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Early Events in the Reductive Dehalogenation of Linear Perfluoroalkyl Substances Daniel J. Van Hoomissen, and Shubham Vyas Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.9b00116 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 21, 2019

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Environmental Science & Technology Letters

1

Early Events in the Reductive Dehalogenation of Linear Perfluoroalkyl Substances

2

Daniel J. Van Hoomissen1 and Shubham Vyas1,*

3 4

1Department

of Chemistry, Colorado School of Mines, 1012 14th St., Golden, CO 80401

*E-mail: [email protected], Tel: 303-273-3632

5 6

Abstract. This letter details the early events in the reductive defluorination of perfluoroalkyl substances

7

(PFASs) and presents a straightforward methodology to predict the reduction behavior of the perfluoroalkyl

8

acids (PFAAs) using electronic structure calculations. Electron attachment to linear perfluorocarboxylic

9

acids (PFxAs) generally occurs at the α-carbon and is energetically not correlated to chain length, contrary

10

to the linear perfluoroalkane sulfonates (PFxSs) where electrons generally insert into other positions.

11

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), two widely studied and scrutinized

12

PFAAs, are therefore predicted to be reduced through diverging pathways. Our protocol can predict the

13

standard reduction potentials of PFAAs, provides a rational basis to probe reaction intermediates, establish

14

free energy relationships, and accounts for PFASs’ inherent structural diversity beyond the linear substrates.

15 16 17 18 19 20 21

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Introduction. Widespread biosphere contamination by PFASs has become a worldwide dilemma1 after

23

decades of pervasive and largely unregulated use. The chemical recalcitrance potential for

24

bioaccumulation2,3 of PFASs have instigated a worldwide effort to detect PFAS in the environment4,5 and

25

also mitigate the pathways of exposure.5 As the epidemiological implications of PFAS-based pollution have

26

become recognizable outside of scientific circles, many studies continue to highlight the troubling aspects

27

of acute and chronic toxicity.6 The increasing evidence of the negative impacts to human physiology,

28

coupled with their continued and worldwide use, warrants a more concerted effort to understand PFAS

29

degradation on a molecular level.7 Studies describing oxidative degradation techniques and their associated

30

mechanisms are prevalent in the literature,8–10 however, theoretical and mechanistic studies concerning

31

reductive techniques are scarce. The semi-empirical work of Forest et al.11, Paul et al.’s analysis of electron

32

attachment to gas-phase n-perfluoroalkanes (n-PFAs)12, and Blotevogel et al.’s recent exploration into the

33

kinetic aspects of PFOA reduction via zero-valent metals13 are some noteworthy exceptions. The current

34

work aims to reframe and expand our knowledge of how PFASs are reduced at the molecular level.

35

Materials and Methods. All stationary points were located with three density functionals, B3LYP14–16,

36

M06-2X17 and ωB97-XD18 with 6-311+G(2d,2p) basis sets using the Gaussian 09(d.01) software suite.19

37

Hessian calculations were used to verify whether the structure was a minimum or maximum and the SMD

38

implicit solvent model20 was utilized throughout to mimic an aqueous environment. The Natural Bond

39

Orbital (NBO 3.121, Gaussian09) method provided atomic spin densities and charges. All PFAAs were

40

assumed to be in a deprotonated (anionic) form (see list of acronyms in the supporting information).

41

Standard reduction potentials (𝐸°𝑠) were calculated against the standard hydrogen electrode (SHE = 4.44V)

42

via Eq. 1.

43

𝐸°𝑠 =

44

Results and Discussion. PFBS and PFPeA as Model Systems for Electron Attachment. First, we

45

qualitatively explored electron (e) attachment to perfluorobutane sulfonate (PFBS) and perfluoropentanoic

∆𝐺(𝑆𝑀𝐷) 𝑛𝐹

(1)

―𝑆𝐻𝐸


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46

acid (PFPeA). Relaxed potential energy surface (PES) scans at the B3LYP level revealed the intermediates

47

following e insertion (Section S2). Upon reduction, the pathways leading to the dissociation of the C—F

48

bond were the most thermodynamically favorable (Figures S1-S16). The geometries and NBO populations

49

of the optimized e adducts for PFPeA and PFBS were indicative of dissociative e attachment processes

50

resulting in weakly bound, non-covalent complexes between the fluoride and the radical anion (Figure 1). PFPeA_0 -0.02 [0.03]

-0.14 [0.12]

-0.97 [0.12]

-0.30 [0.20]

PFPeA_2 -0.89 [0.10] -0.85 [0.02]

-0.14 [0.06] 0.41 [0.73]

PFPeA_3

-0.84 [0.00]

-1.13 [0.09]

-1.14 [0.05]

0.01 [0.01]

-0.13 [0.02]

PFBS_1 -0.72 [0.05]

1.87 [0.25] -0.29 [0.50]

-0.85 [0.13] -0.13 [0.05] 0.37 [0.72]

-0.74 [0.00]

-0.23 [0.00]

-0.35 [0.06]

-0.33 [0.09]

-0.05 [0.03]

-0.07 [0.03]

-1.00 [0.00]

-0.84 [0.00] -0.12 [0.02]

-0.12 [0.01] PFBS_2

-0.04 [0.02]

0.17 [0.81]

0.02 [0.01]

-0.08 [0.01]

PFPeA_4

0.38 [0.74]

-0.33 -0.10 [0.09] [0.05]

-0.06 [0.02]

PFBS_3*

-0.93 [0.09]

-0.87 [0.12]

-0.05 [0.03]

0.37 [0.78]

0.03 [0.00]

-0.35 [0.06]

-0.12 [0.00]

PFBS_0 -1.14 [0.05]

-0.13 [0.05]

0.01 [0.00]

-0.35 [0.06]

-0.20 [0.02]

-0.04 [0.00]

-0.10 [0.05]

-0.33 [0.09]

-0.10 [0.06]

-1.00 [0.00]

PFPeA_1b*

0.38 [0.77]

-0.93 [0.09]

0.50 [0.34] -0.97 [0.12]

-1.00 [0.00]

PFPeA_1a

-0.86 [0.13]

-0.76 [0.02] -0.03 [0.00]

-0.36 [0.05]

-0.93 [0.07]

0.77 [0.77] -0.35 [0.05] -0.13 [0.04]

0.39 [0.72]

-0.30 [0.03]

0.01 [0.00]

-0.39 [0.06]

PFBS_4 -0.05 [0.03]

-0.73 [0.01]

-0.07 [0.03] -0.24 [0.01]

-0.36 [0.05]

-0.12 [0.02]

0.77 [0.77]

-0.91 [0.08]

-0.36 [0.05]

51 52 53 54 55

Figure 1. Intermediate radical-dianion conformations after electron attachment to PFPeA and PFBS with the position indicated by ‘structure_carbon#’. NBO partial atomic charges and excess α-spin densities [in brackets] are given for ‘atom groups’ except where the extra electron is present (bolded values) in which all atomic centers are given. *Denotes the lowest energy conformations using ZPE corrected energies at the B3LYP(SMD)/6-311+G(2d,2p) level.

56

Thermodynamically, the most favorable position for e insertion was directly dependent on whether the

57

compound was a carboxylate or sulfonate (Table 1). Aqueous one-electron reduction potentials were 3 ACS Paragon Plus Environment


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computed by treating the intermediates as complexed (𝐸°𝑠,

59

products; 𝐸°𝑠,

60

computed generally correlated with the BDE of the parent mono-anion’s C—F bond. Electron attachment

61

to PFPeA occurs near the α-(C1)carbon (to the head group) and results in complete dissociation of the C—F

62

bond, while for PFBS, e insertion occurs at the γ-(C3)carbon and forms a stable intermediate.

63

PFPeA_1a/1b are the most stable because the radical is resonance stabilized by the π-system of carboxylate.

64

Blotevogel et al. also predicted reduction the α-carbon for PFOA which was predicated by computing C—F

65

BDEs at the M06-2X level.13 The sulfonate’s trigonal geometry in PFBS_1a cannot impart the same π-

66

stabilization, which is evident by contrasting the spin/charge populations between PFPeA_1a/1b and

67

PFBS_1a. Desulfurization (PFBS_0), proposed in the UV-Sulfite mediated reduction of PFOS22, was also

68

found to be thermodynamically favorable. Structures akin to PFPeA_0 have been previously proposed as

69

photo-reduction intermediates23,24 and was the only observed case of non-dissociative e attachment. NBO

70

analysis of PFPeA_0 illustrates that the additional e is mostly centered on the carboxylate, indicating the

71

influence of the 𝜋𝐶∗ = 𝑂 orbitals on reactivity. A barrier-less transition state structure was identified which

72

connects PFPeA_0 and PFPeA_1b and represents one of many possible mechanisms for e attachment to

73

PFxAs leading to F discharge (Scheme 1 and Figure S18).

74 75 76 77

Table 1. Relative bottom-of-the-well (ΔErel,HF) and zero-point corrected energies (ΔErel,ZPE), the C—F bond length (Å) for each electron attachment location, the C—F bond dissociation energies of the parent molecule (kJ/mol), and the 1 e- reduction potentials for conformations shown in Figure 1 (V) (SHE=4.44V) at the B3LYP(SMD)/6-311+G(2d,2p) level of theory. *denotes lowest energy conformation.

𝑠𝑒𝑝.values

𝑐𝑜𝑚.)

or dissociated/infinitely separated (𝐸°𝑠,

𝑠𝑒𝑝.)

were less negative due to entropic effects. The aqueous reduction potentials

PFPeA_0 PFPeA_1a PFPeA_1b* PFPeA_2 PFPeA_3 PFPeA_4 PFBS_0 PFBS_1 PFBS_2 PFBS_3* PFBS_4

ΔErel,HF 131.5 2.4 0 12.3 7.3 45 44.3 8.8 8.4 0 38.5

ΔErel,ZPE 128.9 2.1 0 12.8 6.9 44.8 42 9.2 8.8 0 38.55

RC—F -3.665 4.001 2.386 2.374 2.501 -3.547 2.336 2.309 2.439

BDEC—F -439.4 441.8 450.5 452.6 490.1 -454.3 440.7 448.8 489.4

𝑬°𝒔,

𝒄𝒐𝒎.

-2.99 -1.50 -1.52 -1.74 -1.64 -2.02 -2.02 -1.61 -1.72 -1.61 -2.00

𝑬°𝒔,

𝒔𝒆𝒑.

--1.22 -1.26 -1.32 -1.34 -1.76 --1.32 -1.19 -1.26 -1.74


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78

Following the first reduction and F release, the geometry of the anion-radical and presence of a protic

79

solvent has sweeping implications on its subsequent reactivity (Scheme 1). Contrary to the mechanism

80

suggested by Qu et al., we found radical H-atom abstractions from a water molecule kinetically and

81

thermodynamically unfavorable.25 However, perfluoro-dianion species created by a second reduction can

82

abstract a proton from water in a kinetically favorable and near-thermoneutral process (Tables S1-S4).

83

Double bond formation, suggested in VB12-mediated reductive defluorination of branched PFxS,26 was

84

also observed following the second reduction of PFPeA_1a, except when explicit water molecules formed

85

hydrogen bonds with the carbanion. Full consideration of the solvent’s role in PFAS reduction was beyond

86

the scope of the current work; nonetheless, these results corroborate previously proposed mechanisms,24,27

87

including those hypothesizing that hydrogen is incorporated into the C—C backbone.23,26,28,29 O O

C F

F C

F C

F

O

e

C F2

CF3

O

F

e

PFPeA

C

F C

2

F C

F

C F2

CF3

O

F O

C

F

F C

C

C F2

F

PFPeA_1a/b

?

O

F

C

C

F

F

C F2

PFPeA_0

O

CF3 O

C F

F C H

F C

C F

C F2

CF3

H2O 2b

C F2

C

e

OH O

F C

O O

1

2a

F C

F

e

e

? O

CF3

CF3

OH O

F

F C

C F

H

C F2

CF3

88 89 90 91 92 93

Scheme 1. Proposed mechanism for two-electron reduction for PFPeA_0 in (1) organic non-protic solvents and (2a) explicit aqueous conditions for PFPeA_1a and (2b) implicit stepwise pathway for PFPeA_1b. After the first reduction and fluoride release (red atom), the carbo-radical can undergo a second reduction, releasing fluoride (blue atom) in organic solvent or in aqueous conditions can undergo reaction with water (green atoms) to form H-incorporated compounds.

94

Benchmarking Computational Methods: n-PFAs and Instantaneous Electron Attachment.

95

Concurrently, we assessed instantaneous e attachment to various (C3-C8) n-PFAs, n-PFxSs and n-PFxAs

96

(C3 excluded) by computing vertical attachment energies (VAE). VAEs, while typically computed in the

97

literature for gas phase species, were computed with the SMD continuum solvation model for a direct

98

chemical comparison. In these cases, the (aq.) descriptor will symbolize electron affinities computed with

99

the SMD model. Previously, the energies and the spatial extent of the unoccupied molecular orbitals in the

100

parent compounds provided early mechanistic conclusions concerning PFAS reduction22 and were used to

OH



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101

develop linear free energy relationships (LFERs).11 We found the distribution and energetics of first

102

occupied and next three unoccupied MOs to be method dependent for all three sub-classes of PFASs;

103

therefore, we felt it necessary to move beyond this qualitative marker (Section S8).

104

Spin density isosurfaces, which provide the spatial distribution of the additional e, showed that inner-chain

105

𝜎𝐶∗ ― 𝐹 orbitals are the most susceptible to reduction in n-PFAs (Table S5, Figure S19). From an energetic

106

standpoint, the VAE(aq)s in PFxAs did not significantly change with increasing chain length in contrast to

107

the PFxSs and n-PFAs (Figure 2). The M06-2X-based results (Figure S20) complimented these trends but

108

the B3LYP-based results showed that all three sub-classes have roughly similar VAE(aq) behavior (Figure

109

S21). The spin-density iso-surfaces and the electrostatic potential maps revealed that PFOS and

110

perfluorooctane stabilize the e along the perfluoroalkyl chain, juxtaposed to PFOA, in which the e

111

appeared delocalized across both π𝐶∗ = 𝑂 and 𝜎𝐶∗ ― 𝐹 orbitals.

n-PFA

0.4

n-PFxS

n-PFxA

0.3

VAE(aq) (eV)

0.2 0.1 C8F18

0.0 -0.1

PFOS

-0.2 -0.3

PFOA

-0.4 3

4

5

6

7

8

# of Fluorinated Carbons

112 113 114 115 116 117

Subsequently, we computed the relaxed structures for the n-PFAs (C3-C8), n-PFxAs (C4-C8) and n-PFxSs

118

(C3-C8) and repeated previous gas-phase computations for the n-PFAs12; both illustrated similar electron

119

attachment behavior (Section S5, S6; Figure S19 and Tables S6-S8). However, the DFT functional chosen

Figure 2. Aqueous vertical attachment energies of n-PFAs, n-PFxSs and n-PFxAs at the ωB97-XD(SMD)/6311+G(2d,2p) level. Inset: Electrostatic potential overlaid on the total electron density (isoval=0.001) and the alphaspin density isosurfaces (isoval=0.001) for PFOA2-•, PFOS2-• and C8F181-•.


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had notable consequences on the energies and geometries (Figure S22). The electron adducts for the PFxSs

121

and PFxAs were computed using two methods: (1) by following the path of the instantaneous attachment

122

from the parent geometry to a minimum and (2) by systematically extending each unique C—F bond of

123

each substrate. Using the latter method, e attachment to the α-carbon position was generally the most

124

favorable for the PFxAs; however, in larger PFxAs, attack in the perfluoroalkyl chain became more

125

thermodynamically favorable (Tables S9-S16). We only observed this behavior for the PFxAs (Figure 3)

126

and again noticed significant entropic effects when the PFxAs α-adducts were treated as infinitely separated

127

(Tables S17-S18). Entropic effects on the trends in Figure 3 were notable although the conclusions

128

remained the same (Figures S23-S24). The results obtained through the first method were dependent on

129

the functional form (Figures S25-S27) and is likely related to the small differences we observed in the

130

distribution of the unoccupied orbitals in the parent compounds (Section S8). For example, for PFOA,

131

ωB97-XD predicted non-dissociative e attachment at the α-carbon, B3LYP predicted non-dissociative e

132

attachment across the chain and M06-2X predicted C—F dissociation at the C5 position. Although, the 1st

133

method provided meaningful insights into the structural motifs possible after e attachment, the second

134

methodology, whose results are presented in Figure 3, allowed for a more consistent evaluation of e

135

attachment to PFAAs. Despite theory’s lack of definitive predictions, the experimental conclusions

136

regarding the chain length of a PFxA and their rate of reduction also remain unclear, e.g. no dependence

137

was noted in UV-Iodide based photoreduction24 but was in UV-Fe(CN)6 based laser flash photolysis work.30

138

Regardless, Figure 3b, Figures S23b and S24b illustrate that a relationship exists between the observed

139

rate constants in the former work and AEA(aq.)s of PFxSs and PFxAs with the same chain length.24 The

140

larger deviation observed for PFOS could be contributed with the larger errors associated with measuring

141

[PFOS], and/or that PFOS is also composed of more easily reducible branched isomers which contributed

142

to larger observed rate constants.24



a 2.58

 (PFxAs)  (PFxSs)

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lowest in chain (PFxAs) lowest in chain (PFxSs)

2.56 2.54

b

3.0x10-3 PFOS

2.5x10-3

2.50

kapp (min-1)

AEA(aq.) (eV)

2.52

2.48

2.0x10-3

1.0x10-3

2.46

PFHS

5.0x10

2.48

PFHA

PFBS

-4

2.44

PFOA

PFBA

1.5x10-3

2.50

2.52

2.54

2.56

2.58

2.60

AEA(aq) (eV)

2.42 2.40 2.38 3

4

5

6

7

8

# of Fluorinated Carbons

143 144 145 146

Figure 3. a) Comparison of AEA(aq.)s of α and the (next) most favorable attachment positions for PFxAs (black) and PFxSs (red). b) The relationship between experimental kapp values24 (UV-Iodide) and theoretical AEAs for PFxAs and PFxSs. and All values are ZPE corrected electronic energies obtained at ωB97-XD(SMD)/6-311+G(2d,2p).

147

Predicting the Redox Behavior of n-PFAAs and Beyond. The preceding analysis allowed for a

148

systematic approach to elucidate e attachment to linear PFAS, however, these parameters are not easily

149

obtained and rationalized through routine experimentation. To provide more meaningful predictions, we

150

devised a cogent, step-wise method to predict the 𝐸°𝑠 of PFASs (Scheme 2). Using 4-(perfluorobutyl)-

151

benzonitrile as an example substrate, the ωB97-XD(SMD)/6-311+G(2d,2p) method predicted an 𝐸°𝑠 within

152

0.1V of the experimental result.27 We found reasonably good agreement between theory and experiment

153

was reached when F and the radical intermediate were treated as infinitely separated species. Therefore,

154

all preceding computations were subject to this constraint. Although not an original focus of this work,

155

Scheme 2 can also predict the oxidation potential of various PFAAs using 𝐸°𝑠,𝑐𝑜𝑚. instead of a 𝐸°𝑠,𝑠𝑒𝑝. in an

156

analogous technique presented recently by Baggioli et al.31 For example, the reduction of PFOA• (𝐸°𝑠 = 2.54

157

V, comp. and exp.32) and PFOS• (𝐸°𝑠 = 3.19V comp., 3.20 exp.33) can be accurately reproduced. Baggioli

158

et al. suggested B3LYP(PCM) or TPSSh(SMD) are more suitable alternatives, and although they predict 8 ACS Paragon Plus Environment

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159

lower 𝐸°𝑠 for PFOA• (~2.2 V), it is in qualitative agreement with cyclic voltammetry experiments of PFOA

160

in acetonitrile.31,34 Accordingly, Scheme 2 in conjunction with the ωB97-XD-based method was utilized to

161

compute both the reduction and oxidation potentials for a diverse set PFAA substrates, including

162

multifunctional and branched species (Figure 4).

163 164 165 166

1-Substrate optimization at *Method(SMD)/6-311+G(2d,2p)

3-Select most probable C—F bond cleavage based on 2. Remove F, optimize radical and F as in 1

2-Compute VAE and f+ atom indices with NBO populations of 1

4-Compute:

∆𝐺 𝐸°𝑠 = 𝑛𝐹 ―𝑆𝐻𝐸

∆G = GA ― (GA• + GF ― )

*Method

𝐸°𝑠1

B3LYP

-1.22

ωB97-XD M06-2X Exp.a

-1.54 -1.93 -1.48

167 168 169 170

Scheme 2. Computing 𝐸°𝑠 of PFASs; example shown for 4-(perfluorobutyl)-benzonitrile (left). Computed standard reduction potential (𝐸°𝑠) vs. SHE (right) shown for *Method(SMD=DMF)/6-311+G(2d,2p). 1SHE = 4.44V aCombellas et al.27 f+ is the Fukui-plus function.

171

Figure 4 shows PFAA oxidation is dependent on the head group and independent of the perfluoroalkyl

172

chain length, with e loss typically occurring at/near the acidic moiety. Conversely, reduction is far more

173

structure dependent; electrons preferentially target the lowest lying 𝜎𝐶∗ ― 𝐹 orbitals, typically tertiary carbon

174

centers, and from resonance-stabilized positions, if present.12 The substrates shown in Figure 4 reflect our

175

recent work on cobalt-mediated reductive defluorination,35 and show that the presence of tertiary C—F

176

bonds does not guarantee outer-sphere reactivity, with C and D being notable exceptions. As the 𝐸°𝑠 of

177

Co(II)/Co(I) in corrin derivatives are typically measured between -0.96 and -1.36V (vs. NHE),36,37 our

178

method illustrates the substrates amenable to Co(I)-reduction. Although 𝐸°𝑠 appears to be estimated correctly

179

on a substrate level, outer sphere e transfers are considered less favorable compared to inner sphere

180

nucleophilic attack in Co-corrin mediated reduction of chlorinated organics.38,39 The kinetic factors of

181

electron transfer and a complete description of the possible inner sphere mechanisms with various reducing

182

agents cannot be ignored in future studies.

183 184



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-1.06

A

B

-0.62

F3C

F C

F C

C F

F2C

COO +2.79

F2C

CF3

D F3C

C F2

185

CF

O

C F2

O

C F2

F2 C

C F2

F2 C

COO +2.56

F C

COO

-1.31

F 3C

F 3C

+2.56

C H2

CF3

-1.10 F2 C

F C CF3

CF3

H2 C

F C

CF2

E

CF3

-1.50

F F3C

C F2

-1.31 F2 C

C

F2 C F C

C F2

F2 C

CF3

C F2

CF

C F2

G C F2

F2 C

C F2

SO3 +3.19 +3.2a

F3C

COO +1.82

COO +2.58

-1.50 F2 C

C F2

F2 C

C F2

F2 C

C F2

COO +2.54 +2.54b

186 187 188

Figure 4. Calculated 𝐸°𝑠 (in V) for various PFAS substrates. The blue atoms represent the proposed site of chemical reduction, while red represents chemical oxidation via Scheme 2. Literature values (italics) are from and aCarter and Farrell33 bGuan et al.32 Structures with red ‘X’ were resistant to Co(I)corrin defluorination as described in Liu et al.35

189

The reductive defluorination of PFAAs has a strong dependence on the polar head group and the

190

perfluoroalkyl chain length, albeit structural characteristics such as branching also play important roles in

191

in the reduction mechanism. Our computational protocol suitably predicts the reactivity of PFAAs with

192

electrons, begins to unravel their redox behavior, and will hopefully help to identify intermediates of

193

reductive degradation. Despite the complexity of electron attachment to linear PFASs, reactivity is mostly

194

dictated by 𝜎𝐶∗ ― 𝐹 orbitals and often results in C—F scission.

195

Supporting Information. Supporting information is provided as described. Geometries and bottom-of-the-

196

well HF energies are provided for the parent structures and their electron adducts to all positions for linear

197

C8F18, PFOA, and PFOS. HOMO, LUMO, LUMO+1 and LUMO+2 orbital distributions (and their

198

energies) provided for the parent molecules at each level of theory in the form of image files. This

199

information is available free of charge on the ACS Publications website.

200

Acknowledgements. This work was funded by National Science Foundation (CHE-1710079 and CHE-

201

1807739). We would like to thank the assistance of John McGroarty (undergraduate researcher) and

202

acknowledge the computational resources allocated by the high-performance computing facility at the

203

Colorado School of Mines.

204 10 ACS Paragon Plus Environment

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