J. Phys. Chem. 1988, 92, 954-959
954
Raman) for its verification. The low excited-state torsional barriers suggest some possibilities for the photochemical dynamics of this elementary conjugated hydrocarbon.
Acknowledgment. This work was supported in part by grants
from the National Science Foundation (CHE-8018318 and CHE-7920433). Registry No. trans- 1,3,5-Hexatriene,821-07-8; cis-l,3,5-hexatriene, 2612-46-6.
Surface-Enhanced Raman Spectra of Pyrazlne, PyrlmMine, and Pyridazine Adsorbed on Sllver Sols Maurizio Muniz-Miranda, Dipartimento di Chimica, Universita Degli Studi di Firenze, 501 21 Firenze. Italy
Natale Neto,* and Ciuseppe Sbrana Centro CNR Composti Eterociclici, 501 21 Firenze, Italy (Received: June 2, 1987)
Raman spectra of pyrazine, pyrimidine, and pyridazine adsorbed on silver sols have been obtained and compared with existing data from corresponding experiments on Ag electrode. These two techniques give similar results except for pyrazine for which strong bands, observed only in the SER spectrum on the electrode, may be due to reduction products coadsorbed on the Ag surface. Chemisorption plays a role in the absorption of diazines as evidence of Ag-N bond formation was found in the SERS of all three molecules. Both N atoms of pyridazine are bound to the substrate and this explains an enhancement, for the latter molecule, of 2 orders of magnitude larger than for the other two compounds. The presence of low-frequency Ag-N modes and predictions of surface selection rules support “edge on”, rather than flat, orientation of diazines on the colloidal particles.
Introduction Since its discovery’*2 surface-enhanced Raman scattering (SERS) has assumed an increasing role for studying the behavior of molecules adsorbed on metal surfaces. Great attention was recently paid to the possibility of establishing the orientation of considering the molecules adsorbed on silver sols or two main sources of enhancement: the electromagnetic and chemical or charge-transfer effect. In this report we examine the S E R spectra of three diazines adsorbed on silver sols under identical experimental conditions and compare the results with existing data obtained from diazines adsorbed on Ag electrode. We intend to demonstrate the equivalence of experiments carried out on colloidal dispersion and on electrodes and, at the same time, obtain information on the molecular orientation of the adsorbate upon the metal surface. Experimental Section Raman spectra were recorded with a Jobin-Yvon HG2s monochromator, a cooled RCA-C3 1034A photomultiplier, a photon-counting system, and a data acquisition facility. An Ar’ laser selecting the 514.5-nm exciting line was employed with a power of 50 mW monitored a t the base of a 10-mm quartz cell containing the samples. Stable silver sols were prepared as in ref 8 by reduction of AgNO, solution with excess NaBH4. After (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 226, 163.
( 2 ) JeanMaire, D. J.; Van Duyne, R. P. J . Electroanal. Chem. 1977,84, 1.
(3) (4) (5) (6) 518. (7)
Takahashi, M.; Fujita, M.;Ito, M. Chem. Phys. Lett. 1984, 109, 122. Takahashi, M.; Ito, M. Chem. Phys. Lett. 1984, 103, 512. Irish, D. E.;Guzonas, D.; Atkinson, G. F. Surf. Sci. 1985, 158, 314. Joo, T. H.; Kim, K.; Kim,H.; Kim, M.S. Chem. Phys. Lett. 1985,117, Takahashi, M.; Niwa, M.; Ito, M. J . Phys. Chem. 1987, 91, 11.
0022-3654/88/2092-0954$01.50/0
addition of diazines, the colloids changed slowly from yellow to red and assumed a final grey-brown-colour. The absorption spectrum showed the usual band at 398 nm and a shoulder, at about 530 nm, characteristic of aggregation. Addition of NaCl (2 X M) produced reversal of particle aggregation, with consequent disappearance of the 530-nm band, and strongly increased the SERS. Enhancement of the Raman signal after chloride addition is a feature already observed in one of the early papers2 on the SERS effect and later confirmed in many subsequent works. A lo-’ M concentration was used for both pyrazine and pyrimidine. At this concentration a quick collapse of the silver colloid occurred for pyridazine for which a much lower concentration, lo4 M, was found to be appropriate. Pyrazine (Merck 99% purity), pyridazine, and pyrimidine (Fluka 98+% purity) were purified by sublimation or by fractional distillation under vacuum, reaching a purity >99.5% as tested by gas chromatography. All aqueous solutions were prepared with triply distilled water, and sodium chloride suprapurum (Merck 99.9%) was used.
Results S E R spectra of the diazines adsorbed on silver sols are shown in Figures 1-3 and compared with those of corresponding 1 M aqueous solutions. Tables 1-111 contain the vibrational assignment of each diazine and a list of fundamental frequencies obtained from silver sols, present paper, and from previous works on electrode surfaces. Vibrations are classified into irreducible representations according to the following convention: all three molecules lie in the yz plane; z passes through nitrogen atoms for pyrazine and coincides with the unique Cz symmetry axis for the other two diazines. Relative intensities are normalized to 100 for the strongest band in each spectrum, observed at 1022, 1020, and 1071 cm-’ in the SERS of pyrazine, pyrimidine, and pyridazine, (8) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J . Chem. SOC., Faraday Trans. 2 1979, 75, 790.
0 1988 American Chemical Society
Spectra of Diazines Adsorbed on Ag Sols
The Journal of Physical Chemistry, Vol. 92, No. 4, 1988 955
TABLE I: Bulk and Surface Fundamental Frequencies of Pyrazine moltena solutionb Raman IR Raman electrodec 3066 3066 3054 3041 1578 1524
3060 (17) 1594 (7) 1529 (5) 1484 1418 1346
1230
1241 (5) 1214 (2)
1135 1118
1120 (0.5) 1063 1021 1017 (100) 922 (1)
1015 919
804 757 703 641 596
758 (1) 703 (6) 615 (0.5) 416
3060 3050 3031 1590 1520 1485 1420 1340 1242 1224 1164 1121 1069 1038 1018 916 797 744 700 635 635 436
SERS enh ratio
sold
species
3072 (7) 1594 (13) 1516 (10) 1362 (1) 1240 (8) 1222 (25) 1124 (3) 1022 (100) 916 (4) 800 (0.5) 700 (5) 636 (1.0) 426 (1) 244 (148)
From ref 23. 1 M aqueous solution, present paper. CFromref 12. d10-2M with addition of salt.
Figure 1. Raman spectrum of pyrazine: lo-* M in Ag colloidal dispersion with addition of NaCl (upper trace); 1 M aqueous solution (lower trace).
Figure 2. Raman spectrum of pyrimidine: M in Ag colloidal dispersion with addition of NaCl (upper trace); 1 M aqueous solution (lower
trace). respectively. These bands have counterparts at 1017,1008,and 1068 cm-I in the Raman spectra of the 1 M aqueous solution. A relative enhancement ratio, defined as the intensity in the colloid divided by the corresponding quantity in the 1 M aqueous solution, characterizes bands in the colloidal samples and is also reported in Tables 1-111. It is useful in comparing the experimental results with predictions of surface selection rules. An absolute en-
lo4 M in Ag colloidal dispersion with addition of NaCl (upper trace); 1 M aqueous solution (lower trace). Figure 3. Raman spectrum of pyridazine:
hancement factor, for the S E R effect, ranging from lo2 to lo3 is derived for pyrazine and pyrimidine, while for pyridazine it is of 2 orders of magnitude larger. Actually in sol spectra of all three diazines, obtained by adding NaCl, the strongest band is observed at about 240 cm-'. This frequency is too low to be associated with a normal mode of the diazines. It is present in the spectrum of the colloidal dispersion after addition of salt but before adsorption of diazines and must be thus assigned to a stretching vibration of C1- bound to the Ag surface. This assignment, in contrast with previous interpretations as an Ag-N stretching, was first given by Wetzel and Gerischer9 who reported a band a t 235 cm-l in SER spectra on silver sols of both AgCl and pyridine + chloride ion. It is possible, however, that AgCl and AgN modes, occurring at very close frequencies, are superimposed in the spectra of .Figures 1-3. This possibility is suggested by the work of Wetzel et al.'O on pyridine and silver halides coadsorbed on Ag electrode. At an applied potential of -0.2V,SERS from both silver chloride and silver chloride pyridine show a strong peak a t about 240 cm-'with a pronounced shoulder at lower frequency when pyridine is coadsorbed. When the applied potential is gradually moved
+
(9) Wetzel, H.; Gerischer, H. Chem. Phys. Lett. 1980, 76, 460. (10) Wetzel, H.; Gerischer, H.; Pettinger, B. Chem. Phys. Lett. 1981, 78, 392.
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The Journal of Physical Chemistry, Vol. 92, No. 4, 1988
Muniz-Miranda et al.
TABLE 11: Bulk and Surface Fundamental Frequencies of Pyrimidine 1iq uid ' solutionb Raman IR Raman electrodeC 3086 3086 3108 (1) 3074 3077 3076 (8) 3053 3052 3028 (2) 3038 1586 1568 1587 ( 5 ) 1568 1575 (7) 1564 1564 1476 (3) 1466 1465 1402 1407 (2) 1397 1397 1375 (0.5) 1370 1225 1231 1233 (5) 1227 1 I63 1168 (2) 1159 1159 1142 1139 1145 (12) 1139 1072 1078 (33) 1076 1071 1061 (19) 1056 1054 1054 996 1008 (100) 1007 99 1 99 1 983 (0.5) 980 980 838 811 813 721 723 708 684 (10) 679 678 678 637 (4) 650 624 623 400 (2) 417 398 349 (1) 344 348
SERS sold
3080 ( 5 )
enh ratio
suecies
0.6
1587 (4) 1566 (40) 1488 (21) 1402 (24)
0.8 5.7 7.0 12.0
1232 (5) 1172 (7) 1142 (64) 1078 (63)
1.o 3.5 5.3 1.9
1062 (36)
1.9
1020 (100) 990 (0.5)
1.o 1.o
684 (10)
1.o
406 (1.5)
0.7
244 (131) 'From ref 2.
1 M aqueous solution, present paper. 'From ref 12.
TABLE 111: Bulk and Surface Fundamental Frequencies of F'yridazine liquid" solutionb Raman IR Raman electrode' 3080 3070 3041 3053 1570 1564 1441 1401 1352 1287 1150 1113 1063 1052 986 970 963 786 775 755 660 622 370 363
1153 1112 1055 1058
SERS sold
enh ratio
species
1576 (24)
1576
1576 (30)
1.2
B2 A1 B2 A1 AI
1456 (4)
1454
1457 (30)
7.5
AI
1388 1287
1362 (10)
3086 (27) 3073 (31) 1555 1540 1440 1408 1340 1283
M with addition of salt; FR = Fermi resonance.
3085 (20) 3070 (70)
0.7 2.2
B2 B2
1292 (9) 1189 (51) 1169 (15) 1128 (5) 1068 (100)
1169 1070
1209 1167 1124 1071
(51) (25) (5) (100)
1 .o 1.7 1 .o 1 .o
A1 B2 AI B2
A1 FR A,
B2 Bl A2
960
977 (108)
982
979 (48)
0.4
A, A2
760 663 632 372
756 669 635 379 369
765 741 670 642 385
(2) (15) (9) (7) (7)
A2 BI 669 (27) 644 (8) 391 (25)
1.8 0.9 3.6
B2
A1 Bl
A2 238 (330)
a
From ref 22.
1 M aqueous solution, present paper.
From ref 7.
to -0.6 V, the peak in the silver chloride spectrum decreases in intensity, following the desorption of chloride ions, and shifts to lower frequency as predicted by coverage-dependent calculations." At the same time the low-frequency shoulder, present when pyridine is coadsorbed, gradually develops into a band still present, around 220 cm-I, at a potential of -0.6 V when a rather complete desorption of C1- ions occurs. This band is attributed to the Ag-N mode of the silver-pyridine system. A closer look at our spectra of Figures 1-3 shows that a similar situation cannot be excluded for diazines absorbed on silver sols; ( I 1) Nichols, H.; Hexter, R. M J . Chem. Phys. 1982, 76, 5595.
M with addition of salt; FR = Fermi resonance.
thus we recorded the SER spectra without addition of salt using the same concentrations employed for the spectra of Figures 1-3. The salt-free spectra are shown in Figure 4 for the 100-300-cm-' region. Due to the complete absence of C1- ions in this case, all visible low-frequency features must be due t o Ag-diazine vibrations. A broad band at 224 cm-I is present in both spectra of pyrazine and pyrimidine. At about the same frequency a shoulder is clearly visible in the case of pyridazine which, however, shows a second pronounced peak at 200 cm-'. Such bands are hidden in the sol spectra of Figures 1-3 and are also, in general, not visible in corresponding experiments on silver electrode because of the presence of a supporting electrolyte. We assign these vibrational
Spectra of Diazines Adsorbed on Ag Sols
> Y .r(
In c Lu Y
C
CI
150
250
Figure 4. SER spectra of pyrazine (a), pyrimidine (b), and pyridazine (c) in the low-frequency region without addition of NaCl (same concentrations as in Figures 1-3).
frequencies in the salt-free colloids to Ag-N stretching of diazines bound to silver particles, in agreement with similar findings for benzyl cyanide on silver sol6 and pyridine on silver electrode.I0 In the latter paper new peaks were found for pyridine, at 170 and 250 cm-’, at more cathodic values of the electrode potential, when chloride ions should be completely absent. Corresponding features are not present in our spectra of diazines which, however, are in general similar to those obtained on Ag electrode at low cathodic values of the potential. The occurrence of two AgN stretchings confirms the adsorption of pyridazine’ through donation to the metal of the lone pair electrons of nitrogen atoms rather than donation of delocalized A electrons. In general our sol spectra of diazines agree with those obtained on Ag electrodes, thus confirming the equivalence of the two techniques. There are, however, points which deserve further comments as will be discussed in what follows when results for each individual molecule will be presented. Pyruzine. Among the three diazines considered in this paper pyrazine presents the most remarkable difference between the SER spectrum on Ag sol and the corresponding spectrum obtained on electrode. As shown in Table I, all bands detected in the colloidal dispersion are also found through Raman spectroelectrochemistry but the reverse is not true. According to ref 12 all infrared-active modes of BIY,Bzu,and B3uspecies gain activity in SERS experiments and a t least three new strong bands are left unassigned. We observe, instead, only three very weak bands not present in the bulk Raman and which may be associated to breakdown of normal selection rules in the SERS of pyrazine. The discrepancy is striking for two strong bands observed12at 1485 and 1420 cm-I, and attributed to Raman modes forbidden under D2h symmetry, completely absent in our spectrum. These bands actually appear stronger than the ring breathing vibration at 1022 cm-’ which dominates our sol spectrum as well as that of bulk pyrazine. In another experiment13 on a Ag electrode no trace exists of the 1420-cm-l peak while only a weak band occurs at 1488 cm-I. In the latter work a band clearly visible at 1317 cm-’ is attributed to a Raman-forbidden normal mode but it is absent in both our spectrum and that of ref 12. Raman-forbidden bands are not visible in a third experiment on SERS of pyrazine on a Ag e1e~trode.l~ The same laser wavelength as in the present paper (12) Dornhaus, R.; Long, M. B.; Benner, R. E.; Chang, R. K. Surf. Sci. 1980, 93, 240. (13) Erdheim, R. G.; Birke, R. L.; Lombardi, J. R. Chem. Phys. Lett. 1980, 69, 495.
The Journal of Physical Chemistry, Vol. 92, No. 4, 1988 951 was used in ref 12 and 14 while the 488.0-nm line was employed in ref 13. In our preliminary work15 on pyrazine on silver sol we found bands due to reduction products superimposed on the SERS of the diazine added to the colloid soon after preparation. This was attributed to the reducing action of excess NaBH4 if not enough time was alllowed for its spontaneous decomposition. The presence of bands which do not belong to the compound used, due to adsorption of reaction products present in low concentration, was also found by LippitschI6 in the sol spectra of pyridine. Owing to the very different conditions for the preparation of Ag substrate, we do not conclude that the same reactions may occur on a Ag electrode. It may be conceivable, however, that the differences between the sol spectrum of Figure 1 and that reported in ref 12, as well as the differences between different spectra obtained on the electrode, may be due to derivatives of pyrazine formed during the reduction step of the AgCl to Ag on the electrode, a possibility not ruled out in ref 12. This is consistent with the observed development in time of the intensity of the Raman-forbidden bands, which appear after the purely Raman-active modes and tend to gain intensity when the Ag electrode is kept for hours at -0.4 V. Except for a band at 636 cm-I, large frequency shifts are not observed between the 1 M solution Raman and the SER spectrum. Two overlapping bands are present both in the SER, at 1222 and 1240 cm-’, and in the Raman spectrum in aqueous solution while only one band is observed at 1230 cm-’ in CS2solution. The two components in aqueous solution are probably due to free pyrazine and hydrogen-bonded pyrazine with a consequent shift to higher frequency as already observed for several IR bands of diazines.” The corresponding splitting in the sol spectrum can be due to hydration of one nitrogen and binding of the second N atom to the Ag substrate. The appearance of weak bands at 1362, 800, and 426 cm-’ is consistent with the presence of a Ag-pyrazine system with lower symmetry, C,, and thus IR bands of pyrazine gain Raman activity when the molecule is adsorbed. However, symmetry considerations are not sufficient to understand the relative enhancement of the bands observed in the sol spectrum. Several surface selection rules have been proposed in the literature. The ones adopted here are due to Creighton18 and deal with enhancement of purely electromagnetic origin for molecules at the surface of small isolated metal spheres. Two possible different orientations of pyrazine, as well as of the other two diazines, are considered here: if the molecule is adsorbed with the z axis perpendicular to the Ag surface, the term “edge on” is used equivalent to the “end on” geometry of ref 19; a flat orientation, with the z axis parallel to the metal surface, is referred to as “face on“. Creighton’s selection rules predict that the least enhanced Raman modes should belong to the B,, species if pyrazine is adsorbed face on and to B,, species for edge on adsorption. In the former case Bl, and B2, modes should appear with comparable intensity while for edge on adsorption this should apply to B, and B3, vibrations. In both cases A, modes should be active but with variable intensity depending on the relative magnitude of the diagonal polarizability components. Our relative enhancement ratios clearly agree with the predictions based on edge on adsorption which is also supported by the observation of a AgN stretching frequency at 224 cm-l. Pyrimidine. A close resemblance exists between the normal Raman and SER spectrum of pyrimidine adsorbed on Ag particles or electrode. Contrary to the case of pyrazine, no evidence was ever found in our experiments of possible reaction derivatives of pyrimidine, Our S E R spectrum is very similar to the reported in ref 12 which is, however, less resolved due to a strong inelastic (14) Sinclair, T. J. Conf. Int. Spectrosc. Raman 7th 1980, 408. (15) Muniz-Miranda, M.; Neto, N.; Sbrana, G. J . Mol. Srruct. 1986, J43, 275. (16) Lippitsch, M. E. Chem. Phys. Le??.1980, 74, 125. (17) Takahashi, H.; Mamoia, K.; Plyer, E. K. J . Mol. Spectrosc. 1966,2J, 217 -.
(18) Creighton, J. A. Surf. Sci. 1983, 124, 209. (19) Hexter, R. M.; Albrecht, M. G. Spectrochim. Acta, Par? A 1979, 35A, 233.
958
Muniz-Miranda et al.
The Journal of Physical Chemistry, Vol. 92, No. 4 , 1988
I
I I
I I
I I
I I I
z
Y
-
I I I 1
Lo
uc c
Y
b
I I
I I
'L'
1020
1060
1100
1140 Raman S P i f t
1180
I I I
1220
I
:m-'
I I I I I
I I I I \
Figure 5. Raman spectrum of progressively diluted aqueous solutions of pyridazine (- pure, --- 10 M, 3 M, 1 M). -.-e-
-..-e.-
1150
radiation background absent in the sol spectrum. A strong doublet at 1076-1054 cm-I in the Raman spectrum of the liquid is attributedZoto Fermi resonance due to interaction of an A I fundamental with a combination of the two B, modes observed at 344 and 721 cm-I. A corresponding doublet is observed in the S E R spectrum with the same relative enhancement ratio. Only A, and B2 modes appear to be enhanced in the sol spectrum while modes assigned to A2 and B, species are either absent or extremely weak. This rules out pyrimidine adsorption with the molecular plane parallel to the Ag surface as, in this case, B2 modes should be the least enhanced.I8 On the other hand our observation of an AgN stretching at 224 cm-' suggests an edge on adsorption through one nitrogen atom with a C, symmetry left for the Agpyrimidine system. In this case modes belonging to irreducible representations which transform like polarizability tensor elements containing the normal component, that is, out of plane A, and B,, are predicted to be less enhanced as observed. Pyridazine. Our SER spectrum of this molecule is very similar to that reported by Takahashi et al.' for pyridazine adsorbed on a Ag electrode when the applied potential was -0.05 V. The assignment of Table 111is thus very closely related to that given by these authors except for the interpretation of bands observed in the 1100-122O-cm~~ region. In this region two bands are present in our Raman spectrum of liquid pyridazine, slightly shifted at lower frequency in CCl., solution, which become three in the 1 M aqueous solution and in the sol spectrum. In Figure 5 we show the Raman spectrum in this region of pyridazine progressively diluted with water, with relative intensities referred to the strong band at about 1070 cm-' which also undergoes a small frequency shift. The first band at 11 19 cm-l slowly moves to 1128 cm-I with decreasing intensity and two peaks gradually replace the 1 161-cm-' band of the pure liquid, finally ending at 1169 and 1189 cm-'. The existing assignment of the vibrational spectrum of pure pyridazine in this region is briefly reviewed as follows. The first Raman polarized band of liquid pyridazine at 11 19 cm-' was explained2' as a combination of 379 B I + 756 B1= 1135 A, in Fermi resonance with a C H bending A I fundamental at about 1150 cm-' actually observed a t 1 16 1 cm-'. A weak band at 1131 cm-' in the infrared spectrum, extremely weak in Raman, was attributed to a C H bending mode of B2 species although, on the basis of the assignment of d2- and d4-deuterio derivatives, a B2 C H bending mode was expected a t a frequency above 1150 cm-*. This assignment was maintained in ref 22 where however, (20) Milani-Nejad, F.; Stidham, H. D. Specrrochim. Acru, Purr A 1975, 31A, 1433. (21) Stidham, H. D.; Tucci, J. V. Spectrochim. Acta, Parr A 1967, 2 3 4
2233. (22) Ozono, Y.; Nibu, Y . ;Shimada, H.; Shimada, R. Bull. Chem. SOC. Jpn. 1986, 59, 2991.
1250
1150
1250
Figure 6. Polarized Raman spectrum of pyridazine in a 6 M (a) and 1 M (b) aqueous solution.
the role of the 1119- and 1131-cm-' bands was reversed. Normal-coordinate calculation carried out in this latter work predicts two vibrational frequencies at 1122 and 1 110 cm-' associated, respectively, to the A, and B, CH bending modes discussed above. Corresponding vibrational frequencies for pyridazine-d, were computed at 966 and 969 cm-'. On this basis, one possibility for explaining the aqueous solution Raman spectrum is to attribute the band at 1189 cm-' to a CH bending of AI species of the molecule bound by hydrogen bond and the 1169-cm-' band to the corresponding mode of free pyridazine. However a 8-cm-I shift of the free molecule mode is not expected and never observed in the infrared spectra of pyridine, pyrazine, pyrimidine, and pyridazine in hydrogen-donor ~olvents.'~ We thus rather propose a n accidental degeneracy of the two A, and B2 C H bending modes at about 1160 cm-I which is lifted in water solution due to different frequency shifts. This interpretation is supported by our observation that the first band, at 1169 cm-' in the Raman spectrum of the 1 M solution, appears depolarized in diluted water solution while the second band remains strongly polarized (Figure 6). This second band is an A, mode, has a large N-N stretching contribution in the potential energy distribution7 and is thus upshifted by hydrogen bonding through the lone pair electrons of the nitrogen atoms. Symmetry considerations exclude a similar contribution to the B2 mode which is thus less affected in aqueous solution. The Fermi resonance discussed above is also gradually lifted due to the considerable frequency shift of the A, fundamental. Thus, the intensity of the combination at 1119 cm-' in the liquid decreases with diluition as shown in Figure 5. This interpretation of the solution Raman spectrum of pyradazine can be readily transferred to the sol spectrum in which adsorption to the Ag substrate through nitrogen atoms replaces hydration. The bands observed in the sol spectrum at 1167 and 1209 cm-' are thus attributed to B2and A, CH bending vibrations, respectively. The latter is the third strongest band in both the normal Raman and SER spectrum of pyridazine but was left unexplained in ref 7. The frequency shifts observed in our sol spectrum are consistent with those found by Takahashi et al.' and, at least for the vibrations of A, species, can be related to the N-N character of the normal modes as proposed by these authors. Through this argument they concluded that pyridazine is probably adsorbed to the substrate through both nitrogen atoms. The presence of two AgN stretching at 200 and 224 cm-' in the salt-free spectrum of the colloidal dispersion supports this con(23) Innes, K. K.; Byme, J. P.;Gross, I. G. J . Mol. Speclrosc. 1967, 22, 125
J. Phys. Chem. 1988, 92, 959-962 clusion. On the other hand, application of Creighton’sl* selection rules to pyridazine suggests that the A2 modes should be the least enhanced ones for edge on adsorption. Actually, our sol spectrum does not show bands belonging to this species while enhancement occurs for normal modes which classify into the remaining representations and present nonvanishing intensity in the 1 M aqueous solution.
Conclusions Raman spectra of 1,4-, and 1,3- and 1,2-diazine adsorbed on Ag sols show characteristic scattering enhancement very similar to that observed in corresponding experiments on silver electrode. Differences between the results obtained with the two techniques were found only for pyrazine and are probably due to adsorption of reaction products. Chemisorption plays a role in the adsorption of diazines since evidence of Ag-N bond formation was found
959
in the low-frequency region of the SER spectra of all three diazines. The assignment of broad bands in the 200-230-cm-’ region to Ag-N stretching, well distinguished from Ag-Cl modes at about the same frequency, clearly supports an edge on adsorption on the Ag substrate in agreement with observed relative enhancement ratios which correspond to Creighton’sl* surface selection rules. Two Ag-N bands are observed for pyridazine and this implies an adsorption through both nitrogen atoms with a relevant contribution of chemisorption to the total enhancement. This is consistent with an enhancement for the latter molecule 2 orders of magnitude larger than that observed for the other two diazines.
Acknowledgment. This work was supported by the Italian Consiglio Nazionale delle Ricerche. Registry No. Ag, 7440-22-4; pyrazine, 290-37-9; pyrimidine, 28995-2; pyridazine, 289-80-5.
Geometrical Structure and Vibrational Frequencies for the Oxygen Analogue of Hexasulfur Charles P. Biahous I11 and Henry F. Schaefer III* Department of Chemistry, University of California, Berkeley, California 94720 (Received: June 5, 1987)
Self-consistent field (SCF)methods with minimum (STO-3G), double { (DZ), and double {plus polarization (DZP) basis sets predict the O6ring to assume chair, twist, and boat conformationsanalogous to similar forms for cyclohexane. All predicted vibrational frequencies for the chair and twist forms are real. Six symmetrically equivalent oxygen atoms are predicted to comprise the lowest energy chair form, with 0-0 bond distances of 1.364 A and bond angles of 104.7O at the DZP SCF level of theory. The boat form is not found to be an energy minimum but rather exhibits one imaginary vibrational frequency which when followed tends toward assumption of the twist form. Energy differences at the DZP SCF level are computed to be 15.9 kcal between the chair and twist forms and 17.5 kcal between the chair and boat. We interpret these results by analogy with cyclohexane and assign the larger energetic discrepanciesto shorter bond distances and inherently greater eclipsing effects for adjacent lone electron pairs than those attributed to bonding electron pairs. Homodesmotic and hyperhomodesmotic reactions devised to predict the decomposition exothermicity of the ring give rather different results, namely, 130 (homodesmotic) and -75 (hyperhomodesmotic) for the heat for formation of 06.
-
Introduction The idea that oxygen rings may in principle be stable molecules takes root in the observation that sulfur, oxygen’s lower neighbor on the periodic table, exists naturally in ringed systems.’ The analogy cannot be too rigidly extended, however, because of the vastly differing character of oxygen-oxygen and sulfur-sulfur bonds. For example, a 2n-membered sulfur ring with a standard sulfur-sulfur bond strength near 54 kcalZlies at an energy level comparable to that of n diatomic sulfur molecules (bond strength 100 kcal/mo13), whereas the relatively weak nature of oxygenoxygen single bonds (35 kcalZ in peroxides) implies that a 2nmembered oxygen ring would lie in energy far above n diatomic oxygen molecules (bond dissociation energy = 118 kca13). One infers, therefore, that oxygen rings, if sufficiently stable, are molecular systems wherein large quantities of energy might be effectively stored (crudely estimatedl8 to be 144 kcal in a sixmembered ring, or 48 kcal per O2molecule). The work described here is an attempt to locate theoretical energy minima which correspond to conformations of such ringed systems comprised of six oxygen atoms. Theoretical Approach Conducting our geometry optimizations from a variety of starting points led us to focus our study on the three optimized conformations of greatest importance. The lowest energy of these, hereafter referred to as the “chair” conformation, optimizes to ‘Author to whom correspondence should be addressed.
0022-3654/88/2092-0959$01 SO/O
D3d symmetry and exhibits the electron configuration 1afgle: 1e: 1af,2afg2e:2e:2af,3a:,
1a:,3e:3a:,3e:4e:4e:4a~~
which, in the lower CZhsymmetry employed in the present geometry optimizations, corresponds to l a i l a i l bi2ai1bi2bi3ai2aZ3bi4ai2bi4b~5a~3a~6ai3bi5bi4ai6b~ 5ai7bZ7al4bi8ai The next lowest in energy, the “twist” form, displays D2 symmetry and has the orbital occupation scheme 1b: 1a:1 bf 1b:2a:2b33a:2b:3b:4a:2b:4b$3b:3b:5a:5b$4b:6af5b: 6b:4b:7a:Sbf6b: A C, “boat” structure was optimized as well with the electron configuration: 1b: 1af 1bf2af 1a$2b$3a:3b:2bf2a$4af4b$3bf5af6af5b:4bf3a$5bf 6bf7a:8a:4af7b: (1) (a) Schaefer, H. F.,to be published. (b) For background on cyclic sulfur structures see Meyer, B. Sulfur, Energy and Enuironment; Elsevier: Amsterdam, 1977. (2) Pine, S . H.; Hendrikson, J. B.;Cram, D. J.; Hammond, C. I. Organic Chemistry, 4th ed.; McGraw-Hill: New York, 1980; p 85. (3) Huber, K.; Herzberg, G. Consrants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.
0 1988 American Chemical Society
