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Thin layer electrochemical Raman study of ion pair formation between the tetrachlorobenzoquinone anion radical and alkaline earth metal cations
Journal of Electroanalytical Chemistry 458 (1998) 191 – 198
Thin layer electrochemical Raman study of ion pair formation between the tetrachlorobenzoquinone anion radical and alkaline earth metal cations Mikito Yamanuki, Toyomasa Hoshino, Munetaka Oyama, Satoshi Okazaki * Department of Material Chemistry, Graduate School of Engineering, Kyoto Uni6ersity, Sakyo-ku, Kyoto 606 -8501, Japan Received 23 June 1998; received in revised form 18 August 1998
Abstract Raman spectroscopy combined with electrochemical measurement has been proved to be effective in the elucidation of complex voltammetric responses. In the reduction of 2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ), complex voltammograms were observed in the presence of alkaline earth metal ions (M2 + ) due to ion pair formation. By using thin layer electrochemical Raman (TLER) measurements synchronized with the reduction of TCQ in the presence of Mg2 + , it was found that an intermediate ion pair, Mg2 + TCQ − , was reduced to an ion pair involving the TCQ dianion, Mg2 + TCQ2 − . In contrast, for the case of Ba2 + , the anion radical salt (Ba2 + TCQ − )2 was found to deposit on the electrode surface, which was confirmed through the Raman measurement of the salts of reduced species produced by a flow-electrolysis synthetic method. The electrochemical behavior was dependent on the metal cations, M2 + . The intermediate ion pair, M2 + TCQ − , may either accept another electron or precipitate on the electrode surface. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Raman spectroscopy; Thin layer electrochemical measurement; Ion pair formation; Reduction of tetrachlorobenzoquinone
1. Introduction Raman spectroscopy is one of the most powerful methods for the observation of structural information of electrogenerated species. In previous papers, we have reported the resonance Raman (RR) measurement of electrogenerated species in solution using a columnelectrolytic continuous-flow method [1 – 3]. With this technique, the RR spectra of electrogenerated species could be successfully observed in controlled electrolysis conditions, and observations were made of the differences in the RR spectra between the cation radical and the dication of N, N%-dimethyl-N, N%-diphenylbenzidine [1], and between the cation radicals and the anion radicals of anthracene derivatives [2,3]. * Corresponding author. Tel: + 81-75-753-5882; fax: +81-75-7534718; e-mail: [email protected].
While the RR measurement of electrogenerated species in solution has become very easy with the columnelectrolysis continuous-flow method, it is still important to observe the electrochemical events directly in the vicinity of electrode surfaces. The specific structural features will become much clearer with the help of the measurements of vibrational structures in homogeneous solution and in the presence of the electrode surface. Van Duyne et al. initiated the methodological approaches to the RR measurement of the electrogenerated species in the vicinity of the electrode, as reported previously [4–7]. Both controlled potential coulometric electrolysis in bulk solution [4–6] and cyclic potential step electrolysis under semi-infinite diffusion mass transfer conditions [4,7] have been proposed as electrogeneration methods. The RR spectra of stable electrogenerated species from reversible redox systems were observed successfully in these pioneering works. How-
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 3 5 1 - 9
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ever, these methods are difficult to apply to the analysis of the complex electrode reaction systems which show non-reversible electrochemical responses due to following chemical reactions or interfacial reactions near the electrode. Thus, the improvement of the methods is still desirable for the Raman measurement of electrochemical events. Recently, RR scattering and absorption spectroscopies were applied to elucidate one- and two-electron oxidation processes of 3,3%,5,5%-tetramethylbenzidine in organic solvents, and consequently, the RR spectra of a charge transfer complex and the dication were characterized [8]. As shown in this work, a promising approach is to apply Raman spectroscopy in a thin layer configuration. By reducing the thickness of the electrolyzed solution layer to a micrometer level in combination with controlled potential electrolysis, the electrochemical events can be observed in the diffusion layer in the steady state. Therefore, we have developed a thin layer electrochemical resonance Raman (TLERR) method to analyze electrode reactions, as reported previously in a communication [9]. Raman measurement using a thin layer spectroelectrochemical cell has an advantage that the species present only in the vicinity of the solution electrode interface can be monitored, so that electrochemical reactions and chemical reactions in the diffusion layer can be directly observed. In the present paper, the useful features of the thin layer electrochemical Raman (TLER) method are presented for the analysis of complex electrochemical reactions. For the ion pair formation between 2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ; chloranil) anion radical (TCQ − ) and Mg2 + , the reaction kinetics were analyzed in homogeneous solution of acetonitrile (AN) by using a pulse-electrolysis stopped-flow method [10]. In the present work, to investigate this type of electrochemical reaction process concerning ion pair formation in detail, we have observed the Raman spectra in the vicinity of the electrode by using the TLER method. The TLER measurements were carried out by applying a negative potential to form TCQ − in the presence of alkaline earth metal cations (M2 + =Mg2+, Ca2+, Sr2+, Ba2 + ).
Fig. 1 shows a schematic view of the cell. The electrode surface is positioned to face a quartz window. The electrode used was a glassy carbon disk electrode mounted in PEEK resin (BAS). The full diameter of the electrode body was 6.0 mm and the diameter of glassy carbon was 3.0 mm. To achieve thin layer conditions, the distance from the electrode surface to the quartz window (diameter 20 mm) was adjusted to be 25 mm by placing a Teflon gasket (thickness 25 mm) which has an oval-shaped hole between the electrode surface and the window. When the cell was filled with a sample solution, the oval area became the effective thin solution layer for electrolysis and Raman measurement. The electrolysis and Raman measurements were carried out for a stationary thin layer solution. The solution was replaced with the flow through method after each measurement. Controlled potential electrolysis was performed with a three electrode system; a Pt wire counter electrode was wound around the working electrode, and a Pt (I − , I3− ) reference electrode was inserted close to the working electrode. In the Raman measurement, the electrochemical cell was set up so that its window faced the entrance slit of the spectrophotometer. The alignment of light collection for TLER measurement is shown in Fig. 2. The argon ion laser line of 488.0 nm for excitation was irradiated on the electrode surface from the side with an incident angle of 25° to the plane of the window. The laser power was 200 mW in all the measurements.
2. Experimental
2.1. Apparatus and method For the TLER measurement, we initially constructed a cell in which solution penetrated between the electrode surface and the quartz window [9]. However, to control the thickness of the thin layer, we constructed a new type cell in the present work.
Fig. 1. Schematic view of TLER cell. (A) Side view, (B) top view. WE, working electrode; CE, counter electrode; RE, reference electrode; SM, spectrophotometer; a, quartz window; b, solution in; c, solution out; d, solution; e, surface of glassy carbon working electrode (diameter, 3.0 mm); f, body of working electrode (diameter, 6.0 mm); g, Teflon gasket whose thickness is 25 mm.
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3. Results and discussion
3.1. Measurement of Raman spectra of sol6ent AN with TLER cell
Fig. 2. Alignment of light collection for TLER measurement. C, TLER cell; WE, working electrode; CE, counter electrode; RE, reference electrode; PS, potentiostat; LS, argon ion laser; SM, spectrophotometer; M, mirror; L1, collection lens for laser light whose focal distance is 8.0 cm; L2, collection lens for scattered light whose focal distance is 3.5 cm; L3, collection lens for the light to the spectrophotometer whose focal distance is 17.0 cm; F, notch filter used to reject the laser light; u, incident angle of 25°.
The incident angle of 25° was chosen to collect the scattering light from the solution layer effectively in the present cell configuration. Two collection lenses and a holographic notch plus filter (Kaiser Optical System) to reject Rayleigh light were placed between the cell and the spectrophotometer. The detection system with a multi-channel detector was described previously [1]. Because the current response was observed to decrease in a very short time of less than 1 s, thin layer conditions were assumed to be achieved in this cell. The Raman signal was accumulated for 3 min after the potential step, i.e. the measurement was carried out for a steady state condition of the electrode surface after applying the potential. The details of the pulse-electrolysis stopped-flow method have been described previously [10,11]. The cyclic voltammograms (CV) were measured using a PAR 174 analyzer.
To evaluate the collection efficiency of Raman scattering light in the present thin layer cell, we observed the Raman spectra of AN, which was used as a solvent. Fig. 3 shows Raman spectra of AN measured with the previous column-electrolysis continuous-flow cell and the present TLER cell. The ratio of the intensities of the two spectra was ca. 1:6 for the TLER cell versus the continuous-flow cell. The light path of the capillary cell perpendicular to the collection lens is 1.4 mm. On the other hand, the thickness of the thin layer is 25 mm, and the length of the oval area perpendicular to the lens is 3.0 mm. Although the method and the efficiency for light collection are different in the two configurations, a spectrum of good quality was observed as shown in this figure, even in the TLER cell with very thin solution layer of 25 mm thickness. This indicates that the present thin layer cell is suitable for the measurement of the electrolyzed solutions.
3.2. Reduction of TCQ in AN and the absorption spectra of TCQ − and TCQ 2 − As a recent paper described the heterogeneous rate constant and the formal potential of TCQ [13], the electrochemical reduction of TCQ in organic solvents has been studied extensively [14–16]. For the reduction of TCQ in AN, two reversible waves were observed in the cyclic voltammogram (as later shown in Fig. 6A Fig. 7A), which are attributed to the formation of TCQ − and TCQ dianion (TCQ2 − ), respectively. TCQ+e − TCQ − TCQ + e TCQ −
−
2−
E= − 0.12 V
(1)
E= − 0.88 V
(2)
2.2. Reagents 2,3,5,6-Tetrachloro-1,4-benzoquinone (Nacalai tesque., GR grade) was used as received. Mg(ClO4)2 and Ba(ClO4)2 (Aldrich), as a source of M2 + , were also used as received. Ca (ClO4)2 4H2O and Sr(ClO4)2 (Mitsuwa Chemicals) were used after drying carefully in a vacuum oven. The purification methods of AN and tetrabutylammonium perchlorate (TBAP) were described previously [11]. The K + TCQ − salt was prepared as described in the literature [12].
Fig. 3. Raman spectra of acetonitrile measured with column-electrolysis continuous-flow cell (A) and TLER cell (B). Gate time, 100 ms; accumulation, 50 times; excitation wavelength, 488.0 nm.
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Fig. 4. Absorption spectra of TCQ − (A) and TCQ2 − (B) in AN measured with a pulse-electrolysis stopped-flow method. [TCQ], 1.0 mM; [TBAP], 0.1 M; optical light path, 0.20 cm. Arrows indicate the excitation wavelength of 488.0 nm.
Therefore, as reported previously [17], the absorption spectra of both reduced species could be observed easily with the column-electroanalysis method, in particular, in their free state, i.e. without the effect of the ion pair formation. The results are shown in Fig. 4. The applied potentials for generating the solutions of TCQ – and TCQ2 − were −0.5 V and −1.2 V (vs. I − /I3− ), respectively. Although the formation of dianion had been difficult in the conventional electrolysis, the quantitative electrolysis in the column-electrode permits the observation of the absorption spectrum of TCQ2 − . From these spectra, a resonance enhancement can be expected with the excitation by a laser line of 488.0 nm in the case of TCQ − . In addition, for TCQ2 − , a pre-resonance enhancement effect might arise with the same excitation.
peared, and the observed Raman spectrum was that of solvent AN, which was similar to the result at the applied potential of + 0.2 V. Although a pre-resonance effect might have been expected for TCQ2 − on the basis of the absorption spectrum in Fig. 4B, the RR spectrum of TCQ2 − could not be measured with the excitation of 488.0 nm. To confirm the present results of TLER measurement for TCQ reduction, we also carried out column-electrolytic continuous flow measurements [1–3]. The same results were obtained with both methods, which indicates that TCQ − exists in the vicinity of the electrode surface with the same vibrational structure as in homogeneous solution. In addition, the RR spectrum measured for TCQ − was similar to that prepared by dissolving K + TCQ − in acetone [12].
3.4. Electrochemical measurement of the reduction of TCQ in the presence of M 2 + While two reversible waves were observed in the reduction of TCQ in AN, the electrochemical response changes drastically with the addition of alkaline earth metal cations reflecting the ion pair formation on or near the electrode surface. Figs. 6 and 7 show the cyclic voltammograms in the reduction of TCQ with the addition of Mg2 + and Ba2 + , respectively. In the case of Mg2 + , with increasing concentration of the metal ion, the reversible peaks (P1, P2) were gradually diminished and a new irreversible cathodic peak (P3) appeared in the potential region slightly positive to P1. On the other hand, in the case of Ba2 + , adsorptive and desorptive peaks were observed for both reduction and oxidation processes, which were seen with an increase in the concentration of Ba2 + . Similar changes were observed in the voltammetric responses in the presence of Ca2 + and Sr2 + .
3.3. TLER measurement in the reduction of TCQ The TLER measurement was carried out by applying electrode potentials of + 0.2, −0.5 and − 1.2 V. At the potentials of − 0.5 and −1.2 V, TCQ is reduced to TCQ − and TCQ2 − , respectively. In the case of neutral TCQ at the potential of +0.2 V, the observed Raman spectrum on the electrode surface was that of solvent AN as shown in Fig. 5A. This is because no resonance enhancement effect was expected for TCQ with excitation at 488.0 nm. By contrast, with the applied potential at − 0.5 V, several Raman peaks were observed as shown in Fig. 5B. Judging from the applied potential and the absorption spectrum of TCQ − in Fig. 4A, the observed Raman spectrum can be attributed to the RR spectra of TCQ − produced on the electrode surface. Next, the applied potential was changed to −1.2 V. Consequently, all the RR peaks of TCQ − disap-
Fig. 5. Raman spectra in the reduction of TCQ measured with the TLER method. Applied potential: (A) + 0.2 V; (B) − 0.5 V. The Raman spectrum obtained at −1.2 V is similar to the one observed in (A). [TCQ], 5.0 mM; [TBAP], 0.1 M. S denotes the Raman band of solvent AN. * is a plasma line of the laser.
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Fig. 8. Changes in absorbance depending on [Mg2 + ]/[TCQ − ]. () 290 nm; () 449 nm. The solution of TCQ − was produced by electrolyzing the AN solution of 0.50 mM TCQ containing 0.1 M TBAP with the applied potential of − 0.5 V for 10 s, and then it was mixed with the AN solution of Mg2 + .
3.5. Reaction of TCQ − and M 2 + in homogeneous solution
Fig. 6. Changes in cyclic voltammograms of TCQ in AN with the addition of Mg2 + measured with the TLER cell. Scan rate, 100 mV s − 1; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Mg2 + ]: (A) 0 mM; (B) 1.0 mM; (C) 2.0 mM; (D) 3.0 mM. Mg2 + was added by dissolving Mg(ClO4)2.
Because these changes were observed at the potential range of the first reduction wave, a strong ion pair interaction between TCQ − and M2 + was assumed to occur in the electrode solution interface.
To clarify the ion pair interaction in homogeneous solution, the reaction of TCQ − with Mg2 + was analyzed using a pulse-electrolysis stopped-flow method [10]. After mixing TCQ − with Mg2 + , the absorption maxima of TCQ − at 320 and 449 nm (Fig. 4A) diminished and another new peak, at 290 nm, was revealed. Fig. 8 shows the changes in the absorbance at 449 nm of free TCQ − and 290 nm of the product when 0.50 mM TCQ − was mixed with various concentrations of Mg2 + . From this result, the molar ratio of the ion pair formed was determined to be Mg2 + :TCQ − of 1:2. Thus, the following two step interactions can be assumed. That is, TCQ − reacts with Mg2 + as expressed by Eq. (3). TCQ − + Mg2 + Mg2 + TCQ −
(3)
Following this reaction, the ion pair, Mg2 + (TCQ − )2, is formed in homogeneous AN solution (Eq. (4)), Mg2 + TCQ − + TCQ − Mg2 + (TCQ − )2
(4)
or an ion pair involving the dianion, Mg2 + TCQ2 − , is formed through the disproportionation reaction (Eq. (5)). Mg2 + TCQ − + TCQ − Mg2 + TCQ2 − + TCQ
(5)
2+
With other M , the same measurement was carried out to clarify the ion pair formation processes. As a result, the molar ratio of M2 + :TCQ − was found to be 1:2 for all M2 + (M= Mg, Ca, Sr, Ba). The changes in the ion pair formation rate in homogeneous solution were observed depending in the nature of metal cations with the pulse electrolysis stopped-flow. The results will be reported elsewhere.
Fig. 7. Changes in cyclic voltammograms of TCQ in AN with the addition of Ba2 + measured with the TLER cell. Scan rate, 100 mV s − 1; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Ba2 + ]: (A) 0 mM; (B) 1.0 mM; (C) 2.0 mM; (D) 3.0 mM. Ba2 + was added by dissolving Ba(ClO4)2.
3.6. TLER measurement for the reduction of TCQ in the presence of Mg 2 + Next, in order to analyze the ion pair formation reaction in the vicinity of the electrode, we carried out
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the TLER measurement for the reduction of 5.0 mM TCQ in AN in the presence of various concentrations of Mg2 + . The applied potential was −0.5 V, which was similar to the TLERR measurement of TCQ − as in Fig. 5. In the presence of Mg2 + , once the ion pairs were formed in solution, the absorption spectrum changed drastically from that of free TCQ − . Therefore, with excitation of 488 nm, the resonance enhancement effect was expected only for free TCQ − . Fig. 9 shows the changes in Raman spectra measured in the reduction of TCQ depending on the concentration of Mg2 + . With increasing concentration of Mg2 + , the RR peaks of free TCQ − decreased as shown in Fig. 9B, C. Thus, the amount of free TCQ − is assumed to diminish through the ion pair formation on the electrode surface as well as in homogeneous solution; i.e. free TCQ − was converted into an ion pair which gave no resonance enhancement effect with the excitation of 488.0 nm. However, the decrease of free TCQ − in the ion pair formation process measured with the TLER method was remarkably different from the results in homogeneous solution as shown in Fig. 8. Fig. 10 summarizes the changes in the RR signals as a function of the molar ratio of [Mg2 + ]/[TCQ]. While the reaction molar ratio of M2 + :TCQ − was 1:2 in homogeneous solution, the decrease of the intensity of RR bands depending on the molar ratio was quite different; i.e. from Fig. 10, the stoichiometry of Mg2 + :TCQ was determined as 1:1. To explain this remarkable difference, the most probable pathway is further reduction of Mg2 + TCQ − once produced on the electrode surface. Judging from the fact that 1 mole of Mg2 + was consumed for each
Fig. 10. Changes in intensity of RR peaks of TCQ − depending on [Mg2 + ]/[TCQ]. () 1019 cm − 1; () 1086 cm − 1. The Raman intensity was normalized using the solvent Raman band at 920 cm − 1, and expressed as a relative value to the result without addition of Mg2 + .
mole of TCQ in the thin layer cell, the electron transfer of Eq. (6) is assumed to proceed at the electrode surface after the formation of Mg2 + TCQ − (Eq. (3)). Mg2 + TCQ − + e − Mg2 + TCQ2 −
(6)
The intermediate ion pair, Mg2 + TCQ − , seems to accept a negative charge and reach electrical neutrality, which is similar to the result in homogeneous solution where Mg2 + TCQ − was not detected in the absorption measurement. This hypothesis is in good agreement with the electrochemical measurements in Fig. 6. That is, the electrode reaction processes can be presented by the following ECE mechanism as summarized by the three equations, Eqs. (1), (3) and (6). The appearance of P3 can be attributed to the reduction of Mg2 + TCQ − . The irreversibility in the CVs is inferred to result from the chemical step of Eq. (3). As the reduction potential of Mg2 + TCQ − was positive of that of TCQ, the electron transfer of Eq. (6) is so thermodynamically favorable that it proceeds soon after the formation of TCQ − . In addition, by taking account of the above relationship of the reduction potentials and by comparing with the reduction mechanisms on the electrode surface, the disproportionation reaction of Eq. (5) seems to be a probable reaction pathway in homogeneous solution.
3.7. TLER measurement for the reduction of TCQ in the presence of Ba 2 +
Fig. 9. Raman spectra observed in the reduction of TCQ in AN with the addition of Mg2 + measured using the TLER method. Applied potential, −0.5 V vs I3− , I − ; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Mg2 + ]: (A) 0, (B) 2.0, (C) 5.0 mM. Total accumulation time of Raman signal was 5 s during 3 min after applying the potential. S denotes the Raman band of solvent AN. * is a plasma line of the laser.
In the presence of Ba2 + , similar TLER measurements were carried out at an applied potential of −0.5 V. Fig. 11 shows the TLER spectrum observed when 1.0 mM Ba2 + was added to 5.0 mM TCQ in AN. Together with the decrease of the RR peaks of free TCQ − , new Raman peaks appeared at 1578 and 1592 cm − 1. This is remarkably different from the behaviour observed in the presence of Mg2 + . In addition, the intensities of these new peaks were found to increase with time for several minutes, and to be dependent on the concentration of Ba2 + .
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Table 1 Raman shifts of TCQ, TCQ−, Ba2+(TCQ−)2 and Sr2+(TCQ−)2 Raman shift/cm−1
Fig. 11. Raman spectrum observed in the reduction of TCQ with the addition of Ba2 + measured using the TLER method. Applied potential, − 0.5 V; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Ba2 + ], 1.0 mM. Total accumulation time of Raman signal was 5 s during 3 min after applying the potential. S denotes the Raman band of solvent AN.
The intensity of this new peak is stronger compared with the RR peak of TCQ − generated in the thin layer. In addition, after the measurement, a dark green colored precipitate was found to be deposited on the electrode surface. Therefore, the increase of the Raman peak at 1592 cm − 1 is assumed to be caused by the formation of this deposited solid. To clarify the origin of the strong Raman peak, we carried out Raman measurements on the Ba2 + salts. Because TCQ − and TCQ2 − are easily prepared selectively and quantitatively in flow-electrolysis, their Ba2 + salts were easily prepared by dropping the solutions directly on solid Ba(ClO4)2. As a result, different colored precipitates could be produced for both species, i.e. a dark green colored precipitate was obtained for TCQ − , and an orange colored precipitate for TCQ2 − . These are attributed to Ba2 + (TCQ − )2, and Ba2 + TCQ2 − , respectively. The results of Raman measurement for these precipitates are shown in Fig. 12. For Ba2 + TCQ2 − , a clear Raman spectrum was not observed because the background light increased, probably due to fluorescence. By comparing the Raman spectra in Figs. 11 and 12, the strong Raman peaks observed in the TLER measurement could be determined to be those of Ba2 + (TCQ − )2, not Ba2 + TCQ2 − . In addition, compared
Fig. 12. Raman spectra of Ba2 + (TCQ − )2 (A) and Ba2 + TCQ2 − (B). These salts were prepared through the flow-electrolysis synthesis as described in the text.
TCQ TCQ− Ba2+(TCQ−)2 Sr2+(TCQ−)2
CC stretching band
CO stretching band
1609 1585 1592 1583
1672 1517 1575 1583
with the Raman peaks of the precipitate Ba2 + (TCQ − )2 in Fig. 12, the higher Raman peak intensities of Ba2 + (TCQ − )2 in Fig. 11 than those of the solvent are assumed to be due to the formation of the solid Ba2 + (TCQ − )2 on the electrode surface. By comparing the Raman bands of Ba2 + (TCQ − )2 with those of the K + TCQ − crystal [12], TCQ − and neutral TCQ, the Raman bands of Ba2 + (TCQ − )2 at 1592 and 1578 cm − 1 are assigned to the CC stretching band and CO stretching band, respectively. In these TCQ related compounds, these two stretching bands are the most characteristic bands, in particular, whose intensities are strong in the solid states. Table 1 summarizes the Raman shifts of TCQ, TCQ − , Ba2 + (TCQ − )2. and Sr2 + (TCQ − )2. From these results, it is assumed that the Raman band of the CO stretching mode shifts significantly reflecting the bond order of each state. The low value for this band in TCQ − indicates that the single bond character is increased in the anion radical. By contrast, for the Ba2 + (TCQ − )2 precipitate, the Raman shift of the CO stretching is between that of TCQ and TCQ − . This result shows that the bond order of TCQ − approaches that of neutral TCQ in the formation of this ion pair, which reflects the changes in the electronic state of the TCQ moiety.
3.8. TLER measurement for the reduction of TCQ in the presence of Ca 2 + and Sr 2 + TLER measurements were also carried out for the cases of Ca2 + and Sr2 + . In the presence of Ca2 + , results identical to those for Mg2 + were obtained. Hence, it was shown that the intermediate ion pair, Ca2 + TCQ − , was reduced to Ca2 + TCQ2 − . By contrast, in the presence of Sr2 + , a strong Raman peak was observed, which was similar to the case of Ba2 + . For the Raman spectrum in this case, which is attributed that of Sr2 + (TCQ − )2, only one strong peak was observed as noted in Table 1. On the basis of the analogy of the TCQ related solid compounds, the observed Raman peak is most probably assigned to the overlapping of CO and CC stretching modes. According to the present results, there exist two distinct electrochemical reactions; i.e. the intermediate
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ion pair, M2 + TCQ − , accepts another electron, or deposits on the electrode surface as the anion radical salt. The former occurs in the presence of Mg2 + or Ca2 + , which have the higher electronegativities. For these cases, because of the strong electronic interaction between M2 + and TCQ − , the M2 + TCQ − formed is assumed to accept another electron easily. By contrast, for the cases of Sr2 + and Ba2 + with lower electronegativities, it is assumed that M2 + TCQ − is not reduced, thus the ion pair, M2 + (TCQ − )2, is made via the reaction with free TCQ − . The Raman shift of the CO stretching mode of Sr2 + TCQ − is a little larger than that of Ba2 + (TCQ − )2. This indicates that the change in the electronic state brought from the ion pair interaction is more significant in the case of Sr2 + with its higher electronegativity. Besides the classification in the electrochemical reactions observed in this work, systematic measurement of the Raman spectra of M2 + (TCQ − )2 might be helpful to understand the differences of ion pair interaction. However, the Mg2 + (TCQ − )2 precipitate could not be produced, though we tried to synthesize it using the flow electrolysis synthetic method. This is probably due to the favorable formation of Mg2 + TCQ2 − in AN.
4. Conclusion In conclusion, the electrochemical reactions concerning the ion pair formation processes, and their dependence on the alkaline earth metal cations, could be observed and clarified through the TLER measurement method. In the presence of Mg2 + and Ca2 + , from the decrease of the RR peak intensities of free TCQ − depending on the metal concentrations, Mg2 + TCQ2 − and Ca2 + TCQ2 − were found to be produced on the electrode surface, which proved the ECE reactions in these systems. On the other hand, in the presence of Sr2 + and Ba2 + , the deposition of Sr2 + (TCQ − )2 and Ba2 + (TCQ − )2 was observed through TLER measurement. Thus, the use of the Raman spectroscopy combined with thin layer electrochemical measurements is a suit-
able method to gain mechanistic information on electrode reactions. The present approach would be useful in analyzing the complex electrochemical responses by making clear the difference between the electrode surface and the bulk solution.
Acknowledgements This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. One of the authors (M.O.) would like to thank a grant from Nissan Science Foundation.
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Thin layer electrochemical Raman study of ion pair formation between the tetrachlorobenzoquinone anion radical and alkaline earth metal cations Mikito Yamanuki, Toyomasa Hoshino, Munetaka Oyama, Satoshi Okazaki * Department of Material Chemistry, Graduate School of Engineering, Kyoto Uni6ersity, Sakyo-ku, Kyoto 606 -8501, Japan Received 23 June 1998; received in revised form 18 August 1998
Abstract Raman spectroscopy combined with electrochemical measurement has been proved to be effective in the elucidation of complex voltammetric responses. In the reduction of 2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ), complex voltammograms were observed in the presence of alkaline earth metal ions (M2 + ) due to ion pair formation. By using thin layer electrochemical Raman (TLER) measurements synchronized with the reduction of TCQ in the presence of Mg2 + , it was found that an intermediate ion pair, Mg2 + TCQ − , was reduced to an ion pair involving the TCQ dianion, Mg2 + TCQ2 − . In contrast, for the case of Ba2 + , the anion radical salt (Ba2 + TCQ − )2 was found to deposit on the electrode surface, which was confirmed through the Raman measurement of the salts of reduced species produced by a flow-electrolysis synthetic method. The electrochemical behavior was dependent on the metal cations, M2 + . The intermediate ion pair, M2 + TCQ − , may either accept another electron or precipitate on the electrode surface. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Raman spectroscopy; Thin layer electrochemical measurement; Ion pair formation; Reduction of tetrachlorobenzoquinone
1. Introduction Raman spectroscopy is one of the most powerful methods for the observation of structural information of electrogenerated species. In previous papers, we have reported the resonance Raman (RR) measurement of electrogenerated species in solution using a columnelectrolytic continuous-flow method [1 – 3]. With this technique, the RR spectra of electrogenerated species could be successfully observed in controlled electrolysis conditions, and observations were made of the differences in the RR spectra between the cation radical and the dication of N, N%-dimethyl-N, N%-diphenylbenzidine [1], and between the cation radicals and the anion radicals of anthracene derivatives [2,3]. * Corresponding author. Tel: + 81-75-753-5882; fax: +81-75-7534718; e-mail: [email protected].
While the RR measurement of electrogenerated species in solution has become very easy with the columnelectrolysis continuous-flow method, it is still important to observe the electrochemical events directly in the vicinity of electrode surfaces. The specific structural features will become much clearer with the help of the measurements of vibrational structures in homogeneous solution and in the presence of the electrode surface. Van Duyne et al. initiated the methodological approaches to the RR measurement of the electrogenerated species in the vicinity of the electrode, as reported previously [4–7]. Both controlled potential coulometric electrolysis in bulk solution [4–6] and cyclic potential step electrolysis under semi-infinite diffusion mass transfer conditions [4,7] have been proposed as electrogeneration methods. The RR spectra of stable electrogenerated species from reversible redox systems were observed successfully in these pioneering works. How-
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 3 5 1 - 9
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ever, these methods are difficult to apply to the analysis of the complex electrode reaction systems which show non-reversible electrochemical responses due to following chemical reactions or interfacial reactions near the electrode. Thus, the improvement of the methods is still desirable for the Raman measurement of electrochemical events. Recently, RR scattering and absorption spectroscopies were applied to elucidate one- and two-electron oxidation processes of 3,3%,5,5%-tetramethylbenzidine in organic solvents, and consequently, the RR spectra of a charge transfer complex and the dication were characterized [8]. As shown in this work, a promising approach is to apply Raman spectroscopy in a thin layer configuration. By reducing the thickness of the electrolyzed solution layer to a micrometer level in combination with controlled potential electrolysis, the electrochemical events can be observed in the diffusion layer in the steady state. Therefore, we have developed a thin layer electrochemical resonance Raman (TLERR) method to analyze electrode reactions, as reported previously in a communication [9]. Raman measurement using a thin layer spectroelectrochemical cell has an advantage that the species present only in the vicinity of the solution electrode interface can be monitored, so that electrochemical reactions and chemical reactions in the diffusion layer can be directly observed. In the present paper, the useful features of the thin layer electrochemical Raman (TLER) method are presented for the analysis of complex electrochemical reactions. For the ion pair formation between 2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ; chloranil) anion radical (TCQ − ) and Mg2 + , the reaction kinetics were analyzed in homogeneous solution of acetonitrile (AN) by using a pulse-electrolysis stopped-flow method [10]. In the present work, to investigate this type of electrochemical reaction process concerning ion pair formation in detail, we have observed the Raman spectra in the vicinity of the electrode by using the TLER method. The TLER measurements were carried out by applying a negative potential to form TCQ − in the presence of alkaline earth metal cations (M2 + =Mg2+, Ca2+, Sr2+, Ba2 + ).
Fig. 1 shows a schematic view of the cell. The electrode surface is positioned to face a quartz window. The electrode used was a glassy carbon disk electrode mounted in PEEK resin (BAS). The full diameter of the electrode body was 6.0 mm and the diameter of glassy carbon was 3.0 mm. To achieve thin layer conditions, the distance from the electrode surface to the quartz window (diameter 20 mm) was adjusted to be 25 mm by placing a Teflon gasket (thickness 25 mm) which has an oval-shaped hole between the electrode surface and the window. When the cell was filled with a sample solution, the oval area became the effective thin solution layer for electrolysis and Raman measurement. The electrolysis and Raman measurements were carried out for a stationary thin layer solution. The solution was replaced with the flow through method after each measurement. Controlled potential electrolysis was performed with a three electrode system; a Pt wire counter electrode was wound around the working electrode, and a Pt (I − , I3− ) reference electrode was inserted close to the working electrode. In the Raman measurement, the electrochemical cell was set up so that its window faced the entrance slit of the spectrophotometer. The alignment of light collection for TLER measurement is shown in Fig. 2. The argon ion laser line of 488.0 nm for excitation was irradiated on the electrode surface from the side with an incident angle of 25° to the plane of the window. The laser power was 200 mW in all the measurements.
2. Experimental
2.1. Apparatus and method For the TLER measurement, we initially constructed a cell in which solution penetrated between the electrode surface and the quartz window [9]. However, to control the thickness of the thin layer, we constructed a new type cell in the present work.
Fig. 1. Schematic view of TLER cell. (A) Side view, (B) top view. WE, working electrode; CE, counter electrode; RE, reference electrode; SM, spectrophotometer; a, quartz window; b, solution in; c, solution out; d, solution; e, surface of glassy carbon working electrode (diameter, 3.0 mm); f, body of working electrode (diameter, 6.0 mm); g, Teflon gasket whose thickness is 25 mm.
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3. Results and discussion
3.1. Measurement of Raman spectra of sol6ent AN with TLER cell
Fig. 2. Alignment of light collection for TLER measurement. C, TLER cell; WE, working electrode; CE, counter electrode; RE, reference electrode; PS, potentiostat; LS, argon ion laser; SM, spectrophotometer; M, mirror; L1, collection lens for laser light whose focal distance is 8.0 cm; L2, collection lens for scattered light whose focal distance is 3.5 cm; L3, collection lens for the light to the spectrophotometer whose focal distance is 17.0 cm; F, notch filter used to reject the laser light; u, incident angle of 25°.
The incident angle of 25° was chosen to collect the scattering light from the solution layer effectively in the present cell configuration. Two collection lenses and a holographic notch plus filter (Kaiser Optical System) to reject Rayleigh light were placed between the cell and the spectrophotometer. The detection system with a multi-channel detector was described previously [1]. Because the current response was observed to decrease in a very short time of less than 1 s, thin layer conditions were assumed to be achieved in this cell. The Raman signal was accumulated for 3 min after the potential step, i.e. the measurement was carried out for a steady state condition of the electrode surface after applying the potential. The details of the pulse-electrolysis stopped-flow method have been described previously [10,11]. The cyclic voltammograms (CV) were measured using a PAR 174 analyzer.
To evaluate the collection efficiency of Raman scattering light in the present thin layer cell, we observed the Raman spectra of AN, which was used as a solvent. Fig. 3 shows Raman spectra of AN measured with the previous column-electrolysis continuous-flow cell and the present TLER cell. The ratio of the intensities of the two spectra was ca. 1:6 for the TLER cell versus the continuous-flow cell. The light path of the capillary cell perpendicular to the collection lens is 1.4 mm. On the other hand, the thickness of the thin layer is 25 mm, and the length of the oval area perpendicular to the lens is 3.0 mm. Although the method and the efficiency for light collection are different in the two configurations, a spectrum of good quality was observed as shown in this figure, even in the TLER cell with very thin solution layer of 25 mm thickness. This indicates that the present thin layer cell is suitable for the measurement of the electrolyzed solutions.
3.2. Reduction of TCQ in AN and the absorption spectra of TCQ − and TCQ 2 − As a recent paper described the heterogeneous rate constant and the formal potential of TCQ [13], the electrochemical reduction of TCQ in organic solvents has been studied extensively [14–16]. For the reduction of TCQ in AN, two reversible waves were observed in the cyclic voltammogram (as later shown in Fig. 6A Fig. 7A), which are attributed to the formation of TCQ − and TCQ dianion (TCQ2 − ), respectively. TCQ+e − TCQ − TCQ + e TCQ −
−
2−
E= − 0.12 V
(1)
E= − 0.88 V
(2)
2.2. Reagents 2,3,5,6-Tetrachloro-1,4-benzoquinone (Nacalai tesque., GR grade) was used as received. Mg(ClO4)2 and Ba(ClO4)2 (Aldrich), as a source of M2 + , were also used as received. Ca (ClO4)2 4H2O and Sr(ClO4)2 (Mitsuwa Chemicals) were used after drying carefully in a vacuum oven. The purification methods of AN and tetrabutylammonium perchlorate (TBAP) were described previously [11]. The K + TCQ − salt was prepared as described in the literature [12].
Fig. 3. Raman spectra of acetonitrile measured with column-electrolysis continuous-flow cell (A) and TLER cell (B). Gate time, 100 ms; accumulation, 50 times; excitation wavelength, 488.0 nm.
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Fig. 4. Absorption spectra of TCQ − (A) and TCQ2 − (B) in AN measured with a pulse-electrolysis stopped-flow method. [TCQ], 1.0 mM; [TBAP], 0.1 M; optical light path, 0.20 cm. Arrows indicate the excitation wavelength of 488.0 nm.
Therefore, as reported previously [17], the absorption spectra of both reduced species could be observed easily with the column-electroanalysis method, in particular, in their free state, i.e. without the effect of the ion pair formation. The results are shown in Fig. 4. The applied potentials for generating the solutions of TCQ – and TCQ2 − were −0.5 V and −1.2 V (vs. I − /I3− ), respectively. Although the formation of dianion had been difficult in the conventional electrolysis, the quantitative electrolysis in the column-electrode permits the observation of the absorption spectrum of TCQ2 − . From these spectra, a resonance enhancement can be expected with the excitation by a laser line of 488.0 nm in the case of TCQ − . In addition, for TCQ2 − , a pre-resonance enhancement effect might arise with the same excitation.
peared, and the observed Raman spectrum was that of solvent AN, which was similar to the result at the applied potential of + 0.2 V. Although a pre-resonance effect might have been expected for TCQ2 − on the basis of the absorption spectrum in Fig. 4B, the RR spectrum of TCQ2 − could not be measured with the excitation of 488.0 nm. To confirm the present results of TLER measurement for TCQ reduction, we also carried out column-electrolytic continuous flow measurements [1–3]. The same results were obtained with both methods, which indicates that TCQ − exists in the vicinity of the electrode surface with the same vibrational structure as in homogeneous solution. In addition, the RR spectrum measured for TCQ − was similar to that prepared by dissolving K + TCQ − in acetone [12].
3.4. Electrochemical measurement of the reduction of TCQ in the presence of M 2 + While two reversible waves were observed in the reduction of TCQ in AN, the electrochemical response changes drastically with the addition of alkaline earth metal cations reflecting the ion pair formation on or near the electrode surface. Figs. 6 and 7 show the cyclic voltammograms in the reduction of TCQ with the addition of Mg2 + and Ba2 + , respectively. In the case of Mg2 + , with increasing concentration of the metal ion, the reversible peaks (P1, P2) were gradually diminished and a new irreversible cathodic peak (P3) appeared in the potential region slightly positive to P1. On the other hand, in the case of Ba2 + , adsorptive and desorptive peaks were observed for both reduction and oxidation processes, which were seen with an increase in the concentration of Ba2 + . Similar changes were observed in the voltammetric responses in the presence of Ca2 + and Sr2 + .
3.3. TLER measurement in the reduction of TCQ The TLER measurement was carried out by applying electrode potentials of + 0.2, −0.5 and − 1.2 V. At the potentials of − 0.5 and −1.2 V, TCQ is reduced to TCQ − and TCQ2 − , respectively. In the case of neutral TCQ at the potential of +0.2 V, the observed Raman spectrum on the electrode surface was that of solvent AN as shown in Fig. 5A. This is because no resonance enhancement effect was expected for TCQ with excitation at 488.0 nm. By contrast, with the applied potential at − 0.5 V, several Raman peaks were observed as shown in Fig. 5B. Judging from the applied potential and the absorption spectrum of TCQ − in Fig. 4A, the observed Raman spectrum can be attributed to the RR spectra of TCQ − produced on the electrode surface. Next, the applied potential was changed to −1.2 V. Consequently, all the RR peaks of TCQ − disap-
Fig. 5. Raman spectra in the reduction of TCQ measured with the TLER method. Applied potential: (A) + 0.2 V; (B) − 0.5 V. The Raman spectrum obtained at −1.2 V is similar to the one observed in (A). [TCQ], 5.0 mM; [TBAP], 0.1 M. S denotes the Raman band of solvent AN. * is a plasma line of the laser.
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Fig. 8. Changes in absorbance depending on [Mg2 + ]/[TCQ − ]. () 290 nm; () 449 nm. The solution of TCQ − was produced by electrolyzing the AN solution of 0.50 mM TCQ containing 0.1 M TBAP with the applied potential of − 0.5 V for 10 s, and then it was mixed with the AN solution of Mg2 + .
3.5. Reaction of TCQ − and M 2 + in homogeneous solution
Fig. 6. Changes in cyclic voltammograms of TCQ in AN with the addition of Mg2 + measured with the TLER cell. Scan rate, 100 mV s − 1; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Mg2 + ]: (A) 0 mM; (B) 1.0 mM; (C) 2.0 mM; (D) 3.0 mM. Mg2 + was added by dissolving Mg(ClO4)2.
Because these changes were observed at the potential range of the first reduction wave, a strong ion pair interaction between TCQ − and M2 + was assumed to occur in the electrode solution interface.
To clarify the ion pair interaction in homogeneous solution, the reaction of TCQ − with Mg2 + was analyzed using a pulse-electrolysis stopped-flow method [10]. After mixing TCQ − with Mg2 + , the absorption maxima of TCQ − at 320 and 449 nm (Fig. 4A) diminished and another new peak, at 290 nm, was revealed. Fig. 8 shows the changes in the absorbance at 449 nm of free TCQ − and 290 nm of the product when 0.50 mM TCQ − was mixed with various concentrations of Mg2 + . From this result, the molar ratio of the ion pair formed was determined to be Mg2 + :TCQ − of 1:2. Thus, the following two step interactions can be assumed. That is, TCQ − reacts with Mg2 + as expressed by Eq. (3). TCQ − + Mg2 + Mg2 + TCQ −
(3)
Following this reaction, the ion pair, Mg2 + (TCQ − )2, is formed in homogeneous AN solution (Eq. (4)), Mg2 + TCQ − + TCQ − Mg2 + (TCQ − )2
(4)
or an ion pair involving the dianion, Mg2 + TCQ2 − , is formed through the disproportionation reaction (Eq. (5)). Mg2 + TCQ − + TCQ − Mg2 + TCQ2 − + TCQ
(5)
2+
With other M , the same measurement was carried out to clarify the ion pair formation processes. As a result, the molar ratio of M2 + :TCQ − was found to be 1:2 for all M2 + (M= Mg, Ca, Sr, Ba). The changes in the ion pair formation rate in homogeneous solution were observed depending in the nature of metal cations with the pulse electrolysis stopped-flow. The results will be reported elsewhere.
Fig. 7. Changes in cyclic voltammograms of TCQ in AN with the addition of Ba2 + measured with the TLER cell. Scan rate, 100 mV s − 1; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Ba2 + ]: (A) 0 mM; (B) 1.0 mM; (C) 2.0 mM; (D) 3.0 mM. Ba2 + was added by dissolving Ba(ClO4)2.
3.6. TLER measurement for the reduction of TCQ in the presence of Mg 2 + Next, in order to analyze the ion pair formation reaction in the vicinity of the electrode, we carried out
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the TLER measurement for the reduction of 5.0 mM TCQ in AN in the presence of various concentrations of Mg2 + . The applied potential was −0.5 V, which was similar to the TLERR measurement of TCQ − as in Fig. 5. In the presence of Mg2 + , once the ion pairs were formed in solution, the absorption spectrum changed drastically from that of free TCQ − . Therefore, with excitation of 488 nm, the resonance enhancement effect was expected only for free TCQ − . Fig. 9 shows the changes in Raman spectra measured in the reduction of TCQ depending on the concentration of Mg2 + . With increasing concentration of Mg2 + , the RR peaks of free TCQ − decreased as shown in Fig. 9B, C. Thus, the amount of free TCQ − is assumed to diminish through the ion pair formation on the electrode surface as well as in homogeneous solution; i.e. free TCQ − was converted into an ion pair which gave no resonance enhancement effect with the excitation of 488.0 nm. However, the decrease of free TCQ − in the ion pair formation process measured with the TLER method was remarkably different from the results in homogeneous solution as shown in Fig. 8. Fig. 10 summarizes the changes in the RR signals as a function of the molar ratio of [Mg2 + ]/[TCQ]. While the reaction molar ratio of M2 + :TCQ − was 1:2 in homogeneous solution, the decrease of the intensity of RR bands depending on the molar ratio was quite different; i.e. from Fig. 10, the stoichiometry of Mg2 + :TCQ was determined as 1:1. To explain this remarkable difference, the most probable pathway is further reduction of Mg2 + TCQ − once produced on the electrode surface. Judging from the fact that 1 mole of Mg2 + was consumed for each
Fig. 10. Changes in intensity of RR peaks of TCQ − depending on [Mg2 + ]/[TCQ]. () 1019 cm − 1; () 1086 cm − 1. The Raman intensity was normalized using the solvent Raman band at 920 cm − 1, and expressed as a relative value to the result without addition of Mg2 + .
mole of TCQ in the thin layer cell, the electron transfer of Eq. (6) is assumed to proceed at the electrode surface after the formation of Mg2 + TCQ − (Eq. (3)). Mg2 + TCQ − + e − Mg2 + TCQ2 −
(6)
The intermediate ion pair, Mg2 + TCQ − , seems to accept a negative charge and reach electrical neutrality, which is similar to the result in homogeneous solution where Mg2 + TCQ − was not detected in the absorption measurement. This hypothesis is in good agreement with the electrochemical measurements in Fig. 6. That is, the electrode reaction processes can be presented by the following ECE mechanism as summarized by the three equations, Eqs. (1), (3) and (6). The appearance of P3 can be attributed to the reduction of Mg2 + TCQ − . The irreversibility in the CVs is inferred to result from the chemical step of Eq. (3). As the reduction potential of Mg2 + TCQ − was positive of that of TCQ, the electron transfer of Eq. (6) is so thermodynamically favorable that it proceeds soon after the formation of TCQ − . In addition, by taking account of the above relationship of the reduction potentials and by comparing with the reduction mechanisms on the electrode surface, the disproportionation reaction of Eq. (5) seems to be a probable reaction pathway in homogeneous solution.
3.7. TLER measurement for the reduction of TCQ in the presence of Ba 2 +
Fig. 9. Raman spectra observed in the reduction of TCQ in AN with the addition of Mg2 + measured using the TLER method. Applied potential, −0.5 V vs I3− , I − ; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Mg2 + ]: (A) 0, (B) 2.0, (C) 5.0 mM. Total accumulation time of Raman signal was 5 s during 3 min after applying the potential. S denotes the Raman band of solvent AN. * is a plasma line of the laser.
In the presence of Ba2 + , similar TLER measurements were carried out at an applied potential of −0.5 V. Fig. 11 shows the TLER spectrum observed when 1.0 mM Ba2 + was added to 5.0 mM TCQ in AN. Together with the decrease of the RR peaks of free TCQ − , new Raman peaks appeared at 1578 and 1592 cm − 1. This is remarkably different from the behaviour observed in the presence of Mg2 + . In addition, the intensities of these new peaks were found to increase with time for several minutes, and to be dependent on the concentration of Ba2 + .
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Table 1 Raman shifts of TCQ, TCQ−, Ba2+(TCQ−)2 and Sr2+(TCQ−)2 Raman shift/cm−1
Fig. 11. Raman spectrum observed in the reduction of TCQ with the addition of Ba2 + measured using the TLER method. Applied potential, − 0.5 V; [TCQ], 5.0 mM; [TBAP], 0.1 M; [Ba2 + ], 1.0 mM. Total accumulation time of Raman signal was 5 s during 3 min after applying the potential. S denotes the Raman band of solvent AN.
The intensity of this new peak is stronger compared with the RR peak of TCQ − generated in the thin layer. In addition, after the measurement, a dark green colored precipitate was found to be deposited on the electrode surface. Therefore, the increase of the Raman peak at 1592 cm − 1 is assumed to be caused by the formation of this deposited solid. To clarify the origin of the strong Raman peak, we carried out Raman measurements on the Ba2 + salts. Because TCQ − and TCQ2 − are easily prepared selectively and quantitatively in flow-electrolysis, their Ba2 + salts were easily prepared by dropping the solutions directly on solid Ba(ClO4)2. As a result, different colored precipitates could be produced for both species, i.e. a dark green colored precipitate was obtained for TCQ − , and an orange colored precipitate for TCQ2 − . These are attributed to Ba2 + (TCQ − )2, and Ba2 + TCQ2 − , respectively. The results of Raman measurement for these precipitates are shown in Fig. 12. For Ba2 + TCQ2 − , a clear Raman spectrum was not observed because the background light increased, probably due to fluorescence. By comparing the Raman spectra in Figs. 11 and 12, the strong Raman peaks observed in the TLER measurement could be determined to be those of Ba2 + (TCQ − )2, not Ba2 + TCQ2 − . In addition, compared
Fig. 12. Raman spectra of Ba2 + (TCQ − )2 (A) and Ba2 + TCQ2 − (B). These salts were prepared through the flow-electrolysis synthesis as described in the text.
TCQ TCQ− Ba2+(TCQ−)2 Sr2+(TCQ−)2
CC stretching band
CO stretching band
1609 1585 1592 1583
1672 1517 1575 1583
with the Raman peaks of the precipitate Ba2 + (TCQ − )2 in Fig. 12, the higher Raman peak intensities of Ba2 + (TCQ − )2 in Fig. 11 than those of the solvent are assumed to be due to the formation of the solid Ba2 + (TCQ − )2 on the electrode surface. By comparing the Raman bands of Ba2 + (TCQ − )2 with those of the K + TCQ − crystal [12], TCQ − and neutral TCQ, the Raman bands of Ba2 + (TCQ − )2 at 1592 and 1578 cm − 1 are assigned to the CC stretching band and CO stretching band, respectively. In these TCQ related compounds, these two stretching bands are the most characteristic bands, in particular, whose intensities are strong in the solid states. Table 1 summarizes the Raman shifts of TCQ, TCQ − , Ba2 + (TCQ − )2. and Sr2 + (TCQ − )2. From these results, it is assumed that the Raman band of the CO stretching mode shifts significantly reflecting the bond order of each state. The low value for this band in TCQ − indicates that the single bond character is increased in the anion radical. By contrast, for the Ba2 + (TCQ − )2 precipitate, the Raman shift of the CO stretching is between that of TCQ and TCQ − . This result shows that the bond order of TCQ − approaches that of neutral TCQ in the formation of this ion pair, which reflects the changes in the electronic state of the TCQ moiety.
3.8. TLER measurement for the reduction of TCQ in the presence of Ca 2 + and Sr 2 + TLER measurements were also carried out for the cases of Ca2 + and Sr2 + . In the presence of Ca2 + , results identical to those for Mg2 + were obtained. Hence, it was shown that the intermediate ion pair, Ca2 + TCQ − , was reduced to Ca2 + TCQ2 − . By contrast, in the presence of Sr2 + , a strong Raman peak was observed, which was similar to the case of Ba2 + . For the Raman spectrum in this case, which is attributed that of Sr2 + (TCQ − )2, only one strong peak was observed as noted in Table 1. On the basis of the analogy of the TCQ related solid compounds, the observed Raman peak is most probably assigned to the overlapping of CO and CC stretching modes. According to the present results, there exist two distinct electrochemical reactions; i.e. the intermediate
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ion pair, M2 + TCQ − , accepts another electron, or deposits on the electrode surface as the anion radical salt. The former occurs in the presence of Mg2 + or Ca2 + , which have the higher electronegativities. For these cases, because of the strong electronic interaction between M2 + and TCQ − , the M2 + TCQ − formed is assumed to accept another electron easily. By contrast, for the cases of Sr2 + and Ba2 + with lower electronegativities, it is assumed that M2 + TCQ − is not reduced, thus the ion pair, M2 + (TCQ − )2, is made via the reaction with free TCQ − . The Raman shift of the CO stretching mode of Sr2 + TCQ − is a little larger than that of Ba2 + (TCQ − )2. This indicates that the change in the electronic state brought from the ion pair interaction is more significant in the case of Sr2 + with its higher electronegativity. Besides the classification in the electrochemical reactions observed in this work, systematic measurement of the Raman spectra of M2 + (TCQ − )2 might be helpful to understand the differences of ion pair interaction. However, the Mg2 + (TCQ − )2 precipitate could not be produced, though we tried to synthesize it using the flow electrolysis synthetic method. This is probably due to the favorable formation of Mg2 + TCQ2 − in AN.
4. Conclusion In conclusion, the electrochemical reactions concerning the ion pair formation processes, and their dependence on the alkaline earth metal cations, could be observed and clarified through the TLER measurement method. In the presence of Mg2 + and Ca2 + , from the decrease of the RR peak intensities of free TCQ − depending on the metal concentrations, Mg2 + TCQ2 − and Ca2 + TCQ2 − were found to be produced on the electrode surface, which proved the ECE reactions in these systems. On the other hand, in the presence of Sr2 + and Ba2 + , the deposition of Sr2 + (TCQ − )2 and Ba2 + (TCQ − )2 was observed through TLER measurement. Thus, the use of the Raman spectroscopy combined with thin layer electrochemical measurements is a suit-
able method to gain mechanistic information on electrode reactions. The present approach would be useful in analyzing the complex electrochemical responses by making clear the difference between the electrode surface and the bulk solution.
Acknowledgements This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. One of the authors (M.O.) would like to thank a grant from Nissan Science Foundation.
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