Source: https://pubs.rsc.org/en/content/articlehtml/2018/sc/c8sc03514h
Timestamp: 2019-04-23 04:30:52+00:00

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There is now an evolving body of evidence suggesting that high molecular weight oligomers from the ozonolysis of alkenes play an important role in new particle formation in the atmosphere. Using high-level quantum chemical calculations and Born Oppenheimer Molecular Dynamics (BOMD) simulations, we suggest that the reactions of anti-substituted Criegee intermediates with amine, especially dimethylamine, could lead to oligomers, which may comprise an unexplored fraction of organic nitrogen-based aerosols in urban polluted environments. The quantum chemical calculations suggest that the barrier for a given Criegee-amine reaction in the gas phase decreases with increase in methyl substitution on the amine to such an extent that the dimethylamine reactions of CH2OO and anti-CH3CHOO occur barrierlessly. The BOMD simulation results suggest that at the air–water interface, which represents a unique reaction medium in the atmosphere, the anti-CH3CHOO–methylamine reaction occurs via multiple mechanisms, which are distinctly different from that in the gas phase. An important implication of these results is that the Criegee-amine chemistries may account for an appreciable fraction of aerosol particles in California's central valley, New York City and Paris areas where significant amounts of nitrogen-based aerosol particles have been detected, but their precise details are still not well understood. Alternatively, these chemistries could also serve as a potential source of the hydroxyl radical and hydrogen peroxide under tropospheric conditions.
Ammonia (NH3) and alkylamines are emitted from various sources, including biological processes in the ocean, animal husbandry, agricultural fertilizers, biomass burning and industrial emissions.21 Laboratory experiments have shown that amines are more efficient than NH3 in enhancing particle formation.22–26 For example, experiments using the CLOUD chamber at CERN have demonstrated that dimethylamine ((CH3)2NH) concentrations exceeding three parts per trillion by volume enhance the NPF rate by more than 3 orders of magnitude relative to that seen with NH3.22 Although NH3 and alkylamines are highly volatile, both participate readily in multiphase reactions with organic species, which impact the aerosol nucleation and growth processes.8,9 The role of multiphase reactions of basic species such as acid–base neutralization, interaction with carbonyls, and particle-phase oxidation reactions in the growth of secondary organic aerosols (SOA) in the atmosphere is well documented in the literature.27 For example, recent quantum chemical calculations28,29 suggest that though amines are at least an order of magnitude less abundant than NH3, their gas-phase complexes with inorganic acids (e.g., methanesulfonic acid and nitric acid) are relatively more abundant. Furthermore, the Born–Oppenheimer Molecular Dynamics (BOMD) simulations suggest that the NH3/amine-induced acid–base chemistries at the air–water interface occur on the time scale of femtoseconds (fs) to picoseconds (ps).28,29 The time scale of these interfacial chemistries is found to depend upon the nature of an acid as well as that of an alkylamine. Though a variety of species have been shown to play a role in NPF events, the involvement of Criegee-amine chemistries in the nucleation and growth of SOA is yet to be considered. Criegee intermediates are transient species that are formed in olefin ozonolysis.30 They impact the tropospheric budgets of OH radicals, organic acids, hydroperoxides, nitrates, sulfates and particulate matter.31,32 In the troposphere, Criegee intermediates participate in various unimolecular and bimolecular chemistries.31,32 Though various bimolecular Criegee reactions have been studied experimentally and theoretically,31,32 the Criegee-amine reactions are yet to be explored both in the gas phase as well as at the air–water interface. Interestingly, the bimolecular reactions of Criegee intermediates with peroxyl and hydroperoxyl radicals in the context of NPF events have been previously discussed.33–35 However, Criegee-amine chemistries have never been explored before from that perspective. It is important to mention here that the Criegee-NH3 reaction has been studied before.36,37 Unfortunately, this reaction was suggested to be tropospherically irrelevant based on computational kinetic analysis.
All quantum chemical calculations reported here were performed using the Gaussian 09 (ref. 49) suite of programs for electronic structure and property calculations. The gas-phase reactions of the simplest Criegee intermediates (CH2OO), anti-CH3CHOO, syn-CH3CHOO and (CH3)2COO with NH3, CH3NH2, and (CH3)2NH were examined. The stationary points on all the reaction profiles were fully optimized at the M062X50/aug-cc-PVTZ51 level of theory. The energetics of these gas-phase reactions were further improved by performing single-point energy calculations at the CCSD(T)52/aug-cc-PVTZ level, for which the M062X/aug-cc-PVTZ optimized geometries were used. Harmonic vibrational frequency analysis at the M062X/aug-cc-PVTZ theoretical level was performed to confirm the authenticity of stationary points in all cases.
As a first step, we examined the Criegee-amine reactions in the gas phase. Specifically, we studied the gas-phase reactions of CH2OO, anti-CH3CHOO, syn-CH3CHOO and (CH3)2COO with NH3, CH3NH2, and (CH3)2NH (Fig. 1a) at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. As pointed out in the Introduction section, although the Criegee-NH3 reaction has been previously studied,36,37 we re-examined the same reaction here in order to facilitate comparative analysis of the Criegee-NH3 and Criegee-amine reactions at the same theoretical footing. The CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated reaction profiles for all these reactions are given in Fig. 1b. The optimized geometries of key species involved in these reactions are given in Table S1.† The results show that the gas-phase Criegee-NH3/amine reactions are concerted exergonic reactions, which are mediated by prereaction and postreaction complexes. The basic mechanism of the gas-phase reaction involves the heterolytic addition of the polar H–N bonds of NH3/amines across the –COO functionality of Criegee intermediates. The CCSD(T)//M06-2X calculated barrier for the CH2OO–NH3 reaction is 2.9 kcal mol−1, which is in good agreement with previously reported CCSD(T) numbers of 3.99–5.40 kcal mol−1. The CH2OO–NH3 reaction is predicted to be 44.1 kcal mol−1 exothermic, which is again consistent with the recently reported CCSD(T)/CBST and CCSDT(Q)/CBS values of 43.7 and 42.1 kcal mol−1, respectively.37 The calculated barriers for the NH3 reactions of CH2OO and anti-CH3CHOO are smaller than those for the syn-CH3CHOO and (CH3)2COO reactions. This is consistent with the well-established notion that the Criegee reactivity towards the bimolecular reactions is determined by the nature and location of substituents.31,32,40 As we move along the NH3 → CH3NH2 → (CH3)2NH series, the barriers for all the Criegee reactions are consistently lowered. This points to an interesting trend: the barriers for the Criegee-NH3/amine reactions correlate inversely with the number of methyl substituents on a given amine (Fig. 2). The Criegee-(CH3)2NH reactions have the lowest barriers among all the amine reactions studied. Interestingly, the impact of this amine-tuned reactivity of the bimolecular Criegee reactions is so pronounced that both the CH2OO–(CH3)2NH and anti-CH3CHOO–(CH3)2NH reactions occur barrierlessly whereas the barriers for the syn-CH3CHOO–(CH3)2NH and (CH3)2COO–(CH3)2NH are 6.6 and 7.1 kcal mol−1 lower than those for the syn-CH3CHOO–NH3 and (CH3)2COO–NH3, respectively. Furthermore, the exothermicities associated with the Criegee-NH3/amine reactions are consistently enhanced as we move along the NH3 → CH3NH2 → (CH3)2NH series. The Criegee-(CH3)2NH reactions have 4.6–7.6 kcal mol−1 larger exothermicities than the analogous Criegee-NH3 reactions. Considering that the barrierless bimolecular Criegee reactions (e.g., Criegee-HNO3 and Criegee-HCOOH) have been previously shown to have rate constants on the order of ∼10−10 cm3 per s per molecule,61–63 both the CH2OO–(CH3)2NH and anti-CH3CHOO–(CH3)2NH reactions are likely to have at least similar rate constants.
Fig. 1 (a) General description of the gas-phase Criegee-amine reactions studied here. (b) The CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated energies (in kcal mol−1 units) of key stationary points for the gas-phase Criegee-amine reactions at 298.15 K and 1 atm.
Fig. 2 The three-dimensional plot showing the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated barrier heights (kcal mol−1) for the gas-phase reactions of various Criegee intermediates with (R3)(R4)NH at 298.15 K and 1 atm. Since the CH2OO–(CH3)2NH and anti-CH3CHOO–(CH3)2NH reactions occur barrierlessly, we have used zero to represent their barrier heights here.
Considering that the aqueous surfaces, which are ubiquitous in fogs, cloud waters, microdroplets and aerosols, provide interesting reaction media for atmospheric chemistries,38–41 we next examined the reaction between anti-CH3CHOO and CH3NH2 on a water droplet of 191 water molecules within the framework of the BOMD simulations. The selection of these particular precursors for the interfacial reaction is inspired by the fact that recent studies predict them to remain undissociated on the water droplet,44,54 thus indicating the possibility of an interfacial reaction between them.
Direct reaction between anti-CH3CHOO and CH3NH2. At the air–water interface, the direct reaction between anti-CH3CHOO and CH3NH2 comprises two steps: (i) C1–N1 bond formation (anti-CH3CHOO + CH3NH2 → anti-CH3CHOO−–CH3NH2+) and (ii) N1 → O2 proton transfer (anti-CH3CHOO−–CH3NH2+ → C(CH3)(H)(OOH)(CH3NH)). Details of this direct reaction are shown in Scheme 1, Fig. 3 and Movie S1.† The prereaction-like complex for the C1–N1 bond forming step is formed at 0.68 ps. At that point, the C1–N1 bond is 2.45 Å long. The CH3NH2 is hydrogen-bonded to the anti-CH3CHOO, as indicated by the H1–O2 bond length of 2.81 Å. See Fig. S1† for additional details. The transition state-like complex for the C1–N1 bond formation is observed at 0.73 ps. The C1–N1 bond has now shrunk to 1.93 Å whereas the hydrogen bonding interaction (O2–H1) between anti-CH3CHOO and CH3NH2 is of 2.71 Å length. The C1–O1 and O1–O2 bonds are 1.27 and 1.50 Å long, respectively. At 0.78 ps, the formation of the C1–N1 bond is complete; the C1–N1 is now 1.53 Å long whereas the C1–O1 bond is now converted into a pure single bond of 1.43 Å length. The O2–H1 hydrogen bond and the O1–O2 bond are 2.07 and 1.48 Å long, respectively. This results in the formation of a Zwitterion-type intermediate, anti-CH3CHOO−–CH3NH2+, which remains stable for 0.37 ps. Subsequently, proton transfer from the N1 of CH3NH2 to the O2 of anti-CH3CHOO occurs, resulting in the formation of an adduct, C(CH3)(OOH)(CH3NH). The activation complex for proton transfer is formed at 1.15 ps. At that point, the N1–H1 and O2–H1 bonds are nearly equidistant, i.e., the N1–H1 is 1.78 Å long whereas the O2–H1 bond is 1.26 Å long. The C1–N1 bond (1.51 Å) remains intact. The C1–O1 bond is 1.45 Å long whereas the O1–O2 bond is lengthened from 1.48 Å at 0.78 ps to 1.62 Å at 1.15 ps. The proton transfer is deemed complete at 1.17 ps; the O2–H1 bond is now transformed into a pure covalent bond of 0.91 Å length whereas the N1–H1 bond has become a hydrogen bonding interaction of 1.78 Å length. The C1–O1 and O1–O2 bonds are now 1.55 and 1.44 Å long, respectively. We have run the BOMD simulations up to 20 ps. The adduct (C(CH3)(OOH)(CH3NH)) remains intact at the air–water interface over the simulated time scale (Fig. S2†) where it is hydrogen bonded to the interfacial water molecules. The snapshots of the adduct as a function of time are shown in Fig. 4. The adduct forms 3.3 average number of hydrogen bonds with the surface water molecules. Precisely, O1, O2 and H1 form one hydrogen bond with the interfacial water molecules on average. The methyl groups or N atom hardly forms any hydrogen bonding interaction with the nearby water molecules.
Scheme 1 Schematic representation of two stepwise pathways predicted from the BOMD simulation-based population analysis of the reaction between anti-CH3CHOO and with methylamine on the water droplet of 191 water molecules. For further details, see also Fig. 3–5.
Fig. 4 Snapshot structures of the adduct (C(CH3)(OOH)(CH3NH)) taken from the BOMD simulations over the 20 ps time scale.
Interfacial water-mediated reaction between anti-CH3CHOO and CH3NH2. The interfacial water-mediated adduct formation between anti-CH3CHOO and CH3NH2 involves a single water molecule and occurs in three steps: (i) C1–N1 bond formation (anti-CH3CHOO + CH3NH2 + H2O·W190 → anti-CH3CHOO−–CH3NH2+ + H2O·W190), (ii) proton transfer from the interfacial water molecule to the terminal Criegee oxygen (O2) (anti-CH3CHOO−–CH3NH2+ + H2O·W190 → C(CH3)(H)(OOH)(CH3NH2+)(OH−) + W190), and (iii) proton transfer from the N1 of CH3NH2 to the interfacial water molecule (C(CH3)(H)(OOH)(CH3NH2+)(OH−) + W190 → C(CH3)(H)(OOH)(CH3NH) + W191). Here W190 represents 190 water molecules of the aqueous interface used in the BOMD simulations. Details of this multi-step addition reaction are illustrated in Scheme 1, Fig. 5 and Movie S2.† The prereaction complex for the first step is formed at 0.96 ps. The C1–N1 bond has now shrunk from 3.20 Å at 0 ps to 2.10 Å. The C1–O1 and O1–O2 bonds are 1.34 and 1.47 Å long, respectively. At that point, the interfacial water molecule is hydrogen-bonded to the N1 of CH3NH2 and the O2 of anti-CH3CHOO, i.e., Ow–H1 = 2.16 Å; O2–Hw = 1.75 Å. Here Ow and Hw are the oxygen and hydrogen atoms of the interfacial water molecule that are involved in the reaction. See Fig. S3† for additional details. The transition state-like complex for the C1–N1 bond formation is observed at 1.03 ps. The C1–N1 bond is now 1.92 Å long whereas the Ow–H1 and O2–Hw hydrogen bonds are 1.71 and 1.62 Å long, respectively. The C1–O1 and O1–O2 bonds are 1.29 and 1.67 Å long, respectively. The C1–N1 bond formation is deemed complete at 1.05 ps; the C1–N1 bond is now 1.63 Å long. H1 is still attached to N1 (N1–H1 = 1.02 Å), which further supports the formation of the Zwitterion-type anti-CH3CHOO−–CH3NH2+ intermediate. The C1–O1 and O1–O2 bonds are 1.35 and 1.38 Å long, respectively. This intermediate remains stable for 0.11 ps. The shorter lifetime of the Zwitterionic intermediate in the water-mediated reaction may be due to the fact that the water-mediated reactions occur faster due to a proton shuttling mechanism. Proton transfer from the Ow of the interfacial water molecule to the O2 of anti-CH3CHOO−–CH3NH2+ then occurs, which results in the formation of C(CH3)(H)(OOH)(CH3NH2+)(OH−). The transition state-like complex for the Ow → O2 proton transfer is formed at 1.16 ps; Ow–Hw = 1.28 Å and O2–Hw = 1.30 Å. The Ow is still hydrogen-bonded to H1 in C(CH3)(H)(OOH)(CH3NH2+)(OH−), i.e., Ow–H1 = 1.54 Å and N1–H1 = 1.11 Å. The C1–O1 (1.44 Å) and O1–O2 (1.60 Å) bonds are pure single covalent bonds at this stage. The formation of the C(CH3)(H)(OOH)(CH3NH2+)(OH−) intermediate is complete at 1.17 ps as the Ow–Hw bond (1.47 Å) has now changed into a hydrogen bonding interaction whereas the O2–Hw bond (1.04 Å) has become a proper single bond. The hydrogen bond involving interfacial Ow and H1 in C(CH3)(H)(OOH)(CH3NH2+)(OH−) is still intact, i.e., Ow–H1 = 1.53 Å and N1–H1 = 1.11 Å. This intermediate remains stable for ∼0.26 ps and then eventually decomposes into C(CH3)(H)(OOH)(CH3NH) and H2O. In this final step, proton transfer from N1 to Ow takes place, which regenerates the interfacial water molecule by converting (CH3CHOOH)(CH3NH2+)(OH−) into C(CH3)(H)(OOH)(CH3NH)(H2O). The transition state-like complex for the N1 → Ow is formed at 1.43 ps, in which both the N1–H1 and Ow–H1 bonds are equidistant, i.e., N1–H1 = 1.28 Å and Ow–H1 = 1.28 Å. The Ow–Hw bond is now a hydrogen bonding interaction (1.81 Å) whereas the C1–N1 bond (1.57 Å) is still intact. At 1.44 ps, the formation of the interfacial water-mediated C(CH3)(H)(OOH)(CH3NH) adduct is complete. At that point, the Ow–H1 is 1.02 Å long whereas the N1–H1 bond is 1.59 Å long. The time evolution of C1–N1, C1–O1, O1–O2, and O2–Hw further supports the formation of C(CH3)(H)(OOH)(CH3NH).
Atmospheric implications. Ozonolysis of alkenes is an important source of SOA in the atmosphere.1–4,64 High-molecular weight oligomers have been recognized as major constituents of SOA from ozonolysis of alkenes.33,34,65–69 However, the identity of these oligomers and their formation mechanisms are yet to be fully established. Our gas-phase calculations and air–water interface inclusive simulations suggest that anti-substituted Criegee intermediates may react with amines, preferably dimethylamine, to form oligomers (Scheme 2). In this mechanism, an amine acts as an agent to initiate oligomerization of Criegee intermediates by reacting with an initial one to form a peroxide functionality, which further reacts with Criegee intermediates. Though the role of bimolecular reactions of Criegee intermediates with carboxylic acids, carbonyls, peroxy and hydroperxy radicals in the particle formation is previously discussed in the literature,31,32 this is the first time the involvement of the Criegee-amine chemistries in aerosol particle formation is being suggested. Though the Criegee-amine reactions in the gas phase may face strong competition from the Criegee-water reaction, these reactions are expected to be favored on the aqueous surfaces, which are ubiquitous in the atmosphere and provide a unique reaction medium. Our recent BOMD simulations suggest that larger Criegee intermediates, >C1 at the aqueous surface do not react with water,44 which makes them available for reactions with other potential precursors. The present BOMD simulations indicate that the N–H bonds in CH3NH2 become sufficiently polar on the water droplet so that it adds to the COO functionality of anti-CH3CHOO and results in C(CH3)(H)(OOH)(CH3NH) over the ps time scale. The reaction between anti-CH3CHOO and CH3NH2 on the water droplet is favored over the reaction between anti-CH3CHOO and H2O because amines are more reactive than water.70 The interfacial water molecules stabilize oligomers originating from the Criegee-amine reactions via hydrogen-bonding and thus, may play a role in lowering their vapor pressures and enhancing the rate of particle formation. Our viewpoint is indirectly supported by laboratory experiments demonstrating that although amines have concentrations at least an order of magnitude lower than that of NH3 in the atmosphere,21 they are more effective than NH3 in enhancing particle formation.22–26 Our results may help in better understanding the aerosol formation in the fog waters of California's central valley, and in polluted urban environments such as New York and Paris where significant nitric acid-amine chemistries have been recently suggested to play a role in the aerosol particle formation in these areas.71–73 In the New York City area, amines are likely emitted from marine sources whereas in Paris, the source of amines is the agricultural activities around the City area. The intense traffic in these urban regions further support the role of the Criegee-amine interactions in the particle formation. Though the nitric acid-amine chemistries have been recently suggested to play a role in the aerosol particle formation in these areas,29 the current results reveal an unexplored source of organic nitrogen.
Scheme 2 Plausible mechanism for the formation of low-volatility oligomers containing stabilized Criegee intermediates as repeat units during the olefin ozonolysis. The oligomer formation involves the sequential addition of Criegee intermediates to amines (preferably dimethylamine because of its greater reactivity). An alternate channel suggests the possibility of the Criegee-amine reaction being a source of the tropospheric hydroxyl radical.
Alternatively, the C(CH3)(H)(OOH)(CH3NH) adduct formed from the reaction between anti-CH3CHOO and CH3NH2 in the gas phase may decompose into C(CH3)(H)(CH3NH)(O) and the OH radical (Scheme 2). The bond dissociation energy for the O–OH bond in Criegee-derived hydroperoxides is ∼40 kcal mol−1.74–76 Considering that the adduct in the Criegee-amine reactions is formed with an additional energy of 38.5–51.9 kcal mol−1, the O–OH bond breakage in C(CH3)(H)(OOH)(CH3NH)-type adducts may occur under tropospheric conditions, which makes these Criegee-amine chemistries a potential source of the tropospheric OH radical. The C(CH3)(H)(OOH)(CH3NH) adduct formed from the Criegee-CH3NH2/NH3 reaction may also decompose into H2O2 and C(CH3)(H)(CH3N)/C(CH3)(H)(H2N) due to the presence of two polar N–H bonds in both CH3NH2 and NH3 (Scheme 2). Both OH radical and H2O2 forming paths from bimolecular Criegee reactions have previously been shown to involve similar energetic demands,74–76 and thus, are equally likely to occur in the troposphere. Since the H2O2 forming pathway requires the presence of two polar N–H bonds in a given amine, such a mechanistic option in the case of the Criegee-(CH3)2NH reactions will not be available.
In summary, we have elucidated the molecular mechanisms of the Criegee-amine reactions both in the gas phase and at the air–water interface using quantum chemical calculations and BOMD simulations, respectively. The quantum chemical calculations suggest that the barrier for a Criegee-amine reaction decreases with the extent of methyl substitution in the amine. This barrier lowering is so pronounced that the dimethylamine reactions of CH2OO and anti-CH3CHOO occur barrierlessly. Though the Criegee-ammonia reactions have been found to be tropospherically insignificant, the facile nature of Criegee-dimethylamine reactions suggests that these chemistries may play a role in the new particle forming events under certain conditions and thus, need to be updated in the existing atmospheric models. Alternatively, these reactions could also serve as a potential source of the OH radical and hydrogen peroxide under tropospheric conditions. The air–water interface inclusive BOMD simulations reveal the diverse mechanistic pathways available for the otherwise simple looking addition reaction between anti-CH3CHOO and CH3NH2 on the aqueous surface. The reaction follows a stepwise mechanism, which may or may not be mediated by the interfacial water molecules and occurs on a picosecond time scale. Overall, these results suggest that the Criegee-amine interactions could contribute towards the organic fraction of the aerosol particles in the atmosphere.
This work was supported by the University of Nebraska Holland Computing Center.
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