Gas-Phase Mechanisms of the Reactions of Reduced Organic

Oct 3, 2016 - Gas-Phase Mechanisms of the Reactions of Reduced Organic Nitrogen Compounds with OH Radicals. Nadine Borduas†, Jonathan P. D. ...
0 downloads 0 Views 1MB Size
Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Gas-Phase Mechanisms of the Reactions of Reduced Organic Nitrogen Compounds with OH Radicals Nadine Borduas, Jonathan P.D. Abbatt, Jennifer Grace Murphy, Sui So, and Gabriel da Silva Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03797 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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

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

Page 1 of 37

Environmental Science & Technology

1

Gas-Phase Mechanisms of the Reactions of Reduced

2

Organic Nitrogen Compounds with OH Radicals

3

Nadine Borduas1*, Jonathan P. D. Abbatt1, Jennifer G. Murphy1, Sui So2, Gabriel da Silva2

4

1

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6

5

2

Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia

6

ABSTRACT: Research on the fate of reduced organic nitrogen compounds in the atmosphere

7

has gained momentum since the identification of their crucial role in particle nucleation and the

8

scale up of carbon capture and storage technology which employs amine-based solvents.

9

Reduced organic nitrogen compounds have strikingly different lifetimes against OH radicals,

10

from hours for amines to days for amides to years for isocyanates, highlighting unique functional

11

group reactivity. In this work, we use ab initio methods to investigate the gas-phase mechanisms

12

governing the reactions of amines, amides, isocyanates and carbamates with OH radicals. We

13

determine that N−H abstraction is only a viable mechanistic pathway for amines and we identify

14

a reactive pathway in amides, the formyl C−H abstraction, not currently considered in structure-

15

activity relationship (SAR) models. We then use our acquired mechanistic knowledge and

16

tabulated literature experimental rate coefficients to calculate SAR factors for reduced organic

17

nitrogen compounds. These proposed SAR factors are an improvement over existing SAR

18

models because they predict the experimental rate coefficients of amines, amides, isocyanates,

19

isothiocyanates, carbamates and thiocarbamates with OH radicals within a factor of two, but

ACS Paragon Plus Environment

1

Environmental Science & Technology

20

more importantly because they are based on a sound fundamental mechanistic understanding of

21

their reactivity.

22

TOC GRAPHIC

Page 2 of 37

23 24

INTRODUCTION

25

Reduced organic nitrogen compounds, characterized by a C−N bond, are important for the study

26

of air quality and climate.1-3 Prominent examples of reduced organic nitrogen compounds in the

27

atmosphere include amines, amides, isocyanates, carbamates, cyanates and their sulfated

28

homologues, thiocarbamates and isothiocyanates. The C−N bond, where the N atom is in its -3

29

oxidation state, cannot typically be formed in the atmosphere. Thus, reduced organic nitrogen

30

compounds are emitted as such, and their fate in the atmosphere is govern by oxidation reactions,

31

particle formation and deposition.

32

These molecules have a wide range of biogenic and anthropogenic sources to the atmosphere and

33

have been identified and quantified in ambient air.4 Mixing ratios of reduced organic nitrogen

34

compounds vary from below pptv up to ppmv levels depending on the molecule, location, time

35

of day and meteorology. Amines are the most commonly measured organic nitrogen molecules

36

in the atmosphere. For example, gas phase trimethylamine concentrations were reported up to 40

37

pptv in rural areas and up to 6 pptv up in urban areas, suggesting an agricultural source.5-7 The

ACS Paragon Plus Environment

2

Page 3 of 37

Environmental Science & Technology

38

significant role of amines in aerosol formation and growth, as well as their recent large scale use

39

as solvents in carbon capture and storage (CCS) technologies, has validated focused attention on

40

understanding their fate in the environment.1,8-11 Indeed, at a CCS plant in Norway, peaks of up

41

to 10 ppbv of monoethanolamine (MEA), 300 ppbv of pyrazine and 800 ppbv of nitromethane

42

were reported at the top of a stripper column.12 It is known that the current benchmark solvent

43

for CCS, MEA, has a short atmospheric lifetime of approximately 2 h, governed by its reactivity

44

towards the OH radical.13-15 Other sources of reduced organic nitrogen molecules to the

45

atmosphere include direct emissions from industrial solvents, biomass burning, cigarettes, and

46

animal husbandry as well as oxidative chemistry of amines.1,4,8,16,17 Indeed, the gas-phase

47

oxidation of amines produces amides as well as isocyanates.13,15,18,19

48

The major atmospheric sinks for reduced organic nitrogen compounds are recognized as being

49

oxidation by OH radicals, and to a lesser extent by NO3 radicals and ozone. Because most

50

amines, amides and isocyanates do not absorb photons of wavelengths in the actinic window,

51

photolysis is not generally competitive.20,21 Loss to aerosol particles is another important sink for

52

some species. For example, amines may act as bases and help nucleate particles and/or contribute

53

to particle growth, which can impact climate directly by scattering light and indirectly by acting

54

as cloud condensation nuclei.20 Reduced organic nitrogen compounds are also thought to be

55

responsible for some of the colouring in brown carbon aerosols, again impacting climate through

56

their light-absorbing properties.22 Some reduced organic nitrogen compounds are also toxic. In

57

particular, methyl isocyanate and isocyanic acid may pose serious health effects if inhaled in

58

mixing ratios above 1 ppbv.23-25 In addition, nitrosamines and nitramines, oxidation products of

59

aminyl radicals reacting with NOx in urban areas for example, are carcinogens.21 Generally,

60

reduced organic nitrogen compounds are not important radiative forcing agents due to their small

ACS Paragon Plus Environment

3

Environmental Science & Technology

Page 4 of 37

61

gas-phase concentrations. However, the highest global warming potential of any compound

62

detected in the atmosphere is currently perfluorotributylamine.26

63

When organic nitrogen is quantified in both the gas and particle phases, the majority is typically

64

found in the particle phase and often in the water soluble fraction of particles.1,10 For example,

65

the concentration of aliphatic amines contributing to dissolved organic nitrogen in rainwater was

66

estimated to be < 1-14 nmol N m -3.3 Wet and dry deposition of organic nitrogen compounds are

67

estimated to be ~ 25% of the atmospheric global nitrogen deposition flux.2

68

A number of laboratory experiments, theoretical calculations and field studies have aimed to

69

better understand the fate of reduced organic nitrogen compounds and were recently reviewed in

70

the context of their use and/or production in CCS plants.4,8,20,21 Their chemical mechanisms have

71

been largely developed by experimental product studies, and more recently are being supported

72

by computational chemistry studies on amines,27-32 as well as recent studies on amide oxidation

73

mechanisms.17,33,34 In this study, we evaluate through computational chemistry, the mechanisms

74

involved in the gas-phase oxidation of reduced organic nitrogen compounds with OH radicals to

75

better understand their overall atmospheric fate. We then compare their mechanisms to highlight

76

the impact adjacent functionalities have on the nitrogen atom. Particular attention is given to

77

amines, amides and isocyanates, common functionalities found in the gas and particle phases.

78

Their lifetimes are also strikingly different, ranging from hours to years depending on the

79

functional group.8,21 We opted to study the simplest molecule of each class, i.e. methylamine,

80

formamide and isocyanic acid for relevance and calculation simplicity. We find that when

81

exposed to OH radicals, oxidation generally occurs on the adjacent C atom of N-containing

82

functionalities. In other words, amines are oxidized to amides and amides are oxidized to

83

isocyanates. We also evaluate a model carbamate molecule, N-methyl methylcarbamate since

ACS Paragon Plus Environment

4

Page 5 of 37

Environmental Science & Technology

84

structure-activity relationship (SAR) analyses for many reduced organic nitrogen compounds

85

originate from the evaluation of carbamate rate coefficients with OH radicals.35 Our mechanistic

86

approach is comprehensive as we consider all possible reaction sites on each functionality

87

including C−H and N−H abstractions, OH additions to carbonyl and OH additions to N atoms,

88

and subsequently identify probable reaction pathways. We then use this insight alongside a

89

compiled database of experimental rate coefficients to build SAR factors that rely on a

90

fundamental understanding of the reactivity of N-containing molecules.

91

COMPUTATIONAL METHODS

92

Computational ab initio methods were employed using the Gaussian 09 code.36 Structures of the

93

reactants were first optimized using the M06-2X density functional, with the 6-31G(2df,p) basis

94

set, and subsequently evaluated using the G3X-K composite theoretical method which combines

95

a series of Hartree-Fock, Møller-Plesset perturbation (MP4/6-31G(2df,p) and MP4/6-31+G(d))

96

and coupled cluster theory calculations (CCSD/6-31G(d)).37,38 A sample input file for the

97

execution of the G3X-K method in Gaussion 09 as well as optimized geometries are presented in

98

the Supplementary Information. G3X-K theory is used in this study because it was specifically

99

designed for thermochemical kinetics and reproduces barrier heights in the DBH24/08 database

100

with an average accuracy of 0.6 kcal mol-1.38 In addition, we did not use diffuse functions (+) in

101

the basis set throughout our calculations, simply to optimize computation time as the energies are

102

similar with and without their incorporation (see Table S1). The energies reported are for 0 K

103

and were calculated from the sum of the electronic and the zero point energies. The accuracy of

104

the energies stated is expected to be within 1 kcal mol-1.38 Bond dissociation energies (BDE) are

105

the difference between ground state energies of the products and the reactants of the bond

106

dissociated reaction.

ACS Paragon Plus Environment

5

Environmental Science & Technology

Page 6 of 37

107

Throughout this work, we describe transition state energy, used interchangeably with barrier

108

height energy, as the energy difference between the transition state and the sum of the reactants’

109

energies. Although many systems exhibit weak pre-complex formation between the reduced

110

organic nitrogen compound and the OH radical, these pre-complexes do not necessarily lie along

111

the reaction coordinate and/or may not be collisionally stabilized. Thus, we assume that their

112

energies do not dominate the overall flux through the transition state.39 While we approach the

113

mechanistic analysis by assuming that the reactivity will largely scale with the height of the

114

transition state, we recognize that entropic factors and tunneling may also play a role,

115

particularly for cases with transition states that are submerged relative to the reactant energy. In

116

addition, we estimate that a 1 kcal mol-1 difference in activation energy translates to

117

approximately a factor of 5 difference in rate coefficients using the Arrhenius equation for room

118

temperature rate coefficients that are on the order of 10-12 cm3 molec-1 s-1. Nonetheless, we

119

emphasize here that the theoretical calculations serve solely to identify reactive pathways and not

120

to calculate rate coefficients.

121

CALCULATION RESULTS

122

1. Bond dissociation energy (BDE)

123

From the energy diagrams of methylamine, formamide, N,N-dimethylformamide, N,N-

124

dimethylacetamide, N,N-dimethylpropanamide, N-methylpropanamide, isocyanic acid and N-

125

methyl methylcarbamate with OH radicals presented in this section (Schemes 1-4 and S1-S4), we

126

calculate bond dissociation energies (BDE) of C−H and N−H bonds in reduced organic nitrogen

127

compounds (Table 1). In general, our values compare well with previously reported

128

literature.40,41 We are aware of only one experimentally determined C−H bond BDE for the

ACS Paragon Plus Environment

6

Page 7 of 37

Environmental Science & Technology

129

reduced organic nitrogen compounds we investigated (methylamine). However, computed BDEs

130

exist and all compare well with our calculated BDEs for C−H bonds.40,42-44 For carbamates for

131

example, the methylamine C−H bond and the methoxy C−H bond were computed at 96 kcal mol-

132

1

133

the experimental BDE value for the N−H bond in formamide is 100.8 kcal mol-1, whereas we

134

calculate a value of 114.6 kcal mol-1.41 Yet, three previous ab initio studies have calculated

135

similar BDEs at 114.5 kcal mol-1, 113.6 kcal mol-1 and 113.2 kcal mol-1.42,43,45 Moreover, the

136

BDE of the N−H bond in amines (like methylamine) is expected to be lower than in amides (like

137

formamide) due to the delocalisation of the nitrogen’s lone pair in the amide functionality as

138

predicted by molecular orbital theory.

139

Table 1: Calculated and experimental bond dissociation energies for reduced organic nitrogen

140

compounds

and 100 kcal mol-1 respectively by Berry et al. and are comparable to our values.44 In addition,

Organic nitrogen

Formyl C−H bond (kcal mol-1)

alkyl C−H bond (kcal mol-1)

N−H bond (kcal mol-1)

Calculated

Calculated

Experimental

Calculated

Experimental

methylamine

NA

91.9

92.746,47

98.8

10042,48,49

formamide

93.3a

114.6

100.841

N,N-dimethylformamide

93.7

N,N-dimethylacetamide

NA

N,N-dimethylpropanamide

NA

N-methylpropanamide

NA

NA 92.5a

?

90.6a 97.9b 90.6a 93.6b 93.3a 95.1b

? ? ? ? ? ?

NA NA NA 105.5

~98.741

ACS Paragon Plus Environment

7

Environmental Science & Technology

Isocyanic acid

141 142 143 144 145

NA

NA

Page 8 of 37

109.8

109.750

NA 93.2a ? N-methyl 106.8 ~10551 c methylcarbamate NA 98.5 ? a b c C−H bond on the N-alkyl side; C−H bond on the amide side C−H bond on the methoxy side; ref 49 is for N-methyl ethylcarbamate51 NA = not applicable; ? = yet to be measured/reported

2. Methylamine + OH

146

Scheme 1 shows the three possible mechanisms for the OH radical to react with methylamine,

147

the simplest of amines, in the gas phase. Both the C−H and the N−H abstraction mechanisms are

148

competitive for this alkylamine, with barriers to reaction close to the entrance level energies of

149

the reactants and within 1 kcal mol-1 of each other. Both reactions are exothermic, as expected

150

due to the formation of water as a by-product. The third mechanism investigated is OH addition

151

to the N atom with a concerted C−N bond cleavage, which is highly endothermic with a clearly

152

inaccessible transition state energy.

153

Methylamine’s gas phase reaction was experimentally investigated by Atkinson et al., by Carl et

154

al. and by Onel et al.27,52,53 These authors find a room temperature rate coefficient of around 2 ×

155

10-11 cm3 molec-1 s-1 (lifetime of ~ 7 h); this fast rate coefficient is consistent with a low transition

156

state energy. Onel et al. also investigated this reaction by ab initio methods and found transition

157

states for both H-abstraction mechanisms slightly below the entrance level energies of the

158

reactants.27 Our results are consistent, albeit slightly higher in energy, a difference in part

159

attributable to the different model chemistries employed and our own theoretical method

160

uncertainty of 1 kcal mol-1.

ACS Paragon Plus Environment

8

Page 9 of 37

Environmental Science & Technology

161 162

Scheme 1. Theoretical energy diagram for methylamine + OH. Energies are 0 K enthalpies in

163

kcal mol−1, at the G3X-K level of theory.

164

3. Amides + OH

165

Amides are known to be products of amine oxidation but our understanding of their fate in the

166

atmosphere is limited.8,15,21,54 Their functionality differs from amines in that amides have a

167

carbonyl moiety adjacent to the nitrogen. The nitrogen’s lone pair is largely delocalized in the

168

carbonyl’s -system, as evidenced by higher BDEs for the N−H bond compared to amines

169

(Table 1). This effect translates to poor nucleophilicity and thus poor reactivity with electrophilic

170

OH radicals, and explains amides’ longer atmospheric lifetimes.17,54 Scheme 2 depicts the energy

171

diagram of formamide’s four possible mechanisms of reaction with OH radicals. The lowest

ACS Paragon Plus Environment

9

Environmental Science & Technology

Page 10 of 37

172

energy transition state is the formyl C−H abstraction, 6 kcal mol-1 lower than the next lowest

173

transition state energy. Thus, this pathway governs the majority of formamide’s reactivity,

174

analogous with the formyl C−H bond in aldehydes.55,56 This mechanism is also quite exothermic

175

compared to the N−H abstraction pathway. In contrast to amines, amides have the potential to

176

react with OH radicals by addition to the carbonyl moiety. We find that this pathway is 9.9 kcal

177

mol-1 above the entrance level energy of the reactants but is exothermic and so is a viable yet an

178

unlikely mechanism. As with amines, the OH addition to the nitrogen in amides is high in energy

179

and endothermic.

180

In previous studies, the room temperature rate coefficient of formamide and OH radicals was

181

measured to be 4.4 × 10-12 cm3 molec-1 s-1 and was supported by ab initio calculations, yet solely

182

focused on the H-abstraction mechanisms.17,34 The formyl C−H abstraction produces a C-

183

centered formyl radical which can go on to react with O2 and form isocyanic acid.17

184

Interestingly, the formyl C−H abstraction is also competitive for N-alkylated amides (see

185

Scheme S1). To complement the previous ab initio work on the reactivity of amides with OH

186

radicals, we also investigated the mechanistic pathways of N,N-dimethylacetamide, N,N-

187

dimethylpropanamide and N-methylpropanamide in Schemes S2, S3 and S4 respectively. Similar

188

mechanistic observations can be made when comparing all these amides: an easy C−H

189

abstraction from methylated amides and an unlikely N−H bond abstraction from amides (in

190

contract to amines).

ACS Paragon Plus Environment

10

Page 11 of 37

Environmental Science & Technology

191 192

Scheme 2. Theoretical energy diagram for formamide + OH. Energies are 0 K enthalpies in kcal

193

mol−1, at the G3X-K level of theory.

194

4. Isocyanic acid + OH

195

Isocyanates differ from the amide functionality by being hybridized sp2 at the nitrogen and sp at

196

the adjacent carbon. In isocyanic acid, the nitrogen’s lone pair is perpendicular to the -system

197

and is therefore not delocalised.57 Isocyanic acid’s OH-reaction energy diagram is presented in

198

Scheme 3. Three mechanisms are plausible: N−H abstraction and OH additions to the carbonyl

199

or to the nitrogen. All three mechanisms are high in energy and so isocyanic acid’s rate

ACS Paragon Plus Environment

11

Environmental Science & Technology

Page 12 of 37

200

coefficient is expected to be very slow at room temperature (Scheme 3). Indeed, the experimental

201

rate coefficient for the reaction of isocyanic acid with OH radicals was measured only at high

202

temperatures and if extrapolated to room temperature is approximately 10-15 cm3 molec-1 s-1,

203

translating to a lifetime of decades.24,58 The OH-addition to the carbonyl (barrier height of + 9.1

204

kcal mol-1) and the N−H abstraction (barrier height of + 19.4 kcal mol-1) pathways are

205

exothermic compared to the OH-addition to the nitrogen which we have now shown is

206

consistently endothermic in amines, amides and isocyanates. No pre-complex could be isolated

207

in our ab initio calculations between isocyanic acid and OH, consistent with all the mechanisms

208

having transition state energies significantly above that of the reactants. Thus, we do not expect

209

gas phase OH radicals to be a sink for isocyanic acid. Rather, it will likely partition to the

210

aqueous phase and undergo further reactions including hydrolysis.59

ACS Paragon Plus Environment

12

Page 13 of 37

Environmental Science & Technology

211 212

Scheme 3. Theoretical energy diagram for isocyanic acid + OH. Energies are 0 K enthalpies in

213

kcal mol−1, at the G3X-K level of theory.

214

5. N-Methyl methylcarbamate + OH

215

We opted to use N-methyl methylcarbamate as our carbamate substrate rather than the simpler

216

methylcarbamate as the latter does not yet have a reported rate coefficient with OH radicals in

217

the literature. Scheme 4 shows the theoretical energy diagram of the five mechanistic pathways

218

relevant to the reaction of N-methyl methylcarbamate with OH radicals. The difference between

219

carbamates and amides is the presence of an oxygen atom next to the amide functionality. This

220

extra oxygen atom provides additional electron density into the carbonyl, decreases the degree of

ACS Paragon Plus Environment

13

Environmental Science & Technology

Page 14 of 37

221

lone pair delocalization at the nitrogen and enhances the reactivity of the molecule towards the

222

electrophilic OH radicals. This claim is supported by the smaller bond dissociation energy (BDE)

223

for the N−H bond in N-methyl methylcarbamate (106.8 kcal mol−1) than in formamide (+ 114.6

224

kcal mol−1) for example (Table 1). Nonetheless, the C−H abstraction mechanisms are expected to

225

dominate the reactivity of carbamates since the two C−H abstraction transition state energies for

226

N-methyl methylcarbamate are below the energies of the reactants. Both OH addition

227

mechanisms, i.e. to the carbonyl or to the nitrogen, require high energy to proceed and are

228

endothermic.

229

We note here that N-methyl methylcarbamate’s experimental rate coefficient (4.3 × 10-12 cm3

230

molec-1 s-1) is slower than that of methylamine (2 × 10-11 cm3 molec-1 s-1) despite the former

231

having calculated lower transition state energies (see Table 2).27,35 Transition state energies are

232

relative to the entrance level energy of the reactants and are therefore not directly comparable

233

between molecules. On the other hand, BDEs are absolute values and we see then that the C−H

234

bond in methylamine has a lower BDE than N-methyl methylcarbamate (see Table 1), consistent

235

with a faster rate coefficient.

ACS Paragon Plus Environment

14

Page 15 of 37

Environmental Science & Technology

236 237

Scheme 4. Theoretical energy diagram for N-methyl methylcarbamate + OH. Energies are 0 K

238

enthalpies in kcal mol−1, at the G3X-K level of theory.

239

MECHANISTIC DISCUSSION

240

We gain insight into the mechanisms governing the reactivity of the four subclasses of reduced

241

organic nitrogen compounds investigated, namely amines (Scheme 1), amides (Scheme 2, S1,

242

S2, S3 and S4), isocyanates (Scheme 3) and carbamates (Scheme 4), by comparing their

243

calculated transition state energies (Table 2). We note here that we use the calculated transition

244

state energy solely to inform us on likely mechanisms and reactive sites.

ACS Paragon Plus Environment

15

Environmental Science & Technology

Page 16 of 37

245

Table 2: Tabulated transition state values from Schemes 1-4 and S1-S4 (with a 1 kcal mol-1

246

accuracy)

Transition state energies (kcal mol-1)

Organic nitrogen + OH

248

N−H abstraction

OH addition to carbonyl

OH addition to nitrogen

Formyl C−H

Nmethylated C−H

methylamine

NA

+ 1.2

+ 0.2

NA

+ 37.4

formamide

+ 0.2

NA

+ 6.3

+ 9.9

+ 41.7

- 0.9

- 1.2

NA

-

-

NA

- 2.4

NA

-

-

NA

- 2.4

NA

-

-

N-methylpropanamide

NA

- 2.2

+ 2.1

-

-

Isocyanic acid

NA

NA

+ 19.4

+ 9.1

+ 32.7

- 1.6 / - 0.1

+ 2.5

+ 14.1

+ 43.6

N,Ndimethylformamide N,Ndimethylacetamide N,Ndimethylpropanamide

247

C−H abstraction

N-methyl NA methylcarbamate NA = not applicable, - = not computed 1. C−H abstraction

249

The C−H abstraction mechanism dominates the reactivity of reduced organic nitrogen

250

compounds when present. This observation is expected as the C−H bond is less polarized than

251

heteroatom X−H bonds, making it more prone to break homolytically and react with the

252

electrophilic OH radical. C−H bonds in organic nitrogen compounds have consistently lower

253

bond dissociation energies than N−H bonds as seen in Table 1. The preference of OH radicals to

ACS Paragon Plus Environment

16

Page 17 of 37

Environmental Science & Technology

254

react with C−H bonds over N−H bonds leads to the oxidation of carbon while the nitrogen

255

remains in the -3 oxidation state throughout oxidative conversion from amines to amides to

256

isocyanates. Building upon this logic, we do not expect oxidation of reduced organic nitrogen

257

compounds to substantially contribute to the production of nitrogen oxides in ambient air. In

258

other words, the N atom in reduced organic nitrogen molecules does not end up as the N atom in

259

nitrogen oxide molecules.

260

2. N−H abstraction

261

The N−H abstraction mechanism which leads to aminyl radicals after the initial OH attack is

262

solely competitive for amines. The increasing trend observed for transition state energies of the

263

N−H abstraction mechanism is as follows: amine < carbamate < amide < isocyanate (see Table

264

2). Indeed, the N−H abstraction in methylamine is the only calculated barrier height which is

265

close to the entrance level energy of the reactants (at + 0.2 kcal mol-1), indicating a rapid and

266

favorable reaction. Importantly, aminyl radicals may go on to react with nitrogen oxides to form

267

nitramines and nitrosamines, which are known carcinogens.60-66 It is noteworthy that amides,

268

isocyanates and carbamates are not expected to form nitramines and nitrosamines directly from

269

OH radicals abstracting N−H bonds, based on the high energy barrier that yields their

270

corresponding aminyl radical. Nonetheless, as Bunkan et al. explicitly discuss, further reactions

271

of amide C-centered radicals may eventually lead to fragmentation and formation of aminyl

272

radicals from amides, yielding nitramines.33

273

3. OH addition to carbonyls

274

The addition of OH radicals to unsaturated bonds is a common mechanistic pathway for alkenes

275

and alkynes. For completeness, we also explored this mechanism for carbonyls, as they exist in

ACS Paragon Plus Environment

17

Environmental Science & Technology

Page 18 of 37

276

reduced organic nitrogen compounds. Although this mechanism is exothermic in formamide and

277

isocyanic acid, it is slightly endothermic for N-methyl methylcarbamate (Schemes 2, 3 and 4).

278

Nonetheless, it possesses a high energy barrier to reaction and is not expected to be competitive

279

with C−H or N−H abstraction pathways. In the case of isocyanic acid, this mechanism dominates

280

its reactivity, but this mechanism is still sufficiently slow at ambient temperatures to be

281

negligible in the context of isocyanic acid’s atmospheric fate.

282

4. OH addition to nitrogen

283

The OH addition to the N atom pathway was also investigated for methylamine, formamide,

284

isocyanic acid and N-methyl methylcarbamate’s reactions with OH radicals. This mechanism

285

was proposed to occur for sulfides and was originally thought to extend to amines.67 We show in

286

Schemes 1 to 4 that this mechanism is consistently high in energy and endothermic for all classes

287

of N-containing compounds studied. This observation remains true even for electron rich amines

288

like trimethylamine, where we calculate at the G3X-K level an activation energy of + 42.3

289

kcal/mol. Therefore, OH additions to N may lead to weak complexes,68 but will not lead to a

290

productive reaction pathway and would simply dissociate.

291

SAR MODELLING DISCUSSION

292

We now use our mechanistic insight to develop structure-activity relationship (SAR) group rate

293

constants and substituent factors to predict the atmospheric fate of reduced organic nitrogen

294

compounds. Our study is constructed on a fundamental mechanistic understanding of reactivity

295

with OH radicals to improve the existing empirical SAR model. We would like to emphasize at

296

this point that we do not calculate rate coefficients with our computed potential energy surfaces.

297

Rather, we use these energy diagrams to identify reactive and non-reactive pathways to lead us in

ACS Paragon Plus Environment

18

Page 19 of 37

Environmental Science & Technology

298

our development of the SAR factors described in this part of the study. We center our discussion

299

on aliphatic reduced organic nitrogen while appreciating that related aromatic compounds are

300

also of environmental importance.

301

1. Description of SAR models

302

Structure-activity relationships (SAR) models were developed as a predictive tool for estimating

303

the room temperature rate coefficients and hence the atmospheric lifetime of organic compounds

304

against the OH radical and later against other atmospheric oxidants.56,68-71 The SAR model’s

305

general approach consists of predicting an overall rate coefficient by summing the individual rate

306

constants of every reactive site on a molecule of interest. Relevant mechanisms for volatile

307

organic compound reactions with OH radicals include H-abstraction, addition to unsaturated

308

carbon-carbon bonds and addition to heteroatoms.68 The SAR method includes two types of

309

factors relevant to our reduced organic nitrogen analysis. First, there are the SAR group rate

310

constants for H-abstractions from any atom (C, N, O, etc.), denoted by a k value with a subscript

311

denoting the site (or functional group such as k-NH-, k-OH) of abstraction. There are three types of

312

SAR group rate constants specifically for C−H abstractions, one for each reaction occurring at a

313

primary (kprim), secondary (ksec) or tertiary (ktert) carbon center. Second, this C−H abstraction

314

SAR group rate constant (kprim, ksec, or ktert) is then multiplied by a substituent factor denoted as

315

F(X) to obtain an overall rate constant specific to the functional group’s reactivity. Sample

316

equations for C−H abstractions are given below and are based on Atkinson et al.68

317

𝑘𝑜𝑣𝑒𝑟𝑎𝑙𝑙 (𝐶𝐻3 − 𝑋) = 𝑘𝑝𝑟𝑖𝑚 𝐹(𝑋)

318

𝑘𝑜𝑣𝑒𝑟𝑎𝑙𝑙 (𝑌 − 𝐶𝐻2 − 𝑋) = 𝑘𝑠𝑒𝑐 𝐹(𝑋)𝐹(𝑌)

ACS Paragon Plus Environment

19

Environmental Science & Technology

319

Page 20 of 37

𝑘𝑜𝑣𝑒𝑟𝑎𝑙𝑙 ( 𝑌𝑍>𝐶𝐻 − 𝑋) = 𝑘𝑡𝑒𝑟𝑡 𝐹(𝑋)𝐹(𝑌)𝐹(𝑍)

320

There currently exist published empirical SAR group rate constants and substituent factors for

321

amines, carbamates or thiocarbamates functionalities, derived from limited experimental rate

322

coefficients.8,35,68,69,72 There are no SAR group rate constants specifically for amides,

323

isocyanates or isothiocyanates. Guided by our ab initio results, we examine the mechanisms

324

governing reduced organic nitrogen molecules’ reactivity in order to build upon past SAR

325

analyses. We tabulate existing experimental rate coefficients for reduced organic nitrogen

326

molecules with OH radicals and formulate SAR model equations defining koverall for each

327

compound (see Table S1 and model equations in the Supplementary Information). We use only

328

experimental rate coefficients in our statistical analysis and omit theoretically calculated rate

329

coefficients although the latter are becoming more reliable (but still few exist for N-containing

330

compounds).71,73 We then solve this overdetermined linear system of equations using a built-in

331

function in the program IGOR Pro (see Supplementary Information). Revised SAR factors are

332

proposed to better capture the differences in reactivity based on substituents on the N atom and

333

to disambiguate the existing factors for N-containing molecules (Figure 1 and Table 3). The SAR

334

factors are reported with two significant figures and without uncertainties as they represent

335

solutions to the overdetermined linear equations, remaining consistent with the SAR

336

literature.68,69,74

337

ACS Paragon Plus Environment

20

Page 21 of 37

Environmental Science & Technology

338 339

Figure 1: The SAR group rate constants represent reactivity at the highlighted H atom in

340

yellow and the SAR substituent factors represent reactivity imparted by the functionality

341

highlighted in blue to the H atom in bold. R1 and R2 can be either H or any alkyl group.

342

Table 3: The proposed SAR group rate constants kX-H and substituent factors F(X) for

343

aliphatic reduced organic nitrogen compounds Functional group Methyla

Group rate constant (cm3 molec-1 s-1) k-CH3 = 0.136 × 10-12 k-CH2- = 0.934 × 10-12 k-CH< = 1.94 × 10-12

Substituent factor (unitless) F(-CH3) = 1.00 F(-CH2) = 1.23 F(-CH