J. Phys. Chem. 1996, 100, 9911-9917
9911
SER Spectra of Bipyrazine Adsorbed on Silver Sols and Silver Electrodes Natale Neto,*,† Maurizio Muniz-Miranda,† and Giuseppe Sbrana‡ Department of Chemistry, UniVersity of Firenze, Via G. Capponi 9, I-50121, Firenze, Italy, and Centro Studi Composti Eterociclici del CNR, Via G. Capponi 9, I-50121, Firenze, Italy ReceiVed: February 7, 1996X
Surface-enhanced Raman spectra of bipyrazine adsorbed on silver colloids were obtained and compared with those measured on Ag electrode at different potentials. The spectra were interpreted on the basis of normal mode calculations using the argentous bipyrazine coordination complex as a model to describe the absorbatesubstrate interactions. It was found that bipyrazine assumes different adsorption geometries at the silver surface, either in aqueous colloidal dispersion after addition of chloride anions or on Ag electrodes at 0 potential. The SER data suggest the presence of two coexisting chemisorbed species, identified by normal mode calculations as bipyrazine bound to silver substrate through the nitrogen atoms located in “ortho” or “meta” position with respect to the inter-ring C-C bond.
Introduction Aromatic molecules containing nitrogen atoms are easily adsorbed to metals, like silver, on which the effect known as surface-enhanced Raman scattering (SERS) takes place. Through this technique, evidence was found for Ag-N bond formation when diazines are adsorbed to silver colloidal particles with enhancement factors related to the number of nitrogens engaged in the silver-molecule bond.1 The role of chemisorption for aza-aromatic molecules was confirmed by our successive investigation of 2,2′-bipyrimidine, hereafter BPM, which is a flexible molecule formed by two diaza-aromatic rings connected by a central C-C bond. Rotation about this bond could, in principle, lead to a nonplanar molecular conformation, which was however excluded, in the solid phase and in solution, on the basis of a complete vibrational analysis of BPM2 based on experimental measurements and normal mode calculations. This vibrational study allowed a detailed explanation of the SER spectrum of BPM adsorbed on silver colloid,3 using the relative enhancement of the Raman bands to assess the conformation of the adsorbed molecule and its orientation on the metal surface. The formation of a metal-adsorbate bond was also observed in the case of the diazines. In this paper we report the results of a similar investigation carried out on bipyrazine (BPZ), which differs from BPM for the relative position of the two nitrogen atoms in each ring. Two possible planar molecular conformations are conceivable for this molecules, namely, the trans and cis forms reported in Figure 1. No experimental structural data are available in the literature, and the only information is provided by ab initio calculations,4 which predict the trans form as the most stable molecular conformation of BPM. In a recent paper5 we have investigated the infrared and Raman spectra of BPZ and proposed a vibrational assignment through frequency calculations closely related to our previous results on BPM. Both experimental results and calculations suggest a centrosymmetric planar structure in solution and in the solid state. The SER spectra of BPZ adsorbed on silver colloidal particles or on electrodes at different potentials are now analyzed and compared with the normal Raman spectrum of a 1:1 BPZ-AgNO3 complex prepared as a suitable model to explain interactions with the metallic substrate. * Author to whom correspondence should be addressed. † University of Firenze. ‡ Centro Studi Composti Eterociclici del CNR. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00378-4 CCC: $12.00
Figure 1. Trans and cis planar conformations of BPZ.
Experimental Section BPZ (Aldrich Co., 98% purity) was purified by repeated water crystallizations obtaining a white crystalline compound with mp 180-81 °C. BPZ-AgNO3 complex was prepared by mixing equal volumes of hot saturated aqueous solutions of BPZ and of AgNO3. The white precipitate thus obtained was washed with water and CHCl3 and dried on phosphorus pentoxide. The elementary analysis (found Ag 31.5%, NO3 20.1%; C8H6N5O3Ag requires Ag 32.8%, NO3 19.0%) indicates the formation of a 1:1 complex. Infrared spectra of the complex were registered with the aid of a FT-IR Bruker Model IFS-120 instrument, using MCT and TGS detectors, in the 50-4000 cm-1 region on samples dispersed in nujol and hexachlorobutadiene mulls and in polythene pellets. Grinding or use of KBr pellets was avoided, as both processes induce decomposition of the complex. A 10-2 M aqueous solution of BPZ was added to stable silver sols prepared by reduction of AgNO3 with an excess of NaBH4.6 Due to the scarce solubility of BPZ in cold aqueous solution, a prolongated stirring of the sol at about 50 °C to favor the adsorption of the molecule onto the colloidal particles was necessary. Raman spectra were obtained with the aid of a Jobin-Yvon Model HG-2S monochromator equipped with a cooled RCA© 1996 American Chemical Society
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Figure 2. Infrared spectra of solid BPZ (lower trace) and AgI-BPZ complex (upper trace).
Figure 3. Raman spectra of solid BPZ (lower trace) and AgI-BPZ complex (upper trace).
C31034A photomultiplier, a photon-counting system, and a data acquisition facility. Raman spectra of solid argentous complex were measured using a rotating device and a defocalized laser beam to avoid decomposition processes. SER spectra of 10-2 M BPZ adsorbed on colloids were registered with the 514.5 nm exciting line of an Ar+ laser with a power of 100 mW at the base of a 10 mm quartz cell containing the samples. Addition of NaCl (10-2 M) increases the enhancement of the Raman bands. SER spectra were also measured on an Ag electrode immersed in a 10-3 M solution of BPZ using KCl (10-1 M) as supporting electrolyte and a saturated calomel reference electrode. The working electrode was a 99.999% pure polycrystalline silver rod 5 mm in diameter. The roughening process of the polished silver electrode consisted of one oxidation-reduction cycle (ORC) between -400 and 150 mV/ SCE. The applied potential was controlled by a Heka PG 28 Series potentiostat.
resemble those of solid BPZ assigned on the basis of a C2h point group symmetry. Several bands of BPZ are shifted due to complex formation, but all new features can be attributed to the NO3- group rather than to a breakdown of the mutual exclusion rule. The formation of Ag-O bonds, with some degree of covalent character, lowers the local symmetry of the NO3- group, which is described by a D3h point group for the free nitrate ion. This leads to frequency shifts and splittings of the infrared vibrational modes of the nitrate ion when organic complexes are formed.12 The assignment of nitrate vibrations is supported by structural data available for several silver complexes13 and, in particular, for AgNO3-pyrazine,14 closely related to our AgI-BPZ complex. The N-O symmetric stretching, which is only Raman active in the spectrum of the free ion and occurs at 1050 cm-1, corresponds to the only new band observed, at 1036 cm-1, in the Raman spectrum of AgI-BPZ. A band at about the same frequency is also present in the IR spectrum, thus confirming that a nitrate group is coordinated to silver with a lower symmetry. We also observe, in the IR spectrum, a band at 1052 cm-1 which can be explained assuming that more than one NO3group is coordinated to silver. The double degeneracy of the other N-O stretching vibration, occurring at 1390 cm-1 for the free ion, should be lifted upon coordination. We find three new strong IR bands in this region, at 1455, 1409, and 1311 cm-1, with a fourth band at 1371 cm-1 partially overlapping a BPZ vibration at about the same frequency. This observation again suggests the presence of more than one nitrate group for each silver, possibly bidentate with different N-O bond lengths. A multiplicity of bands in the N-O stretching region is also reported for Ag-pyridine N-oxides15 and is attributed to a sixcoordinated polymeric structure. To check the validity of this suggestion, we prepared an AgNO3-pyrazine complex whose IR spectrum shows features very similar to those of AgI-BPZ in the two N-O stretching regions. The vibrational frequencies of the nitrate group for the AgNO3-BPZ complex are consistent with a structure similar to that of the corresponding pyrazine complex. For the latter complex, a polymeric crystal structure was determined,14 in which each NO3- interacts with two opposite silver atoms. Two oxygens of a nitrate group are weakly bound to opposite Ag atoms, with an Ag-O distance of 2.72 Å, while the third oxygen forms a bridge between the two silver atoms, with a longer Ag-O distance of 2.94 Å. Thus, asymmetric bidentate nitrate groups are present, with each silver six-coordinated to two NO3groups and to two pyrazine molecules.
Vibrational Assignment The present study of the SER spectra of BPZ is based on a previous vibrational analysis of the isolated molecule.5 A transplanar molecular structure, belonging to the C2h point group, was assumed for BPZ on the basis of the observed infraredRaman mutual exclusion rule, in close analogy with similar conclusions reached for 2,2′-bipyridine.7-9 Experimental data for both BPM and BPZ were used to determine a general valence force field which satisfactorily reproduces the vibrational frequencies of these two aza-aromatic molecules. A complete vibrational assignment was thus obtained for BPZ based on the kinetic, rather than potential, energy distribution10 of each normal mode among different groups of internal coordinates. AgI-BPZ Complex BPZ forms an insoluble 1:1 complex with AgNO3 whose molecular geometry could, in principle, be described by two BPZ molecules coordinated to each silver in a polymeric arrangement. This is the structure assumed3 to interpret the vibrational spectra of a 1:1 AgNO3-BPM complex, later confirmed by a subsequent X-ray study,11 which determined a pseudotetrahedral arrangement for this complex. Since the vibrational data of BPZ suggest a planar, trans, structure for this molecule,5 the assumption of a bidentate silver for the 1:1 AgNO3-BPZ complex would require a cis conformation for BPZ, with consequent IR/Raman coincidence. This possibility is, however, ruled out by the infrared and Raman spectra of the 1:1 AgI-BPZ complex (see Figures 2 and 3), which very closely
Bipyrazine Adsorbed on Silver Sols and Ag Electrodes The new band at 1036 cm-1 in the infrared spectrum of AgIBPZ coincides with the only new peak found in the Raman spectrum of the complex. We assign it to ν1, N-O symmetric stretching, which gains infrared activity when the symmetry of the NO3- group is lowered. A second new infrared band at 817 cm-1 clearly corresponds to ν2, A2′′ out of plane deformation for the symmetric nitrate, somewhat lowered in frequency due to complex formation. Each of the remaining two infrared active degenerate vibrations of the nitrate group splits into several components: the ν3 asymmetric stretching generates two strong bands at 1409 and 1311 cm-1, while the in plane bending ν4 appears as a weak doublet at 700 and 708 cm-1. The latter assignment is confirmed by the presence of two weak bands at about 1740 cm-1. Combinations due to ν1+ν4 appear in this region and are extensively used for structural16,17 diagnosis of metal-nitrate interactions. The presence of two bands in this region with a small splitting, like the one observed for AgIBPZ, is characteristic of a monodentate nitrate group. Overtones of ν1 and combinations of ν1+ν3 were also used,16,17 but in our case, the corresponding bands are too weak and obscured by the absorption patterns of nujol or hexachlorobutadiene mulls. Finally, a broad band observed at 1371 cm-1, clearly due to complex formation although partially overlapping a fundamental of the parent molecule at 1382 cm-1, is also attributed to the NO3- band group. Its assignment as a band of purely ionic nitrate is not consistent with features of the combination region. We rather propose a situation like that found for the AgNO3pyrazine complex.14 It corresponds to a polymeric structure, in which each NO3- interacts with two opposite silver atoms: two oxygens form weak bonds (Ag-O distance of 2.72 Å) with nearby Ag atoms, while the third oxygen weakly interacts (at a distance of 2.94 Å) with both silver atoms. Formation of weak Ag-O bonds is responsible for the distortion from D3h symmetry of the NO3- group, leading to the observed shifts and splittings of the ν1-ν4 bands, while the bridging interaction produces the broad band at 1371 cm-1. Thus, two strong bands at 1409 and 1311 cm-1, observed in the infrared spectrum of the complex but not in the spectrum of the parent molecule, correspond to the E′ mode of the ionic NO3- group and are attributed to asymmetric N-O stretching modes involved in coordination with silver. Hence, the E′ modes of the ionic group split into two nondegenerate vibrations. This assignment closely follows the conclusions reached from a complete structural study13 of several complexes of triphenylphosphine with silver nitrate, in which the NO3- group is present as either a (weakly) coordinated or ionic group. The formation of metal-oxygen bonds with some degree of covalent character lowers the local symmetry of the purely ionic NO3- group for mono- or bidentate nitrate group. The list of experimental data reported in Table 1 shows that a mixing occurs between low lying fundamentals of BPZ and vibrations involving silver atoms, while in the region above 220 cm-1 each vibrational frequency of BPZ has a counterpart in the AgI-BPZ complex. On this basis, we conclude that no evidence exists to support a breakdown of selection rules based on the presence of an inversion center; thus, a trans planar molecular conformation is proposed also for BPZ bonded to silver. Frequency shifts do, however, occur for several normal modes when the Ag complex is formed. To interpret the origin of these shifts, two possible polymeric models for a 1:1 AgI-BPZ complex have been considered (see Figure 4), closely resembling the known structure of the 1:1 AgNO3-pyrazine complex.14 Single-chain frequency calculations have been carried out for two different (Ag-BPZ-)n polymers, both with C2h factor group symmetry, corresponding
J. Phys. Chem., Vol. 100, No. 23, 1996 9913 to model I and II of Figure 4. They differ for the position of the Ag-N bonds, which involve ortho or meta nitrogens with respect to the inter-ring C-C bond of BPZ. An Ag-N distance of 2.213 Å was adopted, as found for the AgI-pyrazine complex, with Ag atoms added along lines bisecting the C-N-C angles. Three force constants were added to the BPZ valence field, related to internal coordinates involving silver atoms, namely, Ag-N stretching (0.6 mdyn Å-1), Ag-N-C bending (0.30 mdyn Å rad-2), and N-Ag out of plane wagging (0.05 mdyn Å rad-2). Normal mode calculations for a simplified model, consisting of an Ag-BPZ-Ag group with C2h symmetry, give for the internal modes values nearly identical to those calculated for the (Ag-BPZ-)n single chain (Table 2). Differences instead occur for low lying frequencies involving the Ag-N bond: two frequencies, observed at 146 (Ag) and 151 (Bu) cm-1 in the complex spectra only, are calculated at 89/120 and 171/171 cm-1 for the polymeric models I/II and at 122/141 and 144/144 cm-1 for corresponding configurations of the Ag-BPZ-Ag group. Since only low lying vibrational modes of BPZ are affected by Ag addition, normal coordinates of the isolated molecule can be used to understand the origin of frequency shifts observed above 500 cm-1 when the spectra of the Ag complex are compared to those of BPZ. The only source of perturbation for each in plane normal coordinate, Qi, of BPZ derives from internal coordinates, Rn, added to form Ag-BPZ. Corresponding shifts toward higher frequencies are thus expected to be proportional to the square of the coefficients (∂Qi/∂Rn), representing the projection of each normal mode on an internal coordinate involving the Ag-N bond. If contributions from C-N-Ag bendings are small, the larger frequency shifts are predicted for normal coordinates with appreciable displacements of N atoms along directions coincident with Ag-N bonds. These considerations apply to the three Ag normal coordinates of BPZ shown on the left-hand side of Figure 5, whose atomic displacements do not appreciably involve C-N-Ag bendings. The ring bending mode calculated at 619 cm-1 for BPZ is expected to undergo consistent frequency shifts for either, “ortho” and “meta”, model assumed for the Ag-BPZ-Ag group as both N atoms of each pyrazine ring move along C-N-C bisectors with comparable amplitudes (Figure 5). The two N atoms move instead along different directions for a second ring bending at 808 cm-1; thus, no frequency change is expected for the “ortho” model, while a large shift should be present for the “meta” model. Exactly the opposite situation applies to the ring stretching mode at 1017 cm-1, which should shift to higher frequency only for an AgI-BPZ complex, consistent with the “ortho” model. These three Ag modes turn out to be the only ones which allow a possible distinction between the two AgBPZ models, as shown in Table 2, where calculated frequency shifts, due to complex formation, are compared with observed values for Raman active modes. This comparison for the three Ag modes somewhat favors model I, but at this stage, this cannot be considered more than a reasonable guess for the molecular structure of the AgI-BPZ complex. Corresponding calculated shifts for Au and Bu modes are not reported since, as expected from the normal modes of BPZ,5 they have very similar values for both models and play no role for the subsequent interpretation of SERS data. SER Spectra SERS data for BPZ adsorbed on silver sols are collected in the last two columns of Table 1. The higher concentration (10-2 M) necessary to obtain a satisfactory Raman scattering for BPZ, compared to the value of 10-4 M used in the SER experiments
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TABLE 1: Infrared and Raman Frequencies of BPZ and 1:1 BPZ-AgNO3 Complex, SERS Data of BPZ Adsorbed on Silver Colloid infrared a
species
BPZ
Bu Ag Ag Bu Bu Ag
3074 m 3047 m 3011 m
Raman BPZ-Agb
SERS from Ag colloid
BPZc
BPZ-Agb
3070 sh 3052 m
3089 m/w 3073 m
3005 w
3061 m/w
1597 sh
1602 sh
1588 m
1594 s
1585 s
1525 s
1521 vs
1518 vs
1492 vw
1487 m
1478 m
1437 w
1439 w
1436 vw
without NaCl
with NaCl
3084 m 3069 w
3074 w 3058 w
1745* d vvw 1741* vvw Ag Bu Bu Ag Ag Bu Ag
1572 vw 1525 w
1465 vs
1595 sh 1590 s 1574 sh
1579 1522
1455
1518 vs 1504 sh 1473 w 1451 w 1435 w
1417 vw Bu Ag Bu
1392 sh
1409* s 1396 sh
1404 vw 1365 vw
1388 vw 1382 s 1371* m
Ag
1322 sh
1332 vw 1315 sh
1320 s
1333 vw 1316 s
1310 s
1300 s
1301 sh
1303 s
1219 m
1258 vw 1233 w 1216 w
1311* s Ag Bu Bu
1286 m 1185 w
1280 sh 1188 m 1198 vw
Bu Ag Bu Ag Bu
1168 w 1152 m
1169 w 1152 m
1089 vs
1096 s
1029 vs
1052 s 1036* m
1170 w 1151 w
1152 w
1152 w
1057 s
1072 s
1063 m
1151 w 1086 m 1063 m
1037* 1024 vs
1019 vs
1010 w
984 w
987 w
Ag Bg 949 w Bg Au Au
872 vw 847 vs
1015 s
1020 vs
1018 vs
1017 vs
981 vw 958 vvw
987 w
970 vw
972 vw 936 w
878 w
874 w
934 vw 875 vw
945 w 867 w
850 s 829 m 817* m
Au Ag Bg Bg Au
802 vvw 800 m 762 vw 755 m
807 m 784 vw 751 vw
805 w 776 vw 748 vw
807 m 781 vw 752 vw
748 m 721 vw 695 vw
709 vw 708* vvw Bu
705 vvw 700* vvw 649 vvw 671 vvw
Bu Ag
Bg
663 w
622 vw 613 m 585 vvw 571 vw
639 m
575 w
652 w 634 w 603 vw 579 vvw
634 sh 613 sh 597 vw 573 vw
Bipyrazine Adsorbed on Silver Sols and Ag Electrodes
J. Phys. Chem., Vol. 100, No. 23, 1996 9915
TABLE 1 (Continued) infrared
Raman
species
BPZa
BPZ-Agb
Ag Au Bg
432 m
444 s
Au Ag Bg Bu
375 w
Au
113 w
183 w
SERS from Ag colloid
BPZc
BPZ-Agb
without NaCl
442 w
449 m
455 w
455 w
407 vw
410 w
420 sh
413 vw 404 vw
335 m 216 vw
348 vw 229 m
326 vw
357 vw
146 w
210 s
238† e vs
with NaCl
368 s 208 w 151 m 124 m
120 sh 99 s Au
90 vw 91 vs 70 vs 47 vs
a
86 sh 64 vw 38 vs
KBr pellet. b Nujol mull. c Solid sample. d * ) bands due to NO3 group.
e†
) Ag-Cl stretching.
TABLE 2: Observed Raman Frequencies of BPZ and 1:1 BPZ-AgNO3 Complex and Values Calculated for the Isolated Molecule and for the Two Polymeric Models Shown in Figure 4 calculated observed species
BPZ
Ag-BPZ
∆ν
BPZ
I
∆ν
II
∆ν
Ag
1588 1525
1594 1521
6 -4
1437
1439
2
1322 1310 1151 1057 1015 800 613 442 335
1315 1299 1152 1072 1021 807 639 449 348 156 987 874 784 751 575 410 229
-7 -11 1 15 6 7 26 7 13
1586 1520 1440 1416 1357 1336 1304 1167 1062 1017 808 619 470 328
1598 1524 1440 1420 1359 1338 1310 1170 1069 1032 810 639 484 341 89 995 844 816 769 576 406 217 43
12 4 0 4 2 2 6 3 7 15 2 20 14 13
1589 1526 1442 1423 1361 1340 1305 1170 1073 1022 826 634 486 333 120 994 844 816 767 585 402 212 49
3 6 2 7 4 4 1 3 11 5 18 15 16 5
Figure 4. Two possible polymeric configurations (AgI-BPZ-)n for the 1:1 AgNO3-BPZ complex.
on BPM,3 indicates that BPZ adsorption on silver is less favored than BPM. As invariably found for all diazines and BPM, the SER spectrum of BPZ without addition of NaCl (Figure 6) shows a broad strong band at about 210 cm-1, characteristic of Ag-N stretching, which overlaps the weak Bg ring rotation of BPZ assigned at about the same frequency.5 This Ag-N frequency is about 70 cm-1 higher than that observed in the AgI-BPZ complex, thus showing a chemisorption with the occurrence of a stable BPZ-metal bond involving a nitrogen lone pair. This also suggests a molecular orientation of BPZ perpendicular to the silver particle. Addition of halide ions produces a stabilization of the colloidal dispersion and a considerable enhancement of the SER spectrum but also a very strong Ag-Cl band which masks the Ag-N vibration revealed in the salt-free sample. In the case of BPZ, addition of NaCl is also responsible for an evident splitting of several Raman bands, present in both spectra of Figure 7. In particular, strong doublets are observed at 807/ 829, 1024/1017, and 1316/1303 cm-1, while single Raman bands are present in corresponding spectral regions of BPZ, AgIBPZ complex, and BPZ adsorbed on silver sol without NaCl. Assuming that two different adsorbed species, X and Y, are present, the bands at 807, 1024, and 1316 cm-1 can be referred to the X species; those at 829, 1017, and 1306 cm-1, to the Y species. Hence, the intensity ratio for each doublet can be considered proportional to the relative abundance of X/Y. The lower spectrum of Figure 7 reflects a larger presence of species
(Ag-BPZ-)n models
Bg
981 878 762 571 407 216
6 -4 -11 4 3 13
990 844 816 767 576 394 212
5 0 0 2 0 12 5
4 0 0 0 9 8 0
X with a small amount of Y, and vice versa for the upper spectrum. This behavior was confirmed by all spectral patterns obtained in several experiments, always with addition of NaCl, of which the two spectra of Figure 7 represent the limiting case. Another doublet at 1063/1086 cm-1 also seems to follow the X/Y behavior, while both components at 1216 and 1233 cm-1 of a last weak doublet, with no visible counterpart in the spectrum of BPZ, should be attributed to the Y species. To verify the hypothesis of two coexisting adsorbed species, a SER experiment was carried out in which the argentous BPZ complex, rather than BPZ, was added to the colloidal dispersion, in the presence of NaCl. This spectrum is not reported here, as it is almost identical to the X spectrum of Figure 7, with less pronounced, although still visible, Y features. Since it is reasonable to assume that the structure of the AgI-BPZ coordination compound does not change upon adsorption, the complex is expected to absorb on the silver particles through the nitrogens not engaged in the coordination bond. Hence, the X species can be associated to an adsorption geometry
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Figure 5. Cartesian displacements and calculated frequencies of three in plane normal modes of free BPZ (first column) and of ortho and meta AgI-BPZ complexes (second and third columns).
Figure 6. SER spectrum of BPZ adsorbed on a salt-free Ag colloid.
Figure 8. SER spectra of BPZ adsorbed on an Ag electrode at different applied potentials.
Figure 7. SER spectra of BPZ adsorbed on Ag sols for two different samples, both with addition of NaCl.
similar to that of the complex, whereas the Y species arises from the interaction of AgI-BPZ with the silver substrate. For a complex with a structure described by model I of Figure 4, the X species corresponds to an adsorption through a nitrogen “ortho” to the C-C central bond of BPZ, while for the Y species a bond is formed with the “meta” nitrogen. The opposite is true if the structure of the complex is consistent with model II of Figure 4; X and Y species are related to “meta” and “ortho” adsorption, respectively.
The above results obtained from silver colloidal dispersion are confirmed by SER experiments on BPZ (10-3 M) adsorbed on silver electrodes. Two different species are identified on the electrode at 0 V potential (Figure 8, lower trace), corresponding to those observed in the silver sols with added NaCl and attributed to molecules adsorbed on the metal surface through an “ortho” or “meta” nitrogen. Actually, the presence of the doublets at 805/830 and 1017/1025 cm-1 accounts for the coexistence of the X/Y species. As shown in the upper trace of Figure 8, the SER spectrum measured on the electrode at -0.3 V is quite different with respect to that at 0 V potential. This is particularly evident in the region below 1000 cm-1, where there is little correspondence between bands of the two spectra. At negative potential, desorption of the chloride anions from the electrode occurs and bipyrazine is supposed to adsorb through a Ag0-BPZ bond formation. The strong Ag-Cl stretching band occurring at ∼240 cm-1 in the spectrum at 0 V vanishes at -0.3 V, allowing the observation of the Ag-N stretching band at 205 cm-1. Hence, this spectrum is very similar to the salt-free colloid spectrum (Figure 6) and cannot be related to the normal Raman spectrum of the AgI-BPZ complex. To understand the origin of the doublets at 807/829 cm-1 and at 1017/1024 cm-1 in the SER spectra of colloids with
Bipyrazine Adsorbed on Silver Sols and Ag Electrodes addition of NaCl, normal mode calculations were carried out assuming BPZ bound to an Ag atom through a nitrogen in the “ortho” or “meta” position as a model of adsorbate-substrate interaction. The valence force field previously employed for the argentous BPZ complex was used. Since the Ag-N stretching mode observed in the SERS is about 70 cm-1 higher than in the normal Raman spectrum of the complex, a larger value was necessary for the corresponding force constant. A value of 2.0 mdyn/Å was then adopted, which produces a reasonable value of 214/213 cm-1 for the Ag-N stretching mode observed in the SERS, with the “ortho” and “meta” models, respectively. In this way calculated frequencies at 811 and 1040 cm-1 were obtained for the “ortho” model of the AgBPZ group, which agree with the experimental frequencies at 807 and 1024 cm-1 attributed to the X species. Corresponding normal modes for the “meta” model were calculated at 834 and 1020 cm-1, which match the experimental values at 829 and 1017 cm-1 associated with the Y species. The other calculated vibrational shifts are similar to those previously obtained for the two models of AgI-BPZ complex and do not allow a distinction between “ortho” and “meta” positions. The overall reasonable agreement, in particular for the two frequencies undergoing marked upshifts as predicted for the two models, is an indication of the validity of a vibrational analysis of the SERS data based on a simplified model of a molecule interacting with a silver “adatom”, similar to the case of the argentous coordination compound. From the analysis of the SER spectra we conclude that bipyrazine undergoes two different types of adsorption on the electrode or silver colloids depending on the presence or the absence of chloride ions. In the absence of chloride anions, the SER spectra give evidence of only one species adsorbed on the electrode or silver colloidal particles, while in the presence of chloride anions two different species are detected, depending on the position of the nitrogen atoms involved in the chemisorption. The effect of Cl- anions on the adsorption mechanism has been recently discussed in ref 18 and related with the formation of positivized silver atoms on the surface. Bipyrazine
J. Phys. Chem., Vol. 100, No. 23, 1996 9917 is adsorbed on these “active sites”, giving rise to Ag+-BPZanion surface adducts, which are responsible for the anioninduced SER effect. In conclusion, the vibrational analysis of the corresponding argentous complex provides a useful guideline for a correct interpretation of the SER data when the halideanion effect is present. Acknowledgment. Financial support from the Italian Consiglio Nazionale delle Ricerche and Ministero della Ricerca Scientifica is gratefully acknowledged. References and Notes (1) Muniz-Miranda, M.; Neto, N.; Sbrana, G. J. Phys. Chem. 1988, 92, 954. (2) Neto, N.; Sbrana, G.; Muniz-Miranda, M. Spectrochim. Acta 1990, 46A, 705. (3) Sbrana, G.; Neto, N.; Muniz-Miranda, M.; Nocentini, M. J. Phys. Chem. 1990, 94, 3706. (4) Barone, V.; Minichino, C.; Filiszar, S.; Russo, N. Can. J. Chem. 1988, 66, 1313. (5) Neto, N.; Muniz-Miranda, M.; Sbrana, G. Spectrochim. Acta 1994, 50A, 357. (6) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 710. (7) Merrit, L. L.; Schroeder, E. D. Acta Crystallogr. 1956, 9, 1981. (8) Emsley, J. W.; Garnett, J. G.; Long, M. A.; Lunazzi, L.; Spunta, G.; Veracini, C. A.; Zanadei, A. J. Chem. Soc., Perkin Trans. II 1979, 853. (9) Neto, N.; Muniz-Miranda, M.; Angeloni, L.; Castellucci, E. Spectrochim. Acta 1983, 39A, 97. (10) Neto, N.; Muniz-Miranda, M.; Sbrana, G. Spectrochim. Acta 1994, 50A, 1317. (11) Sbrana, G.; Muniz-Miranda, M.; Neto, N.; Mangani, S.; Orioli, P. To be published. (12) Parentich, A.; Little, L. H.; Ottewill, R. H. J. Inorg. Nucl. Chem. 1973, 35, 2271. (13) Barron, P. F.; Dyason, J. C.; Healy, P. C.; Engelhardt, L. M.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1986, 1965. (14) Vranka, R. G.; Amma, E. L. Inorg. Chem. 1966, 5, 1020. (15) Ahuja, I. S.; Singh, R.; Singh, R. Spectrochim. Acta 1976, 32A, 547. (16) Curtis, N. F.; Curtis, Y. M. Inorg. Chem. 1965, 4, 804. (17) Lever, A. B. P.; Mantovani, E.; Ramanswany, B. S. Can. J. Chem. 1971, 49, 1957. (18) Muniz-Miranda, M.; Sbrana, G. J. Raman Spectrosc. 1996, 27, 105.
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