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J. phys. Chem. 1981, 85, 248-262
curred. This formal positive charge was satisfied by the addition of a hydride ion to form an S-Hbond. A different situation may exist for the pyridine case. The action of a hydride ion to form a bond while releasing a pair of electrons to the carbon should be less likely because the nitrogen is already a formally negatively charged species and because it is in close proximity to the metal ion that bears a high formal positive charge. The negative charges of the nitrogen and hydride will have an unfavorable effect on the possibility of N-H ond formation, while the presence of a positively char ed metal center will be unfavorable to electron release. It would seem then that, in either manner of breaking the C-N bond, an unfavorable situation develops compared to breaking the C-S bond in thiophene. One might conclude, then, that C-N bond breaking is less likely to occur, permitting further reaction of the carbon chain while the nitrogen remains strongly bound to the site.
9
Conclusion The comparison of the two mechanisms suggests that denitrogenation can be expected to proceed with more difficulty than desulfurization. The specific problems identified in each of the steps in the mechanism arise from fundamental differences in the structure of the molecular orbital manifold for the thiophene and pyridine cases. Merely having a five-membered ring or a six-membered ring may be sufficient to create significant differences between the two heterocycles as they proceed through the mechanism. This effect is entirely due to ring geometry
and is not strongly dependent on the nature of the heteroatom. It had been mentioned previously that it is experimentally known that removal of the nitrogen from pyridine is more difficult than removal of sulfur from thiophene. Aboul-Gheit and Abdou have obtained experimental evidencel0 that it is more difficult to denitrogenate pyrdine than pyrrole. The difference in reactivity between pyridine and pyrrole is qualitatively similar to the difference in reactivity between pyridine and thiophene. I t does not seem to be mere coincidence that the more reactive species are the less basic five-membered heterocycles regardless of the heteroatom. The application of Huckel molecular orbital theory to pyrrole will result in qualitatively the same conclusions as the calculations on thiophene lead one to make. The major difference between pyrrole (or thiophene) and pyridine is not in basicity since the basicity is really a function of the molecular electronic structure of the compounds. Rather, it appears that five-membered rings give rise to molecular orbital patterns which are more suitable for heteroatom removal by the reaction mechanism under examination than do six-membered rings simply by virtue of the connectivity of the molecule. Acknowledgment. I acknowledge the Computation Center of the South Dakota School of Mines and Technology for computer time in support of this project. (10) A. K. Aboul-Gheit and I. K. Abdou, J. Inst. Pet., 69,188 (1973).
Dlthizone Adsorption at Metal Electrodes. 2. Raman Spectroelectrochemlcal Investigation of Effect of Applied Potential at a Silver Electrode Jeanne E. Pemberton and Richard P. Buck' Kenan Laboratories of Chemlstty, UnlversRy of Notfh Carollna, Chapel Hill, North Carollna 27514 (Recelved: March 26, 1980; In Flnal Form: September 15, 1980)
The technique of laser Raman spectroelectrochemistryhas been applied to the study of the adsorption of the diphenylthiocarbazone(dithizone) anion and its redox forms at a silver electrode in aqueous alkaline media. Raman spectra of the adsorbed forms of the dithizone moiety obtained at 100-mV intervals from -1.20 to +1.00 V (vs. Ag/AgCl) are presented. Comparison of these surface spectra with spectra obtained for authentic samples of the anion, silver(1) dithizonate, and the two oxidation products (tetrazolium and disulfide compounds) c o n f i the identity of the surface species at the various applied potentials. The anion, which adsorbs at potentials positive to the potential of zero charge (pzc), is oxidized to the disulfide at potentials greater than 0.00 V, and this disulfide is the predominant species found on the surface in the potential range 0.10-1.00 V. Comparison of vibrational frequencies as a function of applied potential for adsorbed dithizone with authentic silver(1) dithizonate shows that the bonding of Ag to N and S is similar in the two cases. Orientation of surface dithizone can be described approximately.
Introduction Application of the technique of Raman spectroscopy to the study of species at or near electrode surfaces has been the subject of intense investigation since the pioneering efforb of Fleischmann, Hendra, and McQuillanl in 1973. Much of the work has centered on Raman characterization of pyridine adsorbed at a silver electrode surface.2-1* (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. J. Chem. SOC., Chem. Commun. 1973,3,80. 0022-3tmia 1120a5-0248$01.ooio
Jeanmaire and Van Duyne5 and later Albrecht and Creighton4observed that the pyridine/silver spectra were (2) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (3) Fleischmann,M.; Hendra, P. J.; McQuillan, A. J.; Paul, R. L.; b i d , E.S. J. Raman Spectrosc. 1976,4, 269. (4) Albrecht, M. G.; Creighton, J. A. J.Am. Chem. SOC.1977,99,5215. (5) Jeanmaire, D. L.; Van Duyne, R. P. J.Electroanal. Chem. 1977, 84, 1. (6) Albrecht, M. G.; Evans, J. F.; Creighton, J. A. Surf. Sci. 1978, 75, Lll7.
0 1981 American Chemical Society
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Dithizone Adsorption at Metal Electrodes
anomalously intense, and they estimated that the intensity enchancement for the pyridine surface species was on the order of 105-106 times that which would be expected for an equivalent amount of pyridine in solution. Jeanmaire and Van Duyne further showed that this enhancement was general for several nonsymmetric N-containing heteroatom species at a silver electrode. Other adsorbate systems characterized by Raman spectroscopy at electrode surfaces have included Hg2C12and Hg2Br2on a mercury electrode,l thiocyanate,12J3cyanide,14C02,15carbonate,14J5and formate16species on silver electrodes, isomeric cyanopyridines on a silver electrode,16pyridine on a copper carbon monoxidela and i ~ d i n e species ~ ~ l ~ on ~ platinum electrodes, corrosion products on an iron electrode,21 methylene blue on a tin oxide electrodeF2 and p-nitrosodimethylaniline on silver and platinum electrode^.^^ In much of the published work, changes in the applied potential of the electrode have resulted in changes in the Raman spectra observed for the surface species. Most of the observed changes have involved gross changes in the overall intensity of the Raman signal, but in several cases33 the relative intensities of various Raman bands have changed significantly with changes in potential. Jeanmaire and Van Duyne5 showed that the applied potential at which the observed intensity reaches a maximum in the pyridine/silver spectra is different for different bands. All of these results indicate the importance of electrode potential in the observed Raman spectra from adsorbed species. The major thrust of this work was to use Raman spectroscopy to probe the structural changes that occur upon adsorption of the anionic form of diphenylthiocarbazone (dithizone, HDz-, I) and its electrooxidation products at
-S _ _ _ _ _ _ _ H
(OtNl=N2-C=N-N
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4
Ia S
ll
H
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a silver electrode surface and to study the effects that (7)Creighton, J. A.;Albrecht, M. G.; Hester, R. E.; Matthew, J. A. D. Chem. Phys. Lett. 1978,55,55. (8)Marinyuk, V.V.; Lazorenko-Manevich, R. M.; Kolotyrkin, Y. M. Elektrokhimiya 1978,14,1747. (9)Pettinger, B.; Wenning, U.; Kolb, D. M. Ber. Bunsenges. Phys. Chem. 1978,82,1326. (10)Birke, R.L.; Lombardi, J. R.; Gersten, J. I. Phys. Rev. Lett. 1979, 43, 71. (11)Pettinger, B.; Wenning, U.; Wetzel, H. Chem. Phys. Lett. 1979, 67. , 192. (12)Cooney, R. P.; Reid, E. S.; Fleischmann, M. J. Chem. Soc., Faraday Trans. I 1977,1691. (13)Gold, H. S.;Buck, R. P. J.Raman Spectrosc. 1979,8,323. (14)Otto, A. Surf. Sci. 1978,75, L392. (15)McQuillan, A. J.;Hendra, P. J.; Fleischmann, M. J. Electroanal. Chem. 1975,65,933. (16)Allen, C. S.;Van Duyne, R. P. Chem. Phys. Lett. 1979,63,455. (17)Paul. R. L.: McQuillan. A. J.:' Hendra.. P. J.:, Fleischmann. M. J. Electroanal.' Chem. 196,66, 248. (18)Cooney, R. P.;Fleischmann, M.; Hendra, P. J. J. Chem. SOC., Chem. Commun. 1977. 7. 235. (19)Cooney, R. P.; Reid, E. S.; Hendra, P. J.; Fleischmann, M. J.Am. Chem. SOC.1977,99,2002. (20)Cooney, R.P.;Hendra, P. J.; Fleischmann, M. J. Raman Spectrosoc. 1977,6,264. (21)Thibeau, R. J.; Brown, C. W.; Heidersbach, R. H. Appl. Spectrosc. 1978,32,532. (22)Fujihira, M.; Osa, T. J. Am. Chem. SOC.1976,98,7850. (23) Hagen, G.; Glavaski, B. S.; Yeager, E. J.Electroanal. Chem. 1978, 88,269. ~~~
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Figure 1. Schematic dngram of experimental setup for surface Raman spectroelectrochemlstry: M = mirror, P = prism, L = lens, F = laser line interference filter, CL = collection lens.
applied electrode potential had on the observed spectra. Dithizone anion was chosen as the probe molecule in this investigation for several reasons. It is known to be an excellent chelating reagent for a majority of the transition metals.24 It readily forms a 1:l complex with silver ion and, hence, would be expected to interact strongly with the silver metal surface. Dithuone anion is highly colored and is strongly absorbing in the regions of the spectrum accessible with an argon ion laser.25 Thus, by studying the adsorption of HDz- at a silver electrode, one may take advantage of resonance intensity enhancement as well as the surface enhancement that is expected for this heteroatom system a t a silver surface. Raman spectroscopic evidence for the adsorption of the dithizone anion at a silver electrode surface under controlled-potential conditions is presented here and is correlated with the electrochemical behavior of the HDz- species as reported in part 1of this series. This report details the structural changes that occur at the molecular level for the particular adsorbate species existing at the silver electrode surface at different applied potentials. The spectra for the surface forms of HDz- are compared to spectra for authentic samples of HDz- in solution, silver(1) dithizonate metal complex, and the two oxidation products of HDz-. The extent of adsorption at a given potential and changes in the nature of the surface species that occur as a function of electrode potential are studied. The effect of solubility upon the adsorption of HDz- and its redox products is probed by varying the pH of the aqueous alkaline supporting electrolyte system. Experimental Section Spectroscopic and Electrochemical Equipment. The Raman system employed in this study consisted of a Jarrell-Ash Model 25-500 double monochromator used in conjunction with a Princeton Applied Research Model 1121 amplifier/discriminator and a Princeton Applied Research Model 1112 photon counter/processor which was coupled to a Hewlett-Packard recorder. The detector was a Hamamatsu R928 PMT with an extended-red multialkali cathode which was thermoelectrically cooled to 40 "C below the ambient temperature (--20 "C)with a Pacific Photometric PMT housing. The laser used as an excitation source was a Coherent Radiation CR-12 argon ion (24)Irving, H.M. N. H. "Dithizone"; The Chemical Society, Burlington House: London, 1977. (25) Meriwether, L. S.; Breitner, E. C.; Sloan, C. L. J. Am. Chem. SOC. 1966,87,4441.
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TO MONOC HR&
iH?EE Ag ELECTRODE
COLLECTION LENS
CELL
Figure 2. Oblique backscattering geometry employed to obtain Raman spectra from electrode surface.
laser. Laser powers were measured with a Coherent Model 210 broad-band power meter. Laser line interference filters were employed in all cases to remove stray emission lines from the laser output. The Raman spectra were recorded a t a band-pass of 5-6 cm-l with a scan rate of 1.0 cm-l/s and a 2-s RC time constant unless otherwise specified. The plots of Raman intensity vs. applied potential for the individual peaks were obtained by using a larger band-pass of -10 cm-' to minimize the effects of band-maxima shifts with changes in electrode potential on the observed intensity responses. Electrode potentials were controlled by using a system of in-house design and construction consisting of a conventional three-electrode potentiostatB and a triangle wave generator.27 A schematic of the entire Raman spectroelectrochemical system employed in this work is shown in Figure 1. The Raman spectroelectrochemical cell used is a sandwich-type design similar to that reported by Hawkridge, Pemberton, and Blount.% Their design has been altered to allow for spinning of the electrode to minimize effects of thermal desorption and decomposition of the surface species by the laser beam. The position of the electrode surface relative to the flat front glass plate of the cell can be varied to allow different thicknesses of the solution to be sampled by the laser beam. The details of this cell will be reported elsewhere.29 In the work reported here, the electrode was separated from the front glass plate by a thin layer of solution such that a thin-layer configuration existed at the electrode surface. Hence, only a small amount of solution was sampled by the laser beam, and Raman scattering from the dithizone anion in solution is expected to contribute little to the observed surface spectra. The laser beam was focused off-center on the electrode surface at a grazing incidence of -60'. The off-center focusing of the beam ensured that, upon rotation of the electrode, continuously different regions of the surface were being excited by the laser beam. Therefore, the spectra obtained truly represent the average spectra of the surface species. Scattered radiation was collected in the oblique backscattering geometry shown in Figure 2 where the Raman scattered radiation is collected at an angle of 90" with respect to the plane of the electrode surface. The laser beam was focused either to a spot on the surface by using a conventional lens or to a line by using a cylindrical lens. Due to the efficiency of the system used for this work, little (26) Mathis, D. E. Ph.D. Dissertation, University of North Carolina, Chapel Hill, NC, 1978. (27) Woodward, W. S.;Rocklii, R. D.; Murray, R. W. Chem., Biomed. Environ. Instrum. 1979, 9,95. (28) Hawkridge, F. M.; Pemberton, J. E.; Blount, H. N. Anal. Chem. 1977,46,1646. (29) Pemberton, J. E.;Buck, R. P., manuscript in preparation.
difference in the spectra was observed for the two focusing geometries. The incident power of the 488.0-nm argon ion laser line at the cell was -200 mW for the surface spectra presented in this report, although spectra of comparable quality from the surface can be obtained with much less power. The silver working electrode was made from -100 mesh silver powder (Alfa, 99.999%) which had been pressed into a pellet under 1.5 metric tons (-102 000 psi) of pressure for 30 min in a stainless-steel die. This pellet was epoxied into a Teflon shroud using TORSeal (Varian) high-vacuum epoxy. The enshrouded silver electrode was then screwed onto the brass contact of the electrode glass shaft. Electrical contact between the silver pellet and the brass was made with a small spring made of copper wire. The electrode was mechanically polished with successively finer grades of silicon carbide paper, followed by polishing with successively finer grades of diamond paste (Metadi, Buehler) down to 1 pm. The electrode was rinsed with deaerated distilled water and 1 M HCIOl and, finally, copious amounts of distilled water. When introduced into solution, the electrode was potentiostatted at -0.70 V for 5 min prior to spectral data acquisition to remove any residual surface oxides. The resulting surface had a mirror finish and upon visual inspection did not appear different from a silver foil surface. Ac impedance measurements made in this laboratory on similar pressed silver pellet electrodes polished with 1-pm diamond paste gave results comparable to those obtained with silver foil electrodes polished with 1-pm diamond paste.3o This result indicates electrochemicalsimilarity of the surfaces at the microscopic level. No differences are apparent between surface Raman spectra acquired from the pressed pellet electrodes and those acquired from silver foil electrodes. The counter electrode in this system consisted of a Pt wire mounted directly into the body of the spectroelectrochemical cell. The reference electrode used was a AglAgCl half-cell which contacted the test solution through a salt bridge containing saturated KC1 solution. Materials and Reagents. Dithizone (Fisher Scientific) was purified by Soxhlet extraction with ether and precipitation from chloroform with ethanol after the manner of Billman and Cleland.31 The purified product was periodically tested for purity by using TLC on silica gel plates (Eastman) with benzene as an eluent32and repurified as necessary. The pH 10 and 12 aqueous alkaline solutions were prepared from stock solutions of 50% NaOH and distilled water which had been previously boiled for 20 min to remove carbon dioxide. Once prepared, these solutions were stored under Ascarite. The pH of the solutions was checked before each day's experiments. All solutions were deaerated by bubbling with prepurified nitrogen for 30 min. Silver(1) dithizonate, Ag(HDz).H,O, was prepared by dissolution of 0.68 g of AgN03in 100 mL of distilled water, and the pH was adjusted to the optimum value of 133by the addition of dilute nitric acid. This was vigorously shaken with a solution of 0.9 g of purified dithizone in 200 mL of chloroform for 1min. The color of the chloroform layer changed from dark green to reddish orange almost immediately upon introduction of the aqueous silver salt. The chloroform layer was separated and the reddish orange Ag(HDz).H20 was collected by evaporation of the solvent.
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(30) Mathis, D. E.;Rhodes, R. K.; Buck, R. P., unpublished results. (3i) Billman, J. H.; Cleland, E. S. J.Am. Chem. SOC.1943,65,1300. (32) King, H. G.C.; Pruden, G. Analyst (London) 1971,96, 146. (33) Freiser, H.Chemist-Analyst 1961,50, 62.
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Figure 3. Raman spectrum of loe3 M HDz- In pH 12 solution. Conditions: 488.0-nm excitatlon: 200-mW power at sample: 5-cm-l band-pass: 1.0 cm-'/s scan rate; 2.0-9 RC time constant: 0.5-s counting Interval.
The final product was obtained by recrystallization from chloroform. The product was confirmed to be Ag(HDz).H20 by its visible absorption spectrum in chloroform which showed a maximum of 465 nm (lit.% 470 nm). Bis( 1,5-diphenylformazan-3-yl) disulfide (11)was synthesized after the manner of Kiwan and Irving.34 This synthesis involves the controlled oxidation of dithizone by iodine in chloroform solution. The disulfide product was isolated by chromatographing the reaction mixture on a silica column with ether as the eluent. The disulfide is present in the first fast-moving red fraction. The ether was evaporated off, and the residue recrystallized from cyclohexane to give a deep red solid. The visible absorption maximum of this species in cyclohexane was 417 nm (lit.36415 nm). 2,3-Diphenyl-2,3-dihydrotetrazolium-5-thiolate (111) was synthesized according to Irving et al. by oxidation of dithizone with ferricyanide.% Dithizone (1g) was dissolved in 300 mL of chloroform, and 3.2 g of K3Fe(CN)6and 3 g of potassium carbonate were dissolved in 100 mL of distilled water. The two solutions were mechanically stirred together for 6 h, after which the dark green chloroform solution had turned to orange. Separation of the two phases and evaporation of the chloroform left a residue of the tetrazolium species which was recrystallized from hot ethanol to yield bright orange-red crystals. The visible absorption spectrum of this compound in chloroform exhibited an absorption maximum at 465 nm (lit.%468 nm). All other materials were reagent grade or equivalent. Spectral Procedures. The Raman spectra of solid samples were obtained on a spinning Kl3r pellet of the material in a backscattering configuration. Colored solution samples were pumped through a glass tube of small diameter
TABLE I: Raman Spectral Bands and Assignments for lom3 M Dithizone Anion in DH 12 Solution frequency," re1 cm-1 intensitvb assignmentc 210 W 251 m W 305 402 W 427 W 481 m 585 m v(CS) 743 W v(CN) + 6(NH) 838 w 4CS) 982 m v(ring) 1109 S 6 ,v(NCS) 6 ,u(NCS) 1146 m 1162 m v(C,H,-N=) 1198 S 6 ,v(NCS) W 1244 1297 S v(C,H,-NH) 1369 vs vsynl(N= C-N 1 1417 m v(CN) + u ( C S ) 1458 m u(N=N) 1501 S u m(N=C-N) 1588 S v F = c Iring 1625 W u(C=N) a Relative to the 488.0-nm argon ion laser excitation l i e . Measurement uncertainty (standard deviation) = i 2 cm-'. vw = very weak; w = weak; m = medium; s = strong; vs = very strong. u = stretch; 6 = deformation.
(34) Kiwan, A. M.; Irving, H. M. N. H.J. Chem. SOC.E 1971, 901. (35) Irving, H. M. N. H.; Kiwan, A. M.; Rupainwar, D. C.; Sahota, S. S.Anal. Chim. Acta 1971,56, 205. (36) Kiwan, A. M.; Irving, H. M. N. H.J. Chem. SOC. E 1971 898.
Results and Discussion p H 12 Behavior. The resonance Raman spectrum of the dithizone anion (I) in pH 12 solution is shown in Figure
by a peristaltic pump (Gilson) to avoid thermal and photochemical decomposition of the sample. Resonance Raman scattered radiation was collected at right angles to the circulating solution which was excited from above by the laser beam.
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formazans, thioureas, and thioamides. The bands at 982 3, and the major bands of the spectrum are compiled in and 1588 cm-l correspond to the ring “breathing” and Table I. This spectrum was obtained by using an exciv(C=C) modes of the monosubstituted phenyl moiety.42 tation wavelength of 488.0 nm. The absorption maximum The medium-intensity band a t 585 cm-l is attributed of an aqueous alkaline solution of dithizone anion is 470 mainly to the v(CS) mode of the anion. It is expected that nm.24 The 488.0-nm excitation is within the strong abthe sulfur atom, which bears the majority of the negative sorption region of the dithizone anion and therefore gives charge, is engaged in a significant hydrogen-bonding inrise to resonance Raman scattering. To the authors’ teraction with the H atom on N4 (see structure I). This knowledge, this represents the first reported vibrational bonding would decrease the double-bond character of the spectrum of the dithizone anion in solution (Le., not comC-S bond (i.e., lessen the contribution of resonance plexed with a metal). In previous IR spectroscopic studies, structure Ib) and would lower the observed frequency for the spectrum of the neutral form of dithizone has been the C-S stretch. According to the IR and Raman work compared with that of the metal dithizonates in which the reported by Ritchie, Spedding, and Steele on substituted ligand is actually the dithizone Such a comparison may not be entirely valid, because it is ~ ~ o w I Ithioureas,43 ~ ~ ~ this ~ mode also makes a contribution to the very weak band observed at 838 cm-l. The medium-intensity that the structures of the neutral and anionic forms of band at 1417 cm-l may be assigned to the v(CN) mode with dithizone are significantly different. It has been reported minor contributions from the v(CS) vibration. Aitken, on the basis of proton NMR studies of neutral dithizone Duncan, and McQuillanu assigned a similar band at 1415 solutions that the two protons in dithizone are in equivcm-l in the IR spectrum of thiourea to the v(CN) mode. alent environments,4Oand structure IV was suggested for The C-N stretching mode of the dithizone anion also makes a contribution to the weak band observed at 743 cm-l, as is characteristic of similar thioamidic struct u r e ~ .The ~ ~weak ~ ~band ~ a t 1625 cm-l can be assigned to the u(C=N) mode of the dithizone anion by analogy with substituted hydrazone system^.^^?^ lv Meriwether, Breitner, and C01thup~~ assigned the complex series of bands between 1100 and 1200 cm-l for Hgneutral dithizone. The vibrational spectrum of this species (HDz), to various N-C-S vibrational modes of the dithiis expected to be significantly different from that of the zone ligand. The bands of medium and strong intensity metal-complexed anionic form of dithizone, structure V. in the Raman spectrum of the dithizone anion a t 1109, Ph 1146, and 1198 cm-l may be similarly assigned to N-C-S \N-H, modes. The types of vibrational behavior that are expected are the symmetric and asymmetric N-C-S stretches and I ‘! Ph deformations of the N-C-S moiety. No attempt has been made here to distinguish between these different vibrations because of the complexity of such a task. Therefore, the N-C-S vibrations are collectively represented in Table I as G,v(NCS) modes. In the anionic form of dithizone, the two c6H5-N bonds are in different environments. Hence, two distinct C6&N H-N \ stretches should be evident in the Raman spectrum. One Ph of these C6Hb-N bonds is found in an environment similar to that of azobenzene, while the second is in an environV ment like that of a substituted aniline. Using resonance Raman spectroscopy is readily applied to the study of Raman spectroscopy to study a series of azobenzenes, aqueous systems because of their transparency with respect Hacker47has shown that the C6H5-N stretching mode is to Raman scattering, while in IR spectroscopy these generally observed in the frequency region of 1145-1170 aqueous systems are essentially opaque. In the case of cm-l. By analogy with these azobenzene systems, the band aqueous dithizone systems, our results allow a comparison of medium intensity at 1162 cm-’ in the dithizone anion to be made between the Raman spectra of the uncomspectrum can be assigned to the v(C6H6-N=) mode. In plexed anion and the complexed anion, and, hence, effects the vibrational spectra of substituted anilines, the vof complexation of the anion to a metal, which are evident (C6H5-”-) mode appears in the frequency region of in the vibrational behavior, can readily be interpreted. 1250-1315 cm-1.48 Hence, the strong band at 1297 cm-l Vibrational evidence for differences in the structures of can be assigned to the u(C6H5-NH-) mode of the dithizone the various forms of dithizone can also be obtained. Raanion. man spectroscopic investigations of the acid-base forms The band of medium intensity appearing at 1458 cm-l of dithizone in aqueous and nonaqueous media are curmay be assigned to the N=N azo stretch. In the case of rently in progress in this l a b ~ r a t o r y . ~ ~ substituted azobenzenes, Hacker47 has shown that the The vibrational assignments shown in Table I for the dithizone anion have been made on the basis of assign(42) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. “Characteristic Raments reported in the literature for metal dithizonates, and man Frequencies of Organic Compounds”; Wiley: New York, 1974. by comparison of the dithizone structure with the similar (43) Ritchie, R. K.; Spedding, H.; Steel, D. Spectrochim. Acta, Part (37) Fabretti, A. C.; Peyronel, G. J. Inorg. Nucl. Chem. 1976, 37, 603. (38) Dyferman, A. Acta Chem. Scand. 1963,17, 1609. (39) Meriwether, L. S.; Breitner, E. C.; Colthup, N. B. J. Am. Chem. SOC.1965,87,4448. (40) Coleman, R. A.; Foster, W. H.; Kazan, J.; Mason, M. J. Org. Chem. 1970,35, 2039. (41) Pemberton, J. E.; Buck, R. P., manuscript in preparation.
A 1971,27, 1597. (44) Aitken, G. B.; Ducan, J. L.; McQuillan, G . P. J. Chem. SOC.A 1971, 2695. (45) Jenaen, K. A.; Nielsen, P. H. Acta Chem. Scand. 1966, 20, 597. (46) Kitaev, Y. P.; Buzykin, B. I.; Troepol’skaya, T. V. Russ. Chem. Reu. (Engl. Transl.) 1970, 39, 441. (47) Hacker, H. Spectrochim. Acta 1966,21,1989. (48) Tobin, M. C. “Laser Raman Spectroscopy”; Wiley-Interscience: New York, 1971; Chapter 3.
Dithizone Adsorption at Metal Electrodes
N=N stretching mode generally gives rise to a band in the region 1380-1450 cm-'. Resonance structures that can be drawn for the azobenzenes show that the N-N bond possesses some single-bond character which would result in a slight lowering in the frequency of the u(N=N) mode. In the case of the dithizone anion, however, resonance forms in which this N-N bond is a single bond exhibit separation of charge and are not expected to contribute significantly to the overall structure. The N=N bond in dithizone anion has greater double-bond character than that found in the azobenzenes; therefore, the u(N=N) mode of the dithizone anion should, and does, occur at a higher frequency than the same mode of the azobenzenes. The two strong bands at 1369 and 1501cm-' are assigned to the u,,(N=C-N) and u,,(N=C-N) modes, respectively. The assignments are made on the basis of IR and Raman studies on 1,5-diphenylformazan, C6H6-N= N-CH=N-NH-C6H5.4g In this compound, whose structure is closely analogous to the structure of the dithizone anion, the vw(N=C-N) mode is observed at 1378 cm-l, and the corresponding v,,(N=C-N) mode is observed at 1510 cm-l. These stretching modes in 1,5-diphenylformazan are expected to appear at slightly higher frequencies than the corresponding stretching modes in the dithizone anion, because resonance forms in which both C-N bonds are single bonds in the formazan are not expected to contribute much to the structure. However, in the case of the dithizone anion, resonance form Ib has single-bond character in both C-N bonds, and, therefore, the symmetric (1369 cm-l) and asymmetric (1501 cm-l) N=C-N stretching modes are lower in frequency than the corresponding modes in 1,5-diphenylformazan. Figure 4 shows the Raman spectra obtained for a M HDz-/pH 12 solution at a silver electrode surface every 100 mV from -1.20 to +LOO V (vs. Ag/AgCl) and also for the electrode at open circuit. The major Raman bands for the spectra in Figure 4 are listed in Table 11. The silver electrode surface from which these spectra were obtained had been mechanically polished to a mirror finish. No electrochemical roughening procedure to increase the surface area was found necessary to obtain the good quality spectra presented here, as has been reported in the case of pyridine adsorbed at s i l ~ e r . ~ ~ ~ The Raman spectra of the pH 12 background obtained from the silver electrode surface at selected potentials are shown in Figure 5. These spectra do not exhibit the structureless background expected for this aqueous alkaline solution. The thin-layer cell configuration used in these studies in which the cell window is very close to the silver electrode causes interfering bands due to Raman scattering from the cell window to appear in the observed surface spectra. Thus, the broad band at 800 cm-l and the weak broad bands at 1050 and 1200 cm-l in the spectra in Figure 5 are attributed to the cell window. Raman scattering from the window also gives rise to a very strong, rather featureless background from 100 to 500 cm-l and a strong band at 598 cm-' which are not shown in the spectra in Figures 4 and 5. This problem of interference from the cell window of this spectroelectrochemical cell is discussed elsewhere.% It is of importance here to realize that no interfering bands from the cell window appear in the fingerprint region, 900-1600 cm-l. Therefore, the changes with applied electrode potential evident in the fingerprint region in the spectra in Figure 4 are assumed to be due to dithizone anion adsorbate-silver electrode surface interactions. (49) Kukushkina, I. I.; Yurchenko, E. N.; Ermakova, M. I.; Latosh, N. I. Zh.Fzz. Kltirn. 1972, 46, 176.
The Journal of Physical Chemistry, Vol. 85, No. 3, 1981 253
The electrochemical evidence presented in part 160 suggests that the dithizone anion and its redox products are strongly adsorbed at the silver surface. The electrochemical oxidation product of dithizone, bis( 1,5-diphenylformazan-3-yl) disulfide, adsorbs so strongly to the silver surface that the silver is passivated with respect to surface oxidation until potentials in excess of +0.4 V are achieved. The surface Raman spectroelectrochemicaldata shown in Figure 4 further support the adsorption of the dithizone anion and its redox forms at the surface of the silver electrode. Significant changes in the band frequencies and the relative peak intensities between the solution spectra of the dithizone anion (Figure 3) and the silver forms of dithizone are evident. It is assumed that the coverage of the silver surface at this anion concentration does not exceed more than several monolayers of any dithizone surface form. Raman spectroscopy does not generally possess sufficient sensitivity to detect such small quantities of material. Therefore, it appears as if some enhancement mechanism is responsible for the relatively strong scattering intensity observed in this adsorbate system. The dithizone anion is strongly absorbing at the excitation wavelength of 488.0 nm used in this study, and, hence, resonance enhancement of the adsorbed anion is probably a major contribution to the overall enhancement. It is also probable that the mechanism of surface enhancement operative in the case of amines adsorbed at a silver surface5(e.g., pyridine/Ag) occurs in this case as well. However, the contribution that each mechanism makes to the overall enhancement observed in this system is not known. It may be possible to separate these two enhancement effects by systematically changing the excitation wavelength, which will allow the resonance enhancement contribution to be determined. Experiments are currently underway in this laboratory to test this hypothesis. The most striking feature of the spectra in Figure 4 is the change in overall Raman intensity in the fingerprint region with change in the electrode potential. Examination of the fingerprint region in the spectra in Figure 4 at applied electrode potentials negative to the potential of zero charge (pzc) of --0.7 V reported for silver5 reveals no well-defined spectral features. The absence of Raman bands at these potentials suggests that the dithizone anion does not interact strongly with the electrode surface at potentials negative to the pzc in this aqueous pH 12 medium. This result is to be expected on the basis of electrostatic considerations. The extent of interaction of HDzwith the silver appears to increase as the electrode potential is made increasingly more positive with respect to the pzc, as is indicated by the increase in intensity of Raman surface spectra in the fingerprint region as the potential is changed from -0.70 to -0.10 V. As the potential is made more positive than 0.00 V, the intensity of the surface spectra increases even further. Pronounced changes in the band frequencies of the surface spectra suggest that the nature of the surface species has changed at these anodic potentials. Changes in the relative intensities of bands with changes in electrode potential are also evident in the spectra in Figure 4. The bands that change significantly in intensity, relative to other bands in the spectra, are in the frequency range 1340-1410 cm-l. The most drastic change occurs in the band occurring at 1390-1400 cm-l. This band is only of weak to medium intensity at potentials less than 0.00 V, but, at potentials greater than 0.00 V, this band becomes predominant in the (50) Pemberton, J. E.; Buck, R. P., submitted for publication.
214
The Journal of Physlcal Chemistty, Vol. 85, No. 3, 1981
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surface spectra. A shift to higher frequency is also observed for this band when the electrode potential is changed from a value less than 0.00 V to a value greater than 0.00 V. Similar significant (>2 cm-l) shifts are observed with changes in applied potential for several other bands in the spectra as well, as can be seen from the data
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258
The Journal of Physical Chemistry, Vol. 85, No. 3, 1981
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potential is made increasingly more positive. The f i i t step occurs as the potential is made more positive than -4.60 V. At applied potentials of ca. 4.60 to -0.50 V, the surface spectra begin to exhibit significant intensity in the fingerprint region (900-1600 cm-l) of dithizone. The intensities of the surface spectra obtained increase as the applied potential is increased from -0.60 to -0.10 V, but no significant changes in the band frequencies are observed. This suggests that only one surface species exists at the silver electrode in this potential region. The nature of the surface form then appears to change as the potential is made more positive than 0.00 V. This two-step behavior is further borne out by the plots of Raman intensity vs. applied potential shown in Figures 6 and 7. These plots were obtained by observing one band only while continuously changing the potential of the silver
Pemberton and Buck electrode at a rate of 10 mV/s. It was found necessary to change the potential at this slow rate because the kinetics of the surface process appear to be quite slow. Scanning at a faster rate gave Raman intensities which were not truly representative of the surface state of dithizone at the applied potential; i.e., the observed Raman behavior seemed to lag behind the electrode potential at faster rates of potential scan. Detailed investigations of the kinetics of these surface processes are currently under investigation. The plots of Raman intensity vs. applied potential shown in Figure 6 exhibit two distinct steps of increasing intensity with potential. These data were acquired with a large spectrometer band-pass (10 cm-l) to ensure that the effects of band shifts on the observed intensity were minimal. The intensity-potential behavior was reproducible in all cases but was reversible only when the applied potential was kept more negative than 0.00 V. The Raman intensity-potential plots of the silver surface in pH 12 background electrolyte were essentially flat over the entire potential range studied. This suggests that any surface oxides which form contribute little to the observed surface spectra. Classical electrosorption studies51have indicated that the physical property used as an indicator of adsorption coverage should change in a Gaussian manner with changes in potential. This bell-shaped behavior has, in fact, been observed by Jeanmaire and Van D u p e 6 in their Raman spectroelectrochemical investigation of the adsorption of pyridine at a silver electrode. However, this expected behavior is not observed in the case reported here of dithizone at a silver electrode. In this case, the intensity is seen to increase (step 1) as the potential is made more positive up to -4.30 V. The intensity levels off at this point for several hundred millivolts until the second process (step 2) begins to occur at potentials greater than 0.00 V, causing a second increase in the Raman intensity. The leveling off of the Raman intensity around -0.20 V suggests that the expected Gaussian behavior of the intensity-potential plots would be obtained for the first surface process in the absence of the second surface process. The intensity-potential behavior observed for the different Raman bands is not identical. The interpretation of this two-step behavior in the surface Raman spectra, obtained at different electrode potentials, becomes evident after consideration of the electrooxidation behavior of the dithizone anion at silver in aqueous alkaline media. The cyclic voltammetric behavior of the dithizone anion a t silver indicates that no redox processes occur in the potential range between -0.7 and -0.1 V. It is within this potential range that the first step in Raman intensity is observed. The redox behavior in this potential range suggests that this intensity step occurs as a result of electrostatic adsorption of HDz- as the electrode potential is made positive with respect to the pzc. This anion adsorption appears to level off, probably due to saturation coverage, at -4.20 V. Therefore, in the absence of the second surface processes, the maximum coverage of the silver electrode surface by the dithizone anion as indicated by the Raman intensity occurs at -0.20 V. The intensity would be expected to fall off in a Gaussian manner on either side of this maximum. Note that this maximum is positive with respect to the pzc, which is expected for anion adsorption. The second step in the observed Raman intensity begins at -0.00 V. Significant changes in the band frequencies also occur at -0.00 V, suggesting a change in the nature
-
(51) Piersma, B. J. In “Electrosorption”: Gileadi, E., Ed.: Plenum Press: New York, 1967; Chapter 2.
The Journal of Physical Chemistry, Vol. 85, No. 3, 1981 257
Dlthizone Adsorption at Metal Electrodes
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of the adsorbed species. The cyclic voltammetric behavior of the dithizone anion at silver indicates that electrooxidation of the anion begins at -0.00 V in pH 12 solution. Oxidation of the dithizone anions2can occur in two steps.
The first is oxidation of the anion to form the corresponding disulfide compound, bis( 1,5-diphenylformazan(52) Torncsanyi, L. Anal. Chin. Acta 1977,88, 371.
258
The Journal of Physical Chemisfty, Vol. 85, No. 3, 1981
3-yl) disulfide (11). This first oxidation can be carried out
Pemberton and Buck
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