Bridging the gap between electrochemical and organometallic activation: Benzyl chloride reduction at silver cathodes

Article abstract of DOI:10.1021/ja106049c

Integration of voltammetry, surface-enhanced Raman spectroscopy (SERS), and density functional theory (DFT) has allowed unraveling the mechanistic origin of the exceptional electrocatalytic properties of silver cathodes during the reduction of benzyl chloride. At inert electrodes the initial reduction proceeds through a concerted direct electron transfer yielding a benzyl radical as the first intermediate. Conversely, at silver electrodes it involves an uphill preadsorption of benzyl chloride onto the silver cathode. Reduction of this adduct affords a species tentatively described as a distorted benzyl radical anion stabilized by the silver surface. This transient species rapidly evolves to yield ultimately a benzyl radical bound onto the silver surface, the latter being reduced into a benzyl-silver anionic adduct which eventually dissociates into a free benzyl anion at more negative potentials. Within this framework, the exceptional electrocatalytic properties of silver cathodes stem from the fact that they drastically modify the mechanism of the 2e-reduction pathway through a direct consequence of the electrophilicity of silver cathode surfaces toward organic halides. This mechanism contrasts drastically with any of those tentatively invoked previously, and bridges classical electroreduction mechanisms and oxidative additions similar to those occurring during organometallic homogeneous activation of organic halides by low-valent transition-metal complexes.

Full text of DOI:10.1021/ja106049c

Published on Web 11/11/2010
Bridging the Gap between Electrochemical and
Organometallic Activation: Benzyl Chloride Reduction at
Silver Cathodes
†
†
†
†
‡
Yi-Fan Huang, De-Yin Wu, An Wang, Bin Ren, Sandra Rondinini,
,
†
,§
Zhong-Qun Tian,* and Christian Amatore*
State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and
Chemical Engineering, Xiamen UniVersity, 361005 Xiamen, China, Dipartimento di Chimica
Fisica ed Elettrochimica, UniVersit a` degli Studi di Milano, Via Golgi 19, 20133 Milan, Italy,
and D e´ partement de Chimie, Ecole Normale Sup e´ rieure, UMR 8640, 24 Rue Lhomond,
F-75231 Paris Cedex 5, France
Received July 20, 2010; E-mail: christian.amatore@ens.fr; zqtian@xmu.edu.cn
Abstract: Integration of voltammetry, surface-enhanced Raman spectroscopy (SERS), and density functional
theory (DFT) has allowed unraveling the mechanistic origin of the exceptional electrocatalytic properties of
silver cathodes during the reduction of benzyl chloride. At inert electrodes the initial reduction proceeds
through a concerted direct electron transfer yielding a benzyl radical as the first intermediate. Conversely,
at silver electrodes it involves an uphill preadsorption of benzyl chloride onto the silver cathode. Reduction
of this adduct affords a species tentatively described as a distorted benzyl radical anion stabilized by the
silver surface. This transient species rapidly evolves to yield ultimately a benzyl radical bound onto the
silver surface, the latter being reduced into a benzyl-silver anionic adduct which eventually dissociates
into a free benzyl anion at more negative potentials. Within this framework, the exceptional electrocatalytic
properties of silver cathodes stem from the fact that they drastically modify the mechanism of the 2e-
reduction pathway through a direct consequence of the electrophilicity of silver cathode surfaces toward
organic halides. This mechanism contrasts drastically with any of those tentatively invoked previously, and
bridges classical electroreduction mechanisms and oxidative additions similar to those occurring during
organometallic homogeneous activation of organic halides by low-valent transition-metal complexes.
3
-6
Introduction
(Ag, Pd, Cu)
compared to inert electrodes such as glassy
2
a
carbon. Silver cathodes present the highest electrocatalytic
properties among them. Hence, the reduction of benzyl chloride
2
Benzyl chloride (PhCH Cl) reduction has attracted a large
amount of attention in electrochemical investigations of electron-
transfer mechanisms because it represents a paradigm in which
the carbon-chlorine bond which is reductively cleaved is
(
2) (a) Andrieux, C. P.; Gorande, A. L.; Sav e´ ant, J.-M. J. Am. Chem.
Soc. 1992, 114, 6892–6904. (b) Cardinale, A.; Isse, A. A.; Gennaro,
A.; Robert, M.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 2002, 124, 13533–
1
significantly coupled with an aromatic system. For this reason
the unraveling of its exact reduction mechanism at inert
1
3539. (c) Isse, A. A.; Gennaro, A. J. Phys. Chem. A 2004, 108, 4180–
2
a
electrodes has played an important role in the delineation
between concerted and nonconcerted pathways in electrochem-
4186. (d) Sav e´ ant, J.-M. Chem. ReV. 2008, 108, 2348–2378. (e)
Costentin, C.; Louault, C.; Robert, M.; Teillout, A.-L. J. Phys. Chem.
A 2005, 109, 2984–2990.
2
istry. Yet, it has been recently reported that the reduction of
(
(
3) (a) Rondinini, S.; Mussini, P. R.; Crippa, F.; Sello, G. Electrochem.
Commun. 2000, 2, 491–496. (b) Isse, A. A.; Gottardello, S.; Durante,
C.; Gennaro, A. Phys. Chem. Chem. Phys. 2008, 10, 2409–2416.
4) Isse, A. A.; Gottardello, S.; Maccato, C.; Gennaro, A. Electrochem.
Commun. 2006, 8, 1707–1712.
organic halides is greatly facilitated by specific metallic cathodes
†
Xiamen University.
Universit a` degli Studi di Milano.
Ecole Normale Sup e´ rieure.
‡
(5) (a) Rondinini, S.; Mussini, P. R.; Muttini, P.; Sello, G. Electrochim.
Acta 2001, 46, 3245–3258. (b) Falciola, L.; Gennaro, A.; Isse, A. A.;
Mussini, P. R.; Rossi, M. J. Electroanal. Chem. 2006, 593, 47–56.
(6) (a) Isse, A. A.; De Giusti, A.; Gennaro, A.; Falciola, L.; Mussini,
P. R. Electrochim. Acta 2006, 51, 4956–4964. (b) The phenomena
reported in Figures 2 and 4 of ref 6a after long specific preconditioning
of the silver cathode in the presence of benzyl chloride were not
observed under our normal electrochemical conditions, with the
voltammetric wave exhibiting pure diffusional behavior (peak current
proportional to the square root of the scan rate). Furthermore, during
our related SERS investigations 7 we did not observe any change of
the adsorbed species detected along the foot of the wave even after
the electrode has been poised during ca. 4 min at each sampled
potential. This shows that though of interest under the specific
conditions explored in ref 6a the phenomena reported in this reference
are not active under our conditions.
§
(
1) In the classical view the nonconcerted reductive cleavage of aromatic
halides proceeds by electron injection into the low energy π* aryl
orbital. This is then transferred into the σ* C-Cl one when its energy
becomes reduced following its vibrational elongation. However, this
does not account for the requirement of a significant overlap between
the two molecular orbitals which remain orthogonal in such a process.
Thus, an out-of-plane bending of the C-Cl bond in PhCl was shown
recently to be crucial for allowing the cleavage of the C-Cl bond.
See the following: (a) Laage, D.; Burghardt, I.; Sommerfeld, T.; Hynes,
J. T. J. Phys. Chem. 2003, 107, 11271–11291. (b) Burghardt, I.; Laage,
D.; Hynes, J. T. J. Phys. Chem. 2003, 107, 11292–11306. In benzyl
halides, the π* and σ* are nonorthogonal and have a sufficient overlap
to allow a direct electron-transfer into the conjugated orbital hence
leading to a “concerted” rupture of the carbon-halide bond.
10.1021/ja106049c  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 17199–17210 9 17199

A R T I C L E S
Huang et al.
at these electrodes has been the focus of several detailed
4
,5
electrochemical studies. However, the exact origin of its
remarkable properties has remained elusive, mostly because
electrochemical information alone is not sufficient to delineate
and ascertain quantitatively its mechanism.
In this connection, we recently reported that, during this
reaction course, three unsuspected intermediates in close
interaction with the silver cathode surface could be unraveled
by in situ SERS (surface-enhanced Raman spectroscopy)
7
investigations. Though offering significant unsuspected entries
in the mechanistic debate, SERS alone could not offer an
unequivocal mechanistic framework to fully rationalize the
electrocatalytic role played by silver surfaces. We wish to show
that coupling DFT evaluations of the thermodynamic stabilities
of the intermediates identified by SERS with quantitative
voltammetric peak potential analysis allows a precise description
of the main mechanistic features which sustain the exceptional
electrocatalytic properties of silver cathodes. The mechanistic
framework revealed in this work contrasts drastically with any
of those tentatively invoked previously and shows that these
exceptional electrocatalytic properties are a direct consequence
of the electrophilicity of silver surface toward organic halides.
Figure 1. Cyclic voltammetry of PhCH
CH
(E ) -1.76 V vs SCE). The three regions (1-3) represent potential ranges
in which distinct intermediates were observed by SERS (see text); the
gray ones represent zones where intermediates from adjacent zones coexist
as evidenced by the overlap of their individual SERS spectra.
2
Cl, 5 mM in 0.1 M TEAP +
-1
3
CN, at a Ag electrode (2 mm diameter) and a scan rate of 0.2 V s
p
7
a
•
2
-
2
p
inert
PhCH + e f PhCH
[E° > E
]
(2)
(3)
(4)
2
•
2
PhCH + etc. (negligible)
-
2
PhCH f etc. (fast, see eqs 5 - 8)
This work also provides evidence that the boundary between
2
concerted and nonconcerted pathways in electrochemistry is
Previous investigations of the mechanism in eqs 1-4 have
pointed out that the whole electrochemical process is kinetically
limited by the concerted electron-transfer step in eq 1 which
certainly more complex than what may be predicted upon
considering only reactions at inert electrodes.
2
acts as the rate-determining step (rds). Overall, the process is
8
c
bielectronic, showing that the second reduction (eq 2) is fast
at the potential of the voltammetric wave so there may not be
Results and Discussion
9
Delineation of the Main Mechanistic Framework. Let us
any detectable interference by eq 3. The nature and distribution
briefly recall the well documented mechanism of PhCH
reduction at inert electrodes like glassy carbon.
2
Cl
It involves
a sequential pair of single-electron-transfers leading first to the
of electrolysis products is characteristic of the spontaneous
chemical evolution of a benzyl anion in slightly protic
acetonitrile:
2
,4,8
4
,8c
2
1
e-concerted rupture of the carbon-chlorine bond (eq 1) and
-
-
PhCH + CH CN f PhCH + CH CN
(5)
(6)
then to the 1e-reduction of the free benzyl radical formed in
solution (eq 2). This gives rise to a benzyl anion which
2
3
3
2
4
,8
-
-
-
spontaneously evolves to the final products (eq 4):
CH CN + PhCH Cl f PhCH CH CN + Cl
2
2
2
2
•
2
-
p
inert
-
PhCH Cl + e f PhCH + Cl
(rds), [(E° ) , k°inert, E
]
PhCH + PhCH Cl f PhCH CH Ph + Cl
(negligible)
(7)
2
1 inert
2
2
2
2
(
1)
-
-
PhCH + MH f PhCH + M
(8)
(
(
7) (a) Wang, A.; Huang, Y.-F.; Kumar Sur, U.; Wu, D.-Y.; Ren, B.;
2
3
Rondinini, S.; Amatore, C.; Tian, Z.-Q. J. Am. Chem. Soc. 2010, 132,
2
9534–9536. (b) The rapid up-hill formation of a PhCH Cl weak adduct
Here, MH represents any other proton donor than acetonitrile,
viz., residual water, supporting electrolyte, etc.
Voltammetric and electrolytic investigations at silver elec-
trodes showed that these overall features remain essentially
identical except for the drastic anodic shift of the voltammetric
peak potential by 0.5 V (e.g., at V ) 0.2 V s : E silver ) -1.76
V versus SCE compared to Epinert ) -2.26 V versus SCE as
interpolated from ref 2a) irrespective of the scan rate since both
peak-potential variations with log(V) and half-widths are identi-
cal. The voltammetric wave (see Figure 1) remains bielectronic
and still exhibits a pure diffusional behavior controlled by slow
onto the silver surface was indicated by (i) the weakness of its SERS
spectrum intensity, (ii) the fact that its peak frequencies differed only
modestly from those of a free benzyl chloride, and (iii) the constancy
7
a
of this signal with time all along the foot of the voltammetric wave.
8) (a) Isse, A. A.; Falciola, L.; Mussini, P. R.; Gennaro, A. Chem.
Commun. 2006, 344–346. (b) Wipf, D. O.; Wightman, R. M. J. Phys.
Chem. 1989, 93, 4286–4291. (c) In fact since the overall reduction
involves 20% yield in 3-phenylpropanenitrile, a 1e-product, the electron
stoichiometry is 1.8 F/mol; yet by convenience, as previous authors,
we use the formulation “bi-electronic” to recall that the mechanism is
-
1
p
essentially bi-electronic, with 3-phenylpropanenitrile being formed by
4,8c
-
a follow-up reaction due to an S
formed during protonation of PhCH
the parent benzyl chloride. The same is true at silver cathodes since
-phenylpropanenitrile is obtained with the same 20% yield, so that
N
2 reaction of the CH
2
CN anion
-
(
2
by the acetonitrile solvent) onto
4,10
charge-transfer kinetics (R ) 0.3).
These characteristic
3
features establish that under our normal electrochemical condi-
tions the electrocatalytic role of the silver cathode is not due to
any strong alteration of the cathode surface as suggested to occur
we use the same convenient formulation. Note, however, that in both
cases this cannot be detected on the basis of voltammetry alone in the
absence of extreme precision on the electrode surface area and
diffusion coefficient of benzyl chloride. (d) For a discussion about
absolute measurement of electron stoichiometries in transient elec-
trochemistry see the following: Amatore, C.; Azzabi, M.; Calas, P.;
Jutand, A.; Lefrou, C.; Rollin, Y. J. Electroanal. Chem. 1990, 288,
(9) Amatore, C.; Sav e´ ant, J.-M. J. Electroanal. Chem. 1981, 123, 203–
217.
4
5–63.
(10) Nadjo, L.; Sav e´ ant, J. M. J. Electroanal. Chem. 1973, 48, 113–145.
1
7200 J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010

Benzyl Chloride Reduction at Silver Cathodes
A R T I C L E S
in ref 6a after specific preconditioning of the silver cathode or
to 2D or 3D organic-metallic phases as reported by Simonet
et al. for the reduction of organic halides at several electrode
[PhCH Cl···Ag] + e f [PhCH -Ag] +
2 2
Cl- (concerted or sequential) (10)
1
1
•
-
materials including noble metals and silver.
[PhCH -Ag] + e f [PhCH -Ag]
(11)
(12)
(13)
2
2
The product distribution remains essentially identical to that
-
-
[
PhCH -Ag] f PhCH + Ag
2 2
4,8c
observed at inert electrodes, strongly suggesting that it again
results from the spontaneous evolution of a free benzyl anion
-
2
PhCH f etc. (fast; see eqs 5 - 8)
(
eqs 5-8).
1
2
As established by Laviron, the absence of any specific
voltammetric alterations, i.e., as expected when strongly ad-
Conversely, SERS investigations established that at silver
cathodes the first reduction involves a benzyl chloride weakly
1
3a
sorbed intermediates are involved, is not contradictory with
the SERS observation that the first reduction proceeds through
a weak adduct of benzyl chloride onto the silver surface (eq
adsorbed through interaction of its C-Cl bond with the silver
7
surface (zone 1 of Figure 1). As soon as a significant
7
b
9
). Indeed, provided that this species is fleeting, it acts only
electrochemical current starts to flow (zone 2 of Figure 1)
this signal disappears and is replaced by the superimposition
of those of a bound benzyl radical and its corresponding
benzyl anion adduct. The relative intensities of the anionic
adduct characteristic bands vis- a` -vis those of the neutral
adduct increase when the potential is made more negative,
indicating that the steady-state concentration ratio of the two
adducts is displaced in favor of the anionic form. Both signals
then progressively vanish at higher potentials and are replaced
by those of the 3-phenylpropanenitrile/silver adduct (zone 3
of Figure 1). This species may arise only from a sequence
as a transient precomplex formed before the very electron-
transfer step in the very sense of the Debye-Schmoluchowski
description of electron transfers.
The SERS observation of a benzyl radical-silver adduct as
7a
the first observable intermediate does not imply that this
species is generated during the first elementary reduction step.
It may only be the first 1e-intermediate which presents sufficient
stability to be observable spectroscopically. Hence, at this stage,
eq 10 should be viewed either as an elementary step (viz., a
concerted one) or as the transient outcome of a sequential
process. Similarly, though the reduction of the radical adduct
into the anionic one in eq 11 is necessary to account for an
8
d
of reactions akin to those in eqs 5 and 6. Since the product
distribution and the yield of this species are essentially
identical to those observed at inert electrodes, this suggests
that most probably the anionic benzyl adduct dissociates at
sufficiently negative potentials to afford a benzyl anion prone
to proceed along eqs 5-8 as it does at inert electrodes. Due
8
c
overall bielectronic process, it is not yet determined if the
change in reduction potential vis- a` -vis that of a free benzyl
1
4
radical into its free anion (eq 2) allows such a step to proceed
over the whole potential range where the voltammetric wave is
displayed. Hence, the relative magnitude of the adsorption
enthalpies of the adducts involved in eq 11 plays a crucial role
in determining the feasibility of this step.
7
to the strong affinity of the silver surface for nitrile moieties,
3
-phenylpropanenitrile may then adsorb.
In the following we wish to establish that combining
voltammetric data with thermodynamic ones predicted by DFT
15
Hence, the SERS data are perfectly consistent with the
for the different intermediates envisioned in reactions 9-11
allows us to answer the preceding two crucial mechanistic issues.
This will finally serve to establish that the exceptional electro-
catalytic properties of silver cathodes stem from the fact that
they provide a mechanistic bridge between classical electro-
chemical activations of organic substrates and their organome-
tallic counterparts.
identity of the product distributions observed at inert and silver
cathodes, while providing evidence that at silver electrodes the
mechanism proceeds through the crucial involvement of at least
three unsuspected organometallic surface species characterized
by their SERS signals.
Thermodynamic Evaluation of the Stability of Adducts in
Reactions 9-11 by DFT. In our earlier contribution, DFT was
used to predict the structures and SERS spectra of the different
adducts considered above so as to identify them on the basis of
the experimental SERS spectra. DFT may be also used to
evaluate their thermodynamic stabilities relative to the unbound
parent species. These are summarized in Table 1 on the basis
PhCH Cl + Ag f [PhCH Cl···Ag] (up-hill pre-equilibrium)
2
2
7a
(
9)
(
11) (a) Simonet, J.; Peters, D. G. J. Electrochem. Soc. 2004, 151, D7–
D12. (b) Simonet, J. J. Electroanal. Chem. 2005, 583, 34–45. (c)
Simonet, J.; Poizot, P.; Laffont, L. J. Electroanal. Chem. 2006, 591,
4
of the DFT optimized structures of the adducts onto a Ag cluster
1
9–26. (d) Simonet, J. J. Electroanal. Chem. 2006, 593, 3–14. (e)
which was found to correctly represent a silver surface reactivity
Ghilane, J.; Guilloux-Viry, M.; Lagrost, C.; Simonet, J.; Hapior, P.
J. Am. Chem. Soc. 2007, 129, 6654–6661. (f) Poizot, P.; Laffont-
Dantras, L.; Simonet, J. Platinum Met. ReV. 2008, 52, 84–95. (g)
Poizot, P.; Laffont-Dantras, L.; Simonet, J. J. Electroanal. Chem. 2008,
7a
for our present purpose.
(12) Laviron, E. J. Electroanal. Chem. 1995, 382, 111–127.
(13) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New
York, 1980; (a) pp 519-532; (b) p 223; (c) pp 443-451; (d) pp 454-
455 and Figure 11.3.4 therein.
6
22, 204–210. (h) Poizot, P.; Laffont-Dantras, L.; Simonet, J.
Electrochem. Commun. 2008, 10, 864–867. (i) Poizot, P.; Laffont-
Dantras, L.; Simonet, J. J. Electroanal. Chem. 2008, 624, 52–58. (j)
Simonet, J. J. Electroanal. Chem. 2009, 632, 30–38. (k) Simonet, J.
Electrochem. Commun. 2009, 11, 134–136. (l) Jouikov, V.; Poizot,
P.; Simonet, J. J. Electrochem. Soc. 2009, 156, E171-E178. (m)
Simonet, J. J. Appl. Electrochem. 2009, 39, 1625–1632. (n) Poizot,
P.; Jouikov, V.; Simonet, J. Tetrahedron Lett. 2009, 50, 822–824. (o)
Simonet, J. J. Electrochem. Commun. 2010, 12, 520–523. (p) Jouikov.,
V.; Simonet, J. J. Electrochem. Commun. 2010, 12, 331–334. (q)
Jouikov, V.; Simonet, J. Electrochem. Commun. 2010, 12, 781–783.
(14) The reduction potential of the benzyl radical in acetonitrile at a gold
electrode has been reported to lie in between-1.43 and-1.45 V versus
SCE. See the following:(a) Sim, B. A.; Milne, P. H.; Griller, D.;
Wayner, D. D. M. J. Am. Chem. Soc. 1990, 112, 6635. (b) Grampp,
G.; Muresanu, C.; Landgraf, S. Electrochim. Acta 2008, 53, 3149–
3155.
(15) Frisch, M. J. et al. Gaussian09, revision A.02; Gaussian, Inc.:
Wallingford, CT, 2009.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010 17201

A R T I C L E S
Huang et al.
7
Table 1. Thermodynamic Energies of Adsorption on a Naked Ag
4
in zone 3). Competitive adsorption of acetonitrile is then a
parameter to be considered. For example, if one benzyl chloride
displaces m ACN molecules, a better approximation of benzyl
Cluster of the Different Moieties Considered in This Work As
a
Deduced by DFT
energy (kcal/mol.)
m
m)0
chloride Gibbs free energy of adsorption is ∆Gads ) ∆Gads
-
m)0
adduct
∆E
∆H
∆G
m(∆G°ads)CH CN where ∆Gads is the value reported in Table 1
3
for m ) 0. In any instance, since (∆G°ads)CH CN < 0, this makes
benzyl chloride-Ag
4
-3.6
-16.4
-26.4
-8.6
-2.8
-16.0
-26.1
-7.9
5.4
3
benzyl radical-Ag
benzyl anion-Ag
acetonitrile-Ag
4
-6.3
-16.9
-1.5
preadsorption of benzyl chloride even more unfavorable than
reported in Table 1 for m ) 0. The same problem also exists
for the two benzyl adducts. Yet, owing to the similar geometries
of the two adducts (Figure 2), the numbers of ACN molecules
displaced in each case must be equal, so that the value of m
should not affect the change in driving forces between reactions
4
4
a
See Supporting Information in ref
7 for the corresponding
configurations and the coordinates of the models.
1
1 and 2.
Reduction of the Benzyl Radical-Silver Adduct. The feasi-
bility of reducing the benzyl radical adduct into the anionic one
eq 11) over the whole potential range covered by the voltam-
(
metric wave (Figure 1) is a crucial point. This determines the
theoretical possibility of accounting for the observation of an
8
c
overall two-electron wave through this reaction. Indeed, the
wave does not exhibit any feature characteristic of any change
of electron stoichiometry while it unfolds (Figure 1).
Reduction of a free benzyl radical into its free anion (eq 2)
2
is predicted to occur at (E° )inert ) -1.43 V versus SCE in
1
4
acetonitrile; i.e., its driving force is almost null at the foot of
the wave observed at silver cathodes (region 1 in Figure 1).
Conversely, since the adsorption Gibbs free energy of the benzyl
anion is stronger than that of the neutral species (Table 1),
reaction 11 is thermodynamically favored versus that in eq 2:
Figure 2. DFT optimized structures of adducts of benzyl chloride onto
the silver surface as a function of the number of silver atoms used in
modeling the silver surface: (a) Ag ; (b) Ag , n ) 2-5 and 10 (shown for
1 n
n ) 4).
Interestingly, Table 1 reports that the energy and enthalpy
of formation of the benzyl chloride-silver adduct are weak but
negative in agreement with the expected affinity of the silver
surface for chlorine which was previously invoked as part of
(
E°
2
)
silver ) (E°
2
)
2 inert
inert + (∆G°ads,radical - ∆G°ads,anion)/F > (E° )
where ∆G°ads,radical and ∆G°ads,anion are the adsorption Gibbs free
energies of the benzyl radical and the benzyl anion, respectively
3
-5,6a,8a
(Table 1). Using the data in Table 1, one obtains (E°
2
)
silver
)
the electrocatalytic mechanism.
However, the ensuing
Gibbs adsorption free energy results in a positive value (though
still small), thus providing evidence of the constraints enforced
into the organic frame during adsorption (compare Figure 2b).
Such a small positive value of ∆G°ads is perfectly consistent
with regard to the weakness of the SERS intensity of the
spectrum of the benzyl chloride adduct and the fact that its peak
frequencies did differ only modestly from those of a free benzyl
(
16) (a) For quantitative kinetic discussions about oxidative addition
mechanistic pathways in homogeneous organometallic catalysis, see
the following, e.g.: Amatore, C.; Jutand, A. In Handbook of Orga-
noPalladium Chemistry for Organic Synthesis; Negishi, E. I., Ed.;
Wiley, New York, 2002; chapter III.2.19, pp 943-972. (b) For an
evaluation of electron-transfer pathways in organometallic oxidative
additions, see the following, e.g.: Amatore, C.; Pfl u¨ ger, F. Organo-
metallics 1990, 9, 2276–2282, and references therein. (c) For the
importance of precomplexation during oxidative additions, see the
following, e.g.: Amatore, C.; Azzabi, M.; Jutand, A. J. Am. Chem.
Soc. 1991, 113, 1670–1677. For a general review on homogeneous
versus heterogeneous organometallic catalysis in the context of
palladium catalysis, see the following, e.g.: Phan, N. T. S.; Van Der
Sluys, M.; Jones, C. W. AdV. Synth. Catal. 2006, 348, 609–679.
17) (a) Marcus, R. A. J. Chem. Phys. 1965, 43, 679–701. (b) All electron-
transfers considered in this work do not obey the requirement of
Marcus theory for outer-sphere electron-transfers, although this does
not prevent using Marcus formulations for reorganization energies for
the sake of comparison to the experimental values determined. (c)
Similarly, we resort to quadratic correlations between activation
barriers and driving force (eq 19) without implying that the conditions
of application of Marcus theory are obeyed since we do not evaluate
7
chloride molecule.
To evaluate the role of the cluster size (Ag
) on the outcome of DFT-simulated thermodynamic quantities,
n
; n ) 4 in Table
1
the number of silver atoms was varied (n ) 1 to 5 and 10). For
n ) 2-5 and 10, the structures of the adducts were consistently
reproducible for the organic moiety as were the values of the
predicted SERS bands and thermodynamic functions in vacuum.
A four-atom cluster was thus preferred in the larger DFT
simulations which incorporated also solvent contributions treated
as a dielectric continuum (see also Experimental Section).
Conversely, for n ) 1 drastic structural changes were
observed in the adduct as shown for example by comparing
the two structures predicted for the benzyl chloride precomplex
in Figure 2. For the more realistic models (n ) 2-5, 10) benzyl
chloride was found to interact with one silver atom of the cluster
through its C-Cl bond in a configuration strongly reminiscent
(
the coefficient of the quadratic correlations based on Marcus expression
q
of the intrinsic barrier ∆G
0
but determine them experimentally.
Quadratic correlations between activation barriers and driving force
(eq 19) are much more general than the classical Marcus expression
for outer-sphere electron-transfers and stem only from the geometric
properties of the intersection of two potential wells whatever their
shape and origin. In this respect, it must be pointed out that Marcus’
major contribution was not to introduce such quadratic relationships
which were pre-existing (for a discussion see refs 18a, d) but,
importantly, to establish ab initio the fundamental origin of these linear
and quadratic correlations in the particular case of outer-sphere
1
6
of the first step of an oxidative addition. This suggests that
the neighbors of the interacting silver atom participate as ligands
of a reactive metal center would do in a homogeneous
1
6
organometallic oxidative addition.
electron-transfers. This led Marcus to demonstrate (1) the possibility
q
Table 1 also reports the adsorption energy of the acetonitrile
solvent (ACN). Indeed, SERS revealed that the solvent may
competitively adsorb at the electrode surface in zones 1 and 2
of Figure 1 (as well as the 3-phenylpropanenitrile end-product
of evaluating ∆G
0
based on spectroscopic, structural, and thermo-
dynamic physicochemical data, or from isotopic reactions (Marcus’
cross-relationship), and (2) that an inverted region appears when |∆G°|
q
>
4∆G
0
provided that the product system has almost no vibronically
excited states.
1
7202 J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010

Benzyl Chloride Reduction at Silver Cathodes
A R T I C L E S
7
a
Our previous SERS investigations revealed that at suf-
ficiently negative potentials (region 3 in Figure 1) the anionic
adduct signal disappears and is replaced by that of the adsorbed
3
-phenylpropanenitrile product. One may argue that this stems
from the chemical reactivity of the adduct as such. Yet owing
to the close identity between the product distributions at inert
and silver cathodes, one is inclined to believe that the final steps
occur from a dissociated benzyl anion (eqs 5-8) and that the
3
-phenylpropanenitrile adduct is formed by adsorption of the
nitrile product after its formation in solution. Dissociation of
the benzyl anion is certainly enforced by the growing electro-
13a
static repulsion exerted by the increasingly negative electrode
potential onto the negatively charged adsorbed organic moiety
Figure 3. DFT optimized structures of the benzyl radical (left) and benzyl
(
compare to eq 14 for the radical adduct). Note that the same
anion (right) adducts on a Ag
4
cluster.
electrostatic factor certainly explains why no adsorption of
chloride ion, a necessary product of reaction 10, could be
observed by SERS over the potential range of the voltammetric
(
E°
2
)
inert + 0.46 V, i.e., (E° )
2
silver ≈ -0.97 V versus SCE. Such
a value is amply sufficient for the reduction in eq 11 to be
thermodynamically driven all along the voltammetric wave,
including its very foot (region 1 in Figure 1). Nonetheless, such
thermodynamic argument alone is not sufficient to prove the
case, so let us examine now its feasibility from a kinetics point
of view.
7
wave despite the affinity of chloride ions for silver surfaces.
General Considerations about the Mechanism of the First
Reduction Step. SERS investigations established that at silver
electrodes benzyl chloride reduction begins through an electron
transfer onto its weakly bound surface complex to provide,
directly or sequentially, a bound benzyl radical. The above
section confirmed that such benzyl adduct could then be easily
reduced so as to lead to an overall bielectronic voltammetric
wave. This also implies that the fact that signals are observed
for both benzyl adducts is due to kinetics, with the surface
concentration of the neutral species obeying a dynamic steady
state between its formation (eq 10) and its reduction (eq 11) in
which the former decreases while the wave is spanned due to
decreasing diffusion rate of the benzyl chloride to the electrode
while the latter increases with the increasingly negative potential
of the electrode. This is coherent with the progressive decrease
of the neutral benzyl adduct contribution while the wave is
spanned.
In the absence of any kinetics information, this issue may
only be evaluated by comparison to the case of the outer-sphere
reduction in eq 2 since this is known to proceed fast at inert
2a
electrodes. In this respect it is interesting to consider the charge
distribution within the benzyl radical adduct as it is predicted
4
by DFT, e.g., for Ag (eq 14):
•
2
-0.14
2
+0.14 •
- Ag4
PhCH + Ag f [PhCH
]
(14)
4
This shows that in its neutral adduct the benzyl moiety is
already partially charged negatively so that the outer-sphere
reorganization occurring during the reduction in eq 11 is
certainly less than that in eq 2. Hence, its outer-sphere
Hence, this weak adsorption necessarily acts as an uphill
1
7a
13c
reorganization energy, λ
o
*, is certainly much smaller than
preceding step to the electron-transfer one (CE mechanism).
12
that in eq 2. The same is true for the inner-sphere reorganization
Under such conditions no usual voltammetric features related
to adsorption may be observed in agreement with the pure
17a
13a
energy, λ
carbon is already hybridized in sp as it is in the anionic adduct
Figure 3). Conversely, this hybridization must drastically
i
*, since in the neutral state of the adduct the benzylic
3
diffusional appearance of the wave in Figure 1. Scheme 1
illustrates the different mechanistic possibilities which are
consistent with the SERS observations and which may link
mechanistically the benzyl chloride precomplex to the first
intermediate identified by SERS.
(
2
change when a free benzyl radical (sp ) is reduced into a free
anion (sp ) as in eq 2. This is evidence that the intrinsic
3
activation barrier of reaction 11 is much less than that of the
reduction of a free benzyl radical. Hence, one may safely
conclude that the reduction in eq 11 is feasible both thermo-
dynamically and kinetically over the whole potential range of
the voltammetric wave observed at silver cathodes (Figure 1).
To conclude this section, another kinetic possibility needs to
be considered. This would involve a squared-scheme reduction
of the neutral benzyl adduct: predesorption of the benzyl radical,
followed by its reduction in solution and binding of the ensuing
free anion in competition with its fast spontaneous chemical
reactions in eqs 5-8. Yet in this case, the uphill desorption
would disfavor the overall reduction by +6.3 kcal/mol (Table
On the basis of our best knowledge, Scheme 1 considers all
the possible situations which are consistent with present electro-
organic views. These are classified into two categories depending
on the concerted or nonconcerted nature of the elementary
electron-transfer step. Note that, except for the absence of a
chloride ion adsorption which did not occur based on the
absence of the characteristic Ag-Cl features in the SERS
7
a
spectra, the concerted pathways I summarize all those which
have been tentatively considered in previous works to account
for the strong electrocatalytic effect of silver cathodes. To our
best knowledge, the nonconcerted pathways II have never been
considered. For this reason, in the following, we consider all
pathways summarized in Scheme 1 without any preconsideration
about their occurrence.
1
2
), i.e., by ca. 0.27 V. Since (E° )inert ) -1.43 V versus SCE
1
4
for the free radical, the apparent standard reduction potential
for this route would be ca. -1.70 V. This shows that such a
pathway is thermodynamically hampered over most of the
potential range of interest and could not account for the
observation of a full-fledged bielectronic wave. It may then
be safely concluded that the second reduction step occurs
between the benzyl adducts as represented in eq 11.
In the first concerted possibility (case I-1), the reduction of
the initial adduct would give rise to a free benzyl radical such
as that formed at inert electrodes. In this case the electron
transfer would be concerted with the rupture of the weak binding
of the substrate as well as with the rupture of the comparatively
8
c
J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010 17203

A R T I C L E S
Huang et al.
Scheme 1
stronger carbon-chlorine bond. This step would then be favored
existence being ascribed to a stabilization through a close
interaction with the silver surface. This hypothetical species may
then evolve directly to the benzyl radical-silver adduct (case
II-2) or dissociate to generate a free benzyl radical which then
may adsorb onto the cathode surface (case II-1) in competition
with its reduction.
At inert electrodes, electrochemical reduction of benzyl
chloride may not proceed through the corresponding anion
thermodynamically but disfavored kinetically versus that in eq
1
. Indeed, if the uphill precomplexation favors thermodynami-
cally the electron-transfer step, it burdens its activation barrier
since only a fraction R ≈ 0.3 of this gain may help to reduce
the electron-transfer barrier. Furthermore, within the framework
of concerted electron transfers, involving an additional structural
contribution (vs that in eq 1) in a concerted step should result
9
2
a
in a larger intrinsic activation barrier. Hence, it appears
doubtful that mechanism I-1 may outrun that observed at an
inert electrode (eq 1). One cannot conclude so easily about the
occurrence of mechanism I-2. Indeed, here the electron-transfer
step involves in addition a concerted binding of the benzylic
carbon onto the silver surface. This may look awkward within
an electrochemical framework, but this is an exact heterogeneous
mechanistic analogue of homogeneous oxidative additions
during which a weakly coordinated carbon-halogen bond is
reductively cleaved while each moiety binds to the metal
1
radical due to the highly negative value of (E° )inert,NC in Scheme
2a
1. This has indeed been evaluated to be at least -2.9 V versus
2
a
2a
SCE, while (E°
1
)inert ) -0.76 V versus SCE for the concerted
electron transfer in eq 1. Yet, the concerted process has a much
higher intrinsic activation barrier, (∆G )inert ) 20.7 kcal/mol,
0
than that, (∆G
formation of a free anion radical as follows from Marcus’
expression of the corresponding outer-sphere reorganization
energy.
q
2a
q
0
)
inert,NC ≈ 4.5 kcal/mol, of the would-be
1
7a
The important inner reorganization energy of the
concerted pathway (eq 1) explains why the reduction wave of
benzyl chloride at inert electrodes is observed at considerably
1
6
center. Yet, such a step is expected to have an even larger
intrinsic activation barrier than that in case I-1 due to an
additional contribution due to the creation of the carbon-benzyl
bond. Indeed, even if this bond is already prefigured in the
-
1
more negative potentials (-2.26 V vs SCE at 0.2 V s as
interpolated from ref 2a) than its standard potential. In this
q
0
respect it is inferred that if (∆G
)
inert was larger, the concerted
PhCH
2
Cl-Ag adduct (compare Figure 2b), its elongation and
process would not be able to compete with the nonconcerted
one for kinetic reasons though it would remain highly favored
thermodynamically.
its weakness provide evidence that it is far from the strong bond
ultimately present in the benzyl-Ag adduct. Conversely, it
would be highly favored thermodynamically compared to case
I-1 and to the concerted reaction 1 due to the large stabilization
of its product, a gain which also contributes to a decrease of
the effective activation barrier of the CE sequence. Hence,
without any quantitative evaluation of each effect, one cannot
conclude about the overall ability of route I-2 in outrunning
the simple concerted mechanism in eq 1.
Though not generally expressed in this way, this shows that
competition between concerted and nonconcerted mechanisms
is between a kinetically easy but thermodynamically difficult
route (nonconcerted) versus a kinetically difficult but thermo-
dynamically favored one (concerted). Hence, any effect that
would stabilize sufficiently an anion radical intermediate without
increasing too much the corresponding reorganization energy
may well lead to a situation in which the formation of such an
intermediate would outrun the concerted pathway. In this
respect, it is worth noticing that we have shown above that silver
cathode surfaces may donate (benzyl chloride precomplex,
Within the nonconcerted mechanistic set II, a 1e-intermediate
on the way between an anion radical and the fully dissociated
pair, PhCH
•
-
2a
2
+ Cl , is envisioned as a possible primary
product of the first electron transfer, its hypothetical transient
1
7204 J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010

Benzyl Chloride Reduction at Silver Cathodes
A R T I C L E S
benzyl radical adduct) or accept (benzyl anion adduct) electron
density in a similar way as an organometallic center proceeds
The heterogeneous rate constant k varies with the driving
force F(E - E°) via the dependence of its effective activation
1
6
q
18,19
during the activation of organic halides. Hence it is not
unreasonable to envision that similar electronic effects may
contribute to stabilize a complexed anion radical through
stabilizing the π/π*-σ/σ* coupled organic orbitals by partial
donation to the silver cathode. If the carbon-chlorine bond in
such an intermediate is sufficiently elongated though not cleaved,
the energetic demand of its formation may then be drastically
decreased vis- a` -vis that required to create a free anion radical
while not severely burdening the intrinsic electron-transfer
activation barrier. At least at this general hypothetical level,
this justifies considering the hypothetical occurrence of the
nonconcerted pathways in Scheme 1.
barrier (∆G )eff on this parameter:
q
k ) κZ exp[-(∆G ) /RT]
(16)
eff
Here κ is the adiabatic transmission factor (in the following we
consider that κ ) 1 as usual for electrochemical reactions due
to the high quantum coupling between the substrate and the
2
a,18e,f
1/2
metal)
and Z ) (RT/2πM) (where M is the molar mass
3
-1
of PhCH Cl, i.e., Z ) 5.6 × 10 cm s ). When there is a direct
2
q
reduction of the substrate as occurs at inert electrodes, (∆G )
eff
is the activation barrier of the electron-transfer elementary step
q
(E) whatever its intrinsic nature (viz., concerted or not): (∆G )eff
q
)
(∆G )
E
. Conversely, within a CE sequence controlled
In the following, we wish to evaluate quantitatively the
information concealed in the voltammetric peak potentials of
the waves observed at inert electrodes and silver cathodes and
contrast them to the DFT predictions concerning the intermedi-
ates observed by SERS to examine if this allows discriminating
among the four pathways represented in Scheme 1.
1
2,13c
kinetically by the electron-transfer step,
chemical step (C) has a positive Gibbs free energy, (∆G°)
the pre-equilibrium free energy acts as a “work” term in
i.e., when the
C
,
1
8a-d
q
q
Debye-Schmoluchovski theory:
∆G°) . In Scheme 1, the uphill pre-equilibrium step consists
of the adsorption of benzyl chloride onto the silver cathode
(∆G )eff ) (∆G )
E
+
(
C
Quantitative Considerations about the First Reduction
Mechanism. To proceed further, we need first to recall several
basic but crucial features attached to irreversible voltammetric
12
surface, so that (∆G°)
C
) ∆G°ads where ∆G°ads is the
adsorption Gibbs free energy of the benzyl chloride onto the
silver surface (Table 1). Hence, eq 16 is rewritten as follows
for each type of electrode material. At inert electrodes (eq 1)
10,13b
waves (viz., controlled by overall slow charge-transfer kinetics)
since they are essential to the following analysis. Indeed, even if
these are present in textbooks, they are often improperly used.
inert
q
k
) Z exp[-(∆G )E,inert/RT]
(17)
Fully irreversible voltammetric waves are controlled by the
interplay between diffusion, thermodynamics, and the corre-
sponding heterogeneous rate law. The latter may be expressed
as follows for all the cases at hand (Scheme 1 and eq 1),
and
silver
q
k
) Z exp{-[(∆G )
+ ∆G°ads]/RT}
E,silver
(18)
1
0,13b
irrespective of the exact mechanism involved:
at silver electrodes (Scheme 1), irrespective of the exact
mechanism followed.
I ) -FADk[PhCH Cl]
(15)
2
0
Sav e´ ant et al. established that, irrespective of the exact type
Here I is the voltammetric current, A the electrode surface area,
D the diffusion coefficient of the benzyl chloride substrate, k
the effective heterogeneous rate constant at the electrode
potential E under consideration (k°, the standard rate constant,
2a,d,e
of electron transfer considered (viz., concerted or not),
the
q
dependence of (∆G )
E
on the electrode potential, i.e., with F(E
E°), may always be formulated within a Marcusian
framework. The exact nature of the electron-transfer step
concerted or not) is then only reflected by the value of its
19
-
2 0
is the value of k at E ) E°), and [PhCH Cl] the free benzyl
(
chloride concentration in solution at angstrom distances from
the electrode surface, i.e., just before interacting with the very
electrode surface. Note that the use of an irreversible electron
transfer rate law in eq 15 does not imply necessarily that the
backward rate constant is negligible. This may apply also when
the first reaction product is highly unstable since then the overall
kinetics of electron transfer may only proceed in the forward
q
0
standard potential E° and by its intrinsic activation barrier ∆G
q
(
viz., the value of (∆G ) at E ) E°). Thus, irrespective of the
E
q
2a,17b,c,19
situation considered, (∆G ) in eqs 17 or 18 is given by
E
(19) To be rigorous, the electrode potential E in eq 24 should be replaced
by the expression (E-φ) where φ is the Frumkin double-layer potential
term which accounts for the fact that the electron-transfer site is not
located at the very surface of the electrode (where E prevails) but a
few angstroms from it, in proximity to the outer Helmholtz plane
1
0,13d
direction.
(
OHP), where the effective potential is E-φ. Yet, under most
circumstances the site of electron transfer is not known with sufficient
precision so that φ is unknown, especially at solid polycrystalline
electrodes and in organic solvents. Furthermore, this value is also
modified by electrostatic mirror effects due to the metallic surface of
the electrode which acts on the kinetics (see refs 18e, f). Therefore,
this term is generally not accounted for, the ensuing error being then
(
18) (a) Amatore, C. In Organic Electrochemistry; Lund, H., Hammerich,
O., Eds.; M. Dekker: New York, 2000; chapter 1, p 61 and the
following pages. For discussions about the reasons justifying the
combination of “work terms” in the “effective” activation barrier, see.
(
(
b) ref 17a. (c) Marcus, R. A. Discuss. Faraday Soc. 1960, 129, 21.
d) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Phys. Chem. 1986,
0, 37473756. These two latest works consider bimolecular homo-
q
shuttled into the experimental ∆G (E) value, i.e., somewhat corrupting
q
9
the exact meaning of the determined value of ∆G
0
.
geneous reactions, but the concepts involved are readily transposable
to heterogeneous cases within the Marcus framework (upon consider-
ing the electrode as one infinite reactant plane). For a discussion of
this latter point, see the following reference. (e) Kojima, H.; Bard,
A. J. J. Am. Chem. Soc. 1975, 97, 63176324. Yet the metallic nature
of the electrode introduces some electrostatic mirror effects not
accounted for in bimolecular cases. See the following reference. (f)
Hale, J. M. In Reactions of Molecules at Electrodes; Hush, N. S.,
Ed.; Wiley Interscience: New York, 1971, chapter 4, p 229.
(20) This relationship may appear surprising at first glance but should be
readily understood; the observation of a CV wave at a given scan rate
(i.e., of a current flow compatible with a diffusion-limited supply at
this scan rate) requires that the electron-transfer rate constant has the
exact magnitude required for sustaining this precise current flow
(compare eq 15). This condition may be fulfilled only when the
potential-dependent effective activation barrier at the CV peak potential
reaches a constant magnitude (compare eq 22) irrespective of the exact
elementary steps under voltammetric scrutiny.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010 17205

A R T I C L E S
Huang et al.
q
2a
2a
inert ) -0.76 V, (Ep)inert
q
q
0
q 2
0
and with the values of (∆G
)
0
)
inert
,
1
(E° )
(
∆G ) ) ∆G × [1 + F(E - E°)/4∆G ]
(19)
E
2
a
p
-1
-2.26 V, and (E )silver ) -1.76 V versus SCE at 0.2 V s
°
q
though the values of E and ∆G
hand and the latter does not necessarily obey Marcus’ classical
expression since the present electron-transfers do not obey the
outer-sphere requirements.
Under most experimental circumstances the potential range
in which the voltammetric wave may be observable is small
enough versus ∆G
most usual Butler-Volmer formulation. This follows from
a local linearization of its squared bracketed term
0
depend on the exact case at
as well as R ) 0.3, the solution of the linear two-equation-two-
q
unknown system in eqs 24 and 25 affords (∆G
0
)silver ) 10 kcal/
mol and (E° silver ) -1.0 V versus SCE. These results are
1
)
1
7b,c
independent of the exact mechanism considered here. They only
depend on the value of ∆G°ads which has been used and on the
other independently known parameter or experimental data
values. Note that as discussed before we used the minimum
value of ∆G°ads. Increasing ∆G°ads would lead to an even more
q
0
/F that eq 19 may be approximated by its
1
3b
2
1
negative value for (E°
SCE is the most positive value of (E°
voltammetric observations and previously published data.
silver is found to be at least 0.24 V more negative
1
)
silver
.
So (E°
1
)
silver ) -1.0 V versus
1
)silver consistent with the
q
q
2a
(∆G ) ) ∆G + R × F(E - E°)
(20)
E
0
1
Then (E° )
where R, the transfer coefficient, is then the mean value of
than (E° )inert. This result is clearly inconsistent with any of the
concerted pathways envisioned in mechanisms I since by
construction each of them should correspond to a decrease of
the Gibbs free energy of the electron transfer due to the
thermodynamic contribution of the uphill preadsorption step
route I-1) and of the cumulative effects of this step and of the
formation of a stable benzyl radical-silver adduct (Table 1)
1
q
0
R ) [1 + F(E - E°)/4∆G ]/2
(21)
over the whole potential range spanned by a voltammetric wave
for the scan rates, ν, of interest. On the basis of these definitions,
one readily establishes that, at any given scan rate (∆G )eff
the value taken by (∆G )eff at the voltammetric peak potential
(
q
p
,
q
of any irreversible wave under slow charge control is given
(21) In the main text the solution of the 2-equation linear system consisting
2
a,13b
of eqs 24 and 25 was searched in the most general case (viz., being
by
q
valid for either paths I or II) in terms of the values of (∆G
0
)silver and
1
(E° )
silver while imposing ∆G°ads at its minimal value reported in Table
q p
(
∆G ) ) RT(ln Z - 0.78) + 0.5 ln(RT/RFνD) (22)
eff
1 (see the corresponding discussion in text). However, if one restricts
the analysis to the two mechanisms I-1 and I-2 a more simple solution
may be proposed since this does not require fixing an a priori value
for ∆G°ads. Indeed, considering the thermodynamic cycles in Scheme
q
for any values of E° and ∆G
0
, in eq 20 R ≈ 0.3 is
experimentally determined to be approximately equal at silver
electrodes and inert ones, and Z or D are by definition identical
in each situation. Hence, eq 22 establishes that, irrespective of
1
1
, (E° )
silver is a direct function of (E°
1
)
inert and ∆G°ads (mechanism
I-1) or of (E° )inert, ∆G°ads, and ∆G°ads,radical (mechanism I-2):
1
I-1
q
p
(E° )
) (E° )
+ ∆G°ads/F
the exact mechanism followed, the values of (∆G )eff observed
for a given scan rate at each electrode material are equal. Note,
however, that this identity relates two (∆G )eff values which
1 silver
1 inert
(mechanism I - 1) (FTN-1a)
q
I-2
silver
(
E°1)
) (E° )
+ (∆G°ads
-
1 inert
q
0
, and different peak potentials (∆Ep
pertain to different E°, ∆G
∆
G°ads,radical)/F (mechanism I - 2) (FTN-1b)
p
p
)
E
silver - E inert ) 0.5 V). Rewriting this relationship within
Equations FTN-1a,b may be rewritten into a unified form by
introducing a parameter, F, defined such as F ) 0 for mechanism I-1
and F ) -∆G°ads,radical/∆G°ads for mechanism I-2. Then
the framework of eq 20 shows that at any given scan rate
q
p
q
(
∆G )
+ RF(E - E° )
+ ∆G°ads ) (∆G )
+
0
silver
1 silver
0 inert
0
1
I
p
(E° )
) (E° )
+ (1 + F)∆G°ads/F
RF(E - E )
(23)
1 silver
1 inert
inert
(
mechanism I-1 or I-2) (FTN-2)
p
p
Since R ≈ 0.3 for both electrode materials (i.e., ∆E ) E silver
so that eqn 24 becomes
p
-
E
inert is constant), eq 23 may finally be rearranged to afford
q
q
q
(∆G )
+ [1 - R(1 + F)]∆G°ads ) (∆G )
-
a linear equation relating (∆G
0
)silver and (E°
1
)
silver
:
0 silver
0 inert
p
RF∆E (FTN-3)
q
q
(
∆G )
- RF(E° )
) (∆G )
- ∆G°ads
-
p
p
q
0
0
silver
1 silver
0 inert
Considering the numerical values (E )silver ) (E )inert + 0.5 V, (∆G
)
inert
)
2
a
2a
p
p
) 20.7 kcal/mol, (E°
)
inert )-0.76 V versus SCE, (E )inert
1
RF(E°1)
inert
- RF∆E (24)
-1 2a
-
2.26 V versus SCE at 0.2 V s
,
and R ) 0.3, eq FTN-3 becomes
q
0
in which all the right-hand terms are known. Note that, since
this equation follows only from the properties of irreversible
voltammetric waves (viz., from eq 22), it is independent of the
(
∆G )
+ (0.7 - 0.3F)∆G°ads ) 17.2 kcal/mol
silver
(
FTN-4)
q
A second linear relationship between (∆G
by rewriting eq 25 using the same procedure:
0
)silver and ∆G°ads is given
specific mechanism (Scheme 1) underlying the values of
q
0
(
∆G
1 silver
)silver and (E° ) .
q
A second relationship between these two parameters, again
(
∆G )
- 0.6(1 + F)∆G°ads ) 13.8 kcal/mol
silver
0
independent of the exact mechanism involved, is provided by
recalling that R ≈ 0.3 is experimentally determined at each type
of electrode. From eq 21 it then follows that at any scan rate of
interest in this work
(
FTN-5)
Solution of the two-equation-two-unknown linear system in eqs FTN-
, FTN-5 affords ∆Gads ) 2.6 kcal/mol for mechanism I-1 and ∆G°ads
(2.6 + 0.23∆G°ads,radical) kcal/mol for mechanism I-2. Since
4
)
∆
G°ads,radical is necessarily negative (Table 1), this result means that
p
q
p
q
(
E - E° )
/(∆G )
) (E - E° ) /(∆G )
0 inert
∆G°ads needs to be lower than 2.6 kcal/mol for mechanism I-2 (note
that using ∆G°ads,radical ) 6.3 kcal/mol (Table 1) would afford ∆G°ads
1
silver
0 silver
1 inert
(
25)
)
1.15 kcal/mol). Since the value of ∆G°ads ) 5.4 kcal/mol reported
in Table 1 is the minimum one, both results cannot be reconciled with
the present DFT thermodynamic evaluations and with the SERS
evidence. This definitively rules out both concerted mechanisms I.
With a minimal value for ∆G°ads ) 5.4 kcal/mol (viz., upon
neglecting the desorption of acetonitrile molecules, see above),
1
7206 J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010

Benzyl Chloride Reduction at Silver Cathodes
A R T I C L E S
1
4
for route I-2. Hence, in both situations (E°
1
)
silver should be less
facile reduction so as to afford the benzyl radical adduct
observed by SERS. Though this cannot be disproved, it appears
unlikely on the basis of the theoretical outcome of such
1
negative than (E° )inert. More precisely, since 5.4 kcal/mol is
the minimal value of ∆G°ads, the thermodynamic cycle for
I-1
9
mechanism I-1 in Scheme 1 predicts (E°
1
)
silver ) (E°
1
)
inert
+
competitions. Indeed, in such a case, SERS should not yield
any observable signal for the neutral benzyl adduct but should
yield only that of the anionic benzyl adduct. Yet, the signal of
∆
G°ads/F g -0.53 V versus SCE. The outcome is even worse
for mechanism I-2 since the formation of a stable bound benzyl
7
radical (-6.3 kcal/mol, Table 1) necessarily corresponds to a
the neutral species is clearly observed in SERS showing that
I-2
much more positive value: (E°
1
)
silver ) (E°
1
)
inert + (∆G°ads
-
this species is a transient one involved in a dynamic steady state;
this strongly suggests that the neutral benzyl adduct is formed
directly on the silver surface upon fragmentation of the unstable
bound anion radical. The fragmentation of this intermediate is
expected to occur within a few vibrations of its weakened
carbon-chlorine bond, while the exergonic reduction of the
neutral benzyl adduct cannot proceed much more rapidly. It is
then reasonable that such a situation may lead to a steady state
dynamic concentration of the neutral benzyl adduct observable
by SERS.
Within this framework, and irrespective of the issue of the
competition between routes II-1 and II-2 (though we favor case
II-2), the first reduction stage of benzyl chloride at silver
electrodes may be viewed as a missing link, bridging the gap
between activation of organic halides by electrochemical
reduction at inert electrodes and by oxidative additions to
∆
G°ads,radical)/F g -0.25 V versus SCE. Therefore, unless other
effects not envisioned here were to be involved, the two
concerted mechanisms labeled I in Scheme 1 have to be rejected
for not being able to account quantitatively for the magnitude
of the voltammetric wave shift between inert and silver cathodes.
Let us now examine the nonconcerted mechanisms in Scheme
1
. The value of (E°
1
)inert,NC, i.e., that corresponding to the
formation of a free anion radical, has been evaluated at ca. -2.9
2a
V versus SCE or more negative. Compared to (E°
V versus SCE obtained above by taking into account that
E° silver applies to the adsorbed benzyl chloride, this shows
1
)silver ) -1.0
(
1
)
that the primary product of the first electron transfer is stabilized
by ca. 38.5 kcal/mol or more vis- a` -vis a free benzyl chloride
anion radical (as depicted in brackets in Scheme 1). This appears
to be totally excessive for a simple complexation of a classical
benzyl chloride anion radical unless the carbon-chlorine bond
is severely extended. Conversely, the difference between
1
6
homogeneous organometallic centers.
Note that this mechanistic framework may be more general
than the case investigated here though, to the best of our
knowledge, this is the first time this is backed up by quantitative
kinetic and spectroscopic evidence. Indeed, though generally
forgotten in molecular electrochemistry, mechanistic sequences
akin to those labeled II in Scheme 1 have been already proposed
in an electrochemical context. This has for example been done
in rationalizing the observation of smaller but drastically anodic
prewaves during the reduction of several organic halides (RX)
at a mercury electrode. In such cases the main electrochemical
wave observed at higher potentials proceeds through a sequence
analogous to eqs 1-4, i.e., without involving chemically the
mercury surface. Conversely, the drastically anodic prewaves
were accounted for by the involvement of organomercury
species resulting from oxidative addition (eq 26) taking place
(
E°
1
)
silver and (E°
1
)inert shows that the primary product at silver
electrodes is uphill by ca. 11 kcal/mol vis- a` -vis a fully
dissociated pair PhCH
fifth of the ca. 50 kcal/mol or more separating a free anion
radical and a dissociated pair PhCH
•
-
2
+ Cl . This represents only about one-
•
-
2a
2
+ Cl . Hence, describing
•-
tentatively this hypothetical species as an adduct “[PhCH
2
· · ·Cl] /
Ag”, in which the carbon-chlorine bond is considerably
extended but still not fully cleaved, rather than as a fully fledged
anion radical interacting with the silver electrode surface does
not appear unrealistic. Such species would be similar to those
envisioned as transition states in homogeneous organometallic
22
1
6b
oxidative additions.
The much smaller intrinsic activation barrier ∆G
at silver electrodes (10 kcal/mol) versus that at inert ones (20.7
q
0
obtained
2
2
2
a
during the primary electron-transfer event:
kcal/mol) is also compatible with a permanence of a
carbon-chlorine bond in this primary intermediate. Indeed, for
0
I
-
Hg
+ RX + e f R-Hg + X
(26)
a nonconcerted reduction at inert electrodes, one would predict
cathode
q
(
∆G
0
)
NC ≈ 4.5 kcal/mol upon considering Marcus’ expression
17a
I
II
-
for the outer-sphere reorganization energy. Hence, a value
of (∆G
tion of ca. 5.5 kcal/mol, i.e., being about one-third of the
difference (∆G
R-Hg + RX + e f R-Hg -R + X
(27)
(28)
q
0
)silver ) 10 kcal/mol points to an inner-sphere contribu-
II
0
R-Hg -R f R-R + ″Hg ″
q
0
q
)
0
inert - (∆G )NC ) 15.5 kcal/mol. This is
Within the present framework, the global eq 26 may indeed
be viewed as a CE sequence involving a precomplexation-
oxidative addition (C) leading to a transient organomercury
intermediate continuously displaced by its facile reduction (E).
The only difference with the process investigated in this work
consistent with the presence of an elongated carbon-chlorine
bond in the primary intermediate though without undergoing a
full rupture. Nevertheless, this intermediate is certainly highly
unstable and presumably evolves within a few vibrations of its
C-Cl bond which has certainly prevented its observation by
SERS.
(
22) (a) Willett, B. C.; Peters, D. G. J. Electroanal. Chem. 1981, 123, 291–
06. For related examples, see also the following reference. (b)
The fate of this first intermediate is described by the two
limiting possibilities II-1 and II-2 which both lead to the same
3
Mbarak, M. S.; Peters, D. G. J. Electroanal. Chem. 1983, 152, 183–
196. (c) Bart, J. C.; Peters, D. G. J. Electroanal. Chem. 1990, 280,
7
benzyl radical adduct characterized by SERS. However,
1
29–144.
voltammetric data are now helpless in discriminating these two
routes since they occur after the rate-determining step. Yet, by
(
23) (a) Gennaro, A. Electrocatalytic Reduction of Organic Halides, 60th
Annual Meeting of the International Society of Electrochemistry;
Beijing, China; Keynote Lecture, August 18, 2009. (b) Note that there
is a similar drastic positive shift of the reduction wave of aliphatic
chloride at the silver electrodes when shifting from aprotic solvent to
aqueous electrolytes; compare with the following: Scialdone, O.;
Guarisco, C.; Galia, A.; Herbois, R. J. Electroanal. Chem. 2010, 641,
14–22.
16
analogy with organometallic oxidative additions, we favor this
intermediate evolving through a concerted binding of the benzyl
radical onto the silver surface (case II-2). Conversely, the
alternative route II-1 would involve the formation of a free
benzyl radical which then should bind competitively with its
J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010 17207

A R T I C L E S
Huang et al.
Figure 4. Scaled comparison between the energetics of the reaction pathways at inert and silver cathodes (note that the two initial systems have been
arbitrarily set at the same energy to allow direct comparisons). The electron-transfer driving forces, Gibbs energies, and activation barriers have been evaluated
-
1
p
at the voltammetric peak of benzyl chloride (BzCl) observed at silver electrodes at V ) 0.2 V s (E ) -1.76 V versus SCE, Figure 1). Gibbs adsorption
p
energies were taken from Table 1. Electron-transfer ones were evaluated as (∆G)at Ep ) -F(E - E°), where E° represents the standard potentials (vs SCE)
2
a
14
2a
of the redox couples of interest: (E°
1
)
inert ) -0.76 V, (E°
2
)
inert ) -1.43 V, (E°
1
)
silver ) -1.0 V, (E°
2
)
silver ) -0.97 V, (E°
1
)
inert,NC ) -2.9 V. Gibbs
q
q
p
q
2
activation-barrier energies for the first reduction steps at each electrode were evaluated by using the expression (∆G )at Ep ) ∆G
0
0
[1 + F(E - E°)/(4∆G )] ,
q
q
2a
q
where ∆G
0
is the corresponding intrinsic activation barrier: (∆G
0
)
inert ) 20.7 kcal/mol, (∆G )silver ) 10.0 kcal/mol. Direct dashed arrows are
0
7a
indicated for the second reduction steps since activation enthalpies are not known (see text). Intermediates directly observed by SERS are shown
in solid frames.
is that the preceding step C in eq 26 is slow and limits kinetically
of the electron-transfer step versus a concerted mechanism, this
lowers drastically the intrinsic activation barrier of the rate-
determining step.
As seen in Figure 4, the net result of these two contrasting
effects is that at silver cathodes the overall CE sequence has a
smaller effective activation barrier than the concerted pathway
in eq 1. Hence this path is kinetically more efficient and enables
the voltammetric wave to be displayed at drastically less
negative potentials than at inert electrodes.
1
3c
the current intensity of the prewave. This intrinsic slowness
suggests that, in agreement with the comparatively lower lattice
energy of mercury versus silver, this C step is certainly a true
II
oxidative addition (e.g., leading to a R-Hg -X species bound
to the metal surface via its mercury center) rather than a weak
adsorption as observed for benzyl chloride at silver electrodes.
At silver cathodes, the C step has certainly a comparatively
much weaker activation barrier and may then act as a fast uphill
pre-equilibrium so that the prewave may fully develop and
A quantitative analysis based on voltammetric data and
previously reported ones relative to the concerted reduction of
benzyl chloride at inert electrodes showed that, in contrast with
the situation at inert electrodes, the first reduction proceeds
through the formation of a fleeting species most certainly akin
to an anion radical with an elongated carbon-halogen bond
stabilized by the silver surface. In this view, this intermediate
is located on the way between a free benzyl radical anion and
1
2
proceed under slow charge-transfer control.
Similarly, Simonet et al. showed in a series of seminal
1
1
works that many electrode materials traditionally considered
as inert, including noble metals, may lead to the formation of
organic-metallic phases provided adequate electrochemical
conditions are applied. These phases corrode the metal interface
by forming reservoirs of organic radicals or anions which may
be profitably used in electrochemical syntheses. Though not
relevant to our studies, it is worth mentioning here that such
results certainly establish that our classical vision of organic
molecules at metal surfaces is still very limited.
•
-
the fully dissociated product pair, PhCH + Cl , produced via
2
the concerted electron transfer in eq 1. Such a species is thus
expected to fragment within a few vibrations of its weakened
carbon-chlorine bond presumably to afford the benzyl-silver
7
a
radical adduct characterized by SERS (mechanism II-2).
Thermodynamic and kinetic factors concur jointly in suggesting
that this bound benzyl radical is easily reducible to a benzyl
Conclusion
Combining voltammetric, SERS, and DFT data has provided
a coherent mechanistic framework (mechanisms II in Scheme
7
a
anion adduct also characterized by SERS. Hence, the volta-
mmetric wave remains bielectronic over its entire potential
1
, with mechanism II-2 being the most probable) to rationalize
8
c
span.
the exceptional catalytic properties of silver cathodes during
the reduction of benzyl chloride. In this view, the large gain of
ca. 0.5 V provided by silver electrodes results simultaneously
from thermodynamic and kinetic factors. Both stem from
specific interactions between the silver cathode surface and the
substrate as well as with the primary product of the electron-
transfer step. Interestingly, though this lowers the driving force
Finally, since the precomplexation of the substrate onto the
silver surface is an uphill reaction, it has to be disfavored by
the competitive adsorption of any other species (intermediate
or products) prone to bind exergonically to the silver surface,
as has been shown for the acetonitrile solvent and the 3-phe-
7a
nylpropanenitrile final product. This is also true for the benzyl
1
7208 J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010

Benzyl Chloride Reduction at Silver Cathodes
A R T I C L E S
2
7
anion adduct intermediate which necessarily competes with the
substrate precomplexation owing to its favorable adsorption. In
this view, the potential location of the voltammetric wave may
be regarded as resulting from a kinetic self-regulation between
these two conflicting effects. If so, favoring any reaction prone
to decrease the surface concentrations of the benzyl anion adduct
and of the 3-phenylpropanenitrile product should favor the
weaker adsorption of benzyl chloride, i.e., should provoke an
even more positive shift of its reduction wave. It is worth
reaction on single-crystal surfaces of various metal electrodes.
This opens new frontiers in many areas of electrochemical
interest as well as many others in which catalytic interactions
between reactants and metal surfaces are determinant, and
provides additional means to explore even further the gap
between classical electrochemical activation and organometallic
catalysis.
Experimental Section
23a
mentioning in this respect a recent report from Gennaro et al.
establishing that this is actually the case upon submitting the
system to increasing concentrations of CO . This occurs most
Materials and Reagents. Commercial acetonitrile solvent (AR
grade, SCRC Co. Ltd.) was used without further purification.
TEAP (electrochemical grade, Alfa Aesar) and benzyl chloride
2
(
AR grade, SCRC Co. Ltd.) were also used without further
presumably due to a facile electrophilic attack envisioned in
eq 29
purification.
Voltammetric Measurements. All electrochemical experiments
were carried out with silver wires (2 mm diameter; 99.999% from
Alfa Aesar), which were sealed into a Teflon (polytetrafluoroeth-
ylene) stick. The silver electrode was polished to a mirror finish
with emery paper of decreasing grain size followed by alumina
powder with 3, 1, and 0.3 µm particle sizes in this sequence. A
three-electrode electrochemical cell was used in which the reference
-
-
2
[
PhCH -Ag ] + CO f PhCH -CO + Ag
(29)
2
n
2
2
n
that occurs either on the surface (as in organometallic analog
2
4
reactions ) or after desorption of the benzyl anion. Both paths
would decrease the yield of 3-phenylpropanenitrile and favor
the weak adsorption of benzyl chloride. In addition, if reaction
+
electrode was a home-built Ag/Ag pseudoreference electrode and
the counter electrode was a Pt wire. For convenience and
comparison to previous results published in the literature, all
potentials have been converted to the SCE scale by subtracting a
45 mV potential difference as determined by independent calibration
performed after the measurements.
Voltammetric signals were imposed to the cell by a potentiostat
CHI 631B,CH Instrument); currents and potential data were stored
2
9 occurs directly on the surface, it would facilitate even more
this weak binding by imposing a smaller steady-state surface
concentration for the benzyl anion adduct. Water addition was
also reported to lead to a large positive potential shift for the
voltammetric reduction of alkyl halide reduction at silver
2
3b
(
cathodes, although the generated PhCH
3
has no affinity with
the surface. We thus favor the idea that electrophilic attacks
of the anionic benzyl moiety by CO (eq 29) or H O involve a
7
on a computer hard disk for further processing.
DFT Calculations. Cluster models including silver clusters with
different sizes of Ag (n ) 1-5 and 10) were used in the DFT
2
2
n
direct reactivity between these electrophiles and the bound
anionic adduct.
computations as models of the silver electrode surface for optimiz-
ing the structures of surface complexes of possible intermediates
and products. DFT calculations were performed with the hybrid
exchange functional of Becke’s 3 parameters (B3) and Lee-Yang-
Parr’s nonlocal correlation functional (LYP) for finding the stable
Besides the goal of unraveling the origin of the exceptional
electrocatalytic effect of silver cathodes, this work also dem-
onstrated that the integration of SERS measurements, DFT
predictions, and voltammetric investigations offers a perfect
triple tool for precise mechanistic investigations in molecular
electrochemistry when unsuspected surface-bound intermediates
2
8,29
optimization geometries and calculating vibrational spectra.
Basis sets for C, Cl, and H were 6-311+G** with polarization
functions for C, Cl, and H, and with diffuse function only for C
and Cl atoms. For silver atoms, the valence and core electrons were
described by the basis set of LanL2DZ and the corresponding
relativistic effective core potential (ECP), respectively. The solvent
effect was considered by the integral equation formal polarization
continuum model (IEF-PCM). All calculations concerning the
geometry optimization were performed with the aid of the Gauss-
ian09 software package. The ensuing geometries are described
in the Supporting Information material of ref 7a.
Thermodynamic adsorption energies of the benzyl chloride,
benzyl radical, benzyl anion, and acetonitrile have been evaluated
12
are crucially involved. SERS provides in situ molecular-level
information to characterize nonclassical but key surface inter-
mediates whose transient presence eludes voltammetric detec-
28
1
2
tion, while the combination of voltammetry and DFT allows
one to delineate how these species are kinetically involved in
the mechanistic path.
15
1
5
Though intrinsically SERS-active metals are few, this does
not limit the scope of this strategy. Indeed, various SERS-silent
2
5
metals (such as iron, cobalt, nickel, platinum, and palladium)
on silver atom clusters (Ag , n ) 1-5 and 10) without considering
n
may be turned into SERS-active materials through the use of
the role of the solvent. This showed that n ) 4 was sufficient to
core-shell nanoparticles in which the SERS-silent metal of
provide correct bonding contributions compared to more extended
2
6
interest is “wrapped” around a core of a SERS-active one.
4
clusters. Furthermore, we selected Ag clusters to simulate the
Such core-shell nanoparticles have been shown to provide
electrode owing to its structural stability when it was negatively
charged. Indeed, even nowadays it is still very difficult to evaluate
thermodynamic energies of an electrochemical system by DFT,
mostly because of the difficulty of accounting for the electrode
potential. The strategy we used is that commonly adopted by the
whole community. It relies on defining suitable atomic clusters and
adding charges to them in order to represent the electrode charge
density. In a series of preliminary tests we observed that upon
adsorption of the organic moieties such negative charging led to
4
5 26
sufficiently large enhancement factors (∼10 to 10 ) that allow
investigations with an accuracy similar to the present one.
Furthermore, very recently, a new technique (shell-isolated
nanoparticle-enhanced Raman spectroscopy, SHINERS) has
been developed to investigate electrochemical adsorption and
(
(
(
24) (a) Amatore, C.; Jutand, A. J. Am. Chem. Soc. 1991, 113, 2819–2825.
(
b) Amatore, C.; Jutand, A.; Khalil, F.; Nielsen, M. F. J. Am. Chem.
Soc. 1992, 114, 7076–7085.
25) (a) Tian, Z. Q.; Ren, B. Annu. ReV. Phys. Chem. 2004, 55, 197–229.
(27) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.;
Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; Ren, B.; Wang, Z. L.;
Tian, Z. Q. Nature 2010, 464, 392–395.
(
b) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106,
9
463–9483.
26) Wu, D. Y.; Li, J. F.; Ren, B.; Tian, Z. Q. Chem. Soc. ReV. 2008, 37,
025–1041.
(28) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(29) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310.
1
J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010 17209

A R T I C L E S
Huang et al.
geometric deformations in some Ag clusters. Although this did not
alter significantly the structures of organic moieties in the adducts,
this created a problem in estimating the adsorption thermodynamics.
∆SFadsorb ) SFX-Ag4 - SF - SFAg4
X
However, this problem was found to be minimal for the Ag
4
cluster
where “SF” represents the standard thermodynamic function of
interest.
used in the present paper. Furthermore, other functionals (B3PW91
30
or X3LYP) have been used to estimate these values and provided
similar results ((0.5 kcal/mol for the PhCH
2
Cl-Ag
4
adduct) to
Acknowledgment. This work is dedicated to the memory of
the late Jay K. Kochi. It was supported in part by CNRS, UPMC,
and ENS (UMR 8640 and LIA XiamENS) in France and by NSFC
those reported in Table 1.
4
Hence, Ag clusters were used to simulate the active adsorption
site on the electrode and more extensive calculations including a
solvent continuum model were performed (see Supporting Informa-
tion in ref 7a). Adsorption energies and entropies reported in Table
(
20620130427) and MOST (2007DFC40440) in China.
Supporting Information Available: Complete ref 15 including
all authors. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
were then obtained from the equation
(
30) (a) Perdew, J. P.; Burke, K.; Wang, Y. Phys. ReV. B 1996, 54, 16533.
b) Xu, X.; Goddard, W. A. Proc. Natl. Acad. Sci. U.S.A. 2004, 101,
673–2677.
(
2
JA106049C
1
7210 J. AM. CHEM. SOC. 9 VOL. 132, NO. 48, 2010