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A Quantum Chemical Study on •Cl-initiated Atmospheric Degradation of Monoethanolamine Hong-bin Xie, Fangfang Ma, Yuanfang Wang, Ning He, Qi Yu, and Jingwen Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03324 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 24, 2015

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Environmental Science & Technology

1

A Quantum Chemical Study on •Cl-initiated Atmospheric

2

Degradation of Monoethanolamine

3

Hong-Bin Xie†, Fangfang Ma†, Yuanfang Wang†, Ning He‡, Qi Yu† and Jingwen

4

Chen†*

5

†

6

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

7

Technology, Dalian 116024, China

8

‡

9

116024, China

10

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian

Table of Contents (TOC)

11 12

1

ACS Paragon Plus Environment


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ABSTRACT: Recent findings on the formation of •Cl in continental urban areas

14

necessitate the consideration of •Cl initiated degradation when assessing the fate of

15

volatile organic pollutants. Monoethanolamine (MEA) is considered as a potential

16

atmospheric pollutant since it is a benchmark and widely utilized solvent in a leading

17

CO2 capture technology. Especially, •Cl may have specific interactions with the

18

N-atom of MEA, which could make the MEA+•Cl reaction have different pathways

19

and products from those of the MEA+•OH reaction. Hence, •Cl initiated reactions

20

with

21

[CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p)] and kinetics modeling. Results show

22

that the overall rate constant for •Cl initiated H-abstraction of MEA is 5 times faster

23

than that initiated by •OH, and the tropospheric lifetimes of MEA will be

24

overestimated by 6%-46% when assuming that [•Cl]/[•OH] = 1%-10% if the role of

25

•Cl is ignored. The MEA+•Cl reaction exclusively produces MEA-N that finally

26

transforms into several products including mutagenic nitramine and carcinogenic

27

nitrosamine via further reactions with O2/NOx, and the contribution of •Cl to their

28

formation is about 25%-250% of that of •OH. Thus, it is necessary to consider •Cl

29

initiated tropospheric degradation of MEA for its risk assessment.

30

INTRODUCTION

MEA

were

investigated

by

a

quantum

chemical

method

31

Chlorine atoms (•Cl) are historically regarded to be produced primarily from

32

heterogeneous reaction cycles involving sea salt.1-4 A significant production of the •Cl

33

precursor ClNO2 via the reaction of NOx with chloride was observed at night or in the

34

early morning near Boulder, Colorado, an urban location in the middle of North

35

America.5 Accordingly, it was estimated that a total annual ClNO2 production rate for

36

the contiguous US lies in the range of 3.2-8.2 Tg yr-1, providing a photolytic •Cl

37

source of 1.4-3.6 Tg Cl yr-1. This US ClNO2 source is far larger than the first global 2


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38

estimate of 0.06 Tg Cl yr-1, and is similar to the recent 3.2 Tg Cl yr-1 estimated for

39

global coastal and marine regions.5 Therefore, the potential importance of •Cl on the

40

oxidation of tropospheric organic pollutants has been expanded from coastal areas to

41

continental urban areas.6 Previous studies have demonstrated the high oxidation

42

ability of •Cl towards volatile organic pollutants, with rate constants that are, with

43

some exceptions, an order of magnitude larger than those of hydroxyl radicals

44

(•OH).3,6-15 Thus, although the concentration of •Cl in the marine boundary layer is ca.

45

1-10% of [•OH],16,17 the reaction with •Cl can be a significant pathway governing the

46

fate of tropospheric organic pollutants.

47

Similar to the reaction with •OH, the reaction of organic pollutants with •Cl may

48

proceed via either addition of •Cl to unsaturated bonds or H-abstraction.9 However,

49

the favorable reaction pathway for the reaction of one specific compound with •Cl is

50

not always the same as that with •OH. For example, •OH prefers to add to the

51

unsaturated bond of polycyclic aromatic hydrocarbons,18 while •Cl prefers to abstract

52

their hydrogen atoms.19,20 Therefore, the transformation initiated by •Cl may lead to a

53

fate of tropospheric organic pollutants that differs from that by •OH.

54

Monoethanolamine (MEA) is a benchmark and widely utilized solvent in

55

amine-based post-combustion CO2 capture (PCCC), which is a promising technology

56

for reducing CO2 emission from fossil fuel power plants.21-28 Given the possible

57

large-scale implementation of amine-based PCCC, it is likely that there will be

58

relatively significant discharges of MEA or other alcoholamines to the atmosphere

59

from PCCC units because of their relatively high vapor pressure.29 It has been

60

estimated that a CO2 capture plant using MEA as solvent, which removes 1 million

61

tons CO2 per year from flue gas, could potentially emit 80 tons MEA into the

62

atmosphere.30-32

Therefore,

concern

about

the

atmospheric

3


chemistry

and


63

environmental risk of the use of amines associated with the PCCC has been

64

increasing.28,33,34

65

Previous studies have investigated the reaction kinetics and pathways of MEA with

66

•OH.35-38 These studies revealed that through •OH abstracting H on the α and the β

67

site of -NH2 in MEA, the MEA+•OH reaction favorably produces MEA-α and

68

MEA-β radicals. This finally leads to the formation of NHCHCH2OH and

69

NH2CH2CHO by their further reaction with O2.35-38 NH2CH2CHO may photolyze just

70

like other aliphatic aldehydes to form NH2CH2•, which can further react with O2/NO

71

to produce formamide or imine.37,39-42 Formamide could react with ·OH to produce

72

isocyanic acid HNCO,43,44 a potential hazardous compound.45 There are no

73

experimental data available for the gas phase reactions of NHCHCH2OH or other

74

imines.33 However, imines are water-soluble and may be taken up by aqueous aerosols

75

in the troposphere. In aqueous solution, NHCHCH2OH could hydrolyze to produce

76

ammonia and glycolaldehyde.33,46

77

Similar to the MEA+•OH reaction, the MEA+•Cl reaction may also follow the

78

H-abstraction mechanism because MEA has no unsaturated bonds. However, it is

79

unknown which H-atom of MEA can be abstracted favorably. We noted that there

80

could be a particular interaction (2-center-3-electron bond) between the nitrogen lone

81

pair electrons of the amine group and the single electron occupied p-orbital of •Cl,

82

which could make the H-abstraction at the N site become the most favorable,17

83

different from the reaction MEA+•OH. If H-abstraction at the N site of MEA, leading

84

to the formation of N-center radicals, is the most favorable pathway, then the •Cl

85

initiated MEA reaction may finally form carcinogenic nitrosamine via the reaction of

86

N-center radicals with NO.47-49 In addition, the N-center radicals may react with NO2

87

to form nitramine,33,50-53 a potential mutagenic compound.54 Therefore, the formation 4


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88

of N-center radicals will increase the environmental risk of MEA emission from

89

PCCC units. However, to the best of our knowledge, there are no previous studies

90

concerning the atmospheric chemical reaction pathways and kinetics for the

91

transformation of MEA initiated by •Cl.

92

Therefore, it became our purpose to investigate the reaction pathways and product

93

branching ratio of the critical steps in the •Cl-initiated oxidation of MEA using a

94

combination of highly cost-expensive coupled-cluster theory (CCSD(T))55 and

95

kinetics modeling. We further investigated the subsequent reactions of the

96

MEA-radicals formed in the initial steps with O2 and NOx (NO and NO2) as they are

97

important oxidative agents in radical-initiated atmospheric chemical reactions.55

98

COMPUTATIONAL DETAILS

99

Ab Initio Electronic Structure and Kinetics Calculations. All the structures and

100

energy calculations were performed using the GAUSSIAN 09 program package.56 The

101

geometry optimizations and harmonic frequency calculations for the reactants,

102

products,

103

MP2/6-31+G(3df,2p) level.55 The connections of the transition states between

104

designated local minima were confirmed by intrinsic reaction coordinate (IRC)

105

calculation at the MP2/6-31+G(3df,2p) level. Single-point energy calculation was

106

performed at the CCSD(T)/aug-cc-pVTZ level55 based on the geometries at the

107

MP2/6-31+G(3df,2p) level. In the application to atmospheric chemistry problems, the

108

CCSD(T) method is likely the most popular ab initio method in use today.55 It is

109

highly accurate and expensive for energy calculation, and errors of relative energies

110

and bond energies can often be calculated to within 1 kcal mol-1.55 A similar scheme

111

involving MP2 geometry optimization and CCSD(T) single point energy calculation

112

has been successfully used to predict the reaction kinetics of simple mono-, di- and

intermediates

and

transition

states

were

5


performed

at

the


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113

trimethylamine with •Cl.17 Zero point energy at the MP2/6-31+G(3df,2p) level was

114

used to correct the single-point energy. The most stable conformer (Figure 1) of

115

MEA57 was selected as the starting reactant.

116

We employed the MultiWell-2014.1 master equation code58-61 to calculate the

117

reaction rate constants. The master equation method is a powerful tool in calculating

118

the time-dependent, temperature-dependent and pressure-dependent kinetics of a

119

multi-channel and multi-well chemical reaction system. Reaction rate constants for

120

tight transition states were calculated from the RRKM theory,62 based on sums and

121

densities

122

CCSD(T)/aug-cc-pVTZ barrier heights. The energy-grained master equation was

123

solved over 2000 grains of 10 cm−1 each, carried on to 85000 cm−1 for the continuum

124

component of the master equation. N2 was employed as buffer gas. The collision

125

transfer probability between reactive intermediates and N2 was described by the single

126

exponential-down model63 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.64 The overall rate constants k(T) were calculated

129

using

130

of

states

for

the

MP2/6-31+G(3df,2p)

k(T) = k∞(T)(1 - Гreactants)

structures

and

the

(1)

131

where k∞(T) is the high-pressure-limit rate constant for the combination reaction and

132

Гreactants is the fractional yield of reactants.65

133

For both reaction MEA+•Cl and reaction N-center radicals (MEA-N) + NO, the

134

initial combination was found to be barrierless. The calculation for the reaction rate

135

constant of barrierless reactions is always challenging. Several methods can be

136

employed to calculate rate constants for barrierless reactions with respective strengths

137

and weaknesses.66 When the rate constant for a target system or its similar system is 6


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138

known, it is convenient and reliable to use the restricted Gorin model to simulate the

139

kinetics.66 For the reaction of MEA-N radicals with NO, the capture rate for the

140

related •NH2 + •NO association (7×10−11 cm3 molecule−1 s−1) is available.49 The

141

barrierless combination of MEA-N radicals with NO was therefore treated with the

142

restricted Gorin model.67 However, the restricted Gorin model cannot be used for the

143

reaction of MEA with •Cl since no related experimental data are available. We noted

144

that the long-range transition state theory with a dispersion force potential68 had been

145

successfully used in calculating the formation rate constant of pre-complexes for the

146

reactions of mono-, di- and trimethylamine with •Cl.17 Similarly, we also used the

147

long-range transition state theory to calculate the formation rate constant of

148

pre-reactive complexes for the reaction MEA+•Cl. Additional details for the reaction

149

rate constant calculations are presented in the Supporting Information (SI).

150 151

Figure 1. MP2/6-31+G(3df,2p)-optimized geometries for MEA and some important

152

complexes and transition states involved in the reaction of MEA+•Cl.

7



153

RESULTS AND DISCUSSION

154

Initial Reactions with •Cl. In principle, MEA should have seven H-abstraction

155

pathways, as it is constituted by seven H-atoms with different chemical environments

156

(Figure 2). The thermodynamic calculation results (Table S2) indicated that the

157

H-abstraction pathway from the -OH group is endothermic, while the other six

158

pathways are exothermic. Therefore, we excluded the contribution of the

159

H-abstraction pathway from the -OH group to the final products. A schematic

160

potential energy surface for the six remaining H-abstraction pathways is presented in

161

Figure 3, and the optimized geometries for the important species including MEA,

162

complexes and transition states are presented in Figure 1.

163 164

Figure 2. Possible pathways for the reaction of MEA with •Cl. The symbols “TS1-m

165

and P1-m” denote the transition states and products involved in the reaction and m

166

denotes the different species.

167

As shown in Figure 3, every reaction pathway proceeds through a pre-reactive and 8


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168

post-reactive complex. It can be concluded from the activation energies (Ea) that

169

H-abstractions occurring at the N site of MEA via the transition state TS1-1’ and TS1-1

170

are much more favorable than those occurring at the α site and the β site. Therefore,

171

the formation of MEA-N radicals is the most favorable in terms of kinetics. In

172

addition, the pathway via TS1-1’ should be slightly more favorable than that via TS1-1

173

due to its slightly lower reaction energy barrier (-8.3 kcal mol-1) than that (-7.2 kcal

174

mol-1) of the pathway via TS1-1. There is no regularity in reaction barriers occurring at

175

the α and the β site because both the local chemical environment and the

176

σ-electron-withdrawing group (-NH2 and -OH adjacent to the C-H bond) have a great

177

influence on the reaction barrier for the H-abstraction occurring at the α and the β

178

site.69 It deserves mentioning that the energy of the complex (-14.0 kcal mol-1) and Ea

179

(-8.3 kcal mol-1) in the most favorable pathway for the reaction MEA+•Cl are both

180

lower than those of the corresponding process for the reaction of the simplest primary

181

amine CH3NH2 with •Cl.17 This may result from the stabilizing role of the interaction

182

of -OH…•Cl in the pre-reactive complex and the transition state in the reaction

183

MEA+•Cl. As no well-defined transition state was located for the H-abstraction

184

occurring at the β-site (3’), an approximated transition state (TS1-3’) was shown

185

instead in Figure 3. A detailed discussion about the approximation of TS1-3’ is

186

presented in the SI.

187

We observed four pre-reactive complexes involved in the reaction MEA+•Cl

188

(Figure 1). The two complexes RC1-1 and RC1-1’ associated with the interaction

189

between the N-atom and •Cl are the most stable. This should result from the formation

190

of a 2-center-3-electron bond between the nitrogen lone pair electrons and the single

191

electron occupied p-orbital of •Cl. Similar complexes were also observed in the

192

reactions of methylamine, dimethylamine and trimethylamine with •Cl.17 The 9



193

formation of such stable pre-reactive complexes (RC1-1 and RC1-1’) is a reason why

194

the Ea values for the H-abstractions occurring at the N-site are much lower than those

195

at the other sites. A detailed discussion on how such complexes lead to the

196

H-abstraction occurring at the N site is presented in the SI. RC1-1 is 0.3 kcal mol-1

197

lower than RC1-1’ in energy. Structurally, the N-atom in RC1-1 rotates a little bit

198

compared with MEA and RC1-1’(Figure 1). Thus, when MEA rigidly collides with •Cl,

199

RC1-1’ forms first.

200 201

Figure 3. Schematic potential energy surface for the reaction MEA +•Cl calculated at

202

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

203

reactant MEA+•Cl is set as zero (reference state). The symbols “R1, RC1-m, PC1-m,

204

TS1-m and P1-m” stand for reactants, pre-reactive complexes, post-reactive complexes,

205

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

206

different species. ∆E was calculated at 0 K. The green, blue and red pathways 10


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207

correspond to H-abstraction occurring at the N, α and β sites of MEA, respectively.

208

With the master equation method, the overall rate constant (kCl) was calculated to

209

be 3.6×10-10 cm3 molecule-1 s-1 for the reaction MEA+•Cl, which is close to the values

210

for the reactions of methylamine (2.9×10-10), dimethylamine (3.9×10-10) and

211

trimethylamine (3.7×10-10 cm3 molecule-1 s-1) with •Cl.17 It deserves mentioning that

212

although the canonical variational transition state theory within the POLYRATE

213

2010-A program70 usually works well for the barrierless reaction with a pre-reactive

214

complex, a reasonable value could not be calculated for the reaction rate constant of

215

MEA with •Cl, as detailed in the SI. Over the temperature range 200-376 K, the

216

calculated kCl values decrease with increasing temperature (Figure 4). As the

217

temperature range is narrow, there seems an apparent linear relationship between kCl

218

and temperature. For a wide temperature range, the relationship become nonlinear as

219

shown in Figure S3.

A

B

220

The calculated branching ratio Г values for the formation of MEA+•Cl,

221

RC1-1’(RC1-1), MEA-N+HCl, MEA-β+HCl and MEA-α+HCl are 23.37%, 0, 72.38%,

222

0.87% and 3.38% at 298 K and 1 atm, respectively, suggesting a strong preference for

223

the N site H-abstraction. The Г values of the parent MEA+•Cl increase with

224

temperature and the Г values of the main product MEA-N+HCl decrease with

225

temperature. However, the Г values for the other species are still small even though

226

they increase with temperature. This variation of the Г values with temperature could

227

result from the fact that the difference in the transformation rate constants of CP1-1’

228

towards PC1-1(finally to MEA-N+HCl) and other species decreases with increasing

229

temperature, e.g. the difference (k1/k2=2.0×10-2) in rate constants between k1 (for

230

transformation from CP1-1’ to MEA+•Cl) and k2 (from CP1-1’ to PC1-1) at 376 K is

231

smaller than that (k1/k2=2.0×10-6) at 200 K. In addition, the branching ratio of 11



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RC1-1’(RC1-1) is zero although it seems that RC1-1’(RC1-1) lies in a relatively deep well.

233

This can be due to that the excess internal energy (~14 kcal mol-1) along the formation

234

of RC1-1’(RC1-1) is high enough to make RC1-1’(RC1-1) overcome the 6-7 kcal mol-1

235

energy barrier to form other species, but cannot make it deactivated. The similar case

236

was also found for the reactions of CH3NH2, (CH3)2NH, and (CH3)3N with •Cl.17 10 3 -1 -1 kCl×10 (cm molecule s )

232

A

4.0

1.0

MEA-α+HCl MEA+Cl

0.8

3.8

RC1-1' MEA-N+HCl MEA-β+HCl

B

Г 0.6

3.6

0.4

3.4

0.2

3.2

0.0

200

250

300 T(K)

350

200 225 250 275 300 325 350 375 400 T (K)

400

237

Figure 4. Calculated reaction kinetics for the reaction MEA+•Cl. A: Variation of rate

238

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

239

parent MEA+•Cl, RC1-1’, MEA-N+HCl, MEA-β+HCl and MEA-α+HCl at 1 atm.

240

These results confirm our hypothesis that the reaction of •Cl with MEA leads to the

241

formation of N-center radicals (MEA-N) as the primary activated products, in contrast

242

to the reaction of •OH with MEA that forms MEA-β and MEA-α radicals.37 To the

243

best of our knowledge, this is the first report that the reaction of •Cl with MEA, a

244

benchmark and widely utilized solvent in amine-based PCCC, can produce MEA-N

245

radicals. We further investigated the atmospheric transformation of MEA-N radicals

246

as the atmospheric transformation of the minor products MEA-β and MEA-α radicals

247

has previously been investigated.37,46

248

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

249

radicals can subsequently self-isomerize, self-dissociate or react with the main

250

atmospheric oxidants (O2 and NOx). Schematic potential energy surfaces of

251

isomerization and dissociation of the MEA-N radicals are depicted in Figure 5A, and 12


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252

the corresponding reaction enthalpy (∆H) values are listed in Table S2. The Ea values

253

for all the pathways are > 24 kcal mol-1, indicating that the isomerization and

254

dissociation reactions proceed slowly, and consequently the MEA-N radicals have a

255

great chance to react with atmospheric O2 and NOx.

256

The calculated schematic potential energy surface for the MEA-N+O2 reaction is

257

shown in Figure 5B with the corresponding ∆H values listed in Table S2. It can be

258

observed from Figure 5B that there are two kinds of pathways for the MEA-N+O2

259

reaction: one is the H-abstraction pathway in which ground state O2 (at triplet 3O2)

260

abstracts H-atom at the α site of –NH2 to form imine and O2H (P3-1); the other is the

261

addition pathway in which O2 adds to the N-site of MEA-N radicals. The results show

262

that the overall reaction energy barrier (6.4 kcal mol-1) of H-abstraction pathway is

263

much lower than those of the addition pathways (at least 18.4 kcal mol-1). Therefore,

264

the H-abstraction pathway leading to the formation of imine and O2H is more

265

favorable. To the best of our knowledge, this is the first time to reveal that the

266

reaction of N-centered radicals with O2 follows the H-abstraction reaction mechanism.

267

With the H-abstraction pathway, the reaction rate constant kO2 was calculated to be

268

2.9×10-18 cm3 molecule-1 s-1 and the lifetime of MEA-N is 0.07 s. The slow reaction

269

MEA-N+O2 could make the MEA-N radicals have a chance to react with NOx. A

B

13



B

A

270 271

Figure 5. Schematic potential energy surfaces for the isomerization and dissociation

272

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

273

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

274

MEA-N and MEA-N+O2 is set as zero (reference state). The symbols “Rn, TSn-m and

275

Pn-m” stand for reactants, transition states and products involved in the reaction,

276

respectively; m denotes different species, n = 2 is for the isomerization and

277

dissociation reaction and n = 3 is for the reaction MEA-N+O2. ∆E was calculated at 0

278

K.

279

The calculated schematic potential energy surface for the reaction MEA-N+NO is

280

shown in Figure 6, the optimized geometries for the corresponding intermediates and

281

transition states are shown in Figure S4, and the corresponding ∆H values are listed in

282

Table S2. It can be observed from Figure 6 that the MEA-N+NO reaction can

283

barrierlessly proceed to form MEA-N-NO adducts (IM4-1, nitrosamine) with six

284

conformations, depending on the attacking direction of NO. The six conformations of

285

IM4-1 can interconvert into each other (Figure S5).

286

The excess vibrational energy along the formation of nitrosamine IM4-1 could 14


Page 14 of 28

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287

promote IM4-1 to isomerize or dissociate to other species. A recent study also indicates

288

that hot nitrosamine CH3NHNO formed from CH3NH+NO can partly isomerize via a

289

H-shift to CH3NNOH or CH2NHNOH.49 From IM4-1, eight H-shift processes (e.g. 1,3

290

H-shift from the N-atom to the O-atom of -NO, 1,3 H-shift from the Cα-atom to the

291

N-atom of -NO, and 1,4 H-shift from the Cα-atom of -NH2 to the O-atom of –NO)

292

were identified. Among these processes, the 1,3 H-shifts from the N-atom to the

293

O-atom of -NO for three IM4-1 conformations, via respectively TS4-1, TS4-2 and TS4-3,

294

to form three different conformations of IM4-2 are the most favorable. The three

295

formed conformations of IM4-2 are in the trans-form in view of the O-site of -NO and

296

the Cα-site and can interconvert into each other as shown in Figure S6. The trans-form

297

IM4-2g can convert to the cis-form IM4-2h via TS4-9. The cis-form HOCH2CH2NNOH

298

(IM4-2h) can dehydrate via TS4-10 to form diazoethanol (OHCH2CHNN) and water

299

(product set P4-1). This dehydration transition state (TS4-10) is comparable in energy to

300

TS4-9 and TS4-1. Importantly, the dissociation of the chemically activated IM4-1 to

301

OHCH2CHNN+H2O proceeds via the pathways for which the transition states are at

302

least 13 kcal mol-1 below the reactants. Thus, the reaction pathway via the

303

intermediate IM4-1, finally leading to the formation of OHCH2CHNN+H2O, is the

304

most favorable for the reaction MEA-N+NO. It deserves mentioning that the relative

305

energies of IM4-1 and all the transition states in the most favorable channel for the

306

reaction of MEA-N+NO are in general 1.0 kcal mol-1 higher than those of the

307

corresponding species for the reaction CH3NH+NO.49

15



20.0

Page 16 of 28

∆Ε (kcal mol-1) TS 4-8 14.6

10.0

TS4-7 6.9 MEA-N+NO

0.0

IM4-3 7.8

TS4-5 -2.8 TS4-6 -1.4

R4 0.0 TS4-4 -3.1

IM4-2b -13.3 IM4-2e -12.4

-10.0

TS4-9 -13.6

TS4-3 -15.7 TS4-2 -16.2

-20.0

TS4-10 -14.2

IM4-2d -15.7 TS4-1-16.4 OHCH CHNH+HNO 2 P4-2 -19.9

-30.0 TS4-3a -39.4 -40.0 IM4-2f-47.2 IM4-2a -47.2 -50.0

-60.0

IM4-1a -44.5 IM4-1b -45.6 IM4-1c -44.5 IM4-1d -45.8 IM4-1e -45.6 IM4-1f -44.5

IM4-2c -47.3

IM4-2g-45.8

IM4-2h -47.2

P4-1 -54.9 NNCHCH2OH+H2O

308 309

Figure 6. Schematic potential energy surface for the MEA-N+NO reaction calculated

310

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

311

reactant MEA-N+NO is set as zero (reference state). The symbols “R4, IM4-m, TS4-m

312

and P4-m” stand for reactants, intermediates, transition states and products,

313

respectively; m denotes different species. ∆E was calculated at 0 K.

314

To evaluate the branching ratio of important species involved, the formation rate

315

constant of IM4-1 and the overall rate constants (kNO) for the reaction MEA-N+NO, we

316

performed a master equation simulation on the favorable pathway, that is, the pathway

317

via the intermediate IM4-1 to finally form P4-1. The calculated kNO at 298 K and 1 atm

318

is 6.99×10-11 cm3 molecule-1 s-1 and the phenomenological reaction rate constant for

319

the formation of IM4-1 is 6.06 ×10-11 cm3 molecule-1 s-1. As can be seen from Figure 7,

320

in the temperature range of 200-376 K and 1 atm, the calculated kNO values increase

321

with temperature. Therefore, in some cases where temperature and concentration of 16


Page 17 of 28


322

NO are decreased, e.g. from lower-troposphere to mid-troposphere,71 the reaction rate

323

of MEA-N with NO is decreased if we suppose that the concentration of MEA-N

324

remains constant. In the studied temperature range, the reactants predominantly form IM4-1f

326

(nitrosamine) with Г > 70% and the Г values for the other species (IM4-2f, IM4-2g,

327

IM4-2h and P4-1) are small (Figure 7). The product branching ratio of nitrosamine in the

328

reaction MEA-N+NO is about 10% higher than that in the reaction CH3NH+NO at

329

298 K and 1 atm.49 Therefore, under the tropospheric conditions, the reaction of •Cl

330

with MEA can finally lead to the formation of nitrosamine that has been proven to

331

have strong carcinogenic activities.72-74 The formed nitrosamine may also photolyze

332

in daytime.38,50 A recent modelling study predicted that the concentration of

333

nitrosamine is 0.3 pg m-3 in •OH initiated MEA-oxidation based on the assumption

334

that the Г value of MEA-N is 15% in the reaction MEA+•OH, nitrosamine can be

335

formed from the reaction MEA-N+NO and nitrosamine can photolyze in daytime.50

336

Thus, due to the high Г value (72%) of MEA-N in the reaction MEA+•Cl, the

337

concentration of nitrosamine could be higher than that estimated without the

338

consideration of •Cl. 10

3

7 6

IM4-1f IM4-2g

1.0 0.8

8 Г

IM4-2f IM4-2h P4-1

B

0.6 0.4 0.2

5

0.0

4 200

339

1.2

A

9

11

-1 -1

kNO×10 (cm molecule s )

325

250

300 T(K)

350

200 225 250 275 300 325 350 375 400 T (K)

400

Figure 7. Calculated reaction kinetics for the reaction MEA-N+NO. A: Variation of 17



340

reaction rate constants (kNO) at 1 atm; B: Variation of branching ratios (Г) for the

341

formation of IM4-1f, IM4-2f, IM4-2g, IM4-2h and P4-1 at 1 atm.

342

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

343

between MEA-N+O2 and MEA-N+NO. The kO2/kNO ratio is 4.1×10-6, which is close

344

to the corresponding experimental value (1.5×10-6) for the (CH3)2N radicals.52 If we

345

assume that the atmospheric concentration of NO is 5 ppb, which is commonly

346

encountered in urban atmospheres and could be achieved in polluted air masses where

347

MEA emissions are expected to be significant (e.g., coal-fired power stations),46,52

348

then the lifetime of MEA-N with respect to the NO reaction is calculated to be 0.11 s.

349

The lifetime of MEA-N with respect to the NO reaction is comparable to that (0.07 s)

350

with respect to reaction with O2. Therefore, the reaction of MEA with NO is an

351

important removal pathway for MEA-N radicals at NO levels of around 5 ppb or

352

above.

353

The reaction with NO2 can also be a removal pathway for the N-center radicals to

354

form nitramine and imine.33,50-53 We further investigated the potential energy surface

355

(Figure 8) of MEA-N with NO2. The initial combination of MEA-N and NO2

356

resulting in two different adducts (nitramine IM5-1 and N-nitrosooxy amine IM5-2) is

357

barrierless, which means that the reaction rate constant of MEA-N with NO2 should

358

be high, just like the MEA-N+NO reaction. IM5-1 and IM5-2 have three and eight

359

conformations, respectively, depending on the attacking direction of NO2 on MEA-N.

360

The IM5-2 can undergo MEA-NO-NO bond rupture to produce NO and nitroxide

361

radicals without any transition state. However, IM5-1 needs to overcome a high

18


Page 18 of 28

Page 19 of 28


362

reaction barrier to form imine and HONO(P5-1). By comparing the potential energy

363

surfaces, it was found that nitramine IM5-1 in the reaction MEA-N+NO2 is trapped in

364

a deeper well than nitroamine in the reaction MEA-N+NO. Therefore, nitramine IM5-1,

365

a potentially mutagenic compound,54 can be formed by collisional deactivation, just

366

like the formation of nitroamine in the reaction of MEA-N with NO. We can conclude

367

that the reaction of MEA-N with NO2 will mainy lead to the formation of nitramine

368

and nitroxide radicals with high reaction rate constants like in case of other N-center

369

radicals.52-53

370 371

Figure 8. Schematic potential energy surface for the MEA-N+NO2 reaction calculated

372

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

373

reactants MEA-N+NO2 is set as zero (reference state). The symbols “R5, IM5-m, TS5-m

374

and P5-m” stand for reactants, intermediates, transition states and products involved in

375

the reaction, respectively; m denotes different species. ∆E was calculated at 0 K.

376

Implications. The calculated kCl of the MEA+•Cl reaction is higher than that (kOH) of

377

the MEA+•OH reaction36,37 by a factor of 5 (at 298K) and is close to the rate

378

constants for the reactions of methylamine (2.9×10-10), dimethylamine (3.9×10-10) and 19



379

trimethylamine (3.7×10-10 cm3 molecule-1 s-1) with •Cl. As in the marine boundary

380

layer, •Cl concentrations [•Cl] are estimated to be as much as 1-10% of the [•OH],16,17

381

the contribution of •Cl to the transformation of MEA is about 5-50% (estimated by

382

kCl[•Cl]/kOH[•OH]) of the contribution of •OH. Based on the atmospheric

383

concentrations of •OH (9.7 × 105 molecules cm-3)75 and •Cl (9.7 × 103 - 9.7 × 104

384

molecules cm-3),16,17 the tropospheric lifetimes of MEA with respect to the reaction

385

with •OH (τOH) and a total tropospheric lifetimes with respect to both reaction with

386

•OH and •Cl (τOH,Cl) were calculated to be τOH = 3.8 hours and τOH,Cl = 2.6-3.6 hours.

387

Thus, the tropospheric lifetimes of MEA will be overestimated by 6-46 % if the role

388

of •Cl is ignored, which further proves the importance of •Cl in the transformation of

389

MEA, especially when considering the huge area of oceans (71% of the earth’s

390

surface) as well as the potential source of •Cl from continental urban areas.5

391

This study for the first time reveals that the N-center radicals MEA-N are produced

392

most favorably from the MEA+•Cl reaction. The yield of the MEA-N radicals in the

393

reaction of MEA with •Cl is about 5 times higher than that of the reaction

394

MEA+•OH.36 The subsequent atmospheric reaction of the MEA-N radicals with O2

395

and NOx finally leads to the formation of imine, nitroxide radicals, nitramine and

396

nitrosamine, and the contribution of •Cl to their formation is about 25-250%

397

(estimated by kCl[•Cl]ГCl,MEA-N/kOH[•OH]ГOH,MEA-N, where ГCl,MEA-N and ГOH,MEA-N are

398

the branching ratios of MEA-N in the reaction MEA+•Cl and MEA+•OH,

399

respectively) of that of •OH in marine environments. Thus, if •Cl is not considered

400

when assessing the risk of MEA, the risk will be underestimated significantly.

401

This study further proves that the reaction pathways and products for some organic

402

pollutants with •Cl can be different from those with •OH, implying that more studies

403

should be performed on •Cl initiated tropospheric degradation of volatile organic 20


Page 20 of 28

Page 21 of 28


404

pollutants for their fate assessment, although it actually has already been taken into

405

consideration for decades.2

406 407

ASSOCIATED CONTENT

408

Supporting Information. Texts, figures, and tables giving detailed discussion about

409

an approximated transition state for the H-abstraction occurring at the β site, master

410

equation calculations and long-range transition state theory treatment, reaction

411

process of the H-abstraction occurring at the N site of MEA, variational transition

412

state theory calculation for the reaction MEA+•Cl, geometries involved in the reaction

413

MEA-N+NO, potential energy surfaces for the interconversion of different

414

conformers, and energetic values for the reactions. This material is available free of

415

charge via the Internet at http://pubs.acs.org.

416 417

AUTHOR INFORMATION

418

Corresponding Author

419

*

420

ACKNOWLEDGEMENTS

421

We thank Prof. Willie Peijnenburg (Leiden University) for improving the contents of

422

the manuscript and Prof. John R. Barker (University of Michigan) for providing the

423

MultiWell-2014.1 program and instructions on the calculation. The study was

424

supported by the National Natural Science Foundation of China (21207016,

425

21325729), and Program for Changjiang Scholars and Innovative Research Team in

426

University (IRT_13R05).

Phone/fax: +86-411-84706269; e-mail: [email protected].

427

21



428

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