Temperature Dependent Kinetics of the Reaction of Criegee

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Temperature Dependent Kinetics of the Reaction of Criegee Intermediate with Propionaldehyde: A Computational Investigation Revathy Kaipara, and Balla Rajakumar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06603 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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The Journal of Physical Chemistry

Temperature Dependent Kinetics of the Reaction of Criegee Intermediate with Propionaldehyde: A Computational Investigation Revathy Kaipara 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: The temperature dependent kinetics for the reaction of Criegee intermediate (CH2OO) with propionaldehyde (CH3CH2CHO) was investigated using Canonical Variational Transition state theory (CVT) in conjunction with Small-Curvature tunneling (SCT) method and the Interpolated Single point energy (ISPE) method at the CCSD(T)/AUG-cc-pVTZ//B3LYP/6311G (d, p) level of theory. A rich chemistry was depicted by the title reaction, though the contributions of all the reaction pathways were limited to atmospheric pressure conditions. The reaction of CH2OO with CH3CH2CHO was identified to proceed via the formation of Secondary Ozonide (SOZ), which then underwent a sequence of unimolecular isomerization and decomposition reactions to form a variety of products. The obtained rate coefficient for the formation of SOZ at 298 K was determined to be, k = 2.44×10-12 cm3 molecule-1 s-1. At low temperature, collisionally stabilized SOZ was found to be the more stable product. Contrarily, at high temperature, SOZ degraded to HCHO, CH3CH2COOH was found to be major products. The complete degradation mechanism and thermochemistry for the reaction of CH2OO with CH3CH2CHO along with their rate coefficients over the temperature range of 200-1000 K are reported.

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1. INTRODUCTION Emission of VOCs in the Earth’s atmosphere takes place via both biogenic and anthropogenic sources.1,2 Removal of these molecules from the Earth’s atmosphere occurs mainly via photo-oxidation processes, which are primarily initiated by atmospheric oxidants such as OH and NO3 radicals, O3 molecules and Cl atoms.1-4 They are the major cleansing agents in the Earth’s atmosphere. The reaction of OH radicals with VOCs takes place during the day time, whereas NO3 radical chemistry is dominant in night.3, 4 Moreover, photo-oxidation of VOCs due to Cl atoms is significant in Marine Boundary Layer (MBL) conditions.5,6 Similar to these atmospheric oxidants, Criegee intermediate (CH2OO, CI) is another cleansing agent and has immense significance in determining the tropospheric chemistry. In 1949, Rudolf Criegee, proposed carbonyl oxide biradical (CH2OO) as a key intermediate in alkene ozonolysis.7 CH2OO is formed in the Earth’s atmosphere via 1, 3-cycloaddition reaction across the double bond, which initially forms the Primary Ozonide (POZ).8,9 Subsequently, POZ undergoes rapid dissociation to yield CH2OO and the corresponding aldehyde.9 Due to its high oxidation capacity, it causes non-photolytic production of OH radicals, either via unimolecular decomposition or bimolecular reactions with other available VOCs.10,11 In addition to this, CH2OO are also important for the formation of organic acids and aerosols in the troposphere.12 Therefore, the chemistry of CH2OO is at the vanguard of modern research in atmospheric chemistry. Out of all the emitted VOCs, carbonyl compounds, in specific, aldehydes are an important class of VOCs which are omnipresent in the Earth’s atmosphere. Emission of aldehydes in the Earth’s atmosphere not only happens via biogenic and anthropogenic sources, but also by the usage of “gasohol” in the fuel combustion, which produces carbon monoxide


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(CO) and aldehydes. Furthermore, photo-oxidation of methane and isoprene also contributes towards atmospheric concentration of aldehydes.13 In particular; effect of an emitted molecule can be gauged based on its residence time and global warming capacity. Aldehydes, in general, have lifetimes in the order of few hours. Propionaldehyde (CH3CH2CHO) is the third most abundant aldehyde in the Earth’s atmosphere with an atmospheric lifetime of 12-15 hours. The tropospheric concentration of CH3CH2CHO was found to be in the range of (1-2) × 1010 molecules cm-3.14-18 Additionally, for the direct measurement of the kinetics of CH2OO, carbonyl compounds were frequently employed as scavengers in an O3-H2 blend.19 The scavenger undergoes cycloaddition reaction with CH2OO to form Secondary Ozonide (SOZ), which further isomerizes and decomposes into more stable products.20-23 Taatjes et al.23 first investigated the reaction of CH2OO with CH3CHO at 4 Torr and 293 K using laser photolysis/synchrotron photoionization mass spectrometry. They reported the rate coefficient to be (9.5±0.7) ×10-13 cm3 molecule-1 s-1. Stone et al.24 measured the kinetics of CH2OO with CH3CHO as a function of pressure at 295 K using flash photolysis/laser induced fluorescence (FP-LIF) of CH2I2-O2-N2 gas mixtures with an excess amount of co-reagent (CH3CHO) and obtained the rate coefficient at 25 Torr of pressure as (1.48±0.04) × 10-12cm3 molecule-1 s-1. Furthermore, Wen et al.25 explored the reaction mechanism of CH2OO with CH3CHO theoretically and reported HCHO and CH3COOH as major products. Surprisingly, there were no studies reported on the reaction of CH2OO with propionaldehyde (CH3CH2CHO), to the best of our knowledge. Therefore, in this work, we employed the dual level of theory to investigate the reaction kinetics, thermodynamics and the degradation mechanism for the reaction of CH2OO with CH3CH2CHO.



2. COMPUTATIONAL METHODOLOGY All the Geometries of the reactants, pre and post reactive complexes, transition states and products involved in the various reaction pathways were optimized using Becke’s three parameter Lee- Yang-Parr hybrid functional theory B3LYP with Poples’ basis set 6-311G(d, p). B3LYP method has been extensively utilized for the study of various CH2OO systems because of its higher accuracy. It also performs better in computing thermodynamics and reaction kinetics, when compared to other DFT level of theories.26-33 Harmonic vibrational frequency calculations were carried out to confirm the stationary nature of the optimized structures and are tabulated in Table S2 of Supporting Information (SI). In order to endorse the fact that the local minima connect to delineated transition states, the minimum energy path (MEP), defined as the steepest drop from the saddle point to both reactant and product route in the mass- weighed Cartesian coordinate system, was contrived with intrinsic reaction coordinates (IRC) calculations and are shown in Figure S1.34,

35

Relax Potential Energy Surface scan (RPES) was carried out for

CH3CH2CHO (300 increment) as a function of the corresponding dihedral angle at the B3LYP/6311G(d, p) level of theory, and is shown in Figure S3. The obtained RPES scan energy was further used to identify the lowest energy conformer. Moreover, one dimensional hindered rotor (1D-HR) corrections were computed using the McClurg method.36 The single point energy calculations, the couple-cluster calculation with single, double and triple excitation CCSD (T) method with Dunning’s AUG-cc-pVTZ basis set were performed using the geometries obtained at B3LYP/6-311G (d,p) level of theory.37 Cartesian coordinates and harmonic vibrational frequencies for all the reactants, pre and post reactive complexes, transition states and products are given in Table S1. T1-diagnostic values given in Table S3 justified the reliability of singlereference character in CCSD (T) wave function. This value conferred a qualitative estimation of


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the importance of nondynamical electron correlation. To be more specific, CH2OO is considered as a closed shell species. T1 diagnostic values for a closed shell species less than 0.04, suggest that the result obtained from a couple cluster method or any other single reference method is acceptable.38-40 In this work, T1-diagnostic values were found to be not greater than 0.04 for any of the species. Gaussian 09 suite program was used for carrying out all the electronic structure calculations and normal mode frequency analysis.41 The vibrational frequencies were viewed using Gauss View 5.0. 2.1 Rate constant calculations The methodology used in this work for kinetics assumes equilibrium between the reactants (CH2OO and CH3CH2CHO) and the pre-reactive complex.

where, k1 and k-1 are the forward and the reverse rate coefficients for the corresponding equilibrium between the reactants and the pre-reactive complex, and k2 corresponds to the unimolecular decomposition rate for the pre-reactive complex to the final products. The equilibrium rate constant (Keq) was calculated using the following equation,

= =

exp (−

(

(1)

For the calculation of Keq, the partition functions were calculated using the vibrational frequencies and rotational constants computed at the B3LYP/6-311G(d, p) level of theory. On the other hand, the unimolecular rate constant (k2) was determined using

the Canonical

Variational Transition State Theory (CVT) with Small Curvature Tunneling (SCT) corrections along with Interpolated Single Point Energy (ISPE) calculations over the temperature range of



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200-1000 K. In variational transition state theory, the dividing surface is a dynamical bottleneck to flux in the direction of products, and the one allowing least flow of flux is identified variationally. The dividing surfaces are defined to be orthogonal to the reaction path, where the reaction path is defined as the minimum energy path connecting the saddle point with both the reactant and product regions. This path is located by following the path of steepest descents in both the directions from the saddle point in a mass-weighted coordinate system.46-48 The generalized expression for the thermal rate constant at a particular temperature T is given as a function of the location ‘s’ along the reaction coordinate,

k GT (T , s ) = κ SCT σ

 V (s)  k BT  Q GT (T , s )    exp − MEP  R h  Φ (T )  k BT  

(2)

Where, κ SCT is the small-curvature tunneling (SCT) transmission coefficient, σ is the symmetry factor, ‘s’ is defined as the distance along the MEP with the origin located at the saddle point, where the ‘s’ value is positive on the product side and negative on the reactant side, h is the Planck’s constant, ΦR is the reactant partition function, VMEP is the classical potential energy (also called the Born-Oppenheimer potential) along with its zero of energy at the reactants, and QGT is the internal partition function for the generalized transition state at ‘s’ with the local zero of energy at VMEP(s). Both ΦR (T) and QGT(T,s) partition functions are approximated as product of electronic, vibrational and rotational partition functions. In canonical variational transition state theory, the rate constant in the above equation is minimized with respect to‘s’, given as below. k CVT (T ) = min k GT (T , s ) s

(3)

To further incorporate quantum mechanical effects for motion along the reaction coordinate, CVT rate coefficients [kCVT(T)] were multiplied by a temperature dependent transmission coefficient κ(T), given as


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kCVT/SCT (T) = κSCT (T) × kCVT(T)

(4)

Moreover, the ratio of CVT and TST rate coefficients explained the effect of variational effect (VaG = VMEP + ZPE) on the calculated rate coefficients over the studied temperature range. To further incorporate the effect of quantum mechanical tunneling on the computed rate coefficients, the ratio of CVT and CVT/SCT rate constants was calculated. POLYRATE 2008 and GAUSSRATE 2009A programs were used to calculate the TST, CVT and CVT/SCT rate coefficients over the temperature range of 200-1000 K.49, 50. 3. RESULTS AND DISCUSSION 3.1 Potential energy surface Energetically seven distinct reaction pathways were studied for the reaction of CH2OO with CH3CH2CHO and are shown in Scheme-1. The reaction of CH2OO with CH3CH2CHO undergoes both bimolecular as well as unimolecular reactions. The potential energy surface (PES) for the title reaction is shown in Figure 1a and the corresponding geometries optimized at B3LYP/6-311G (d, p) are given in Figure 2. Relative energies (∆Er) for the transition states (TSs), intermediates (IMs) and products obtained at the dual level of theory, CCSD (T)/AUG-ccpVTZ// B3LYP/6-311G (d, p) are listed in Table 1a. As seen from Figure 1a, the reaction entrance pathway-1involves the formation of a weak Van der Waal complex (IM1, -7.9 kcal mol1

), formed directly from the reaction of CH2OO with CH3CH2CHO, leading to the formation of

Secondary Ozonide (SOZ, IM2, -51.5 kcal mol-1). A 1, 3-cycloaddition of CH2OO across the carbonyl bond in CH3CH2CHO goes via a five-membered ring transition state (TS1) signifying the formation of SOZ, with an energy barrier of -6.9 kcal mol-1. It can be seen from Figure 3, to further comprehend the formation of SOZ, the bond distance between C1-O11 atoms and O4-C5 atoms in TS1 was found to change from 2.54 A0 to 1.40 A0 and 2.51 A0 to 1.41 A0 respectively.



The formed SOZ undergoes subsequent isomerization and decomposition reactions to yield stable end products. The isomerization channel involves an intramolecular 1, 3- H shift from the carbonyl carbon (C5) to oxygen (O7) via a four-membered ring transition state (TS2) with an energy barrier of -12.6

kcal mol-1. The formation of intermediate IM3 (hydroxy methyl

propionate, HMP) via TS2 involves the increase of H6-O7 bond distance by 93 % and C5-H6 bond by 8 % respectively. The formed HPF further decomposes via an intramolecular 1,5- H shift sixmembered TS (TS3, -107.8 kcal mol-1) to yield HCHO and CH3CH2COOH as the final products with an ∆Er value of -114.4 kcal mol-1. As shown in Figure 3, the decomposition transition state TS3, was denoted along the MEP with an increase of O7-H6 bond by 27 % and a decrease of O4H6 by 19 % respectively. In addition to this, SOZ also depicted three other isomerization pathways to yield various stable products. From the obtained PES, pathway-2 characterizes a concerted three-membered intramolecular 1, 2-H shift TS (TS4, -10.5 kcal mol-1), followed by decomposition, to form HCOOH and CH3CH2CHO as end products. Comparison of barrier heights for the two distinct channels (pathway-1 and pathway-2) illustrated their reaction combativeness. The difference between the activation barriers of pathway-1 and pathway-2 was not more than ~3 kcal mol-1. Furthermore, in pathway-3, HCHO and CH3CH2CHOO were the end products formed with an increase in C1-O7 and C5-O11bond distances by 78 % and 98 % via TS5, with a barrier of -8.3 kcal mol-1. Due to the high energy barrier height for pathway-3 compared to other pathways, it was considered to be a minor contributor to the total reaction. Moreover, pathway-4 involved a 1, 2-ethyl shift three-membered ring TS (TS6, -6.9 kcal mol-1), followed by bond dissociation, to give HCHO and CH3CH2OCHO as stable products. The TS was identified with an increase of C5-C8 bond by 16 %, and C8-O4 bond by 55 % respectively as shown in Figure S4.This channel


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also had higher barrier height compared to other competing pathways (1 and 2), and hence was assumed to contribute very less to the overall reaction. The results obtained herewith were also in good agreement with the available literature data. Jalan et al.9 theoretically studied the reaction of CH2OO with CH3CHO and shown that the isomerization and decomposition pathway indeed exist. A similar conclusion was drawn experimentally by Taatjes et al.23 that the chemically activated SOZ either isomerizes or decomposes into stable products, when the energy barrier height of isomerization and decomposition channels was near or below the entrance reaction channels.23 In addition to this, there exists another pathway-5 which illustrated the formation of ozone and 1C4H8 as the final products. In this pathway, cycloaddition reaction involving the oxygen atom (O3) connected to the carbon atom in CH2OO to the carbonyl oxygen (O10) in CH3CH2CHO formed a five-membered ring TS (TS7, 15.6 kcal mol-1) through a 2-ethyl-1,2,3-trioxolane (IM5, POZ, -2.7 kcal mol-1) to give another TS (TS8, 55.7 kcal mol-1) to finally form the end products. In the process of formation of products, the bond distances of C1-O2 was found to increase by 89 % from its actual bond length and the C6-O10 bond was elongated by 98 % respectively, as shown in Figure S5. As the obtained barrier height for this pathway was energetically inaccessible in tropospheric conditions, it explains the unfeasibility of this pathway. Furthermore, two additional pathways (pathway-6 and pathway-7) were studied for the unimolecular decomposition of SOZ. It can be seen from Table 1a and Figure S6 that, pathway 6 involves the elongation of C5-O11 bond distance in SOZ by 43% from its actual bond length leading to the cleavage of C5-O11 bond via TS9 with a ∆Er of -73.2 kcal mol-1. This decomposition yields to the formation of methylenebis (oxy) and CH3CH2CHO as final products with a ∆Er of -98.6 kcal mol-1 and as shown in Table 1a. Due to the high energy barrier height of



this reaction compared to other reaction pathways, it was considered to be a minor contributor to the total reaction. Furthermore, methylenebis (oxy) is a biradical, which further decompose via TS10 with a ∆Er of 90.1 kcal mol-1 to form H2 and CO2 as products with a ∆Er of -94.0 kcal mol1

respectively and as shown in Table 1b and Figure 1b. The TS was identified with an increase of

C1-O2 bond by 48% and decrease of H2-H3 bond by 40 % respectively as shown in Figure S7. The formation of H2 and CO2 were evident from the theoretical study reported by Crehuet et al. 51

on the reaction of methylenebis (oxy) with ethylene, which explains the formation of H2 and

CO2 from the rapid decomposition of methylenebis (oxy). In addition to this, there exists another pathway-7, which involves the formation of HCHO and propylenebis (oxy) as final products and the formed propylenebis (oxy) further decomposes to form CO2 and C2H6 as end product. As shown in Figure S6, involves the increase in the bond distance of C1-O11 in SOZ by 34 % via TS11 with a ∆Er of -7.2 kcal mol-1. This leads to the formation of HCHO and propylenebis (oxy) as final products with ∆Er of -26.3 kcal mol-1.This pathway also has high ∆Er value when compared to other reaction pathways. Therefore, this pathway was considered to have minor contribution to the overall reaction. Further, it can be seen in Table 1c and Figure 1c that, decomposition of propylenebis (oxy) via TS12 (∆Er = 89.0 kcal mol-1) to form CO2 and C2H6 as end products with a ∆Er of -95.4 kcal mol-1. The TS was identified with an increase in the bond distance of C2-C5 by 40 % and by stretching the bond distance of H3-C5 by 27%. Among all, pathway-1 was found to be the lowest energy pathway for the title reaction. Therefore, HCHO and CH3CH2CHO are the major products formed in the reaction of CH2OO with CH3CH2CHO. This result was clearly supported by the study carried out by Wei et al.25 who investigated the complete degradation mechanism for the reaction of CH2OO with CH3CHO and drawn similar conclusions.


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3.2 Thermochemistry Thermochemical parameters (reaction entropies (S), Gibbs’ free energies (∆G0) and reaction enthalpies (∆H0)) for the studied reaction pathways determined at the B3LYP/6-311G(d, p) level of theory are given in Table 1a. The standard reaction enthalpies (∆H0) and Gibbs free energies (∆G0) are two important parameters used to explain the spontaneity of a reaction channel. Exothermicity (∆H0< 0) and endothermicity (∆H0 > 0) can be explained based on the values of reaction enthalpies, whereas exergonicity (∆G0< 0) and endergonicity (∆G0> 0) are explained in terms of ∆G0 values. Hence, the heat of reaction and the feasibility of the studied pathways were predicted using the calculated thermodynamic parameters. It can be seen from Table 1a that, pathway-1 is exothermic (∆H0 = -113.1 kcal mol-1) as well as thermodynamically more feasible (∆G0 = -113.0 kcal mol-1) in comparison to other pathways. Therefore, the products (HCHO and CH3COOH) formed via subsequent decomposition and isomerization TSs in pathway-1 was observed to be more favorable. The stability of the products from this pathway was evident due to the 1,3-H shift, which involves the formation of a strainfree five-membered ring TS (TS2). This behavior was previously reported by Taatjes et al. 23 and similar observations were made from the experimental results for the reaction of CH2OO with acetaldehyde (CH3CHO). Similarly, Wei et al. 25 proposed the complete pathway for the reaction of CH2OO with CH3CHO and concluded that the formation of HCHO and CH3COOH was more spontaneous when compared to other channels. In addition to the most feasible channel (pathway-1), thermodynamic parameters for other isomerization channels were also computed. From Table-1, formation of HCOOH and CH3CH2CHO via pathway-2 was also found to be exothermic (∆H0 = -65.3 kcal mol-1) and spontaneous (∆G0 = -79.6 kcal mol-1). In this particular pathway, an intramolecular 1,2-H shift



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involving via a three-membered ring TS, was found to be comparatively less feasible than the 1,3-H shift observed in pathway-1. On the other hand, the formation of HCHO and CH3CH2CHOO in pathway-3 was found to be endothermic (∆H0 = 45.4 kcal mol-1) and endergonic (∆G0 = 31.3 kcal mol-1) in nature. Non-spontaneous behavior of this pathway was explained based on the instability of CH3CH2CHOO radical. Furthermore, pathway-4 involving the formation of HCHO and CH3CH2OCHO (ethyl formate), via three-membered intramolecular 1,2-ethyl shift TS (TS6) was also found to be exothermic (∆H0 = -56.3 kcal mol-1) and exergonic (∆G0 = -70.3 kcal mol-1) in nature. Moreover, in pathway-5, the formation of ozone and 1-C4H8 was found to be endothermic (∆H0 = 59.8 kcal mol-1) and non-spontaneous (∆G0 = 60.6 kcal mol-1). Hence, the formation of these products via pathway-5 was found to be thermodynamically not feasible. Moreover, It can be seen from Table 1a that, the formation of methylenebis (oxy) and CH3CH2CHO as products in pathway-6 was found to be exothermic (∆H0 = -21.6 kcal mol-1) and exergonic (∆G0 = -21.4 kcal mol-1).

Furthermore, the

decomposition of unstable methylenebis (oxy) to form H2 and CO2 in pathway-6 was found to be exothermic (∆H0 = -100.5 kcal mol-1) and exergonic (∆G0 = -106.8 kcal mol-1) and these values are given in Table 1b. This high enthalpy may be due to the unstable nature of the biradical which rapidly dissociate to form products. Similar conclusion was drawn by Crehuet et al.51 on the reaction of methylenebis (oxy) with ethylene. They reported that the methylenebis (oxy) and its derivatives are highly unstable and rapidly decompose to form products. Furthermore, they explained that the possibility for the formation of H2 and CO2 from the dissociation of methylenebis (oxy) in the gas phase.51 Moreover, it can be observed from Table 1a that, the ∆H0 and ∆G0 values for the yield of propylenebis (oxy) and HCHO as end products in pathway-7 was also exothermic (∆H0 = -21.4 kcal mol-1) and exergonic (∆G0 = -20.6 kcal mol-1). Furthermore,


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the ∆H0 and ∆G0 for the decomposition of propylenebis (oxy) to yields CO2 and C2H6 as final products were also calculated. This decomposition channel was also found to be exothermic (∆H0 = -104.4 kcal mol-1) and exergonic (∆G0 = -114.8 kcal mol-1), due to the unstable nature of the biradical. 4. RATE COEFFICIENTS Rate coefficient calculations were performed for the title reaction using the optimized geometries obtained at B3LYP/6-311G(d, p) level of theory. Intermediate in each of the pathways were also considered for calculating the reaction rate coefficients. The classical potential energy curve (VMEP), the ground- state vibrational adiabatic energy ( VaG ) and zero point energy curve (ZPE) are plotted as a function of intrinsic reaction coordinates ‘s’ (amu)1/2bohr and are shown in Figure S2 of SI. The maxima of the VaG andVMEP curves are located at different positions and the shapes of these two curves also varied along the reaction coordinate, which indicated that the variational effects are significant. ZPE curve was almost constant at the proximity of the saddle point, as the intrinsic reaction coordinate varied. It can be seen from Table S3 that, the TST and CVT based rate constants over the studied temperature range were found to be different. Moreover, in the case of bimolecular reaction pathways, tunneling effect was found to be significant. Therefore, to incorporate the slight variational effect and significant tunneling coefficients in the bimolecular reactions, CVT/SCT along with ISPE method and hindered rotor correction was employed to evaluate the rate coefficients over the studied temperature range of 200-1000 K and are tabulated in Table S4. The specific temperature range was chosen in this current work to understand the stability of collisionally stabilized SOZ in both low and high temperature regions.



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As mentioned earlier, the reaction goes through both unimolecular (pathway-2, 3, 4, 6 and 7) as well as bimolecular pathways (pathway-1 and 5). Hence, the rate coefficients were computed for both the pathways and the room temperature rate coefficients of all reaction channels are tabulated in Table 2. The reaction channel R1 involves the formation of SOZ via TS1.

To calculate the rate

coefficient of R1, equilibrium constant (Keq) between IM1 and reactants were considered and is given below,

%$ + !"1 &' ()

(R1)

The equilibrium between the reactants (CH2OO + CH3CH2CHO) and the pre-reactive complex (IM1) was found to be a fast equilibrium with an equilibrium constant Keq = 1.00×10-3 cm3 molecule-1. Therefore, the equilibrium was considered to be negligible for the reaction channel R1. Similar, conclusion was drawn by Jalan et al.9 in the reaction of CH2OO with CH3CHO. Hence, the rate coefficient for the formation of SOZ via TS1 was computed with respect to reactants. The kinetics obtained for the reaction channel R1 were fitted to a three-parameter fit equation over the temperature range of 200-1000 K and is shown in Figure 4. A negative temperature dependent Arrhenius behavior was observed for R1. The temperature dependent Arrhenius equation for this channel was obtained by fitting the calculated rate coefficients using three parameter fit equation and is given below, k(cm3 molecule-1 s-1) = 9.67×10-20 T1.3exp(2728.6/T) Presence of a pre-reactive complex (IM1) which stabilized the submerged TS (TS1), explains the negative temperature dependent behavior. The high–pressure limit rate coefficient obtained at


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298K was found to be k = 2.44×10-12 cm3 molecule-1 s-1, which is higher by a factor of 1.4 on comparison to the reactivity of CH2OO with CH3CHO (k∞ = 1.70×10-12 cm3 s-1).24 This can be attributed to an increase of methyl group in the parent chain. Furthermore, the reaction for the formation of ozone and 1-C4H8 from the bimolecular reaction of CH2OO with CH3CH2CHO is given as, *+

$-

$/

+ !"4 &' !"5 &' + = − −

(R2)

The equilibrium between the reactants (CH2OO + CH3CH2CHO) and the pre-reactive complex (IM4) was also found to be a fast equilibrium with an equilibrium constant Keq = 6.65×10-3 cm3 molecule-1. Therefore, the equilibrium was considered to be negligible for the reaction channel R2.9 The formation of the 4-ethyl-1, 2, 3-trioxolane or primary ozonide (IM5, POZ) was formed from the cycloaddition of reactants via TS7. Further it decomposes via TS8 to form ozone and 1C4H8 as products. Arrhenius plot for the reaction channel R2 is given in Figure S8. A negative temperature dependent Arrhenius behavior was observed for the formation of ozone and 1-C4H8 via TS7 and TS8, The temperature dependent Arrhenius equation for R2 was obtained by fitting the calculated rate coefficients using a three parameter fit and is given below, k(cm3 molecule-1 s-1) = 7.07×10132 T-46.6exp(30821/T) The rate coefficient acquired for TS7 and TS8 in R2 was found to be kTS7 = 4.28× 10-24 s-1 and kTS8 = 4.90× 10-26 s-1 respectively. Therefore, rate coefficient for the consecutive reaction for the formation of ozone and 1-C4H8 in R2 was found to be k = 2.06× 10-25s-1. Moreover, the formation of POZ was twelve orders of magnitude smaller than the formation of SOZ in the reaction of CH2OO with CH3CH2CHO. Therefore, this result reveals that the pathway leading to



Page 16 of 44

the formation of ozone and 1-C4H8 is also kinetically unfavorable. Hence, the reaction of CH2OO with CH3CH2CHO leads to a minimal amount of ozone formation in the troposphere. Eventually, SOZ undergoes subsequent isomerization and decomposition reactions to form stable products. The nine reaction channels were discussed in PES formed from SOZ and are shown below: $

() &' "0

(R3)

$

"0 &' + $1

() &' +

(R4) (R5)

$2

(R6)

$3

(R7)

$4

(R8)

() &' + () &' + () &' 567ℎ9:6;6(?@9 + $%A

"67ℎ9:6;6 (?@9 &B' + $%%

() &B' CD?C9:6;6(?@9 + $%

0D?C9:6;6 (?@9 &B' + 3

(R9)

(R10)

(R11)

In reaction channels R3, HMP is formed from the isomerization reaction of SOZ via TS2. The Arrhenius plot for the reaction channel R3 is given in Figure 5. A positive temperature Arrhenius behavior was observed and the temperature dependent Arrhenius equation for R3 was obtained by fitting the calculated rate coefficients using the four parameter fit equation52 and is given below,


Page 17 of 44 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


E(>

%

K + 809.6 %1./ 1.37 × 101 (K + 809.6 = 6.04 IJ O P exp I− P K + 6.55 × 102 300

The rate coefficients obtained for the reaction channel R3 at 298 K was found to be k = 5.17 s-1. On the other hand, in reaction channel R4, the formed HMP further undergo isomerization followed by decompose to form HCHO and CH3CH2COOH as final products. The Arrhenius plot for the reaction channel R4 is given in Figure 6. A positive temperature dependent Arrhenius behavior was noticed. The temperature dependent Arrhenius equation for the reaction channel R4 was obtained by fitting the calculated rate coefficients by three parameter fit and is given below, k(s-1) = 2.82 T3.12 exp (1356.3/T) The rate coefficients for the reaction channels R4 at 298 K was computed to be k = 1.76 × 106s-1. The evidence for the formation of HMP via SOZ and its further decomposition reaction channel to form HCHO and CH3CH2COOH as final products was explained by Jalan et al.9 in the reaction of CH2OO with CH3CHO. Moreover, the temperature dependent Arrhenius equation for the formation of HCOOH and CH3CH2CHO in reaction R5 via TS4 was obtained by fitting the calculated rate coefficients using four parameter fit equation 52 and is given below and shown in Figure S9. E(>

%

= 2.70 × 10

%

K + 101.9 1. 6.53 × 10 (K + 101.9 IJ O P exp I− P K + 1.03 × 101 300

A positive temperature dependent Arrhenius behavior was observed for the reaction channel R4. The computed rate coefficient for this channel at 298 K is k=5.04×10-4 s-1.



Page 18 of 44

Furthermore, the temperature dependent Arrhenius equation for reaction channel R6 leading to the formation of HCHO and CH3CH2CHOO as the end products via TS5, was obtained by fitting the calculated rate coefficients using linear least squares method and is given below and shown in Figure S10. k( s-1)= 2.28×1015exp (16876/T) A positive temperature dependent Arrhenius behavior was further noticed for this channel. This behavior may be due to the high energy barrier of the reaction channel R6 compared to other unimolecular isomerization reaction channels. The calculated rate coefficient for the reaction channel R6 at 298 K is k= 5.74×10-10 s-1. Moreover, in reaction channel R7, HCHO and CH3CH2OCHO as products formed via TS6. The temperature dependent Arrhenius equation was obtained by fitting the calculated rate coefficients using a three parameter fit and is given below, and plotted in Figure S11. k (s-1) = 1.11×10-15 T7.3exp (1263/T) A positive temperature dependent Arrhenius behavior was observed for this channel. The calculated rate coefficients for the reaction channel R7 at 298 K is k = 7.36×104 s-1. In addition to these channels, SOZ also undergoes decomposition reactions to form products. In the reaction channel R8 involves the formation of methylenebis (oxy) and CH3CH2CHO as products formed via TS9 from the decomposition of SOZ is given as, $4

() &' 567ℎ9:6;6 (?@9 +


(R8)

Page 19 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Arrhenius plot for the reaction channel R8 is given in Figure S12. A positive temperature Arrhenius behavior was observed, due to the high energy barrier height for the corresponding transition state (TS9). The temperature dependent Arrhenius equation for this channel was obtained by fitting the calculated rate coefficients using linear least squares method and is given below, k (s-1)= 9.42×109exp (11518/T) The computed rate coefficient for the reaction channel R8 at 298 K was found to be k = 1.54× 10-7 s-1. Furthermore, the reaction channel R9 for the decomposition of methylenebis (oxy) to form H2 and CO2 via TS10 is given as, $%A

"67ℎ9:6;6 (?@9 &B' +

(R9)

The Arrhenius plot for the reaction channel R9 is given in Figure S13. A slight positive temperature Arrhenius behavior was observed. The temperature dependent Arrhenius equation for this channel was obtained by fitting the calculated rate coefficients using the four parameter fit52 and is given below, E(>

%

= 3.00 × 10

%

K + 796.4 %1.1 9.90 × 10 (K + 796.4 IJ O P exp I− P K + 6.34 × 102 300

The rate coefficient obtained for this reaction channel at 298 K is k = 1.41×10-25 s-1. In addition to this, the kinetics for the formation of propylenebis (oxy) and HCHO in reaction channel R10 from the unimolecular decomposition of SOZ via TS11 is given as,



$%%

() &B' CD?C9:6;6 (?@9 +

Page 20 of 44

(R10)

The Arrhenius plot for the reaction channel R10 is given in Figure S14. A positive temperature Arrhenius behavior was observed. The temperature dependent Arrhenius equation for this channel was obtained by fitting the calculated rate coefficients using linear least square method and is given below, k (s-1)= 1.13×1011exp (11028/T) The rate coefficient obtained for this reaction channel at 298 K was found to be k = 8.60×10-6 s1

. Furthermore, the formed unstable biradical propylenebis (oxy) undergoes further

decomposition to form CO2 and C2H6 as end products via TS12 and is shown below, $%

0D?C9:6;6 (?@9 &B' + 3

(R11)

A positive temperature dependent Arrhenius behavior was observed and as shown in Figure S15. The temperature dependent Arrhenius equation for this channel was obtained by fitting the calculated rate coefficient by four parameter fit equation52 and is given below, E(> % = 1.98 × 10%1 IJ

K + 825.9 1%.2 4.94 × 10 (K + 825.9 O P exp I− P K + 6.82 × 102 300

The obtained rate coefficient for the reaction channel R11 at 298 K was found to be k = 6.94× 10-21 s-1. Hence, the formation of methylenebis (oxy) and propylenebis (oxy) formed from the decomposition reaction of SOZ was found to be kinetically and thermodynamically less feasible compared to the other reaction channels emerged from SOZ.


Page 21 of 44 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


Therefore, based on the rate coefficients obtained for the various unimolecular isomerization pathways via SOZ, for the formation of HCHO and CH3CH2COOH as final products via reaction channels R3 and R4 was found to be the major route in the reaction of CH3CH2CHO with CH2OO. 4.1 Branching Ratios The branching ratios of eight reaction channels (R1, R3-R8 and R10) are plotted over the temperature range of 200-1000 K and are shown in Figure 7. Branching ratios are defined as the ratio of partial decay constants to the total decay constants. The expression for branching ratio is given as,

Branching ratios (in %) =

ki ×100 ktotal

The concerted unimolecular isomerization and decomposition of collisionally stabilized SOZ takes place via seven reaction channels, as described in the previous section. The stability of SOZ depends on the temperature. At low temperatures (below 300 K), collisionally stabilized SOZ was not found to be degraded via any of the above reaction channels, as shown in Figure S16. On the contrary, at high temperatures (above 300 K), the products (formation of HMP via R3, HCHO + CH3CH2COOH via R4, HCOOH + CH3CH2CHO via R5, HCHO + CH3CH2CHOO via R6, HCHO + CH3CH2OCHO via R7, methylenebis (oxy) + CH3CH2CHO via R8 and propylenebis (oxy) + HCHO via R10) were observed to be formed. At high temperature, the formation of HCHO + CH3CH2COOH via R4 was found to be the major products, when compared with products formed via other reaction channels. Furthermore, it can be seen from Figure 7 that, above 300 K, the yield for the formation of HCHO + CH3CH2COOH was found to



increase from 2% at 400 K to 90% at 850 K and was observed to decrease thereafter till 1000 K (80%). Simultaneously, an observable increase in the yield of HCHO + CH3CH2CHOO was seen from 775 K (1.5%) to 18% at 1000 K and negligible at 298 K. Moreover, the formation of propylenebis (oxy) from SOZ was also found to be an observable increase in the yield above 900 K and is shown in Figure S17. Moreover, in comparison to the reaction channels (R4 and R6), the products formed from other reaction channels were found to be negligible at studied temperature range. 5. CONCLUSIONS In this work, the reaction of CH2OO with CH3CH2CHO over the temperature range of 200-1000 K was studied. Seven distinct reaction pathways were observed for the title reaction. From the computed energetics, thermochemical parameters and reaction kinetics obtained at CCSD (T)/AUG-cc-pVTZ//B3LYP/6-311G (d, p) level of theory, pathway-1 leading to the formation of HCHO and CH3CH2COOH as products was found to be kinetically labile as well as thermodynamically feasible. The computational calculations predict that the collisionally stabilized SOZ was more stable at low temperatures. Contrarily, at high temperature, the products formed via unimolecular isomerization and decomposition of SOZ was found to be more feasible. Furthermore, the reaction of CH2OO with CH3CH2CHO confers a new approach for the formation of organic acids. Simultaneously, a profound reaction mechanism gives a novel directionality towards the formation of various Oxygenated Volatile Organic Compounds in the atmosphere above 300 K.


Page 22 of 44

Page 23 of 44 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


SUPPORTING INFORMATION Optimized geometries and vibrational frequencies of the reactants, pre and post reactive complexes, transition states and products are given in Table S1 and Table S2. Intrinsic reaction coordinates (IRC) plots, variational effects plot of all transition states and Rotamer scan of CH3CH2CHO are shown in Figure S1 to Figure S3. T1-Diagnostic values and rate coefficients for the title reaction are shown in Table S3 and Table S4. Furthermore, the variations in the bond length of all pathways (except pathway-1) with respect to reaction coordinate are shown in Figure S4 to Figure S7. Arrhenius plots for all the reaction channels (except R1, R3 and R4) are given in Figure S8 to Figure S15. Moreover, the branching ratios for the formation of SOZ and the products formed from the reaction channels (R3, R5, R7, R8 and R10) are given in Figure S16 and Figure S17 respectively.

ACKNOWLEDGEMENTS We would like to thank Prof. Donald G. Truhlar for providing POLYRATE 2008 and GAUSSRATE 2009A suite of programs. Both the authors thank Mr. V. Ravichandran and his team for providing high-performance computational resources for the work. RK also thank IITM for research fellowship.



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41) Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Gill, P.W.M.; Johnson, B.G.; Robb, M.A.; Cheeseman, J.R.; Keith, T.A.; Petersson, G.A.; Montgomery, J.A., et al. Gaussian 09.2009 Gaussian, Inc., Wallingford, CT. 42) Garrett, B.C.; Truhlar, D.G. Generalized transition state theory: Bond energy-bond order method for canonical variational calculations with application to hydrogen atom transfer reactions. J. Am. Chem. Soc. 1979, 101, 4534–4548. 43) Garrett, B.C.; Truhlar, D.G.; Grev, R.S.; Magnuson, A.W. Improved treatment of threshold contributions in variational transition-state theory. J. Phys. Chem. 1980, 84, 1730–1748. 44) Liu, Y.P.; Lynch, G.C.; Truong, T.N.; Lu, Da-H.; Truhlar, D.G.; Garrett, B.C. Molecular modeling of the kinetic isotope effect for the [1, 5]- Sigmatropic rearrangement of cis-1, 3-Pentadiene. J. Am. Chem. Soc. 1993, 115, 2408–2415. 45) Lu, D.H.; Truong, T.N.; Melissas, V.S.; Lynch, G.C.; Liu, Y.P.; Grarrett, B.C.; Steckler, R.; Issacson, A.D.; Rai, S.N.; Hancock, G.C., et al. A new version of a computer program for the calculation of chemical reaction rates for polyatomics. Comput. Phys. Commun. 1992, 71, 235–262. 46) Truhlar, D.G.; Garrett, B.C. Variational transition state-theory. Acc. Chem. Res. 1980, 13, 440. 47) Truhlar, D.G.; Garrett, B.C. Variational transition state-theory. Annu. Rev. Phys. Chem. 1984, 35, 159. 48) Truhlar, D. G.; Isaacson, A. D.; Garrett, B. C. In Generalized transition state theory. Eds. CRC Press: Boca Raton, FL, 1985, Vol. 4, p 65.



49) Zheng, J.; Zhang, S.; Corchado, J.C.; Chuang, Y.Y.; Coitiño, E.L.; Ellingson, B.A.; Truhlar, D.G. GAUSSRATE, version 2009-A 2010, University of Minnesota, Minneapolis. 50) Zheng, J.; Zhang, S.; Lynch, B.J.; Corchado, J.C.; Chuang, Y.Y.; Fast, P.L.; Hu, W.P.,; Liu, Y.P.; Lynch, G.C.; Nguyen, K.A., et al. POLYRATE, version, 2008, University of Minnesota, Minneapolis. 51) Crehuet, R.; Anglada, J.M.; Cremer, D.; Bofill, J. M. Reaction Modes of Carbonyl Oxide, Dioxirane, and Methylenebis (oxy) with Ethylene: A New Reaction Mechanism. J. Phys. Chem. A 2002, 106, 3917-3929. 52) Zheng, J.; Truhlar, D. G. Multi-path variational transition state theory for chemical reaction rates of complex polyatomic species: ethanol + OH reactions. Faraday Discuss. 2012, 157, 59−88.


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Scheme 1 Seven different reaction pathways were studied for the reaction of CH2OO with CH3CH2CHO.



Figure1a Potential energy surface for the reaction of CH2OO with CH3CH2CHO computed at the CCSD (T)/AUG-cc-pVTZ//B3LYP/6-311G(d, p) level of theory.


Page 32 of 44

Page 33 of 44 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


Figure1b Potential energy surface for the reaction of decomposition of methylenebis (oxy) computed at the CCSD (T)/AUG-cc-pVTZ//B3LYP/6-311 G(d, p) level of theory.

Figure1c Potential energy surface for the reaction of decomposition of propylenebis (oxy) computed at the CCSD (T)/AUG-cc-pVTZ//B3LYP/6-311 G(d, p) level of theory.



CH2OO

CH3CH2CHO

IM3

Page 34 of 44

IM2, SOZ

IM1

IM4 IM5 ,

TS2

TS1

TS3 TS4

TS5

CH3CH2COOH

CH3CH2CH=CH2

TS6

TS7

HCHO

HCOOH

CH3CH2CHO

CH3CH2CHOO


TS8

CH3CH2OCHO

O3

Page 35 of 44 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


TS9

TS12

TS11

TS10

Methylenebis (oxy)

CO2 C2H6

Propylenebis (oxy)

H2

Figure 2 Optimized geometries of reactants (CH2OO and CH3CH2CHO), pre and post reactive complexes (IM), transition states (TS) and products obtained at B3LYP/6-311G (d, p) level of theory.



Page 36 of 44

Table 1a Calculated Relative energies (∆Er), zero-point energy corrected relative energies (∆Er, ZPE)

IF (Imaginary Frequencies) and thermochemical parameters [entropies (S0), changes in

enthalpies (∆H0) and Gibbs free energies (∆G0)] of pre and post reactive complexes (IM), reactants, transition states (TS) and products obtained at CCSD(T)/AUG-cc-pVTZ//B3LYP/6311G(d, p) level of theory.

Species

IF

ZPE

∆Er

∆G0

∆H0

S0

*

∆Er, ZPE

CCSD(T)/AUGB3LYP/6-311G(d, p)

cc-pVTZ

CH2OO+CH3CH2CHO

-

72.2

0.0

0.0

0.0

130.5

0.0

TS1

182.3i

74.3

-4.8

6.8

-5.6

88.7

-6.9

TS2

888.3i

71.9

-3.9

8.8

-5.1

83.9

-12.6

TS3

1038.9i

74.1

-106.9

-95.2

-108.0

87.3

-107.8

TS4

883.1i

72.5

-8.6

3.8

-9.5

85.8

-10.5

TS5

63.8i

74.1

-0.7

4.7

-7.9

88.3

-8.3

TS6

476.4i

73.3

-6.3

5.7

-7.1

87.7

-6.9

TS7

422.1i

74.8

20.2

32.9

18.9

83.5

15.6

TS8

128.2i

73.6

58.1

69.9

57.4

88.3

55.7

TS9

376.3i

74.5

10.9

9.9

23.3

85.5

-73.2

TS11

436.3i

73.2

-6.3

-7.0

5.7

87.6

-7.2

IM1

-

73.7

-6.1

4.1

-6.1

96.4

-7.9

IM2(SOZ)

-

77.7

-43.4

-30.6

-44.8

82.7

-51.5


Page 37 of 44 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


IM3 (HMP)

-

77.7

-43.3

-30.4

-44.8

82.4

-122.1

IM4

-

73.6

5.2

17.8

3.7

82.7

-9.5

IM5 (POZ)

-

76.8

5.0

17.5

3.6

82.8

-2.7

HCHO+CH3CH2COOH

-

73.4

-113.2

-113.0

-113.1

130.1

-114.4

HCOOH+CH3CH2CHO

-

73.7

-110.1

-79.6

-65.3

130.4

-111.0

HCHO+CH3CH2CHOO

-

71.7

0.3

31.3

45.4

129.6

0.1

HCHO+CH3CH2OCHO

-

73.2

-101.3

-70.3

-56.3

129.4

-101.5

CH3CH2CH=CH2+O3

-

72.3

59.9

60.6

59.8

127.8

54.2

Methylene bis (oxy) +

-

73.0

-21.5

-21.4

-21.6

129.7

-98.6

-

72.4

-21.4

-20.6

-21.4

127.7

-26.3

CH3CH2CHO Propylene bis (oxy) + HCHO *∆Er, ZPE = ∆ECCSD(T)/AUG-cc-pVTZ + ZPEB3LYP/6-311G(d, p) Table 1b Calculated Relative energies (∆Er), Zero-Point energy corrected relative energies (∆Er, ZPE)

IF (Imaginary Frequencies) and thermochemical parameters [Entropies (S0), changes in

Enthalpies (∆H0) and Gibbs free energies (∆G0)] of reactants, transition states (TS) and products for the decomposition of methylene bis (oxy) obtained at CCSD(T)/AUG-cc-pVTZ//B3LYP/6311G(d, p) level of theory. Species

IF

ZPE

∆Er

∆G0

∆H0

S0

B3LYP/6-311G(d, p) Methylene TS10 H2 + CO2

1612.4i -

20.3 14.0 13.6

0.0 52.8 -101.8

0.0 -107.0 -106.8

0.0 -100.0 -100.5


58.8 59.6 82.2

*

∆Er, ZPE

CCSD(T)/AUGcc-pVTZ 0.0 90.1 -94.0


Table 1c Calculated Relative energies (∆Er), Zero-Point energy corrected relative energies (∆Er, ZPE),

IF (Imaginary Frequencies) and thermochemical parameters [Entropies (S0), changes in

Enthalpies (∆H0) and Gibbs free energies (∆G0)] reactants, transition states (TS) and products for the decomposition of propylene bis (oxy) obtained at CCSD(T)/AUG-cc-pVTZ//B3LYP/6311G(d, p) level of theory. Species

IF

ZPE

∆Er

∆G0

∆H0

S0

B3LYP/6-311G(d, p) Propylene TS12 CO2 + C2H6

1416.7i -

55.7 51.4 54.0

0.0 87.5 -105.3

0.0 86.8 -114.8

0.0 87.9 -104.4

74.1 77.9 109.0

*

O4-C5 (TS1) 0

C1-O11 (TS1)

2.0 O7-6H (TS2) H6-O4 (TS3) 1.5

H6-C5 (TS2) O7-6H (TS3)

1.0 -2.5

-2.0

-1.5

-1.0

-0.5 1/2

0.0

0.5

1.0

Reaction coordinate (amu )bohr Figure 3 Variation in the bond length of pathway-1 with respect to reaction coordinate (amu1/2bohr) at B3LYP/6-311G (d, p) level of theory.


∆Er, ZPE

CCSD(T)/AUGcc-pVTZ 0.0 89.0 -95.4

2.5

Bond length (A )

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 38 of 44

Page 39 of 44 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 2 Rate coefficients for each pathways obtained at CCSD (T)/AUG-cc-pVTZ//B3LYP/6311G (d, p) for the reaction of CH2OO with CH3CH2CHO at 298 K (ak in units of cm3 molecule1 -1 b

s , k in s-1).

a

Reaction channels

kCVT/SCT

CH2OO + CH3CH2CHO → SOZ

2.44×10-12

b

SOZ→HMP

5.17

HMP →HCHO+CH3CH2COOH

1.76×106

b

SOZ→ HCOOH + CH3CH2CHO

5.04×10-4

b

SOZ→ HCHO + CH3CH2CHOO

5.74×10-10

b

SOZ→ HCHO + CH3CH2OCHO

7.36×104

b

a

b

CH2OO + CH3CH2CHO → POZ

4.28×10-24

b

POZ→ O3 + CH2=CH-CH2CH3

4.90×10-26

SOZ→ Methylenebis (oxy) + CH3CH2CHO

1.54×10-7

b

b

Methylenebis (oxy)→ H2 + CO2

1.41×10-25

SOZ→ Propylenebis (oxy) + HCHO

8.60×10-6

b

6.94×10-21

Propylenebis (oxy)→ CO2 + C2H6



1000

600 500

400

300

250

200

4

5

-22 -24 -26 -28

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 40 of 44

-30 -32 -34

1

2

3

-1

1000/T (K ) Figure 4 Arrhenius plot for the formation of SOZ via TS1 in reaction channel R1.


Page 41 of 44

1000

600 500

400

300

250

200

4

5

30

20 -1

ln (k in 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


10

0

-10

-20 1

2

3

-1

1000/T(K ) Figure 5 Arrhenius plot for the formation of HMP via TS2 in reaction channel R3.


The Journal of Physical Chemistry 1000

600 500

400

300

250

200

4

5

22 20

-1

ln (k in 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 42 of 44

18 16 14 12 10 1

2

3

-1

1000/T(K )

Figure 6 Arrhenius plot for the formation of HCHO and CH3CH2CHO from HMP via TS3 in reaction channel R4.


Page 43 of 44

100

80

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


60

40

20

0 200

400

600

T(K)

800

1000

Figure 7 Branching ratios studied as a function of temperature (200-1000 K) for the products formed from the unimolecular isomerization and decomposition of SOZ. In the figure, sky blue color for the formation of SOZ via TS1, navy blue color depicts the formation of HCHO and CH3CH2COOH via TS3 and orange color for the formation of HMP from SOZ via TS2 in pathway-1, green color for the yield of HCOOH and CH3CH2CHO via TS4 in pathway-2, red color for HCHO and CH3CH2CHOO via TS5 in pathway-3 and yellow color for the yield of HCHO and CH3CH2OCHO via TS6 in pathway-4, pink color for the formation of methylenebis (oxy) via TS9 in pathway-6 and black color for the yield of propylenebis (oxy) via TS11 in pathway-7.



TOC Graphics


Page 44 of 44

Temperature Dependent Kinetics of the Reaction of Criegee

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