Atmospheric Fate of Criegee Intermediate Formed During Ozonolysis

Subscriber access provided by University of Sunderland

A: Environmental, Combustion, and Atmospheric Chemistry; Aerosol Processes, Geochemistry, and Astrochemistry

Atmospheric Fate of Criegee Intermediate Formed During Ozonolysis of Styrene in Presence of HO and NH: The Crucial Role of Stereochemistry 2

3

Tahamida Banu, Kaushik Sen, and Abhijit Kumar Das J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06835 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 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 Journal Journal of of Physical Physical Chemistry Chemistry The

Atmospheric Fate of Criegee Intermediate Formed During Ozonolysis of Styrene in Presence of H2O and NH3: The Crucial Role of Stereochemistry Tahamida Banu, Kaushik Sen,* and Abhijit K. Das* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

ABSTRACT A gas phase mechanistic investigation of the unimolecular, water/ammonia assisted GHFRPSRVLWLRQ UHDFWLRQV RI WKH .-hydroxy hydroperoxides (HPs) and hydroperoxide arylamines (a-HPs) produced during the styrene ozonolysis has been carried out theoretically in the present article. The instrumental role of stereochemistry in controlling the outcome of individual reactions has been discussed. Thermodynamic parameters ('G298K, 'H298K, 'E0K) associated with individual reactions have also been computed. The rate constants estimated for individual reactions using conventional transition state theory (TST) combined with statistical mechanics provide a comprehensive understanding of the reaction mechanism and also elucidate the atmospheric fate of Criegee intermediates. Considering the feasibility of reactions from thermodynamic and kinetic point of view, while aldehyde (PhCHO) formation pathway originating from bimolecular decomposition of HP is found to be kinetically favored, benzoic acid formation pathway remains favored thermodynamically. A similar consideration for the bimolecular reactions of a-HP reveals the phenylmethanimine formation pathway to be kinetically favored, while the benzamide formation pathway being favored thermodynamically. Our findings appear to be in excellent agreement with the experimental observations.

*Corresponding AuthorV¶ E-mail: [email protected], [email protected] 1 ACS ACS Paragon Paragon Plus Plus Environment Environment


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

Page 2 of 31

INTRODUCTION It is now widely known that atmospheric aerosol particles have significant LPSDFW RQ (DUWK¶V climate. They not only DIIHFW (DUWK¶V UDGLDWLYH EDODQFH DQG VXUIDFH WHPSerature1,2 but are also found to cause adverse health issues.3 A large number of air-pollution borne diseases are caused by exposure to aerosol particles.4 Formation of secondary organic aerosols (SOA) from the reactions of the emissions of gas-phase pollutants are among the key sources of these ultrafine particles.5 In this connection it is worth mentioning here that aromatic compounds constitute a larger proportion contributing to the total SOA formation compared to that of alkanes and alkenes.6 The case of styrene is particularly interesting in this context as it is not only a hazardous air pollutant but has also been identified to be the second most efficient species forming SOA, just after Toluene.6,7 The main sources of styrene in the atmosphere are fugitive emissions from petrochemical facilities and industrial processes involving styrene or its polymers.8,9 Among several possible reactions of styrene, the reaction with ozone is particularly important as it provides access to the one of the most important chemical species, carbonyl oxide, popularly known as Criegee intermediate. The stabilized Criegee intermediate (sCI) can undergo a number of bimolecular reactions in the atmosphere with atmospheric molecules such as H2O, SO2, CO2, NH3, H2SO4 etc.10-18 However, the reactions of Criegee intermediate with water and ammonia are especially critical because of the abundance of water and ammonia in the atmosphere and also because these bimolecular reactions of stabilized Criegee intermediate significantly influence the formation of aerosol in the atmosphere.17,19,20 In view of their relative structural simplicity and the fact that the contribution of aromatic compounds to the total SOA formation is larger compared to alkanes and alkenes, the styreneozone system is ideal to investigate the potential impacts of atmospheric molecules on SOA production. But, unfortunately, styrene-ozone system has been the topic of interest in only a few experimental investigations

16,17

which attempted to account for the influence of ammonia and

water on secondary organic aerosol formation from the said system. Cocker and co-workers

17

experimentally investigated the formation of aerosol particles from the reaction of styrene with ozone in presence of ammonia and water. They observed a decrease in the number of aerosol particles when styrene decomposes in presence of NH3 compared to its decomposition in the 2 ACS ACS Paragon Paragon Plus Plus Environment Environment

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


absence of NH3. Inspired by their work, Jørgensen and Gross theoretically investigated the reaction mechanisms between NH3 and secondary organic aerosol precursors, viz. SOZ (secondary ozonide) and HSE (hydroxyl substituted ester), generated from the ozonolysis of alkenes.21 In another work, Jørgensen and Gross investigated the reaction mechanism and associated kinetics for the interaction between ammonia and simple carbonyl oxide and its methyl derivatives.22 Very recently, Aranda and co-workers16 have experimentally investigated the effect of water vapor on the ozonolysis reaction of styrene leading to the formation of aerosol particles. However, to the best of our knowledge there is no theoretical investigation available which attempt to scrutinize the effect of ammonia and water on the aerosol formation from the ozonolysis of styrene. In the present work we have theoretically explored the mechanistic details featuring the formation of secondary organic aerosol from styrene ozonolysis in presence of ammonia and water. We have taken into account the influence of stereochemistry on the reaction mechanism. The ozonolysis of styrene proceeds through the formation of molozonide as shown in Figure 1, leading to the formation of two Criegee intermediates, CI and CIc.

Figure 1. Schematic representation of styrene ozonolysis and formation of HP. The phenyl substituted carbonyl oxide (CI) is found to be more stable than the other one (CIc) and additionally no theoretical investigations have attempted to examine the effect of presence of

3 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 2 of 31

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


phenyl ring in the Criegee intermediate on the reaction mechanism of subsequent reactions with water and ammonia. These prompted us to explore the reactions of CI with water and ammonia. The reaction of CI ZLWK ZDWHU DQG DPPRQLD LQLWLDOO\ OHDGV WR WKH IRUPDWLRQ RI .-hydroxy hydroperoxide (HP) and hydroperoxide arylamine (a-HP), respectively. The subsequent unimolecular decompositions of HP and a-HP, taking into account their stereochemical orientations, are schematically shown in Figure 2 and Figure 3 respectively. The catalytic effects of water and ammonia on the decomposition of HP and a-HP leading to the same end products have also been examined in the present study and the schematic representations of these catalyzed reaction routes are also included in Figure 2 and Figure 3 respectively. Our study thus explains how the stereochemical orientation of the species controls the outcome of individual reaction pathways. In addition to these, we have evaluated the stepwise rate constants of the unimolecular and bimolecular reactions employing conventional transition state theory (TST) combined with statistical mechanics. Elucidating the kinetics of the reactions is crucial not only for a comprehensive understanding of the reaction mechanism but also for fully estimating the atmospheric fate of Criegee intermediate.

Figure 2. Stereochemical representations of the unimolecular and water-assisted reaction channels of HP1 and HP2 produced from reaction of stabilized CI with H2O.


Page 4 of 31

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


Figure 3. Stereochemical representations of the unimolecular and ammonia-assisted reaction channels of a-HP1 and a-HP2 produced from reaction of stabilized CI with NH3.

COMPUTATIONAL DETAILS All equilibrium geometries of the reactants, transition states, intermediates and products are computed using Gaussian 09 suite of quantum chemistry package.23 Geometries of all the species, involved in the present study have been optimized and reported using the hybrid metaGGA M06-2X functional,24 in conjunction with the aug-cc-pVDZ basis set. As composite CBSQB3 method25 is utilized to calculate thermodynamic parameters very accurately,26 it has been applied to obtain more reliable energies of the species and the minimal free energy profile diagrams have subsequently been constructed based on these CBS-QB3 values. It is important to mention here that the experimentally estimated activation energy values for the reaction of simple CI with water vapor are reported to show excellent agreement with the theoretical estimates at CBS-QB3 level.14 Therefore, the CBS-QB3 energy values have also been used to evaluate the rate constants of all the reactions investigated in the present work. This composite method uses B3LYP/CBSB7 geometries and vibrational frequencies with appropriate scaling for 5 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 4 of 31


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

accurate single-point energy calculations. The frequencies used in CBS-QB3 method are scaled by a factor of 0.99. Frequency calculations have been carried out at the same CBS-QB3 level to confirm whether the optimized structures are local minima (no imaginary frequency) or transition states (one imaginary frequency) on the free energy surfaces and to evaluate the zero-point vibrational energy (ZPVE). The connecting first order saddle points that are the transition states between the equilibrium geometries have been obtained by the synchronous transit-guided quasi-Newton (STQN) method. The intrinsic reaction coordinate (IRC) calculations have been carried out to confirm the connections between the transition state and the local minima.27,28 In our effort towards the exploration of the reaction kinetics, we have applied steady state approximation to the water- and ammonia-assisted bimolecular reactions of Criegee Intermediates (CI1, CI2) to calculate the rate constants of the corresponding reactions. We have also calculated the stepwise rate constants for the bimolecular decomposition reactions of hydroperoxides (HP, a-HP) using the same approximation. All these reactions are assumed to follow the following multistep mechanisms,

The first step involves a fast pre-equilibrium between the reactants A, B and the intermediate AB. On the other hand, the intermediates AB, C and D are in thermal equilibrium. The steady state principle can be applied to the first step of the reaction because of its fast pre-equilibrium nature, i.e., (d[AB])/(dt §

The rate constant for the individual step (GE ) can be evaluated using

the conventional transition state theory (TST) equation, GE = Á:6;ê

G> 6 365 ATL>F:'65 F '4 ;¤G> 6? D 34

Where the constant 1 represents the symmetry factor accounting for the number of possible identical reaction paths. + 7 is the tunneling factor that originates from the quantum mechanical tunneling through the potential energy barrier along the reaction coordinate, and QTS and QR represent the partition functions for the transition state and the reactant. One dimensional Wigner tunneling correction is used here,29 which assumes that the tunneling can only occur along the reaction coordinate,


Page 6 of 31

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


1 DåE 2 l p Á:6; = 1 + 24 G> 6

i

is the magnitude of the imaginary frequency of transition state. Using the basic statistical

thermodynamic equation Keq can be evaluated, -AM = ê

3#$ ATL cFk'#$ 3# 3$

:'# + '$ ;ogWG> 6

QA and QB are the partition functions for the reactants and QAB is the partition function for the pre-equilibrium complex, AB. EAB, EA and EB denote the total energies of the pre-equilibrium complex AB and the reactants A and B respectively. All the room temperature kinetic calculations based on CBS-QB3 energy values have been performed at 1 atm pressure and these results are expected to be fairly close to that for the high pressure limit. The values obtained for the equilibrium constant (Keq) and the step-wise rate constants (ki) are believed to be reliable as they have been evaluated using the fundamental statistical thermodynamic principles and TST, respectively. RESULTS AND DISCUSSION The optimized structures of the reactants, stable intermediates, and products, obtained at M062X level of theory, are summarized in Figure S1 in the supporting information. The important geometrical parameters along with the optimized geometries of the transition states are displayed in Figure 6 and Figure 9 and Figure 11 respectively. All the schematic free energy profile diagrams, depicted in Figures 4, 5, 7, 8 and 10, are based on relative Gibbs free energy change ('G298K) values obtained at CBS-QB3 level of theory. It should be noted here that in case of bimolecular reactions of HP and a-HP, as the association complexes, from which different reaction channels originate, differ from one another in their stereochemical orientation (apparent from Figure 2 and Figure 3), we did not show them in the corresponding free energy surfaces in Figure 5, 8, and 10. The other important thermodynamic parameters ('H298K, 'E0K) are collected in Table 1 and Table 2 respectively. For each of the decomposition reactions, the estimated equilibrium constant (Keq) for the first step and the step-wise rate constants (ki) are summarized in Table 3 and Table 4. Table S1 in the supporting information depicts all the relative energies associated with the decomposition reactions at CBS-QB3 level. As already mentioned, these 7 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 6 of 31


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

values are used to calculate the equilibrium constants and the stepwise first order rate constants of all the associated reactions. Table S2 and Table S3, on the other hand, illustrate all the partition functions, imaginary frequencies of the transition states along with the Wigner tunneling corrections. A. H2O Assisted Reactions 1. Formation of HP from the reaction between stabilized Criegee intermediate and water Figure 4 provides a schematic representation of the energetics of the reaction between Ph(H)COO (CI) and H2O. As already shown in Figure 1, there are two different possible conformers of CI, anti (CI1) and syn (CI2), which can undergo bimolecular reactions with H2O in the atmosphere. The syn conformer (CI2) is found to be slightly more stable than anti conformer (CI1) by an amount of 2.5 kJ/mol and these two isomers are interconnected by a transition state with an activation barrier of 154.4 kJ/mol. The reactions between CI (both CI1 and CI2) and H2O are found to be initiated by the barrier-less formation of weak van der Waals intermediates, IM1H2O and IM2H2O, situated at a free energy level of 7.9 and 8.3 kJ/mol, respectively (Figure 4) with respect to the initial reactants.

Figure 4. CBS-QB3 based schematic free energy profiles for the water-assisted reactions of stabilized CI at 298K. 8 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 8 of 31

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


IM1H2O and IM2H2O, thus formed, subsequently traverse activation free energy barriers of 49.1 (TS1H2O) and 75.2 kJ/mol (TS2H2O), leading to the generation of two different conformers of Dhydroxy hydroperoxides, HP1 and HP2. It is important to note that, although several other conformational isomers of hydroxyl hydroperoxide are possible to be formed, HP1 and HP2 are found to be the most stable ones among them and therefore we have considered only the decomposition channels arising from these two conformers. Energetically, HP1 is found to be more stable than HP2 by an amount of 6.1 kJ/mol (refer to Figure 5). The difference in stability is well explained considering the orientation of the phenyl ring in HP1 and HP2. The phenyl ring is positioned anti to the ±OOH group (staggered conformation) in HP1 which minimizes the steric interaction. On the other hand, the position of phenyl ring in HP2 is sterically demanding and this raises its free energy. This is also true for the NH3-assisted reaction which will be discussed later. It is important to note here that, HP1 and HP2 can interconvert into one another, but, this interconversion pathway is not a single step one, rather it is a multistep process (also applicable for a-HP1 and a-HP2). This can be attributed to the presence of intramolecular Hbonding in both the systems. Rotation about the C-O bond of HP1 would lead to an altogether different conformer of hydroxy hydroperoxide, devoid of any intramolecular H-bond. As displayed in Table 3, the equilibrium constants for the formation of IM1H2O and IM2H2O are both in the order of ~10-6 L mol-1. The stepwise rate constant values (ki) for the formations of HP1 and HP2 are calculated to be 2.33 u 106 s-1 and 7.05 u 101 s-1 respectively. It is apparent from Figure 4 that HP1 and HP2 are formed with an excess of free energy of 113.7 and 104.9 kJ/mol respectively, which is sufficient for their subsequent unimolecular decompositions. Alternatively, vibrationally excited HP1 and HP2 can become collisionally stabilized and undergo a further bimolecular reaction. An account of the subsequent unimolecular and water-assisted bimolecular decomposition of HP1 and HP2 is detailed in the following sections. It is worth mentioning here that the overall reaction features in the bimolecular decompositions remain almost unaltered including the stereochemical orientation of hydroxyl hydroperoxides during the course of the reactions; the only difference being the transfer of H atoms through the additional water molecule. So, the additional water molecule acts as a via media for the H atom transfer mechanism. The catalytic role of the water molecule is also apparent from the following discussions involving energetics of the reactions and from the 9 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 8 of 31

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


Page 10 of 31

fact that it is generated back at the end of the reaction. These facts also hold for the NH3-assisted mechanisms, discussed in the respective section. 1.1. Unimolecular and H2O assisted reaction channel from most stable isomer HP1 The unimolecular decomposition of HP1 has two possible product channels. The peroxide bond ‡

‡

cleavage may directly result in the generation of PhCH(OH) O along with O H radicals ('GBDE = 151.0 kJ/mol). Alternatively, the unimolecular decomposition of HP1 may pass through the transition state, TS1PhCOOH, producing PhCOOH and H2O (Figure 5). In this case, the transition state, TS1PhCOOH, adopts an eclipsed conformation with the peroxide bond (O3-O4) and C1-H5 bond aligning parallel to each other (see Figure 2). This stereochemical orientation of the transition state favors the transfer of the H5 atom bonded to C1 to the O4 atom of the ±OOH group (see Figure 6). The peroxide bond (O3-O4 bond) is also simultaneously cleaved, releasing one molecule of H2O. This consequently results in the formation of PhCOOH molecule. This reaction pathway is found to be associated with a free energy barrier of 190.2 kJ/mol relative to the reactant, HP1. It should be noted here that, in this case, hydroxyl group does not play any role in the formation of PhCOOH; it remains intact to the molecular backbone. The exothermicity of this reaction ('G ~ -359.1 kJ/mol) is calculated to be very high relative to the reactant HP1 and the value of stepwise rate constant (ki) is of the order of ~10-21 s-1.


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


Figure 5. CBS-QB3 based schematic free energy profile for the unimolecular and water-assisted reactions of HP1 and HP2 at 298K temperature.

The water-assisted bimolecular decomposition of HP1 needs to pass through a six-member cyclic transition state (TS1cPhCOOH) (see Figure 6) to give rise to the formation of benzoic acid along with water molecules. The barrier height associated with this bimolecular process is about 193.3 kJ/mol with respect to the starting materials, HP1 + H2O. The underlying mechanism of this bimolecular pathway involves the breakage of the peroxide bond followed by the generation of ± OH fragment, which subsequently captures one of the hydrogen atoms of the external H2O molecule. The H2O molecule in turn takes up one H atom (H5) from the molecular backbone. Thus, two molecules of H2O are generated in the overall conversion. The energetics of this bimolecular process indicates no significant catalytic role of the additional H2O molecule. The stepwise reaction rate constant for this PhCOOH generation pathway is calculated to be 8.68 u 10-19 s-1 (Table 3).


Page 10 of 31

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


Page 12 of 31

Figure 6. Optimized geometries of the transition states associated with the decomposition of HPs obtained at M06-2X/aug-cc-pVDZ level.

1.2. Unimolecular and H2O assisted reaction channels from less stable isomer HP2 ‡

‡

Apart from peroxide bond cleavage producing PhCH(OH) O and O H radicals ('GBDE = 146.0 kJ/mol), the unimolecular decomposition of HP2 may lead to the formation of an aldehyde (PhCHO) + H2O2 and an acid (PhCOOH) + H2O (shown in Figure 5).

1.2.1. PhCHO formation pathway Unimolecular decomposition of HP2 traverses through the transition state, TS2PhCHO, giving rise to the formation of PhCHO+H2O2 (see Figure 5). TS2PhCHO is found to adopt an eclipsed 12 ACS ACS Paragon Paragon Plus Plus Environment Environment

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


conformation along this pathway and is associated with an activation barrier of 176.1 kJ/mol with respect to HP2. The transition state is characterized by a four-member cyclic structure. The important geometrical parameters associated with TS2PhCHO are displayed in Figure 6. The eclipsed conformation of the transition state facilitates the transfer of the H atom of the hydroxyl group to the O3 of peroxide group with concomitant elongation of the C1-O3 bond to 1.807 Å. This explains the formation of a H2O2 molecule along this pathway. During the formation of H2O2, the C1-O2 bond length is simultaneously shortened by an amount of 0.102 Å, indicating the formation of PhCHO as an end product. The overall reaction pathway is calculated to be almost thermoneutral ('G=1.1 kJ/mol). As summarized in Table 3, the stepwise rate constant (ki) of this unimolecular reaction channel is calculated to be 2.88 u 10-18 s-1. The water-assisted bimolecular decomposition of HP2 may proceed through a six-member cyclic transition state, TS2cPhCHO, which is associated with a free energy barrier of 132.3 kJ/mol with respect to the initial reactants HP2 + H2O (see Figure 5). This bimolecular decomposition apparently results in the formation of PhCHO and H2O2 with the water molecule being regenerated at the end of the reaction. An investigation of the reaction mechanism reveals that the H atom from hydroxyl group is transferred to the H2O molecule during the course of the reaction; H2O in turn transfer one of its H atoms to the peroxide O-O(H) bond which subsequently leads to the removal of H2O2. The substantial lowering of activation free energy by an amount of 43.8 kJ/mol compared to the unimolecular decomposition firmly establishes the pronounced catalytic role of water molecule on this decomposition pathway of HP. The stepwise reaction rate constant for this PhCHO generation pathway is calculated to be 2.40 u 10-9 s-1 (Table 3).

1.2.2. PhCOOH formation pathway In an alternate decomposition pathway through TS2PhCOOH, HP2 leads to the formation of benzoic acid (PhCOOH) along with one molecule of H2O. Unlike TS1PhCOOH, this transition state has a staggered conformation and is associated with an activation free energy barrier of 192.9 kJ/mol with respect to HP2. TS2PhCOOH is characterized by a five-member ring fused with a three-member cyclic structure. The staggered conformation of TS2PhCOOH allows H atom (H7) of the hydroxyl group to be in the close proximity of O4 atom of the ±OOH group. This renders the removal of H2O through a simultaneous rupture of both peroxide (O3-O4) and hydroxyl (O2-H7) 13 ACS Paragon ACS Paragon Plus Plus Environment Environment

Page 12 of 31


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

bonds. The accompanying H atom (H5) transfer from C1 to O3 points towards the eventual formation of PhCOOH. Unlike the PhCOOH formation pathway from HP1 reported before, here hydroxyl group participates actively in the reaction mechanism. This overall conversion is observed to be highly exothermic, by an amount of -359.1 kJ/mol ('G) with respect to HP1. The calculated ki value of this unimolecular reaction channel is of the order of a10-21 s-1 (Table 3). Analogous to the unimolecular decomposition, the water-assisted bimolecular decomposition of HP2 in an alternative decomposition pathway may lead to the formation of PhCOOH along with two molecules of H2O (see Figure 5). This alternate decomposition pathway passes through the transition state, TS2cPhCOOH, which is characterized by fused three- and seven-member cyclic structures. A similar kind of bond breaking and bond making mechanism akin to the unimolecular decomposition is also found to be operative here. The repetition of the corresponding unimolecular decomposition mechanism takes place through an intervening water molecule. The activation free energy barrier associated with this conversion is estimated to be 168.1 kJ/mol relative to the initial reactants HP2 + H2O, which clearly indicates that the external H2O molecule has mild catalytic effect on this bimolecular reaction process, unlike the one in the corresponding bimolecular process of HP1. The stepwise rate constant value for this benzoic acid generation channel is 9.54 u 10-16 s-1.

B. NH3 Assisted Reactions 1. Formation of a-HP from the reaction between stabilized Criegee intermediate and ammonia A schematic free energy diagram for the reaction between the different conformers of Criegee intermediate (Ph(H)COO) and NH3 leading to the formation of hydroperoxide arylamine (a-HP) is depicted in Figure 7. The initial barrier-less association between CI1 (anti form) and NH3 leads to the formation of an intermediate, IM1NH3, which subsequently passes through a five-member cyclic transition state, TS1NH3, giving rise to a-HP1. This conversion has an activation barrier of 43.7 kJ/mol with respect to the initial reactants (CI1 + NH3).


Page 14 of 31

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


Page 14 of 31

Figure 7. CBS-QB3 based schematic free energy profile for the ammonia-assisted reactions of stabilized CI at 298K temperature.

Following the same reaction pattern, CI2 (syn-form) gives rise to the generation of a slightly less stable conformer a-HP2, the stability difference being 6.6 kJ/mol between a-HP1 and a-HP2 (Figure 10). This involves the intermediate and transition state IM2NH3 and TS2NH3, respectively. Alike TS1NH3, TS2NH3 is also a five-member cyclic transition state with slightly higher activation free energy of 60.1 kJ/mol. The a-HP1 and a-HP2, so produced, have an excess free energy of 115.0 and 105.7 kJ/mol, respectively, as evident from Figure 7. This is sufficient to trigger their subsequent unimolecular decompositions. However, as discussed earlier, in the case of hydroxy hydroperoxides (HP1 and HP2), the vibrationally excited a-HP1 and a-HP2 can also become collisionally stabilized and undergo a further bimolecular reaction. The excess Gibbs free energy ‡

of a-HP1 and a-HP2 is sufficient to cleave their peroxide bonds and produce PhCH NH 2 O and ‡

OH

radicals ('GBDE = 161.9 kJ/mol and 153.1 kJ/mol). The other plausible unimolecular

decomposition pathways along with the catalyzed ones originating from a-HP1 and a-HP2 are reported in the following sections. As summarized in Table 4, the equilibrium constants for the formation of IM1NH3 and IM2NH3 are both in the order of ~10-7 L mol-1. The stepwise rate 15 ACS ACS Paragon Paragon Plus Plus Environment Environment


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

constants for the formations of a-HP1 and a-HP2 are calculated to be 7.78 u 107 s-1 and 2.02 u 105 s-1, respectively.

1.1. Formation of phenylmethanimine (PhCHNH) + H2O2 from a-HP1 Our investigation reveals that the unimolecular decomposition of a-HP1 may lead to the formation of an imine (PhCHNH) along with H2O2, passing through the transition state, TSimine, as shown in Figure 8.

Figure 8. CBS-QB3 based schematic free energy profile for the unimolecular and ammoniaassisted reactions of a-HP1 at 298K temperature.

TSimine possesses a four-member cyclic structure and is associated with an activation barrier of 194.1 kJ/mol with respect to the initial reactant, a-HP1. Stereochemically, this transition state is neither found to resemble an eclipsed form nor a staggered form. This might be due to an early rupture of the C1-O3 bond that takes place well before this transition state is reached along the free energy surface. A simultaneous transfer of one of the H atoms attached to N to the O3 of ± OOH group results in the removal of H2O2 yielding the phenylmethanimine. The overall reaction process is found to be slightly endothermic (15.9 kJ/mol). It should be noted here that we were


Page 16 of 31

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


not able to locate any imine formation pathway from a-HP2. The value of stepwise rate constant (ki) for this unimolecular reaction is calculated to be 6.40 u 10-22 s-1.

Figure 9. Optimized geometries of the transition states associated with the decomposition of aHP1 obtained at M06-2X/aug-cc-pVDZ level.

Phenylmethanimine formation is also rendered in presence of molecular NH3 as a catalyst. Our investigation on the bimolecular reaction between a-HP1 and NH3 reveals a similar mechanistic scenario as already encountered in the case of corresponding unimolecular decomposition. Association between a-HP1 and NH3, resulting in the formation of PhCHNH, proceeds through a six-member cyclic transition state, TScimine with an activation barrier of 167.4 kJ/mol (see Figure 8). H2O2 is generated as a by-product of this reaction along with the regeneration of molecular 17 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 16 of 31


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

NH3. As evident from Figure 3, the nearly staggered conformation of the transition state favors the transfer of one of the H atoms of the NH2 group, attached to the C backbone, to the external NH3 which simultaneously releases one of its H atoms to be taken up by the departing ±OOH fragment. Thus the H atom transfer mechanism is mediated by the external NH3 molecule. It should be noted here that the presence of NH3 lowers the barrier height by an amount of only ~27 kJ/mol compared to the unimolecular decomposition. This implies only a mild catalytic effect of NH3 on the reaction. For this phenylmethanimine generation reaction pathway, the value of stepwise rate constant is calculated to be 7.02 u 10-16 s-1.

1.2. Formation of PhCHO + NH2OH from a-HP1 a-HP1 in an alternative decomposition pathway leads to the generation of benzaldehyde (PhCHO) along with NH2OH (see Figure 8). This pathway is associated with a four-member cyclic transition state, TSaldehyde, featuring the simultaneous breakage of peroxide and C-N bond which eventually leads to the removal of NH2OH from the system. This process requires surmounting over an activation free energy barrier of 201.5 kJ/mol to achieve this transformation. The overall reaction process is observed to be moderately exothermic (-104.0 kJ/mol). In addition to the unimolecular reaction pathway, we also explored the possibility of formation of benzaldehyde in presence of molecular NH3 as a catalyst. We were indeed successful in locating a bimolecular reaction pathway giving rise to PhCHO formation. The transition state (TScaldehyde) located for this conversion is found to consist of a six-member cyclic structure with an activation free energy of 263.2 kJ/mol. Thus, NH3 does not show any catalytic activity in this case. A close look at the mechanism of this bimolecular reaction reveals the role of NH3 in the overall conversion. The reaction proceeds through a stereochemically staggered conformation of a-HP1 followed by coordination with the external NH3 through H-bonding (see Figure 3). The nitrogen atom of the NH3 molecule then captures the ±OH fragment by rupturing the peroxide bond. One of the H atoms of NH3 is simultaneously transferred to the -NH2 group of a-HP1 leading to the breakage of C-N bond. This explains the formation of PhCHO along with the generation of NH2OH and NH3. This aldehyde formation pathway is found to be noticeably absent in case of a-HP2. The stepwise reaction rate constants for these two reaction channels are estimated to be 3.73 u 10-23 s-1 and 1.44 u 10-32 s-1, respectively. 18 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 18 of 31

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


1.3. Formation of PhCONH2 + H2O from a-HP1 and a-HP2 Amide formation pathway may follow from both a-HP1 and a-HP2. In addition to the unimolecular decomposition mechanism, the catalytic activity of NH3 is analyzed in this case also. Following is a detailed discussion on the PhCONH2 formation pathways from both a-HP1 and a-HP2.

Figure 10. CBS-QB3 based schematic free energy profile for the unimolecular and ammoniaassisted reactions of a-HP1 and a-HP2 leading to PhCONH2 at 298K tempearture.

1.3.1. Pathway1 This decomposition pathway is generated from the most stable isomer a-HP1. Our investigation located a four-member ring containing transition state, TS1amide for this conversion, the estimated activation free energy barrier being ~209 kJ/mol relative to a-HP1 (see Figure 8). The eclipsed form of the transition state is found to facilitate the transfer of H5 atom to the O4 atom of ±OOH group (see Figure 3) followed by the subsequent elimination of H2O. This ultimately gives rise to the generation of PhCONH2. The corresponding bimolecular reaction pathway traverses through a six-member cyclic transition state, TS1camide, which is associated with an activation barrier of 181.0 kJ/mol (see Figure 8). The presence of NH3 lowers the barrier by an amount of 27.8 kJ/mol compared to the 19 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 18 of 31


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

unimolecular decomposition. A careful analysis of the TS1camide reveals that the H5 atom from the C backbone migrates to the externally added NH3 which in turn transfers one of its H atoms to the fragmented ±OH moiety, generated from the cleavage of peroxide bond. Thus, ammonia not only assists the H-transfer mechanism but also exerts a moderate catalytic effect on the decomposition of a-HP1. It should be noted here that both the unimolecular and catalyzed pathways are calculated to be highly exothermic (-357.1 kJ/mol). The associated stepwise reaction rate constants for these two reaction channels are calculated to be of the order of ~10-24 and ~10-18 s-1 respectively.

Figure 11. Optimized geometries of the transition states associated with the decomposition of aHP2 obtained at M06-2X/aug-cc-pVDZ level. 20 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 20 of 31

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


1.3.2. Pathway2 The transition state, TS2amide, associated with this unimolecular decomposition pathway of aHP2, could not be obtained using conventional QST algorithm for closed-shell systems. However, we were able to obtain this concerted transition state with unrestricted broken spin symmetry wavefunction which is believed to produce an accurate description for open shell singlet systems.30 Specifically, we have used the guess=mix option, as implemented in the Gaussian09 software package,23 to generate this wavefunction for the open-shell singlet TS. As evident from Figure 11, the transition state, TS2amide is characterized by a fused three- and fivemember ring structures and is associated with an activation barrier of 174.3 kJ/mol with respect to a-HP1 (see Figure 10). It is apparent from the geometry of the transition state that the ±OH fragment, generated through the breakage of the peroxide bond, captures one of the H atoms attached to the ±NH2 group, which in turn abstracts the H5 atom from the molecular backbone, finally resulting in the formation of PhCONH2 along with H2O. Noticeably, the transition state possesses a nearly staggered conformation (see Figure 3) which favors the underlying mechanism. This overall conversion is found to be highly exothermic by an amount of -357.1 kJ/mol. In the corresponding ammonia-assisted bimolecular decomposition pathway of a-HP2, the association between a-HP2 and NH3 passes through a fused three- and seven-member ring containing TS, TS2camide (shown in Figure 11) and leads to the formation of benzamide along with H2O and NH3. The characteristic features of the overall reaction mechanism remain the same as that of the unimolecular one, differing only in the transfer of H atom through NH3. As shown in Figure 10, the estimated activation barrier associated with this conversion is 254.1 kJ/mol w. r. t. a-HP1 + NH3 and this clearly suggests that the additional NH3 molecule does not have any catalytic effect on the reaction pathway; rather the reaction rate is decreased with the increase in barrier height by an amount of 79.8 kJ/mol. The associated stepwise reaction rate constants for these two reaction pathways are calculated to be of the order of ~10-17 and ~10-26 s-1 respectively.

1.3.3. Pathway3 We were able to identify an alternate unimolecular decomposition pathway of a-HP2 (via TS3amide) that may result in the formation of the same benzamide found in pathway2. The 21 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 20 of 31


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

transition state associated with this reaction pathway is also found to be a concerted one and we obtained this open-shell singlet TS with unrestricted broken spin symmetry wavefunction using the guess=mix option as used for TS2amide. It is apparent from Figure 11 that TS3amide is also characterized by fused three- and five-member cyclic structures and has a similar stereochemical conformation as that of TS2amide (see Figure 3). This unimolecular decomposition pathway differs from that of Pathway2 only in the fact that instead of migrating to the N of NH 2 group, the H5 atom attached to the molecular backbone migrates to the O3 of ±OOH resulting in the formation of the enol form of benzamide (iminol), which subsequently undergoes tautomerization to form benzamide (shown in Figure 11). TS3amide is associated with an activation free energy barrier of 201.3 kJ/mol with respect to a-HP1 (shown in Figure 10), which is slightly higher than TS2amide. Similar to Pathway2, while analyzing the catalytic role of NH3 on this reaction pathway, we were also successful in locating the corresponding transition state (TS3camide) for this bimolecular reaction. Analogous to Pathway2, here the association between a-HP2 and NH3 was also found to pass through a fused three- and seven-member cyclic transition state structure TS3camide (shown in Figure 11) leading to the generation of the enol form of benzamide (iminol) which finally tautomerizes to form benzamide (Figure 10). Here also the characteristic features of the overall reaction mechanism remains the same as that of the corresponding unimolecular one differing only in the transfer of H atom through NH3 (see Figure 11). The activation free energy value associated with TS3camide is calculated to be 180.2 kJ/mol relative to a-HP1 + NH3. Unlike Pathway2, here, the decrease in free energy barrier by 21.1kJ/mol compared to the unimolecular one clearly suggests the potential catalytic role of NH3 on this particular reaction channel. The associated stepwise reaction rate constants for these two reaction channels are calculated to be of the order of ~10-21 and ~10-13 s-1 respectively.

CONCLUSIONS In the present study, we have elucidated how the reactions between phenyl substituted carbonyl oxide (Criegee intermediate, CI), formed from the ozonolysis of styrene, with water and ammonia leads to the formatLRQ RI .-hydroxy hydroperoxide (HP) and hydroperoxide arylamine (a-HP), respectively. The very fast stepwise rate constant (Ki ~ 104-107 sec-1) associated with the decomposition of the intermediate formed from the reaction between stabilized CI and H2O/NH3 22 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 22 of 31

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


implies the transient existence of CI in nature. HP and a-HP are found to be formed with a substantial amount of initial excess free energy which triggers their subsequent unimolecular decomposition. An amount HP and a-HP may also become collisionally stabilized and undergo further bimolecular reactions with atmospherically abundant water and ammonia. Our investigation provides significant insight into the mechanism and energetics associated with the unimolecular and bimolecular decompositions of HP and a-HP. The influence of stereochemistry on the reaction mechanism has also been elucidated. The stereochemical orientations of the species are found to be the key factor in controlling the outcome of individual reaction pathways. As dictated by the energetics of the reactions involving HP1 and HP2, the water-assisted bimolecular reaction of HP2 leading to the formation of PhCHO is found to be kinetically favored among others. This is also consistent with the rate constant values estimated theoretically in the present work (see Table 3). However, atomospheric reactions are known to be much slower and take place over a long period of time. This suggests the dominance of benzoic acid formation pathway from thermodynamic point of view. Our conclusion is well corroborated with the experimental findings of Cocker et al.,17 who observed a relative increase in the amount of benzoic acid formation during styrene ozonolysis under high H2O concentration. Similarly, for the NH3-assisted reaction channels of a-HP1 and a-HP2, although the phenylmethanimine formation pathway is found to be kinetically favored over others, as also supported by the estimated rate constants; the benzamide formation pathway (thermodynamically controlled) would dominate in the long run. Cocker and coworkers, in their experimental investigation, showed that addition of excess NH3 after secondary aerosol formation leads to rapid and significant decrease in aerosol volume. According to them this is due to the nucleophilic attack of NH3 on two major aerosol forming products, 3,5-diphenyl-1,2,4-trioxolane and hydroxylsubstituted ester, resulting in their rapid decomposition. Their experimental observation regarding the decrease in SOA formation in presence of NH3 corroborates well with our theoretical findings. It is evident from our theoretical investigation that NH3 captures highly reactive Criegee intermediate to form hydroperoxide arylamine (a-HP) which subsequently undergoes further decomposition. Thus NH3 helps in reducing the availability of Criegee intermediate to react with benzaldehyde to produce the above mentioned aerosol forming products.


Page 22 of 31


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

Page 24 of 31

ASSOCIATED CONTENT Supporting Information (1) Additional optimized geometries (reactants, reactant complexes, products) (2) Tables of relative energies ('E0K), relative free energies ('G298K) and relative enthalpies ('H298K), (3) Partition functions (Q) (4) Imaginary frequencies (Ki RI WKH WUDQVLWLRQ VWDWHV DQG :LJQHU¶V tunneling correction

: ; and (5) explanation for the fast pre-equilibrium of the isolated

reactants (CI + H2O/NH3) with the association complexes, IM1 and IM2. ACKNOWLEDGEMENTS Tahamida and Kaushik are grateful to Indian Association for the Cultivation of Science (I.A.C.S.), for providing them research fellowships.

REFERENCES (1) Pilinis, C.; Pandis, S. N.; Seinfeld, J. H. Sensitivity of direct climate forcing by atmospheric aerosols to aerosol size and composition. J. Geophys. Res. 1995, 100 (D9), 18739-18754. (2) Appel, B. R.; Tokiwa, Y.; Hsu, J.; Kothny, E. L.; Hahn, E. Visibility as related to atmospheric aerosol constituents. Atmos. Environ. 1985, 19 (9), 1525-1534. (3) Seaton, A.; Godden, D.; Macnee, W.; Donaldson, K. Particulate air pollution and acute health effects. Lancet 1995, 345 (8943), 176-178. (4) Bates, J. T.; Weber, R. J.; Abrams, J.; Verma, V.; Fang, T. ; Klein, M.; Strickland, M. J.; Sarnat, S. E.; Chang, H. H.; Mulholland, J. A.; Tolbert, P. E.; Russell, A. G. Reactive oxygen species generation linked to sources of atmospheric particulate matter and cardiorespiratory effects. Environmental Science & Technology 2015, 49 (22), 13605-13612. (5) Cheung, H. C.; Chou, C. C. K.; Huang, W. R.; Tsai, C. Y. Characterization of ultrafine particle number concentration and new particle formation in an urban environment of Taipei, Taiwan. Atmospheric Chemistry and Physics 2013, 13, 8935±8946. (6) Sun, J.; Wu, F.; Hu, B.; Tang, G.; Zhang, J.; Wang, Y. VOC characteristics, emissions and contributions to SOA formation during hazy episodes. Atmospheric Environment 2016, 141, 560570. (7)

EPA

2017,

Initial

list

of

hazardous

air

pollutants

with

modifications.

https://www.epa.gov/haps/initial-list-hazardous-air-pollutants-modifications. (8) Knighton, W. B.; Herndon, S. C.; Wood, E. C.; Fortner, E. C.; Onasch, T. B.; Wormhoudt, 24 ACS ACS Paragon Paragon Plus Plus Environment Environment

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


J.; Kolb, C. E.; Lee, B. H.; Zavala, M.; Molina, L.; Jones, M. Detecting fugitive emissions of 1,3-butadiene and styrene from a petrochemical facility: An application of a mobile laboratory and a modified proton transfer reaction mass spectrometer. Industrial & Engineering Chemistry Research 2012, 51 (39), 12706±12711. (9) Zhang, Z.; Wang, H.; Chen, D.; Li, Q.; Thai, P.; Gong, D.; Li, Y.; Zhang, C.; Gu, Y.; Zhou, L.; Morawska, L.; Wang, B. Emission characteristics of volatile organic compounds and their secondary organic aerosol formation potentials from a petroleum refinery in Pearl River Delta, China. Science of the Total Environment 2017, 584-585, 1162-1174. (10) Kroll, J. H.; Sahay, S. R.; Anderson, J. G.; Demerjian, K. L.; Donahue, N. M. Mechanism of HOx formation in the gas-phase ozone-alkene reaction. 2. Prompt versus thermal dissociation of carbonyl oxides to form OH. J. Phys. Chem. A 2001, 105 (18), 4446-4457. (11) Aplincourt, P.; Ruiz-López, M. F. Theoretical Investigation of Reaction Mechanisms for Carboxylic Acid Formation in the Atmosphere. J. Am. Chem. Soc. 2000, 122 (37), 8990-8997. (12) Crehuet, R.; Anglada, J. M.; Bofill, J. M. Tropospheric formation of hydroxymethyl hydroperoxide, formic acid, H2O2, and OH from carbonyl oxide in the presence of water vapor: A theoretical study of the reaction mechanism. Chem. Eur. J. 2001, 7 (10), 2227-2235. (13) Anglada, J. M.; Aplincourt, P.; Bofill, J. M.; Cremer, D. Atmospheric formation of OH radicals and H2O2 from alkene ozonolysis under humid conditions. ChemPhysChem. 2002, 3 (2), 215-221. (14) Hasson, A. S.; Chung, M. Y.; Kuwata, K. T.; Converse, A. D.; Krohn, D.; Paulson, S. E. Reaction RI FULHJHH LQWHUPHGLDWHV ZLWK ZDWHU YDSRU í $Q DGGLWLRQDO VRXUFH RI 2+ UDGLFDOV LQ alkene ozonolysis? J. Phys. Chem. A 2003, 107 (32), 6176-6182. (15) Hatakeyama, S.; Kobayashi, H.; Akimoto, H. Gas-phase oxidation of sulfur dioxide in the ozone-olefin reactions. J. Phys. Chem. 1984, 88 (20), 4736-4739. (16) Díaz-de-Mera, Y.; Aranda, A.; Martínez, E.; Rodríguez, A. A.; Rodríguez, D.; Rodriguez, A. Formation of secondary aerosols from the ozonolysis of styrene: Effect of SO2 and H2O. Atmospheric Environment 2017, 171, 25-31. (17) Na, K.; Song, C.; Cocker III, D. R. Formation of secondary organic aerosol from the reaction of styrene with ozone in the presence and absence of ammonia and water. Atmospheric Environment 2006, 40 (10), 1889-1900.


Page 24 of 31


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

(18) Kurtén, T.; Bonn, B.; Vehkamäki, H.; Kulmala, M. Computational study of the reaction between biogenic stabilized Criegee intermediates and sulfuric acid. J. Phys. Chem. A 2007, 111 (17), 3394-3401. (19) Sakamoto, Y.; Inomata, S.; Hirokawa, J. Oligomerization reaction of the Criegee intermediate leads to secondary organic aerosol formation in ethylene ozonolysis. J. Phys. Chem. A 2013, 117

í

(20) Heaton, K. J.; Sleighter, R. L.; Hatcher, P. G.; Hall IV, W. A.; Johnston, M. V. Composition domains in monoterpene secondary organic aerosol. Environ. Sci. Technol. 2009, 43 (20), í (21) Jørgensen, S.; Gross, A. Theoretical investigation of reactions between ammonia and precursors from the ozonolysis of ethane. Chemical Physics 2009, 362 (1-2), 8±15. (22) Jørgensen, S.; Gross, A. Theoretical investigation of the reaction between carbonyl oxides and ammonia. J. Phys. Chem. A 2009, 113 (38), 10284±10290. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. (2013) Gaussian 09, revision D.01, Gaussian, Inc., Wallingford, CT. (24) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functional and 12 other functionals. Theor. Chem. Acc. 2008, 120 (1-3), 215-241. (25) Montgomery Jr, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. A complete basis set model chemistry. VI. Use of density functional geometries and frequencies. J. Chem. Phys. 1999, 110 (6), 2822-2827. (26) Sen, K.; Mondal, B.; Pakhira, S.; Sahu, C.; Ghosh, D.; Das, A. K. Association reaction between SiH3 and H2O2: a computational study of the reaction mechanism and kinetics. Theor. Chem. Acc. 2013, 132:1375. (27) Gonzalez, C.; Schlegel, H. B. An improved algorithm for reaction path following. J. Chem. Phys. 1989, 90 (4), 2154-2161. (28) Gonzalez, C.; Schlegel, H. B. Reaction path following in mass-weighted internal coordinates. J. Phys. Chem. 1990, 94 (14), 5523-5527. 26 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 26 of 31

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


(29) Wigner, E. P. Crossing of Potential Thresholds in Chemical Reactions. Z. Phys. Chem., Abt. B, 1932, 19 (1), 203-216. (30) Kuwata, K. T.; Guinn, E. J.; Hermes, M. R.; Fernandez, J. A.; Mathison, J. M.; Huang, K. A computational re-examniation of the Criegee intermediate-sulfur dioxide reaction. J. Phys. Chem. A 2015, 119, 10316±10335.

Table 1. Important thermodynamic parameters associated with the water and ammonia-assisted reaction channels of CI1 and CI2 calculated at CBS-QB3 level in kJ/mol unit.


Page 26 of 31

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


Table 2. Important thermodynamic parameters associated with the water and ammonia-assisted reaction channels of HPs and a-HPs calculated at CBS-QB3 level in kJ/mol unit.


Page 28 of 31

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


Table 3. The equilibrium constant (Keq) of the first step, the stepwise rate constants (ki) for water-assisted reaction channels of styrene based Criegee Intermediates at 298 K, calculated at CBS-QB3 level.


Page 28 of 31


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

Table 4. The equilibrium constant (Keq) of the first step, the stepwise rate constants (ki) for ammonia-assisted reaction channels of styrene based Criegee Intermediates at 298 K, calculated at CBS-QB3 level. 30 ACS ACS Paragon Paragon Plus Plus Environment Environment

Page 30 of 31

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


TOC Graphic


Page 30 of 31

Atmospheric Fate of Criegee Intermediate Formed During Ozonolysis

Recommend Documents