Water oxidation surface mechanisms replicated
by a totally inorganic tetraruthenium–oxo
molecular complex
Simone Piccinina,1, Andrea Sartorelb, Giuliana Aquilantic, Andrea Goldonic, Marcella Bonchiob,1, and Stefano Fabrisa,1
a
Consiglio Nazionale delle Ricerche (CNR)–Istituto Officina dei Materiali (IOM) DEMOCRITOS Simulation Center and Scuola Internazionale Superiore di Studi
Avanzati (SISSA), 34146 Trieste, Italy; bInstitue of Membrane Technology of the Italian National Council of Research (ITM–CNR) and Department of Chemical
Sciences, University of Padova, 35131 Padova, Italy; cSincrotrone Trieste SCpA, 34149 Trieste, Italy
Solar-to-fuel energy conversion relies on the invention of efficient
catalysts enabling water oxidation through low-energy pathways.
Our aerobic life is based on this strategy, mastered by the natural
Photosystem II enzyme, using a tetranuclear Mn–oxo complex as
oxygen evolving center. Within artificial devices, water can be
oxidized efficiently on tailored metal-oxide surfaces such as
RuO2. The quest for catalyst optimization in vitro is plagued by
the elusive description of the active sites on bulk oxides. Although
molecular mimics of the natural catalyst have been proposed, they
generally suffer from oxidative degradation under multiturnover
regime. Here we investigate a nano-sized Ru4–polyoxometalate
standing as an efficient artificial catalyst featuring a totally inorganic molecular structure with enhanced stability. Experimental
and computational evidence reported herein indicates that this is
a unique molecular species mimicking oxygenic RuO2 surfaces.
Ru4–polyoxometalate bridges the gap between homogeneous
and heterogeneous water oxidation catalysis, leading to a breakthrough system. Density functional theory calculations show that
the catalytic efficiency stems from the optimal distribution of the
free energy cost to form reaction intermediates, in analogy with
metal-oxide catalysts, thus providing a unifying picture for the
two realms of water oxidation catalysis. These correlations among
the mechanism of reaction, thermodynamic efficiency, and local
structure of the active sites provide the key guidelines for the rational design of superior molecular catalysts and composite materials designed with a bottom–up approach and atomic control.
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artificial photosynthesis ab initio simulations
spectroscopy electrocatalysis
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P
| X-ray absorption
hotocatalytic water splitting offers a bioinspired strategy for
replacing fossil fuels with clean energy vectors (1–3). The
overall reaction entails a sequence of light-promoted electron
and proton transfers coupled with cleavage and formation of
molecular bonds, ultimately splitting H2O molecules into O2 and
H2. The application of such technology for a viable solar-fuel
economy is at the forefront of a very intense research effort. The
main issue is the design and optimization of innovative catalysts
enabling the half reaction of water oxidation (2H2O → O2 + 4H+ +
4e−) (1) at low overpotential, high turnover frequencies, and longterm operation stability. Considering its high thermodynamic cost
[E0 = −1.23 V at pH = 0 vs. normal hydrogen electrode (NHE)]
and mechanistic complexity, water oxidation catalysis (WOC)
poses severe challenges for artificial photosynthesis applications.
In plants, water oxidation is catalyzed by the Mn4CaO4 oxygen
evolving complex of Photosystem II (PSII) enzymes. The natural
catalyst exhibits a functional asset of four redox active metal
centers with adjacent μ–oxo bridges to enable water oxidation
with a maximal turnover frequency of 400 s−1 per O2 molecule
(4). The drawback lies in the intrinsic weakness of the biological
components, requiring multiple repair strategies of both the inorganic core and the enzyme proteins. Similar stability issues due
www.pnas.org/cgi/doi/10.1073/pnas.1213486110
to oxidative damage during water oxidation affect most of the
synthetic homogeneous molecular catalysts (5, 6).
In the artificial transposition of the natural process, the attention is actually focused on metal-oxide heterogeneous catalysts,
which conjugate robustness and efficiency, with the most prominent examples being RuO2, IrO2, or cobalt phosphate (7, 8). In
particular, RuO2-based materials for electrocatalytic water oxidation have been the subject of intense investigation, including
single- or polycrystalline systems (4, 9, 10), compact RuO2 films,
and composite oxides (4, 11, 12). The structure and composition
of the active sites in these materials are strongly dependent on
surface morphology, preparation, and doping conditions, which
hinder the precise mapping of surface-absorbed species and/or
detection of short-lived high-valent intermediates. As a result, the
mechanism for water oxidation promoted by metal-oxide heterogeneous catalysts such as RuO2 remains controversial. This
translates into severe hurdles to fundamental studies of elementary steps and to their optimization in terms of rates and efficiency
ultimately hampering the rational design of the light-absorption/
conversion interface.
In this paper we identify a well-defined molecular analog to
guide our understanding of the fundamental principles governing
WOC at metal-oxide materials, while offering an unprecedented
bridge between homogeneous and heterogeneous catalysis. Our
perspective is based on the highly promising class of polyoxometalate (POM) catalysts, which comprise multinuclear
metal–oxo cores embedded in totally inorganic molecular scaffolds. One of the most efficient and robust catalytic cores reported
so far is a tetraruthenate oxo fragment [Ru4O4(OH)2(H2O)4]6+
sandwiched between two inert polyoxotungstate ligands [Ru4–
POM; Fig. 1A], displaying a discrete and nano-sized structure.
Ru4–POM has been shown to oxidize water in the homogeneous phase with small overpotential (0.35 eV), high turnover
frequency (>450 h−1), and no degradation (13, 14). By virtue of
the molecular nature, when coupled to photogenerated oxidants,
it displays photo-induced electron transfer rates in the nano/
microsecond time domain (15), thus approaching the natural
paradigm, while preserving its activity also when it is interfaced
with functionalized carbon nanotubes (16).
Our combined computational and spectroscopic study traces
a parallel scenario of water oxidation at molecular Ru4–POM
and crystalline RuO2 surfaces. We identify the thermodynamic
origins and kinetic pathways governing the high efficiency of
Author contributions: S.P., A.S., M.B., and S.F. designed research; S.P. and A.S. performed research; G.A. and A.G. contributed new reagents/analytic tools; S.P., A.S.,
G.A., A.G., M.B., and S.F. analyzed data; and S.P., M.B., and S.F. wrote the paper.
The authors declare no conflict of interest.
†
This Direct Submission article had a prearranged editor.
1
To whom correspondence may be addressed. E-mail: [email protected],
[email protected], or [email protected].
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1213486110/-/DCSupplemental.
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Edited† by Thomas J. Meyer, University of North Carolina, Chapel Hill, NC, and approved February 6, 2013 (received for review August 3, 2012)
Fig. 1. (A) Structure of the Ru4–POM in the resting state S0. (B–F) Octahedral environment around one Ru center of Ru4–POM, ligands involved in water
oxidation and schemes of the corresponding electronic structure of the highest occupied molecular orbitals for the Ru–ligand moyeties.
the molecular Ru4–POM and demonstrate that its elementary
tetraruthenate–oxo fragment complies with the same optimal
thermodynamic requirement for water oxidation displayed by
crystalline RuO2 surfaces. Disclosing the atomistic origins of the
high catalytic efficiency and stability of the molecular analog
opens the way for a predictive catalyst upgrade and its integration in innovative photosynthetic materials.
Results and Discussion
Ru K-Edge X-Ray Absorption Spectroscopy Analysis. We begin by
demonstrating that the local atomistic and electronic structures
of the metal centers in molecular Ru4–POM and crystalline
RuO2 surfaces are equivalent. X-ray absorption near edge
spectroscopy (XANES) has been performed at Ru K-edge on
Ru4–POM as isolated crystals of the cesium salt, exhibiting all
Ru(IV) centers [Ru(IV)POM in Fig. 2], on a related Ru(III)
complex [Ru(H2O)SiW11O39]5−, and on hydrous ruthenium oxide as prepared (RuO2·xH2O) and after thermal activation
(RuO2 act.) at 150 °C for 5 h. The latter is a key treatment for
enhancing the catalytic properties of RuO2 crystals.
The XANES spectra (Fig. 2) have been recorded, for solid
samples, on the XAFS beamline at the Elettra synchrotron
source. The Ru(IV)POM and hydrous RuO2 display identical
XANES edge and line shape. The edge position is known to
depend on the ruthenium oxidation state, shifting to higher
Fig. 2. XANES spectra at the Ru K edge for different Ru4–POM [Ru(IV)POM
(red) and Ru(III)POM (blue)], RuO2 hydrous (green), and thermally activated
(black). The inset shows the pre-edge and edge region of the spectra.
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photon energy as the valence state increases (17, 18). It can be
concluded that the Ru ions in the two materials have the same
Ru(IV) oxidation state. These data compare well with those
reported for related Ru(IV) systems (18). Instead, the main
absorption edge of Ru(III)POM is shifted to lower photon energies (by 2 eV), which is consistent with a lower oxidation state
of Ru. It is worth noting that the pre-edge feature is practically
the same for all of the samples. Moreover, the absence of adventitious Ru(III) species and or mixed valence states is evident
from the position of the first absorption edge and from the line
shape of the first absorption peak [note that in Ru(III)POM, the
first structure is higher than the second one]. This is consistent
with a silent EPR behavior of the freshly isolated crystals (19).
In addition, X-ray diffraction spectra shows that the Ru(IV)
ions in both the Ru4–POM and RuO2·xH2O materials have the
same RuO6 octahedral local environment with the same average
Ru–O bond length, 1.98 Å (SI Text and Table S1). To complete
the analogy, note that every Ru center of the Ru4–POM core
coordinates a water ligand and can be viewed as a minimal,
highly hydrated, RuO2 nanocrystal. Interestingly, thermally activated RuO2 shows a shift of the peaks in the main absorption
edge to higher photon energy, suggesting the evolution to less
hydrated or anhydrous materials, with changed structure, in
agreement with literature observation (20).
Reaction Intermediates and Thermodynamics. The equivalence in
the short-range atomistic and electronic structures of the molecular and bulk ruthenium–oxide catalysts is validated by our
density functional theory (DFT) calculations. These predict a
distorted octahedral geometry around the Ru metal centers in
the Ru4–POM active core (21) in excellent agreement with experimental data (SI Text and Table S1) and similar to the one of
RuO2 crystalline surfaces. In the absence of an applied potential
or without oxidants in solution, the calculations show that every
Ru atom of the Ru4–POM is in oxidation state IV and coordinates a water ligand, thus resulting in four Ru(IV)–H2O
groups (Fig. 1A) (13, 14, 21). In this configuration, the d4 electrons formally residing on the metal ion in the Ru(IV)–H2O
moiety occupy the t2g-like orbitals with two unpaired spins (Fig.
1B). This reference structure is denoted as S0.
From this resting state, we perform a thermochemical analysis
of water oxidation adopting the computational method proposed
by Norskov and coworkers (22, 23). We aim at identifying the
four Ru4–POM intermediates (Si, Fig. 3) that, through a cycle of
four stepwise proton-coupled electron-transfer oxidations
(PCET), make the formation of the O–O bond and the release of
O2 thermodynamically favorable.
The first cycle we consider begins from S0 and involves all of
the four Ru sites of the Ru4–POM core: S0 → S4 + 4H+ + 4e−
(see cycle A in Fig. 3). The first oxidation of S0 leads to the
Piccinin et al.
Fig. 3. (Upper) Calculated free energy of the four catalytic cycles for water
oxidation at increasing oxidation state of Ru4–POM. Black dots represent the
available experimental values (19). Only C and D satisfy the thermodynamic
requirements for water oxidation (dashed horizontal line). (Lower) Scheme
of the tetraruthenium–oxo core in S0 and description of reaction intermediates Si.
formation of one Ru(V)–OH moiety (Fig. 1C) according to
Ru(IV)–H2O → Ru(V)–OH + H+ + e−. We denote this state as
S1. The oxidized Ru(V) d3 ion has three unpaired electrons in
the t2g-like orbitals. The successive three oxidations via PCET
steps transform the other Ru(IV)–H2O groups of S1 into the
Ru(V)–OH ones of S2, S3, and S4 states (cycle A in Fig. 3).
The energetics of these four initial oxidation steps is shown in
Fig. 3 (cycle A, S0 − S4). The calculated values of the free energy
differences (ΔG) are in very good agreement with the available
experimental data (black circles) (Note that only the S0−S3
portion of the cycle is accessible experimentally) (19). This validates the high accuracy of the present computational approach
[Becke 3 Lee Yang Parr (B3LYP) functional] for predicting the
reaction thermodynamics. It turns out that the calculated ΔG for
the S0/S4 couple is 3.38 eV (green line in Fig. 3), which is 1.18 eV
below the thermodynamic limit for water oxidation (dashed line
in Fig. 3) (While the experimental value the free energy change
at standard conditions associated to the reaction 2H2O→O2+
2H2 is 4.92 eV, the B3LYP value, including zero point energy
corrections and entropic contributions is 4.56 eV. Consistently,
we will compare our B3LYP results for the energetics of the
intermediates with this value rather than the experimental one. ).
This indicates that promoting water oxidation requires catalyst
oxidation states higher than S4. It therefore rules out cycle A,
which involves the simultaneous participation of the four Ru
centers of the Ru4–POM core.
Further oxidation of S4 to S5 leads to the formation of one
formal Ru(VI)–oxo moiety (Fig. 1D) via the reaction Ru(V)–
OH → Ru(VI)–O + H+ + e−. S5 is the highest valent intermediate in the the S1/S5 catalytic cycle B (Fig. 3), which
entails only three Ru centers. This PCET step changes the spin
density localized at the Ru atom, from 1.9 (in Ru–OH) to 1.0
Piccinin et al.
Reaction Mechanisms for Water Splitting and O2 Evolution. Once the
catalyst has reached the activated intermediate capable of oxidizing water (S6), the reaction can then proceed with the key
O–O bond formation step (Fig. 4). There are several possible
mechanisms for this step, either intramolecular (e.g., involving
an oxo ligand and an O atom of the metal-oxide core) or intermolecular (e.g., a nucleophilic attack on the Ru–oxo moiety
by a solvent water molecule). The direct formation of the O–O
bond from two oxo ligands in the same or in neighboring Ru4–
POM molecules can be excluded on the basis of the very large
distance between the two oxo atoms in the core (> 5.26 Å) and of
the reaction kinetics, respectively (13).
The lowest energy path for the O–O bond formation at one of
the two Ru–oxo moieties of S6 was determined by metadynamics
calculations (27) using as collective variable (CV) the coordination number of the oxo ligand with any other O in the
system, either from the solvent or from the catalyst (Figs. S1 and
S2 and other details in SI Text). This CV allows us to capture
both the intra- and intermolecular mechanisms introduced
above. This metadynamics simulation predicts that the O–O
bond is preferentially formed via an intermolecular mechanism
(Fig. 5A). It consists in the nucleophilic attack on the Ru–oxo
moiety by a solvent water molecule, which evolves to a hydroperoxo ligand and liberates a proton in solution. This path is
energetically more favorable than any intramolecular reaction
mechanism (SI Text). Indeed the electronic structure of the
catalyst shows the presence of low-energy empty states localized
around the Ru–oxo moiety, suitable for a nucleophilic attack by
water (Fig. S3).
To estimate the reaction barrier more precisely, the metadynamics simulation described above was refined by a second calculation. This time two CVs were biased: (i) the coordination
number of one of the oxo ligands with the oxygen atom of the
water molecule involved in the nucleophilic attack (CV1) and (ii)
the coordination number of the oxygen atom of this water molecule with its two hydrogens (CV2). The free energy landscape as
a function of these CVs is displayed in Fig. 5B. It shows that the
nucleophilic attack proceeds via a concerted mechanism, in
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(in Ru–O), and a net spin polarization appears on the oxo ligand. Note that on the basis of the charge and spin analysis, it is
not possible to unambiguously associate the Ru–O moiety to
a Ru(VI)–oxo or to a Ru(V)–oxyl radical (Table S2). The oxyl
radical, however, has been proposed to be a key intermediate in
catalytic water splitting by ruthenium complexes as in the “blue
dimer” (24, 25) and also for the oxygen evolving complex of
PSII (26). Similarly to cycle A, also the calculated free energy
of cycle B (4.21 eV) is well below the thermodynamic limit for
splitting water (Fig. 3).
The calculated thermodynamics predict that water oxidation
can only be promoted by the higher valent Ru4–POM intermediates emerging in catalytic cycles C or D (Fig. 3). These cycles
require the active participation of two and one Ru centers, respectively. The computed free energy difference for both the S2/S6
and S3/S7 couples is 4.82 eV. In both these cycles, the most demanding oxidation is the formation of the first oxo moiety (S4 →
S5, ΔG = 1.53 eV), while a lower energy cost is required to form
the second Ru–oxo group (S5 → S6, ΔG = 1.31 eV).
In the Nørskov approach, kinetic barriers separating the intermediate states are neglected, and the overpotential is approximated by the free energy cost of the most demanding oxidation
step along the cycle. In this framework, the reaction overpotential
of both cycles C and D is determined by the formation of the first
Ru–oxo moiety, and can be computed as η = ΔG(S4 → S5) – ΔG
(H2O)/4= 0.39 eV, and is in excellent agreement with the experimental value of 0.35 eV (16). Here ΔG(H2O) is the free energy
change for the reaction 2H2O → O2 + 2H2 at pH = 0 and room
temperature.
Fig. 4. Free energy diagram for cycles C and D with a schematic representation of the reaction intermediates. ΔG(H2O) is the free energy change in the
reaction 2H2O → O2 + 2H2 at pH = 0 and room temperature.
which the formation of the O–O bond and the deprotonation of
the solvent water molecule happen concurrently (Fig. S2), in
analogy with what has been proposed for single-center Ru
complexes (28).
Of the two Ru–oxo groups initially present in the activated S6
state, one converts to Ru–OOH via nucleophilic attack (as
shown in our metadynamics), the other to Ru–OH by a proton
transfer. In the resulting S*6 intermediate (Fig. 4), all of the Ru
atoms are in oxidation state V (spin density analysis in Table S2).
These simulations predict an activation energy of 0.96 eV
[Perdew–Burke–Ernzerhof (PBE) functional]. Single point
B3LYP calculations of two configurations representing the initial
and transition states allow us to refine the free-energy barrier to
0.79 eV. By inserting this value in an Arrhenius-type equation,
assuming a prefactor of 1013 s−1 and room temperature, we estimate a turnover frequency of 0.4 s−1, which is in good agreement with the experimental rate, 0.12 s−1 at pH = 0 (13). On
RuO2, for the same non-Faradaic step leading to the formation
of the OOH ligand, Fang et al. estimated the activation barrier to
decrease linearly with the applied bias (29), with a slope consistent
with the experimental Tafel slope. Extrapolating their results to
zero bias one obtains a barrier of 0.74 eV, in the same range as our
activation energy, suggesting similar kinetic requirements for the
formation of the O–O bond in both systems.
By analyzing the final state predicted by these metadynamics
simulations (Fig. 5A), we conclude that the relevant precursor of
O2 evolution is the hydroperoxo ligand of S*6 (Fig. 1E). Its formation is slightly endothermic, ΔG = 0.16 eV, as shown by the
calculated thermodynamics for the S6 → S*6 step (Fig. 4). Once
the hydroperoxo has been formed, the reaction cycle is downhill
in energy and does not involve any net transfer of electrons or
protons to the electrode or solution (dashed blue line in Fig. 4).
Therefore, an external potential is not required to close cycle C.
Oxygen evolution from the hydroperoxo intermediate entails
two further steps. First, one of the Ru(V)–OH is converted back
to a Ru(IV)–H2O through the migration of a proton and an
electron from the hydroperoxo ligand. This leads to a O•−
2
superoxo bound to the Ru(V) center (S8 in Figs. 4 and 1F).
Then, an electron transfers from the superoxo to the neighboring
Ru ion, and the resulting O2 ligand is exchanged with a solvent
water molecule. This leads to O2 evolution and to the S2
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intermediate, thus closing the catalytic cycle. Overall, the proton
exchange and O2 evolution steps are exothermic by –0.05 and
–0.20 eV, respectively.
Atomistic and Thermodynamic Origins of Catalytic Efficiency: Unifying
Concepts in WOC. The formation of the key hydroperoxo in-
termediate through the S5 → S6 → S*6 sequence could also be
envisioned as a single PCET step coupled to the nucleophilic
attack (S5 → S*6 , ΔG = 1.47 eV; Fig. 4). This is the same
Fig. 5. Reaction mechanisms for water splitting and O–O bond formation.
(A) Nucleophilic attack: Initial (S6), transition (TS), and final states (FS). (B)
Free energy surface of the reaction as a function of the two CVs reconstructed from the metadynamics simulation. The two activation energies
refer to the PBE/B3LYP functionals, respectively.
Piccinin et al.
mechanism of water oxidation proposed for ideal metal and
metal-oxide surfaces (30–32).
The similarity between the homogeneous and heterogeneous
reaction mechanisms becomes even more evident by considering
the S3 → S4 → S5 → S*6 → S7 path (cycle D in Fig. 3), which is
energetically equivalent to the S2 → S*6 one (cycle C) described
above. Cycle D involves the same four PCET steps proposed for
metal-oxide surfaces (32):
ΔG1
2H2 O ƒƒƒƒ! OH* + H+ + e− + H2 O;
ΔG2
OH* + H+ + e− + H2 O ƒƒƒƒ! O* + 2H+ + 2e− + H2 O;
[1]
[2]
ΔG3
[3]
ΔG4
[4]
O* + 2H+ + 2e− + H2 O ƒƒƒƒ! OOH* + 3H+ + 3e− ;
OOH* + 3H+ + 3e− ƒƒƒƒ! O2 + 4H+ + 4e− :
In both the crystalline RuO2 and molecular Ru4–POM cases
(cycle D), the oxygen evolution cycle takes place at a single ruthenium site. It involves the hydroxo (OH*), oxo (O*), and
hydroperoxo (OOH*) intermediates, which are formed electrocatalytically, and the formation of the O–O bond proceeds
through the nucleophilic attack of a water molecule. This analogy sheds light on the origins of the efficiency of the Ru4–POM
complex and provides a unifying scenario governing water oxidation at molecular and crystalline metal-oxide catalysts.
In the theory of surface catalysts, maximum thermodynamic
efficiency is achieved by thermodynamic stairways of metal-based
intermediates that equally distribute the free energy of water
oxidation (4.92 eV at pH = 0) among the four elementary steps
of Eqs. 1–4. In this way the same minimal potential (1.23 V at
pH = 0, NHE) can drive all of the PCET steps, avoiding thermodynamic barriers due to uneven stability of some intermediates.
For a wide class of crystalline metal and metal-oxide surfaces,
the binding energies of metal-aquo/hydroxo and oxo species are
shown to depend linearly one to another (32, 33). As a result, the
free energy of each oxidation (ΔGi in Eqs. 1–4) can be expressed
as linear functions of the oxygen binding energy (ΔEO), which is
taken as the reaction descriptor (32). [This linear dependence
has also be shown to lead to ΔG2 + ΔG3 = 3.2 ± 0.2 eV (33).
Multi-site mechanisms, favored for proximal metal-group interactions and leading to intramolecular O–O coupling, would not
obey the linear relationships described above and could thus
brake the predicted 3.2 eV constraint.]
Fig. 6 reports the free energies for the two most demanding
oxidations steps—the formation of the oxo (ΔG2) and of the
hydroperoxo (ΔG3) intermediates—as a function of ΔEO. The
resulting volcano plot (shaded area) determines the thermodynamic overpotential (i.e., the largest ΔGi) for a given value of the
descriptor ΔEO. The thermodynamic requirements at the top of
the volcano plot (ΔEO such that ΔG2 = ΔG3) set the optimal
catalyst with the smallest overpotential (32). Extended screening
of metal-oxide crystalline surfaces show that RuO2(110) surfaces
satisfy well these optimal thermodynamic requirements [black
dot in Fig. 6; data from ref. 33 obtained using the restricted PBE
(RPBE) functional for an O-covered surface]. [The black dot in
Fig. 6 corresponds to the RuO2–3O structure in (32), where O
ligands are present at three out of four Ru surface sites. The
oxygen adsorption energy is strongly influenced by the surface
coverage: as the coverage is reduced, oxygen is more strongly
bound and the corresponding point on the volcano moves to left
(32).] This rationalizes why this material is among the ones displaying the smallest, albeit finite, overpotential (33).
Piccinin et al.
Fig. 6. (A) Free energies of the four PCET steps (reactions 1–4) for water
oxidation on the crystalline RuO2 surface (black line) and Ru4–POM (blue). (B)
Negative of free energy cost of the most demanding oxidation steps (ΔG2
and ΔG3) as a function of the oxygen binding energy (ΔEO). The shaded area
(volcano plot) sets the overpotential for a given value of ΔEO.
Our calculations show that Ru4–POM perfectly complies with
the set of linear relationships established for metal oxide surfaces
and is almost on top of the volcano, in close proximity with
RuO2(110) surfaces (Fig. 6). This is a clear indication that the
tetraruthenate–oxo core of this molecular catalyst can be viewed
as an elementary RuO2 unit, promoting the same reaction mechanism as the parent oxide, with very similar overpotential as
shown both experimentally and theoretically. In Fig. 6 we report
for Ru4–POM both data obtained with the B3LYP and PBE
functionals. While for this system the use of different functionals
results in significant differences in the energetics (with only hybrid
functionals being able to accurately reproduce the experimental
overpotential and the free energy change of the S0 → S3 transformation) (21), both data points lie on the volcano plot. This
suggests that the linear relationships established using one particular functional (RPBE) (32) might be of general application.
Single- vs. Multisite Mechanisms by Molecular Cores. The thermodynamic equivalence of the two reaction pathways shown in
Fig. 4 suggests that the efficiency of multicenter metal–oxo cores
is not necessarily determined by multisite mechanisms involving
oxidation of several metal centers of the core. Instead, the
minimum overpotentials can be achieved also by reaction pathways in which a single metal site within the tetraruthenium core
promotes a multielectron process, like cycle D for the Ru4–
POM. In this cycle, the nonparticipating sites remain in the
Ru(V)–OH state. While metal oxidation states on the bulk material have hardly been discussed, this observation traces a parallel between cycle D and water oxidation on fully hydroxylated
RuO2 surfaces, which are predicted to be stabilized under applied bias (29, 32).
Indeed a single-site Ru–POM molecule was recently shown to
catalyze the oxidation of water in the homogeneous phase (34),
suggesting that cycle D could be a common mechanism at play
on both single- and multicenter catalysts. Although the thermodynamic efficiency of the Ru4–POM can be controlled by only
one or two sites, the presence of multiple metal centers clearly
plays an important function, as shown by the significantly superior performance of Ru4–POM (13, 14) compared with the single
center catalysts (34). The difference in their efficiency is likely
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and
to arise from the local environment around the active sites,
shaped and reinforced by the metal–oxo connectivity among
them. This, together with the thermodynamic analogy between
the homogeneous Ru4–POM and the heterogeneous metal–oxo
surfaces presented above, suggests that the Ru–O connectivity in
the tetraruthenium–oxo core could be enough to mimic, at the
active site, the structural and electronic effects of an extended
RuO2 surface.
In conclusion, we show that the efficiency of the Ru4–POM
complex, a unique tetranuclear water-oxidation catalyst, stems
from thermodynamic and kinetic origins that are common to
extended metal–oxide surfaces. The stepwise oxidation of the
tetraruthenate core is instrumental to activate a single Ru–oxo
moiety, triggering the water nucleophilic attack and O2 release.
The overpotential is minimized because the ruthenium centers
can catalyze the formation of the oxo and hydroperoxo key
intermediates with an almost identical cost, within the molecular
tetrarutenium–oxo cores and on extended RuO2 surfaces.
We identify a parallel scenario of water oxidation between two
efficient classes of homogeneous and heterogeneous inorganic
catalysts. The core of Ru4–POM can be viewed as an optimal
RuO2 cluster in which every metal center is exposed to the solvent,
thus exhibiting the highest surface area for catalysis. Although not
directly involved in the chemical reaction, the metal spectators
play a crucial role, providing the electronic stabilization for the
building-up of a high-valent ruthenium–oxo site, capable of water
oxidation. In the same environment, the hydroperoxo intermediate is also produced at low thermodynamic and kinetic cost
for the overall cycle. Understanding the guidelines to enhance
such interplay between the performance of the catalyst and local
atomic environment around the active site will be crucial for the
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6 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1213486110
design of the next generation catalysts: molecules working as
activated surfaces.
Materials and Methods
Simulations were based on DFT, using CP2K code (35). Our previous
benchmarks (21) show that unrestricted B3LYP/aug–triple zeta valence
double polarized (TZV2P) provides an accurate description of the energetics
of the reactions, with errors of the order of 0.1 eV per PCET step. The
thermodynamics of the catalytic cycle was studied with the Norskov protocol
(22), which has already been applied to study water oxidation (30–32) and
oxygen reduction (22) on metal and metal–oxide surfaces. Details on the
application of this methodology to Ru4–POM can be found in ref. 21. To
reduce the computational cost of hybrid functional calculations, the energy
differences among the various intermediates are evaluated on the simplified
Ru4–Cl model first (where POM ligands are substituted with Cl− ions; ref. 21;
SI Text and Fig. S4). The small contribution to the energy differences introduced by the presence of the POM ligands is evaluated at the PBE level
(21). All calculations for the thermodynamics of the catalytic cycle are performed in vacuum, since solvent effects on the energetics of PCET reactions
for this system have been shown to be negligibly small (21).
ACKNOWLEDGMENTS. We thank the Distributed European Infrastructure
for Supercomputing Applications (DEISA)/Partnership for Advanced Computing in Europe (PRACE) for computational resources, A. Laio for help in the
metadynamics calculations, and A. Cognigni for preliminary XANES experiments. We thank G. Scoles, M. Prato, and their teams for collaboration in the
broader context of innovative nanomaterials for energy applications. This
work was partially supported by the Seventh Framework Programme Marie
Curie International Reintegration Grant (IRG) Program (Grant PIRG04-GA2008-239199); by the European Cooperation in Science and Technology
(COST; action number CM1104); by the Italian Ministero dell’Istruzione,
dell’Universitá e della Ricerca (Firb Nanosolar RBAP11C58Y_003, PRIN HIPHUTURE 2010N3T9M4_001); and by Fondazione CARIPARO (NanoMode,
progetti di Eccellenza 2010).
19. Sartorel A, et al. (2009) Water oxidation at a tetraruthenate core stabilized by polyoxometalate ligands: Experimental and computational evidence to trace the competent intermediates. J Am Chem Soc 131(44):16051–16053.
20. Cormier ZR, Andreas HA, Zhang P (2011) Temperature-dependent structure and
electrochemical behavior of RuO2/carbon nanocomposites. J Phys Chem C 115(39):
19117–19128.
21. Piccinin S, Fabris S (2011) A first principles study of water oxidation catalyzed by
a tetraruthenium-oxo core embedded in polyoxometalate ligands. Phys Chem Chem
Phys 13(17):7666–7674.
22. Norskov JK, Rossmeisl J, Logadottir A, Lindqvist L (2004) Origin of the overpotential
for oxygen reduction at a fuel-cell cathode. J Phys Chem B 108(46):17886–17892.
23. Norskov JK, Bligaard T, Rossmeisl J, Christensen CH (2009) Towards the computational
design of solid catalysts. Nat Chem 1(1):37–46.
24. Liu F, et al. (2008) Mechanisms of water oxidation from the blue dimer to photosystem II. Inorg Chem 47(6):1727–1752.
25. Yang X, Baik M-H (2006) cis,cis-[(bpy)2RuVO]2O4+ catalyzes water oxidation formally
via in situ generation of radicaloid RuIV-O. J Am Chem Soc 128(23):7476–7485.
26. Siegbahn PEM, Crabtree RH (1999) Manganese oxyl radical intermediates and o-o
bond formation in photosynthetic oxygen evolution and a proposed role for the
calcium cofactor in photosystem ii. J Am Chem Soc 121(1):117–127.
27. Laio A, Parrinello M (2002) Escaping free-energy minima. Proc Natl Acad Sci USA
99(20):12562–12566.
28. Chen Z, et al. (2010) Nonaqueous catalytic water oxidation. J Am Chem Soc 132(50):
17670–17673.
29. Fang YH, Liu ZP (2010) Mechanism and Tafel lines of electro-oxidation of water to
oxygen on RuO2(110). J Am Chem Soc 132(51):18214–18222.
30. Rossmeisl J, Logadottir A, Norskov JK (2005) Electrolysis of water on (oxidized) metal
surfaces. Chem Phys 319(1–3):178–184.
31. Rossmeisl J, Nørskov JK, Taylor CD, Janik MJ, Neurock M (2006) Calculated phase diagrams for the electrochemical oxidation and reduction of water over Pt(111). J Phys
Chem B 110(43):21833–21839.
32. Rossmeisl J, Qu Z-W, Zhu H, Kroes G-J, Norskov JK (2007) Electrolysis of water on oxide
surfaces. J Electroanal Chem 607(1–2):83–89.
33. Man IC, et al. (2011) Universality in oxygen evolution electrocatalysis on oxide surfaces. ChemCatChem 3(7):1159–1165.
34. Murakami M, et al. (2011) Catalytic mechanism of water oxidation with single-site
ruthenium-heteropolytungstate complexes. J Am Chem Soc 133(30):11605–11613.
35. VandeVondele J, et al. (2005) QUICKSTEP: Fast and accurate density functional calculations using a mixed Gaussian and plane waves approach. Comput Phys Comm 167
(2):103–128.
Piccinin et al.
Supporting Information
Piccinin et al. 10.1073/pnas.1213486110
SI Text
Synthesis of Ru4-polyoxometalate and RuO2
Crystals of Ru4–polyoxometalate (POM), isolated as cesium salt,
were collected according to the literature protocol (1). RuO2, in
the hydrated and thermally activated form, was synthesized according to the procedure by Harriman et al. (2). For RuO2 hydrated, RuCl3 (100 mg) was dissolved in 100 mL of water, and
the pH was adjusted to 7.0 with NaOH. The solution was heated
at 60 °C for 5 h, and then the precipitate was collected by filtration. For RuO2 thermally activated, a sample of RuO2 hydrated was annealed at 150 °C for 5 h.
Synthesis of Na5[RuIII(H2O)SiW11O39]
A total of 2.0 g of K8SiW11O39·13H2O (620 mmol) and 0.25 g of
Ru(acac)3 (620 mmol) were dissolved in 50 mL of H2O in
a closed reactor (HPR-1000/10S, Milestone) and irradiated inside the cavity of a Microwave Ethos-1600 labstation (Milestone)
according to the following parameters: initial power, 350 W;
initial time, 2 min; final power, 300 Watt; Tbulk, 200 °C; reaction
time, 5 min. The reaction mixture was eluted through an ion
exchange resin (Na+), and the crude product was isolated after
solvent removal. The purification of Na5[RuIII(H2O)SiW11O39]
can be achieved by cromatography on a Sephadex G-15 column,
with water as eluent, isolating the product in 70% yield.
Structure of the S0 Intermediate
In Table S1 we report the distances between one of the Ru ions in
Ru4–POM (in the resting state S0) and the six oxygen atoms
surrounding it. The experimental data, obtained through X-ray
diffraction of Ru4–POM crystals (1), reveal a distorted RuO6
octahedron. The Ru–O distances are accurately reproduced by
our Density Functional Theory (DFT) [Perdew–Burke–Ernzerhof (PBE)] calculations, both in vacuum (gas phase) and in solution. The d(Ru–O6H2 O ) distance is considerably overestimated
when comparing the experimental data and DFT simulations in
gas phase, while a better agreement is with DFT simulations in
solution. The latter are performed with the quantum mechanics
molecular mechanics (QMMM) approach described in our earlier publication (3). Experimental data for RuO2 reveal that both
hydrous and anhydrous RuO2 have the same Ru–O distance (4).
Electronic Structure of the Intermediates
S0–S4. The evolution of the oxidation states of the Ru atoms along
the catalytic cycle was analyzed through the Mulliken populations. In Table S2 we show the differences of up and down
Mulliken spin density localized on the Ru atoms for the S0–S4
intermediates. In the initial state S0 each of the four Ru ions is in
oxidation state IV, with two unpaired spins in the t2g-like orbitals.
A spin polarization of ∼1.4 (instead of 2.0, as expected for two
unpaired electrons fully localized at the Ru atoms) indicates
a partial delocalization of the spin density on the oxygen ligands.
The Ru atoms are ferromagnetically coupled across the μ–hydroxo
bridges [i.e., Ru1–Ru2 and Ru3–Ru4 are ferromagnetic (FM)
coupled]. Each oxidation increases the absolute value of the spin
polarization of the Ru ions, which is consistent with the removal
of one electron from the spin minority, leading formally to three
unpaired electrons on the Ru ion. As evident from Table S2, the
FM coupling across the μ–hydroxo bridges is preserved.
S5–S7. In Table S2 we show the Mulliken spin densities on the
Ru ions also for the S5–S8 intermediates. In the case of S5, the
Piccinin et al. www.pnas.org/cgi/content/short/1213486110
oxidation of Ru(V)–OH by a proton-coupled electron-transfer
(PCET) would formally lead to a Ru(VI)–O moiety. In a highspin configuration, we would expect the two unpaired spin of the
Ru(VI) ion (Ru4) to lead to a spin density of ’ −1.4, consistently
with the Ru(IV) case. We find, on the other hand, a considerably
lower spin density (−0.69). Our calculations show that also the
oxygen atom involved in the oxo ligand has a net spin polarization, of about 0.5 electrons, and it is FM coupled to the corresponding Ru(VI) atom. This is due to a partial charge transfer of
spin-down electrons from the oxygen atom to the corresponding
Ru(VI) atom, which is partially reduced. This explains the lower
than expected spin polarization on the Ru ion, which is therefore
in an intermediate oxidation state between Ru(V) and Ru(VI).
A complete transfer of one spin-down electron would have
resulted in a Ru(V)–oxyl radical moiety (RuVI–O → RuV − O•−),
which theoretical calculations have predicted to be a key intermediate in oxidation reactions catalyzed by several metal–oxo
systems such as the “blue dimer” (5), the Oxygen Evolving Center
(OEC) of Photosystem II (PSII) (6), as well as Mn–porphyrins (7).
The same considerations apply also in the case of the S6 intermediate, where two oxo ligands are present, even though the
spin densities on the corresponding Ru ions are slightly larger. In
the S*6 , S7, and S8 intermediates, all Ru atoms have a spin polarization consistent with the Ru(V) oxidation state, as displayed
in Table S2.
Metadynamics Simulations
Given the high computational cost of hybrid functionals, the
metadynamics simulations are performed using the PBE functional and double zeta valence polarized (DZVP)–molecular
optimized (MOLOPT) basis set. Single point calculations of the
energy difference between initial and transition states are then
used to obtain the Becke 3 Lee Yang Parr (B3LYP)/aug-triple
zeta valence double polarized (TZV2P) correction of the PBE/
DZVP–MOLOPT estimate of the free energy barriers. Metadynamics was used to reconstruct the reaction-free energy in the
space of a few collective variables (CVs) described in the text,
namely the coordination number (CN) between groups of atoms.
The Ru4–Cl model is solvated in 70 explicit water molecules.
The hydrogen atoms were treated as deuterium atoms, which
enabled us to use a time step of 1.0 fs. The temperature was set
at 300 K using the “canonical sampling velocity rescaling” thermostat of Bussi et al. (8). To speed up the calculation we used
the multiple walkers technique (9), using eight walkers. The
initial configurations of the walkers were selected from snapshots
of an equilibrated free dynamics, separated by 100 fs.
First we studied the formation of the O–O bond between an
oxo ligand and any other O atom in the system, either from the
solvent or from the catalyst. This was achieved by selecting the
CN of the oxo ligand with the other O atoms as the CV. This
choice allows us to capture both the intra- and intermolecular
mechanisms. CNs between atoms of group A and atoms of group
B are defined as:
X n rij ;
[S1]
CN =
i∈ A; j∈ B
where n(r) = (1 − (r/rc)p)/(1 − (r/rc)q) is a continuous function of
the distance that is ∼1 for small r within a cutoff radius rc and
zero otherwise. We used P = 6 and q = 12 and rc = 3.0 Bohr.
With this choice of parameters, the initial state corresponds to
CV = 0.1 and the final state to CV = 0.7. The width of the hills is
1 of 5
0.05, their height 1 mHa (0.027 eV), and a new hill was deposited
every 20 fs.
Fig. S1 shows the time evolution of the CV. The transition to
the final state takes place after about 9 ps. By monitoring which
O atom binds to the oxo ligand, we found that a water molecule
had lost one hydrogen and bonded to the oxo ligand, giving rise
to a hydroperoxo intermediate. The cleavage of the O–H bond,
however, was not biased in this simulation, resulting in an
overestimation of the height of the barrier (about 1.5 eV).
In the second metadynamics simulation, we used as the first CV
the CN between one of the oxo ligands and the oxygen atom of
one water molecule, the closest one to the oxo ligand (CV1). The
second CV is the CN between oxygen and hydrogen in the water
molecule closest to the oxo ligand (CV2). For CV1 we used the
same parameters as in the first metadynamcis simulation, while
for CV2 we reduced the cutoff rc to 2.5 Bohr.
The free energy difference between S6 and FS in Fig. 5 is not
reported in the main text because the simulation was stopped
after the first crossing of TS.
In Fig. S2 we show the time evolution of the CVs during the
metadynamics simulation. Each panel represents one of the eight
walkers. We can see that within the length of this simulation only
a fraction of the walkers escape the initial state (characterized by
CV1 ’ 0.1, CV2 ’ 1.8) and reach the final state (CV1 ’ 0.7,
CV2 ’ 0.9). Notice how in all cases in which the final state is
reached, the transformation of the two CVs from the initial value
to the final is concerted, implying that the formation of the O–O
bond and the cleavage of one of the two O–H bonds in water
happen simultaneously.
The B3LYP estimate of the activation barrier shown in Fig. 5
has been obtained using single point calculations. We extracted
from the metadynamics simulations two snapshots representative
of the initial and transition state configurations, and considered
the Ru4–Cl cluster and the two water molecules involved in the
1. Sartorel A, et al. (2008) Polyoxometalate embedding of a tetraruthenium(IV)-oxo-core
by template-directed metalation of [γ-SiW10O36]8-: A totally inorganic oxygen-evolving
catalyst. J Am Chem Soc 130(15):5006–5007.
2. Harriman A, Pickering IJ, Thomas JM, Christensen PA (1988) Metal oxides as heterogeneous
catalysts for oxygen evolution under photochemical conditions. Faraday Trans 1(84):
2795–2806.
3. Piccinin S, Fabris S (2011) A first principles study of water oxidation catalyzed by
a tetraruthenium-oxo core embedded in polyoxometalate ligands. Phys Chem Chem
Phys 13(17):7666–7674.
4. Cormier ZR, Andreas HA, Zhang P (2011) Temperature-dependent structure and
electrochemical behavior of RuO 2 /carbon nanocomposites. J Phys Chem C 115(39):
19117–19128.
5. Yang X, Baik M-H (2006) cis,cis-[(bpy)2RuVO]2O4+ catalyzes water oxidation formally
via in situ generation of radicaloid RuIV-O. J Am Chem Soc 128(23):7476–7485.
Piccinin et al. www.pnas.org/cgi/content/short/1213486110
nucleophilic attack. We performed single point calculations at
the PBE and B3LYP level and computed a correction of 0.17 eV
to be subtracted from the PBE value.
Nudged Elastic Band Calculations
To confirm that the intramolecular path has a higher activation
energy that the nuclephilic attack, we performed a nudged elastic
band (NEB) calculation. Also in this case we used the simplified
Ru4–Cl model, and since no solvent water molecule is involved
in this reaction path, we modeled the reaction in vacuum. We
computed the reaction path for the formation of molecular oxygen starting from the S6 intermediate, and involving an oxo ligand bridging oxygen atom of the Ru–oxo core. The activation
energy is in this case 2.7 eV, which is considerably higher than
the nucleophilic path (0.96 eV).
Electronic Structure of S6
In Fig. S3 we show the isosurface corresponding to the lowest
unoccupied molecular orbital (LUMO) of the intermediate S6.
Here we consider a configuration obtained from a snapshot of
the metadynamics run, which is representative of the initial state.
We notice how the LUMO is delocalized over a good portion of
the catalyst and has significant weight over both the Ru–oxo
moyeties present on this intermediate, making these sites prone
to a nucleophilic attack from water.
Simplified Ru4–Cl Model
In Fig. S4 we show the structure of the Ru4–POM molecule and
the simplified Ru4–Cl model in the initial state S0. In Ru4–Cl
each of the two POM ligands [SiW10O36]8− have been replaced
with four Cl− ions. In the S0 state, the charge of Ru4–POM is 10−,
while the charge of Ru4–Cl is 2−. A comparison of the energetics
obtained with the full and simplified models is discussed in our
earlier work (3).
6. Siegbahn PEM, Crabtree RH (1999) Manganese oxyl radical intermediates and O-O
bond formation in photosynthetic oxygen evolution and a proposed role for the
calcium cofactor in Photosystem II. J Am Chem Soc 121(1):117–127.
7. De Angelis F, Jin N, Car R, Groves JT (2006) Electronic structure and reactivity of
isomeric oxo-Mn(V) porphyrins: Effects of spin-state crossing and pKa modulation.
Inorg Chem 45(10):4268–4276.
8. Bussi G, Donadio D, Parrinello M (2007) Canonical sampling through velocity rescaling.
J Chem Phys 126(1):014101.
9. Raiteri P, Laio A, Gervasio FL, Micheletti C, Parrinello M (2006) Efficient reconstruction
of complex free energy landscapes by multiple walkers metadynamics. J Phys Chem B
110(8):3533–3539.
2 of 5
1
0.8
CV
0.6
0.4
0.2
0
0
Fig. S1.
2000
4000
6000
Time [fs]
8000
Time evolution of the CVs during the metadynamics simulation.
2
2
2
1.5
1.5
1.5
1.5
1
1
1
0.5
0.5
0.5
CV
2
1
CV1
CV2
0.5
0
0
500 1000 1500
0
0
500 1000 1500
0
0
500 1000 1500
0
2
2
2
1.5
1.5
1.5
1.5
1
1
1
1
0.5
0.5
0.5
0.5
CV
2
0
0
10000
500 1000 1500
0
0
500 1000 1500
0
0
500 1000 1500
0
0
500 1000 1500
0
500 1000 1500
Time [fs]
Fig. S2. Time evolution of the two CVs, CV1 and CV2, during the metadynamics simulation. Each panel represents one of the eight walkers used in the
simulation.
Piccinin et al. www.pnas.org/cgi/content/short/1213486110
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Fig. S3. Isosurface of the LUMO of a snapshot of the S6 intermediate. The white ellipses highlight the two Ru-oxo moieties present on this state. Yellow and
blue surfaces represent positive and negative values of the orbital, respectively. Red, green, white, and cyan spheres represent O, Ru, H, and Cl atoms,
respectively.
Fig. S4. Comparison of the structural model of (A) Ru4–POM and (B) Ru4–Cl. Green, red, white, cyan, purple, and yellow spheres represent Ru, O, H, W, Cl, and
Si atoms, respectively.
Piccinin et al. www.pnas.org/cgi/content/short/1213486110
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Table S1. Comparison among the distances between the Ru ion and the six oxygen atoms
surrounding it in the RuO6 octahedron in Ru4–POM and RuO2
Atoms
dðRu OPOM
Þ
1
dðRu OPOM
Þ
2
dðRu Ocore
Þ
3
dðRu Ocore
Þ
4
dðRu OOH
5 Þ
2O
dðRu OH
Þ
6
davg(Ru–O)
avg
d
Ru4–POMExp
Ru4 POMDFT
vac
Ru4 POMDFT
sol
RuOExp
2
2.05
2.07
1.84
1.86
1.98
2.06
1.98
2.08
2.13
1.89
1.88
1.96
2.31
2.04
2.07
2.08
1.86
1.86
2.01
2.18
2.01
1.98
(Ru–O) is obtained by averaging the six Ru–O distances. Values are in Å.
Table S2. Differences of up and down Mulliken spin density (μ) localized on
the Ru atoms for the various intermediate states Si
Atoms
μ(Ru1)
μ(Ru2)
μ(Ru3)
μ(Ru4)
μ(O2)
μ(O3)
μ(O4)
ð1Þ
S0
ð2Þ
S1
ð3Þ
S2
ð2Þ
S3
ð1Þ
S4
ð2Þ
S5
ð1Þ
S6
1.43
1.39
1.94
1.86
1.87
1.88
1.85
1.41
1.89
1.94
1.85
1.90
1.03
0.99
−1.40 −1.36 −1.35 −1.76 −1.89 −1.85 −1.00
−1.43 −1.32 −1.37 −1.31 −1.88 −0.69 −1.85
0.40
0.49
—
−0.46
—
0.46
*ð1Þ
S6
ð2Þ
S7
ð2Þ
S8
1.89
1.87
1.89
1.88
1.90
1.34
−1.86 −1.89 −1.93
−1.70 −1.88 −1.90
—
—
—
−0.38
0.36
0.33
−0.11
0.69
0.67
Subscript indicates the spin multiplicity of each intermediate.
Piccinin et al. www.pnas.org/cgi/content/short/1213486110
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