Atmospheric Oxidation of Piperazine Initiated by Â·Cl: Unexpected High

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Environmental Processes

Atmospheric Oxidation of Piperazine Initiated by •Cl: Unexpected High Nitrosamine Yield Fangfang Ma, Zhezheng Ding, Jonas Elm, Hong-bin Xie, Qi Yu, Cong Liu, Chao Li, Zhiqiang Fu, Lili Zhang, and Jingwen Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02510 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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Environmental Science & Technology

1

Atmospheric

Oxidation

of

Piperazine

Initiated

by

·Cl:

2

Unexpected High Nitrosamine Yield

3

Fangfang Ma†, Zhezheng Ding†, Jonas Elm‡, Hong-Bin Xie†*, Qi Yu†, Cong Liu†,

4

Chao Li§, Zhiqiang Fu†, Lili Zhang† and Jingwen Chen†*

5

†

6

Education), School of Environmental Science and Technology, Dalian University of

7

Technology, Dalian 116024, China

8

‡

9

§

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of

Department of Chemistry and Climate, Aarhus University, Aarhus 8000, Denmark State Environmental Protection Key Laboratory of Wetland Ecology and Vegetation

10

Restoration, School of Environment, Northeast Normal University, Changchun

11

130117, China

12

Table of Contents (TOC)

13 14

ABSTRACT. Chlorine radicals (·Cl) initiated amine oxidation play an important role

15

for the formation of carcinogenic nitrosamine in the atmosphere. Piperazine (PZ) is

16

considered as a potential atmospheric pollutant since it is an alternative solvent to

17

monoethanolamine (MEA), a benchmark solvent in a leading CO2 capture technology.

18

Here, we employed quantum chemical methods and kinetics modeling to

19

investigate ·Cl-initiated atmospheric oxidation of PZ, particularly concerning the

20

potential of PZ to form nitrosamine compared to MEA. Results showed that

21

the ·Cl-initiated PZ reaction exclusively leads to N-center radicals (PZ-N) that mainly

22

react with NO to produce nitrosamine in their further reaction with O2/NO. Together 1

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23

with the PZ + ·OH reaction, the PZ-N yield from PZ oxidation is still lower than that

24

of the corresponding MEA reactions. However, the nitrosamine yield of PZ is higher

25

than the reported value for MEA when [NO] is < 5 ppb, a concentration commonly

26

encountered in polluted urban atmosphere. The unexpected high nitrosamine yield

27

from PZ compared to MEA results from a more favorable reaction of N-center

28

radicals with NO compared to O2. These findings show that the yield of N-center

29

radicals cannot directly be used as a metric for the yield of the corresponding

30

carcinogenic nitrosamine.

31

INTRODUCTION

32

Chlorine radicals (·Cl) have a high oxidation ability toward volatile organic

33

pollutants, with rate constants that are most often an order of magnitude larger than

34

those of hydroxyl radicals (·OH).1-15 ·Cl were historically considered to be produced

35

primarily from heterogeneous reaction cycles involving sea salt,1,16-19 and the

36

concentration of ·Cl ([·Cl]) in the marine boundary layer is approximately 1−10% of

37

[·OH].20-22 Therefore, the importance of ·Cl on the oxidation of tropospheric organic

38

pollutants had been thought to be limited to the marine boundary layer. However, in

39

the last seven years, a significant ·Cl source from ClNO2 has been identified in

40

mid-continental areas, such as North America, Central Europe and Western

41

Europe,23-26 and very recently even higher concentration of ClNO2 was detected in the

42

urban atmosphere of northern China,27,28 suggesting ·Cl also play an important role in

43

transforming atmospheric organics in continental areas. A very recent study also

44

demonstrated that ·Cl regionally can be more important than ·OH for the alkane

45

oxidation.28,29 In addition, the transformation of tropospheric organic pollutants

46

initiated by ·Cl may lead to a fate that differs from that by ·OH.21,30-32 These imply

47

that reactions with ·Cl have become a significant pathway governing the fate of 2


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48

tropospheric organic pollutants.

49

Piperazine (PZ) is one of 160 amines detected in the atmosphere.33,34 Its aqueous

50

solution is being developed as an alternative solvent to monoethanolamine (MEA), a

51

benchmark and widely used solvent in post-combustion CO2 capture (PCCC).35,36

52

Given the possible large-scale implementation of amine-based PCCC, it is likely that

53

there will be relatively significant discharges of PZ into the atmosphere from PCCC

54

units because of its relatively high vapor pressure (21 Pa at 20°C).37 As for the

55

atmospheric emission of PZ, one of the concerning environmental risks is the

56

formation of carcinogenic nitrosamines via atmospheric oxidation.21,30-32 Particularly,

57

as alternatives to MEA, the potential to form carcinogenic nitrosamine compared to

58

MEA is of concern.

59

·OH and ·Cl are found to be two main daytime initiators for the formation of

60

N-center radicals, the precursor of carcinogenic nitrosamine, in the atmospheric

61

oxidation of reported amines.8,21,30-32,38,39 Generally, reactions of amines with ·OH

62

mainly form C-center radicals with N-center radicals as minor products.8,30,32,38,39

63

However, the reactions with ·Cl mainly form N-center radicals.21,31 Our recent study

64

on MEA oxidation identified that the contribution of ·Cl to form carcinogenic

65

nitrosamine is about 25−250% compared to ·OH depending on the concentration

66

of ·Cl.31 This implies that ·Cl could play a dominant role in the formation of

67

carcinogenic nitrosamine when [·Cl] is high, like in the urban atmosphere of northern

68

China. PZ could follow the atmospheric oxidation process of other reported

69

amines8,21,30-32,38,39, i.e. the daytime formation of nitrosamine from PZ will mainly be

70

initiated by ·OH and ·Cl. The overall daytime yield of nitrosamine from PZ oxidation

71

by ·OH and ·Cl should depend on: 1) The branching ratios of N-center radicals from

72

PZ oxidation by both ·OH and ·Cl; 2) The yield of nitrosamine in the resulting 3



73

reactions of the formed N-center radicals; 3) The ratio of reaction rate constant

74

(kOH/kCl) between reactions of PZ with ·OH (kOH) and ·Cl (kCl). Being a cyclic

75

diamine, the electronic structure of PZ differs from previously investigated amines.

76

This implies that the above mentioned three determining factors will be different,

77

leading to a different overall daytime yield of nitrosamine from PZ oxidation by ·OH

78

and ·Cl. Therefore, to evaluate the potential of PZ to form nitrosamine, mechanistic

79

and kinetic information for the PZ oxidation initiated by both ·OH and ·Cl are

80

essential.

81

Recently, Onel et al. investigated the PZ + ·OH reaction.40 They found that kOH

82

value of PZ +·OH reaction is (2.38 ± 0.28) × 10-10 cm3 molecule-1 s-1 at 298 K, which

83

is about 4 times higher than that of MEA + ·OH reaction. More importantly, the yield

84

of N-center radicals was found to be 9% ± 6%, which is much lower than that of

85

MEA + ·OH reaction (36% ± 4%). Therefore, they concluded that the atmospheric

86

oxidation of PZ initiated by ·OH has a lower potential to form carcinogenic

87

nitrosamine, compared to MEA. However, to the best of our knowledge, there are no

88

previous studies concerning the reaction of PZ initiated by ·Cl although it could play

89

an important role in the formation of carcinogenic nitrosamine. In addition, no

90

previous studies have investigated the reactions of a cyclic amine or diamine with ·Cl.

91

Therefore, to extend the current knowledge of amines + ·Cl reactions and evaluate the

92

daytime potential of atmospheric PZ oxidation to the formation of carcinogenic

93

nitrosamine, the information about the oxidation of PZ initiated by ·Cl is crucial.

94

In this study, we investigated the kinetics and product branching ratios of the

95

critical steps in the ·Cl-initiated oxidation of PZ using a combination of quantum

96

chemistry calculations and kinetics modeling. For the PZ-radicals formed in the initial

97

steps, we investigated the subsequent reactions, including isomerization and 4


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98

dissociation. Furthermore, to probe the yield of carcinogenic nitrosamine, we studied

99

the bimolecular reactions of the PZ-radicals with O2 and NO which are important

100

oxidative agents in radical-initiated atmospheric chemical reactions.

101

COMPUTATIONAL DETAILS

102

Ab Initio Electronic Structure Calculations. All electronic structure calculations

103

were performed with the GAUSSIAN 09 package.41 The geometry optimizations and

104

vibrational frequency calculations of reactants, products, intermediates and transition

105

states were performed at the MP2/6-31+G(3df,2p) level of theory.42 Intrinsic reaction

106

coordinate (IRC) calculations were employed to confirm the connection of each

107

transition state between designated local minima. Zero point energy corrections were

108

obtained at the MP2/6-31+G(3df,2p) level of theory and the single point energies

109

were calculated at the CCSD(T)/aug-cc-pVTZ level of theory.42 The combination of

110

MP2/6-31+G(3df,2p) and CCSD(T)/aug-cc-pVTZ has previously been used to

111

reliably predict the reaction kinetics of mono-, di-, tri-methylamine, MEA, formamide

112

and N-methylformamide with ·Cl.9,21,31 To account for the effect of spin-orbit

113

coupling, a literature value of 0.8 kcal mol-1 was applied to the isolated ·Cl. This

114

effect is quenched in the transition state structures and in other regions of the reaction

115

pathways.21 Atomic charges in all transition states of the PZ + ·Cl reaction is based on

116

analysis of natural bond orbital (NBO)43 calculations.

117

Kinetics Calculations. We employed the MultiWell-2014.1 program suite to

118

calculate the reaction rate constants.44-46 For the multi-channel and multi-well

119

chemical reactions, the Rice-Ramsperger-Kassel-Marcus (RRKM) theory47 within

120

MultiWell master equation code was used to calculate the reaction rate constants for

121

tight

transition

states

based

on

sums

and

densities

5


of

states

for the


122

MP2/6-31+G(3df,2p) structures and the CCSD(T)/aug-cc-pVTZ barrier heights. The

123

energy-grained master equation was solved over 2000 grains of 10 cm−1 each, carried

124

on to 85000 cm−1 for the continuum component of the master equation. N2 was used

125

as the buffer gas and the collision transfer was described by the single

126

exponential-down model48 with the average transfer energy ∆Ed = 200 cm-1. The

127

Lennard-Jones parameters for intermediates were calculated from an empirical

128

method proposed by Gilbert et al.49 For the barrierless entrance pathways, two

129

different theories were used to calculate their reaction rate constants. When the rate

130

constants for the studied system or similar system is available, the Hindered Gorin

131

model was used.50 If not, the long-range transition-state theory with a dispersion force

132

potential was used.51 Both theories have previously been applied to calculate the

133

reaction rate constants of barrierless entrance pathways.9,21,31,52,53 Details for the

134

reaction rate constant calculations of the barrierless entrance pathways were presented

135

in the Supporting Information (SI). It deserves mentioning that the initial energy

136

distribution for the master equation simulations was the chemical activation

137

distribution for the combination reaction of reactants producing the pre-reactive

138

complex. In this manner, the reactants should yield non-zero branching ratio,

139

representing the reverse reaction. This approach is different from previous studies

140

where the branching ratio of reactants is considered to be zero.54,55 For the reactions

141

with only one step, the canonical transition state theory (TST) within the Thermo

142

module of MultiWell-2014.1 suite was employed to calculate the rate constants.44-46

143

Tunneling effects were taken into account in all of the reaction rate constants 6


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144

calculations for the reactions involving H-shift or H-abstraction by using a one

145

dimensional unsymmetrical Eckart barrier.56 The tunneling effects on the reaction rate

146

constants and branching ratios of important species, at 1 atm and 298 K, were

147

presented in the SI.

148

Global Minimum Search. The reactant PZ has a range of conformations. Here,

149

global minimum of PZ was selected as its starting conformation for the further study

150

of the reaction mechanism and kinetics. The employed scheme for global minimum

151

search is similar to our previous studies.52,53 Ab initio molecular dynamics (AIMD)

152

within TURBOMOLE program package57 was used to generate a range of gas-phase

153

conformations of PZ. The conformations from AIMD were selected as the starting

154

points for geometry optimization at the MP2/6-31+G(3df,2p), followed by

155

CCSD(T)/aug-cc-pVTZ single point energy calculations. Our identified global

156

minimum structure of PZ is consistent with the structure obtained by Sarma et al.39

157

and is presented in Figure 1A.

158 159

Figure 1. (A): Optimized geometries of PZ, the pre-reactive complex and transition

160

states in the PZ + ·Cl reaction at the MP2/6-31+G(3df,2p) level of theory (The

161

distance shown is in Å); (B): Diagram showing the start and end point in the 7



162

identified six H-shift transition states from IM4-1a and IM4-1b. (The blue arrows present

163

the direction of H-shift).The white balls represent H atoms; the gray ones represent C

164

atoms; the blue ones represent N atoms; green ones represent Cl atoms and red ones

165

represent O atoms.

166

RESULTS AND DISCUSSION

167

Initial Reactions with ·Cl. ·Cl can abstract H-atoms connected to a C-atom or

168

N-atom of PZ with a total of 10 H-abstraction pathways. Considering the C2h

169

symmetry of PZ, the number of pathways that need to be considered can be reduced to

170

three, i.e. abstracting a H-atom on the axial and equatorial position of –CH2– and –

171

NH–. The calculated potential energy surface for these three H-abstraction pathways

172

is presented in Figure 2, and the optimized geometries for the PZ, the pre-reactive

173

complex and transition states are shown in Figure 1A. As can be seen from Figure 2,

174

each reaction pathway proceeds through a pre-reactive and post-reactive complex.

175

Interestingly, all reaction pathways proceed via the same pre-reactive complex, which

176

is stabilized by 2-center-3-electron (2c-3e) bonds between the lone pair electrons on

177

the N-atom of PZ and the single electron occupied p-orbital of ·Cl. It can be

178

concluded from the activation energies (Ea) that the H-abstraction from the –NH– site

179

via the transition state TS1-3, leading to N-center radicals, PZ-N, is much more

180

favorable than those occurring at the –CH2– site. Therefore, the formation of PZ-N

181

radicals is the most favorable in terms of kinetics, different from ·OH initiated

182

oxidation of PZ where C-center radicals are the main products.39 In addition, the Ea

183

value of H-abstraction from the axial position of –CH2– (TS1-2) is lower than that

184

from the corresponding equatorial position (TS1-1), similar to the PZ + ·OH reaction.39

185

It deserves mentioning that the energies of the pre-reactive complex (-18.0 kcal mol-1)

186

associating with 2c-3e bonds and Ea (-14.6 kcal mol-1) in the H-abstraction pathway 8


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187

occurring at the –NH– site for the PZ + ·Cl reaction are both lower than the

188

corresponding reported values for the MEA + ·Cl reaction.31

189 190

Figure 2. Schematic potential energy surface for the PZ + ·Cl reaction calculated at

191

the CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p) level of theory. The total energy of

192

the reactants PZ + ·Cl is set to zero (reference state). The symbols “R1, RC1-1, PC1-m,

193

TS1-m and P1-m” refer to reactants, the pre-reactive complex, post-reactive complexes,

194

transition states and products involved in the reaction, respectively; m denotes

195

different species.

196

Since the N-H bond energy (92.7 kcal mol-1) of PZ is higher than C-H bond

197

energy (91.3 kcal mol-1) and all pathways proceed via the same 2c-3e bonds complex,

198

it is interesting to discuss why the Ea value for the formation of N-center radicals is

199

much lower than that for the formation of C-center radicals. By analyzing the NBO

200

charges of the transition states TS1-1, TS1-2 and TS1-3, we found that a significant

201

charge transfer (the atomic charge of ·Cl is -0.791 e) occurs at TS1-3, but not in TS1-1

202

and TS1-2 (see SI). Similar to the stabilizing role of the previously studied

203

charge-transfer complex,58,59 the electrostatic interaction induced by a significant

204

charge transfer between ·Cl and PZ should be a main reason that TS1-3 is more stable 9



205

than TS1-1 and TS1-2. To the best of our knowledge, this is the first to point out that a

206

charge transfer mechanism is the main reason for the favorable formation of N-center

207

radicals in the amines + ·Cl reactions.

208

With the master equation method, the overall rate constant (kCl) and branching

209

ratios (Γ value) for the PZ + ·Cl reaction were calculated at 1 atm and relevant

210

atmospheric temperatures 230−330 K.60,61 The kCl value was calculated to be 4.7 ×

211

10-10 cm3 molecule-1 s-1 at 1 atm and 298 K, which is close to the reported values for

212

the

213

10-10 cm3 molecule-1 s-1), trimethylamine (3.7 × 10-10 cm3 molecule-1 s-1), MEA (3.6 ×

214

10−10 cm3 molecule-1 s-1) and N-methylformamide (2.3 × 10-10 cm3 molecule-1 s-1)

215

with ·Cl.9,21,31 Over the temperature range 230−330 K, the calculated kCl values

216

present a positive temperature dependence (Figure 3A). The calculated Γ values for

217

the formation of R1, RC1-1, P1-1, P1-2 and P1-3 are 0.0974%, 0.0%, 0.0106%, 0.0840%

218

and 99.8% at 1 atm and 298 K, respectively, suggesting that N-center radicals are

219

exclusively formed. In addition, Γ values of all species change negligibly with

220

temperature in the range 230−330 K (Figure 3B). Therefore, under realistic

221

tropospheric conditions, the PZ +·Cl reaction exclusively forms N-center radicals

222

PZ-N. To the best of our knowledge, this study is the first to identify that the reaction

223

of ·Cl with a diamine or cyclic amine exclusively produce N-center radicals.

reactions of methylamine (2.9 × 10-10 cm3 molecule-1 s-1), dimethylamine (3.9 ×

10


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(B)

(A) 1.00

4.8

Γ 0.95

4.7

RC1-1

R1

P1-3

P1-2

P1-1

10

3

-1

kCl × 10 cm molecule s

-1

4.9

4.6

0.00

4.5 230 240 250 260 270 280 290 300 310 320 330 T (K)

230 240 250 260 270 280 290 300 310 320 330 T (K)

224

Figure 3. Calculated reaction kinetics for the PZ + ·Cl reaction. A: Variation of rate

225

constants (kCl) at 1 atm; B: Variation of branching ratios (Г value) for the formation of

226

the R1, RC1-1, P1-1, P1-2 and P1-3 at 1 atm.

227

Subsequent Reactions of PZ-N radicals. The chemically activated PZ-N radicals

228

can subsequently self-isomerize/dissociate or react with main atmospheric oxidants

229

such as O2 and NO. The schematic potential energy surface for the isomerization and

230

dissociation of the PZ-N radicals is shown in Figure 4A. The Ea values for all

231

considered pathways are above 35.7 kcal mol-1, yielding reaction rate constants (kuni)

232

below 3.2 × 10-13 s-1. This indicates that the isomerization and dissociation reactions

233

of PZ-N radicals proceed very slowly, and consequently the PZ-N radicals could have

234

a large potential to react with atmospheric O2 and NO.

235

For the PZ-N + O2 reaction, two pathways are considered: 1) direct H-abstraction

236

pathway in which O2 directly abstracts H-atom from the α site of –N– to form a cyclic

237

imine (cyc-N=CHCH2NHCH2CH2) and ·O2H (P3-1); 2) O2 addition pathway in which

238

O2 barrierlessly adds to the –N– site of PZ-N radicals to form five different adducts

239

(IM3-1a, IM3-1b, IM3-1c, IM3-1d, IM3-1e) depending on the direction of O2 attack. The

240

calculated schematic potential energy surface for the PZ-N + O2 reaction is shown in

241

Figure 4B. The Ea value (10.6 kcal mol-1) of the direct H-abstraction pathway is much

242

lower than those of the addition pathways (at least 23.5 kcal mol-1). Therefore, the 11



243

H-abstraction pathway leading to the formation of a cyclic imine and ·O2H is the most

244

favorable. A similar direct H-abstraction mechanism has previously been found in the

245

MEA-N + O2 reaction.31 The calculated reaction rate constant (kO2) for the

246

H-abstraction pathway is 2.4 × 10-21 cm3 molecule-1 s-1 at 298 K, which is about three

247

orders of magnitude lower than that of the MEA-N + O2 reaction.31 A similar trend in

248

reaction rate constant of PZ-N + O2 greater than that of MEA-N + O2 is further

249

identified using composite methods such as CBS-QB3, G3B3 and G4. It has

250

previously been identified that the reaction of MEA-N radicals with O2 competes with

251

its reaction with NO when the atmospheric concentration of NO is 5 ppb.31 In a

252

similar manner, the PZ-N radicals could likely react with NO.

253 254

Figure 4. Schematic potential energy surfaces for the isomerization and dissociation

255

of PZ-N radicals (A) and the PZ-N + O2 reaction (B) calculated at the

256

CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p) level of theory. The total energy of the

257

reactants PZ-N radicals and PZ-N + O2 is set to zero (reference state), respectively.

258

The symbols “Rn, TSn-m and Pn-m” refer to reactants, transition states and products

259

involved in the reactions, respectively; m denotes different species, n = 2 is the

260

isomerization and dissociation reaction and n = 3 is the PZ-N + O2 reaction.

261

The calculated schematic potential energy surface of the PZ-N + NO reaction is

262

presented in Figure 5 and the geometries of the important intermediates and transition 12


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263

states are shown in Figure S1. It is observed from Figure 5 that NO directly

264

abstracting a H-atom from the –CH2– site adjacent to the –N– at least need to

265

overcome a 0.5 kcal mol-1 energy barrier to form P4-1 (HNO + cyclic imine) and NO

266

addition to PZ-N forming PZ-N-NO adducts (IM4-1a and IM4-1b, nitrosamine) is a

267

barrierless process. Obviously, the formation of PZ-N-NO adducts is more favorable

268

in the initial attack of NO on PZ-N. The formed adducts can have two different

269

isoenergetic conformations (IM4-1a and IM4-1b) depending on the direction of NO

270

attack. Interconversion of IM4-1a and IM4-1b proceeds via TS4-3, with an energy barrier

271

of 21.2 kcal mol-1. The excess vibrational energy along the formation of nitrosamine

272

IM4-1a and IM4-1b could promote their isomerization or dissociation. Previous studies

273

have indicated that the formed nitrosamine from MEA-N + NO and CH3NH + NO

274

reactions can partly isomerize via a H-shift pathway, followed by dissociation

275

processes to form final products.31,62 From IM4-1a and IM4-1b, six H-shift pathways

276

were identified as shown in Figure 1B. Among these reaction pathways, the H shifts

277

from axial and equatorial site of –CH2– of IM4-1a, ortho position of –NNO, to O-atom

278

via TS4-4 and TS4-5 respectively, to form IM4-2a and IM4-2b are the most favorable. The

279

formed lower energy IM4-2a need to overcome very high reaction energy barriers to

280

form other species e.g. IM4-4, P4-3 and P4-2. This mechanism is different from the

281

MEA-N + NO and CH3NH + NO reactions, for which the fragment products can be

282

formed via relatively low reaction energy barriers.31,62

13



283 284

Figure 5. Schematic potential energy surface for the PZ-N + NO reaction calculated at

285

the CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p) level of theory. The total energy of

286

the reactants PZ-N + NO is set to zero (reference state). The symbols “R4, IM4-m,

287

TS4-m and P4-m” refer to reactants, intermediates, transition states and products,

288

respectively; m denotes different species.

289

To evaluate the overall reaction rate constant (kNO) and the Г values of the

290

important species involved in the PZ-N + NO reaction, master equation and TST

291

method were employed to investigate the kinetics of the addition (proceeding via

292

IM4-1) and direct H-abstraction pathway, respectively. The calculated reaction rate

293

constants for the addition and direct H-abstraction are 7.2 × 10-11 and 9.0 × 10-16 cm3

294

molecule-1 s-1, respectively. Therefore, the direct H-abstraction pathway almost has no

295

contribution to the formation of the final products and kNO is assumed to be equal to

296

the reaction rate constant of the addition pathway. The Γ value for the formation of

297

IM4-1 is 99.97% with negligible contributions from other species in the addition

298

pathways at 1 atm and 298 K. Therefore, PZ-N radicals reaction with NO 14


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299

predominantly form the nitrosamine (IM4-1). The high Γ value of IM4-1 is

300

understandable since both IM4‑1a and IM4-1b lie in a very deep potential well in the

301

addition pathway. In addition, the Γ value for the formation of IM4-1 remains almost

302

constant in the temperature region 230−330 K as shown in Figure S2. It deserves

303

mentioning that the Γ value of nitrosamine (Γnitro) formation in the PZ-N + NO

304

reaction (99.97%) is significantly higher than that of the nitrosamine formation in the

305

MEA-N + NO (86%) and CH3NH + NO (~ 60%) reactions at 1 atm and 298 K.31,62

306

With the calculated values of kuni, kO2 and kNO, we evaluated the competition

307

among the different pathways of PZ-N radicals transformation including

308

self-isomerization/dissociation, bimolecular reactions with O2/NO. To effectively

309

compare these pathways, the bimolecular rate constants of the PZ-N + O2 and PZ-N +

310

NO reactions were multiplied by the concentration of O2 ([O2], 4.92 × 1018 molecules

311

cm-3) and NO ([NO], ~ 5 ppb, 1.23 × 1011 molecules cm-3), respectively, to obtain the

312

pseudo-first order rate constants. The calculated pseudo-first order rate constant of

313

PZ-N with NO (8.9 s-1) is much higher than that of reaction with O2 (0.01 s-1) and its

314

unimolecular self-isomerization/dissociation rate constant (3.2 × 10-13 s-1) at 298 K.

315

Therefore, PZ-N radicals exclusively react with NO in the atmosphere when the

316

concentration of NO is 5 ppb or above, which is commonly encountered in polluted

317

urban atmospheres. Even when the atmospheric concentration of NO is as low as

318

0.005 ppb, the PZ-N reaction with NO (0.009 s-1) remains competitive with the

319

reaction with O2. Our previous study found that the MEA-N reaction with NO only

320

can compete with the reaction with O2 when the atmospheric concentration of NO

321

reaches about 5 ppb.31 Combined with a higher Γ value for the nitrosamine formation

322

in the PZ-N + NO reaction compared to that in the MEA-N + NO reaction at 1 atm

323

and 298 K, we can conclude that PZ-N radicals can more effectively produce 15



324

nitrosamine than MEA-N radicals. This also implies that the feasibility for the

325

formation of nitrosamine from N-center radicals highly depends on the specific

326

electronic structure of the compounds. Therefore, the yield of the N-center radicals in

327

the reaction cannot directly be used as a metric for the corresponding yield of

328

nitrosamine.

329

Uncertainty Analysis. Besides the underlying assumption of RRKM and TST, the

330

errors in the calculated reaction energy barriers could be an important factor to cause

331

the uncertainty in predicted reaction rate constants and Г values.9,63,64 Errors in

332

reaction energy barriers at the CCSD(T)/aug-cc-pVTZ level of theory can often be

333

considered to be within 1 kcal mol-1.42 Assuming ± 1 kcal mol-1 errors in the reaction

334

energy barriers, we estimated the uncertainty of the calculated reaction rate constants

335

and Г values of the important species (PZ-N radicals and nitrosamine). Table S6

336

presents the data for the uncertainty analysis and it can be seen that the ± 1 kcal mol-1

337

errors have little effects on kCl, kNO and the Г values of PZ-N radicals and nitrosamine

338

involved in the PZ + ·Cl and PZ-N + NO reactions at 1 atm and 298 K. However, the

339

potential errors lead to a change of about one order of magnitude in kuni and kO2. Since

340

the kuni value is more than ten orders of magnitude lower than the pseudo-first order

341

rate constants of PZ-N reacting with NO (0.005−5 ppb) and O2. This suggests that

342

unimolecular self-isomerization/dissociation of PZ-N can’t compete with the PZ-N

343

reactions with NO and O2 even when considering an error of ± 1 kcalmol-1 in the

344

reaction energy barrier. However, the change in kO2 will affect the competition

345

between PZ-N + O2 and PZ-N + NO reactions. This leads to a change in atmospheric

346

concentration of NO that can make the PZ-N + NO reaction compete with the PZ-N +

347

O2 reaction. The required atmospheric concentration of NO become 0.002 ppb and

348

0.03 ppb when +1 kcal mol-1 and -1 kcal mol-1 errors are considered for the PZ-N + 16


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Page 17 of 26


349

O2/NO reactions, respectively. All in all, the uncertainty caused by possible errors in

350

reaction energy barriers will not influence the conclusions drawn.

351

Implications. The calculated kCl of the PZ + ·Cl reaction is a factor of 2 higher than

352

kOH of the PZ + ·OH reaction,40 and is close to the kCl (cm3 molecule-1 s-1) of the

353

reactions of methylamine (2.9 × 10-10), dimethylamine (3.9 × 10-10), trimethylamine

354

(3.7 × 10-10), MEA (3.6 × 10-10) and N-methylformamide (2.3 × 10-10) with ·Cl.9,21,31

355

The calculated yield of N-center radicals of the PZ + ·Cl reaction is found to be

356

99.8%, which is about 10 times higher than that of the PZ + ·OH reaction.40 In the

357

marine boundary layer, [·Cl] is estimated to be around 1−10% of [·OH]. The

358

contribution of ·Cl to the transformation of PZ is found to be 2−20% (estimated by

359

kCl[·Cl]/kOH[·OH]) and the contribution to the formation of PZ-N radicals is found to

360

be 20−200% (estimated by kCl[·Cl] × ΓN,Cl/kOH[·OH] × ΓN,OH, ΓN,Cl and ΓN,OH is the

361

yield of N-center radicals initiated by ·Cl and ·OH, respectively), relative to the

362

contribution of ·OH. The same as the contribution of ·Cl to the formation of PZ-N

363

radicals, the contribution of ·Cl to the formation of nitrosamine is 20−200% of that

364

of ·OH. Based on the calculated reaction rate constants (kCl and kOH) and branching

365

ratios of N-center radicals (ΓN,Cl and ΓN,OH) for the PZ + ·Cl/·OH reactions, the overall

366

yield of N-center radicals of PZ with respect to both reactions with ·OH and ·Cl are

367

estimated {kCl[·Cl] × ΓN,Cl/(kOH[·OH] + kCl[·Cl]) + kOH[·OH] × ΓN,OH/(kOH[·OH] +

368

kCl[·Cl])} to be 0.11−0.24. This value is higher than the corresponding reaction

369

considering only OH (0.09). Therefore, if ·Cl are not considered in the atmospheric

370

transformation of PZ, the yield of N-center radicals will be significantly

371

underestimated. In addition, the daytime yield of N-center radicals from PZ oxidation

372

by ·OH and ·Cl is lower than that of the corresponding MEA reactions (0.38−0.48), in

373

agreement with the conclusion made by only considering the reaction initiated 17



374

by ·OH.

(A) 0.25

(B) 0.30

PZ ([⋅Cl]/[⋅OH]=1%) MEA ([⋅Cl]/[⋅OH]=1%)

PZ ([⋅Cl]/[⋅OH]=10%) MEA ([⋅Cl]/[⋅OH]=10%)

0.25

Γnitro, overall

0.20

Γnitro, overall

Page 18 of 26

0.15 0.10

0.20 0.15 0.10

0.05

0.05

0.00

0.00 5

10

15

50 75 100

5

5000 10000 15000 20000

10

15

50 75 100

5000 10000 15000 20000

[NO] (ppt)

[NO] (ppt)

375

Figure 6. Estimated overall yield of nitrosamine (Γnitro, overall) for the PZ and MEA

376

reactions initiated by ·OH and ·Cl in two extreme [·Cl] conditions ([·Cl]/[·OH]=1%

377

(A) and [·Cl]/[·OH]=10% (B).

378

This study reveals that the reaction of N-center radicals from PZ with NO can

379

compete with the reaction with O2 even at low NO concentration, e.g. 0.005 ppb, and

380

the N-center radicals reaction with NO exclusively forms nitrosamine. This

381

mechanism differs from the case of N-center radicals from MEA for which the

382

reaction with NO can only compete with the reaction with O2 at high NO

383

concentration, e.g. 5 ppb and the N-center radicals reaction with NO can partly form

384

fragmented products besides nitrosamine.31 This implies that the competition of the

385

N-center radicals reaction with O2 and NO will be highly affected by the specific

386

molecular structure, which makes the yield of nitrosamine from various N-center

387

radicals different. When the concentration of NO is < 5 ppb (Figure 6), the estimated

388

overall yield of nitrosamine {Γnitro, overall = ΓN,OH,Cl × Γnitro × kNO[NO]/(kNO[NO] +

389

kO2[O2]), ΓN,OH,Cl is the overall yield of N-center radicals initiated by ·OH and ·Cl} for

390

the reactions of PZ initiated by ·OH and ·Cl are higher than that of the corresponding

391

MEA reactions in both extreme [·Cl] conditions ([·Cl]/[·OH] = 1% and [·Cl]/[·OH] =

392

10%). Specially, in the NO concentration range 5−1000 ppt, the yield of nitrosamine

393

from MEA oxidation is much lower than that from PZ one. Therefore, the potential of 18


Page 19 of 26


394

PZ to form nitrosamine by ·OH and ·Cl oxidation is significantly higher than the

395

corresponding MEA oxidation in the urban and rural atmosphere, in contrast to

396

previous speculation based on the lower N-center radicals yield of PZ than that of

397

MEA. Even when considering the potential errors in the calculated reaction energy

398

barriers for the reactions, the Γnitro, overall of ·OH and ·Cl initiated PZ oxidation is still

399

higher than that of the corresponding MEA in both extreme [·Cl] conditions (see

400

Figure S3). Therefore, this study shows that the yield of N-center radicals cannot be

401

translated directly into the yield of the corresponding nitrosamine. This implies that

402

more studies should be performed on reactions of N-center radicals to probe the

403

formation of atmospheric nitrosamine.

404

ASSOCIATED CONTENT

405

Supporting Information.

406

Details for the reaction rate constant calculations; NBO charges for all the transition

407

states of the PZ + ·Cl reaction; Tunneling effects on the reaction rate constants and

408

branching ratios of important species; Uncertainty analysis for the calculated reaction

409

rate constants and branching ratios of important species; Calculated variation of

410

branching ratios with the temperature of the PZ-N + NO reaction; Optimized

411

geometries for the important intermediates and transition states involved in the PZ-N

412

+ NO reaction; Comparison of the overall yield of nitrosamine between the reactions

413

of PZ and MEA in two extreme [·Cl] conditions when ± 1 kcal mol-1 errors in the

414

predicted reaction energy barriers are considered; Cartesian coordinates of the

415

transition states for all reactions. This material is available free of charge via the

416

Internet at http://pubs.acs.org.

417

AUTHOR INFORMATION

418

Corresponding Author 19



419

*

420

ACKNOWLEDGEMENTS

421

We thank Prof. John R. Barker (University of Michigan) for providing the

422

Multiwell-2014.1 program. The study was supported by the National Natural Science

423

Foundation of China (21677028, 21325729), the Major International (Regional) Joint

424

Research Project (21661142001), the Program for Changjiang Scholars and

425

Innovative Research Team in University (IRT_13R05), and the Programme of

426

Introducing Talents of Discipline to Universities (B13012) and Supercomputing

427

Center of Dalian University of Technology.

Phone/fax: +86-411-84707844; E-mail: [email protected], [email protected]

20


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REFERENCES

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Atmospheric Oxidation of Piperazine Initiated by Â·Cl: Unexpected High

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