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A combined Raman, DFT and MD study of the solvation dynamics and
the adsorption process of pyridine in silver hydrosolsw
Marco Pagliai,a Luca Bellucci,a Maurizio Muniz-Miranda,a Gianni Cardini*ab and
Vincenzo Schettinoab
Received 19th July 2005, Accepted 27th September 2005
First published as an Advance Article on the web 17th October 2005
DOI: 10.1039/b509976e
The adsorption of pyridine onto silver colloids has been investigated by Raman spectroscopy
experiments and by ab initio DFT and MP2 calculations. The solvation dynamics of the pyridine
in water has been studied by a molecular dynamics simulation. The results are compared with the
latest available experimental and theoretical data. It is found that the pyridine is essentially
hydrogen bonded to one solvent molecule. Calculations based on pyridine–water and pyridine–
Ag1 complexes allow the reproduction of the experimentally observed Raman features and
explain the adsorption process of the ligand in silver hydrosols.
1. Introduction
Metal nanoparticles dispersed in aqueous media are able to
adsorb several different species from the solution, in particular
organic molecules. The adsorption is favored by the presence
of heteroatoms such as N or O in the molecular structure. The
adsorption process in a Ag hydrosol can be monitored by
various spectroscopic techniques. In the UV-Vis spectrum an
intense band usually occurs around 400 nm, due to the
plasmon resonance of the electrons localized at the surface
of Ag colloidal particles with 10–20 nm average diameters, as
shown in Fig. 1A. The presence of a ligand adsorbed onto the
Ag nanoparticles is able to induce a colloidal aggregation,
which is detected by the occurrence of a secondary plasmon
band at longer wavelengths (Fig. 1B). This spectral feature is
more evident for molecules that are chemically adsorbed on
the Ag surface.
The adsorption of organic ligands can be studied by means
of the surface-enhanced Raman scattering (SERS) effect. The
molecules adsorbed on nanostructured surfaces of Ag, Au or
Cu undergo a huge intensification of their Raman signals. This
effect is generally explained on the basis of two different
mechanisms:1 the electromagnetic enhancement, and the
charge-transfer effect between the molecule and metal. In the
first case a giant enhancement of the electric field occurs near
the surface of the metal nanoparticles, in the second a reso-
nance effect involves the energy levels of both the metal and
molecule. The electromagnetic mechanism is estimated to
provide enhancement factors with a magnitude of 106 for the
Raman signals of molecules approaching the metal surface,
even without a chemical interaction with the latter. The
charge-transfer effect provides a further Raman enhancement
that can reach a factor of 102 for chemisorbed molecules.
Consequently, these exhibit more intense SERS spectra in
comparison with physisorbed molecules. In addition, chemi-
sorbed molecules show significant frequency shifts for some
bands compared with the corresponding normal Raman bands
of the free molecule. This effect is due to the chemisorption
that modifies some geometrical parameters and the electron
density of the molecule with a consequent perturbation of the
force constants. On the other hand, frequency shifts of vibra-
tional bands are also induced by interactions with molecules in
the surrounding environment, mainly with those of the sol-
vent. When a molecule is dissolved in aqueous solution, it is
solvated by water molecules, but in the presence of suspended
metal particles it can adsorb onto the metal surface through a
chemical interaction that successfully competes with the aqu-
eous medium. The aim of this work is to evaluate the possible
chemisorption of organic ligands onto a Ag hydrosol by a
computational approach based on the density functional
theory (DFT).2,3 Here, DFT calculations are employed to
reproduce the frequency shifts observed in the Raman spectra
of aqueous solutions and in the SERS spectra in Ag hydrosols
with respect to the normal Raman spectra of the free mole-
cules, i.e. without interactions with the solvent molecules as
well as with the Ag nanoparticles. On the basis of a satisfac-
tory evaluation of these frequency shifts, it is possible to find a
suitable explanation of the different capabilities of organic
ligands to adsorb onto a Ag hydrosol, instead of remaining
bound to the water molecules. Actually, DFT calculations
allow the evaluation of the structural changes of the molecule
upon interaction with water or Ag, as well as the charge-
transfer effects. In both solvation and chemisorption processes
the formation of charge-transfer complexes has to be taken
into account. If the charge-transfer is larger in the molecule–
metal interaction than in the molecule–water interaction, a
more stable complex is formed on the metal surface. The
analysis of the SERS spectra of some organic molecules has
been previously performed with ab initio calculations without
a Laboratorio di Spettroscopia Molecolare, Dipartimento di Chimica,Universita di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino,Florence, Italy
b European Laboratory for Nonlinear Spectroscopy (LENS), ViaNello Carrara 1, 50019 Sesto Fiorentino, Florence, Italy.E-mail: gianni.cardini@unifi.it
w Electronic supplementary information (ESI) available: Calculatedand measured vibrational frequencies of pyridine, pyridine–water,pyridine–Ag1 and pyridine–Ag(0) (Tables S1–S4). See DOI: 10.1039/b509976e
This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 171–178 | 171
PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics
taking solvent effects into account, and only comparing the
frequency shifts on moving from the free organic molecule to
the complex with the metal or metallic clusters, both in their
ionic or neutral form.4–7 Although this procedure usually
allows a correct reproduction of the Raman spectra, the
information on the solvent effects is completely neglected. In
this work the solvation dynamics of pyridine (Py), which was
the first molecule to exhibit SERS enhancement in both an
electrochemical cell8 and a Ag hydrosol,9 have been studied by
a molecular dynamics (MD) simulation. Although the struc-
tural reorganization of the water close to the pyridine molecule
has been studied through Monte Carlo simulations,10,11 a
dynamical view is still lacking. A detailed description of the
H-bond dynamics is essential for characterizing the stability
and strength of the H-bond with the solvent and to set up a
simple model to explain the Raman spectra of pyridine in
water. This model represents a starting point for understand-
ing the SERS spectra of pyridine by DFT calculations. The
SERS spectra of pyridine in Ag colloids activated by coad-
sorbed chloride anions are quite similar to the normal Raman
spectrum of the Ag(I) coordination compound,9 suggesting
that the interaction with the metal surface closely resembles
that of pyridine interacting with a Ag ion. In activated Ag
colloids, the presence of active sites constituted by positively-
charged Ag atoms has been previously ascertained.12,13
2. Experimental
Stable Ag hydrosols were prepared by the reduction of AgNO3
(99.9999% purity, Aldrich) with an excess of NaBH4 (99.9%
purity, Aldrich) in extra-pure distilled water according to
Creighton’s procedure,14 and taking precautions to avoid
reduction products.15 The addition of 10�3 M NaCl
(99.999% purity, Aldrich) ensured a stronger Raman enhance-
ment of pyridine (99.9þ% purity, Aldrich) at a concentration
of 10�3 M in the Ag hydrosols. Pyridine (1M) was also
dissolved in an aqueous and a CCl4 (99.9þ% purity, Aldrich)
solution. Raman measurements were performed using a Jobin-
Yvon HG-2S monochromator, a cooled RCA-C31034A
photomultiplier and the 514.5 nm exciting line was supplied
by an Arþ laser with a power of 50 mW. Power density
measurements were performed with a power meter (model 362,
Scientech, Boulder, CO), giving an B5% accuracy in the
300–1000 nm spectral range.
Absorption spectra in the 200–800 nm region were mea-
sured with a Cary5 Spectrophotometer in order to detect the
surface plasmon bands of the Ag colloidal particles. TEM
measurements of Ag colloids in the absence and presence of
pyridine were performed using a Philips EM 201 instrument
with an electron beam emitted at 80 kV, after placing a drop of
the colloidal sample onto a carbon–Cu grid.
3. Computational details
The DFT calculations2,3 have been performed with the Gaus-
sian 98 rev. A.7 suite of programs,16 using a combination of
the BLYP,17,18 B3LYP19–21 or B3PW9119,22–24 exchange and
correlation functionals along with the LANL2DZ25–27 or
CEP-31G28–30 basis sets. The LANL2DZ basis set consists
of the Dunning–Huzinaga31 full double-z contraction on first
row elements and Los Alamos pseudopotentials for core
electrons in conjunction with a double-z contraction for the
other elements, whereas the CEP-31G basis set consists of
effective core pseudopotentials in conjunction with a double-zcontraction for the valence electrons. MP232 calculations were
also performed with both the CEP-31G and LANL2DZ basis
sets, using the Gaussian 98 rev. A.7 suite of programs.16
Structure optimizations, with a very tight criterion, and nor-
mal frequencies calculations have been performed using an
improved grid in the numerical evaluation of the integrals,
INTEGRAL(GRID ¼ 199974). The calculated frequencies
have been uniformly scaled using a 0.983 factor for the
CEP-31G basis set and a 0.972 factor for the LANL2DZ basis
set for both the B3LYP and B3PW91 calculations. For the
BLYP calculations, 1.022 and 1.018 scaling factors have been
adopted for the CEP-31G and LANL2DZ basis sets, respec-
tively. For all of the MP2 calculations, a 1.01 scaling factor
was used. The Raman intensities of the vibrational modes,
computed on the basis of the double harmonic approximation,
i.e. without taking into account the electric and mechanical
anharmonicity, correspond to spatially averaged values ac-
cording to the usual formulae reported in standard text-
books.33
In order to estimate the number of water molecules H-
bonded to the N atom of pyridine, a classical molecular
Fig. 1 UV-visible absorption spectra of Ag colloids in the absence of ligand (A) and with 10�3 M pyridine (B). The corresponding transmission
electron microscopy (TEM) images are reported as insets.
172 | Phys. Chem. Chem. Phys., 2006, 8, 171–178 This journal is �c the Owner Societies 2006
dynamics (MD) simulation has been performed, adopting the
standard OPLS 11 site potential10 for the solute and modeling
the water molecules with the TIP4P potential.34,35 The model
system, consisting of one pyridine and 345 solvent molecules in
a periodic cubic box with sides of 21.8828 A (at the experi-
mental density of water, 0.9966 g cm�3), has been thermalized
at B298 K for B50 ps by velocity scaling. The trajectory has
been collected for 200 ps, saving the atomic coordinates every
2 fs, with a time step of 0.2 fs in the NVE ensemble. The MD
simulations, when keeping all of the molecules rigid, have been
performed with the Moldy program.36
4. Raman spectra of pyridine
Fig. 2 shows the Raman spectra of pyridine as a pure liquid, in
aqueous and in CCl4 solutions, in comparison with the SERS
spectrum of the molecule adsorbed on Ag colloids activated by
the addition of chloride anions. All of the Raman spectra are
dominated by two very intense bands around 1000 and 1030
cm�1, attributed to the ring breathing and ring trigonal mode,
respectively.
Significant frequency shifts of some bands are detected upon
going from the liquid to water solutions or to a Ag hydrosol,
whereas no appreciable frequency shift occurs in the Raman
spectrum in CCl4 solution. This indicates that the interactions
with the molecules of an apolar solvent like CCl4 or with other
pyridine molecules in the pure liquid can be considered similar
when compared with those occurring when pyridine interacts
with water molecules or Ag particles. Hence, it is reasonable to
compare the DFT calculated frequencies of pyridine in aqu-
eous solutions or Ag hydrosols with the vibrational data in the
liquid. Moreover, as shown in Fig. 2, the frequency shifts
observed in the SERS spectrum are larger than those observed
in the Raman spectrum of the aqueous solution, indicating a
stronger interaction with the Ag surface. Finally, the intensity
ratio (1.49) between the two strongest bands observed in the
Raman spectrum of the liquid at 992 and 1031 cm�1 markedly
changes in the Raman spectrum of the aqueous solution (2.06)
and is only slightly affected in the SERS (1.45). The DFT
calculations are employed here to reproduce the frequency
shifts, as well as the relative intensities of the observed bands.
5. Results and discussion
The adsorption of pyridine molecules onto Ag colloids occurs
in an aqueous environment and the process is in competition
with the pyridine–water solvation. The analysis of the SERS
spectra of pyridine (frequency shifts, relative intensities)
should be made in comparison with the spectra in water.
Therefore, a classical molecular dynamics simulation has
initially been performed in order to evaluate the structure
and dynamics of pyridine solvation as a basis for the con-
struction of a reliable model for the subsequent DFT calcula-
tions. The pair radial distribution functions and the running
integration number obtained from 200 ps simulations are
reported in Fig. 3.
The first peak position and coordination number obtained
from the H� � �N and O� � �N pair radial distribution functions
are in perfect agreement with the latest experimental measure-
ments of Bako et al.37 In fact, the first H� � �N peak position is
in the 1.8–1.9 A range, in agreement with experiment, and the
coordination numbers are 1.3 and 1.4 for the H� � �N and
O� � �N contacts, respectively. For comparison, the measured
Fig. 2 Raman spectra of pyridine in: (a) CCl4 solution, (b) pure
liquid; (c) aqueous solution; (d) Ag hydrosol. Excitation line: 514.5
nm. The Raman intensity has been enhanced and reported (grey line)
for a better visualization of the spectral features.
Fig. 3 Radial distribution functions (unbroken line) and running
integration number (dashed line) for the H� � �N (grey) and O� � �N(black) intermolecular contacts.
This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 171–178 | 173
value is 1.2 � 0.1. This result is a consequence of the presence
of only one water molecule directly bound to the N atom of
pyridine, with at least another water molecule close to this, as
shown in Fig. 4 where a snapshot of the MD simulation is
reported.
A specific analysis has been performed to characterize the
pyridine–water H-bond in depth, starting from the pair angu-
lar distribution function. In fact, the H-bond is a directional
interaction38 and its strength depends not only on the intera-
tomic distances but also on the distribution of the HO� � �Nangle.
The angular distribution g(y) related to all of the contacts
(top panel of Fig. 5) and the H-bonded molecule (bottom
panel of Fig. 5), selected on the basis of H� � �N and O� � �Ndistances lower than the first peak minimum in their g(r), does
not show a minimum at 301 as hypothesized by Fileti et al.,11
but only a flex. The choice of 301 as a limiting value of HO� � �N
angle appears to be restrictive. In fact, as can be seen in Fig. 5
(lower panel), the HO� � �N angle distribution extends up to
501, a quite large value which is, however, very close to that
found in pure water.39 Starting from these results it is possible
to describe the configurational H-bond space and to obtain the
average lifetime related to the formation and breaking of the
H-bond. To this end the H-bond can be described by the
traditional classical40 approach based on the following geo-
metrical criteria:
1. r(H� � �N) o 2.6 A
2. r(O� � �N) o 3.3 A
3. HO� � �N o 501
Alternatively, the H-bond could be characterized by the
function introduced recently to describe the H-bond network
in liquid methanol41 and the Cl� þ CH3Br reaction in water42
FjHB ¼ A(r(t)) � B(y(t)) (1)
with A(r(t)) and B(y(t))
AðrðtÞÞ ¼ e�ðre�rjðtÞÞ2=ð2s2r Þ if ðre � rjðtÞÞo0
AðrðtÞÞ ¼ 1 if ðre � rjðtÞÞ � 0
BðyðtÞÞ ¼ e�ðye�yjðtÞÞ2=ð2s2yÞ if ðye � yjðtÞÞo0
BðyðtÞÞ ¼ 1 if ðye � yjðtÞÞ � 0
8>>>><>>>>:
The values of the parameters re, ye, sr and sy are directly
extracted from the non-normalized pair radial, h(r), and
angular distribution function, h(y).In the above definitions, re is the position of the first peak in
the h(r)H� � �N function and ye is the position of the first peak in
the h(y) function, whereas sr and sy are the half width at half
maximum in the h(r) and h(y) functions, respectively.Fig. 6 shows the configurational H-bond space described by
the two approaches. It is clear from this result that the
HO� � �N angle extends to at least 301. However, larger values
of the angle should also be considered according to the
classical approach. These arise from the continuous exchange
of water molecules that enter or leave the H-bond region.
The continuous exchange of the water molecules bound to
pyridine is depicted in Fig. 7 where the H-bonded water
molecules, out of 345 molecules of the simulated sample, are
identified at each time of the simulation. The lifetime of the
Fig. 4 Snapshot of the pyridine molecule H-bonded to a water
molecule extracted from the MD simulation.
Fig. 5 Angular distribution function of the HO� � �N angle. Top
panel, all of the contacts. Bottom panel, only H� � �N and O� � �Ndistances lower than the position of the first minimum in the respective
g(r).
Fig. 6 H-bond configurational space obtained with the geometrical
criteria (left surface) and with the new weighted function (right
surface). A color image of this figure is available in the html.
174 | Phys. Chem. Chem. Phys., 2006, 8, 171–178 This journal is �c the Owner Societies 2006
H-bond to the pyridine has been calculated to be 359 and
306 fs according to the classical and alternative approach,
respectively. These values are the averages of a number of
processes with different lifetimes.
The solvent reorganization in the neighborhood of the N
atom during the simulation can be easily observed from Fig. 8,
showing that the surface spanned by the O atom of the water
molecules closest to the N atom is approximately a spherical
cap with a large extension.
The molecular dynamics simulation described above clearly
shows that the pyridine molecule is directly bound to a single
water molecule by a strong H-bond. This suggests the adop-
tion of a bimolecular pyridine–water complex as a model to
mimic the solvent effect on the structural and vibrational
properties of the system. The starting point to validate this
model is represented by the recent results of Dkhissi et al.43
and Schlucker et al.44 showing that the B3LYP exchange and
correlation functionals in conjunction with the 6-31þþG(d,p)
basis set satisfactorily reproduce both the structure and the
energy of the pyridine–water H-bonded complexes. The accu-
racy of the B3LYP functional in describing the structural and
electronic properties of H-bonded systems has been recently
assessed and described by Rabuck and Scuseria.45 In order to
compare the frequency shifts moving from the pyridine to the
water system and to the pyridine–Ag1 complex, it is not
possible to adopt the same computational approach, and a
more appropriate basis set is required. To further validate the
model and assess the charge-transfer influence on both the
vibrational frequencies and Raman intensities, the calculations
have also been performed on the pyridine–Ag(0) complex,
although the surface active sites of the nanoparticles are
considered to be positively charged by the adsorption of
Ag1 ions counterbalanced by coadsorbed anions.12,13 There-
fore, DFT calculations have been carried out for the above
systems using different functionals and the CEP-31G and
LANL2DZ basis sets and it turns out that these reproduce
the frequency shifts accurately. This further justifies the im-
portance of the direct bonding of a water molecule to the N
atom of pyridine.
The optimized structures show the N atom of pyridine
involved in the interaction with the water molecules as well
as with the Ag ion, as expected when considering the fact that
the conjugation between the lone-pair and the p electrons of
the aromatic ring rather weakens the basicity of the N atom.
The DFT calculation of pyridine bound to one water mole-
cule, even given the simplicity of the model, leads to a H� � �NH-bonding distance (B1.80 A) comparable with the first peak
position (1.85 A) obtained from the MD calculations shown in
Fig. 3. The calculated Ag–N distance in the pyridine–Ag1
complex (2.17 A) is similar to that in the Ag(I) coordination
compound (2.16 A),46 whereas the calculated Ag–N distance
in the pyridine–Ag(0) complex is rather longer, as reported in
Table 1.
The calculated Ag1� � �N distances are very close to those
reported by Wu et al.47 (2.198 A) and Yang et al.48 (2.196 A),
from DFT and MP2 calculations, respectively.
The vibrational frequencies of pyridine, pyridine–water and
pyridine–Ag1 have been calculated using the CEP-31G and
LANL2DZ basis set and different functionals. The results are
reported in Tables S1 and S2 of the ESI.w The vibrational
frequencies calculated at the MP2 level are reported in Table
S3 of the ESI.w The average discrepancy between calculated
and observed frequencies of pyridine nobs � ncalc is summar-
ized in Table 2, showing that the DFT approach is more
efficient, and that the LANL2DZ basis set seems to work
better than the CEP-31G basis set.
In Tables 3 and 4 the frequency shifts, observed in the
Raman spectrum of the aqueous solutions and in the SERS
spectrum with respect to the Raman spectrum of the liquid,
are compared with the results of DFT calculations. It can be
seen that the general trend of the frequency shift is pretty well
reproduced by the approach of this paper. The frequencies of
the pyridine–Ag(0) complex are not reported in Tables 3 and
4, but are instead reported in Table S4 of the ESIw, since theydo not reproduce the observed frequency shifts and relative
Raman intensities.
The calculated Raman spectra of pyridine, pyridine–water
and pyridine–Ag1 using B3LYP with the LANL2DZ basis set
are shown in Fig. 9. These should be compared with the
experimental spectra of Fig. 2. It can be seen that the agree-
ment is quite satisfactory for not only the peak positions but
Fig. 7 Coordination number averaged every 0.2 ps (upper panel) and
water molecules H-bonded to the solute during the simulation (lower
panel). All of the solvent molecules in the sample are labeled from 1 to
345. The figure identifies the molecules directly H-bonded to the
pyridine during the simulation.
Fig. 8 Distribution of the water molecules around the N atom for
values of the H� � �N and O� � �N distances and HO� � �N angle that
satisfy the geometrical criteria for the presence of the H-bond.
This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 171–178 | 175
also their relative intensities. In particular the intensity ratio of
the 992 and 1031 cm�1 peaks changes in the B3LYP/
LANL2DZ calculation in pretty close agreement with experi-
ment. The calculated Raman spectrum of pyridine–Ag(0) has
been shown in the upper panel of the Fig. 9 for comparison.
Although the main features of the Raman spectra are accu-
rately reproduced, it is not possible to reproduce the absolute
Raman intensities, since the effects due to the atoms that make
up the Ag nanoparticles are missing in the present approach.
The overall results on the frequencies, frequency shifts and
relative intensities show that the simple models of the pyri-
dine–water and pyridine–metal interactions are rather appro-
priately used in a DFT approach to describe the processes of
pyridine solvation and chemisorption on metal particles.
In both cases, charge-transfer processes are rather impor-
tant; in the case of pyridine the charge-transfer effect to the
metal particles is much larger than that to the water molecules.
The charge-transfer obtained by the natural bond orbital
(NBO)49 analysis is summarized in Table 5. It is seen that
with the B3LYP functional there is a predicted charge-transfer
of 0.130 e� and 0.047 e� to Ag and water, respectively. On the
contrary, for the complex with the metal atom, the charge-
transfer is slightly lower (0.036 e�) than that in water.
These results give a picture of the chemisorption process of
an organic ligand such as pyridine onto colloidal Ag. The
tendency to adsorb on Ag, removing the coordinated water
molecule, is linked to the formation of a more stable complex
with the metal surface. In this respect the stabilization energy
related to the formation of the pyridine–water and pyridine–
Ag1 complexes are �32.8 and �229.8 kJ mol�1, respectively.
Since the binding energy for the pyridine–Ag(0) complex is
Table 1 H� � �N, Ag1� � �N and Ag(0)� � �N distances at the levels of theory and basis sets adopted in the calculations. The CEP label refers to thecalculations with the CEP-31G basis set, while LANL refers to the LANL2DZ basis set
MP2 BLYP B3LYP B3PW91
CEP LANL CEP LANL CEP LANL CEP LANL
H� � �N/A 1.877 1.871 1.811 1.805 1.803 1.800 1.765 1.770Ag1� � �N/A 2.200 2.240 2.168 2.174 2.170 2.176 2.158 2.164Ag(0)� � �N/A 2.378 2.447 2.428 2.483 2.434 2.484 2.400 2.454
Table 2 Average discrepancy between the observed and calculatedfrequencies of pyridine
CEP-31G/cm�1 LANL2DZ/cm�1
BLYP 12.3 10.2B3LYP 12.8 9.3B3PW91 13.3 7.4MP2 13.3 27.5
Table 4 Computed and measured frequency shifts on moving from liquid pyridine to the water solution and from liquid pyridine to the colloidalsolution. Calc. A ¼ BLYP/LANL2DZ, Calc. B ¼ B3LYP/LANL2DZ, Calc. C ¼ B3PW91/LANL2DZ and Calc. D ¼ MP2/LANL2DZ
Dn liq. PY - water Dn liq. PY - colloid
Ramancm �1
Obs.cm �1
Calc. Acm �1
Calc. Bcm �1
Calc. Ccm �1
Calc. Dcm �1
Obs.cm �1
Calc. Acm �1
Calc. Bcm �1
Calc. Ccm �1
Calc. Dcm �1
A1 n(10) 604 w/m þ11 þ6 þ15 þ17 þ16 þ19 þ30 þ40 þ42 þ40B2 n(22) 653 m �4 �14 �2 �2 �1 �4 �19 �6 �6 �2A1 n(9) 992 vvs þ8 þ12 þ17 þ20 þ20 þ16 þ26 þ30 þ34 þ35A1 n(8) 1031vs þ1 �4 þ2 þ3 þ6 þ5 þ7 þ17 þ15 þ30A1 n(7) 1069 w 0 �4 0 0 þ10 0 �6 �2 þ1 þ22A1 n(6) 1217 m �1 �4 þ1 0 þ2 þ4 þ6 þ8 þ8 þ14A1 n(5) 1483 w �3 0 þ5 þ6 þ8 þ4 þ8 þ11 þ12 þ17B2 n(16) 1573 w/m þ1 þ1 þ2 þ3 þ3 þ2 �3 �3 �2 þ4A1 n(4) 1582 m þ10 þ12 þ15 þ9 þ11 þ17 þ26 þ22 þ23 þ34
Table 3 Computed and measured frequency shifts on moving from liquid pyridine to the water solution and from liquid pyridine to the colloidalsolution. Calc. A ¼ BLYP/CEP-31G, Calc. B ¼ B3LYP/CEP-31G, Calc. C ¼ B3PW91/CEP-31G and Calc. D ¼ MP2/CEP-31G
Dn liq. Py - water Dn liq. Py - colloid
Ramancm �1
Obs.cm �1
Calc. Acm �1
Calc. Bcm �1
Calc. Ccm �1
Calc. Dcm �1
Obs.cm �1
Calc. Acm �1
Calc. Bcm �1
Calc. Ccm �1
Calc. Dcm �1
A1 n(10) 604 w/m þ11 þ14 þ13 þ15 þ13 þ19 þ38 þ37 þ39 þ39B2 n(22) 653 m �4 �2 �2 �3 �3 �4 �7 �6 �6 �4A1 n(9) 992 vvs þ8 þ19 þ17 þ21 þ19 þ16 þ33 þ30 þ34 þ34A1 n(8) 1031vs þ1 þ2 þ1 þ1 þ6 þ5 þ13 þ14 þ11 þ31A1 n(7) 1069 w 0 0 0 0 �1 0 �2 0 þ3 �3A1 n(6) 1217 m �1 þ3 þ3 þ1 þ3 þ4 þ13 þ12 þ12 þ18A1 n(5) 1483 w �3 þ7 þ6 þ6 þ8 þ4 þ15 þ15 þ15 þ21B2 n(16) 1573 w/m þ1 þ1 þ2 þ2 þ3 þ2 �3 -4 �3 þ6A1 n(4) 1582 m þ10 þ12 þ12 þ9 þ12 þ17 þ26 þ26 þ27 þ36
176 | Phys. Chem. Chem. Phys., 2006, 8, 171–178 This journal is �c the Owner Societies 2006
only �19.71 kJ mol�1, the chemisorption process will occur on
positively charged sites of the colloid, as experimentally
hypothesized.12,13 As a consequence, the variation of the
electronic structure is much more pronounced in pyridine–
Ag1 than in pyridine–water. This behavior is illustrated in Fig.
10, where the electron density differences for the two com-
plexes show that the electronic structure of the ring is strongly
perturbed by presence of the Ag1 ion.
If charge-transfer and chemisorption are closely related, the
in-plane ring deformation modes should undergo the largest
frequency shifts, since the atoms of the ring are those most
involved in the electron rearrangement. As shown in Tables 3
and 4, the largest frequency shifts really occur for these
vibrational modes, i.e. ring bending, n(10); ring breathing,
n(9); trigonal ring deformation, n(8); and quadrant ring de-
formation, n(4).
6. Conclusions
In the past, the interpretation of the SERS effect has been
mainly based on the modeling of the metal surface, where the
ligand molecules were considered to be physisorbed.50,51 On
the other hand, DFT calculations have been successfully
employed in the study of the Raman enhancement related to
the chemisorption process, which results in the formation of
complexes between the ligand and active sites on the metal
surface.6,43,47,48,52–56 In the present work we have shown that
the DFT method is not only a convenient approach for
correctly interpreting the SERS data, but it also allows the
description of the possible chemisorption of a ligand in a Ag
hydrosol, thus exhibiting Raman enhancements on the basis of
the molecule–metal ion charge-transfer effect.
It has been observed that water binding and the model
adopted for the description of the solvation play a central role
in the understanding of both frequency shifts and chemisorp-
ton processes. Molecular dynamics simulations in conjunction
with ab initio calculations have allowed the development and
justification of a simple model that reproduces the experimen-
tal findings.
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Table 5 Charge-transfer at the levels of theory and basis sets adopted in the calculations. The CEP label refers to the calculations with theCEP-31G basis set, while the LANL label refers to the LANL2DZ basis set
MP2 BLYP B3LYP B3PW91
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178 | Phys. Chem. Chem. Phys., 2006, 8, 171–178 This journal is �c the Owner Societies 2006