A combined Raman, DFT and MD study of the solvation dynamics and the adsorption process of pyridine...

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: [email protected]

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.


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.


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.


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


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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Fig. 9 Calculated Raman spectra of (from top to bottom) pyridine–

Ag1, pyridine–water and pyridine. The DFT calculations have been

performed using the LANL2DZ basis set with the B3LYP functional.

The Raman spectrum of pyridine–Ag(0) is also shown (grey line).

Fig. 10 Electron density differences for the pyridine–water (left) and

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figure is available in the html.

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

CEP LANL CEP LANL CEP LANL CEP LANL

Dq(H2O) /e� 0.037 0.038 0.051 0.051 0.047 0.047 0.054 0.052

Dq(Ag1) /e� 0.078 0.075 0.178 0.175 0.130 0.131 0.122 0.122

Dq(Ag(0)) /e� �0.004 0.001 0.048 0.052 0.034 0.036 0.029 0.031


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A combined Raman, DFT and MD study of the solvation dynamics and the adsorption process of pyridine...

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