Sm

La

b

a

ARRA

KDEFFS

1

edmaAtomf

imc

dRoo

i

a

1d

Spectrochimica Acta Part A 79 (2011) 226–231

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

urface-enhanced Raman scattering and DFT investigation of Eriochrome Black Tetal chelating compound

ászló Szabóa,b,∗, Krisztian Hermana, Nicolae Leopolda, Claudia Buzumurgab, Vasile Chis a

Faculty of Physics, Babes-Bolyai University, Kogalniceanu 1, 400084 Cluj-Napoca, Romania“Nicolae Stancioiu” Heart Institute, Motilor 19-21, 400001 Cluj-Napoca, Romania

r t i c l e i n f o

rticle history:eceived 14 December 2010eceived in revised form 7 February 2011

a b s t r a c t

The surface-enhanced Raman scattering (SERS) spectra of Eriochrome Black T (EBT) and its Cu(II), Fe(III),Mn(II) and Pb(II) complexes were recorded using a hydroxylamine reduced silver colloid. Moleculargeometry optimization, molecular electrostatic potential (MEP) distribution and vibrational frequencies

ccepted 16 February 2011

eywords:FTriochrome black TTIR

calculation were performed at B3LYP/6-31G(d) level of theory for the EBT molecule and its Cu(EBT),Fe(EBT) and Mn(EBT) metal complexes. Differentiation between EBT complexes of Cu(II), Fe(III), Mn(II)and Pb(II) is shown by the SERS spectral features of each complex.

© 2011 Elsevier B.V. All rights reserved.

T-RamanERS

. Introduction

Metal ions determination represents an area of interest in sev-ral fields, like environmental protection, food safety or clinicaliagnostics. Analytical methodologies for direct determination ofetal ions were established over the last decades including atomic

bsorption or emission spectroscopy and mass spectrometry [1].lthough these methods are sensitive and accurate, they require

edious sample pre-treatment and expensive equipment. On thether hand, recent years have seen an increase in the develop-ent of optical chemical sensors for heavy metals, in order for easy

abrication, low cost, good selectivity and sensitivity [1–3].Eriochrome Black T (EBT), with the structure shown in Fig. 1,

s a non-selective azo dye, widely used as colorimetric reagent foretal ions because it forms very stable, water-soluble and highly

olored complexes with the vast majority of transition metals [4,5].In a previous work [6], a fiber-optic-based system for Raman

etection in liquid chromatography was described. The resonanceaman spectra of the two azo dyes, eriochrome black T and eri-chrome blue SE, were used to test the system and detection limitsf 160 ng for these analytes were found [6].

Eriochrome Black T dye was used previously to reduce silver ionsn an aqueous solution, producing a stable colloidal suspension [7].

Surface-enhanced Raman scattering (SERS) offers a suitablelternative to overcome the low sensitivity inconvenience of

∗ Corresponding author. Tel.: +40 264405300/5153; fax: +40 264591906.E-mail address: laszlo.szabo@phys.ubbcluj.ro (L. Szabó).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.02.041

Raman spectroscopy. When the molecules are adsorbed to roughmetal surfaces, the Raman cross section is enhanced several ordersof magnitude [8–11]. The potential to combine high sensitivity withthe structural information content of Raman spectroscopy makesSERS spectroscopy a powerful tool in a variety of fields.

Carron et al. [12] reported the sensing of EBT-metal complexesusing surface-enhanced resonant Raman scattering (SERRS).

In this work, SERS spectroscopy is used in conjunction withquantum chemical calculations in order to characterize the struc-ture, electronic properties and vibrational spectra of the EBTmolecule and its complexes with Cu(II), Fe(III), Mn(II) and Pb(II).Thus, the Raman and SERS spectra of EBT, as well as the SERS spectraof EBT-Cu(II), -Fe(III), -Mn(II) and -Pb(II) complexes, were assignedusing DFT calculations at B3LYP/6-31G(d) level of theory. EBT com-plexes with Cu(II), Fe(III), Mn(II) and Pb(II) are differentiated bytheir SERS spectra, each metal complex showing a particular SERSspectral fingerprint.

2. Experimental

2.1. Chemicals

All chemicals used were of analytical reagent grade. The silvercolloid was prepared according to the previously reported proce-

dure [13]. The pH value of the silver colloid, measured immediatelyafter preparation, was found to be 8.

For the metal complexes preparation, the following metal saltswere used: copper (II) sulfate pentahydrate, iron (III) chloride hex-ahydrate, manganese (II) chloride tetrahydrate and lead (II) nitrate.

L. Szabó et al. / Spectrochimica Acta Part A 79 (2011) 226–231 227

of EB

Ep11[

2

rGD

gaapr

Rbwsfiw

3

pwuebttwt

R

I

Fig. 1. B3LYP/6-G(d) optimized geometry

BT complexes with Cu(II), Fe(III), Mn(II) and Pb(II) were pre-ared by adding 1 ml dilutions of 10−3 M metal salt solution toml 10−3 M EBT solution, up to obtaining finally 2 ml mixtures at:1 EBT:metal salt molar ratio, EBT chelating metal ions at 1:1 ratio6].

.2. Methods

The FTIR/ATR spectrum of EBT powder sample was recorded atoom temperature on a conventional Equinox 55 (Bruker OptikmbH, Ettlingen, Germany) FTIR spectrometer equipped with aTGS detector.

The FT-Raman spectrum of EBT was recorded in a backscatteringeometry with a Bruker FRA 106/S Raman accessory equipped withnitrogen cooled Ge detector. The 1064 nm Nd:YAG laser was useds excitation source, and the laser power measured at the sampleosition was 300 mW. The FT-Raman and FTIR/ATR spectra wereecorded with a resolution of 4 cm−1 by co-adding 32 scans.

SERS spectra were recorded using a DeltaNu Advantage 532aman spectrometer (DeltaNu, Laramie, WY) equipped with a dou-led frequency NdYAG laser emitting at 532 nm. The laser poweras 6 mW and the spectral resolution of 10 cm−1. For all SERS mea-

urements 25 �l of analyte were added to 0.5 ml silver colloid, thenal concentration of the metal-complexes in the colloidal solutionas 4.8 × 10−5 M.

. Computational details

Molecular geometry optimizations, molecular electrostaticotential (MEP) distributions and vibrational spectra calculationere performed with the Gaussian 03 W software package [14] bysing density functional theory (DFT) methods with B3LYP hybridxchange-correlation functional [15,16] and the standard 6-31G(d)asis set. No symmetry restriction was applied during geome-ry optimizations. The vibrational frequencies were computed athe optimized geometries to ensure that no imaginary frequenciesere obtained, confirming that they correspond to true minima on

he potential energy surfaces.

The calculated Raman activities (Si) were converted to relative

aman intensities (Ii) using the following relationship:

i = f (�0 − �i)4Si

�i

[1 − exp

(−(hc�i/kT)

)] (1)

T with atom and ring numbering scheme.

where f is a normalization factor for all peak intensities, �0 isthe exciting laser wavenumber, �i is the wavenumber of the ithvibrational mode, c is the speed of light, h and k are Planck’s andBoltzmann’s constants and T is the temperature [17–19].

The computed wavenumbers have been scaled by 0.9614 as pro-posed by Scott and Radom [20]. To aid in mode assignment, webased on the direct comparison between the experimental and cal-culated spectra by considering both the frequency sequence andintensity pattern and by comparisons with vibrational spectra ofsimilar compounds [21,22].

4. Results and discussion

Experimental FTIR/ATR and calculated IR spectra of EBT in the650–1700 cm−1 region are shown in Fig. 2, while the most impor-tant IR experimental and calculated vibrational modes of EBTmolecule, together with the proposed assignments are summarizedin Table 1.

The predicted B3LYP/6-31G(d) theoretical wavenumbers are inconsistently good agreement with the experimental values. Theaverage of the absolute deviations between experiment and the-ory is 9.41 cm−1, while the root mean square deviation (RMSD) ofresiduals is 12.92 cm−1 (standard deviation (SD) of 12.38 cm−1). Onthe other hand, the normalized RMSD is 1.38%. The largest discrep-ancy between experiment and theory is observed for the IR band at1049 cm−1 (absolute deviation of 40 cm−1), corresponding to a nor-mal mode that mainly involves the sulfonic group. Other importantdeviations are observed for the bands at 904, 1099 and 790 cm−1.

As shown in Fig. 2 and Table 1, the most representative FTIR/ATRbands for the EBT are the following: 1049 cm−1, corresponding tothe symmetric stretching vibration of the two S O bonds in theSO3H group coupled with OH bending in the same group; the broadband centered at 1201 cm−1 due to the CC stretching vibrations ofthe naphthalene ring2 and the band at 1337 cm−1 correspondingto the symmetric stretching vibration of NO2 group. The band at1145 indicates also the symmetric stretching vibration of the S Obonds and OH bending in the sulfonic group. The C–C stretchingvibrations of the naphthalene ring2 contribute significantly to the

normal modes giving rise the IR bands at 1099, 1201, 1232, 1274and 1303 cm−1.

The asymmetric stretching vibration of the NO2 group is shownat 1565 and 1617 cm−1. In the 1445–1617 cm−1 range appear theC–C stretching vibrations of the naphthalene rings. The bands in the

228 L. Szabó et al. / Spectrochimica Acta Part A 79 (2011) 226–231

Table 1Selected experimental FTIR/ATR bands (cm−1) and B3LYP/6-31G(d) calculated wavenumbers (cm−1) of EBT.

Experimental wavenumbers (cm−1) Calculated wavenumbers (cm−1)

FTIR/ATR B3LYP Band assignment

681 676 op. bending (ring1, ring2), op. bending (CH ring1)739 750 ip. (ring1, ring2) deformation790 774 �(SO31H), ip. ring2 deformation803 802 op. bending (O27H), ip. ring2 deformation827 831 op. bending (CH ring1)884 897 ip. ring2 deformation, �(C21NO2), �(C16SO3H)904 926 op. bending (C22H,C15H)982 989 ip. ring2 deformation, �(C16S), ı(CH ring2)1049 1089 ı(O31H), �s(SO3), ı(CH ring2), �(C16SO3)1099 1117 �(CC ring2), ı(O31H), ı(CH ring2), �(C13N12)1145 1130 ı(O31H), �s(SO3), ı(CH ring1, ring2)1201 1204 �(CC ring2), ı(CH ring2)1232 1232 �(C8N11), �(CC ring2, ring1), ı(CH ring2, ring1)1274 1262 ı(CH ring1, ring2), �(CC ring2), ı(O26H)1303 1299 �(C13N12), ı(O27H), �(CC ring2), ı(CH ring1, ring2)1337 1340 �s(NO2), �(C21N23)1379 1387 �as(CC ring2), ı(O27H), ı(CH ring2), �(N12N11)1405 1407 �(N12N11), �(CC ring2), ı(CH ring1, ring2), ı(O26H)1445 1438 �(CC ring1), ı(CH ring1), ı(O27H)1501 1505 �(CC ring1), ı(CH ring1)1565 1574 �(CC ring2), ı(O27H), �as(NO2), ı(CH ring2)1617 1614

�—stretching, �as—asymmetric stretching, �s—symmetric stretching, ı—in-planering2—naphthalene(C13–C22).

Fig. 2. Experimental FTIR/ATR (top) and B3LYP/6-31G(d) (bottom) calculated IRspectra of EBT.

�(CC ring1, ring2), �as(NO2), ı(CH ring1, ring2), ı(O26H)

bending, op.—out of plane, ip.—in plane, ring1—naphthalene (C1–C10),

739–982 cm−1 range are due to the superposition of the in planedeformation vibrations of both naphthalene rings. The bands at 681and 827 cm−1 are due to the out of plane CH bending vibrationcharacteristic of the naphthalene ring1.

The molecular electrostatic potential (MEP) is widely used asa reactivity map displaying most probable regions for the elec-trophilic attack of charged point-like reagents on organic molecules[23]. As it can be seen in the MEP distribution of the EBT moleculeobtained from DFT calculations depicted in Fig. 3, the negativecharge is located mainly on the NO2 and SO3H groups. Thus whenadded to the silver colloidal solution, the adsorption of the moleculeto the silver surface is supposed to occure through the NO2 andSO3H groups. The adsorption through the –N N– chromophorecannot be completely ruled out but it seems however less prob-able due to the required necessary deformation of the molecule inorder to arrange with the N N bond in the close vicinity of the silversurface.

It was observed that Raman spectrum of EBT is overlapped bystrong fluorescence emission when the 532 nm laser excitation linewas used. Therefore, the FT-Raman spectrum of EBT was recordedusing a Nd:YAG laser, emitting at 1064 nm, from solid polycrys-talline sample. This spectrum, together with the SERS spectrum ofEBT is shown in Fig. 4.

The assignment of the EBT FT-Raman and SERS bands was per-formed based on the direct comparison between experimental andDFT calculated bands by considering both, the frequency sequenceand the intensity pattern. Table 2 shows the most important FT-Raman and SERS bands as well as the B3LYP/6-31G(d) calculatedwavenumbers of EBT with their assignment.

The SERS spectrum of EBT is shown in Fig. 4. The SERS spectrumof EBT is dominated by several bands assigned to the naphthalenering2 at 1144, 1293, 1335, 1447 and 1594 cm−1. These bands willbe used as marker bands in the following discussion. The bandat 1144 cm−1 indicates the symmetric stretching vibration of theSO3H group. The CC asymmetric and symmetric stretching vibra-

tions of the naphthalene ring2 are present at 1293, 1335, 1374 and1594 cm−1. The in plane OH bending vibration of the OH groups isshown at 1293 cm−1. Besides the above mentioned vibrations, inthe range from 1144 to 1614 cm−1 appear in plane CH bendings of

L. Szabó et al. / Spectrochimica Acta Part A 79 (2011) 226–231 229

Table 2Selected experimental FT-Raman and SERS bands of EBT and B3LYP/6-31G(d) calculated vibrational modes with considerable Raman intensity.

Experimental wavenumbers (cm−1) Calculated wavenumbers (cm−1)

SERS FT-Raman B3LYP Band assignment

973 979 982 (ring2, ring1) breathing, ı(CH ring2, ring1), ı(O26H)1007 1034 1018 ip. ring1 deformation, ı(CH ring1)1047 1059 1080 ı(CH ring2), �(C21N23), ip. ring2 deformation1144 1145 1130 ı(O31H), �s(SO3), ı(CH ring1, ring2)

1172 1183 ı(CH ring1), ı(O26H), ip. ring1 deformation1240 1216 1230 �(C8N11), ı(CH ring1, ring2), ı(C14O27H)1293 1273 1298 �as(CC ring2), ı(O27H), ı(O26H), �(C13N12), ı(CH ring2)1335 1334 1346 �as(CC ring2), ı(C14O27H), ı(O26H), �(N12N11), �(C13N12), ı(CH ring2)1374 1370 1386 �as(CC ring2), ı(C14O27H), �(N12N11), ı(CH ring2)1419 1402 1408 �(N12N11), �(CC ring2), ı(CH ring1, ring2), ı(O26H), �(C7O26H)1447 1444 1443 ı(CH ring1, ring2), ı(C14O27H), �(CC ring1, ring2)1470 1471 1458 ı(CH ring1, ring2), ı(C14O27H), �(N12N11), �(CC ring1, ring2)1505 1512 �(CC ring2), ı(O27H), �as(NO2), ı(CH ring2)1558 1529 1562 �(CC ring2), ı(O27H), �as(NO2), ı(CH ring2)1594 1576 1585 �(CC ring2), � (NO ), ı(CH ring2)

� laner

taa

1t

Fao

1614 1616 1613

—stretching, �as—asymmetric stretching, �s—symmetric stretching, ı—in-ping2—naphthalene(C13–C22).

he naphthalene rings. In the range between 1505 and 1614 cm−1

ppears the asymmetric stretching vibration of the NO group,

2lways coupled to CC ring stretchings and OH or CH bendings.

The SERS spectrum of EBT shows a broad band in the200–1400 cm−1 region. In our opinion this band is due to con-ribution of naphthalene ring2 CC stretching vibration.

ig. 3. B3LYP/6-31G(d) calculated 3D molecular electrostatic potential of EBT (in.u.) mapped on the electronic density isosurface of 0.02 a.u. and schematically viewf EBT adsorption geometry on the silver surface.

as 2

�(CC ring1, ring2), �as(NO2), �(CH ring1, ring2)

bending, op.—out of plane, ip.—in plane, ring1—naphthalene (C1–C10),

A more comprehensive assignment of the EBT SERS and FT-Raman bands is shown in Table 2.

DFT calculations were also performed on the EBT-Cu(II) com-plex. After geometry optimization, the MEP distribution and thevibrational modes of the complex were calculated. As shown inFig. 5, for EBT-Cu(II) complex, the negative charge is located on the

Fig. 4. Experimental FT-Raman spectrum (middle) of polycrystalline EBT, SERS spec-trum (top) of EBT and B3LYP/6-31G(d) calculated Raman spectrum of EBT (bottom)in gas-phase.

230 L. Szabó et al. / Spectrochimica Acta Part A 79 (2011) 226–231

Fcs

O

ts

Et

Fo

ig. 5. B3LYP/6-31G(d) calculated 3D molecular electrostatic potential of EBT-Cu(II)omplex (in a.u.) mapped on the electronic density isosurface of 0.02 a.u. andchematically view of EBT-Cu(II) complex adsorption geometry on the silver surface.

26, O27 and N11 atoms involved in the metal ion coordination [6].In a similar manner as for the EBT molecule, the MEP distribu-

ion of the EBT-Cu(II) complex sustains an adsorption to the silver

urface by the NO2 and SO3H groups.

The SERS spectrum of the EBT-Cu(II) complex, obtained from theBT:CuSO4 1:1 molar ratio, is shown in Fig. 6.The SERS spectrum ofhe EBT-Cu(II) complex shows several differences compared to the

ig. 6. B3LYP/6-31G(d) calculated Raman spectrum of EBT (bottom), SERS spectrumf the EBT (middle) and SERS spectrum of the EBT-Cu(II) complex (top).

Fig. 7. B3LYP/6-31G(d) calculated Raman spectra and SERS spectra of EBT, EBT-Cu(II), -Fe(III), -Mn(II) and -Pb(II) complexes, prepared at EBT:metal salt molar ratioof 1:1.

SERS spectrum of EBT. Several bands are shifted up to ∼30 cm−1

and relative intensities are modified. The most important shiftsare: from 1293 to 1337 cm−1, from 1335 to 1362 cm−1 and from1374 to 1402 cm−1. The band at 1337 cm−1 indicates the �(CC ring1,ring2), �(C7O26), ı(C10H, C15H, C22H, C3H) vibrations. The �(CCring2, ring1), ı(CH ring1, ring2), �(N11N12) vibrations appear at1362 cm−1. The �(C7O26), �(CC ring2), �(N11C8), ı(CH ring1, ring2)vibrations are present at 1402 cm−1.

The assignment of the SERS bands of the EBT-Cu(II) complex wasperformed by comparing the experimental bands and the DFT cal-culated spectrum of the EBT-Cu(II) complex. Table 3 shows the mostimportant, EBT and EBT-Cu(II) complex SERS bands and B3LYP/6-31G(d) calculated wavenumbers, as well as the assignment of thebands as obtained from the DFT calculation.

The EBT-metal complexes can be differentiated by their SERSspectral features. Fig. 7 shows the SERS spectra of EBT-Cu(II), -Fe(III), -Mn(II) and -Pb(II) complexes.

As seen in Fig. 7, most SERS bands of the EBT-Fe(III), -Mn(II) and-Pb(II) complexes show Raman shifts that are close to those cor-responding to the EBT-Cu(II) SERS spectrum, the largest observeddeviation being 12 cm−1. The SERS bands assignments for thesecomplexes is similar to those given in Table 3 for the EBT-Cu(II)complex, as revealed also by the DFT based calculations for EBT-Fe(III) and -Mn(II) complexes.

Each EBT-metal complex SERS spectrum from Fig. 7 shows acharacteristic spectral fingerprint. Several SERS bands are repre-

sentative for each EBT-metal complex, like 1242, 1337, 1362, 1402,1510, 1590 cm−1 for EBT-Cu(II), 1240, 1336, 1352, 1400, 1510,1564 cm−1 for EBT-Fe(III), 1217, 1327, 1358, 1400, 1591 cm−1 forEBT-Mn(II) and 1216, 1327, 1352, 1388, 1591 cm−1 for EBT-Pb(II)complex.

L. Szabó et al. / Spectrochimica Acta Part A 79 (2011) 226–231 231

Table 3Selected SERS bands of EBT and EBT-Cu(II) complex, together with B3LYP/6-31G(d) calculated vibrational modes of EBT-Cu(II) complex.

Experimental wavenumbers (cm−1) Calculated wavenumbers (cm−1)

SERS EBT-Cu(II) SERS EBT B3LYP-Cu(II) Band assignment

502 505 ip. (ring1, ring2) deformation, �(CuO27C14)579 591 601 ip. (ring2, ring1) deformation, �(CuO2), �(C16SO3)668 693 633 ip. (ring2, ring1) deformation, �(CuO2), �(C16SO3)694 713 694 �(Cu41O2), ip. deformation ring3, ı(C14O), ı(C15H, C22H), �(S28O31H)790 789 781 �(S28O31H), �(CuO27)822 822 867 ring2, ring1 breathing, �(CuN11), ı(C8N11N12), ı(C10H, C15H, C22H, C3H)879 880 891 ring1, ring2 breathing, �(C16SO3), �(C21NO2)968 968 957 ring2 breathing1043 1047 1026 ı(CH ring1), �(CC ring1)1147 1144 1174 ı(CH ring1, ring2), �(CC ring1)1242 1240 1246 ı(CH ring1), �(CC ring1)1337 1293 1298 �(CC ring1, ring2), �(C7O26), ı(C10H, C15H, C22H, C3H)1362 1335 1356 �(CC ring2, ring1), ı(CH ring1, ring2), �(N11N12)1402 1374 1406 �(C7O26), �(CC ring2), �(N11C8), ı(CH ring1, ring2)1440 1447 1462 ı(CH ring1), �(CC ring1), �(C7O26), �(N12N11)1510 1505 1532 �(CC ring1, ring2), �(N12N11), ı(CH ring1, ring2)1564 1558 1572 �(CC ring2), � (NO ), ı(CH ring2)

� nding,

5

nMo

pe

ato

sm

A

P

R

[[

[

[[

[[[[

[

[

1590 1594 15951613 1614 1598

—stretching, �as—asymmetric stretching, �s—symmetric stretching, ı—in-plain be

. Conclusions

Optimized geometries, molecular electrostatic potential andormal modes of EBT and EBT-metal complexes of Cu(II), Fe(III) andn(II) were calculated by theoretical DFT B3LYP/6-31G(d) meth-

ds.The FT-Raman and SERS spectra of EBT and EBT-metal com-

lexes were safely assigned, due to a good match betweenxperimental and DFT calculated vibrational modes.

The calculated MEP distributions indicate for the EBT moleculend EBT-Cu(II) complex the highest electronegativity localized onhe NO2 and SO3H groups, thus, the adsorption to the silver surfaceccuring by these groups.

Each EBT-metal complex SERS spectrum shows a characteristicpectral fingerprint, permitting thus discrimination between EBT-etal ion complexes.

cknowledgement

This work was supported by CNCSIS-UEFISCSU, project numberN II RU PD 445/2010.

eferences

[1] A.A. Ensafi, M. Fouladgar, Sens. Actuators B 113 (2006) 88.[2] P.C.A. Jeronimo, A.N. Araujo, M. Conceicao, B.S.M. Montenegro, Sens. Actuators

B 103 (2004) 169.

[3] P.C.A. Jeronimo, A.N. Araujo, M.C.B.S.M. Montenegro, C. Pasquini, I.M.Jr.

Raimundo, Anal. Bioanal. Chem. 380 (2004) 108.[4] X. Zhai, S. Efrima, J. Phys. Chem. 100 (5) (1996) 1779.[5] Ck. Chong, Ck. Mann, Tj. Vickers, Appl. Spectrosc. 46 (2) (1992) 249.[6] J. Ghasemi, H. Peyman, M. Meloun, J. Chem. Eng. Data 52 (2007)

1171.

[

[

[

as 2

�(CC ring1), ı(CH ring1)�s(CC ring2), �as(NO2), ı(CH ring2)

ip.—in plane, ring1—naphthalene (C1–C10), ring2—naphthalene(C13–C22).

[7] Dr. Asif Husain, Lecturer, Theoretical Basis of Analysis: Complexometric Titra-tions, Dept. of Pharmaceutical Chemistry, Faculty of Pharmacy, New Delhi,2007.

[8] G.C. Schatz, R.P. Van Duyne, Electromagnetic mechanism of surface-enhancedspectroscopy, in: J.M. Chalmers, P.R. Griffiths (Eds.), Handbook of VibrationalSpectroscopy, Wiley, New York, 2002, pp. 759–774.

[9] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Chem. Rev. 99 (1999) 2957.10] K. Kneipp, H. Kneipp, H.G. Bohr, Top. Appl. Phys. 103 (2006) 261.11] N. Leopold, S. Cînta Pînzaru, M. Baia, E. Antonescu, O. Cozar, W. Kiefer, J. Popp,

Vib. Spectrosc. 39 (2005) 16.12] K. Carron, K. Mullen, M. Lanouette, H. Angersbach, Appl. Spectrosc. 45 (1991)

420.13] N. Leopold, B. Lendl, J. Phys. Chem. B 107 (2003) 5723.14] Gaussian 03, Revision E.01, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuse-

ria, M.A. Robb, J.R. Cheeseman, J.A. Jr. Montgomery, T. Vreven, K.N. Kudin, J.C.Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G.Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M.Klene, X. Li, J.E. Knox, H.P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G.Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D.Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford,J. Cioslowski, B.B. Stefanov, G. Liu, A. 10 Liashenko, P. Piskorz, I. Komaromi, R.L.Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challa-combe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople,Gaussian, Inc., Wallingford CT, 2004.

15] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.16] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.17] P.L. Polavaru, J. Phys. Chem. 94 (1990) 8106.18] V. Krishnakumar, G. Keresztury, T. Sundius, R. Ramasamy, J. Mol. Struct. 702

(2004) 9.19] S.D. Williams, T.J. Johnson, T.P. Gibbons, C.L. Kitchens, Theor. Chem. Acc. 117

(2007) 283.20] A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502.

21] C.F. Leypold, M. Reiher, G. Brehm, M.O. Schmitt, S. Schneider, P. Matousek, M.

Towrie, Phys. Chem. Chem. Phys. 5 (2003) 1149.22] N. Leopold, L. Szabó, A. Pîrnau, M. Aluas, L.F. Leopold, V. Chis, O. Cozar, J. Mol.

Struct. 919 (2009) 94–99.23] P. Politzer, D.G. Truhlar, Chemical Application of Atomic and Molecular Elec-

trostatic Potentials, Plenum, New York, 1981.