J. Phys. Chem. 1990, 94. 3706-3710
3706 30 CTAB:HSal:NoBr / W
surfactant systems,42and they also discussed their experimental results on the basis of the salting-out power and lyotropic series. However, the minimum amount of NaBr necessary for the present CTAB:HSal:NaBr/W system to separate into the Ll-L2 two phases is much smaller than that in Imae’s data; therefore, the mechanism of liquid-liquid phase separation for the CTAB: HSal:NaBr/W system would not be salting-out but coagulation. On the other hand, the CTAB:NaSal/ W system never separated into two liquid-liquid phases in the concentration range equivalent to Figure 3. This again suggests that the CTAB:NaSal micelles have no net charges or electrostatic interactions to be screened by additive NaBr. Concluding Remarks
C,
/M
Downloaded by UNIV OF CAMBRIDGE on September 14, 2015 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/j100372a064
Figure 7. Relationship between the asymptotic volume fraction V L I -of the L, phase and C,.
additive simple salts induce and enhance the liquid-liquid phase ~ e p a r a t i o n . ) ~Some ~ ’ workers3841reported that simple salts affect the liquid-liquid phase separation in nonionic surfactant systems following the lyotropic series. Recently, Imae et al. reported salt-induced liquid-liquid phase separation in aqueous cationic (31) Clunie, J. S.;Corkill, J. M.; Goodman, J. F.; Symons, P. C.; Tate, J. R. Trans. Faraday SOC.1967, 63, 2839. (32) Ekwall, P. Advances in Liquid Crysrals; Brown, G . H., Ed.: Academic Press: New York, 1975. (33) Lang, L. C.: Morgan, R. D.J. Chem. Phys. 1980, 73, 5849. (34) Corti, M.; Minero, C.; Degiorgio, V. J. Phys. Chem. 1984,88, 309. (35) Imae, T.; Konishi, H.; Ikeda, S. J . Phys. Chem. 1986, 90, 1417. (36) Imae, T.; Ikeda, S. J. Colloid Interface Sci. 1986, 113, 449. (37) Herrmann, K. W . J. Phys. Chem. 1964, 68, 1540. (38) Schich, M. J. J. Colloid Interface Sci. 1962, 17, 801. (39) Tokiwa, F.; Matsumoto, T. Bull. Chem. Soc. Jpn. 1975, 48, 1645. (40) Deguchi, K.; Meguro, K. J . Colloid Interface Sci. 1975, 50, 223. (41) Zulauf, M. Physics of Amphiphiles, Micelles, Vesicles, and Microemulsions; Degiorgio, V., Corti, M. Eds.;North-Holland: Amsterdam, 1985.
In the CTAB:HSal/W system with low CD,dissociation of Bris complete at C , = 0.5CD so that Br- from CTAB forming threadlike micelles is replaced with HSal by the mole ratio of 2:l. Dissociation of HSal molecules in the first location on the micelle surface is 60-70% and that in the second location in the micelle interior is rather low. The threadlike CTAB:HSal micelles thus possess net positive charges on their surface. The CTAB: HSal:NaBr/ W system shows liquid-liquid two-phase separation induced by addition of NaBr beyond a certain level. One of the liquid phases contains very little CTAB, but the other viscoelastic phase contains threadlike micelles in a highly condensed state. This phase behavior also suggests that the CTAB:HSal micelles have electrostatic interaction to be screened by an additive salt. On the other hand, threadlike micelles of the CTAB:NaSal/W system have no electric charges, because they consist of essentially 1: 1 complexes between CTA+ and Sal-.
Acknowledgment. T.S. thanks the Yukawa Scholarship Association, Faculty of Science, Osaka University, for a scholarship during 1988 that enabled him to carry out this study. (42) Imae,
T.:Sasaki, M.; Abe, A.; Ikeda, S. Langmuir 1988, 4 , 414.
Surface-Enhanced Raman Spectra of 2,2’-Bipyrimidine Adsorbed on Silver Sol G . Sbrana,* Centro Studi sui Composti Eterociclici del C N R , Via G . Capponi 9, 1-50121 Firenze, Italy
N. Neto, Department of Chemistry, Universitri di Potenza, Via N . Sauro 85, I-85100 Potenza, Italy
M. Muniz-Miranda, and M. Nocentini Department of Chemistry, Universitri di Firenze. Via G . Capponi 9, I-50121 Firenze. Italy (Received: June 27, 1989) The surface-enhanced Raman spectrum of 2,2’-bipyrimidine adsorbed on silver sol has been obtained and analyzed by using a vibrational assignment previously determined for this azoaromatic molecule. Evidence was found for Ag-N bond formation with the silver substrate and a model is proposed for the adsorption on the basis of a very close similarity of the SER data with the normal Raman spectrum of a 1:l complex with AgNO,. Normal-mode analysis suggests a planar conformation for the adsorbed species, with the molecule perpendicular to the metal, bound through N atoms. Variation of relative intensities of Raman bands with different exciting lines is consistent with a charge-transfer contribution to the enhancement of the scattering cross section. introduction
Surface-enhanced Raman scattering (SERS) is a well-established method for studying properties of molecules adsorbed on metals like silver, gold, and copper. Through comparison of SER data of the adsorbed species with those obtained from ordinary Raman experiments on solutions or solid samples, information can be gained on the molecular conformation and on the orien0022-3654/90 f 2O94-37O6$02.50f 0
tation of the adsorbate on the metal substrate. Different types of surfaces are commonly used and, among them, colloidal particles are particularly simple to prepare and provide with a substantial enhancement of the Raman cross section interpreted in terms of the electromagnetic theory’ and of a dynamical charge transfer ( I ) Wang, D. S.: Kerker, M.; Chew, H. W . Appl. Opr. 1980, 19, 4159.
0 1990 American Chemical Society
SERS of 2,2’-Bipyrimidine Adsorbed on Silver Sol
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between the substrate and the adsorbate.2 In the latter mechanism the source of the enhancement is specific of the molecular species involved in the absorption, producing a chemisorption with formation of a metal-adsorbate bond of the kind found for the diazines., A “chemical effect” should also be present in the case of 2,2’-bipyrimidine, which is an azoaromatic molecule formed by two pyrimidine rings connected in 2-position through a C-C single bond. The presence of nitrogen lone pairs causes a destabilization, due to conjugative interactions through the aromatic rings, at least in the gas phase. The molecule is in fact planar in the solid state while in the gas phase a torsional angle of 48’ about the inter-ring bond was determined4 assuming a C, symmetry. SERS data on silver sols are analyzed in the present paper to study the conformation of the adsorbed molecule, the formation of chemical bond with the substrate and the orientation on the metal surface. For this purpose we heavily rely on the vibrational assignment of 2,2’-bipyrimidine previously obtained through normal-mode analysis5 of both isolated molecule and crystal. A parallel vibrational study of a 1:l complex with AgNO,, with corresponding normal-mode calculations, accounts for frequency shifts observed in the SER spectra and provides with a model for chemisorption on Ag particles. Experimental Section
2,2’-Bipyrimidine supplied by Alfa Co. (99% purity) was purified by sublimation under vacuum giving a white crystalline powder, m.p. 1 15-1 16 OC. 2,2’-Bipyrimidine/AgNO, complex was prepared by mixing equal volumes of hot aqueous solutions (10-1 M) of silver nitrate and 2,2’-bipyrimidine. The yellow powder thus obtained was repeatedly washed with water and dried on phosphorus pentoxide. The salt, almost insoluble in the usual organic solvents, decomposes at about 120 OC and decomposition occurs also by grinding. The elementary analysis (found: Ag, 32.4; NO,, 19.5; C8H6N503Agrequires Ag, 32.8, NO,, 19.0) indicates the formation of the 1:l complex. Stable silver sols were prepared by reduction of A g N 0 , with excess NaBH46 using precautions to avoid reduction products.’ A lo-‘ M concentration of 2,2’-bipyrimidine in the aqueous colloidal dispersion was used. Addition of NaCl ( M) increased the stability of the colloidal dispersion. The Raman instrument was equipped with a JobinYvon HG-2S monochromator, a cooled RCA-C3 1034 photomultiplier, and a data acquisition system. SER spectra were obtained with several exciting lines supplied by Ar+ and Kr+ lasers (457.9, 488.0, 514.5, 521.2, 568.9, 647.1, and 676.0 nm), with quartz cell a power of 50 mW monitored at the base of a IO-” containing the samples. All spectra were corrected to account for monochromator and photomultiplier efficiency. Raman spectra of the complex were measured on a solid sample by using a rotating device and a power of 50 mW supplied by the 514.5-nm Ar+ laser line. Corresponding IR spectra in Nujol mull and polythene pellet, prepared without grinding to avoid decomposition, were registered with an FT-IR Bruker Model IFS-120 instrument in the 204 0 0 0 - ~ m -region. ~ Results
SER spectra of 2,2’-bipyrimidine adsorbed on silver sols were obtained with and without addition of NaCl. Addition of halide ions in Ag colloidal systems produces a considerable enhancement of Raman bands, as observed for pyridine,8 2,2’-bi~yridine,~ and (2) Otto. A. Light ScatteringinSolids; Cardona, M., Guntherodt, G.,Eds.; Springer: Berlin, 1984; Vol. IV. (3) Muniz-Miranda, M.; Neto, N.; Sbrana, G.J. Phys. Chem. 1988, 92, 954. (4) A353, (5) (6)
Fernholt, L.; Romming, C.; Samdal, S. Acta Chem. Scand. 1981, 706.
Neto, N.; Sbrana, G.;Muniz-Miranda, M. Spectrochim. Acta, in press. Creighton, J. A.; Blatchford, C. G.;Albrecht, M. G. J . Chem. Soc., Faraday Trans. 2 1979, 75, 790. (7) Muniz-Miranda, M.; Neto, N.; Sbrana, G. J . Mol. Struct. 1986, 21,
275. (8) Heard, S.M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J . Phys. Chem. 1985, 89, 389.
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3101
100
300
500
?OD R ~ . P
900 5n.t:
1100
1300
500
:?OC
1cv-1,
Figure 1. SER spectra of 2,2’-bipyrimidine of the same Ag colloidal sample before (lower trace) and after (upper trace) addition of NaCl
M).
diazines.j Spectra without addition of NaCl are, on the other hand, useful to reveal the possible occurrence of Ag-N stretching modes otherwise overlapping with the strong Ag-C1 vibration. However, spectra without addition of salt are not easily reproducible and the colloid tends to collapse more quickly. The visible absorption spectrum of 2,2’bipyrimidine/Ag sol shows a peak at -400 nm and a secondary broad maximum at 620 nm characteristic of aggregation. Addition of NaCl causes a visible reversal of the usual aggregation trend, with corresponding disappearance of the second broad band. The difference between SER spectra of 2,2’-bipyrimidine with and without addition of salt is evident from Figure 1, in which the two corresponding spectra, obtained from the same sample of 2,2’-bipyrimidine in Ag colloidal dispersion, are reported. The upper spectrum is that of the sample containing NaCl ( M) and was recorded immediately after running the salt-free experiment. A marked difference is evident as two strong bands at 614 and 923 cm-I, present in the salt-free spectrum, completely disappear after salt is added. The same behavior is observed for a weak band at 1380 cm-I and also the broad background scattering in the 1100-1700-~m-~ region is eliminated by addition of NaC1. These three bands match the vibrational frequencies of the borate ionlo and correspond precisely to peaks observed in the 2,2’-bipyridine/Ag colloid system, when no ion is added,g and attributed to reaction side products. In view of our experiment, these are bands due to borate ion, present in solution when NaBH, reduces AgN03, whose adsorption is removed by chloride ions. A similar behavior of chloride, which removes citrate from colloidal surfaces, was reported* in the case of pyridine adsorbed on Carey Lea silver sols. Actually a competing adsorption process occurs, in the salt-free colloidal samples, as intensities of bands due to borate ion progressively diminish in time while those of 2,2’-bipyrimidine are intensified; thus the presence of chloride accelerates this process. However, strong background scattering in the 1100-1700-cm-’ region is a permanent feature in our SER spectra without salt. Both spectra in Figure 1 display an extremely intense band at about 220 cm-’ which cannot be due to the weak BZgring deformation of 2,2’-bipyrimidine observed at about the same frequency in both solution and powder spectra. This band is instead assigned to AgCl stretching in presence of salt, masking the corresponding Ag-N vibration as revealed in the salt-free spectrum, in analogy to what is observed in the case of diazines., The complete set of observed Raman frequencies for 2,2’-bipyrimidine and their vibrational assignment, taken from ref 5, are reported in Table I, together with the vibrational data for the powder spectrum of the 1:l complex with AgNO,. Corresponding spectra of the molecule adsorbed on silver sol or engaged in (9) Kim, M.; Itoh, K. J . Chem. Phys. 1987, 97, 126. (10) Steele, W.C.; Decius, J. C. J . Chem. Phys. 1956, 25, 1184
3708 The Journal of Physical Chemistry, Vol. 94, No. 9, I990 I
TABLE I: Raman Frequencies of 2,2’-Bipyrimidine solution CDCI, aqueous 3095 m 3077 m 3064 m 3049 m 3026 w 3016 w 1590 sh 1570 vvs 1567 vvs 1550 sh 1458 vvs 1451 vvs
0.80 0.3 0.4 0.39 0.3
1336 s 1298 w 1172 w 1112 w 1095 sh 1078 s
1332 s 1291 vw I I72 vvw 1105 m
0.16 0.7 0.9 0.64
1075 s
0.02
1006vs
998 vs
846 w Downloaded by UNIV OF CAMBRIDGE on September 14, 2015 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/j100372a064
p
0.1
786 m
845 vw 0.74 809 vw 1.0 794 vvw 1.0 782 m 0.1
634 m 550 w
629 w 550 vw
0.96 1.0
400 w
401 vw
0.96
342 w 218 w
339 vw 222 w
0.3 1.0
1584 vw 1562 s
complex powder 3087 m 3078 m 3062 m 1585 vw 1564 ws
1448 m
1449 ws
1330s 1302 vw 1184 vw
1330 ws 1305 vw 1181 m 1119 w 1100 m 1079 s 1044 vs 1023 ws
powder 3062 m 3047 m 3030 m 1577 sh I564 ws 1544 sh 1445 ws 1432 sh 1335 sh 1326s 1292 w 1167 w 1120 m 1101 vw 1078 s
sol 3126 3054 m
995vs 990 sh 849 w 807 vw 784 sh 777 m
1018s 1000 sh
629 m 553 w 401 sh 396w 345 w 337 w 213 m
636vw
98 s 70 sh 56 vs 32 vs
IO96 w lO8Om
853 vw 809 vw 790 sh 786 s
414 w
783 vs 707 vw 639 m 552 vw 404 vw 417vw
364 w
362 w
222 s
234 s 67 vs
I
I
I
f species”
B1, A, A, B,. A, A, BI, BI, A, Bl, B,,
kO,, A, . B3, B2, B3, B3, A, B,, B3, B,, A, B3, (Ag-N) lattice lattice lattice lattice
“Assignment from ref 5.
4
Sbrana et al.
! 1
Figure 2. Raman spectrum (514-nm excitation) of 2,2’-bipyrimidine: 1 M aqueous solution (a), S E R spectrum with addition of salt (b), and powder spectrum of Ag-2,2’-bipyrimidine complex (c) (dot marks a band of the NO3 ion).
complex formation are shown in Figure 2, compared with the normal Raman spectrum of 1 M water solution. The Raman spectra of the colloidal dispersion and of the Ag complex are very similar to each other and to corresponding spectra of both the isolated molecule and the crystal, and this immediately suggests a planar molecular conformation, for 2,2’-bipyrimidine bound to Ag, as found for the crystaL4 This conclusion is supported by the fact that no new IR band, possibly due to violation of mutual exclusion selection rules under Du molecular symmetry, is observed in the infrared spectrum of the Ag complex. It is evident from Figure 2 that in both the normal Raman and SER spectra only totally symmetric Ag modes display a considerable intensity while all other vibrations are weak. Two B i g
I
250
I
I
I
150
E
WAVENUMBERS cm-‘
Figure 3. Infrared spectra in the low-frequency region of 2,2‘-bipyrimidine (upper trace) and of the 2,2’-bipyrimidine/Ag complex (lower trace) in polythene pellets.
vibrations, at 400 and 634 cm-l in the aqueous solution spectrum, are clearly visible in the SER spectrum while the assignment to this species of other very weak bands, at 1184, 1302, and 1584 cm-l in the SER spectrum, is dictated by the previous vibrational study of 2,2’-bipyrimidine. Thus five out of seven BI fundamentals in the 100-1600-cm-’ region are present in the JER spectrum while no evidence is found for vibrations belonging to B2 and B36 species. On this basis Creighton’s surface selection rulesiq exclude the possibility of an adsorption with the molecular plane parallel to silver particles, as in this case Blg modes should be the less enhanced. Of the two possible orientations for a molecule perpendicular to the silver particle, only one is consistent with the formation of Ag-N bond which gives rise to a very strong band at 222 cm-’. Thus we conclude that 2,2’-bipyrimidine is perpendicular to the metal, with the nitrogen lone pairs engaged in a chemical bond with silver. The Raman spectrum of the 2,2’-bipyrimidine/AgN03 complex is reported in Figure 2 together with the SER spectrum of the molecule adsorbed on silver sol, both compared with data from 1 M aqueous solution. The Ag complex displays a new band, with respect to the free molecule, at 234 cm-’, which can be assigned to Ag-N stretching in analogy to the description given above for the band at 222 cm-l in the sol spectrum. A corresponding new intense IR peak is observed at 223 cm-I, as shown in Figure 3. The Raman intensity of the Ag-N vibration for the complex is much lower than that observed in the SER spectrum of the sol. Although Raman spectra of both complex and adsorbed species are dominated by strong Ag vibrations, the relative intensities differ in the two cases as SERS selective enhancement affects the sol spectrum. The validity of surface selection rules is confirmed by the fact that weak BZgand B36bands in solution are also present in the spectrum of the complex while absent in the SER spectrum. Relative intensities of SER bands depend on the excitation wavelength as shown in Figure 4, where spectra of a colloidal sample, containing NaC1, are reported for several different laser lines in the 300-1 700-cm-’ region. In order to follow the relative intensity of the Ag-N vibration, in Figure 5 SER spectra without addition of salt are reported in the 100-900-cm-’ region. Corresponding variations in the upper region are not reported as they closely follow the pattern of Figure 4. Changes in relative intensities of observed bands are particularly relevant in the 1300-1600-~m-~ region. However, we do not observe the same kind of dramatic variation with the excitation wavelength observed for phthalazine.’* Intensities of weak Bigbands seem independent ( I 1 ) Creighton, J. A. Surf. Sci. 1983, 124, 209. (12) Moskovits, M.: Suh, J. S. J . Phys. Chem. 1984,88, 5526.
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3709
SERS of 2,2'-Bipyrimidine Adsorbed on Silver Sol
be briefly discussed in the last section.
I
I
/I
I
5oc
300
-00 %mal
900
sn:':
::GO
2300
:50C
~:n-:'
Downloaded by UNIV OF CAMBRIDGE on September 14, 2015 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/j100372a064
Figure 4. SER spectra of 2,2'-bipyrimidine adsorbed on silver sol M NaCl added) with different excitation laser lines.
I/
160
300
' 500 ' Raman S h i f t
700
900
(cm-1) Figure 5. SER spectra of 2,2'-bipyrimidine adsorbed on silver sol (without addition of NaCI) with different excitation laser lines. of the excitation wavelength while most of the A, modes reach a maximum for the 514-560-nm excitation range, with the relevant exception of the band at 1450 cm-' which steadily increases with wavelength and finally becomes the more relevant spectral feature in the 1300-1600-cm-' region. A steady increase in intensity, relative to the band at 786 cm-', is also observed for the Ag-N vibration at 220 cm-l. The plasma resonance contribution to SERS, predicted by the electromagnetic theory,' cannot discriminate between normal modes of the same symmetry species. According to this theory, the excitation profile should match the extinction spectrum and this seems not to be the case of the profiles reported in Figures 4 and 5. Thus an additional resonance contribution must be invoked, of the type proposed by Lombardi et aI.,l3 related to a charge-transfer mechanism between adsorbate and substrate bound by chemical Ag-N bond. This aspect will (13) Lombardi, J. R.; Birke, R. L.; Lu, T.;Xu,J . J . Chem. Phys. 1986, 84, 4174.
Normal-Mode Calculation In the previous section conclusions on the conformation and orientation of the adsorbed species were drawn through comparison of the spectral data for the isolated molecule with those obtained from the SER of the colloidal dispersion and the normal Raman spectrum of the Ag complex. In this section we check these conclusions with the help of normal-mode calculations carried out using a valence force field recently obtained for the molecule under cons id era ti or^.^ In order to study the possibility of a nonplanar molecular conformation for the adsorbed molecule, vibrational frequencies were obtained for the isolated molecule by introducing nonzero twist angles about the central C-C bond, up to the value determined by electron diffraction4 in the gas phase. We found that normal modes belonging to A,, A,, and B,, molecular species are not affected by variation of this torsion angle while most of the remaining vibrations are only slightly perturbed by imposing a distortion from planarity. Marked frequency shifts are instead predicted for four normal modes listed below, with observed values from CDCI3 solution and calculated frequency ranges caused by a continuous variation of the twist angle from 0' to 50': B1, in-plane ring stretching observed at 401 cm-I, range from 414 to 482 cm-I; Bl, out-of-plane ring deformation observed at 395 cm-I, range from 396 to 329 cm-I; B3, out-of-plane ring deformation observed at 222 cm-I, range from 226 to 27 1 cm-l; B3, in-plane ring stretching observed at 167 cm-I, range from 177 to 143 cm-I. Since these frequencies are observed in the IR or Raman spectrum of both solutions and solid sample and do not appreciably change when the complex is formed or when adsorption on Ag occurs, a planar molecular conformation is suggested by our vibrational experiments. On the other hand such predictions can be used to check the occurrence of a distorted conformation, possible in the gas phase or in solution of nonpolar solvents. In both complex and sol spectra a few bands are considerably shifted toward higher frequencies with respect to the values observed in solution. To account for the observed frequency shifts, calculations have been carried out for a model which resembles the possible structure of the Ag complex. Due to the Occurrence of an insoluble 1:l complex with AgN03, one possibility is a pseudotetrahedral geometry for two bidentate bipyrimidine molecules coordinated to each Ag in a polymeric arrangment. The tetrahedral coordination geometry of Ag(1) with N-heterocyclic ligands is not uncommonI4 and, in particular, it is found for 4,4',6,6'-tetramethy1-2,2'-bipyridyl,l5closely related to the molecule under consideration here. On this basis the model assumed for 2,2'-bipyrimidine bound to silver substrate involves the binding of two nitrogen atoms. A planar structure for this molecule, averaged from X-ray data: was used by adding an Ag atom only on one side of the molecule, symmetrically placed with respect to the two nitrogens and lying in the molecular plane. For an Ag-N distance of 2.318 A, as found from X-ray studies of the above-mentioned Ag-bipyridyl complex, a "bite" angle N-Ag-N of 72.1' was obtained, identical with that reported in ref 15, with a resulting C, symmetry for the Ag/bipyrimidine roup. Three force constants, Ag-N stretching (2.0 mdyn/ ), N-Ag-N bending (0.5 mdyn A/rad2), and interaction between Ag-N stretchings (-0.3 mdyn/A), were added to account for planar vibrations. With these values two Ag-N stretching frequencies are calculated at 217/219 cm-I, close to the value observed in the Raman spectra of both the sol and Ag complex of bipyrimidine and in the IR spectrum of the complex. Vibrational frequencies calculated for the A, and B,, molecular species are reported in Table I1 together with corresponding values obtained for the complex. Thus computed frequency shifts may be compared with those observed going from solution to sol spectrum. The agreement is fairly satisfactory except for a B,, mode calculated at 663 cm-l for the Ag-bipyrimidine model while observed at 636 and 639
1
(14) Lancashire, R. J. ComprehensiueCoordination Chemistry; Wilkimon, G., Ed.; Pergamon: Oxford, 1987; Vol. 5. ( 1 5 ) Gocdwin, K. V.; McMillin, D. R.; Robinson, W . R. Inorg. Chem.
1986, 25, 2033.
3710 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990
Sbrana et al.
./
1442
1581
1335
+
w\
1-
+ - -
++++
+ p - - o +
5-
Amr, v
4
1034
1095
w
- t - - o 6 - -
*)ae Y n
T
I W
797
369
“$p’
9
&.?-
T*--
217
Figure 6. Cartesian displacements for in plane normal modes of the 2,2’-bipyrimidine/Agcomplex corresponding to A, molecular modes (8,X 8 per change In normal coordinate)
Downloaded by UNIV OF CAMBRIDGE on September 14, 2015 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/j100372a064
TABLE II: Comparison between Observed In-Plane, A, and B,, Frequencies (cm-’) and Values Calculated for the Isolated 2,Z’iBipyrimidine Molecule and Its Ag Complex‘
species A,
observed molecule sol 1562 1567 1451 1332
1075 998 782
B,,
339 1557.
1432. 133.5. 1291
1448
1330 1080 1018 786 364
1584
Av
-5 -3 -2
5 20 4 25 7
calculated molecule complex 1575 1580 1442
1333 1084 1009 791 342 1575
459 354 280
1158 623 414
629 40 I
636
12 7
412
11
582
1351
1279
1 I72
1095 034 797 369
1456 11
1302 1184
1442 1335
Av
5 0 2 11 25 6 27 7 3 3 1
163
5
663 425
40 11
“Observed data from normal Raman spectrum of CDC13 solution and SERS from aqueous silver sol; starred frequencies from powder spectrum. cm-I in the sol and complex spectrum, respectively. On the other hand the IR spectrum of the complex displays a new strong band at 667 cm-I (calculated at 681 cm-I for the model) which replaces a band of corresponding intensity at 642 cm-’ (calculated at 649 cm-I for the B3”species) in the IR spectrum of 2,2’-bipyrimidine. Since these two frequencies are both assigned to in-plane ring bending, with identical atomic displacements within each ring,5 the disagreement for the B mode can be ascribed to interaction terms missing in the simpli8ed interaction field, for the molecule bound to silver, given above. For all other normal modes, the reproduction of frequency shifts, observed in the sol spectrum, is substantially correct and confirms that the orientation of the molecule adsorbed on silver, as previously indicated on the basis of the surface selection rules, is reasonable. Normal modes for the Ag complex are shown in Figure 6 for those vibrations corresponding to A, modes of the isolated molecule, including the Ag-N stretching mode characteristic of complex formation. The presence of the heavy silver atom has, as expected, very little influence on the individual atomic displacements which are practically the same as those of the isolated m o l e c ~ l e .These ~ displacements will be referred to in what follows.
Conclusions The Raman spectrum of 2,2’-bipyrimidine is little affected by adsorption on silver substrate and this is a considerable advantage for the assignment of the observed frequencies based on results previously obtained for the isolated molecule. The SER spectrum is thus interpreted in terms of a planar molecular conformation, as found from X-ray studies. On other hand, minor differences in intensities, when spectra of the adsorbed species are compared to those of both the Ag complex and the isolated molecule, suggest the validity of SERS surface selection rules based on D a symmetry and allow conclusions on the molecular orientation of the adsorbed species, perpendicular to the Ag substrate and bound through nitrogen atoms. These results are confirmed by model calculation for the molecule bound to silver, which satisfactorily account for the observed frequency shifts of Raman bands observed in the SER spectrum. Relative intensities of SER bands, for vibrations definitely belonging to the A, irreducible representation, do not undergo the same variation with the exciting wavelength. This variation is, in our experiment, fairly independent of the extinction spectrum which shows a secondary maximum that disappears when salt is added. Following a charge-transfer scheme, if donation of lone pair electrons of the nitrogen atoms to metal occurs, then totally symmetric vibrations involving nitrogen displacements can mix metal and excited molecular states, leading to a resonant contribution tunable with the exciting frequency. This seems to be the case of all observed A bands, except for the C H bending mode at 1450 cm-I, which dehnitely does not fit into this resonant scheme. However, this normal mode is actually the only one not involving displacements of nitrogen atoms (see Figure 6), is thus unaffected by charge transfer, and experiences only a plasma resonance contribution to SERS. This is also the case of the Ag-N normal mode, approximately described by translation of molecule and Ag atom in opposite directions, with relative intensity steadily increasing with wavelength. Although it is tempting to think of a scale of charge-transfer efficiency for A, normal modes, this would stretch too far a qualitative explanation. It is, however, an indication of the validity of normal-mode calculations which, when combined with infrared and Raman experimental data, allow a better understanding of the mechanism underlying SERS spectroscopy . Acknowledgment. Financial support from the Italian Consiglio Nazionale delle Ricerche and Minister0 della Pubblica Istruzione is gratefully acknowledged.
