Raman spectroscopic study of 1,2-ethanedithiol adsorbed on silver surface

Raman spectroscopic study of 1,2-ethanedithiol adsorbed on silver surface

Journal of Molecular Structure, 197 (1989) 171-180 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands 171 R A M A N S P E C ...

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Journal of Molecular Structure, 197 (1989) 171-180 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

171

R A M A N S P E C T R O S C O P I C S T U D Y OF 1 , 2 - E T H A N E D I T H I O L A D S O R B E D ON S I L V E R S U R F A C E

CHI~OL KEE KWON, KWAN KIM and MYUNG SO0 KIM

Department of Chemistry, Seoul National University, Seou1151-742 (Korea) YOON SUP LEE

Department of Chemistry, Korea Advanced Institute of Science and Technology, Seou1130-650 (Korea) {Received 30 September 1988)

ABSTRACT Surface-enhanced Raman scattering of 1,2-ethanedithiol in silver sol has been investigated. 1,2Ethanedithiol has been found to be chemisorbed dissociatively on the silver surface by rupture of its two S-H bonds. 1,2-Ethanedithiolate anion formed upon adsorption was bound to silver atom (s) via its two sulfur atoms. Conformers of 1,2-ethanedithiol adsorbed selectively on the surface, the gauche conformer around the C-C bond being more likely adsorbed when the bulk concentration of the molecule increased.

INTRODUCTION

1,2-Ethanedithiol is very important as a suitable model molecule for the analysis of the cystine-cysteine interconversion in vivo. From this point of view, a large number of reports have appeared in relation to its molecular structure [ 1-6 ] and vibrational analysis [2,3,5 ]. However, there still remains some controversy about the vibrational assignment in connection with the rotational isomerism of the molecules. Understanding the interaction between sulfur compounds and various metals has been of great concern in catalyst chemistry [7]. Despite the realization that sulfur adversely affects catalyst performance, there is little understanding of the poisoning mechanism. In particular, the amount of sulfur compounds which can poison catalysts is so little that such studies are frequently limited by the sensitivity of the available analytical methods. Surface-enhanced Raman scattering (SERS) [8-10] has become a useful technique for the observation of vibrational spectra of molecules adsorbed on metal surfaces at monolayer or submonolayer coverage. The SERS technique was also demonstrated to be a powerful means to provide a useful information 0022-2860/89/$03.50

© 1989 Elsevier Science Publishers B.V.

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on the vibrational assignment of the molecule in liquid phase. From the concentration dependence of the SER spectra, the vibrational bands for various monomercaptans can be identified in relation to their rotational isomerism [11-151. We report here on the SERS of 1,2-ethanedithiol in aqueous silver sol. One of the major objectives of this work is to investigate whether the observations made for monomercaptans can be extended to dimercaptans. The structure and conformation of the adsorbed species and the nature of the chemical bonding between the adsorbate and substrate are investigated. In order to help assign the vibrational bands in liquid phase, the Raman spectra of silver 1,2ethanedithiolate and lead 1,2-ethanedithiolate salts were also recorded. EXPERIMENTAL

The method of preparation of the silver sol and details of the Raman measurements have been reported previously [16]. A small amount (10 ttl) of 10-2-10 -3 M 1,2-ethanedithiol in methanol/water (1:4 v/v) was added to 1 ml of silver sol solution. A small amount (20-40 ttl) of 4% poly (vinyl)pyrrolidone solution was added to stabilize the sol solution. To prepare silver 1,2-ethanedithiolate salt, a 1-M solution of 1,2-ethanedithiol in methanol was mixed with 1-M aqueous solution of A g N Q in a 1 : 2 volume ratio. The precipitate was filtered and washed with water and methanol successively. Raman scattering was measured using a pellet made of the precipitate. Lead 1,2-ethanedithiolate salt was prepared by mixing a 1-M aqueous solution of PbC12 with 1-M methanol solution of 1,2-ethanedithiol in a 1 : 1 volume ratio. To prepare 1,2-ethanedithiol-(SD)2, 1 ml of 1,2-ethanedithiol was dissolved in 10 ml of 4-M NaOD in D20. After adding a sufficient amount of DC1 in D20, the resulting 1,2-ethanedithiol- ( SD ) 2phase was separated. The procedure was repeated several times to achieve better than 90% deuteration. All the chemicals used were reagent grade and triply distilled water was used for the preparation of solutions. The infrared (IR) spectrum was recorded on a Perkin Elmer Model 283 IR spectrophotometer. RESULTS AND DISCUSSION

The ordinary Raman spectra of neat 1,2-ethanedithiol and 1,2-ethanedithiol-(SD)2 are shown in Fig. 1 (a) and 1 (b), respectively. The SER spectra of 1,2-ethanedithiol obtained at the bulk concentrations of 4 × 10-6 and 2 × 10-5 M in silver sol are shown in Fig. 2(a) and 2(b), respectively. The reason for recording the SER spectrum at two different bulk concentrations was to investigate the change in the conformations of the adsorbed species with surface coverage.

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Fig. 1. The Raman spectra of (a) 1,2-ethanedithiol and (b) 1,2-ethanedithiol- (SD) 2.

As was the case for the monomercaptans investigated so far [11-18], 1,2ethanedithiol adsorbed on silver surface exhibited strong enhancement of the R a m a n bands. For example, based on the intensity of the CCS deformation mode [3 ] appearing at 403 and 418 c m - 1 in the ordinary R a m a n and the S E R spectra, respectively, the S E R enhancement is estimated to be ca. 5 X 105. One of the conspicuous features in the S E R spectrum is the complete absence of the S - H stretching band which appears at 2558 c m - 1 in the ordinary R a m a n spectrum. This indicates that 1,2-ethanedithiol binds dissociatively on the silver surface with both of its two S - H bonds cleaved. For monomercaptans, the sole S - H bonds were observed previously to be cleaved upon the surface adsorption. Another conspicuous feature in the S E R spectrum is the substantial redshifts of the C - S stretching frequencies. For example, the band at 612 c m - 1 in the S E R spectrum (Fig. 2 (a)) which can be correlated with the C - S stretching band (see below) centered at 638 cm -1 in the ordinary R a m a n spectrum (Fig. 1 (a)) exhibits a red-shift of 26 cm -1. Similar observations were made for the monomercaptans. Such a phenomenon was used previously to conclude that a thiolate anion formed upon adsorption was bound to the silver surface via its sulfur atom. The same conclusion can also be drawn for 1,2-ethanedithiol. However, for dimercaptans, both the sulfur atoms bind to the surface silver atom (s). The vibrational-band assignment of 1,2-ethanedithiol is very complicated

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Fig. 2. The S E R spectra of 1,2-ethanedithiol at (a) 4 × 10 -6 M and (b) 2 X 10 -'~ M.

due to the presence of several conformational isomers around the C-C and two C-S bonds. A total of six distinct conformers are possible in the liquid state, namely TTT, GTG, GTG', GGG, TGG, and TGT conformations. The second letter indicates trans (T) or gauche (G) conformation around the C-C bond, and the first and the third letters indicate the conformation around the two respective C-S bonds. In GTG', two thiol hydrogens lie at the opposite sides of the molecular plane formed by the S-C-C-S skeleton. According to the CNDO/2 spd' and sp calculations [19], the most stable form of 1,2-ethanedithiol is the TGT conformer. On the other hand, a recent ab initio calculation [20] suggests that for the rotation about the C-S bond, there is no actual energy difference between the G and T forms and that for the internal rotation about the C-C bond, the T conformation is more stable than the G form by about 3 kcal mol-1. Specifically, the GTG' form was proposed to be the most stable conformation. Although the stability with regard to the internal rotation about the C-C bond is still a matter of controversy, as described above, the rotational isomerism about the C-S bond is generally considered to be unimportant. Hence, in this work, the vibrational bands of 1,2ethanedithiol have been assigned considering the rotational isomers around

175 the C-C bond only. The present vibrational assignment is based on the work of Hayashi et al. [3] with some modifications. The reasons for these will be explained later. The vibrational bands in the 600-750 cm-1 spectral region are mostly due to the C-S stretching modes of the various conformers. In the ordinary Raman spectrum (Fig. 1 (a) ) six bands are observed in this region, namely three strong peaks at 638, 722, and 745 cm -1, two weak peaks at 669 and 682 cm -1, and one shoulder peak at 700 c m - 1. Two of the six bands, centered at 682 and 745 c m - 1, were not mentioned in the work of Hayashi et al. [3], even though their presence was reported in a later publication from the same laboratory. The remaining four bands were assigned by Hayashi et al. to the C-S stretching modes of the two conformers, T T T and TGT, i.e. the bands at 638 and 669 c m - 1 were ascribed to the TGT conformer and the bands at 700 and 722 c m - 1to the T T T conformer. However, considering the recent ab initio calculation mentioned previously, the above conformer assignment looks suspicious. Also, the appearance of other additional peaks in this region suggests the presence of other conformers in addition to those considered by Hayashi et al. [3]. In the SER spectrum (Fig. 2 (a)), the spectral pattern in the C-S stretching region is much simplified, probably due to the fact that only the two conformations, T and G, around the C-C bond are possible for the adsorbed 1,2ethanedithiolate. Three distinct peaks and one shoulder peak appearing in this region, namely at 612, 646, 688, and 715 cm -1, may be correlated with those at 638, 669, 722, and 745 cm-1 in the ordinary Raman spectrum, respectively. The bands in the SER spectrum are thus red-shifted by 20-30 c m - 1 from the corresponding bands in the ordinary Raman spectrum. Such huge red-shifts in the C-S stretching modes, which were also observed for the adsorbed monomercaptans, are due to the binding of sulfur atoms to the silver surface. It is to be noted that in the SER spectrum the relative intensities of the bands at 688 and 715 cm-1 are much reduced compared to those at 612 and 646 cm-1. This, we believe, is mainly due to different stability of the T and G conformers adsorbed on the silver surface. When 1,2-ethanedithiolate adsorbs as the T form, a strong steric interaction between the methylene hydrogens and the surface may be present. For the G conformer adsorbed through its sulfur atoms the methylene hydrogens are conjectured to be directed away from the surface. Also, according to the surface selection rule [21,22 ], the C-S stretching vibration of the T form, parallel to the metal surface, may exhibit less enhancement than that of the G form which is, to a large extent, perpendicular to the metal surface [ 23 ]. It seems then reasonable to assign the bands at 612 and 646 cm-1 to the G conformer and the bands at 688 and 715 cm -1 to the T conformer. In our previous SERS investigation on aliphatic monomercaptans, one conformer was found to dominate over the others as the surface coverage increased. In this context, it is to be noted that the band at 688 c m - 1 disappears completely in the SER spectrum (Fig. 2 (b)) obtained at higher bulk concen-

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tration of 1,2-ethanedithiol. The 711 c m - a band in Fig. 2 (b), to be discussed later, is believed to have nothing to do with the T form. Additional evidence supporting the above conformer classification will also be presented later in the analysis of the CH2 rocking modes. The vibrational bands in the 750-1000 cm-1 spectral region of the ordinary Raman spectrum are due to the CHe rocking modes and the CSH bending modes. The bands at 819 and 899 cm -1 were assigned to the CSH bending modes by Hayashi et al. [3]. Even though it is tempting to correlate these bands with the bands at 825 and 900 cm -1 in the SER spectrum (Fig. 2(a)), respectively, such an assignment is not reasonable due to the absence of S-H hydrogen atoms in the adsorbate. To confirm the vibrational assignment of the CSH bending modes, the ordinary Raman spectrum of 1,2-ethanedithiol- (SD)2 was recorded (Fig. 1 (b)). No band appears in the 819 and 890 cm -1 regions in the Raman spectrum of the deuterated compound indicating that the assignment of the 819 and 899 cm-1 peaks to the CSH bending modes is reasonable. Two new peaks at 598 and 615 cm-1 in the Raman spectrum of the deuterated compound can be assigned to the CSD bending modes. An additional band appears at 877 c m - 1 in the Raman spectrum of the deuterated sample, which can be correlated with the shoulder band at 855 c m - 1 in the Raman spectrum of the undeuterated compound. This, we believe, can be assigned to one of the CH2 rocking modes. We also assign the bands at 768, 945, and 976 cm -1 in Fig. l ( a ) to the CHe rocking modes. Hayashi et al. [3] assigned the bands at 768 and 976 cm -1 to the CH2 rocking modes of the G conformer around the C-C bond, while the band at 945 c m - 1 was not assigned. In the SER spectrum (Fig. 2 (a)), three bands appear distinctly in this region, namely at 825, 900, and 930 cm-1. These bands, when correlated with the bands at 855, 945, and 976 cm-1, respectively, in the ordinary Raman spectrum, exhibit red-shifts of 30-50 c m - 1. Such assignments are not unreasonable because the extent of the red-shifts are comparable to the values observed for the metal complexes of 1,2-ethanedithiol. For example, in the IR spectrum of Pb (SeC2H4) shown in Fig. 3 (a), two bands appearing in the CH2 rocking region, namely at 823 and 913 cm -1, can be correlated with the bands at 825 and 901 c m - 1, respectively, in the SER spectrum of 1,2-ethanedithiol (Fig. 2 (a)). It is known that the S-C-C-S skeleton assumes the G conformation in Pb ($2C2H4). In fact, the bands at ca. 830 and ca. 900 c m - 1 are regarded as the gauche marker bands for the metal complexes of the XCH2CH2Y type ligands [24-28]. Hence, the bands at 825 and 901 cm -1 in the SER spectrum, which correlate with those at 855 and 945 c m - ~in the ordinary Raman spectrum, are assigned to the CH2 rocking modes of the G conformer around the C-C bond. Then, the remaining band at 930 cm-1, which correlates with the 976 c m band in the ordinary Raman spectrum, should be ascribed to the CH2 rocking mode of the conformer. This band disappears completely in the SER spectrum (Fig. 2 (b)) obtained at high bulk concentration. This is in line with our pre-

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Fig. 3. (a) Infrared and (b) Raman spectra of Pb(S2C2H4). (c) Raman spectrum of Ag2(S2C2H4).

vious conformer assignment of the C-S stretching modes. That is, at low surface coverage, both the gauche and the trans forms are present on the surface even though the population of the former dominates the latter. On the other hand, only the gauche form is present at high surface coverage. The other CH2 rocking mode of the T conformer, which is expected to be very weak in the SER spectrum taken at low bulk concentration, cannot be assigned easily. The rather broad shoulder peak at 715 cm-1 which was assigned previously to the C-S stretching of the T conformer may contain some CH2 rocking character originating from the 768 cm-1 peak in the ordinary Raman spectrum. Alternatively, that band may be entirely due to the CH2 rocking mode of the T conformer. Since the mass of an SH group is nearly equal to the mass of a chlorine atom, the vibrational spectra of 1,2-ethanedithiol should be close to those of 1,2dichloroethane, which have been examined in detail previously [29,30]. Our assignment of the CH2 rocking modes of 1,2-ethanedithiol is indeed more similar to that of 1,2-dichloroethane than is the assignment made by Hayashi et al. [3]. In the SER spectrum (Fig. 2(b)) obtained at high bulk concentration, a medium intensity peak appears at 711 cm -1. This band grows with the bulk concentration. In the SER spectrum recorded at the bulk concentration of

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M, its intensity is approximately half that of the band at 621 cm -1. According to our reasoning, it cannot be ascribed to the T form. This band can be correlated with that at 736 cm-1 in the ordinary Raman spectrum (Fig. 3 (c) ) of silver 1,2-ethanedithiolate salt. In fact, the ordinary Raman spectrum of the silver salt correlates extremely well with the SER spectrum (Fig. 2 (b) ) at high bulk concentration. This band can not be correlated at all with the bands in the vibrational spectra (Fig. 3(a) and 3(b) ) of Pb(S2C2H4). Otherwise, the vibrational peaks of silver salt correlate well with those of lead salt. For example, the 664 and 662 c m - 1 peaks of lead salt which arise from the CS stretchings can be correlated with the 635 and 663 c m - 1 peaks of silver salt. The 823 and 914 cm -1 peaks of lead salt which are due to the CH2 rocking vibrations can be correlated with the 844 and 914 cm -1 peaks of silver salt. These observations suggest that both the high concentration SER spectrum (Fig. 2 (b)) and the silver salt spectrum (Fig. 3 (c)) originate entirely from the G conformer around the C-C bond, regardless of the existence of the 711 and 736 cm-1 peaks, respectively, in the former and the latter spectra; otherwise, the above two spectra would have been more complex than as observed. It is to be noted, however, that the structure of Pb ($2C2H4) is rather compact, $2C2H42- being attached to Pb atoms as a bidentate ligand [28]. In the case of Ag2 ($2 C2 H4), two silver atoms interact with $2 C2H24-, on average. Therefore, in the crystal structure of Ag2 ($2 C2 H4) two different types of sulfur atoms in terms of their interaction environment with silver atoms may exist. If this is the case, the bands at 711 cm -~ in Fig. 2(b) and at 736 cm -1 in Fig. 3 (c) may be due to the C-S stretching modes of 1,2-ethanedithiolate which are less influenced by the silver atoms than the species responsible for the 621 and 650 cm -~ bands in Fig. 2 (b) and the 635 and 663 cm -1 bands in Fig. 3 (c). Such an argument may be checked by performing an X-ray crystallographic study of Ag2(S2C2H4). However, efforts to prepare a single crystal of Ag2 ($2 C2 H4) have been fruitless so far due to the extremely low solubility of the salt. Also, the semi-empirical MNDO force-field analysis performed to obtain information on the existence of two different sulfur atoms in the salt was indecisive. In summary, it was observed that 1,2-ethanedithiol exhibits relatively large Raman enhancement in aqueous silver sols. 1,2-Ethanedithiol is chemisorbed dissociatively on the silver surface by rupture of its two S-H bonds and the 1,2-ethanedithiolate anion formed upon adsorption is bound to silver atom (s) via its two sulfur atoms. It was concluded that conformers of 1,2-ethanedithiol adsorbed selectively on the silver surface, the gauche conformer around the CC bond being more likely adsorbed when the bulk concentration of the molecule increased. The assignment of the vibrational-bands is listed in Table 1. It has been demonstrated that the SERS data can be used to refine the conflicting vibrational assignments of 1,2-ethanedithiol in the liquid phase. In order to clarify some unclear points in the spectra, relating to the C-S stretching re10 - 4

180 gion, an X-ray crystallographic study of the silver 1,2-ethanedithiolate single crystal seems to be urgently needed. ACKNOWLEDGEMENTS

This work was supported by the Ministry of Education, Republic of Korea.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

R.N. Nandi, C.F. Su and M.D. Harmony, J. Chem. Phys., 81 (1984) 1051. M. Ohsaku and H. Murata, J. Mol. Struct., 52 (1979) 143. M. Hayashi, Y. Shiro, M. Murakami and H. Murata, Bull. Chem. Soc. Jpn., 38 (1965) 1734. I. Hargittai and Gy. Schultz, J. Chem. Soc. Chem. Commun., (1972) 323; Acta Chim. Acad. Sci. Hung., 75 (1973) 381. D. Welti and D. Whittaker, J. Chem. Soc., (1962) 4372. S.T. Mizushima, T. Ichishima, I. Nakagawa and J.V. Quagliano, J. Phys. Chem., 59 (1955) 293. M.M. Labes, P. Love and L.F. Nichols, Chem. Rev., 79 (1978) 1. R.K. Chang and T.E. Furtak (Eds.), Surface-enhanced Raman Scattering, Plenum Press, New York, 1982. J.A. Creighton, Surf. Sci., 124 (1983) 209. H. Nichols and R.M. Hexter, J. Chem. Phys., 75 {1981) 3126. T.H. Joo, K. Kim and M.S. Kim, J. Phys. Chem., 90 {1986) 5816. T.H. Joo, K. Kim and M.S. Kim, J. Mol. Struct., 158 (1987) 265. T.H. Joo, M.S. Kim and K. Kim, J. Mol. Struct., 160 (1987) 81. T.H. Joo, K. K i m a n d M . S . Kim, J. Mol. Struct., 162 (1987) 191. C.K. Kwon, M.S. Kim and K. Kim, J. Mol. Struct., 162 (1987) 201. T.H. Joo, K. Kim and M.S. Kim, Chem. Phys. Lett., 112 (1984) 65. T.H. Joo, M.S. Kim and K. Kim, J. Raman Spectrosc., 18 (1987) 57. C.J. Sandroff, S. Garoffand K.P. Leung, Chem. Phys. Lett., 96 (1983) 547. M. Ohsaku, N. Bingo, W. Sugikawa and H. Murata, Bull. Chem. Soc. Jpn., 52 (1979) 355. M. Ohsaku, J. Mol. Struct., 138 (1986) 283. M. Moskovits, J. Chem. Phys., 75 (1981) 3126. J. Gersten and A. Nitzan, J. Chem. Phys., 84 (1986) 2942. M. Moskovits, Rev. M. Phys., 57 (1985) 783. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, WileyInterscience, New York, 1978, p. 206. M. Ikram and D.B. Powell, Spectrochim. Acta, Part A, 28 (1972) 59. A. Finch, R.C. Poller and D. Steele, Trans. Faraday Soc., 61 (1965) 2628. G. Newman and D.B. Powell, J. Chem. Soc., 61 (1962) 3447. K. Nahara, S. Nakayama, F. Watari and K. Aida, J. Inorg. Nucl. Chem., 35 {1973) 2610. C. Tanaka, J. Tanaka and K. Hirao, J. Mol. Struct., 146 (1986) 309. S. Mizushima, T. Shimanouchi, I. Harada, Y. Abe and H. Takeuchi, Can. J. Phys., 53 ( 1975 ) 2085.