Surface Orientation of Pyrazine Adsorbed on Silver from the Surface

3100

Langmuir 2002, 18, 3100-3104

Surface Orientation of Pyrazine Adsorbed on Silver from the Surface-Enhanced Raman Scattering Recorded at Different Electrode Potentials J. Soto, D. J. Ferna´ndez, S. P. Centeno, I. Lo´pez Toco´n, and J. C. Otero* Department of Physical Chemistry, Faculty of Sciences, University of Ma´ laga, E-29071-Ma´ laga, Spain Received March 30, 2001. In Final Form: November 26, 2001 The observed shifts of the vibrational SERS frequencies with respect to those of the Raman spectrum of pyrazine have allowed us to conclude that the molecule adsorbs to the metal through the unshared electron pair of one of the nitrogen atoms. The same conclusion is reached by observing the behavior of the molecular vibrations when the electrode potential is changed. Only one type of molecules has been found with orientation perpendicular to the metal surface in the whole range of potentials studied. These conclusions are supported by the comparison of the experimental data with the ab initio results obtained for the vibrational frequencies of pyrazine isolated and coordinated to several conveniently selected silver clusters.

Introduction One of the still basic open questions in surface-enhanced Raman scattering (SERS) concerns whether the observed Raman signal in a particular spectrum comes from molecules adsorbed on the surface or located in its vicinity but not directly bonded to the metal. This is a quite relevant question because, for instance, surface adsorption may be important in connection with heterogeneous catalysis, corrosion, etc. Moreover, adsorption is related to both of the enhancement mechanisms proposed to explain the SERS phenomena, namely, the electromagnetic (EM) and the charge transfer (CT) mechanisms.1-3 Irrespective of the fact that some evidence exists to suppose that the EM mechanism operates even at distances significantly apart from the metal,4 the existence of adsorption is the basic hypothesis of the so-called propensity rules derived from that mechanism.5 These rules allow one to get insight concerning the surface orientation from the changes in the relative intensities of Raman and SERS spectra of a molecule. On the other hand, the changes in relative intensities are alternatively explained by the CT mechanism on the basis of a resonant charge-transfer process between the metal and the molecule.3,6-8 Although CT processes without close contact between donor and acceptor are possible, it is generally assumed that surface * To whom correspondence should be addressed. Phone: +34 952132019. Fax: + 34 952132047. E-mail: [email protected]. (1) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (2) Creighton, J. A. In Spectroscopy of Surfaces; Clark R. J. H., Hester R. E., Eds.; Wiley: Chichester, U.K., 1988; p 37 (The Selection Rules for Surface-Enhanced Raman Spectroscopy). (3) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143. (4) Murray, C. A. In Surface-Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; p 203 (Molecule-Silver Separation Dependence). (5) Moskovits, M.; DiLella, D. P.; Maynard, K. J. Langmuir 1988, 4, 67. (6) Creighton, J. A. Surf. Sci. 1986, 173, 665. (7) (a) Arenas, J. F.; Woolley, M. S.; Otero, J. C.; Marcos, J. I. J. Phys. Chem. 1996, 100, 3199. (b) Arenas, J. F.; Lo´pez Toco´n, I.; Otero, J. C.; Marcos, J. I. J. Phys. Chem. 1996, 100, 9254. (c) Arenas, J. F.; Lo´pez Toco´n, I.; Woolley, M. S.; Otero, J. C.; Marcos, J. I. J. Raman Spectrosc. 1998, 29, 673. (d) Arenas, J. F.; Woolley, M. S. Lo´pez Toco´n, I.; Otero, J. C.; Marcos, J. I. Vib. Spectrosc. 1999, 19, 213. (8) Arenas, J. F.; Lo´pez Toco´n, I.; Otero, J. C.; Marcos, J. I. J. Chem. Phys. 2000, 112, 7669.

adsorption is necessary for that CT taking place, which is why one accepts that the CT mechanism is limited to directly adsorbed molecules of the first layer.4 Likewise, surface adsorption has been invoked to explain the observation in SERS of Raman inactive modes, for instance, the “u” fundamentals in centrosymmetric molecules such as pyrazine, in such a way that the operating symmetry is not that of the isolated molecule but that of the metal-molecule complex, usually a lower one.8,9 Alternatively, the SERS activity of such fundamentals has been suggested to arise from a strong electric field gradient in the interphase.10 The latter mechanism is supported by the fact that frequency shifts observed in SERS are moderate and, therefore, adsorption should not be strong and the operating symmetry should not be quite different from that of the isolated molecule. As can be inferred, surface adsorption is a question closely related to the key questions in SERS. In this work we have recorded the SERS spectra of pyrazine on a silver electrode and studied the dependence of the observed frequencies on the electric potential of the interphase. The experimental results have been successfully compared with the ab initio computed frequencies for several silverpyrazine complexes with different compositions, orientations, and charges of the metallic clusters. It has been found that the experimental behavior is satisfactorily explained by assuming that pyrazine is adsorbed through the unshared electron pair of one of the nitrogen atoms. This means a perpendicular orientation of the pyrazine ring with respect to the metal surface as well as a descent in symmetry from D2h to C2v, which in turn may be responsible for the observation in SERS of B3u, Au, and, especially, B1u modes inactive in D2h.8 To confirm these conclusions, the frequency shifts originated by protonation of pyrazine have been compared and discussed too.11,12 (9) See for instance: Moskovits, M.; DiLella, D. P. J. Chem. Phys. 1980, 73, 6068. (10) (a) Sass, J. K.; Neff, H.; Moskovits, M.; Holloway, S. J. Phys. Chem. 1981, 85, 621. (b) Moskovits, M.; DiLella, D. P. J. Chem. Phys. 1982, 77, 1655. (11) Montan˜ez, M. A.; Lo´pez Toco´n, I.; Otero, J. C.; Marcos, J. I. J. Mol. Struct. 1999, 482-483, 201. (12) Brolo, A. G.; Irish, D. E. Z. Naturforsch. 1995, 50a, 274.

10.1021/la010489p CCC: $22.00 © 2002 American Chemical Society Published on Web 03/16/2002

Surface Orientation of Pyrazine Adsorbed on Ag

Langmuir, Vol. 18, No. 8, 2002 3101

Experimental Section Pyrazine (Aldrich) has been purified by distillation under reduced pressure. SERS records have been obtained by using a three-electrode cell monitored by a PAR model 173 potentiostat and a PAR model 175 programmer, fitted with a platinum (Metales Preciosos, SA) counter electrode and a PAR No. K0260 saturated Ag/AgCl/KCl reference electrode. The pure silver (Metales Preciosos, SA) working electrode has been successively polished with 1.00, 0.30, and 0.05 µm alumina (Bueler) and then electrochemically roughened in a 1.0 M KCl aqueous solution by maintaining initially the surface potential at -0.50 V and then subjecting it to 10 2-s pulses at +0.60 V. Water has been degassified, deionized, and triply distilled. SERS have been recorded from 1.0 M KCl and 0.1 M pyrazine aqueous solutions. A Jobin-Yvon U1000 spectrometer has been used always, and the 514.5 nm exciting line from an Spectra Physics 2020 Ar+ laser has been selected. The power reaching the sample has been kept at 30 mW.

Figure 1. Molecular models of the N and Φ complexes of pyrazine with silver clusters.

Results and Discussion Theoretical Model. Ab initio calculations of the frequencies of coordinated and isolated pyrazine (Pz) have been carried out by using the GAUSSIAN 94 program package13 at the RHF/3-21G level of theory, which allowed one to compute analytically the force constants and to compare the results with the published ones involving frequency shifts originated by water dilution and protonation.11 To check the reliability of these results, we have carried out B3-LYP/3-21G as well as RHF and B3-LYP calculations with the LanL2DZ pseudopotential. The conclusions derived from the different levels of theory are quite similar as can be seen in the corresponding tables submitted as Supporting Information. A previous question is the selection of a molecular model to simulate the surface adsorption and the effect of the changes in the electrode potential. There exists some controversy dealing with the nature of the SERS active adsorption sites, and several models have been proposed ranging from isolated adsorbed metal atoms (adatoms14) up to complex metallic clusters such as Agn+ (3 < n < 6) which may exhibit different structures with similar energies.15 The simplest model of the silver-pyrazine complex is the Ag-Pz supermolecule, but it shows the trouble that a silver atom possesses an unpaired electron and the UHF calculations involving silver atoms exhibit an important spin contamination which invalidates this level of theory. Because of the same reason, the effect of the change of the electrode potential cannot be simulated by keeping constant the number of silver atoms and changing the charge of the metallic cluster. As a consequence, we have selected the series of complexes shown in Figure 1, in which a metal atom is coordinated to a nitrogen atom of the pyrazine (Ag-N) or to the π-system (Ag-Φ) giving rise to perpendicular or parallel orientations with respect to the metal surface, respectively. All of the chosen models (Ag+-Pz, Ag3+-Pz, Ag2-Pz, and Ag3-Pz) are closed-shell systems, and the metallic (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanow, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J.; Reploge, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Steward, J. J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. GAUSSIAN 94, revision E.2; Gaussian Inc.; Pittsburgh, PA, 1995. (14) Otto, A.; Pockrand, I.; Billmann, J.; Pettenkofer, C. In SurfaceEnhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; p 147 (The Adatom Model: How Important Is Atomic Scale Roughness?). (15) Dong, S. Y.; Wang, G.; Wang, W.; Zhang, Z.; Zheng, J. Appl. Phys. 1989, B49, 553.

Figure 2. Relationship between the charge of pyrazine (Pz) and the charge of the Ag1 atom in the N (white circles) and Φ (black circles) complexes. Table 1. Calculated RHF/3-21G Charges of Pyrazine (Pz) and Silver Atoms in the N and Φ Complexes complex N

Φ

charge Pz charge Ag1 Ag2 Ag3 charge Pz charge Ag1 Ag2 Ag3

Ag+-Pz

Ag3+-Pz

Ag2--Pz

Ag3--Pz

0.17 0.83

0.14 0.50 -0.13 0.5 0.12 0.57 -0.13 0.44

0.10 -0.11 0.01

0.07 -0.99 0.46 -0.53 0.01 -0.67 0.12 -0.46

0.16 0.84

0.06 -0.01 -0.05

clusters have linear structures orientated along the axis C2(z) (Ag-N) or C2(x) (Ag-Φ). In such a way, the possible specific interactions between pyrazine and silver atoms nondirectly bonded are minimized and, moreover, structures such as the nonlinear Ag3 clusters liable to undergo Jahn-Teller effect are avoided.16 The chosen complexes allow one to simulate properly the change in the electrode potential and, therefore, the change in the surface charge excess on the metal. The complex is formed by charge donation from pyrazine to the metallic cluster. Although the selected clusters have different composition and net charges, they are classified according to the amount of the donated charge, which in turn relates to the charge of the silver atom directly bonded to pyrazine (Ag1) (Figure 2). The net charges of Pz and the respective silver atoms of the clusters are summarized in Table 1. That of Ag1 range from +0.83 or +0.84 in the Ag+-N or Ag+-Φ complexes, respectively, to -0.99 or -0.67 in the respective complexes with Ag3-, being close to zero in the neutral ones with Ag2. Both types of (16) See for instance: Wedum, E. E.; Grant, E. R.; Cheng, P. Y.; Willey, K. F.; Duncan, M A. J. Chem. Phys. 1994, 100, 6312.

3102

Langmuir, Vol. 18, No. 8, 2002

Soto et al.

Table 2. Frequency Shifts (cm-1) of the Raman Spectrum of Pyrazine in HCl Aqueous Solution and in the SERS at 0.0 V with Respect to the Pure Liquid Raman Spectrum, RHF/3-21G Calculated Frequencies of Isolated Pyrazine (Pz), and the Respective Shifts Corresponding to the N Complexes with H+ and Ag+ exptl mode 20b;ν(CH),B2u 2;ν(CH),Ag 7b;ν(CH),B3g 13;ν(CH),B1u 8a;νring,Ag 8b;νring,B3g 19a;δ(CH),B1u 19b;δ(CH),B2u 3;δ(CH),B3g 9a;δ(CH), Ag 14;νring,B2u 18a;νring,B1u 15;νring,B2u 12;δring,B1u 1;νring,Ag 17a;γ(CH),Au 5;γ(CH),B2g 10a;γ(CH),B1g 11;γ(CH),B3u 4;τring,B2g 6b;δring,B3g 6a;δring,Ag 16b;τring,B3u 16a;τring,Au a

PED (%)a

liquid

100 ν(CH) 100 ν(CH) 100 ν(CH) 100 ν(CH) 68 νring, 24 δ(CH) 76 νring, 13 δring 72 δ(CH), 28 νring 68 δ(CH), 30 νring 92 δ(CH) 72 δ(CH), 26 νring 100 νring 42 νring, 38 δring 62 νring, 32 δ(CH) 66 δring, 32 νring 96 νring 80 γ(CH) 92 γ(CH) 100 γ(CH) 96 γ(CH) 95 τring 94 δring 90 δring 91 τring 90 τring

3070b 3055 3040 3018b 1580 1525 1490b 1418b 1346 1233 1148b 1125b 1067b 1022b 1016 997c 983 927 804b 756 704 602 417b 350c

PED contributions from the RHF/3-21G scaled frequencies.8

complexes with Ag- are unstable and dissociate in the optimization process. Frequency Shifts Originated by Protonation. The vibrational frequency shifts of azines (azobenzenes) have been used in SERS to deduce how the molecule interacts with the metal17,18 on the basis of criteria similar to those used to explain the shifts originated by protonation or dilution in water.11,12 Aromatic molecules with heteroatoms, as pyrazine and pyridine, can interact with the metal through the unshared electron pairs of the nitrogen atoms or through the π-system. If they are bonded through the π-system, the aromatic ring should be weakened because of the bonding nature of such electrons, and therefore, the ring stretching vibrations should shift toward lower frequency as observed in benzene and toluene. On the other hand, it has been suggested that if the interaction takes place through the heteroatom, then the aromatic ring is strengthened because of the nonbonding nature of the unshared electron pairs, therefore originating an increase of the frequencies of the ring stretching fundamentals. The latter shifts are the same that can be expected in aqueous solutions because of the hydrogen bonding between the heteroatoms and the molecules of the solvent. The same effect, even stronger, should be observed in acid aqueous solutions given that in this case the hydrogen bonds between the nitrogen atoms and the hydrogen ions are stronger. In a previous work, we have studied the effect of the dilution in water and that of the protonation on the vibrational frequencies of pyrazine with the help of the ab initio RHF/3-21G calculations.11 We have found that the observed shifts in aqueous solution correspond quite well with the calculated ones for pyrazine coordinated to two molecules of water through both nitrogen atoms. On the other hand, protonation originates larger frequency shifts than coordination to neutral water does. In the latter case the experimental data are taken from aqueous solutions of HCl by Irish and (17) Kellog, D. S.; Pemberton, J. E. J. Phys. Chem. 1987, 91, 1120. (18) Gao, P.; Weaver, M. J. J. Phys. Chem. 1989, 93, 6205.

RHF/3-21G

HCl

SERS 0.0 V

+33

+13

+34 -34

+14 -5

-42 -10 +25 -11

+12

+10 -4

+27 +5

-20 -22 -15 +7

b

0 -10 +38 +24

Pz

H+-N

Ag+-N

3408 3418 3387 3391 1732 1679 1644 1558 1509 1362 1111 1249 1179 1114 1111 1193 1178 1087 931 881 794 678 514 443

+35 +30 +41 +39 +27 -38 -11 -30 -77 -15 +4 -23 -3 +15 -10 +14 +47 -21 -45 -30 -27 +1 -30 +2

+17 +14 +12 +13 +10 -6 +2 +4 +4 +4 -12 -9 +13 +28 -1 +9 +10 -9 +10 -4 -6 +28 +12 -4

IR liquid. c IR solid.

Brolo12 and are summarized in Table 2. As can be seen, protonation originates much more complex effects than expected by the previously mentioned rule. For instance, there are ring stretching fundamentals, e.g. 8a (+34 cm-1) and 14 (+25 cm-1), which shift blue as expected, but important red shifts are observed in cases of fundamentals with a large participation of ring stretching coordinates, e.g. 8b (-34 cm-1) and 19b (-42 cm-1), as well as in other fundamentals such as 4;τring (-22 cm-1) and 10a;γ(CH) (-20 cm-1). Generally speaking, the effect of protonation is complex and there are some modes which shift red and others which shift blue in a similar number and amount. Despite this complex behavior, the RHF/3-21G calculated frequencies of pyrazine and pyrazine coordinated to only one hydrogen ion through the unshared electron pair of one of the nitrogens (H+-N) reproduce satisfactorily the sign and magnitude of the observed shifts. Table 2 summarizes the calculated frequencies of isolated and protonated pyrazine as well as the assignment of the vibrational spectrum and its PED as calculated from the scaled RHF/3-21G force field.8 For instance, the calculated shifts for modes 8a, 14, 8b, 19b, 4, and 10a amount to +27, +4, -38, -30, -30, and -21 cm-1, respectively, in good agreement with the experimental values. These results point out that monoprotonated pyrazine is the majority species even at high HCl concentrations. Moreover, the C2v symmetry of that species (H+-N) allows all the fundamentals to be Raman active and can explain why “u” modes such as 19b, 14, 18a, and 12 are observed. On the contrary, the calculated shifts for diprotonated pyrazine are much larger than the experimental ones, and its symmetry is still centrosymmetric. Frequency Shifts Originated by Coordination with Silver. The shifts observed in the SERS spectrum recorded at 0.0 V are also shown in Table 2. It can be seen that they are quite different from those originated by protonation. The largest shifts are now positive and correspond to modes 12, 6a, and 16b, amounting to +27, +38, and +24 cm-1, respectively, but none of the mentioned

Surface Orientation of Pyrazine Adsorbed on Ag

Langmuir, Vol. 18, No. 8, 2002 3103

Figure 3. Correlation between the RHF/3-21G calculated and observed shifts of the frequencies of pyrazine originated by protonation (black circles) and by coordination with silver (white circles). The latter plot has been obtained from the experimental shifts of the SERS at 0.0 V and the computed ones for the Ag+-N complex.

Figure 4. Dependence of the observed frequency shifts of pyrazine on the electrode potential.

modes is essentially a ring stretching vibration. This behavior is also followed by the calculated frequencies of the species Ag+-N, in which the atom Ag1 bears the greatest part of the positive charge (+0.84). As can be seen in Table 2, the largest blue shifts are calculated for modes 12, 6a, and 16b, amounting to +28, +28, and +12 cm-1, respectively. Likewise, the behavior of modes 2 and 8a is reproduced satisfactorily; the calculated values show smaller shifts in pyrazine coordinated to silver (+14 and +10 cm-1, respectively) than in monoprotonated pyrazine (+30 and +27 cm-1), in good agreement with the experimental values observed in SERS (+13 and +14 cm-1) and in HCl solution (+33 and +34 cm-1). The remaining vibrations show small shifts; for instance, modes 8b, 9a, 1, and 4, show different signs (-5, +12, +5, and -10 cm-1, respectively) which are reproduced satisfactorily by the theoretical results (-6, +4, -1, and -4 cm-1). The only significant disagreement involves vibration 11, which does

not shift noticeably but its calculated value amounts up to +10 cm-1. Generally speaking, the shifts of the outof-plane fundamentals appear to be underestimated by the ab initio calculations. Figure 3 shows the good correlation found between the calculated and observed frequency shifts of pyrazine in HCl aqueous solution and in the SERS at 0.0 V and the different behavior for both types of coordination. Table 3 summarizes the shift of the SERS frequencies recorded at electrode potentials of 0.0, -0.25, -0.50, and -0.75 V, respectively. It can be seen that almost every fundamental shifts toward the red as the electrode potential becomes more negative, i.e., as the surface negative charge excess increases (Figure 4). However, mode 6a, which is the most sensitive one in pyrazine coordinated to silver at 0.0 V, shifts still +24 cm-1 in the spectrum recorded at -0.75 V. These values can be compared with the calculated shifts for the complexes

Table 3. Comparison between the Effect of the Electrode Potential on the Frequency Shifts (cm-1) of Pyrazine Observed in SERS and the RHF/3-21G-Calculated Ones for Different Complexes with Silver SERS (V) mode 20b;ν(CH),B2u 2;ν(CH),Ag 7b;ν(CH),B3g 13;ν( CH),B1u 8a;νring,Ag 8b;νring,B3g 19a;δ(CH),B1u 19b;δ(CH),B2u 3;δ(CH),B3g 9a;δ(CH),Ag 14;νring,B2u 18a;νring,B1u 15;νring,B2u 12;δring,B1u 1;νring,Ag 17a;γ (CH),Au 5;γ( CH),B2g 10a;γ (CH),B1g 11;γ(CH),B3u 4;τrin g,B2g 6b;δr ing,B3 g 6a;δr ing,Ag 16b;τring,B3u 16a;τring,Au a

RHF/3-21G

0.0 -0.25 -0.50 -0.75 ∆exp a Ag+-N Ag3+-N Ag2-N Ag3--N ∆cal b Ag+-Φ Ag3+-Φ Ag2-Φ Ag3--Φ ∆calb +13

+13

+5

-5

-18

+14 -5

+16 -5

+10 -5 -7

+4 -3 -10

-10 +2

+12

+14

+8

+4

-8

+27 +5

+23 +5

+21 +3

+11 +1

-16 -4

0 -10

-1 -10 -4 +36 +22

-1 -14 -7 +32 +20 +7

-7 +24 +12 +5

-14 -12

+38 +24

+17 +14 +12 +13 +10 -6 +2 +4 +4 +4 -12 -9 +13 +28 -1 +9 +10 -9 +10 -4 -6 +28 +12 -4

+14 +11 +12 +11 +11 -4 +2 +4 +5 +3 -9 -8 +11 +24 -1 +9 +11 -3 +10 -4 -6 +24 +11 -3

+9 +7 +11 +10 +9 -1 0 +1 +1 0 -4 -4 +4 +15 -1 +7 +10 +1 +8 -2 -4 +16 +8 0

+3 +1 +2 +2 +1 0 -3 0 -1 -4 0 -2 -2 +3 -2 +8 +8 +3 +6 0 -2 +5 +3 0

-14 -13 -10 -11 -9 +6 -5 -4 -5 -8 +12 -7 +15 -25 -1 -1 -2 +12 -4 +4 -4 -23 -9 +4

+15 +11 +19 +19 -32 -27 -22 -14 -10 -15 +24 -18 -17 -20 -23 -2 -6 -24 +22 -18 -14 -12 -14 +2

+15 +12 +19 +19 -24 -20 -17 -9 -6 -11 +15 -14 -14 -4 -12 +2 -5 -11 +16 -13 -12 -8 -12 -1

Shifts between the SERS recorded at -0.75 and 0.0 V. b Shifts between the Ag3--Pz and Ag+-Pz complexes.

+11 +10 +13 +13 -10 -9 -6 -3 -2 -4 -2 -6 -7 -5 -10 -4 -6 +7 +4 -7 -6 -4 -9 -8

+2 +2 +1 +1 0 0 0 0 +1 0 +5 +1 -1 +1 -1 -7 -7 -10 -7 -2 0 +1 -2 -5

-13 -9 -18 -18 +32 +27 +22 +14 +11 +15 +19 +9 +16 +21 +22 +5 +1 +14 -29 +16 +14 +13 +12 -7

3104

Langmuir, Vol. 18, No. 8, 2002

Soto et al.

agreement with the experimental values -18, -10, -8, -16, -14, and -12 cm-1, respectively, observed in SERS when the electrode potential changes from 0.0 to -0.75 V. In the case of the Ag3--Pz complex the charge donated by pyrazine is quite small, which explains that the calculated frequencies for the Ag3--N complex are quite similar to those of the isolated molecule. This encourages us to suppose that, in the electrode potentials studied, the charges of the silver atoms directly bonded to Pz in the SERS active sites range between that of the Ag1 atoms of the Ag+-N and Ag3--N complexes. Finally, Table 3 includes also the calculated shifts for the Φ-Ag complexes, which appear to be quite different from the experimental ones: the magnitude, the sign, and the effect of the charge of the cluster being just the opposite of the observed SERS results. Therefore, the Ag-Φ coordination has been definitively discarded. Conclusions Figure 5. Dependence of the calculated frequency shifts on the charge of pyrazine (Pz).

Figure 6. Plot of the RHF/-3-21G computed frequency shifts of pyrazine between the Ag+-N and Ag3--N complexes (∆cal) versus the observed ones in the SERS at -0.75 V with respect to that at 0.0 V (∆exp).

Ag+-N, Ag3+-N, Ag2-N, and Ag3--N, where the atom Ag1 supports a charge amounting to +0.83, +0.50, -0.11, and -0.99, respectively. Generally speaking, as the negative charge supported either by the Ag1 atom or by the metallic cluster increases, the calculated frequencies of pyrazine shift red systematically in agreement with the experimental behavior (Figure 5). In Table 1 one observes that a charge donation from pyrazine to the metallic cluster takes place. It amounts to -0.17 in the Ag+-N complex and is responsible for the observed frequency shifts. As the negative charge on either atom Ag1 or the cluster increases, the charge donated by pyrazine decreases and, consequently, the calculated shifts decrease too. Table 3 includes also two columns showing the experimental (∆exp) and calculated (∆cal) values for the frequency shifts of pyrazine originated by the change in the electrode potential from 0.0 to -0.75 V and the calculated ones between Ag+-N and Ag3--N clusters, respectively. Figure 6 shows the correlation between experimental and calculated ∆. Likewise, frequency shifts between the complexes Ag+-N and Ag3--N of the most representative modes, namely 2, 8a, 9a, 12, 6a, and 16b, are calculated to be -13, -9, -8, -25, -23, and -9 cm-1, respectively, in good

We think that it is clearly shown that the pyrazine molecules responsible for the SERS effect are directly bonded to silver atoms of the metal surface, that the bonding takes place by charge donation from the lone pair of one of the nitrogen atoms to the metal, and that such donation accounts for the observed shifts of the vibrational frequencies. Moreover, the extent of the charge donation is related to the surface charge excess, which in turn is controlled by the electrode potential. Finally, the obtained results allow us to deduce that all the bands recorded in SERS, including in-plane and out-of-plane vibrations, seem to be originated by only one type of molecule adsorbed with a orientation perpendicular to the metal surface in the whole range of the studied electrode potentials. Coordination through the nitrogen atom means that adsorbed pyrazine should be oriented perpendicular to the metal surface and, therefore, adsorption originates a descent in symmetry from D2h down to C2v which may be responsible for the observed Raman activity of “u” modes. For instance, B1u modes 12 and 19b become A1 and therefore SERS active. Finally, our results show clearly that complexes bonded through the π-electronic system of the pyrazine ring do not play any relevant role at all. Consequently, SERS activity of out-of-plane modes 16b;B3u, 16a;Au, and 4;B2g must not be explained by an orientation parallel to the surface as it should be derived on the basis of the propensity rules of the EM mechanism. The activity of the 16a,b pair of fundamentals in the EELS of benzene has been explained on the basis of an incipient ring puckering of benzene radical anion.8,19 Provided that the CT mechanism of SERS can be considered similar to an EELS experience under resonance conditions, the activity of these fundamentals should have a similar origin in both cases. This conclusion confirms again the participation of the CT mechanism in the SERS spectra of pyrazine, which obviously is favored by the adsorption. Acknowledgment. This research has been supported by the Spanish MCYT through Project BQU 2000-1353. S.P.C. wishes to thank the Spanish MECD for a fellowship. Supporting Information Available: Three tables with the RHF/LanL2DZ, B3-LYB/3-21G, and B3-LYP/LanL2DZ results corresponding to the Ag-N complexes. This material is available free of charge via the Internet at http://pubs.acs.org. LA010489P (19) Wong, S. F.; Schulz, G. J. Phys. Rev. Lett. 1975, 35, 1429.