Computational Study of the Reactions of Chlorine Radicals with

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Computational Study of the Reactions of Chlorine Radicals with Atmospheric Organic Compounds Featuring NHx−π-Bond (x = 1, 2) Structures Hong-Bin Xie,† Fangfang Ma,† Qi Yu,† Ning He,‡ and Jingwen Chen*,† †

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology and ‡State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: Among 160 organic NHx-containing compounds (x = 1, 2) detected in the atmosphere, there are about 80 species for which the molecules contain p−π conjugate substructures of NHx−π-bonds. Here, chlorine radical (·Cl)initiated reactions for formamide, N-methylformamide, ethenamine, and aniline, as their cases, were investigated by a quantum chemical method [CCSD(T)/aug-cc-pVTZ//MP2/631+G(3df,2p)] and kinetics modeling. The calculated overall rate constants are 5.5 × 10−11, 2.3 × 10−10, 2.7 × 10−10, and 1.7 × 10−10 cm3 molecule−1 s−1 for formamide, N-methylformamide, ethenamine, and aniline, respectively, and agree well with experimental values for available ones. Importantly, the results show that the reactions of two amides with ·Cl mainly lead to C-center radicals via ·Cl abstracting the −CHO hydrogen of amides. However, both ethenamine + ·Cl and aniline + ·Cl reactions mainly produce delocalized radicals with the radical center on the C-site and N-site via a ·Cl addition and the −NHx hydrogen abstraction pathway, respectively. Therefore, this study reveals that reactions of organic NHx-containing compounds with ·Cl have various reaction mechanisms, in contrast to our previous understanding that −NHx hydrogen abstraction pathways, leading to N-center radicals, are the most favorable. The unveiled reaction mechanisms should be of significance for the risk assessment of atmospheric organic NHx-containing compounds and enriching ·Cl chemistry.

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acids, and amides have been identified in the atmosphere.15 The most common and abundant organic NHx-containing compounds in the atmosphere are low-molecular-weight aliphatic amines such as methylamine (MA) and dimethylamine (DMA). Their concentration in ambient air is on the ppt level.15 The reactions of organic NHx-containing compounds with ·Cl could form various products via either the addition of ·Cl to unsaturated bonds or H abstraction, and our current understanding of the atmospheric reaction mechanism of organic NHx-containing compounds comes from reactions of amines with ·Cl, for which N-center radicals formed by Habstraction pathways are the main products.7,13 The formation of N-center radicals was ascribed to the formation of a prereactive complex associating the particular two-center− three-electron (2c−3e) bonds (Scheme 1) between the lonepair electrons of N atoms in amines and the semioccupied p orbital of ·Cl,16 which decreases the overall reaction energy barrier for H abstractions occurring at the N site. It deserves mentioning that in assessing the environmental risk caused by atmospheric organic NHx-containing compounds, N-center

INTRODUCTION Atmospheric chlorine radicals (·Cl) have high reactivity toward volatile organic pollutants, with rate constants that are, with some exceptions, 10 times higher than those of hydroxyl radicals (·OH).1−4 Historically, ·Cl was regarded as being formed primarily from heterogeneous reaction cycles involving sea salt,1,5 and their concentration is estimated to be ca. 1−10% of that of ·OH.6,7 Therefore, the role of ·Cl in atmospheric oxidation has been traditionally thought to be limited to the marine boundary layer. In recent years, a significant ·Cl source was found in continental regions of American, Canada, and Germany.8−10 The new findings of ·Cl sources have expanded the potential importance of ·Cl from coastal areas to continental urban areas. Thus, ·Cl will play a more important role in governing the fate of tropospheric organic pollutants than what was previously thought. So far, ·Cl atmospheric chemistry for many volatile organic pollutants including hydrocarbons, halogenated hydrocarbons, and oxygenated hydrocarbons has been investigated.11,12 However, a little attention has been paid to the ·Cl-initiated transformation of organic NHx-containing compounds (x = 1, 2).7,13,14 A recent review revealed that about 160 different organic NHx-containing compounds including aliphatic amines, aromatic amines, piperazine, pyrrolidine, piperidine, amino © 2017 American Chemical Society

Received: November 13, 2016 Revised: January 29, 2017 Published: February 6, 2017 1657

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Scheme 1. Demonstration of the Molecular Orbitals and Structural Characteristics of N−Cl Two-Center−Three-Electron (2c− 3e) Bonds by Taking Methylamine as a Casea

(A) Simple molecular orbital representation. (B) Contributing electronic structures. (C) σ-bonding orbital diagram of 2c−3e bonds. (D) σantibonding orbital diagram of 2c−3e bonds. a

radicals are among the most concerning products since they are hard to react with O2 and therefore can react mainly with NOx (x = 1, 2) to form carcinogenic nitrosamines and nitramines in the atmosphere.13,17−21 In principle, all organic NHx-containing compounds can form 2c−3e bonds with ·Cl and the bond strength depends on the electron-donor ability of N atoms. In view of the chemical reaction, the strong 2c−3e bonds tend to make the formation of N-center radicals more favorable in the reactions of organic NHx-containing compounds with ·Cl by decreasing the overall reaction energy barrier. In addition, according to the Evans− Polanyi relationship,22 a strong N−H bond generally decreases the feasibility of the formation of N-center radicals. As for atmospheric organic NHx-containing compounds, the electrondonor ability of N atoms and the N−H bond strength could vary with the functional groups attached to the −NHx (x = 1, 2) group. As a result, the reactions of atmospheric organic NHx-containing compounds toward ·Cl may not always favorably lead to N-center radicals, the precursors of carcinogenic nitrosamines and nitramines. Among 160 organic NHx-containing compounds detected in the atmosphere, there are about 80 species for which the molecules contain substructures of −NHx (x = 1, 2) connecting to π bonds, featuring a p−π conjugate structure. Amides could be the most abundant in these p−π conjugate compounds. A very recent study found that the concentration of amides can reach the few ppbv level in in urban Shanghai, China.23 In view of electronic structures, the lone pair of electrons on N atoms of the p−π conjugate compounds can be delocalized more or less to their adjacent π bonds, which tends to decrease the electron-donor ability of N atoms.24 Therefore, the p−π conjugate compounds should form weaker 2c−3e bonds with ·Cl than will the reported alkyl amines.7,13 In addition, the N− H bond enthalpy of the p−π conjugate compounds could be different from that of the alkyl amines because of their unique electronic structures. For example, the N−H bond enthalpy of amides is usually higher than that of the alkyl amines, which could result from the difference in types of hybridization of N atoms between amides (sp2) and alkyl amines (sp3).25 However, according to our preliminary calculations, the N−H bond enthalpy of aromatic amines and alkene amines is lower than that of alkyl amines. Compared to alkyl amines, the decrease in the 2c−3e bond strength and change in the N−H bond enthalpy could alter the reaction energy barrier to form N-center radicals. This could cause the p−π conjugate compounds to present various reaction mechanisms toward ·Cl, not like alkyl amines with N-center radicals as the main products.7,13

In this study, following our previous study on the reaction of monoethanolamine, a kind of alkyl amine, with ·Cl, we investigated the reaction mechanisms and kinetics of the organic NHx-containing compounds featuring NHx−π conjugate substructures (x = 1, 2) with ·Cl by taking two simple amides (formamide and N-methylformamide), one simple alkene amine (ethenamine), and one aromatic amine (aniline) as cases. All selected organic NHx-containing compounds have been detected in the ambient atmosphere.15 The concentration of amides and aniline were found to be up to a few ppbv23 and tens of pptv,26 respectively. Combined high-cost coupledcluster theory (CCSD(T)) calculations and kinetics modeling were employed in this study. The results of this study will enhance our understanding of the reaction mechanism and kinetics of organic NHx-containing compounds, be of significance for the environmental risk assessment of organic NHx-containing compounds, and enrich ·Cl chemistry.

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COMPUTATIONAL DETAILS: AB INITIO ELECTRONIC STRUCTURE AND KINETICS CALCULATIONS The Gaussian 09 program package27 was employed to perform all of the structure and energy calculations. The geometry optimization and harmonic frequency calculations for the reactants, products, intermediates, and transition states were performed at the MP2/6-31+G(3df,2p) level.28 The connection of the transition states between designated local minima was confirmed by intrinsic reaction coordinate (IRC) calculation at the MP2/6-31+G(3df,2p) level. A single-point energy calculation was performed at the CCSD(T)/aug-cc-pVTZ level28 based on the geometries at the MP2/6-31+G(3df,2p) level. The single-point energy was corrected by the zero-point energy at the MP2/6-31+G(3df,2p) level. In the application to atmospheric chemistry problems, the CCSD(T) method is likely the most popular ab initio method in use today.28 It is very accurate and expensive for energy calculations, and the errors in relative energies and bond energies can often be calculated to within 1 kcal mol−1.28 A previous study found that predicted reaction rate constants based on the energy data from a similar scheme involving MP2 geometry optimization and the CCSD(T) single-point energy calculation for mono-, di-, and trimethylamine with ·Cl agree well with experimental values.7 To account for the effect of spin−orbit coupling, a literature value of 0.8 kcal mol−1 was applied for isolated ·Cl, and this effect is quenched in the transition structure and in other regions of the reaction pathways.7 The most stable conformers were selected as the starting reactant. The electron number for lone pair electrons of N atoms in the target compounds based on the natural bond orbital (NBO)29 analysis were calculated. 1658

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The Journal of Physical Chemistry A In addition, the Gibbs free energies of reaction (ΔG) for the formation of intermediates and products were calculated at 298 K and are presented in the Supporting Information (SI). The MultiWell-2014.1 master equation code30−33 was employed to calculate the reaction rate constants. The master equation method is a powerful tool in calculating the timedependent, temperature-dependent, and pressure-dependent kinetics of a multichannel and multiwell chemical reaction system. The reaction rate constants for tight transition states were calculated from the RRKM theory34 on the basis of sums and densities of states for the MP2/6-31+G(3df,2p) structures and the CCSD(T)/aug-cc-pVTZ barrier heights. The energygrained master equation was solved over 2000 grains of 10 cm−1 each, carried on to 85 000 cm−1 for the continuum component of the master equation. N2 was employed as a buffer gas. The collision transfer probability between reactive intermediates and N2 was described by the single-exponentialdown model35 with an average transfer energy of ΔEd = 200 cm−1. The Lennard-Jones parameters for intermediates were calculated from an empirical method proposed by Gilbert et al.36 The rate constants for the barrierless entrance/exit pathways were calculated by the long-range transition-state theory with a dispersion force potential,37 which had been successfully used in calculating the reaction rate constants of the barrierless entrance pathways for the reactions of mono-, di-, and trimethylamine7 and MEA13 with ·Cl. Details for the rate constant calculation with long-range transition-state theory were presented in the SI.

Figure 1. Schematic potential energy surface for the reaction of formamide + ·Cl calculated at the CCSD(T)/aug-cc-pVTZ//MP2/631+G(3df,2p) level [The total energy of the reactants formamide + ·Cl is set as zero (reference state). Symbols R1, RC1‑m, PC1‑m, TS1‑m, and P1‑m stand for reactants, prereactive complexes, postreactive complexes, transition states, and products involved in the reaction, respectively; m denotes different species. ΔE was calculated at 0 K. The 2c−3e bonds between ·Cl and N atoms are highlighted by the red dashed line in the schematic diagram of molecular structure.]

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RESULTS AND DISCUSSION Reactions of Amides with ·Cl. In principle, ·Cl could either add to unsaturated CO bonds or abstract H atoms from the −CH3, HCO, and −NHx (x = 1, 2) groups of the target amides. However, it failed to locate intermediates formed from ·Cl addition to the O site of the CO bonds although lots of attempts have been made, suggesting the unfeasibility of such an addition. We noted that there is also no such addition process in the reaction of methacrolein with ·Cl.2 Therefore, the possible pathways for the reactions of the amides with ·Cl include ·Cl addition to the C site of CO bonds and H abstractions. The schematic potential energy surfaces including the possible pathways for the two amides are presented in Figures 1 and 2, respectively, and the optimized geometries for the important species including reactants, complexes, intermediates, and transition states are presented in Figure 3. Reaction of Formamide with ·Cl. It can be concluded from the overall reaction energy barriers (Figure 1) that the Habstraction occurring at the C-site of HCO via transition state TS1−3 is much more favorable than those occurring at the N site and addition pathway. In view of thermodynamics, only the H-abstraction occurring at the C-site is exothermic, and other pathways are endothermic. Therefore, the formation of Ccenter radical ·CONH2 via H abstraction at the C site of HCO is the most favorable in terms of both kinetics and thermodynamics and is exclusive. Reaction of N-Methylformamide with ·Cl. Similar to the reaction of formamide with ·Cl (Figure 2), an N−H hydrogen abstraction and addition pathway are energetically uncompetitive and a HCO hydrogen abstraction is the most favorable for the reaction of N-methylformamide with ·Cl in terms of both thermodynamics and kinetics. In addition, the abstraction of a −CH3 hydrogen is exothermic, and the corresponding overall reaction energy barrier is about 2 kcal mol−1 higher than that of

Figure 2. Schematic potential energy surface for the reaction of Nmethylformamide + ·Cl calculated at the CCSD(T)/aug-cc-pVTZ// MP2/6-31+G(3df,2p) level [The total energy of the reactants Nmethylformamide + ·Cl is set as zero (reference state). The symbols “R2, RC2‑m, PC2‑m, TS2‑m and P2‑m” stand for reactants, prereactive complexes, postreactive complexes, transition states and products involved in the reaction, respectively; m denotes different species. ΔE was calculated at 0 K. The 2c−3e bonds between ·Cl and N atoms are highlighted by the red dashed line in the schematic diagram of molecular structure.]

the most favorable pathway. Therefore, for the reaction of Nmethylformamide with ·Cl, ·CONHCH3 could be the major products and HCONHCH2· radicals are the minor ones. Clearly, the reactions of ·Cl with formamide and Nmethylformamide cannot lead to N-center radicals, in contrast to the previous study on reactions of ·Cl with the alkyl amines that form N-center radicals.7,13 To the best of our knowledge, this is the first report in which the atmospheric reactions of ·Cl with amides, a kind of organic NHx-containing compounds, cannot produce N-center radicals. It deserves mentioning that NH hydrogen abstraction pathways for the reactions of these two amides with ·Cl still proceed via a prereactive complex with 1659

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2c−3e bonds between ·Cl and N atoms (Figures 1 and 2) although they are uncompetitive. Reaction of Ethenamine with ·Cl. ·Cl could either add to unsaturated CC bonds or abstract H atoms from CH2 CH− and −NH2 of ethenamine. All of the located reaction pathways are shown in Figure S1, and the main reaction pathways are depicted in Figure 4. The optimized geometries for the important species are presented in Figure 3. Addition Pathways. We made many attempts to locate the transition state for ·Cl addition to the terminal C atom of C C bonds to form an addition intermediate. However, all attempts failed; instead, most attempts lead to the terminal addition intermediate, implying that ·Cl addition to the terminal C atom of CC bonds is barrierless. A recent study also showed that ·Cl addition to the terminal C atom of CC bonds of methacrolein is barrierless.2 In addition, the located plausible transition state for ·Cl addition to the central C atom of CC bonds was confirmed to be a Cl-shift one from the terminal C atom to the central C atom of CC bonds by IRC calculation. Therefore, a central addition intermediate should be formed via the Cl-shift process in the reaction, which is similar to the case of the reaction of ·Cl + methacrolein.2 As shown in Figure 4, the ·Cl addition to the terminal C atom of CC bonds forms addition intermediates (IM3−1) with two conformations, depending on the attacking direction of ·Cl. The two conformations of IM3−1 can easily interconvert into each other with an energy barrier of about 0.3 kcal mol−1. The excess vibrational energy along the formation of IM3−1 could promote IM3−1 to isomerize or dissociate to other species. From IM3−1, two Cl-shift processes [e.g., 1,2 Cl shift from the terminal C atom to the central C atom of CC bonds to form IM3−2; 1,3 Cl shift from the terminal C atom to the N atom of −NH2 to form RC3−1 associated with the 2c−3e bonds between the N atoms and ·Cl] and three H-abstraction processes [e.g., ·Cl abstracts two H atoms from CH2 via

Figure 3. MP2/6-31+G(3df,2p)-optimized geometries for formamide, N-methylformamide, ethenamine, aniline, and some important complexes, intermediates, and transition states involved in the reactions of formamide + ·Cl, N-methylformamide + ·Cl, ethenamine + ·Cl, and aniline + ·Cl. [The 2c−3e bonds between ·Cl and N atoms are highlighted by red dashed line. The distances are in angstroms.]

Figure 4. Schematic potential energy surface for the main reaction pathway of ethenamine with ·Cl calculated at the CCSD(T)/aug-cc-pVTZ// MP2/6-31+G(3df,2p) level. [The total energy of reactants ethenamine + ·Cl is set as zero (reference state). Symbols R3, RC3−1, IM3‑m, PC3‑m, TS3‑m, and P3‑m stand for reactants, prereactive complexes, intermediates, postreactive complexes, transition states, and products involved in the reaction, respectively; m denotes different species. ΔE was calculated at 0 K. The 2c−3e bonds between ·Cl and N atoms are highlighted by the red dash line in the schematic diagram of molecular structure.] 1660

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from −NH2 were considered. Because of the Cs symmetry of aniline, the pathways that need to be considered were further reduced to four addition pathways (addition to para, meta, ortho, and ipso sites of the −NH2 group) and one H abstraction from −NH2. In addition, previous studies indicated that there is no potential barrier in the addition pathway for the reactions of ·Cl with benzene, anthracene, and pyrene;38,39 addition pathways for the reactions of ·Cl with aniline should also be barrierless. The schematic potential energy surface for the possible pathways is presented in Figure 5. The optimized geometries for the important species are presented in Figure 3.

TS3−3 and TS3−3′ (Figure S1) and one H atom from −NH2 via two successive transition states TS3−2 and TS3−6 to finally form P3−3, P3−3′ (C-center radicals ·CHCH2NH2 + HCl), and P3−2 (delocalized radicals ·CH2CHNH + HCl)] were identified. We note that the energy of TS3−6 is lower than that of PC3−2 at the CCSD(T) level, different from that at the MP2/6-31+G(3df,2p) level (TS3−6 is 0.02 kcal mol−1 higher than PC3−2), indicating that the transformation from PC3−2 to PC3−3 is barrerless at the CCSD(T) level. Therefore, the H abstraction from −NH2 via the IM3−1 intermediate should be a one-step process at the CCSD(T) level. The two intermediates/ complexes, IM3−2 and RC3−1, formed in the Cl-shift processes can further dissociate to products of the delocalized radical + HCl via H-abstraction processes. It deserves mentioning that the spin density of the C atom of the −CH2 group in delocalized radical ·CH2CHNH is 0.96, which is much higher than 0.77 for the N atom (see all atomic spin density of ·CH2CHNH in SI). Therefore, the radical center of the delocalized radical ·CH2CHNH is mainly on the C cite of the −CH2 group. Direct H-Abstraction Pathways. Apart from the −NH2 hydrogen abstraction via RC3−1 formed from addition intermediate IM3−1, ·Cl can direct abstract −NH2 hydrogen via RC3−1 to form delocalized radical ·CH2CHNH as shown in Figure 4. It deserves mentioning that the energy of RC3−1 with the 2c−3e bonds between the N atom and ·Cl is about 12.4 kcal mol−1 less stable than that of IM3−1, which could make the entrance reaction of ethenamine + ·Cl prefer to form IM3−1 in view of thermodynamics. Therefore, −NH2 hydrogen abstraction via RC3−1 could be considered to start from IM3−1. In addition, direct H-abstraction processes from CH2CH− are thermodynamically unfeasible (Figure S1) and thus make no contribution to the final products. By comparing the overall reaction energy barriers (Figure S1) of various addition and H-abstraction pathways, we conclude that the pathway proceeding via intermediate IM3−1 to directly form P3−2 (delocalized radicals ·CH2CHNH and HCl) is the most favorable, followed by the pathways to form P3−1 and P3−2 via two successive intermediates IM3−1 and RC3−1. The other pathways contribute very little to the final products because of their high overall reaction energy barriers. Interestingly, all of the favorable pathways lead to delocalized radical ·CH2CHNH via ·Cl abstracting −NH2 hydrogen. To the best of our knowledge, this is the first time to report that the products of NH hydrogen abstraction can be favorably formed via addition intermediates but not complexes associating with the 2c−3e bonds between the N atoms and ·Cl. Therefore, this study unveils a new pathway for the formation of the NH hydrogen abstraction product from the reaction of organic NHx-containing compounds with ·Cl. Reaction of Aniline with ·Cl. ·Cl could either add to a C atom or abstract a H atom from the phenyl and −NH2 group of aniline. Previous studies have demonstrated that for the reactions of chemicals containing phenyl group (e.g., benzene and polycyclic aromatic hydrocarbons) with ·Cl, H abstraction from the phenyl group is thermodynamically unfeasible.38,39 Our calculations also showed the H abstraction from the phenyl group of aniline is endothermic. For example, H abstractions from the C site of para, meta, and ortho sites of the −NH2 group are endothermic by 19.2, 18.8, and 18.9 kcal mol−1, respectively. Therefore, we excluded the contribution of the H abstraction from the phenyl group. Only the pathways of ·Cl addition to C atoms of the phenyl group and H abstraction

Figure 5. Schematic potential energy surface for the aniline + ·Cl reaction calculated at the CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p) level. [The total energy of the aniline + ·Cl reactants is set as zero (reference state). Symbols R4, RC4−1, PC4‑m, TS4‑m, IM4‑m, and P4‑m stand for reactants, prereactive complexes, postreactive complexes, transition states, intermediates, and products involved in the reaction, respectively; m denotes different species. ΔE was calculated at 0 K. The 2c−3e bonds between ·Cl and N atoms are highlighted by the red dashed line in the schematic diagram of the molecular structure].

As can be seen in Figure 5, the overall reaction energy barrier for direct −NH2 hydrogen abstraction that proceeds via a complex RC4−1 with the 2c−3e bonds between the N atoms and ·Cl to form delocalized radicals (C6H5NH·) with the radical center on the N atom (SI) and HCl is −3.9 kcal mol−1. The energy of RC4−1 is lower than that of addition intermediates formed in the entrance pathway, different from the case for the reaction of ethenamine with ·Cl. Therefore, in the entrance pathway, the reaction prefers to form RC4−1 in view of thermodynamics. Similar to the reactions of other aromatics with ·Cl, the formed addition intermediates except for IM4−2 cannot transform to fragmental products for the aniline + ·Cl reaction.38,39 IM4−2 can finally lead to products of delocalized N-center radicals and HCl via two successive transition states TS4−2 and TS4−3, a similar process to the reaction of ethenamine with ·Cl. The overall reaction energy barrier for the transformation of IM4−2 is −0.8 kcal mol−1, which is about 3.0 kcal mol−1 higher than that of the direct −NH2 hydrogen abstraction pathway. We noted that the ΔG values shown in the SI for the formation of all of the addition intermediates except IM4−2 are >0 at 298 K. In principle, the addition intermediates that are thermodynamically unfeasibly formed and cannot further transform to thermodynamically 1661

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Figure 6. Variation of calculated reaction rate constants with temperature at 1 atm.

ethenamine and aniline is lower than that of the alkyl amines. Therefore, the unfavorable factors of both decreased 2c−3e bond strength and increased N−H bond enthalpy for the formation of NH hydrogen abstraction products can explain the increase in the overall reaction energy barriers of the H abstractions from the −NHx of amides compared to those of the alkyl amines.7,13 The synergetic effects of one favorable (decreased N−H bond enthalpy) and one unfavorable factor (decreased 2c−3e bond strength) for the formation of NH hydrogen abstraction products is the reason that the overall reaction energy barriers (−7.3 and −3.9, respectively) of the H abstractions from the −NH2 of ethenamine and aniline are comparable to those of the alkyl amines. Kinetics. To evaluate the product branching ratio and the overall rate constant (kCl) for the reactions of ·Cl with formamide, N-methylformamide, ethenamine, and aniline, we performed master equation simulations on the favorable pathways, that is, the pathways to form P1−3 (·CONH2 + HCl) for formamide, pathways to P2−3 (·CONHCH3 + HCl) and P2−3′ (HCONHCH2· + HCl) for N-methylformamide, pathways to P3−1 and P3−2 (·CH2−CHNH + HCl) via intermediate IM3−1 for ethenamine, and a direct H-abstraction pathway to P4−1 (C6H5NH· + HCl) for aniline. The calculated kCl values are 5.5 × 10−11, 2.3 × 10−10, 2.7 × 10−10, and 1.7 × 10−10 cm3 molecule−1 s−1 for formamide, N-methylformamide, ethenamine, and aniline at 298 K and 1 atm, respectively. There are experimental kCl values available for two amides (4.5 × 10−11 and 9.7 × 10−11 cm3 molecule−1 s−1 for formamide and N-methylformamide, respectively),14 which agree well with the corresponding computational values. The factors between the computational values and experimental values are 1.2 and 2.4, respectively. This could further support the rationality of our selection in the computational scheme. As can be seen from Figure 6, over the temperature range of 200−376 K, the calculated kCl values decrease with increasing temperature. In addition, as shown in Figure S2, branching ratios of the intermediate or complex are negligibly small for the ·Cl + formamide, ·Cl + N-methylformamide, and ·Cl + ethenamine reactions over the temperature range of 200−376 K, indicating that these three reactions predominantly form fragmental

feasible fragmental products could return to reactants. Therefore, all addition intermediates except for IM4−2 can return to reactants. All in all, the direct H-abstraction pathway is the most favorable for the reaction of aniline with ·Cl. We also compared the direct H-abstraction pathways occurring at the N site between the four target organic NHxcontaining compounds and previously reported alkyl amines.7,13 The direct H-abstraction pathways occurring at the N site for the target compounds still proceeds via a prereactive complex with 2c−3e bonds between ·Cl and N atoms, similar to those for the alkyl amines.7,13 However, the 2c−3e bond length (N−Cl bond in Figure 3) for the four target compounds is longer than those of reported alkyl amines.7,13 The relative energies [(−3) − (−11) kcal mol−1)] of the prereactive complex to corresponding reactants for the target organic NHx-containing compounds are higher than those (∼−14 kcal mol−1) of the corresponding alkyl amines,7,13 which means that the 2c−3e bond strength formed from the target compounds and ·Cl are lower than those from the alkyl amines with ·Cl. This should result from a delocalization of lone pair electrons of N atoms of the target organic NHxcontaining compounds to their adjacent π bonds, which decreases the electron donor ability of N atoms of the target compounds toward ·Cl. According to the NBO analysis, the numbers of lone-pair electrons on the N atoms of formamide, N-methylformamide, ethenamine, and aniline are 1.807e, 1.766e, 1.897e, and 1.904e, respectively, which are all smaller than 2e. Therefore, the lone-pair electrons of the N atoms do delocalize to their adjacent π bonds. In addition, the overall reaction energy barriers (>8.7 kcal mol−1) of the H abstractions from the −NHx of amides is much higher than those (∼−7 kcal mol−1) of the alkyl amines. However, the overall reaction energy barriers (−7.3 and −3.9, respectively) for ethenamine and aniline are comparable to those of the alkyl amines.7,13 The various overall reaction energy barriers should be mainly ascribed to the different N−H bond enthalpies of the four target compounds besides the various 2c−3e bond strengths. The N−H bond enthalpy (∼115 kcal mol−1) of amides is higher than that of the alkyl amines (∼105 kcal mol−1),25 and the N−H bond enthalpy (∼87.2 and 93.1 kcal mol−1) of 1662

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The Journal of Physical Chemistry A products. P1−3 is the only product of the ·Cl + formamide reaction, and the product ratios between P2−3 and P2−3′ for ·Cl + N-methylformamide and between P3−1 and P3−2 for ·Cl + ethenamine are calculated to be 31.3 and 644 000 at 298 K, respectively. Therefore, under tropospheric conditions, P1−3 is an exclusive product for the ·Cl + formamide reaction; P2−3 is a main product and P2−3′ is a minor one for the ·Cl + Nmethylformamide reaction, and P3−2 is a main product and P3−1 is a negligible one for the ·Cl + ethenamine reaction. It deserves mentioning that P3−2 is mainly produced via the R3 → IM3−1 → PC3−2 → PC3−3 → P3−2 pathway, confirming that P3−2 is favorably formed in the pathway proceeding via addition intermediate IM3−1 but not in the complex associating with the 2c−3e bonds. For the ·Cl + aniline reaction, complex RC4−1 except for P4−1 takes a nonnegligible branching ratio at low temperatures. When the temperature is higher than 298 K, the reaction predominantly forms P4−1. Comparison with Previous Experiments and Implications. As far as we know, only products and kinetics information for the reactions of two amides with ·Cl are available among the four reactions investigated in this study. Thus, we compared our mechanistic findings with the experimental data for these two reactions. This study found that for the reaction of formamide with ·Cl, ·CONH2 radicals are exclusive products whereas for N-methylformamide, ·CONHCH3 radicals are major products and HCONHCH2· radicals are minor ones. A recent study indicated that the reaction of ·CONH2, ·CONHCH3, and HCONHCH2· radicals with atmospheric O2 can produce isocyanic acid, methyl isocyanate, and N-formylformamide, respectively.40,41 Thus, the ·Cl-initiated atmospheric transformation of formamide will finally form isocyanic acid, and the ·Cl-initiated N-methylformamide reaction will finally form methyl isocyanate as a major product and N-formylformamide as a minor one. This prediction is consistent with the corresponding experimental product identification performed by Barnes et al.,14 which in turn verifies that the revealed reaction mechanisms that proceed via the H-abstraction pathways to form C-center radicals for the two amides under study are reliable. It deserves mentioning that isocyanic acid and methyl isocyanate are compounds that are potentially hazardous to human health.42−46 Therefore, the transformation of formamide and N-methylformamide initiated by ·Cl will increase the environmental risk of formamide and N-methylformamide. We noted that the NH2CO· formed via H abstraction from the −CHO group is the exclusive product for the reaction of formamide with ·OH, ·CONHCH3 via H abstraction from the −CHO group is the major product, and HCONHCH2· is a minor product for the reaction of N-methylformamide with ·OH.40,41 Thus, the reaction mechanism of these two amides with ·OH is similar to that with ·Cl. However, the reaction energy barrier of the most favorable pathway for the reactions of formamide and N-methylformamide with ·Cl is about 2−3 kcal mol−1 lower than those of corresponding reactions with ·OH, further indicating the high reactivity of ·Cl.1−4 Reactions of ethenamine and aniline with ·Cl favorably produce delocalized C-center radical ·CH2CHNH and N-center radical C6H5NH·, respectively. Because of the delocalized character, ·CH2CHNH and C6H5NH· could have reactivities of both C-center and N-center radicals. It is known that typical Ncontaining C-center radicals and N-center radicals can be removed from the atmosphere primarily through bimolecular reactions with O2 and NOx (x = 1, 2), respectively.13,47,48

Because the concentration of atmospheric O2 is much higher than that of NOx, ·CH2CHNH and C6H5NH· could be removed from the atmosphere primarily through reactions with O2 as a result of their C-center radical reactivity. ·CH2CHNH could react with O2 to form CO + ·OH + CH2NH in a similar way as ·CH2CHO.49 The reaction products of C6H5NH· with O2 could be complicated. A possible product could be ·OH + CHOCHCHCHCHCN involving a ring-opening process. Further study is necessary to investigate the atmospheric fate of delocalized C-center radical ·CH2CHNH and N-center radical C6H5NH·. In addition, although there is no direct support for new mechanistic findings for the reaction of ethenamine with ·Cl, indirect support can be found in a recent computational study of the ·Cl + methacrolein reaction.2 It was found that H abstraction from −CHO of methacrolein via a terminal addition intermediate is more favorable than direct H-abstraction from the −CHO of methacrolein, which can well explain previous experimental observations.50 This is similar to our case, where the −NH2 hydrogen abstraction proceeding via a terminal addition intermediate is more favorable than direct −NH2 hydrogen abstraction by ·Cl. As for kinetics, kCl values for the two amides are about 10 times higher than their reaction rate constants with ·OH (kOH) (4.4 × 10−12 and 1.01 × 10−11 cm3 molecule−1 s−1 for formamide and N-methylformamide, respectively).40,41 As in the marine boundary layer, ·Cl concentrations [·Cl] are estimated to be as much as 1−10% of [·OH],6,7 and the contribution of ·Cl to the transformation of the two amides is about 10−100% (estimated by kCl[·Cl]/kOH[·OH]) of the contribution of ·OH. Therefore, ·Cl plays an important role in transforming amides, and the contribution of ·Cl has to be considered in the fate assessment of amides. There is no available experimental results including kOH values and mechanistic information for the reaction of ethenamine and aniline with ·OH, and we cannot make a comparison for the contribution of ·Cl and ·OH in transforming them. Probably, ·Cl also plays an important role in transforming ethenamine and aniline under conditions where ·Cl is expected to be important. This study indicated that the four target organic NHxcontaining compounds can form weaker 2c−3e bonds [(−3) − (−11) kcal mol−1] with ·Cl than do alkyl amines because of delocalization of the N lone-pair electrons to their adjacent π bonds. The change in N−H bond enthalpy and the decreased 2c−3e bond strength cause the reactions of the four target organic NHx-containing compounds with ·Cl to have various reaction mechanisms. This study can be the first time to point out that the reactions of organic NHx-containing compounds with ·Cl can favorably form C-center radicals and NH hydrogen-abstraction products (delocalized C-center radicals) via addition intermediates. Because the four target organic NHx-containing compounds cover the main p−π conjugate types of lone pair electrons of −NHx (x = 1, 2) to π bonds (lone pair electrons to CO, CC, and the phenyl group) for the detected 80 atmospheric p−π conjugate organic NHxcontaining compounds, it is reasonable to propose that the revealed reaction mechanisms can be applicable to other atmospheric p−π conjugate organic NHx-containing compounds. In addition, this study implies that the reactions of atmospheric organic NHx-containing compounds with ·Cl have disparate reaction mechanisms that lead to different products and risks. Therefore, to comprehensively understand the risk of 1663

DOI: 10.1021/acs.jpca.6b11418 J. Phys. Chem. A 2017, 121, 1657−1665

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The Journal of Physical Chemistry A

(7) Nicovich, J. M.; Mazumder, S.; Laine, P. L.; Wine, P. H.; Tang, Y.; Bunkan, A. J. C.; Nielsen, C. J. An experimental and theoretical study of the gas phase kinetics of atomic chlorine reactions with CH3NH2, (CH3)2NH, and (CH3)3N. Phys. Chem. Chem. Phys. 2015, 17, 911−917. (8) Thornton, J. A.; Kercher, J. P.; Riedel, T. P.; Wagner, N. L.; Cozic, J.; Holloway, J. S.; Dubé, W. P.; Wolfe, G. M.; Quinn, P. K.; Middlebrook, A. M.; Alexander, B.; Brown, S. S. A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry. Nature 2010, 464, 271−274. (9) Mielke, L. H.; Furgeson, A.; Osthoff, H. D. Observation of ClNO2 in a Mid-Continental Urban Environment. Environ. Sci. Technol. 2011, 45, 8889−8896. (10) Phillips, G. J.; Tang, M. J.; Thieser, J.; Brickwedde, B.; Schuster, G.; Bohn, B.; Lelieveld, J.; Crowley, J. N. Significant concentrations of nitryl chloride observed in rural continental Europe associated with the influence of sea salt chloride and anthropogenic emissions. Geophys. Res. Lett. 2012, 39, L10811. (11) Poutsma, M. L. Evolution of Structure−Reactivity Correlations for the Hydrogen Abstraction Reaction by Chlorine Atom. J. Phys. Chem. A 2013, 117, 687−703. (12) Saiz-Lopez, A.; von Glasow, R. Reactive halogen chemistry in the troposphere. Chem. Soc. Rev. 2012, 41, 6448−6472. (13) Xie, H.-B.; Ma, F. F.; Wang, Y. F.; He, N.; Yu, Q.; Chen, J. W. Quantum Chemical Study on ·Cl-Initiated Atmospheric Degradation of Monoethanolamine. Environ. Sci. Technol. 2015, 49, 13246−13255. (14) Barnes, I.; Solignac, G.; Mellouki, A.; Becker, K. H. Aspects of the Atmospheric Chemistry of Amides. ChemPhysChem 2010, 11, 3844−3857. (15) Ge, X.; Wexler, A. S.; Clegg, S. L. Atmospheric amines − Part I. A review. Atmos. Environ. 2011, 45, 524−546. (16) McKee, M. L.; Nicolaides, A.; Radom, L. A. Theoretical Study of Chlorine Atom and Methyl Radical Addition to Nitrogen Bases: Why Do Cl Atoms Form Two-Center-Three-Electron Bonds Whereas CH3 Radicals Form Two-Center-Two-Electron Bonds? J. Am. Chem. Soc. 1996, 118, 10571−10576. (17) Onel, L.; Dryden, M.; Blitz, M. A.; Seakins, P. W. Atmospheric Oxidation of Piperazine by OH has a Low Potential To Form Carcinogenic Compounds. Environ. Sci. Technol. Lett. 2014, 1, 367− 371. (18) Onel, L.; Blitz, M.; Dryden, M.; Thonger, L.; Seakins, P. Branching Ratios in Reactions of OH Radicals with Methylamine, Dimethylamine, and Ethylamine. Environ. Sci. Technol. 2014, 48, 9935−9942. (19) Karl, M.; Svendby, T.; Walker, S. E.; Velken, A. S.; Castell, N.; Solberg, S. Modelling atmospheric oxidation of 2-aminoethanol (MEA) emitted from post-combustion capture using WRF−Chem. Sci. Total Environ. 2015, 527−528, 185−202. (20) Nielsen, C. J.; Herrmann, H.; Weller, C. Atmospheric chemistry and environmental impact of the use of amines in carbon capture and storage (CCS). Chem. Soc. Rev. 2012, 41, 6684−6704. (21) da Silva, G. Formation of Nitrosamines and Alkyldiazohydroxides in the Gas Phase: The CH3NH + NO Reaction Revisited. Environ. Sci. Technol. 2013, 47, 7766−7772. (22) Evans, M. G.; Polanyi, M. Equilibrium constants and velocity constants. Nature (London, U. K.) 1936, 137, 530−531. (23) Yao, L.; Wang, M.-Y.; Wang, X.-K.; Liu, Y.-J.; Chen, H.-F.; Zheng, J.; Nie, W.; Ding, A.-J.; Geng, F.-H.; Wang, D.-F.; Chen, J.-M.; Worsnop, D. R.; Wang, L. Detection of atmospheric gaseous amines and amides by a high resolution time-of-flight chemical ionization mass spectrometer with protonated ethanol reagent ions. Atmos. Chem. Phys. 2016, 16, 14527−14543. (24) Borduas, N.; Abbatt, J. P. D.; Murphy, J. G.; So, S.; da Silva, G. Gas-Phase Mechanisms of the Reactions of Reduced Organic Nitrogen Compounds with OH Radicals. Environ. Sci. Technol. 2016, 50, 11723−11734. (25) Bråt en, H. B.; Bunkan, A. J.; Bache-Andreassen, L.; Solimannejad, M.; Nielsen, C. J. Final Report on a Theoretical Study

atmospheric organic NHx-containing compounds, more studies should be performed on ·Cl-initiated tropospheric degradation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b11418. Details for long-range transition-state theory calculation; reaction rate constants for the entrance/exit pathways; Gibbs free energies of reaction (ΔG); schematic potential energy surface for the reaction of ethenamine with ·Cl; atomic spin density of NH hydrogen abstraction products (delocalized C-center and N-center radicals); variation of the calculated product branching ratio with temperature and Cartesian coordinates of stationary points on the potential energy surface for the target reactions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-411-84706269. E-mail: [email protected]. ORCID

Hong-Bin Xie: 0000-0002-9119-9785 Jingwen Chen: 0000-0002-5756-3336 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. John R. Barker (University of Michigan) for providing the MultiWell-2014.1 program. This study was supported by the National Natural Science Foundation of China (21677028, 21325729), the Major International (Regional) Joint Research Project (21661142001), the Fundamental Research Funds for the Central Universities, and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05).

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REFERENCES

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