Electron attachment to pentafluorophenol and pentafluoroaniline via reaction with atomic metal anions

Article abstract of DOI:10.1016/j.cplett.2014.09.026

Electron transfer to pentafluorophenol and pentafluoroaniline was effected by their reaction with the metal anions Fe^-, Cs^-, Rb^-, Ag^- and Cu^-, the latter formed by electrospray ionisation of solutions of the corresponding oxalate salt. Both electron transfer and dissociative electron attachment was observed. The results are compared to a recent electron attachment study to these two molecules by ómarsson et al. Based on the appearance of product ions in our study, clear differences are observed between the two methods of formation of the organic anions. Several reactions due to direct reaction with the metal anions are observed.

Full text of DOI:10.1016/j.cplett.2014.09.026

Chemical Physics Letters 614 (2014) 186–191
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Electron attachment to pentafluorophenol and pentafluoroaniline via
reaction with atomic metal anions
J.M. Butson, P.M. Mayer∗
Chemistry Department, University of Ottawa, Ottawa, Canada K1N 6N5
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2014
In final form 8 September 2014
Electron transfer to pentafluorophenol and pentafluoroaniline was effected by their reaction with the
metal anions Fe−, Cs−, Rb−, Ag− and Cu−, the latter formed by electrospray ionisation of solutions of the
corresponding oxalate salt. Both electron transfer and dissociative electron attachment was observed.
The results are compared to a recent electron attachment study to these two molecules by Ómarsson
et al. Based on the appearance of product ions in our study, clear differences are observed between the
two methods of formation of the organic anions. Several reactions due to direct reaction with the metal
anions are observed.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
studied by Curtis et al. suggested that the reactions between
metal anions and neutral molecules can follow three main reac-
The reactions of atomic metal anions with gas phase molecules
represent a novel frontier of chemistry, sparingly researched due
to the difficulty associated with the production of a significant flux
of atomic metal anions (AMAs). Early pioneers of this field include
Sallans et al. [1], who were able to produce V−, Cr−, Fe−, Co− and
Mo− from their corresponding parent metal-carbonyl anions in an
FT-ICR mass spectrometer. Once formed, the metal anions were
allowed to react with their neutral parent carbonyls as well as neu-
tral acid molecules, allowing the determination of the heterolytic
and homolytic bond energies of the corresponding metal hydrides
[1]. Miller et al. [2] were able to determine the gas phase acid-
ity/bond energy of FeH via reactions of Fe− with neutral acids in
a Selected Ion Flow Tube (SIFT) apparatus. Both of these studies
utilised atomic metal anions produced via collision induced dis-
sociation, limiting the resultant flux of metal anions and thus the
downstream chemistry that could be explored.
In 2010–11, Curtis et al. and Attygalle et al. discovered that
a solution containing a metal salt and oxalic acid could produce
metal anions utilising an electrospray ionisation (ESI) source in
negative mode [3–5]. This discovery enabled the reactions of K−,
Rb−, Cs−, Fe−, Co−, Ni−, Cu− and Ag− with methyl chloride, ethyl
chloride, methyl bromide, nitromethane and acetonitrile to be
probed in the collision hexapole of a bench-top triple-quadrupole
mass spectrometer [4]. The experiments on the neutral compounds
tion mechanisms: electron transfer, proton abstraction and bond
activation [4]. In 2012, Ómarsson et al. [6,7] published a study
of the direct attachment of 0–14 eV energy electrons to neutral
pentafluorophenol (PFP) and pentafluoroaniline (PFA) in the gas
phase. Ómarsson et al. [6,7] found that electron attachment to
PFP produced anionic products of m/z 184, 183, 165, 164, 145,
136, 117 and 19. Electron attachment to PFA produced anionic
products of m/z 183, 182, 163, 143, 124, 74, 26 and 19 [6,7]. In
their studies, Ómarsson et al. [6,7] found that low energy elec-
tron attachment reactions in PFP and PFA can be induced when
coupled with exothermic dissociation channels, allowing high
attachment cross sections of the anions. Furthermore, they found
that intramolecular hydrogen bonding played a pivotal role in dis-
sociative electron attachment resulting in the formation of HF from
either PFP or PFA [6,7]. Their studies provide us with a unique
opportunity to directly compare dissociative electron transfer reac-
tions using atomic metal anions with those employing isolated
electrons. It is the goal of this letter to connect and explain the
reactions that are known to be driven by electron transfer vs.
those that involve the metal anion chemically interacting with the
reactant.
2. Materials and methods
2.1. Experimental procedures
∗
Electrospray ionisation mass spectrometry experiments were
accomplished with a Micromass Quattro-LC triple-quadrupole
Corresponding author.
E-mail address: pmmayer@uottawa.ca (P.M. Mayer).
http://dx.doi.org/10.1016/j.cplett.2014.09.026
0009-2614/© 2014 Elsevier B.V. All rights reserved.

J.M. Butson, P.M. Mayer / Chemical Physics Letters 614 (2014) 186–191
187
mass spectrometer utilising a Z-spray source while running the
MassLynx 3.5 operating system. Metal oxalate solutions were pre-
pared by combining 2 moles of aqueous oxalic acid to 1 mole of a
metal salt solution at concentrations on the order of 10−1 mol/L in
methanol. Solutions were subsequently placed on a Daiger Vortex-
Genie 2 shaker and allowed to shake for approximately 30 min
before being diluted to a final working concentration of 10−4 mol/L
or lower. Solutions containing iron typically require a 3:1 ratio for
best results, likely due to the binding of two oxalic acid molecules
to the 3+ oxidation state of the iron salt used, rather than one acid
required for 1+ oxidation state metals. A flow rate of 50 ␮l/min was
used with the capillary, cone and extractor voltages set to 2.96 kV,
42 V and 7 V, respectively. The source temperature was set to 80 ◦C
in order to limit clustering [8]. The metal anion m/z is selected
with the first quadrupole. The purity of the metal anion beam is
tested via collisions with argon at 8 × 10−4 bar, using lab frame
collision energies up to 75 eV. Entrance and exit voltages of the
hexapole collision cell are set to 50 eV, collision energy remains
variable. Polyatomic impurities at the set m/z will dissociate at
these collision energies and result in fragment ions, while the pure
atomic metal anion will not yield product ions. Only if no frag-
ments are detected (indicating a pure metal anion) will the selected
m/z be allowed to undergo reactions with neutral substrates in
the hexapole collision cell in the absence of argon. Entrance and
exit voltages of the hexapole collision cell are always set to 50 eV,
with the exception of the reactions of Cu−, Ag− and Cs− with
pentafluorophenol, where they were set to 10 eV. Collision energy
remains variable throughout the gas phase experiments of AMAs
with neutral reactants. Pressures of a neutral collision gas partner
are kept constant throughout an experiment. The median inten-
sity of the peaks present in a mass spectrum is first computed to
estimate the background present. Only m/z peaks possessing inten-
sity larger than twice the median intensity are used for further
analysis.
16
0
133Cs-
8
6
4
2
0
A
183C6F5O-
184C6F5OH-
130 132 134
165C6F4OH-
145C6F3OH- 164C6F4O-
150
180
87Rb-
12
0
8
6
4
2
0
B
C
183C6F5O-
86 88 90
165C6F4OH-
145C6F3OH-
164C6F4O-
184C6F5OH-
150
10
180
6
4
2
0
56Fe-
183C6F5O-
0
184C6F5OH-
164C6F4O-
52 56 60
160
180
183C6F5O-
D
E
4
2
65Cu-
0
6
80
120
160
183C6F5O-
107Ag-
4
2
0
120
160
m/z
Figure 1. Characteristic mass spectra of (A) Cs−, (B) Rb− (C) Fe− (D) Cu− and (E) Ag−
reacting with pentafluorophenol (PFP). The Y axis represents total ion abundance
(absolute intensity) while the X axis represents mass to charge ratio (m/z). Pressures
of gaseous PFP range from: (A) 1.8 × 10−4, (B) 1.1 × 10−4, (C) 1.1 × 10−4, (D) 5 × 10−4
and (E) 5 × 10−4 torr.
3. Results and discussion
2.2. Computational methods
Throughout the discussion, the words ‘threshold energy’ will be
used to discuss the various neutral/anion product energies relative
to the starting neutral/anion reactants. The identity of the various
neutral product fragments were determined on the basis of thresh-
old energies calculated at the CBS-4M level and evidence found in
the relevant scientific literature. Calculated electron affinities of
pertinent product ions/neutral partner fragments can be found in
the supplemental section, Table S1. Reactions of Fe−, Cs−, Rb−, Ag−
and Cu− with PFP and PFA will be discussed.
Figures 1 and 2 display characteristic mass spectra from each
combination of metal anion and neutral gas phase collision part-
ner, PFP and PFA. The main reaction products are the fragments
pertaining to H and/or F abstraction for PFA and PFP respectively,
as well as the fragment associated with HF loss for PFP and the par-
ent anion of PFP. Figure 3 exhibits the collision energy dependence
of the product ions in the reaction of Fe− with PFP. Figures 4–7
provide a direct comparison of Ómarsson et al.’s electron energy
resolved attachment data with our centre-of-mass collision energy
data for common reaction products.
Reaction threshold energies were calculated with the Complete
Basis Set 4M level of theory [9–11] using the gaussian 09 suite
of programmes [12]. In this approach, geometries are optimised,
and harmonic vibrational frequencies calculated, using UHF/3-
21G(*). A series of single point energies are then calculated at
the MP4SDQ/6-31G, MP2/6-31 + G** using the minimal population
localisation method analysis, and HF/CBSB1 levels of theory. The
errors associated with CBS-4M are comparable to other methods
(mean absolute deviation of 3.26 kcal/mol on the G2/97 test set) but
can be applied to much larger systems [11] making it ideal for the
study of metal anions and their reactivity with neutral molecules.
The reaction threshold energies and electron affinities calculated in
this study were used for the comparison of various potential neu-
tral partners for a given product ion. Thus, they should be taken
as qualitative, their purpose being to determine chemical formulas
of the lowest energy possible reaction products. Pictorial repre-
sentations of the reactions highlight the lowest energy structures
used to calculate the threshold energies. Starting structures used
for calculations were initialised at the optimised geometry of the
intact neutral molecule or anion, less a combination of H, F, O or
N atoms, depending on the reaction in question. K− was used as
a surrogate for calculating threshold energies of potential reaction
products involving Rb− and Cs− due to limitations within the CBS-
4M method’s treatment of inner core electrons [9]. Similarly, only
reactions with Cu− were calculated due to limitations in dealing
with Ag−. Given the similarity in the EA of K, Cs and Rb as well as
Cu and Ag, this is not expected to result in significant error as long
as reaction intermediates are not probed.
3.1. Formation of parent anions from Cs−, Rb−, Fe−
Cs−, Rb− and Fe− were all observed to transfer an electron to
form [PFP]•− (R1) as shown below (m/z 184 in Figure 1).
(R1)

188
J.M. Butson, P.M. Mayer / Chemical Physics Letters 614 (2014) 186–191
R1) [C6F5OH]-
400
200
0
R3) [C6F5O]-
133Cs-
134
A
B
4
2
0
A
C
E
B
D
F
3
2
1
0
80
40
0
3
2
1
6
3
182C6F5NH-
130
132
136
-
164C6F4NH2
144C6F3NH-
0
(eV)
0
(eV)
0
4
8
12
0
0
0
4
8
12
140
150
160
170
180
190
R7) [C6F4O]-
R9) [C6F3O]-
87Rb-
30
15
8
20
10
0
10
5
10
5
10
5
4
0
80
85
90
95
100
164C6F4NH2
-
182C6F5NH-
0
(eV)
0
(eV)
0
4
8
12
4
4
8
12
12
[C5F4]-
[C5F3]-
150
160
170
180
190
C
D
20
10
0
160
80
0
2
1
8
4
12
6
56Fe-
113FeF3
-
0
0
0
8
4
0
(eV)
(eV)
0
4
8
12
8
60
65Cu-
80
100
120
F-
G
50
25
0
103CuF2
-
2
1
0
(eV)
70
80
90
100
110
0
4
8
12
m/z
Figure 2. Characteristic mass spectra of (A) Cs−, (B) Rb−, (C) Fe− and (D) Cu−
reacting with pentafluroaniline (PFA). The Y axis represents total ion abundance
(absolute intensity) while the X axis represents mass to charge ratio (m/z). Pressures
of gaseous PFA range from: (A) 2.2 × 10−4 and (B) 2.2 × 10−4, (C) 1.1 × 10−4, and (D)
1.7 × 10−4 torr.
Figure 4. Intensity (left/right axis) vs. electron energy (eV) of the product anions
from electron attachment (naked line, left axis) courtesy of Ómarsson et al., and
reaction with Cs− (line with circles, right axis, x-axis now in centre of mass collision
energy) to pentafluorophenol (PFP). (A) m/z 184, denoting [C6F5OH]−, (B) m/z 183,
denoting [C6F5O]−, (C) m/z 164, denoting [C6F4O]−, (D) m/z 145, denoting [C6F3O]−
,
(E) m/z 136, denoting [C5F4]−, (F) m/z 117, denoting [C5F3]−, (G) m/z 19, denoting
F
−. Pressure of gaseous PFP was set to 1.8 × 10−4 torr.
15
10
5
m/z
R1) [C6F5OH]-
R3) [C6F5O]-
164
183
184
A
C
B
100
50
0
30
15
0
6
3
0
2
1
0
(eV)
(eV)
0
5
10
0
5
10
0
R7) [C6F4O]-
R9) [C6F3O]-
0
2
4
6
8
D
2
10
5
20
10
0
6
3
0
Centre of Mass Collision Energy (eV)
1
Figure 3. Plot of the relative intensity (%) of product ions vs. centre of mass collision
energy (eV) in reactions between Fe− and PFP. Pressure of gaseous PFP was set to
1.8 × 10−4 torr.
0
0
(eV)
(eV)
0
5
10
0
5
10
Figure 5. Intensity (left/right axis) vs. electron energy (eV) of the product anions
from electron attachment (naked line, left axis) courtesy of Ómarsson et al., and
reaction with Rb− (line with circles, right axis, x-axis now in centre of mass collision
energy) to pentafluorophenol (PFP). (A) m/z 184, denoting [C6F5OH]−, (B) m/z 183,
Calculations at the CBS-4M level using K− as a stand in for Cs−
and Rb− suggest a threshold energy of 0.43 and 0.63 eV for the for-
mation of [PFP]•− and [PFA] •−, respectively (Table 1). Replacing K−
with Fe− yielded a threshold energy of −0.14 eV for the formation
of [PFP]•− (Table 1). The intensity of the PFP anion produced from
Cs−, Rb− and Fe− is such that it is possible to rule out 13C contrib-
utions from the deprotonated pentafluorophenol anion (Table S2).
In addition, the same fragment anions observed during electron
attachment experiments to PFP are observed in the reactions with
Cs−, Rb− and Fe− (Figures 1, 4 and 5), providing further evidence
suggestive of electron transfer.
denoting [C6F5O]−, (C) m/z 164, denoting [C6F4O]−, (D) m/z 145, denoting [C6F3O]−
Pressure of gaseous PFP was set to 1.1 × 10−4 torr.
.
high enough to rule out 13C contributions from the deprotonated
pentafluoroaniline anion at m/z 182 (Table S2).
(R2)
While observable in the mass spectrum of Rb− and Cs− with
PFA (Figure 2), formation of [PFA]•− in the reactions with Rb− and
Cs− (R2) cannot be confirmed since the intensity of m/z 183 is not
Fragment anions of m/z 182, 163, 143, 124, 74 were observed
in the mass spectra produced via electron attachment to PFA [6].

J.M. Butson, P.M. Mayer / Chemical Physics Letters 614 (2014) 186–191
R4) [C6F5NH]-
189
R8) [C6F4NH]-
R4) [C6F5NH]-
R8) [C6F4NH]-
A
C
E
B
D
A
C
E
G
B
D
F
4
2
4
2
20
10
0
4
20
10
0
2
1
0
10
5
2
1
0
2
0
(eV)
0
0
(eV)
0
4
8
12
12
0
4
8
12
0
4
8
12
0
4
8
12
(eV)
(eV)
R11) [C6F3N]-
[C4F4]-
R11) [C6F3N]-
[C4F4]-
8
8
4
8
2
1
20
10
0
6
3
18
2
1
4
0
4
0
9
0
0
(eV)
0
(eV)
0
(eV)
0
(eV)
0
4
8
0
4
8
12
12
0
4
4
4
8
12
12
12
0
4
8
12
[CN]-
[C3F2]-
[C3F2]-
8
4
30
15
0
6
3
30
15
0
20
10
0
4
2
0
(eV)
0
(eV)
0
(eV)
0
4
8
0
8
F-
0
4
8
12
Figure 7. Intensity (left/right axis) vs. electron energy (eV) of the product anions
from electron attachment (naked line, left axis) courtesy of Ómarsson et al., and
electron transfer from Rb− (line with circles, right axis, x-axis now in centre of mass
collision energy) to pentafluoraniline (PFA). (A) m/z 183, denoting [C6F5NH]−, (B)
m/z 163, denoting [C6F4NH]−, (C) m/z 143, denoting [C4F3N]−, (D) m/z 124, denoting
[C4F4]−, (E) m/z 74, denoting [C3F2]−. Pressure of gaseous PFA was 2.2 × 10−4 torr.
20
10
0
2
1
0
(eV)
0
8
Figure 6. Intensity (left/right axis) vs. electron energy (eV) of the product anions
from electron attachment (naked line, left axis) courtesy of Ómarsson et al., and
electron transfer from Cs− (line with circles, right axis, x-axis now in centre of mass
collision energy) to pentafluoraniline (PFA). (A) m/z 183, denoting [C6F5NH]−, (B)
m/z 163, denoting [C6F4NH]−, (C) m/z 143, denoting [C6F3N]−, (D) m/z 124, denoting
[C4F4]−, (E) m/z 74, denoting [C3F2]−, (F) m/z 26, denoting [CN]−, (G) m/z 19, denoting
3.2. Loss of H+ via formation of neutral MH
Proton abstraction was observed as a reaction product channel
for collisions of Fe−, Cs−, Rb−, Ag− and Cu− with PFP (R3) as well as
Cs− and Rb− with PFA (R4). Fe−, Cu− and Ag− were not observed to
abstract a proton from PFA.
F
−. Pressure of gaseous PFA was 2.2 × 10−4 torr.
The observation of these same m/z fragments in the mass spectra
produced via the collisions of Cs− and Rb− (Figures 2, 6 and 7) with
PFA infers that they are due to electron transfer to PFA from these
AMAs.
OH
F
O
F
F
F
F
F
F
F
F
F
+
+ MH
M
Due to the high EA of the neutral metal atom, collisions of Ag−
and Cu− did not result in electron transfer to PFP/PFA.
(R3)
Table 1
Calculated (CBS-4M) reaction threshold energies for the most likely reaction products of Fe− and K−. K− was used as a surrogate for calculations involving Cs− and Rb− due
to limitations within the computational method chosen. Approximate electron attachment resonance energies (eV) for similar product anions observed by Ómarrsson et al.
during dissociative electron attachment experiments to PFP and PFA are included for comparison.
Reaction number
Incident anion
Collision partner
Neutral product
Product anion
m/z
Threshold energy
eV
Ómarrson et al.
eV
Cu−
Fe−
C6F5OH
C6F5OH
(R3)
CuH
[C6F5O]−
183
−0.88
0
(R1)
(R3)
(R7)
Fe
FeH
Fe + HF
[C6F5OH]−
[C6F5O]−
184
183
164
−0.14
−1.63
0.16
0
0.4–2.0
0
[C6F4OH]−
K−
C6F5OH
(R1)
(R3)
(R5)
(R7)
(R9)
K
KH
KF
K + HF
KFHF
[C6F5OH]−
[C6F5O]−
[C6F4OH]−
[C6F4O]−
[C6F3O]−
184
183
165
164
145
0.43
−0.63
−2.33
0.72
0
0.4–2.0
0
−2.32
K−
C6F5NH2
(R2)
(R4)
(R6)
(R8)
(R10)
(R11)
K
KH
KF
K + HF
KFHF
K + 2HF
[C6F5NH2]−
[C6F5NH]−
[C6F4NH2]−
[C6F4NH]−
[C6F3NH]−
[C6F3N]−
183
182
164
163
144
143
0.63
0.46
0.9
0.6–1.6
−1.97
0.61
0–1.0
−0.83
2.74
1.6–2.0

190
J.M. Butson, P.M. Mayer / Chemical Physics Letters 614 (2014) 186–191
3.4. Loss of F+ via formation of neutral MF
NH2
F
NH
F
F
F
F
F
F
F
F
F
Only Cs− and Rb− were observed to produce fluorine cation loss
from PFP and PFA (R5) and (R6).
+
+ MH
M
(R4)
OH
OH
F
F
F
F
F
F
F
+
+
+ MF
+ MF
M
M
There are a number of possible pathways for the reactions
resulting in H+ loss (Table S3). The lowest energy pathway for a
loss of H+ is abstraction by the metal anion. Calculated threshold
energies suggest that H+ abstraction from PFP and PFA (R3) is 1.7 eV
more favourable than producing a neutral M and H atom from PFP•−
and PFA•− via a dissociative pathway. H+ abstraction from PFP via
Fe− and Cu− is energetically favourable by approximately 2.2 and
4.7 eV, respectively. Thus abstraction, rather than dissociative elec-
tron transfer, is likely occurring. This is consistent with the collision
energy dependence of this reaction, which occurs near the collision
energy threshold, while dissociative electron attachment from PFP
requires upwards of 4 eV (Figures 4 and 5) [6,7]. Ag− cannot be mod-
elled using CBS-4M methods [9,10]. Thus the energetics of forming
AgH via H+ abstraction by Ag− (R3) can only be inferred based on
the similarity between the EA of Ag and Cu. Further evidence sup-
porting H+ abstraction is the relatively low abundances of neutral
H dissociation in electron attachment experiments compared with
the high abundances noted in the reactions with Cs−, Rb−, Fe−, Cu−
and Ag− with PFP as well as Cs− and Rb− with PFA (Figures 1 and 2)
[6]. Thus, it can be stated that the metal anions react with PFP and
PFA by the formation MH via proton abstraction.
F
F
(R5)
(R6)
NH2
NH2
F
F
F
F
F
F
F
F
F
There are a number of possible pathways for the reactions
resulting in F+ loss (Table S3). As showcased above, the lowest
energy pathways for a loss of F+ is F+ abstraction via the metal
anion. Calculations predict that F+ abstraction from PFP and PFA
is ∼5.0 eV more favourable than a pathway involving the dissocia-
tion of PFP•− and PFA•−. Further evidence supporting F+ abstraction
is the fact that no neutral F dissociation was observed in electron
attachment experiments with PFP and PFA (Figures 4 and 6) com-
pared with the high abundances noted in the reactions with Cs−
and Rb− (Figures 1 and 2).
3.5. Loss of HF after electron transfer to PFP and PFA
The formation of a metal hydride via proton abstraction was
not observed during collisions of Fe−, Cu− and Ag− with PFA. The
gas-phase acidities, ꢀacid Ho (ꢀH for the reaction MH → M− + H+)
relate to the heterolytic bond strengths in neutral molecules. Since
all of the acidities are similar, Ag− (ꢀacid Ho (AgH) = 14.57 0.08 eV
[13]), Rb− (ꢀacid Ho (RbH) = 14.890 0.022 eV [13]), Cs− ꢀacid
Ho (CsH) = 14.9428 0.00004 eV [13]), Fe− ꢀacid Ho (FeH)
14.91 0.19 eV [13]) and Cu− (ꢀacid Ho (CuH) = 15.04 0.11 eV
[13]), acidities alone cannot explain the fact that deprotonation is
observed for Rb− and Cs−, but not for the transition metals. Rather,
Fe− and Cu− can access alternate reactions forming [FeF3]− and
[CuF2]− that can out-compete deprotonation. For Ag−, the high EA
means it remains unable to participate in this chemistry.
The mass spectra resulting from Rb−, Cs− and Fe− colliding with
neutral PFP or PFA indicates that loss of HF•+ from PFP and PFA
takes place (Figures 1 and 2). Calculated reaction threshold energies
(Table S3) suggest that electron transfer leading to PFP•− or PFA•−
,
followed by HF elimination, is the least endothermic (Table 1).
Experimentally this is observed via the collision of Fe− with PFP.
Referring to Figure 3, the appearance of [C6F4O]− corresponds with
the appearance of [C6F5OH]−, suggesting the formation of these
two ions are related, which would only be the case if [C6F4O]− is
formed directly from [C6F5OH]•−. The dissociative electron trans-
fer reaction producing the loss of HF from either PFP•− or PFA•− are
presented below (R7) and (R8).
OH
OH
O
F
F
F
F
F
F
F
F
F
F
F
F
+
+ M
+ M + HF
M
3.3. Formation of metal-fluorine anions
F
F
The product anions [FeF3]− and [CuF2]− were the only ones
observed from collisions of Fe− and Cu− with PFA at any centre
of mass collision energy explored (Figure 2C and D). FeH and CuH
have higher acidities than Cs− and Rb−, yet both Cs− and Rb−
reacted to produce neutral hydrides while Fe− and Cu− did not.
This suggests that [FeF3]− and [CuF2]− formation are kinetically
favoured relative to neutral hydride formation. The reactions of
hydride formation in both cases are endothermic, with ꢀH for
(R4) being 0.21 0.28 eV for Fe− and 0.08 22 eV for Cu−. Thus
it is likely that halogen abstraction is an exothermic process.
The observation of these multiply substituted products without
(R7)
NH2
F
NH2
F
NH
F
F
F
F
F
F
F
F
F
F
F
F
+
+ M
+ M + HF
M
the observation of FeF−, FeF2 or CuF− suggests that these are
(R8)
−
concerted reactions and not stepwise reactions requiring multiple
collisions with PFA. Calculations at the CBS-4M level suggest a
trigonal planar and linear geometry for the product anions [FeF3]−
and [CuF2]−, respectively, represented pictorially below.
Direct observation of PFA− is not possible due to 13C contrib-
utions from the deprotonated species, and thus the onset energies
of [C6F4NH]− and [C6F5NH2]− corresponding to HF loss and par-
ent anion formation from PFA cannot be compared, but must be
inferred based on chemical similarity to PFP and evidence from
the study by Ómarsson et al. [6] suggesting a dissociative electron
transfer, rather than an abstraction process, is taking place.
F
F
Fe
F
F
Cu
F

J.M. Butson, P.M. Mayer / Chemical Physics Letters 614 (2014) 186–191
191
Cu− and Ag− did not react to produce any dissociative electron
transfer products. This is likely due to their higher electron affinity
relative to the other metals tested.
in these reactions. In addition, other metal-based reactions such as
H+, F+ or FHF+ abstraction (Table 1) have been identified.
The trend observed within the calculations associated with the
reactions of Cs− and Rb− with PFP and PFA suggest that product
anions resulting from dissociative electron transfer are relatively
endothermic when compared to reactions where the metal anion
plays a direct role. This is experimentally evident in Figures 4–7
where metal-mediated fragments appear at low centre-of-mass
collision energies (Figures 4B and D, 5B and D, 6A and 7A), whereas
the related dissociative electron transfer fragments appear at high
electron energies (Figures 4A, E and F, 6B–F and 7B–F). Under-
standably, the comparison of electron and collision energy in these
two experiments is fraught with uncertainty, but the trends are
consistent with the theoretical results. The formation of F− is the
only dissociative electron transfer fragment observed at 0 eV cen-
tre of mass collision energy that is observed at higher electron
attachment energies (Figures 4G and 6G). A novel, metal-mediated
reaction channel could be the root of this observation.
3.6. Loss of FHF+ from PFP and PFA
The final reaction channel to be discussed involving both PFP and
PFA involves the loss of FHF+. The two reaction channels with the
lowest threshold energies (Table S3) involve the concerted abstrac-
tion of FHF+ from neutral PFP and PFA by the metal anion and the
abstraction of an F+ atom followed by the dissociation of an HF
molecule from the de-halogenated organic ion. The threshold ener-
gies of these reactions are separated by ∼1.20 eV, favouring the
formation of M-FHF rather than the two step process required for
the formation MF followed by the loss of HF. The FHF+ abstraction
reaction is presented in reactions (R9) and (R10).
OH
O
F
F
F
F
Experimental evidence provided in this communication indi-
cates that metal anions are able to undergo an extensive suite of
chemical reactions that are similar in character to low energy elec-
tron attachment as well novel reactions involving molecular H and
F atom abstraction.
+
+
+ MFHF
+ MFHF
M
M
F
F
F
F
(R9)
NH2
NH
F
F
F
F
F
Acknowledgements
F
F
F
(R10)
PMM thanks the Natural Sciences and Engineering Research
Council of Canada for continuing financial support. Acknowledge-
ment is made to the Donors of the American Chemical Society
Petroleum Research Fund for partial support of this research. The
authors are grateful to Ómarsson et al. for providing original data
for Figures 4–7.
FHF loss is observed in electron attachment experiments with
PFP at electron energies of 8 eV (Figure 4) and is not observed from
electron attachment to PFA (Figure 6) [7]. The loss of FHF is observed
in metal anion collision experiments with PFP and PFA at 0 eV, indi-
cating that a different mechanism, that of metal anion abstraction,
is occurring in these systems.
Appendix A. Supplementary data
4. Loss of 2 HF from PFA
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2014.09.026.
There are many potential reaction channels that could result
in [PFA-2(HF)]− (Table S3). The lowest energy reaction, based on
threshold energies involves electron transfer to PFA followed by
the sequential loss of 2 HF molecules (R11).
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N
F
F
F
F
F
F
F
F
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F
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+ M
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5. Conclusions
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Global similarities in the fragmentation products of PFP and
PFA from collisions with Cs−, Rb− and Fe− in comparison to those
observed by Ómarsson et al. [6,7] from direct electron attachment
is a strong indication that dissociative electron transfer is occurring