pathways of methyl chavicol + hydroxyl radical reaction.

11

Pathways, R4, R5 and R6 corresponds to the hydrogen atom abstraction reaction from the allyl group carbon atoms, C7, C8 and C9 of MC to form the intermediates, I4, I5 and I6 through the transition states, TS4, TS5 and TS6 with an energy barrier of 6.63, 9.92 and 10.06 kcal/mol. On comparing the geometrical parameters of the transition states, TS4, TS5 and TS6, the elongation of C-H bonds is found to be 0.06 Å, 0.1 Å and 0.14 Å, respectively with respect to the reactant complexes, and the distance between the cleaved H-atom and O-atom of the OH radical is longer by 0.5 Å, 0.4 Å and 0.3 Å than the OH bond length of the H2O molecule. The reaction enthalpy and Gibbs free energy for the abstraction of hydrogen atom from C7 carbon atom is highly favourable with ∆H298 =-29.97 kcal/mol and ∆G298 =-30.27 kcal/mol. For the removal of a hydrogen atom from C8 and C9 carbon atoms in allyl group the ∆H298 and ∆G298 are found to be -10.42 and -11.78 kcal/mol and -6.37 and -7.51 kcal/mol. The elongation of C-H bond is smaller than the elongation of O-H bond indicating that the transition states involved in the primary hydrogen atom abstraction reactions of MC by OH radical are reactant like, in accordance with Hammond’s postulate [42] applied to an exothermic reaction. Earlier studies on the atmospheric degradation of various organic compounds reveal that most of the hydrogen atom abstraction processes are exothermic in nature [43-45]. From, Fig.3 it is observed that the hydrogen atom abstraction from allyl group carbon atom, C7 (pathway R4) is the most favourable pathway with an enthalpy of -29.97 kcal/mol and the radical, I4 formed in this pathway is stabilized due to conjugation with the double bond [19]. The hydrogen atom abstraction from methoxy carbon atom, C10 (pathway R1) is the second most favourable reaction pathway with an enthalpy of -19.67 kcal/mol. Therefore, from the energy barriers and thermodynamic consideration for the above mentioned six H-atom abstraction pathways, the order of reaction competition ability is R4 > R1 > R5 > R6 > R2 > R3. Because of electron withdrawing nature of double bond, energy barrier for the hydrogen atom abstraction from phenyl ring [46] carbon atoms

12

(pathway R2 and R3) is found to be higher and hydrogen atom abstraction from phenyl ring of methyl chavicol is the least favourable.

Fig. 3: Potential energy profile for the OH addition and hydrogen atom abstraction reactions of methyl chavicol (MC) with hydroxyl radical calculated at M06-2X/6-311+G(d,p) level of theory. The values in parantheses are in kcal/mol obtained at the CCSD(T)/cc-pVDZ level of theory. As illustrated in Scheme 1, the OH radical can bind with MC at the C-C double bonds present in phenyl ring and of allyl group through six different pathways, R7-R12. Pathway, R7 corresponds to the addition of OH radical to ipso carbon atom, C4 of MC leading to the formation of MC-OH adduct, I7 through the transition state, TS7 with an energy barrier of

13

5.83 kcal/mol. As shown in Fig. 4, in TS7 the distance between the O atom of the OH radical and the carbon atom of methyl chavicol is 2.02 Å (O2-C4) which is shorter by 0.6 Å with

14

Fig. 4: The optimized structure of the transition states involved in the initial reaction of methyl chavicol with hydroxyl radical. respect to the reactant complex. Also, the addition of OH radical to MC elongates the C-C bond adjacent to the site of addition, indicating a fraction of electron density transfer to the newly formed C-O bond. For the intermediate adduct, I7, C4-C5 and C3-C4 bond distances 15

distances are increased by 0.1 Å. The formation of MC-OH adduct, I7 is exothermic with an enthalpy of -19.99 kcal/mol and exoergic with ΔG298 of -9.92 kcal/mol. Similar behaviour is observed for the pathways, R8, R9 and R10, which represents the electrophilic addition of OH radical at ortho (C5), meta (C6) and para (C1) carbon atoms of MC which leads to the formation of MC-OH adducts, I8, I9 and I10. The energy barrier corresponding to the transition states, TS8, TS9 and TS10 is 2.9, 4.84 and 3.08 kcal/mol, respectively. In the transition states TS8-TS10 the distance between O atom of OH radical and carbon atoms, C5, C6 and C1 of MC is around 2 to 2.1 Å. These reaction pathways are exothermic with relative enthalpy around -18 to -19 kcal/mol and exoergic with a Gibbs free energy around -8.5 kcal/mol. Pathways, R11 and R12 corresponds to the OH radical addition at the allyl group carbon atoms, C8 and C9 of MC to form MC-OH adduct intermediates, I11 and I12 through the transition states, TS11 and TS12 with an energy barrier of 1.8 and 1.4 kcal/mol. As shown in Fig. 4, in TS11 and TS12 the distance between the oxygen atom of the OH radical and the carbon atom of the MC is 2.14 Å (O2-C8) and 2.17 Å (O2-C9), which is shorter by 0.45 and 0.68Å with respect to the reactant complex. The MC-OH adducts, I11 and I12 are formed in an exothermic reaction with an enthalpy of -29.81 and -30.7 kcal/mol and exoergic with ΔG298 value of -19.73 and -20.78 kcal/mol. Upon comparing the energy barrier and relative enthalpy values of all the studied six addition pathways, it is observed that the enthalpy of formation of MC-OH adduct follow the order I12 < I11 < I7 < I10
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considering the energy barrier and thermodynamics of all the twelve reaction pathways it is concluded that the reactivity of phenyl ring carbon atom towards H-atom abstraction and OHaddition is insignificant for the overall system. The calculations of the temperature dependence of rate constants have been performed at M06-2X/6-311+G(d,p) and B3LYP/6-311+G(d,p) level of theories for the reactions of methyl chavicol with hydroxyl radical. The rate constant for the favourable H-atom abstraction and OH-addition reactions, R1, R4, R11 and R12 are calculated using canonical variational transition state theory (CVT) with small curvature tunnelling correction, SCT (CVT/SCT) in the temperature range from 278 to 350 K with zero point corrected energies, gradient and Hessians calculated at M06-2X/6-311+G(d,p) level of theory. The rate constant calculated using the transition state theory (TST), CVT, TST/SCT and CVT/SCT methods are summarized in Table S4 of Supplementary material. As observed from Table S4 (Supplementary material), the ratio between the rate constants calculated from CVT with SCT (CVT/SCT) and CVT is in the order of 1, showing a negligible tunnelling effect on the rate constants calculated for the studied reactions, whereas an appreciable variational effect is noticed. For instance, the observed variational effect for the favourable H-atom abstraction reaction and OH-addition reaction is around 0.4. Arrhenius plot for the rate constant calculated over the temperature range of 278 to 350 K is shown in Fig. 5. As shown in Fig.5, a positive temperature dependence is observed for the rate constants. At 298 K the rate constant calculated at M06-2X/6-311+G(d,p) level of theory, for the formation of intermediates I1and I4 from hydrogen atom abstraction reaction is 1.13x10-13, 6.62x10-13 and for the intermediates I11 and I12 formed from OH radical addition reaction is 6.04x10-12 and 1.73x10-11 cm3molecule-1s-1. The rate constant calculated for Pathway R12 (formation of MCOH intermediate I12) agrees well with the experimental rate constant value reported by

17

Fig. 5: Arrhenius plot for the rate constant obtained for the formation of the intermediates I1, I4, I11 and I12 over the temperature range of 278-350 K at M06-2X/6-311+G(d,p).

Gai et al. [19] and Environmental Protection Agency [17. For comparison purpose, the rate constant for the favourable H-atom abstraction pathway, R7 and OH-addition reaction pathway, R12 was calculated using B3LYP/6-311+G(d,p) level of theory. At 298 K the rate constant calculated for the formation of radical intermediates, I4 and I12 is 1.49x10-11 and 1.27x10-10 cm3molecule-1s-1, which is higher by two to one order of magnitude than that obtained from M06-2X method, which is presumably due to lower barrier height calculated at B3LYP method. As mentioned above, our results clearly infer that M06-2X method provides a reliable kinetic results and B3LYP method overestimates the rate constants. Previously it is reported [49-51] that the kinetic results obtained from B3LYP functional 18

significantly disagree with the experimental data. The calculated rate constant values for OH addition and H-atom abstraction reactions and the corresponding thermochemical data strongly suggest that the reactions corresponding to the formation of intermediates, I4, I11 and I12 are favourable and hence the subsequent secondary reactions were studied for these intermediates. Subsequent reaction from intermediate, I4 The chemically active radical intermediate, I4 produced in the initial H-atom abstraction reaction can subsequently undergo bimolecular reaction with O 2. The possible reaction pathways for the intermediate, I4 is depicted in Scheme 2 and the optimized geometry of the reactive species involved in the reactions are shown in Figs. 6 and S6. The relative energy, enthalpy and Gibbs free energy are summarized in Table S7 (Supplementary material) and the profile of the potential energy surface is depicted in Fig. 7. Similar to the

Scheme 2: Possible secondary reaction from alkyl radical intermediate, I4.

19

secondary reactions of typical C-centred radical, O2 is added at C7 site of I4 to form a peroxy radical intermediate, I13 through the transition state, TS13 with an energy barrier of 6.51 kcal/mol. In the transition state, TS13 the C7-O2 bond distance is 2.112 Å which is 0.6 Å longer than that of the intermediate, I13. The reaction correspond to the formation of I13 is exothermic and exoergic with ∆H298 =-13.13 and ∆G298 =-10.35 kcal/mol. At lower NO concentration the formed peroxy radical, I13 can react with HO2, producing a stable product, 1-(4-methoxy-phenyl)-prop-2-en-1-yl-hydroperoxide (P1) and O2. Due to high electron

Fig. 6: The optimized structure of the transition states involved in the secondary reactions of the alkyl radical intermediates I4.

20

affinity, the terminal oxygen atom of peroxy radical abstracts the hydrogen atom of HO 2 and 1-(4-methoxy-phenyl)-prop-2-en-1-yl-hydroperoxide (P1) is formed along with an oxygen molecule. A transition state, TS14 with an energy barrier of 8.53 kcal/mol was identified for this product formation channel. In TS14, the breaking O5-H12 bond was elongated by 0.04 Å with respect to the intermediate complex, I13+HO2, and the newly formed O3-H12 bond in the product, P1 gets shortened by 0.6 Å. The product, P1 is formed in an exothermic reaction with an enthalpy of -24.54 kcal/mol and Gibbs free energy of -27.15 kcal/mol.

Fig. 7: Relative energy profile for the subsequent reactions of radical, I4. The relative energy (kcal/mol) calculated at M06-2X/6-311+G(d,p) level of theory is given for the reactive species. As shown in Scheme 2 the peroxy radical, I13 can oxidize by NO• to form alkoxy radical, I14 and NO2 in a barrier less reaction. The formation of alkoxy radical, I14 is exothermic and exoergic with an enthalpy and free energy of -34.68 and -41.6 kcal/mol. As shown in Scheme 2, the exit pathway for the alkoxy radical, I14 is its reaction with O 2 and decomposition reaction. The reaction between I14 and O 2 leads to the formation of the 21

product, 1-(4-methoxy-phenyl)-propenone (P2) along with HO2 through the transition state, TS15 with an energy barrier of 10.44 kcal/mol. The distance between H8 and O3 atoms in TS15 is 1.47 Å and in the product, P2 the bond length (H8-O3) is 0.993 Å. The product, P2 is formed in a highly exothermic and exoergic reaction with ∆H298 =-47.14 and ∆G298 =-49.05 kcal/mol. The second exit pathway studied for the alkoxy radical, I14 is the decomposition reaction. The decomposition of alkoxy radical results in the cleavage of C7C8 bond to generate 4-methoxy benzaldehyde (P3) and a carbon centred radical, I15 which surmounts a high energy barrier of 18.03 kcal/mol. In the transition state structure, TS16 the C7-C8 bond length is 2.116 Å which was 1.513 Å in the alkoxy radical, I14. The decomposition reaction is found to be endothermic and endoergic with a reaction enthalpy of 11.68 kcal/mol and Gibbs free energy of 2.3 kcal/mol. By comparing ΔETot and ∆H298 values calculated for the subsequent reactions of I4, it is found that the possible products formed from H-atom abstraction reactions are 1-(4-methoxy-phenyl)-prop-2-en-1-yl-hydroperoxide (P1) and 1-(4-methoxy-phenyl)-propenone (P2). Subsequent reactions from intermediate, I11 The scheme for the secondary reactions of the MC-OH adduct, I11 is shown in Scheme 3. The optimized structure of the transition states and other reactive species are shown in Figs. 8 and S8 (Supplementary material). The relative energy, enthalpy and Gibbs free energy are summarized in Table S9 (Supplementary material) and the profile of the potential energy surface is depicted in Fig. 9. Under atmospheric condition, the reaction of MC-OH adduct intermediate, I11 with O2 leads to the formation of a peroxy radical intermediate, I16. The reaction corresponding to the formation of the peroxy radical intermediate, I16 is a barrier less reaction with an enthalpy of formation, -33.99 kcal/mol.

22

Scheme 3: Scheme for the possible secondary reactions of intermediate adduct, I11. The main chemical fate of peroxy radical, I16 depend on the level of NO• x (x=1,2) in the atmosphere. When the concentration of NO• x is sufficiently high the loss of I16 is dominated by the reaction with NO•, leading to the formation of alkoxy radical intermediate, I17 in a barrier less reaction with an exothermicity of -17.95 kcal/mol, and the formation is exoergic with ∆G298=-23.91 kcal/mol. The formed alkoxy radical intermediate, I17 could further react with HO2 to form the product, 3-(4-methoxy- phenyl)-propane-1,2-diol (P4) along with O2 through the transition state, TS17 with an energy barrier of 8.31 kcal/mol. In TS17, the newly forming O3-H14 bond length is 1.553 Å which is shorter by 0.4 Å than that of the intermediate complex, I17+HO2. The product, P4 is formed in an exothermic and exoergic process with ∆H298 =-43.77 and ∆G298 =-46.34 kcal/mol. Reaction of peroxy radical with

23

Fig. 8: The optimized structure of the transition states involved in the secondary reactions of the alkyl radical intermediates, I11. HO2 is of central importance in the atmosphere, as it serve as sink for HO2 radical and terminate the ozone generating chain reactions [52].

The reaction of I16 with

hydroperoxy radical will lead to the formation of a stable product, 1-hydroperoxy-3-(4methoxy-phenyl)-propan-2-ol (P5) along with O2 through the transition state, TS18 with an energy barrier of 13.52 kcal/mol. The distance between O4 and H14 atoms is 2.265, 1.65 and 0.968 Å, respectively in I16+HO2, TS18 and P5. The observed decrease in bond length shows the formation of a strong bond between O4 and H14 atoms in P5. The product, P5 is formed in an exothermic reaction with a reaction enthalpy of -28.16 kcal/mol and Gibbs free energy of -30.54 kcal/mol. Thus, the reaction of peroxy and alkoxy radical with HO2• is the exit pathway for MC-OH adduct (I11), resulting in the formation of a stable products, 3-(4-methoxyphenyl)-propane-1,2-diol (P4) and 1-hydroperoxy-3-(4-methoxy-phenyl)-propan-2-ol (P5).

24

Fig. 9: Relative energy profile for the reaction of intermediate adduct, I11 with O2, NO, HO2 radical. Subsequent reactions from intermediate I12 MC-OH adduct intermediate, I12 was found to be the most promising intermediate formed by the reaction of methyl chavicol with OH radical. So, the subsequent reactions of I12 with atmospheric reactive species is more important. The reaction scheme for I12 is shown in Scheme 4. The optimized geometry of the transition states, intermediates and products are shown in Figs. 10 and S10 (Supplementary material) and the relative energy profile is shown in Fig. 11. The energy values are summarized in Table S11 (see Supplementary material). As shown in Scheme 3, the intermediate adduct, I12 is expected to react rapidly with O2 to yield a peroxy radical, I18 in a barrier less reaction. The process is exothermic and exoergic with ∆H298 =-29.73 and ∆G298 =-26.89 kcal/mol. The dominant fate of peroxy radical in the atmosphere is its reaction with either HO2 or NO radical. The formed peroxy radical, I18 at 25

Scheme 4: Possible secondary reaction scheme for the intermediate adduct, I12. lower NO• concentration react with HO2 to produce a stable product, 2-Hydroperoxy-3-(4methoxy-phenyl)-propan-1-ol (P6) and O2. The product formation is exothermic with an enthalpy of -6.99 kcal/mol. The energy barrier for this reaction pathway is 12.37 kcal/mol. The O3-H14 bond length in P6 is 0.7 Å which is shorter than that of the transition state, TS19. The reaction of peroxy radicals with nitric oxide is of profound importance in the atmosphere, since it leads to the production of ozone [53]. As shown in Scheme 4, in the process of formation of alkoxy radical, I19, the nitric oxide (NO) radical directly abstracts the

26

terminal oxygen atom of peroxy radical, I18 without the formation of a transition state. The formation of alkoxy radical, I19 is exothermic with ∆H298 =-23.15 and exoergic with ∆G298 =-34.58 kcal/mol.

Further, the formed alkoxy radical can undergo two different

decompositions, the first reaction is corresponding to the formation of (4-Methoxy-phenyl)acetaldehyde and the second reaction is leading to the formation of 4-methoxy benzyl radical.

Fig. 10: The optimized structure of the transition states involved in the secondary reactions of the alkyl radical intermediate, I12.

27

As shown in Fig. 11, the formation of product, (4-Methoxy-phenyl)-acetaldehyde (P7) in the first decomposition pathway needs to overcome an energy barrier of 9.98 kcal/mol. As shown in Fig. 10, in the transition state, TS20 the distance between C8 and C9 atoms is increased by 0.5 Å with respect to that of alkoxy radical, I19. The product, P7 is accompanied with the methyl radical, I20 with an exothermicity of -6.76 kcal/mol.

Fig. 11: Relative energy profile for the reaction of intermediate adduct, I12 with O2, NO•, HO2 radicals and decomposition reaction of alkoxy radical, I19. In the second decomposition pathway, 4-methoxy benzyl radical (I21) is formed by the breaking of C7-C8 bond present in the alkoxy radical, I19 through the transition state, TS21. In the transition state, TS21 the C7-C8 bond was elongated by 0.5 Å with respect to that of alkoxy radical, I19. This reaction has an energy barrier of 7.56 kcal/mol and is exothermic by -5.39 kcal/mol. Thus formed 4-methoxy benzyl radical (I21) could further react with HO2 radical to form the product, 4-methoxy toluene (P8) along with O2 through the 28

transition state, TS22 with an energy barrier of 13.59 kcal/mol. The distance between C7 and H10 atoms is 1.287 and 1.099 Å in TS22 and P8, respectively. The reaction corresponding to the formation of product, 4-methoxy toluene (P8) is an exothermic and exoergic process with ∆H298 =-13.33 and ∆G298 =-12.91 kcal/mol. The products identified in the present investigation are identical with that reported by Gai et al.[19]. That is, the oxidation of methyl chavicol leads to the formation of the products, 4-methoxybenzaldehyde (P3), (4-Methoxyphenyl)-acetaldehyde (P7) and 4-methoxy toluene (P8) through the secondary reaction of intermediates, I4 and I12 which significantly contribute to total ozone reactivity and aerosol loading in the global atmosphere. Conclusion The gas phase reaction of methyl chavicol with OH radical is investigated by using high level quantum chemical methods and variational transition state theory. Twelve reaction pathways were studied for the initial reaction. The calculated thermodynamic parameters show that all the studied initial reactions are exothermic in nature. The results indicate that the reactions corresponding to the hydrogen atom abstraction from allyl carbon atom, C7 (leading to the formation of I4) and OH radical addition at the allyl carbon atoms, C8 and C9 (leading to the formation of I11 and I12) are the favourable reaction pathways. The rate constant calculated for the initially formed intermediate I4, I11 and I12 at 298 K is 6.62x1013

, 6.04x10-12 and 1.73x10-11 cm3 molecule-1s-1. The subsequent secondary reactions were

studied for the initially formed alkyl radicals I4, I11 and I12, resulting in the formation of stable products,

1-(4-methoxy-phenyl)-prop-2-en-1-yl-hydroperoxide (P1), 1-(4-methoxy-

phenyl)-propenone (P2), 4-methoxy benzaldehyde (P3), 1-hydroperoxy-3-(4-methoxyphenyl)-propan-2-ol (P4), 1-hydroperoxy-3-

(4methoxyphenyl)

propan-2-ol (P5),

2-

hydroperoxy-3-(4-methoxy-phenyl)-propan-1-ol (P6), (4-Methoxy-phenyl)-acetaldehyde (P7) and 4-methoxy toluene (P8). The products, P3, P7 and P8 were identified as a SOA in the

29

oxidation reaction of methyl chavicol by OH radical which agrees well with that experimentally reported products by Gai et al.[19]. The reported reaction pathways, rate constants and products identified in this work clearly show the important role of methyl chavicol in the regional atmospheric chemistry. Further, the studied reaction pathways corresponding to the formation of intermediates and products will help to understand the atmospheric chemistry of BVOCs and experimental studies on BVOCs. Acknowledgements The authors are thankful to UGC and Department of Science and Technology (DST), India, for funding to establish the high performance computing facility under the SAP and PURSE programs. Supplementary Information The bond distances, bond angles and dihedral angles of Methyl chavicol molecule is summarized in Table S1. Relative energy, ΔETot (kcal/mol), enthalpy, ΔH298 (kcal/mol) and Gibbs free energy, ΔG298 (kcal/mol) for the initial reactions of conformer, CI and CII of methyl chavicol with OH radical is summarized in Table S2 and S3. The TST, CVT, TST (SCT), and CVT (SCT) rate constants (cm3 molecule-1 s-1 ) for the initial H-abstraction reaction (R1 and R4) and OH addition reactions(R11 and R12) of methyl chavicol with OH radical are given in Table S4. Optimized structure of the reactant complex, intermediate complex and intermediates involved in the primary reactions of methyl chavicol with hydroxyl radical is shown is Figure S5. Optimized structure of the reactive species involved in the secondary reactions of intermediate, I4, I11 and I12 are shown in Figure S6, S8, S10. Relative energy, ΔETot (kcal/mol), enthalpy, ΔH298 (kcal/mol) and Gibbs free energy, ΔG298 (kcal/mol) for the reactions of radical intermediate, I4, I11 and I12 calculated at M06-2X, ωB97XD, B3LYP level of theories are summarized in Table S7, S9 and S11.

Keywords: Methyl chavicol, Biogenic volatile organic compound, OH radical, kinetics, SOA products. 30

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Graphical Abstract

35

Highlights 

Gas-phase oxidation mechanism of methyl chavicol with OH radical were studied using DFT calculations.



Electrophilic addition of OH radical to the methyl chavicol is the energetically favourable reaction pathway.



The rate constant for the favourable reaction pathways

were calculated using

canonical variational transition state theory including quantum tunnelling effect. 

The subsequent secondary reactions were studied for the initially formed alkyl radical intermediates.



The products identified in the present study contribute to secondary organic aerosol loading into the atmosphere.

36