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Environmental Processes

The mechanism of C-H bond oxidation by aqueous permanganate Jens Blotevogel, Anthony K. Rappe, Arthur N. Mayeno, Tom Sale, and Thomas Borch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03157 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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Environmental Science & Technology

The mechanism of C-H bond oxidation by aqueous permanganate

Jens BlotevogelA,*, Anthony K. RappéB,*, Arthur N. MayenoC, Tom C. SaleA, Thomas BorchA,B,D,*

A

Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80523, USA

B

Department of Chemistry, Colorado State University, Fort Collins, Colorado, CO 80523, USA

C

Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, CO 80523, USA

D

Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523, USA

C─ H

C─ OH oxygen rebound

C=O

TOC Art

1 ACS Paragon Plus Environment


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ABSTRACT

2

The permanganate ion (MnO4-) has been widely used as reagent for water treatment for

3

over a century. It is a strong enough oxidant to activate carbon-hydrogen bonds, one of the most

4

important reactions in biological and chemical systems. Our current textbook understanding of the

5

oxidation mechanism in aqueous solution involves an initial, rate-limiting hydride abstraction by

6

permanganate followed by reaction of the carbocation with bulk water to form an alcohol. This

7

mechanism fits well into the classic oxidation sequence of alkane → alcohol → aldehyde →

8

carboxylate, the central paradigm for both abiotic and biotic alkane oxidation in aqueous

9

environments. In this study, we provide three lines of evidence through (1) a broken-symmetry

10

density functional theory approach, (2) isotope labeling experiments, and (3) kinetic network

11

modeling to demonstrate that aqueous permanganate can circumvent prior alcohol formation and

12

produce aldehydes directly via a reaction path that bifurcates after the initial transition state. In

13

contrast to classic transition state theory, the rate-limiting step is found to not determine product

14

distribution, bearing critical implications for pathway and rate predictions. This complex reaction

15

network provides new insights into the oxidation mechanisms of organic compounds involving

16

transition metal complexes as well as enzyme or metal oxide surface active sites.

17 18 19

INTRODUCTION

20

The permanganate anion MnO4- is a classic chemistry textbook oxidant that has been

21

widely used since the mid-19th century for drinking and wastewater treatment,1,2 water

22

disinfection,3 and groundwater remediation,4,5 as well as in chemical synthesis and medical

23

applications.6-8 Permanganate’s popularity is founded on the activation of rather unreactive carbon-

24

hydrogen bonds, a significant focus area of fundamental and industrial research for several

25

decades9 owing to its pivotal role in the functionalization of alkanes,10,11 metabolism of

26

endogenous and exogenous molecules,12 and destruction of environmental contaminants.13,14,15

27

Despite decades of intensive research, however, the exact mechanism by which

28

permanganate and many other transition metals oxidize C-H bonds is still a center of scientific

29

debate as the intermediates of these reactions are short-lived and thus not easily detected.13,16-17 In

30

1995, Gardner and Mayer18 reported that in aqueous solution the kinetics of toluene C-H bond

31

activation were consistent with an initial, rate-limiting hydride (H-) abstraction generating a water2 ACS Paragon Plus Environment

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stabilized carbocation intermediate, likely forming benzyl alcohol (Pathway 1 in Scheme 1). The

33

observed products were benzaldehyde and benzoic acid. These observations fit well into the classic

34

oxidation sequence of alkane → alcohol → aldehyde → carboxylate, the central paradigm for

35

abiotic and biotic alkane oxidation in aqueous environments.

36

37 38

Scheme 1.

39 40

In a previous study we investigated the oxidation of hexamethylphosphoramide (HMPA)

41

by aqueous-phase permanganate in excess under pseudo-first-order conditions.19 HMPA is an

42

industrial solvent and groundwater contaminant,20,21 and well suited for C-H bond oxidation

43

experiments as its six methyl substituents are located on the outside of the molecule, shielding

44

other potentially oxidizable functional groups.20 In an effort to establish a predictive model for this

45

reaction under environmentally relevant conditions, however, we were unable to obtain a

46

satisfactory kinetic network model fit for our experimental data under the assumption of the classic

47

oxidation sequence suggested above (see Fig. S1 in the Supporting Information). This led us to

48

hypothesize that there is an additional mechanism and pathway of C-H bond activation by aqueous

49

permanganate. As early as 1963, Wiberg & Fox22 had proposed oxygen rebound as possible

50

mechanism, in which the carbocation and hypomanganate (i.e., HMnVO42- after hydride

51

abstraction) would react to form an ester intermediate (Pathway 2 in Scheme 1), but failed to

52

provide conclusive experimental support. Furthermore, the potential fate of an ester intermediate 3 ACS Paragon Plus Environment


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remained unexplained. Thus, the objective of our study was to provide multiple lines of both

54

theoretical and experimental evidence in order to finally unravel the mechanism of C-H bond

55

oxidation by aqueous permanganate, and in a broader context, to advance our understanding and

56

predictive capabilities regarding organic chemical oxidation involving transition metals.

57 58 59

EXPERIMENTAL SECTION

60

Isotope Labeling Experiments. For the experiment determining pathway, kinetics, and

61

origin of oxygen in the aldehyde (i.e., formyl-PMPA), 1 mL of H218O (97.2%, Cambridge Isotope

62

Laboratories) was transferred into an HPLC vial. KMn16O4 (certified ACS, Fisher Scientific) and

63

HMPA (99%, MP Biomedicals) were added to yield final concentrations of 60 mM and 2.5 mM,

64

respectively. The reaction was carried out under pseudo-first-order kinetics19 and non-agitated at

65

22 °C. Samples (15 μL) were quenched by adding 1.485 mL of 5 mM Na2S2O3 and filtered (0.2

66

µm, nylon).

67

To validate the findings of the experiment above and determine the source of oxygen in the

68

alcoholic species, we synthesized KMn18O4 by dissolving 50 mg KMn16O4 in 1 mL H218O (97.7%,

69

Cambridge Isotope Laboratories) within a sealed 2-mL amber glass ampule. The ampule was

70

heated in an oven at 115 °C for 11 days. The 18O-content of the permanganate after evaporation of

71

the solvent water was determined to be 91.4% (see below). Subsequently, the synthesized

72

KMn18O4 and HMPA were dissolved in 1.5 mL H216O to yield final concentrations of 60 mM and

73

2.5 mM, respectively. The reaction was carried out non-agitated at 22 °C. Samples (15 μL) were

74

quenched by adding 1.485 mL of 5 mM Na2S2O3 and filtered (0.2 µm, nylon).

75

To quantify the natural exchange of oxygen isotopes between the alcoholic / aldehydic

76

oxidation products and the solvent water under conditions of the kinetic experiment, the sample

77

taken after five minutes during the isotope experiment described above was analyzed after

78

quenching, KMn18O4 precipitation and filtration over the course of 65 minutes, the duration of the

79

kinetic experiments described above. The decreasing

80

dissolved in H216O revealed only minor oxygen exchange of 1.0% in the alcohol (HM-PMPA) and

81

0.5% in the aldehyde (formyl-PMPA).

18

O-content in the two oxidation products

82

To quantify the natural exchange of oxygen isotopes between permanganate and the solvent

83

water under conditions of the kinetic experiment, KMn16O4 was added to 1 mL of H218O (97.5%, 4 ACS Paragon Plus Environment

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Cambridge Isotope Laboratories) at a final concentration of 60 mM. Over the course of 65 minutes,

85

the 18O-content in KMn16O4 increased by only 0.21%.

86

Kinetic Network Model. To determine pseudo-first-order rate constants k’ for HMPA and

87

its reaction products, an analytical solution for networks of irreversible first-order reactions23 was

88

implemented in Mathcad (Version 14.0.0.163, PTC). This algorithm involves the solution of a

89

coefficient matrix, in which the individual (pseudo-)first-order rate constants for all reactions

90

within the network are the coefficients, for the direct calculation of species concentrations at any

91

time of interest. The matrix coefficients were iterated to fit the experimental data until the pooled

92

unweighted non-linear R2 as well as the non-linear R2 for each individual compound, both of which

93

based upon the use of the total corrected sum of squares, were maximized.

94

Chemical Analyses. HMPA and its phosphoramide-based oxidation products were

95

analyzed with an Agilent 1100 Series liquid chromatograph equipped with a 150 mm × 2.1 mm

96

XTerra phenyl column, 3.5 µm particle size (Waters) coupled to an Agilent G3250AA MSD TOF

97

system with an electrospray ionization (ESI) source in positive ion mode (LC/ESI+-TOF-MS).

98

Separation was carried out isocratically with 0.01% formic acid in water / acetonitrile (98:2).24

99

Reference masses for mass spectrometric calibration were 121.050873 and 922.009798. Further

100

details are provided in the Supporting Information.

101 102 103

COMPUTATIONAL DETAILS

104

The majority of quantum chemical calculations were conducted via Gaussian 09 (Rev.

105

B.01). All optimization and frequency calculations were performed without symmetry constraints

106

for 295.15 K (i.e., the temperature of our experimental system) at the B3LYP level of theory25,26

107

with the 6-31+G(d) basis set27 for O, N, C and H as well as the effective core potential (ECP)-type

108

LANL2DZ basis set28,29 for P, Mn and K (K added to balance the negative charge on the oxyanion).

109

To account for solvation effects on reaction pathways, mechanisms and kinetics, a cluster-

110

continuum model based on the SMD solvation model was applied.20,30 One potassium ion was

111

added as counterion to balance each anionic manganese species in the respective calculation. Wave

112

function stability/symmetry was tested using the stable=opt keyword and reoptimized using a

113

corrected wave function as needed. To test the broken symmetry computational model (see Results

114

and Discussion for more details on broken-symmetry wave functions), a complete active 5 ACS Paragon Plus Environment


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space/perturbation theory model was used to compute the barrier height. For this study the ORCA

116

electronic structure suite31 was used, and the 6-31 basis set was applied for all “non-active” centers.

117

For Mn, the four oxo oxygens, and the reactive C-H, a set of p (H) and d (O, C) polarization

118

functions were added. For both the ground state and transition state, a CAS(14,12) wave function

119

optimization was followed by a NEVPT2 perturbation treatment.32-34 The SMD solvation model

120

was used here as well. A detailed description of computational methods is provided in the

121

Supporting Information.

122 123 124

RESULTS AND DISCUSSION

125

To generate a first line of evidence for our hypothesis of an additional C-H bond oxidation

126

mechanism by aqueous permanganate, we performed a B3LYP density functional theory (DFT)

127

study on HMPA (compound GS1 in Figs. 1-3). Beyond the transition state (TS(1-2/4) in Figs. 1-

128

3) for initial hydride abstraction from any of the methyl (-CH3) functional groups of HMPA by

129

permanganate, the intrinsic reaction coordinate (IRC) suggested oxygen rebound as the exclusive

130

mechanism (magenta circles, Fig. 1), in contradiction to the previously suggested reaction of the

131

carbocation with bulk water to form an alcohol.18 Furthermore, the addition of explicit solvent

132

water molecules and hydroxide ion in our calculations did not stabilize the intermediate

133

carbocation. To validate this surprising finding, we conducted a spin stability study of the transition

134

state wave function, which indicated that the closed shell DFT wave function did not accurately

135

describe this system, and thus failed to provide either the lowest-energy transition state or the

136

lowest-energy product ground state(s). Bond pairs, described by molecular orbitals, are a mixture

137

of covalent and ionic contributions. As a bond pair is stretched during the course of a reaction, at

138

a certain point known as the Coulson-Fischer point,35,36 on-site repulsion due to the ionic

139

contribution to the wave function is no longer compensated for by favorable kinetic energy and

140

nuclear attraction terms due to the ionic contribution. At this point a wave function emerges as the

141

ground state wherein different spins occupy different orbitals. This unrestricted wave function is

142

referred to as a broken-symmetry wave function. As shown in Fig. S2 in the Supporting

143

Information, β spin density builds up on the Mn center in the broken symmetry wave function

144

while α spin density delocalizes over the hydrogen, carbon, and adjacent phosphoramide oxygen

145

centers. The density-mapped electrostatic potential surface highlights build-up of positive charge 6 ACS Paragon Plus Environment

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on potassium and negative charge on the hydroxide ion. The DFT spin-natural orbitals and CAS

147

natural orbitals in Fig. S2 support a model for the wave function consisting of diradical character

148

(occupations of roughly 1.5 and 0.5) but with significant ionic character as well. Given this

149

diradical ionic mixture, it is not surprising that the computed kinetic isotope effect of kCH3/kCD3=5.6

150

for the methyl group(s) in HMPA falls between the experimental carbocationic (small enthalpy of

151

activation, large entropy of activation, and large kinetic isotope effect, kC7H8/kC7D8=9.7) and

152

hydrogen atom transfer (larger enthalpy of activation, smaller entropy of activation, and moderate

153

kinetic isotope effect, kC7H8/kC7D8=2.5) range for the methyl substituent in toluene.18

154

Relative Energy (kcal/mol)

20

0

156 157 158 159 160 161 162

GS1

-10

hydride abstraction

hydride abstraction

oxygen rebound

-20 -30

Hydride Abstraction IRC (BS-B3LYP) PES Scan Water Stabilization (BS-B3LYP) PES Scan Oxygen Rebound (BS-B3LYP) Hydride Abstraction IRC (B3LYP)

-40 -50 -60

155

TS(1-2/4)

10

0

20

40

60

80 100 Coordinate 120 140 160 Reaction

INT2 GS4 180

200

220

Figure 1. Potential energy surface (PES) for C-H bond activation by aqueous permanganate. The spin-restricted intrinsic reaction coordinate (IRC) falsely implies exclusive oxygen rebound after initial hydride abstraction from HMPA, leading to manganate ester formation only. The total energy for the corrected IRC levels off after hydride abstraction. Relaxed PES scans show that either oxygen rebound or (solvent) water addition proceed without energy barrier before a carbocation ground state is produced. Transition state and bifurcation point geometries are provided in the Supporting Information.

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Consequently, we used a broken-symmetry wave function (BS-B3LYP)37 to revisit the C-

165

H bond oxidation reaction by permanganate. Here, the IRC leveled off after hydride/hydrogen

166

abstraction without indication of oxygen rebound (blue diamonds in Fig. 1), despite the close

167

proximity of opposite charges on the forming hypomanganate anion and carbocation centers. The

168

barrier height of the initial hydride/hydrogen abstraction was calculated to be 7.1 kcal/mol (Fig.

169

2), compared to 8.1 kcal/mol from the CAS/NEVPT2 calculation (see the Supporting Information).

170

As described in the Supporting Information, the broken-symmetry approach has been shown to

171

produce results similar to those obtained from multi-reference ab initio calculations for molecular

172

systems with spin instability,38 yet at much lower computational cost. Relaxed scans of the

173

potential energy surface (PES) starting from the downward energy slope of the IRC toward

174

hypomanganate and solvent waters were conducted, in which the distances between the

175

carbocation center and the nearest oxygen atom in hypomanganate as well as between the

176

carbocation center and the nearest oxygen atom in a solvent water molecule were decremented.

177

These suggested that both water stabilization (yellow squares, Fig. 1) and oxygen rebound (green

178

triangles, Fig. 1) pathways proceed without an additional energy barrier.

179


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MnO3 O H H2CR

H 2O -

Relative Energy (kcal/mol)

TS(1-2/4) (7.1)

Mn(OH)O2

MnO3

O

O

H2CR

H

GS1 (0.0)

(-37.9)

OH

TS(2-4) INT2 Mn(OH)O2

(-47.1)

H2CR

TS(2-3) (-38.4)

(-41.9)

GS4

O

O

(-90.2)

H2CR

HCR

(-95.3)

TS(3-5) -

OH

H

INT3

O

Mn(OH)O2

Mn(OH)O2

O

O

-

O HCR

GS5 (-119.1)

HCR

181 182 183 184 185

TS(4-5)

(-50.5)

Mn(OH)O2

MnO3

180

HC(OH)R

HCR

Reaction Coordinate Figure 2. BS-B3LYP reaction profile for C-H bond oxidation by aqueous permanganate. Orange: Solvent water binding after initial hydride abstraction producing an alcohol. Red: Oxygen rebound leading to alcohol production. Blue: Oxygen rebound circumventing alcohol production. Complete ground state, intermediate and transition state geometries are provided in the Supporting Information.

186 187

Thus, the basic DFT approach without consideration of broken symmetry wrongfully

188

provided an asymmetric PES wherein the IRC diverted according to the mass-weighted steepest

189

descent path to one specific product ground state,39 leaving the parallel pathway hidden. The BS-

190

B3LYP-corrected three-dimensional PES now reveals a post-transition state bifurcation on a 9 ACS Paragon Plus Environment


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symmetric coordinate (Fig. 3), in which a single transition state (TS(1-2/4)) leads to formation of

192

more than one product without additional energy minima or barriers in between, generally referred

193

to as "two-step (or two-event), no-intermediate" mechanism.40,41

194

195 196 197

Figure 3. Reaction potential energy surface. The computed minimum energy paths are plotted in red; the remainder of the surface is conceptual.

198 199

Beyond the reaction path bifurcation (Fig. 2), oxygen rebound produces a hypomanganate

200

ester (INT2), alternatively water stabilization leads to the formation of an alcohol (GS4), which is

201

further oxidized through hydride abstraction by permanganate (TS(4-5)), with concurrent

202

deprotonation of the hydroxyl substituent) to an aldehyde (GS5). The symmetric nature of the

203

calculated PES would suggest a product ratio of hypomanganate ester to alcohol of 1:1.39,42

204

While the formation of a hypomanganate ester had been suggested previously,22 direct

205

experimental evidence for it has not been provided until now. The BS-B3LYP calculations suggest

206

two energetically similar reactions (Fig. 2). Backside hydrolysis of the Mn-O bond (TS(2-4)) leads 10 ACS Paragon Plus Environment

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207

to the generation of the same alcoholic species GS4 as after initial water stabilization. Slightly

208

more favorable is hydride abstraction from the carbon center by a second permanganate ion (TS(2-

209

3)), making the organic substituent a better leaving group (INT3). Subsequent (base-catalyzed)

210

backside hydrolysis of the Mn-O bond (TS(3-5)) then directly leads to aldehyde formation (GS5).

211

Furthermore, the reaction network suggests that all oxidations yield hypomanganate HMnVO42- as

212

product. Consequently, analyses of Mn speciation were not attempted as they would be

213

unsupportive of mechanistic investigations.

214

Thus, isotopic labeling studies were carried out to complement the theoretical predictions.

215

First, HMPA was oxidized with Mn16O4- in H218O. Two isotopes of the aldehydic species formyl-

216

pentamethylphosphoramide (formyl-PMPA) were detected via liquid chromatography coupled

217

with electrospray ionization time-of-flight mass spectrometry (LC/ESI+-TOF-MS). One aldehyde

218

contained 16O, while the other contained 18O in the formyl substituent (Table 1). Appearance of

219

both isotopes confirmed that both water and permanganate are sources of oxygen in the aldehydic

220

product.

221

pentamethylphosphoramide (HM-PMPA) product was hindered by the fact that the alcoholic

222

species underwent neutral loss of water ([M+H]+ - H216/18O) during electrospray ionization mass

223

spectrometry.19 In direct injection mode, however, the major adduct of the alcohol was [M+Na]+,

224

and the detection of 18O-HM-PMPA provided evidence that water was at least one of the sources

225

of oxygen in the alcohol. Secondly, the presence of

226

determined to support hypomanganate ester hydrolysis as a source of the alcohol (TS(2-4)), since

227

ESI+-TOF-MS was unable to resolve the

228

overlapping (sodiated) 18O-formyl-PMPA signal. The oxidation experiment was thus repeated with

229

freshly prepared Mn18O4- in H216O. Here,

230

confirmation that C-H bond activation by aqueous permanganate indeed proceeds through at least

231

three parallel pathways as shown in Fig. 2. Oxygen exchange between the detected oxygenated

232

reaction intermediates and the solvent water as well as between permanganate and water over the

233

experimental duration of 65 minutes was confirmed to be negligible (see the Supporting

234

Information).

Support

for

the

two

predicted

16

oxygen

16

sources

in

the

hydroxymethyl-

O-HM-PMPA could not be conclusively

O-HM-PMPA accurate mass at m/z of 218 from the

18

O-HM-PMPA was successfully detected, providing

235



236 237

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Table 1. Accurate mass spectrometric data for HMPA and its products detected during the isotope labeling experiments. Compound

Molecular ion peak

HMPA

[M+H]+

18

O-HM-PMPA

16

O-formyl-PMPA

16 18

Measured m/z

Calculated exact m/z

Error (ppm)

C6H19N3OP+

180.1256

180.1260

2.2

[M+Na]+

C6H18N3Na16O18OP+

220.1077

220.1071

2.7

[M+H]+

C6H17N316O2P+ +

O-formyl-PMPA

[M+Na]

O-formyl-PMPA

+

[M+H]

Molecular ion peak formula

194.1053

194.1053

0.0

16

+

216.0874

216.0872

0.9

18

+

196.1095

196.1095

0.0

C6H16N3Na O2P 16

C6H17N3 O OP

238 239 240

To determine observed rate constants k’ for the parallel reactions, we applied a first-order

241

kinetic network model23 based on the HMPA oxidation experiment with Mn16O4- in H218O, which

242

enabled discrimination between formyl-PMPA but not HM-PMPA isotopes (Fig. 4). The vastly

243

improved pooled unweighted R2 value of 0.9934 compared to the initial fit for water addition only

244

(R2 = 0.8950, Fig. S1) further supported the validity of the BS-B3LYP pathway prediction. The

245

kinetic network model revealed that the initial hydride abstraction by permanganate is the rate-

246

limiting step during C-H bond oxidation,18 supported by the experimental detection of HM-PMPA

247

and formyl-PMPA as the two first stable intermediates with the other five methyl substituents

248

intact. The ester hydrolysis steps are more than four orders of magnitude faster. Moreover, the

249

best-fit rate constants of 0.133 min-1 for both water stabilization and oxygen rebound confirmed

250

the predicted symmetric nature of the PES after reaction path bifurcation, leading to an equal ratio

251

of hypomanganate ester to alcohol generation. The overall observed rate constant for HMPA

252

oxidation of 0.133 min-1 + 0.133 min-1 = 0.266 min-1 matched well the previously determined rate

253

in unlabeled systems of 0.249 ± 0.022 min-1.19

254


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1.0

X

R2 = 0.9992

0.9

hydroxymethyl R2 = 0.9940

k* ≥ 1500

0.8 0.7 0.6

C/C0

alkyl

16O-formyl

R2 = 0.9892 18O-formyl

R2 = 0.9898

0.5 0.4 0.3 0.2

pooled R2 = 0.9931

0.1 0.0

0

10

20

255 256 257 258 259 260

30

Time (min)

40

50

60

Figure 4. C-H bond oxidation by Mn16O4- in H218O. Symbols represent experimental, lines represent pseudo-first-order kinetic network model data of normalized HMPA concentration over time. Insert shows setup of the kinetic network model with best-fit rate constants k’ (min-1). Rate constant k*, determined during statistical fitting, has a lower limit of 1500 min-1 (i.e., an increase of k* above 1500 did not improve or change the model fit).

261 262

Implications. Given that post-transition state bifurcations occur after the rate-limiting step, they

263

are difficult to establish experimentally. Advances in quantum chemical methods and

264

computational resources have permitted computational studies to suggest their presence.43 This

265

bears critical implications for classic two-dimensional transition state theory,44 which has been

266

traditionally used by organic and environmental chemists for the prediction of transformation rates,

267

pathways, and product distributions. As a shared transition state structure may lead to the formation

268

of more than one product after post-transition state bifurcation, the assumptions imposed by

269

transition state theory of one distinct transition state per product and the reaction pathway being

270

independent of PES shape fall apart.



271

As implied by our findings, metal-mediated undirected C-H bond activation selectivities

272

do not necessarily depend exclusively on the rate-limiting step in their mechanisms,16 but at times

273

on post-transition state PES shape and dynamic effects.45 Given solvent (water) participation in

274

one of the bifurcated paths, and transition state diradical-ionic mixed character, production

275

distribution should be sensitive to the solvent. Other factors such as the relative concentration of

276

species and non-covalent interactions during reaction may affect product ratios in these systems as

277

well.41

278

Here, we have unraveled the modes of C-H bond oxidation by one metal oxo species after

279

over a century of extensive application and decades of scientific controversy. However, our

280

findings likely have far broader implications for oxidation mechanisms of organic compounds

281

involving transition metal complexes as well as enzyme or metal oxide surface active sites. Reports

282

of reaction path bifurcations in biological systems are emerging,46 and recognition of their role in

283

heterogeneous catalysis is anticipated.

284 285 286

AUTHOR INFORMATION

287

Corresponding Authors

288

* (J.B.) E-mail: [email protected]. Phone: +1-970-491-8880. Fax: +1-970-491-8224.

289

* (A.K.R.) E-mail: [email protected]. Phone: +1-970-491-6292. Fax: +1-970-491-

290

1801.

291

* (T.B.) E-mail: [email protected]. Phone: +1-970-491-6235. Fax: +1-970-491-5676.

292

Notes

293

The authors declare no competing financial interest.

294 295 296

ACKNOWLEDGMENTS

297

Funding for this work was provided by E.I. du Pont de Nemours and Company. We thank Y.

298

Desyaterik for discussion of mass spectrometric analyses and K. Karimi Askarani for graphical

299

assistance.

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ASSOCIATED CONTENT

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Supporting Information. The Supporting Information is available free of charge on the ACS

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Publications website at DOI: …

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Alternative kinetic fit without oxygen rebound, additional analytical details, additional

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computational details, as well as values and optimized structures of broken-symmetry ground

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states, transition states, intermediates, and the bifurcation point.

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REFERENCES

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