Formation of Temporary Negative Ions and Their Subsequent Fragmentation upon Electron Attachment to CoQ0 and CoQ0H2

Abstract

Ubiquinone molecules have a high biological relevance due to their action as electron carriers in the mitochondrial electron transport chain. Here, we studied the dissociative interaction of free electrons with CoQ0, the smallest ubiquinone derivative with no isoprenyl units, and its fully reduced form, 2,3‐dimethoxy‐5‐methylhydroquinone (CoQ0H2), an ubiquinol derivative. The anionic products produced upon dissociative electron attachment (DEA) were detected by quadrupole mass spectrometry and studied theoretically through quantum chemical and electron scattering calculations. Despite the structural similarity of the two studied molecules, remarkably only a few DEA reactions are present for both compounds, such as abstraction of a neutral hydrogen atom or the release of a negatively charged methyl group. While the loss of a neutral methyl group represents the most abundant reaction observed in DEA to CoQ0, this pathway is not observed for CoQ0H2. Instead, the loss of a neutral OH radical from the CoQ0H2 temporary negative ion is observed as the most abundant reaction channel. Overall, this study gives insights into electron attachment properties of simple derivatives of more complex molecules found in biochemical pathways.

Full text

Formation of Temporary Negative Ions and Their
Subsequent Fragmentation upon Electron Attachment to
CoQ0and CoQ0H2
João Ameixa,*[a, b, c] Eugene Arthur-Baidoo,[a, b] João Pereira-da-Silva,[c] Júlio C. Ruivo,[d]
Márcio T. do N. Varella,[d] Martin K. Beyer,[a] Milan Ončák,[a] Filipe Ferreira da Silva,[c] and
Stephan Denifl*[a, b]
Ubiquinone molecules have a high biological relevance due to
their action as electron carriers in the mitochondrial electron
transport chain. Here, we studied the dissociative interaction of
free electrons with CoQ0, the smallest ubiquinone derivative
with no isoprenyl units, and its fully reduced form, 2,3-dimeth-
oxy-5-methylhydroquinone (CoQ0H2), an ubiquinol derivative.
The anionic products produced upon dissociative electron
attachment (DEA) were detected by quadrupole mass spec-
trometry and studied theoretically through quantum chemical
and electron scattering calculations. Despite the structural
similarity of the two studied molecules, remarkably only a few
DEA reactions are present for both compounds, such as
abstraction of a neutral hydrogen atom or the release of a
negatively charged methyl group. While the loss of a neutral
methyl group represents the most abundant reaction observed
in DEA to CoQ0, this pathway is not observed for CoQ0H2.
Instead, the loss of a neutral OH radical from the CoQ0H2
temporary negative ion is observed as the most abundant
reaction channel. Overall, this study gives insights into electron
attachment properties of simple derivatives of more complex
molecules found in biochemical pathways.
Introduction
The mitochondria are organelles best known for their role in
producing adenosine triphosphate (ATP), that upon conversion
to adenosine diphosphate (ADP) releases energy required to
power most processes in cells. These organelles, in addition to a
chain of four proteins embedded in the inner mitochondrial
membrane, identified as complexes I, II, III and IV, make use of
mobile molecules – ubiquinone (also called coenzyme Q10) and
cytochrome c– to shuttle electrons down the chain. This
electron-transport chain generates an electrochemical proton
gradient across the inner mitochondrial membrane.[1] The
enzyme ATP synthase harnesses the energy stored in the
electrochemical proton gradient to synthesize ATP. As de-
scribed by Morton,[2] the simplest electron carrier agent
involved in the electron transport chain is coenzyme Q10
(CoQ10). This molecule consists of a p-benzoquinone (p-BQ,
C6H4O2) derivative head-group accounting for the electron
transfer ability, with an attached side group of ten isoprenoid
units making this molecule mobile within the inner mitochon-
drial membrane. In a multi-step reaction, each complex I and II
transfers one electron to a CoQ10 molecule reducing it to the
intermediate ubisemiquinone radical CoQ10H*, and then to the
fully reduced form known as ubiquinol, CoQ10H2(see Figure 1).
Subsequently, complex III (cytochrome bc1) receives a pair of
[a] Dr. J. Ameixa, Dr. E. Arthur-Baidoo, Prof. Dr. M. K. Beyer, Dr. M. Ončák,
Prof. Dr. S. Denifl
Institut für Ionenphysik und Angewandte Physik
Leopold-Franzens Universität Innsbruck
Technikerstraße 25, 6020 Innsbruck, Austria
E-mail: j.ameixa@campus.fct.unl.pt
Stephan.denifl@uibk.ac.at
[b] Dr. J. Ameixa, Dr. E. Arthur-Baidoo, Prof. Dr. S. Denifl
Center for Biomolecular Sciences Innsbruck (CMBI)
Leopold-Franzens Universität Innsbruck
Technikerstraße 25, 6020 Innsbruck, Austria
[c] Dr. J. Ameixa, J. Pereira-da-Silva, Prof. Dr. F. Ferreira da Silva
Centre of Physics and Technological Research
Departamento de Física, Faculdade de Ciências e Tecnologia
Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal
[d] J. C. Ruivo, Prof. Dr. M. T. d. N. Varella
Instituto de Física
Universidade de São Paulo
Rua do Matão 1731, 05508-090 São Paulo, Brazil
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cphc.202100834
© 2022 The Authors. ChemPhysChem published by Wiley-VCH GmbH.
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
Figure 1. Pathways for the formation of the three forms of the electron
carrier ubiquinone, CoQ10.
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electrons from CoQ10H2to regenerate CoQ10.[3] In chloroplasts,
plastoquinone, a p-BQ derivative, acts as electron carrier in the
light-dependent reactions of photosynthesis.[4,5] More recently,
the potential use of quinones derivatives for energy harvesting
and storage in batteries has been investigated.[6,7]
The ability of an electron carrier molecule to attach an
electron depends on the availability of a low-lying vacant
molecular orbital.[8] The capture of an electron with kinetic
energy below the ionization potential, and thus termed low-
energy electron (LEE), by a target molecule leads to the
formation of a temporary negative ion, TNI#, also called
resonance. Since TNIs are formed in an electronically or vibra-
tionally excited state (as indicated by the superscript #), they
tend to subsequently decay within 1015 up to 102s.[9,10] The
decay of a TNI#into anionic fragments and one or more
neutral counterparts through dissociative electron attachment
(DEA) takes place when the time required for dissociation is
sufficiently shorter compared to the time for spontaneous
emission of the extra electron (autodetachment)[11] which often
yields the neutral parent molecule in an excited state.[12–14] The
DEA process was shown to play an important role in radiation
damage of biomolecules upon interaction of ionizing radiation
with matter, including DNA,[15] proteins[16] as well as many
radiosensitizing agents.[17–20] It should be noted, however, that
already a small hydration shell changes both energetics and
dynamics of the DEA process.[21,22]
The lifetime of the TNI#is conferred by the electron capture
mechanism of the target molecule. If the incoming electron
electronically excites the target molecule, a core-excited TNI#is
formed by concomitant capture of the incoming electron in a
previously vacant molecular orbital (MO). In the gas-phase, the
lifetime of a core-excited Feshbach resonance is relatively long
with respect to the time for autodetachment, which in turn
favors the dissociation of the TNI#, i. e. DEA.[23] Otherwise, a
shape resonance is produced when the incoming electron is
captured by the target molecule in a potential barrier created
by the electron-molecule interaction. Usually, a TNI#created by
a shape resonance has a lifetime ranging from 1015 up to
1010 s.[12,14]
The formation of resonances by electron attachment to p-
BQ as well as their lifetimes and decay channels have been
studied extensively using a broad range of experimental
methods, such as electron attachment or transmission
spectroscopy,[24–33] photoelectron spectroscopy,[34–36] action
spectroscopy,[37–39] as well as theoretical calculations, namely
quantum chemical models[40–42] or electron scattering
calculations.[43–45] Based on time-resolved photoelectron spec-
troscopy and ab initio calculations, Horke et al.[46] demonstrated
that the excitation of gas-phase p-BQ*anions at 2.58 eV
(480 nm) and 3.10 eV (400 nm) yields excited states, namely the
shape resonance 2Aulying at 0.7 eV and the core-excited
resonance 2B3u at 1.35 eV above the ground state of the neutral,
that decay eventually on a sub-40 femtosecond timescale via a
series of conical intersections to the ground state of the anion,
thereby preventing autodetachment. Further, with the same
combination of methods, Bull et al.[47] showed that a similar
mechanism is also operative in CoQ0(2,3-dimethoxy-5-methyl-
p-benzoquinone), an analogue of CoQ10 holding no side-chain.
Although p-BQ has an electron affinity of about 1.91 eV,[48] the
formation of a long-lived molecular anion p-BQ*through
stabilization of the core-excited resonance 2B3u at about 1.35 eV
was reported by several attachment studies, while no attach-
ment of thermal electrons was observed.[25,26,33,49] In addition to
the molecular anion p-BQ*, Khvostenko et al.[50] identified more
than twenty DEA channels. Finally, the recent electron attach-
ment study by Pshenichnyuk et al.[51] (using a standard ion
source with magnetic mass spectrometer for mass analysis of
anions) involving shorter-tail analogues CoQn(n=1, 2, 4)
revealed that elongation of the side chain leads to an increase
of the lifetime of the isolated molecular anions CoQ�
nformed
at 1.2 eV as well as to a reduction of the efficiency of
dissociative pathways producing fragment anions.
In the present study, we assess the formation of TNIs and
their subsequent decay into anionic fragments through the
study of the interaction of LEEs with the smallest analogue
CoQ0in the gas phase. Anion efficiency curve for fragment
anions resulting from the decay of TNI states are determined
with a crossed electron-molecular beam setup coupled with a
quadrupole mass spectrometer. Quantum chemical calculations
provide thermochemical thresholds for subsequent comparison
with the experimentally determined thresholds of the observed
DEA reactions. Calculations on elastic electron scattering by
CoQ0as well as empirical correction of orbital energies are
employed to predict the position of short-lived TNI states.
Moreover, we further studied electron attachment to the
reduced analogue of CoQ0, the hydroquinone derivative CoQ0H2
(2,3-dimethoxy-5-methylhydroquinone). A comparison between
both CoQ0and p-BQ, CoQ0H2and hydroquinone (HQ, C6H4(OH)2)
will also be provided, respectively.
Experimental Section
Dissociative electron attachment set-up
The anion efficiency curves for mass-selected fragment anions
formed in electron attachment to CoQ0and CoQ0H2were measured
with a crossed molecular-electron beam setup, which consists of a
hemispherical electron monochromator (HEM) coupled with a
quadrupole mass spectrometer.[17] CoQ0(182 u) and CoQ0H2(184 u)
sample were purchased from Sigma-Aldrich and placed as received
in an external container kept at 313 K. Given that CoQ0/CoQ0H2
exist in a natural equilibrium, each sample might contain traces
quantities of the other. From electron ionization mass spectra at
the electron energy of 70 eV carried out before the measurements
for anions, we observe a negligible contribution of <0.2 % of CoQ0
in the CoQ0H2sample at 313 K. The sample vapor is introduced into
the interaction region of the HEM by a 1 mm-diameter stainless
steel capillary attached to a gas inlet coupled with a precision valve.
Within the interaction region of the HEM, the effusive neutral beam
crossed orthogonally with the electron beam. The anions formed
therein were extracted by a weak electrostatic field towards the
quadrupole mass spectrometer for mass selection. At last, a channel
electron multiplier operated in single-pulse counting mode was
employed for ion detection. For a given mass-selected anion, a
complete anion efficiency curve was measured in the electron
energy range of 0 to 20 eV as a first step, and subsequent
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measurements were performed only for the electron energies
showing ion yield intensity. The HEM was tuned to generate an
electron beam with an energy resolution of 120 meV for trans-
mitted electron currents of 5 up to 30 nA as monitored with a
picoammeter connected to a Faraday plate placed after the
interaction region. The electron energy resolution was determined
by measuring the full-width at half maximum of the well-known
~0 eV resonance for the formation of Clfrom CCl4.[52] This reaction
was also used to calibrate the electron energy scale of the anion
efficiency curves. Finally, the experimental onsets for the observed
fragments were determined from the Gaussian fittings of the ion
yields, as presented elsewhere.[53] The ion yields presented in this
work are shown in arbitrary units (a.u.) and thus ion intensities are
not comparable between CoQ0and CoQ0H2.
Theoretical Methods
Different structures of the CoQ0molecule were searched using the
engine built in the Avogadro software[54] and subsequently
optimized with density functional theory (DFT), employing the
B3LYP functional and the aug-cc-pVDZ basis set. We obtained three
conformers A–C, see Figure 2, that all lie within 4 meV. The
differences in geometry are not expected to significantly impact
the π* resonances of interest. The calculated vertical electron
affinity (VEA, see below) does not considerably differ among the
three conformers (<0.1 eV), suggesting similar spectra of π* shape
resonances. The dipole moment of the conformer A is significantly
smaller than those of the conformers B and C. We have chosen the
most stable A conformer for further calculations as (i) the geometry
would not significantly impact the positions of the shape
resonances; and (ii) the smaller dipole moment is expected to make
the signatures of the anion states more evident in the integral cross
section (ICS), in view of the smaller dipolar contribution to the
background. For the electron scattering calculations, we utilized the
Schwinger Multichannel method[55] implemented with the Bachelet-
Hamann-Schlüter pseudo-potentials,[56] SMCPP. The scattering wave
function was expanded in a basis of configuration state functions
(CSFs), i.e., spin-adapted (N+1)-electron Slater determinants. In the
static-exchange (SE) approximation, the trial basis set only
comprises CSFs of the type F0ij jfji, where F0is the target ground
state, described in the Hartree-Fock (HF) level, and fjis a scattering
orbital (the product is properly anti-symmetrized). Since the target
is kept frozen in the SE approximation, the correlation-polarization
effects are not accounted for. In the static-exchange plus polar-
ization approximation (SEP), the CSF space is augmented with
configurations given by Fnij jfji, where Fnis a singly excited target
state. While we considered both singlet- and triplet-coupled
excitations, only CSFs with total spin S¼1=2 were included in the
calculation. The SEP configuration space was chosen based on the
energy criterion described by Kossoski and Bettega[57] and was
composed of 14131 CSFs. Modified virtual orbitals (MVOs) obtained
from cationic cores with charge +6ewere employed as particle
orbitals for the target excitations as well as scattering orbitals. The
Gaussian basis sets employed in the target and scattering
calculations were the same described in previous study with the p-
BQ molecule, see Ref..[45] The target geometry used in the scattering
calculations was optimized at the MP2/aug-cc-pVDZ level with the
Gaussian09 package.[58]
The characters of the resonance states of CoQ0were assigned from
the inspection of the pseudo-eigenstates of the scattering Hamil-
tonian represented in the CSF space. Especially for narrow
resonances, there is usually one such pseudo-state that lies close in
energy (<0.2 eV) to the ICS peak related to the resonance, with
large coefficients on a few CSFs, typically one 1 to 3, given by
products of the target ground state with a MVO. To a reasonable
approximation, the orbital occupied by the additional electron to
form the shape resonance (�res) can be written as �res ¼Sicici,
where ciare the singly occupied VOs of the CSFs of interest, and
the coefficients ciare taken from the pseudo-eigenstate. VEA
estimates, computed as empirically corrected virtual orbital ener-
gies, were also obtained according to Scheer and Burrow.[59] The
geometry optimizations and virtual orbital energy (VOE) calcula-
tions were calculated with the B3LYP/6-31G* model.
The shape and core-excited resonances of the CoQ0molecule were
also computed with the complete active space self-consistent-field
(CASSCF) method implemented in the OpenMOLCAS package.[60]
The dynamic electronic correlation was further accounted for with
second-order perturbation theory (CASPT2). The calculations used
the extended relativistic atomic natural orbital (ANOL) basis set in
the [3s2p1d] contraction scheme. The active space comprised the
occupied πand norbitals as well as the virtual π* orbitals,
amounting to 13 electrons and 10 orbitals, i. e., CASSCF(13,10). The
CASPT2 calculations used the state-average CASSCF wave function
as reference, with the imaginary shift[61] set to 0.2 a.u. The
ionization-potential electron-affinity (IPEA) shift was not employed,
as recommended by Zobel et al.[62] for organic chromophores.
In Figure 2, selected isomers of CoQ0H2are also shown, DF. Here,
energy differences are more pronounced and depend on the
orientation of OH groups; VEAs are negative due to the limited
basis set. Energies of reaction channels upon electron attachment
were calculated at the B3LYP/aug-cc-pVDZ level, and the stability of
the wave function was tested for all structures. The character of
located structures was confirmed through vibrational analysis,
relative and reaction energies as well as electron affinities include
zero-point correction; vertical electron attachment energies are
reported without the correction. DFT calculations were performed
in the Gaussian software package.[58]
Results and Discussion
Using mass spectrometry, we have identified the anionic
species formed by electron attachment to CoQ0and its reduced
form, CoQ0H2. Also, the intact CoQ0molecular anion was
observed; however, in the present contribution, we focus on
Figure 2. Relative energy E, dipole moment μ, vertical electron affinity (VEA)
and electron affinity (EA) in three optimized isomers of CoQ0(A–C) and
CoQ0H2(D–F). Calculated at the B3LYP/aug-cc-pVDZ level.
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the DEA pathways alone. In contrast, the molecular anion of
CoQ0H2is not observed in the present experiment. This result is
supported by the present calculations that predict a negative
electron affinity for CoQ0H2(note that the value depends
considerably on basis set quality and the negative value may be
a computational artefact). In such case, fast autodetachment
prevents the experimental detection in a μs timescale. DEA to
CoQ0produced six detectable anion fragments through (i)
single-bond ruptures, namely dehydrogenation and single as
well as double demethylation reactions, and (ii) rearrangement
reactions that yield the fragment anions observed at m/z124
and m/z108. The observed anions produced in DEA to CoQ0are
listed in Table 1, along with the positions of peak maxima
observed in the anion efficiency curves, experimental onsets
and calculated thermochemical thresholds. It should be noted
that the thermochemical thresholds predicted at the B3LYP
level consider the initial (neutral molecule) and final states
(negative ion fragment and neutral counterpart(s)) of the DEA
process. However, rearrangement reactions might involve
transition states above the thermochemical threshold which are
not considered at the B3LYP level of theory. For CoQ0H2, we
have observed the formation of seven anionic fragments
resulting from single-bond cleavages, namely the dehydrogen-
ated molecular anion, as well as a set of fragments formed due
to the release of OH, OH, O, and CH3. In addition, more
complex reactions involving several bond ruptures and rear-
rangement reactions yield two fragment anions at m/z152 and
m/z26. Table 2 summarizes the properties of the observed
anions upon DEA to CoQ0H2. Figure 3 shows possible molecular
structures of the dissociation products produced due to DEA to
both a) CoQ0and b) CoQ0H2.
Electron scattering calculations for conformer A of CoQ0are
shown in Figure 4. The SE calculations are known to over-
Table 1. Mass-to-charge ratio (m/z) of the fragment anions formed upon
DEA to CoQ0, along with the peak positions comprising the ion yields
(sorted by increasing energy) as well as the experimental onsets and
thermochemical thresholds obtained (zero-point corrected reaction ener-
gies) at the B3LYP/aug-cc-pVDZ level of theory.
Peak position
[eV]
Thermochemical threshold
[eV]
m/z Anion 1. 2. 3. Exp. Theory
181 (CoQ0
H)1.8 7.0 1.1 1.05
167 (CoQ0
CH3)1.8 0.8 -0.85
152 (CoQ0
2CH3)2.9 3.5 5.2 2.2 0.78
124 C6H4O-
35.5 6.6 4.6 3.08
108 C6H4O25.3 7.0 9.3 3.7 2.04
15 CH38.1 9.7 6.0 1.90
Table 2. Mass-to-charge ratio (m/z) of the fragment anions formed upon
DEA to CoQ0H2, along with the peak positions comprising the ion yields
(sorted by increasing energy) as well as the experimental onsets and
thermochemical thresholds obtained (zero-point corrected reaction ener-
gies) at the B3LYP/aug-cc-pVDZ level of theory.
Peak position[a,b]
[eV]
Thermochemical thresh-
old [eV]
m/z Anion 1. 2. 3. Exp. Theory
183 (CoQ0H2
H)1.7 2.4 1.3 1.31
167 (CoQ0H2
OH)1.6 0.7 0.29
152 (CoQ0H2
CH3OH)2.5 3.5 4.7 1.7 0.76
26 C2H22.0 5.9 2.9 2.95
17 OH7.0 9.3 4.0 2.58
16 O4.0 4.13
15 CH39.0 6.3 1.63
[a] The peak at 2.0 eV for the anion with m/z 26, C2H2, can be assigned to
an impurity, see text for further details. [b] The exact peak positions for
the anion with m/z 16, O, are not given due to the relatively weak ion
yield intensity.
Figure 3. Suggested dissociation pathways in a) CoQ0and b) CoQ0H2as
calculated at the B3LYP/aug-cc-pVDZ level.
Figure 4. Integral cross section (ICS) for elastic electron scattering by the
CoQ0molecule. The red and blue lines correspond, respectively, to the
results obtained in the SE and SEP approximations. Virtual orbitals associated
with the shape resonances (π2* and π3*) are also shown. The arrows connect
these plots to the SE and SEP level peaks arising from those anion states,
and the peak positions in both approximations are indicated alongside the
orbital plots. In the SEP cross section, we also assign the peaks around
2.50 eV, 3.08 eV, 3.61 eV and 3.78 eV to Feshbach (F) resonances.
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estimate the energies of shape resonances since the dynamical
response of the target electrons to the projectile (correlation-
polarization effects) is neglected. The SE-level cross section
points out three resonances, labelled π1* to π3* in order of
increasing energy, around 0.05 eV, 3.5 eV and 4.2 eV. The SEP
results, which incorporate the correlation-polarization effects,
show only two shape resonances, at 1.30 eV (π2*) and 2.06 eV
(π3*). The lowest lying π1* anion state is stable and found at
1.9 eV below the neutral form, according to the diagonalization
of the scattering Hamiltonian represented in the CSF basis
employed in the calculations.
The orbitals in Figure 4 provide insights into the resonance
characters of the CoQ0molecule. Although they were obtained
with a compact basis set (B3LYP/6-31G* calculation), their
amplitudes are consistent with those of the resonance orbitals,
as inferred from the pseudo-eigenstates of the scattering
Hamiltonian. The SEP cross sections in Figure 4 also show
several higher-lying peaks, starting at about 2.5 eV. Since these
structures have no counterparts in the SE cross sections, they
might be either pseudo-resonances, which can arise because
the open electronic channels are treated as closed in the SEP
calculations, or Feshbach resonances. Most of the structures
could be assigned as Feshbach resonances employing the same
procedure described in Ref.[45] for p-BQ.
However, the description of Feshbach resonances in SEP
calculations is generally poor. The resonance positions are
significantly overestimated since the parent states are described
by single excitations. In view of these limitations, we also
performed CASSCF/CASPT2 calculations for the CoQ0anion
isomer. To this end, we considered the geometry of the neutral
isomer A optimized with second order Møller-Plesset perturba-
tion theory (MP2). The results are shown in Table 3, along with
the SEP results. The Feshbach resonance positions obtained in
the SEP calculations are largely overestimated with respect to
the CASPT2 excitation energies, as expected, and some states
described by the latter method are not present in the SEP-level
cross section. The CASPT2 results also indicate a resonance with
mixed character at 1.15 eV, (π3*)1/(π3*)1, (π1*)2, which corre-
sponds to two resonances with predominant shape (2.06 eV)
and Feshbach (3.08 eV) characters in the SEP computations. The
lowest-lying triplet states at the CASPT2 level lie around 2.58 eV
to 2.81 eV, so the CASPT2 anion states around and above 3.5 eV
have core-excited shape character.
The unusually large number of low-lying Feshbach reso-
nances found in CoQ0should allow for the coupling among the
shape and core-excited anion states. Although one cannot
obtain resonance widths for the CASSCF/CASPT2 calculations,
they seem to account for those couplings better than the SEP
scattering calculations, which poorly describe the energies of
the parent neutral states (single excitations). It should be noted
that the rotation of the methoxy groups, which mainly
distinguishes three conformers A–C, seems to have a mild
impact on the anion state energies. We further investigated the
effect of geometry on the energies of the anion states with
different theory levels, as shown in Table S1.
While we did not perform high-level calculations for CoQ0H2,
the positions of shape resonances were investigated with
empirical VEA estimates based on B3LYP/6-31G* computations.
We obtained two π* shape resonances around 1.0 eV (π1*) and
1.6 eV (π2*), as shown in Figure 5. The comparison of the
empirical VEAs for the shape resonances of CoQ0and CoQ0H2is
available in Table S2. It should also be mentioned that, unlike
CoQ0, the CoQ0H2molecule has no vertical bound state.
Dehydrogenation of CoQ0and CoQ0H2
In our DEA study with CoQ0, we detected anion yield at m/z
181, assigned to the dehydrogenated CoQ0molecular anion,
(CoQ0
H). The anion efficiency curve shown in Figure 6
indicates resonances at 1.8 eV and 7.0 eV. The experimental
onset of 1.1 eV agrees with the calculated thermochemical
threshold of 1.05 eV, assuming H-abstraction from one of the
methyl groups, see Figure 3 including suggested dissociation
pathways (H abstraction from the ring or an OCH3group lies
higher by more than 0.7 eV). Due to the symmetric shape of the
first peak and its position at 1.8 eV, we assign the DEA ion yield
rather to the decay of the π3* resonance (located at 2.00 eV, see
SEP results in Figure 4), without significant contribution of π2*
at 1.30 eV. The peak at 7.0 eV is above the range of energies
considered in the scattering calculations but can be likely
assigned to core-excited resonance. For electron attachment to
Table 3. Energies of the CoQ0anion states for conformer A, in units of eV,
obtained from the CASSCF/CASPT2//MP2/aug-cc-pVDZ calculations. For
each state, the main electronic configurations, and the respective weights
(squared coefficients) are indicated. Whenever possible, the resonance
positions obtained from SEP-level scattering calculations are also shown.
The state with mixed character, (π3*)1/(π3*)1, (π1*)2, shows up as two
resonances with prevailing shape (S) and Feshbach (F) character in the SEP
results.
Anion state weight CASPT2 [eV] SEP [eV]
(π2*)10.82 0.85 1.30
(n2)1(π1*)20.87 1.11 3.61
(π3*)1
(π3)1(π1*)2
0.31
0.39
1.15 2.06 (S)
3.08 (F)
(n1)1(π1*)20.88 1.16 3.78
(π4)1(π1*)20.70 1.46 2.50
(π3*)1
(π4)1(π1*)1(π2*)1
0.35
0.24
1.69
(n1)1(π1*)1(π2*)10.57 3.58
(n2)1(π1*)1(π2*)10.59 3.61
(π2)1(π1*)20.56 3.64
Figure 5. Calculated orbitals relevant to the two lowest lying shape
resonances in CoQ0H2predicted at the B3LYP/aug-cc-pVDZ level.
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p-BQ, Khvostenko et al.[50] have observed the formation of (p-
BQH)through resonances centered at 1.7, 4.86 and 6.46 eV.
By comparing the peak positions in the anion efficiency curves
for the dehydrogenation of both CoQ0(obtained here) and p-
BQ,[50] it can be derived that the dehydrogenation of both
quinone analogues occurs at similar electron energies close to
1.8 and 7.0 eV while the core-excited resonance at 4.86 eV is
absent in CoQ0. In DEA studies to HQ, Pshenichnyuk et al.[63]
reported the formation of the dehydrogenated molecular anion
of HQ at 1.6 eV as the most intense dissociation pathway, in
addition to a weaker contribution at 4.2 eV. Here, we have
observed the dehydrogenation of CoQ0H2through a single
asymmetric contribution comprised of two peaks at 1.7 and
2.4 eV, whereas the resonance at 4.2 eV seems not to be
present in the more complex analogue, CoQ0H2. The loss of an
H atom from CoQ0H2due to an OH bond cleavage has a
thermochemical threshold of 1.31 eV, i. e., close to the exper-
imental onset. Therefore, we assign the measured yield to the
π2* shape resonance of CoQ0H2(located at 1.6 eV, see Figure 5).
We just note that the release of H2or H +H from CoQ0as well
as CoQ0H2was not detected here while such pathway was
observed in electron attachment to HQ[63] but not for p-BQ.[50]
Demethylation of CoQ0and CoQ0H2
The demethylation channel resulting in the release of one or
two methyl groups is the most intense fragmentation channel
observed in CoQ0. In Figure 7-a), the anion efficiency curve for
the formation of (CoQ0
CH3)shows an intense peak positioned
at 1.8 eV arising from DEA, as well as a gradually increasing ion
signal for electron energies above ~ 9.0 eV which can be
assigned to non-resonant ion-pair formation. Through the latter
process, a fragment negative ion is formed along with a
positively charged counterpart ion and an electron. The minor
peak at ~0 eV can be assigned as an artefact or to form upon
DEA to thermally excited CoQ0. Like for (CoQ0
H), the major
peak at 1.8 eV is close to the energies of the π3* resonance in
CoQ0(2.0 eV). The most probable demethylation pathway
proceeds from a methoxy group of CoQ0, with the threshold of
0.85 eV. The release of the methyl group directly from the
CoQ0ring costs 1.92 eV and might thus also contribute to the
tail at higher electron energies. Demethylation in p-BQ
appeared to proceed through a resonance at 1.78 eV, in
addition to two further resonances at 5.78 and 6.78 eV.[50] For p-
BQ, Khvostenko et al.[50] proposed that the release of a CH3
group involves the transfer of a hydrogen atom to a carbon
atom followed by a ring opening reaction, while the demeth-
ylation of CoQ0upon electron attachment seems to result from
a single bond cleavage alone.
DEA to CoQ0also yields a fragment anion at m/z152 due to
a concurrent loss of two methyl units. The anion efficiency
curve shown in Figure 7-b) is comprised of a very weak ~ 0 eV
peak (which is assigned to an artefact), and a feature
constituted by two peaks appearing at about 2.9 and 3.5 eV. In
this energy range, the CASPT2 calculations suggest several
Feshbach resonances around 3.5 eV (see Table 3). A broad
Figure 6. Anion efficiency curve for the dehydrogenated molecular anion
formed upon DEA to CoQ0and CoQ0H2, top panel (CoQ0
H), and bottom
panel (CoQ0H2
H).
Figure 7. Anion efficiency curves as a function of the electron energy for the
formation of the following fragment anions: a) (CoQ0
CH3)from CoQ0, the
inset (x20) shows the signal for ion-pair formation; b) (CoQ0
2CH3); c) CH3
from CoQ0and d) CH3from CoQ0H2.
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resonance between 4.0 and 7.0 eV with a maximum at 5.2 eV is
also observed. The DEA reaction with the loss of two CH3units
due to two OCH3bond cleavages to form (CoQ0
2CH3)is
calculated to require 0.78 eV and lies thus energetically below
the experimental threshold of 2.2 eV. When the CH3unit from
the ring is released along with the second CH3unit from one of
the methoxy moieties, the thermochemical threshold rises to
3.17 eV.
As shown in Figure 7-c), the anion efficiency curve for the
formation of the counterpart CH-
3from CoQ0exhibits a slightly
asymmetric feature possibly comprised of two resonances
centered at 8.1 and 9.7 eV, as obtained from a fitting of the ion
signal with two Gaussian functions. The onset for CH-
3
formation due to DEA to CoQ0was experimentally estimated to
be 6.0 eV, which is above the respective calculated threshold of
1.90 eV for release from a methoxy moiety. For electron
energies above 15.0 eV, CH-
3is also produced by a non-
resonant ion-pair formation process.
For CoQ0H2, we surprisingly have not observed the release
of one or two neutral methyl groups producing demethylated
molecular anions. From a computational point of view, the DEA
reaction leading to the release of one neutral methyl group
would be even exothermic with 0.38 eV. The release of two
neutral methyl groups in DEA to CoQ0H2would be only slightly
endothermic with 0.93 eV, i.e., the non-observation of these
species cannot be explained by thermodynamic arguments.
However, CH-
3is produced in DEA to CoQ0H2through a rather
weak resonance centered at 9.0 eV, shown in Figure 7-d). The
experimental onset of 6.3 eV lies substantially above the
thermochemical threshold of 1.63 eV for CH-
3formation from
CoQ0H2. The electron attachment study by Pshenichnyuk et al.
did not report the loss of a single methyl unit upon DEA to HQ,
which would require a complex DEA reaction with multiple
bond cleavage and formation of new bonds.[63]
Loss of OH and O upon DEA to CoQ0H2
In the present study with CoQ0H2, the most intense DEA
channel is the formation of (CoQ0H2
OH)along with loss of a
hydroxyl radical through a single peak at 1.6 eV. This energy
matches with the suggested π2* shape resonance in CoQ0H2,
see Figure 5. The counterpart anion OHis also detected, and
both efficiency curves are shown in Figure 8. Both channels are
not detected in DEA to CoQ0. OHformation proceeds through
a weak resonance centered at 7.0 eV as well as through an ion-
pair formation pathway at electron energies above 15.0 eV. In
previous DEA studies with HQ,[63] OHwas reported to be
formed in a single resonance at 10.2 eV, but the loss of neutral
OH radical was not observed. Instead, the authors in Ref.[63]
observed the formation of (HQH2O)with a maximum at
1.3 eV. However, the intensity of this anion was rather weak
(0.57% of the (HQH), which was the most abundant anion
observed). At last, anionic oxygen Owas also detected. Note
that under the present experimental conditions, the ion yield
intensity is relatively small, and thus hindering the identification
of resonances in the anion signal observed between 4 and
12 eV. Calculated energy thresholds agree well with the
experimental ones (Table 2).
Other Anions Formed upon DEA to Either CoQ0or CoQ0H2
In addition to single-bond cleavages, DEA to CoQ0also
proceeds through more complex reaction pathways, which
involve several bond cleavages followed by rearrangements. As
shown in Figure 9, we obtained anion yield at m/z 124, which
we assign to C6H4O3. The formation of this anionic fragment
occurs through two resonances at 5.5 and 6.6 eV, with an
experimental onset of 4.6 eV. The fragmentation pathway may
include the concomitant loss of neutral CH3and COCH3units by
multiple bond ruptures within CoQ0, namely a cleavage of CO
bond within a methoxy moiety, along with release of the
second methoxy moiety accompanied by a carbon atom from
the ring. The thermochemical threshold for such dissociation
reaction, yielding a five membered ring anion, is 3.08 eV, i. e.,
this channel might be open at the observed resonance
energies. For the ion yield obtained at m/z108, which we assign
to C6H4O2, the anion efficiency curve shows three different
resonances at 5.3, 7.0 and 9.3 eV. Assuming the ring opens
upon loss of CO2from the initially formed (CoQ0
2CH3)ion, an
energy threshold of 2.04 eV is obtained (see Figure 3).
In the case of the hydroquinone derivative CoQ0H2, we
detected two additional anionic fragments at m/z152 and at
m/z26 whose anion efficiency curves are presented in Fig-
ure 10. The first species, (CoQ0H2
CH3OH)is most probably
formed through intramolecular hydrogen transfer from OH to
CH3O forming CH3OH that leaves the temporary negative ion,
with resonances at 2.5, 3.5 and 4.7 eV. The respective calculated
Figure 8. Anion efficiency curve as a function of the electron energy, for the
formation of the following fragment anions in DEA to CoQ0H2: top panel –
(CoQ0H2
OH), centre panel – OHand bottom panel – O.
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reaction energy is 0.76 eV. We also observed weakly abundant
anion yield at m/z26, which shows anion formation over a
broad energy range. In detail, a first asymmetric peak is
observed close to 2 eV, which can be most likely ascribed to an
impurity.[64] A second resonance is found at 5.9 eV, on top of a
non-resonant ion signal resulting from ion-pair formation. The
fragment anion is assigned to the vinylidene anion C2H-
2which
has an electron affinity of 0.48 eV.[65] The ring has to decompose
during ion formation and a complicated decomposition path-
way might be expected, one thermochemically possible scenar-
io takes place through dissociation of two water molecules
(Figure 3).
Conclusion
In the present study, we investigated the formation of
temporary negative ions of 2,3-dimethoxy-5-methyl-p-benzo-
quinone (CoQ0) and 2,3-dimethoxy-5-methylhydroquinone
(CoQ0H2), as well as their dissociation into fragment anions in
the gas phase. Both prototypal molecules are respectively
associated with ubiquinone serving as a mobile electron carrier
within the mitochondrial electron transport chain, and its fully
reduced form, ubiquinol. We observed a variety of DEA
channels, however, the presence of the two hydrogen atoms as
in CoQ0H2substantially alters fragmentation channels. Note-
worthy, the most abundant DEA fragment anion of CoQ0,
(CoQ0
CH3), is fully quenched in CoQ0H2and, instead,
(CoQ0H2
OH)is observed as the most intense fragment anion.
These fragment anions share a similar resonance peak with its
maximum around 1.6–1.8 eV.
The loss of a neutral methyl group was also observed for
other derivatives of the coenzyme Q0, CoQ1, CoQ2and CoQ4.[51]
Interestingly, the authors in Ref.[51] also observed an anion with
a mass 30 u lower than the parent mass, which they assigned to
(CoQx
OCH2)(x=1,2,4), i.e. the abstraction of a formaldehyde
molecule. They predicted exothermic reaction energies of about
2.0 eV for this channel, irrespective of the number of the
isoprenyl units. Our quantum chemical calculations based on
the endothermic release of two single methyl groups are in line
with the experimentally obtained threshold of 2.2 eV.
For CoQ0, the shape resonance π3* predicted at 2.0 eV by
the SMCPP scattering calculations decays by DEA reactions
leading to loss of a hydrogen atom and a single methyl unit. In
CoQ0H2, the shape resonance π2* can decay by the loss of either
a hydrogen atom or a hydroxyl group due to single bond
cleavages. We did not detect anions formed by the decay of
shape resonances π2* in CoQ0and π1* of CoQ0H2with quadru-
pole mass spectrometry.
When considering the present results and previous ones in
Ref.,[50] it also may be concluded that the total number of
abundant fragment anions does not seem to increase with the
size and the number of the isoprenyl units. This supports the
view of the p-BQ unit acting as the electrophore. However,
while gas-phase studies allow to identify TNIs and subsequent
dissociation products unambiguously, it is important to further
understand how the incorporation of intermolecular interac-
tions, such as within a microsolvated biomolecular cluster,
affects the dynamics and energetics of the TNIs of CoQ0and
CoQ0H2observed in the gas-phase.
Acknowledgements
This work was supported by the FWF, Vienna (P30332). JCR
acknowledges support from the Brazilian National Council for
Scientific and Technological Development (CNPq), grant No.
153377/2016-0. MTNV also acknowledges CNPq (grant No.
304571/2018-0). The calculations were partly performed with HPC
resources from STI (University of São Paulo), partly using the HPC
infrastructure LEO of the University of Innsbruck. JA, JPS, and FFS
Figure 9. Anion efficiency curves for the formation of the anionic fragments
with m/z 124, C5HO3CH3(top), and m/z 108, C6H4O2(bottom) from CoQ0.
Figure 10. Anion efficiency curves for the formation of anionic fragments at
m/z 152 (CoQ0H2
CH3OH)(top panel), and at m/z 26 C2H-
2(bottom).
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acknowledge the Portuguese National Funding Agency FCT-MCTES
through the research grant PTDC/FIS-AQM/31215/2017 as well as
Radiation Biology and Biophysics Doctoral Training Programme
(RaBBiT, PD/00193/2012); UID/Multi/04378/2019 (UCIBIO); UID/FIS/
00068/2020 (CEFITEC). JA and JPS also acknowledge FCT-MCTES
through the PhD grants PD/BD/114447/2016 and PD/BD/142768/
2018, respectively.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
Keywords: electron carrier molecules ·ubiquinone ·
dissociative electron attachment ·electron scattering ·quantum
chemistry
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Manuscript received: November 19, 2021
Revised manuscript received: January 5, 2022
Version of record online: February 10, 2022
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