Surface-Enhanced Raman Spectroscopy of Halogenated Aromatic

Figure 1 displays the Raman spectrum of neat FBT and the SER spectra of FBT ...... Hua Tan, Lingkai Wong, Mei Ying Lai, G. S. M. Kiruba, Weng Kee Leon...
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Langmuir 1998, 14, 3580-3589

Surface-Enhanced Raman Spectroscopy of Halogenated Aromatic Thiols on Gold Electrodes Cory A. Szafranski,† Weslene Tanner, Paul E. Laibinis,‡ and Robin L. Garrell*,† Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received March 6, 1997. In Final Form: April 6, 1998 Surface-enhanced Raman spectroscopy (SERS) has been used to characterize monolayers of p-substituted benzenethiols (XBTs) and p-substituted benzenemethanethiols (XBMTs) (substituent ) X ) F, Cl, Br) on gold electrodes. Detailed vibrational assignments have been made for the Raman and SER spectra of all six compounds. All of these molecules exist on the surface as thiolates, with the aromatic ring tilted relative to the surface normal. Monolayers of the XBTs and XBMTs remain intact on the surface throughout the potential range between the oxidation of the gold surface at ∼ +800 mV vs SCE and the reduction of water at ca. -1000 mV at neutral pH. Monolayers of ClBT and BrBT can be partially reduced electrochemically to form mixed monolayers of the halogenated BT and benzenethiol itself. The reductive elimination of the halide occurs at potentials more positive than are required for reduction of the same molecules in solution. FBT, FBMT, and ClBMT cannot be reduced at the surface, and the BrBMT monolayer is only slightly reduced. The electrochemical reactivities of the XBT and XBMT monolayers are explained in terms of facilitated electron transfer from the metal to the adsorbed thiolate, the properties of the leaving group (halogen), and the electronic consequences of having a methylene spacer group between the sulfur and the aromatic ring. This work shows the feasibility of modifying aromatic self-assembled monolayers in situ to form mixed monolayers. It also provides a framework for designing and fabricating monolayers with prescribed stabilities and electroactivities.

Introduction Thiols spontaneously assemble onto gold substrates from solution and form organized monolayers. Self-assembled monolayers derived from alkanethiols have been used as model systems in fundamental studies of adsorption, wetting, adhesion, and friction.1,2 The formation of well-defined monolayers on electrode surfaces by this method has been used to control and study electrochemical processes.3,4 Recent studies have focused on the use of aromatic and substituted aromatic thiols as an alternative (and possibly superior) class of adsorbates.5 To fully exploit the potential of organized monolayers based on aromatic thiols, it is important to understand the adsorbate-surface interactions for this class of molecules, including the adsorbate orientation, its reactivity, and the stability of the monolayers themselves. * To whom correspondence should be addressed. † University of California, Los Angeles. ‡ Massachusetts Institute of Technology. (1) (a) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (b) Ulman, A. An Introduction to Ultrathin Films: From LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (c) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (d) Whitesides, G. M.; Gorman, C. B. In Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC Press: Boca Raton, FL, 1995; Chapter 52. (e) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (2) (a) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570-579. (b) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (c) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990-1995 and references therein. (d) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663-7676. (e) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960-10965. (3) (a) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135-6136. (b) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823-10824. (4) (a) Chidsey, C. E. D. Science 1991, 251, 919-922. (b) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173-3181. (c) Cheng, J.; Sa`ghi-Szabo´, G.; Tossell, J. A.; Miller, C. J. J. Phys. Chem. 1996, 100, 680-684 and references therein.

In the preceding paper, we used surface-enhanced Raman spectroscopy (SERS) to characterize the orientation and stability of monolayers formed from benzenethiol (BT), benzenemethanethiol (BMT), and p-cyanobenzenemethanethiol on gold electrodes.6 We found that the S-H bonds of these molecules cleave upon adsorption and that the product thiolates adsorb through the sulfur atom with the phenyl ring tilted relative to the gold surface. For these molecules, desorption occurs at potentials greater than +800 mV vs SCE and less than -1000 mV vs SCE, coincident with (or because of) oxidation of the surface and reduction of water, respectively. In this study, we describe SERS studies of monolayers on gold derived from various para-halo-substituted aromatic thiols. The aims are to determine the effects of substitution on the molecular orientations and stability of the monolayers. The molecules studied include pfluoro-, p-chloro-, and p-bromobenzenethiol (FBT, ClBT, and BrBT) and p-fluoro-, p-chloro-, and p-bromobenzenemethanethiol (FBMT, ClBMT, and BrBMT). The electron density on the sulfur atom of each molecule is expected to be influenced by the electronegativity and electron donor properties of the substituent groups. We have therefore characterized the effects of the substituent groups on monolayer structure (molecular orientation), stability, and reactivity. The in situ electrochemical modification of substituted aromatic thiol monolayers is also demonstrated and is shown to provide a new route for fabricating mixed monolayers. (5) (a) Sabatani, E.; Cohen-Boulakis, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974-2981. (b) Chang, S. C.; Chao, I,; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792-6805. (c) Tour, J. M.; Jones, L., II.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 95299534. (d) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320. (6) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570.

S0743-7463(97)00251-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/28/1998

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Experimental Section Materials. Chemicals were obtained from Aldrich and used as received unless noted below. NaOCH3 was obtained from Fisher, p-bromobenzylbromide from Matheson, and K2CO3 from Sigma. FBT and ClBMT were distilled under reduced pressure. BrBT and ClBT were purified by sublimation under reduced pressure. Melting points are reported uncorrected. The progress of intermediate steps in all reactions was followed by thin-layer chromatography. p-Fluorobenzenemethanethiol (FBMT). A solution of thioacetic acid (3.22 g, 42.3 mmol) and NaOCH3 (1.90 g, 35.2 mmol) in 50 mL of methanol was added to a solution of p-fluorobenzyl bromide (6.10 g, 32.3 mmol) in 10 mL of methanol. The solution was refluxed for 4 h. Distilled water (50 mL) was added, and the solution was extracted with CH2Cl2 (3 × 25 mL). The organic extracts were combined and concentrated to give an oil. The oil was dissolved in methanol (50 mL) and purged with N2 for 30 min. K2CO3 (0.5 g) was added, and the mixture was stirred for 4 h under N2. The reaction was quenched by addition of acetic acid (5 mL) and concentrated. FBMT was obtained by vacuum distillation (40 °C/1 Torr; lit.7 45 °C/1 Torr) as a colorless liquid (1.78 g, 12.5 mmol, 39%). 1H NMR (300 MHz, CDCl3) δ 7.28 (ddd, 2 H), 6.98 (ddd, 2 H), 3.70 (t, 2 H), 1.74 (t, 1 H). p-Bromobenzenemethanethiol (BrBMT). A solution of thioacetic acid (2.04 g, 26.8 mmol) and NaOCH3 (1.24 g, 22.9 mmol) in 50 mL of methanol was added to a solution of p-bromobenzyl bromide (5.03 g, 20.1 mmol) in 10 mL of methanol. The solution was refluxed for 12 h. Distilled water (50 mL) was added, and the solution was extracted with CH2Cl2 (3 × 25 mL). The organic extracts were combined to give an oil. The oil was dissolved in methanol (50 mL) and purged with N2 for 30 min. K2CO3 (0.5 g) was added, and the mixture was stirred for 4 h under N2. The reaction was quenched by addition of acetic acid (5 mL), and the reaction mixture was concentrated. BrBMT was obtained by vacuum distillation (80 °C/1 Torr) as a colorless liquid which partially solidified upon standing (2.79 g, 13.7 mmol, 68%). Mp 21-23 °C (lit.7 mp 25 °C); 1H NMR (250 MHz, CDCl3) δ 7.42 (d, 2 H), 7.19 (d, 2 H), 3.57, (t, 2 H), 1.74 (t, 1 H). Instrumentation and Sample Preparation. The experimental apparatus and procedure used to obtain the Raman and SER spectra of the samples have been described elsewhere.6,8 Full details of the spectral acquisition are given in the preceding paper in this issue.6 To generate the monolayers, the roughened gold electrode was immersed in 1 mM thiol/ethanol solutions for ∼30 min. The electrode was rinsed with fresh ethanol and subsequently transferred to the spectroelectrochemical cell, which contained 0.1 M KCl (aqueous), for SER spectroscopy. A potential of -600 mV vs SCE was used to obtain the initial SER spectrum of each compound, as this potential yielded spectra with the highest resolution. Complete SER spectra were then obtained sequentially as the potential was stepped positively in 100 mV increments to a final potential of +500 mV or +800 mV vs SCE. Similarly, a freshly derivatized electrode was used to obtain SER (7) Brembilla, A.; Roizard, D.; Schoenleber, J.; Lochon, P. Can. J. Chem. 1984, 62, 2330-2336. (8) Garrell, R. L.; Szafranski, C.; Tanner, W. In Raman and Luminescence Spectroscopies in Technology II; Adar, F., Griffiths, J. E., Eds.; SPIE Proceedings Vol. 1336; SPIE: Bellingham, WA, 1990; pp 264-271.

Figure 1. (a) Raman spectrum of liquid FBT. SER spectra of FBT adsorbed on a gold electrode at (b) -600 mV and (c) -1000 mV. spectra at -600 mV and sequentially at more negative potentials (to -1000 mV) in 100 mV increments. The scan rate was 1 cm-1/ s; acquiring each full spectrum took ∼30 min. Repeat spectra scans at a given potential were identical, indicating that the adlayer coverage and composition had fully equilibrated.

Results and Discussion p-Fluorobenzenethiol (FBT). Figure 1 displays the Raman spectrum of neat FBT and the SER spectra of FBT obtained at -600 and -1000 mV vs SCE. The vibrational frequencies, relative intensities, and assignments for the Raman spectrum of neat FBT and the SER spectra at +800, -600, and -1000 mV are tabulated in the Supporting Information; the vibrational assignments are made for frequencies between 150 and 3200 cm-1 and are based on the Raman spectral assignments for p-fluorobenzenethiol made by Varsanyi.9 Between -600 and +800 mV, the SER bands did not exhibit significant shifts in frequency (0-4 cm-1), and we use the SER spectrum obtained at -600 mV to highlight the differences between adsorbed and free FBT. (9) Varsanyi, G. In Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; John Wiley & Sons: New York, 1974.

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The most notable difference between the Raman spectrum and the SER spectra is that the δCSH and νSH modes at 919 and 2563 cm-1, respectively, in the Raman spectrum are absent in all of the SER spectra. This absence indicates that the S-H bond of FBT cleaves upon adsorption on gold to form p-fluorobenzenethiolate, which is adsorbed through the sulfur atom. This behavior is the same as that for the adsorption of BT, BMT, and p-cyanobenzenemethanethiol onto gold.6,8 In the Raman spectrum of FBT, there are two bands, ν6a (814 cm-1) and ν7a (375 cm-1), whose vibrations contain contributions from the C-S stretching mode. Upon adsorption, these modes shift -13 and +2 cm-1, respectively. For BT, ν6a shifted -7 cm-1 and ν7a shifted upward slightly upon adsorption.6 The similarity of these shifts indicates similar interactions for BT and FBT with gold. Many ring modes in the -600 mV SER spectrum of adsorbed FBT are shifted downward significantly from their positions in the Raman spectrum of free FBT. Upon adsorption, the symmetric ring breathing modes, ν1 (10981064 cm-1), ν12 (634-614 cm-1), and ν8a (1590-1569 cm-1), exhibited shifts of 20 cm-1 or more to lower wavenumbers. These downshifts can be explained as resulting from two factors: charge transfer from the adsorbate to the metal, which decreases the electron density in the ring bonding orbitals, and back-donation from the metal into ring antibonding orbitals. The shifts for FBT are about 10 cm-1 larger than those for BT.6 The larger shifts could be due to a greater interaction of the ring of FBT with the electrode surface compared to that for the ring of BT. More likely, it is because the presence of an electronegative fluorine atom in the para position leads to greater metaladsorbate back-donation. The ν9a (1159-1141 cm-1) and ν19a (1489-1470 cm-1) bands, whose vibrations primarily involve in-plane CCH bending, also decrease by ∼20 cm-1 upon adsorption. The cause of this shift is unclear; however, we observed similar shifts for the other haloaromatic thiols (vide infra). The presence of strong bands in the C-H stretching region (above 3000 cm-1) in the SER spectrum and the fact that the out-of-plane modes are not enhanced more than the in-plane modes in the -600 mV spectrum are evidence that FBT does not lie flat on the surface.10-13 By analogy with the analysis for BT,6,8 we conclude that FBT, like BT, adsorbs in a tilted orientation on the gold surface. As is the case for BT, the potential dependence of the frequencies and the band shapes of the ring modes can be used to support the inferred adsorbate orientation.6,8,11 Over the entire potential range studied, the bands assigned to ν1, ν6a, ν7a, ν8a, ν9a, ν12, and ν19a shift between 0 and 5 cm-1. These shifts with applied potential are much smaller than those observed for π-systems that interact strongly with a metal surface.11 For benzene adsorbed in a flat orientation and alkylbenzenes adsorbed nearly flat on gold, Gao and Weaver reported substantial band broadening in addition to large negative frequency shifts and, in some cases, splitting of one band into two. We do not observe these trends in our spectra of FBT, supporting a tilted orientation in which the ring does not coordinate to the gold surface. (10) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1983, 87, 1540-1544. (11) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985, 89, 50405046. (12) (a) Patterson, M. L.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5046-5051. (b) Moskovits, M.; DiLella, D. P.; Maynard, K. J. Langmuir 1988, 4, 67-76. (13) (a) Joo, T. H.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1987, 18, 57-60. (b) Joo, T. H.; Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1989, 93, 1422-1425. (c) Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1990, 94, 2552-2556.

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Between -600 and +800 mV, the SER bands did not shift significantly in frequency (0-4 cm-1). The only differences were increases in the relative intensities of the ν8a, ν9a, and ν18a (996 cm-1) bands and decreases in the relative intensities of the ν1, ν6a, ν7a, and ν12 bands; these changes were not large. The close similarities between the spectra indicate that the average molecular orientation is independent across this potential range. As the potential is shifted from -600 to -1000 mV, a number of out-of-plane modes that were observed only weakly, or not at all, increase in relative intensity. The νCH modes at 3044 cm-1 (ν20b) and 3055 cm-1 (ν2) disappear at -1000 mV. The relative intensities of bands due to the in-plane modes ν1, ν7a, and ν12 increase, while those for the in-plane modes ν8a and ν19a do not change. The increases in relative intensity can be explained as resulting from a fraction of the adsorbed FBT molecules tilting to a flatter orientation upon disruption of the monolayer by H2 evolution (reduction of water) at ca. -1000 mV.6 The behavior of ν13 of FBT must be noted. This ring mode primarily involves the C-F stretching vibration.9 It red shifts 15 cm-1 (from 1224 to 1209 cm-1) upon adsorption (Figure 1b). Its position is potential-dependent, shifting +10 cm-1 when the potential is stepped from -600 to +800 mV and -6 cm-1 when the potential is stepped from -600 to -1000 mV (Figure 1c). These shifts result from charge movement from the fluorine atom into and out of the C-F bond at positive and negative potentials, increasing and decreasing the strength of the bond, respectively. The intensity of ν13 remains constant for potentials between -600 and +800 mV; however, its intensity increases when shifting the potential from -600 to -1000 mV. Such a dependence of the vibrational intensity on applied potential is typical of modes sensitive to molecule-to-metal charge transfer. As the potential is shifted positive of +800 mV or negative of -1000 mV, the overall intensity of the spectrum decreases substantially. Like BT, FBT desorbs at potentials > +800 and < -1000 mV, coincident with oxidation of the metal surface or interfacial reduction of water. p-Chlorobenzenethiol (ClBT). Figure 2 displays the Raman spectrum of ClBT and the SER spectra of ClBT at -600 and -1000 mV. The frequencies, relative intensities, and assignments for the Raman spectrum of solid ClBT and SER spectra obtained at +800, -600, and -1000 mV are tabulated in the Supporting Information; the assignments are based on those of Varsanyi for pchlorobenzenethiol and agree with those of Nyquist and Evans.9,14 As for the adsorption of BT and FBT onto gold, the δCSH and νSH modes are absent from the SER spectra of adsorbed ClBT. Modes such as ν6a (741-736 cm-1), ν7a (326-321 cm-1), and ν20a (542-538 cm-1) that are associated with C-S stretching exhibit shifts of ca. -5 cm-1 upon adsorption. The shift in ν6a is smaller than the shift of -13 cm-1 for FBT and is comparable to the shift of -7 cm-1 for BT, while the shift in ν7a sharply contrasts the slight positive shift observed upon adsorption of FBT. The difference in the behavior of ν7a for FBT and ClBT arises because a “heavy” atom (chlorine), rather than a “light” atom (fluorine), is para to the heavy sulfur atom.9 Varsanyi found that when both substituents on the benzene ring are heavy, as in ClBT, ν20a and ν7a are much more substituent-sensitive than is the case for “lightheavy” substituted rings, such as FBT. For ClBT, just as for BT and FBT, the absence of νSH and δCSH, along with (14) Nyquist, R. A.; Evans, J. C. Spectrochim. Acta 1961, 17, 795801.

Halogenated Aromatic Thiols on Gold Electrodes

Figure 2. (a) Raman spectrum of crystalline ClBT. SER spectra of ClBT adsorbed on a gold electrode at (b) -600 mV and (c) -1000 mV. (d) SER spectrum of benzenethiol (BT) adsorbed on a gold electrode at -600 mV.

the shifts in modes involving C-S stretching, indicates that ClBT adsorbs on gold as the thiolate through the sulfur atom. As for BT and FBT, several of the in-plane ring modes of ClBT red-shift upon adsorption: ν12 shifts from 1064 to 1058 cm-1, ν1 from 1092 to 1080 cm-1, and ν8a from 1572 to 1563 cm-1. These shifts are less than those observed for FBT and are closer to those for BT. A few other bands associated with in-plane modes also decrease in frequency upon adsorption: ν9a shifts from 1180 to 1168 cm-1, ν19a from 1476 to 1464 cm-1, and ν18b from 1100 to 1093 cm-1. The shifts in ν9a and ν19a are less than those for FBT (-18 cm-1) and similar to those for BT (-10 cm-1) upon adsorption. (ν18b is not observed in the SER spectra of FBT and therefore cannot be compared.) As for BT and FBT, the downshifts of the ClBT ring modes upon adsorption are caused by adsorbate-to-metal charge transfer, perhaps combined with delocalization of the negative charge on sulfur into ring antibonding orbitals and/or metal-to-adsorbate back-donation into π*-orbitals.

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The shifts are less than those for FBT, as chlorine is less electronegative than fluorine. The overall similarity of the frequency shifts for BT, FBT, and ClBT upon adsorption indicates that the three molecules interact with the surface in a similar way, contacting the surface through the sulfur and not the benzene ring. Upon shifting the potential positive of -600 mV, the positions of the ring modes do not change by more than 3 cm-1. These small shifts indicate that the ring does not interact with the gold surface directly but instead is most likely tilted like adsorbed BT and FBT. The most significant spectral changes as the potential is shifted from -600 to +800 mV are the decreases in the relative intensities of the bands attributable to ν20a (νCS) at 538 cm-1, ν12 at 1058 cm-1, ν1 at 1080 cm-1, and ν8a at 1563 cm-1. For comparison, ν12 and ν1 of FBT decrease in relative intensity, but ν8a increases in relative intensity. When the potential is shifted to +800 mV, the overall intensity of the ClBT spectrum slightly decreases, and the original spectrum is not recovered upon returning the potential to +500 mV or -600 mV. Some of the monolayer apparently desorbs at potentials g 500 mV. The generally similar potential dependence of the SER spectra of ClBT and FBT indicate that ClBT and FBT adsorb in a similar orientation and interact with the surface in similar ways. Upon stepping the potential from -600 to -1000 mV, there are no substantial band shifts (Figure 2c), but the relative intensities of some bands do change. The outof-plane modes ν16b (486 cm-1), ν4 (692 cm-1), and ν10a (810 cm-1) are more intense. The symmetric ring breathing mode, ν1, becomes the most intense in-plane ring mode. The CCC bending mode, ν14, and the CCH bending mode, ν19b, which were not observed at -600 mV, are observed as weak-to-moderate (1276 cm-1) and very weak (1383 cm-1) bands, respectively. The most significant differences between the -600 and -1000 mV spectra are the appearance of three new bands of medium-to-strong intensity at 416, 993, and 1019 cm-1. These bands cannot be assigned to ClBT, even allowing for the possibility that the adsorbed molecules have completely reoriented. In fact, the positions and relative intensities of these bands are nearly identical to those of the strong bands in the SER spectrum of BT at 416, 995, and 1018 cm-1. We therefore attribute the new features in the ClBT spectrum to BT formed by reduction of some of the ClBT monolayer, forming adsorbed BT and dissolved Cl-. A -600 mV SER spectrum of BT is shown in Figure 2d for comparison. The increase in intensity and broadening of the band at 1080 cm-1 in the -1000 mV SER spectrum of ClBT are most likely due to the appearance of, and overlap with, the strong 1070 cm-1 band of BT. Also, one of the features in the ν8a region (most likely at 1561 cm-1) can be attributed to the strong ν8a band of BT. These new features in the SER spectrum of ClBT at -1000 mV decrease in intensity when the potential is shifted back to -600 mV, just as they do in the SER spectrum of pure BT.6 The reductive elimination of Cl- in adsorbed ClBT to form BT occurs at a potential about 1.5 V more positive than that for the reduction of chlorobenzene in solution to benzene and Cl-.15 The more facile reduction is partly due to the presence of the sulfur para to chlorine but also because ClBT is adsorbed, facilitating metal-to-molecule electron transfer. Having the sulfur bonded to the ring is necessary to facilitate metal-to-molecule charge transfer to form the radical ion intermediate, with the subsequent (15) Fry, A. J. In Synthetic Organic Electrochemistry, 2nd ed.; John Wiley & Sons: New York, 1989; Chapter 4.

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elimination of the halogen substituent by an established mechanism.15 By comparison, the FBT monolayer could not be reduced, presumably because fluorine is a poorer leaving group. We note that even when the applied potential is held at -1000 mV for prolonged periods, halide elimination is never complete: that is, we never reduce all of the ClBT molecules to BT. This suggests that some surface sites on the rough electrode are more electrochemically active than others. Higher conversions to BT are not possible as more negative potentials lead to the formation of H2 and the desorption of the monolayer. The optimal conditions may be at -1000 mV, where the reductive elimination of the halogen in adsorbed ClBT occurs without substantial thiolate desorption. In contrast with the 15 cm-1 downshift for the ring-F stretch (ν13) of FBT upon adsorption, the downshifts of the ring-Cl stretching modes of ClBT, ν20a (from 542 to 538 cm-1) and ν7a (from 326 to 321 cm-1), are only 4-5 cm-1. Between -1000 and +800 mV, these modes shift -1 and +8 cm-1, respectively, in contrast with the +16 cm-1 shift for the ring-F stretch of FBT. The differences in the potential dependence of the halogen-ring stretching modes of ClBT and FBT are due to differences both in the electronegativities of FBT and ClBT and in the nature of the normal modes in light-heavy and heavy-heavy parasubstituted rings.9 A smaller potential dependence of ring modes is expected for ClBT, since the effect of the surface potential on the electron density on the ring can be better compensated by Cl, which can act either as an electrondonating group or as an electron-withdrawing group. At potentials outside the range +800 to -1000 mV, ClBT irreversibly desorbs from the gold surface, as evidenced by the disappearance of spectral features assignable to ClBT. The fact that the ClBT monolayer can be electrochemically modified in situ to form a mixed monolayer is remarkable. Present electrochemical modifications to monolayers have been limited to redox cycling4,16 (i.e., electron transfer without the formal formation or breaking of covalent bonds) or the electropolymerization of the monolayer with a bulk monomeric phase.3 The present method may provide a general route toward fabricating mixed monolayers with electrochemically controlled compositions. The pure and mixed monolayers of ClBT are stable over a wide potential range, which allows for the possibility of subsequent modification of the outer surface of the monolayer, perhaps electrochemically. p-Bromobenzenethiol (BrBT). Figure 3 displays the Raman spectrum of solid BrBT and the SER spectra of BrBT at -600 and -1000 mV and at -600 mV after excursion to -1000 mV. The frequencies, relative intensities, and assignments for the Raman spectrum of BrBT and the SER spectra obtained at +500, -600, and -1000 mV are provided in the Supporting Information; the assignments are based primarily on those for the Raman spectrum of p-bromobenzenethiol.9 As for adsorbed BT, FBT, and ClBT, the δCSH and νSH modes are absent from the SER spectra. The ν6a mode, which has a small contribution from the C-S stretching vibration, shifts -9 cm-1 upon adsorption (from 727 to 718 cm-1), which is comparable to the shifts of -13 cm-1 for FBT and -5 cm-1 for ClBT. The ν20a mode (493 cm-1), which has large contributions from the C-S and C-Br stretching vibrations, does not shift upon adsorption but does increase in relative intensity. By comparison, ν20a (16) Finklea, H. O.; Ravenscroft, M. S.; Snider, D. A. Langmuir 1993, 9, 223-227 and references therein.

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Figure 3. (a) Raman spectrum of crystalline BrBT. SER spectra of BrBT adsorbed on a gold electrode at (b) -600 mV, (c) -1000 mV, and (d) -600 mV after excursion to -1000 mV. (e) SER spectrum of BT adsorbed on a gold electrode at -600 mV.

of ClBT shifts only 4 cm-1. The ν7a band, which also involves C-S and C-Br stretching, downshifts 29 cm-1 (from 267 to 238 cm-1) and decreases greatly in relative intensity. This shift is larger than those for FBT (+2 cm-1) and ClBT (-5 cm-1), reflecting the effect of increasing the mass of the para substituent. As with the other

Halogenated Aromatic Thiols on Gold Electrodes

halo-BT compounds, the absence of δCSH and νSH and the downshifts of the modes involving C-S stretching indicate that BrBT adsorbs on gold through the sulfur atom as the thiolate. In the Raman spectrum of solid BrBT (Figure 3a), we observe three peaks in the ν8b/ν8a frequency range: 1555 (w-m, sh), 1566 (m), and 1586 (w, sh) cm-1. Varsanyi reported two peaks in this region of the Raman spectrum of BrBT: ν8b at 1561 cm-1 and ν8a at 1577 cm-1.9 By comparison with our ν8a assignments for FBT (1590 cm-1, m) and ClBT (1572 cm-1, vvs), we assign the BrBT medium-intensity band at 1566 cm-1 as ν8a. The band at 1555 cm-1 is assigned to ν8b, while the shoulder at 1586 cm-1 is left unassigned. In the SER spectra of BrBT, the ν8b mode is missing. As with FBT and ClBT, ν8a exhibits a red shift (in this case, -13 cm-1 for the -600 mV spectrum) upon adsorption. The band at ∼1576 cm-1 in the SER spectra of BrBT is most likely the unassigned 1586 cm-1 band of neat BrBT. As with BT, FBT, and ClBT, the in-plane ring modes of BrBT downshift upon adsorption. For example, ν18a shifts from 1008 to 1005 cm-1, ν1 from 1069 to 1063 cm-1, ν12 from 1096 to 1072 cm-1, and ν8a from 1566 to 1553 cm-1. These shifts are comparable to those observed for BT and the other halo-BT compounds, with the downshifts of ν1 and ν12 being slightly larger than those for ClBT. Of the halo-BT compounds, FBT exhibits the largest ring mode shifts upon adsorption, most likely because of the higher electronegativity of fluorine. The observation that the ring mode red shifts for ClBT, BrBT, and BT are comparable implies that the compounds adsorb in a similar orientation, irrespective of the para substituent. Upon adsorption, several out-of-plane ring modes are observed that are absent from the spectrum of neat BrBT: these include ν16a at 412 cm-1, ν16b at 475 cm-1, ν4 at 686 cm-1, and ν17b at 803 cm-1. The presence of these modes, along with the relatively strong intensities of the in-plane ring modes (ν1, ν12, and ν8a), and the presence of νCH modes indicate that BrBT is oriented neither perpendicular nor flat and therefore must be tilted relative to the mean surface normal. When the potential of the electrode is shifted from -600 to +500 mV, few changes occur in the SER spectrum of BrBT. Frequency shifts of no more than 3 cm-1 are observed for the ring modes (as was the case for FBT and ClBT), supporting a tilted orientation for BrBT. Slight spectral changes occur at +500 mV with the appearance of ν17a as a new band at 980 cm-1 and a slightly more intense ν17b at 803 cm-1. The changes in these out-ofplane modes are reversible when the potential is shifted back to -600 mV, indicating either that a temporary reorientation of the molecules occurs at +500 mV to a flatter orientation or that metal-molecule charge redistribution occurs, perturbing these ring vibrations. At +800 mV, there is a dramatic decrease in the intensity of the SER spectrum of BrBT. This change is not reversed when the potential is shifted back to -600 mV, indicating that desorption has occurred. In contrast, the SER spectra of FBT and ClBT were still intense at +800 mV. The BrBT adlayer is apparently less stable at positive potentials. When the applied potential is shifted from -600 to -1000 mV (Figure 3c), the SER spectrum changes dramatically. First, the changes in the relative intensities of ν12 (1072 cm-1), ν1 (1063 cm-1), and ν8a (1553 cm-1) are opposite those seen when the potential was shifted from -600 to +500 mV. Second, ν10b (out-of-plane) decreases in intensity and shifts from 283 to 279 cm-1 while ν7a (in-plane) increases slightly in intensity and from 238 to

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258 cm-1. The most important change is the appearance of new bands at 350, 613, 896, 993, 1018, 1103, 1149, 1427, and 1566 cm-1. These bands have no counterparts in the Raman spectrum of BrBT; however, they are very similar to bands in the SER spectrum of BT (Figure 3e). These same BT features were observed in the ClBT SER spectrum at -1000 mV but not in the FBT spectrum at -1000 mV. We deduce that, at -1000 mV, part of the BrBT monolayer is reduced to BT, with elimination of Br-. The greater intensity of the BT bands in the BrBT -1000 mV spectrum compared with those in the ClBT -1000 mV spectrum may indicate that reductive elimination of the halide is more complete for the brominated monolayer. This is consistent with bromine being a better leaving group than chlorine. When the potential is returned to -600 mV after about 30 min at -1000 mV (Figure 3d), the BT peaks decrease significantly in intensity, indicating that some of the BT that formed has desorbed. Almost certainly this occurred during the hold at -1000 mV, prior to stepping the potential back to -600 mV. Some BT remains with BrBT in what is now a mixed monolayer. The reduction of adsorbed BrBT to BT occurs at ∼1.3 V more positive than the reduction of bromobenzene to benzene and Br-.15 Again, this is partly due to the presence of the sulfur as a ring substituent, but largely because the thiol is bonded to the electrode through the sulfur atom This link facilitates electron transfer to the ring and formation of the radical anion intermediate prior to halide elimination. We studied the formation of BT from BrBT and ClBT in two separate experiments. After the monolayers were prepared and spectra were obtained at -600 mV, the potential was stepped to -1000 mV and the 975-1100 cm-1 frequency range was scanned (∼1 spectrum/min). For BrBT, the BT peaks at 993, 1018, and 1072 cm-1 grew in with time, leveled in intensity after 9 min, and remained constant for 2 h, whereupon the potential was then returned to -600 mV. The spectrum at -600 mV (Figure 4a) shows clearly that a mixture of BT and BrBT remains on the gold surface. At more negative potentials (-1200 mV), the SER signals from BT and BrBT were lost, indicating the desorption of both thiolates. For the ClBT monolayer as well, holding the potential at -1000 mV caused the BT peaks to grow with time. They achieved maximum intensity after 9 min and began to steadily decrease in intensity after ∼40 min. Figure 4b shows the SER spectrum after 2 h at -1000 mV and a return to -600 mV. A mixture of ClBT and BT remains on the surface, and the residual BT peaks are less intense than those for the BrBT monolayer. As noted earlier, it is significant that neither the BrBT monolayer nor the ClBT monolayer is completely reduced to BT. p-Fluorobenzenemethanethiol (FBMT). Figure 5 shows the Raman spectrum of FBMT and SER spectra for adsorbed FBMT at -600 and +800 mV. The frequencies, relative intensities, and vibrational assignments for the Raman spectrum and SER spectra of FBMT at +800, +500, 0, -600, and -1000 mV are given in the Supporting Information; the assignments are primarily based on those for p-fluorotoluene and analogous para-substituted benzene derivatives.9 Upon adsorption, the νSH mode at 2566 cm-1 in the Raman spectrum of FBMT disappears. The νCS mode (assigned to the medium-to-strong band at 664 cm-1 by analogy to the assignment of νCS in BMT to a very strong band at 678 cm-1)6 shifts -14 cm-1 (to 650 cm-1) in the SER spectrum at -600 mV. This shift is similar to the -13 cm-1 shift observed for BMT under the same condi-

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Figure 4. SER spectra of (a) BrBT and (b) ClBT adsorbed on a gold electrode at -600 mV after 2 h at -1000 mV.

tions. The νCS mode shifts +4 cm-1 when the potential is shifted from -600 to -1000 mV and -1 cm-1 when the potential is shifted from -600 to +500 mV. The potential dependence of νCS, along with the absence of νSH in the SER spectra, indicates that FBMT adsorbs on gold as the thiolate, just as BMT does.6 Also supporting the bonding of FBMT to gold through the sulfur atom is the appearance of a band at ∼275 cm-1 in the SER spectra. This mode is not observed in the spectrum of neat FBMT and may be assigned to the Au-S stretch.17 Comparison of the Raman and SER (-600 mV) spectra shows that the ring modes of FBMT are perturbed more than those of BMT upon adsorption. For FBMT, the inplane ring modes shift as follows: ν1 from 848 to 840 cm-1, ν12 from 758 to 743 cm-1, and ν8a from 1598 to 1589 cm-1. For BMT, the shifts were 0 or +1 cm-1.7 Because these shifts, especially that of ν1, are much smaller than those observed by Gao and Weaver when benzene adsorbed flat on a gold surface,11 we do not view them as evidence for a flat BMT ring orientation. Instead, the shifts are probably a consequence of the electron-withdrawing power of fluorine. The shifts are smaller than those for FBT (-32, -20, and -21 cm-1, respectively) because the methylene spacer in FBMT inhibits delocalization of electrons from the sulfur atom onto the ring. Upon shifting the potential negative of -600 mV, the frequencies and relative intensities of the following key modes do not change significantly: ν1 at about 840 cm-1, ν19a at 1499 cm-1, and ν8a at 1589 cm-1. This constancy indicates that adsorbed FBMT does not reorient when the potential is shifted negative of -600 mV and that the ring of FBMT is tilted away from the surface, as in adsorbed BMT.6,8,11 No bands are observed at -1000 mV for FBMT that can be assigned to BMT, indicating that adsorbed FBMT is resistant to electrochemical reduction. (17) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740.

Figure 5. (a) Raman spectrum of liquid FBMT. SER spectra of FBMT adsorbed on a gold electrode at (b) -600 mV and (c) +800 mV.

Figure 5b and c shows the spectral changes as the potential is stepped from -600 mV to +800 mV. At an intermediate potential of 0 mV (not shown), the most noticeable changes are increases in the relative intensities of ν13 (C-CH2 stretch) at 1185 cm-1, ν9a at 1144 cm-1, and ν8a at 1585 cm-1 and the appearance of new weak bands at 577, 890, 943, 992, and 1161 cm-1; these bands are as yet unassigned. At +500 mV (not shown) and +800 mV, further changes in intensity are observed. These spectral changes are the result of either changes in the moleculesurface charge-transfer interactions or a reorientation of the adsorbed moleculessthe latter being more likely at +800 mV due to the large spectral changes that occur between +500 and +800 mV. As for BMT, FBMT completely desorbs at about -1200 mV and at about +1000 mV. This process is irreversible when there is no reservoir of FBMT in the electrolyte solution. The potential window over which the FBMT monolayer remains on gold is wider than that of FBT, indicating that FBMT forms a more stable or durable monolayer. p-Chlorobenzenemethanethiol (ClBMT). Figure 6 displays the Raman spectrum and the -600 and +800 mV SER spectra of ClBMT. The assignments for ClBMT are provided in the Supporting Information and were made by comparison with those for p-chlorotoluene and other similar para-substituted benzene derivatives.9 Upon adsorption, we observe the disappearance of the νSH mode at 2567 cm-1 for ClBMT and a shift in its νCS mode from 692 to 666 cm-1; this shift of -26 cm-1 is

Halogenated Aromatic Thiols on Gold Electrodes

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Figure 6. (a) Raman spectrum of liquid ClBMT. SER spectra of ClBMT adsorbed on a gold electrode at (b) -600 mV and (c) +800 mV.

substantially larger than the -13 and -14 cm-1 shifts that occur upon adsorption of BMT and FBMT on gold. The νCS mode for ClBMT exhibits a similar shift with applied potential to that of BMT: it does not change position from -1000 to -600 mV but shifts -6 cm-1 over the range -600 to +800 mV. As for BMT and FBMT, the results indicate that ClBMT adsorbs on gold as the thiolate. Upon adsorption, all of the ring modes of ClBMT exhibit shifts similar to those of FBMT, indicating similar interactions of ClBMT and FBMT with the electrode surface. As with FBMT, there are few noticeable changes in the SER spectrum when the electrode potential is shifted to -1000 mV. The only significant changes are the decrease in the relative intensity of ν8a at 1581 cm-1 and the slight decrease in the intensity of νCS at 666 cm-1; these changes are irreversible upon return to -600 mV. As was the case for FBMT, no bands assignable to BMT appear at -1000 mV in the ClBMT spectrum, indicating that the monolayer is not reduced. As the potential is shifted positive of -600 mV, there are substantial changes in the ClBMT spectrum (Figure 6), with many of the changes being similar to those observed for FBMT and BMT. The similarity of the general shape of the ClBMT spectrum obtained at +800

Figure 7. (a) Raman spectrum of crystalline BrBMT. SER spectra of BrBMT adsorbed on a gold electrode at (b) -600 mV, (c) -1000 mV, and (d) -600 mV after excursion to -1000 mV. (e) SER spectrum of benzenemethanethiol (BMT) adsorbed on a gold electrode at -600 mV.

mV to the spectra of FBMT and BMT at the same potential indicates that the ClBMT monolayer has undergone similar structural changes and reorientation; the differences in the spectra of FBMT and ClBMT (Figures 5a and 6a) arise from differences in the mass and electronegativity of the para substituent. As with BMT and FBMT, complete and irreversible desorption occurs at potentials positive of +800 mV and negative of -1000 mV. p-Bromobenzenemethanethiol (BrBMT). Figure 7 displays the Raman spectrum of solid BrBMT and the SER spectra of BrBMT at -600 and -1000 mV and at -600 mV after excursion to -1000 mV. The frequencies,

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relative intensities, and assignments for the Raman spectrum of BrBMT and the SER spectra obtained at +800, -600, and -1000 mV are provided in the Supporting Information; the assignments are based on those for p-bromotoluene and other similar benzene derivatives.9 Upon adsorption, the νCS band in the Raman spectrum (683 cm-1) shifts -14 cm-1 (at -600 mV); this shift is similar to the -14 and -13 cm-1 shifts observed for FBMT and BMT, respectively. As is the case for the other BMT derivatives, the νSH mode at 2563 cm-1 in the spectrum of solid BrBMT is not present in the initial -600 mV SERS spectrum (or others). The absence of νSH in the SER spectra and the shift in the νCS band indicate that BrBMT, like the other aromatic thiols, adsorbs as the thiolate. The changes in frequency of the major in-plane ring vibrations of BrBMT are similar to those of FBMT and ClBMT upon adsorption (Figure 7b), indicating similar interactions for the halo-BMTs with the electrode surface. The only difference is the ν12 mode of BrBMT, which shifts from 606 to 615 cm-1 upon adsorption. This blue shift is unique among the ring modes of the thiols in this study. Its origin is not clear but may be related to the electron-donating ability of bromine. As with BrBT, there are two bands in the ν8a region in the Raman and SER spectra. We assign the more intense one (at 1588 cm-1 in the Raman spectrum and 1583 cm-1 in the SER spectrum) as ν8a, on the basis of on the position of ν8a in the spectra of the other para-substituted haloaromatics (vide supra). When the potential of the electrode is shifted from -600 to +800 mV, the signal-to-noise ratio in the BrBMT spectrum (not shown) increases greatly, as does the overall intensity. The bands due to ν1 (1070 cm-1), ν9a (1178 cm-1), and ν8b (1565 cm-1), along with ωCH2/ν13 (1212 cm-1), increase dramatically in relative intensity. The same spectral changes were observed for ClBMT; in fact, the +800 mV spectra of BrBMT and ClBMT look very similar. This indicates that ClBMT and BrBMT interact with the gold surface in similar ways, as do ClBT and BrBT. The spectral changes that occur at +800 mV for BrBMT are not reversed upon returning to -600 mV, suggesting that BrBMT adopts a new orientation at +800 mV, perhaps facilitated by partial desorption of the monolayer. Shifting the potential from -600 to -1000 mV causes only subtle changes in the SER spectrum of BrBMT (Figure 7c). Increases in the relative intensities for some of the out-of-plane modessfor example, ν16a, ν16b, and ν17a at 405, 510, and 986 cm-1, respectivelyssuggest that the molecules may reorient the ring to be more parallel to the gold surface. At -1000 mV, the largest changes are in relative intensity, with ν18a (998 cm-1), ν9a (1173 cm-1), and ν8a (1583 cm-1) becoming stronger. The frequencies of these in-plane ring modes decrease only 1-3 cm-1 as the potential is shifted from -600 to -1000 mV. All of the spectral changes that occur at -1000 mV are irreversible when the potential is shifted back to -600 mV. This irreversibility is consistent with the desorption of some molecules at -1000 mV. An alternative explanation for the irreversible changes in the SER spectra of BrBMT is that the BrBMT monolayer may be partially reduced. When the BrBMT monolayer is held at -1000 mV, bands at 998, 1026, and 1594 cm-1 increase in relative intensity. These peaks can be assigned to adsorbed BMT in the BrBMT monolayer (see Figure 7e for comparison). Although the 999 cm-1 band is assigned to ν18a of BrBMT, there could also be a contribution from the ν12 mode of BMT. The 302 cm-1 band in the -1000 mV SER spectrum of BrBMT could be assigned to δCCS of adsorbed BMT. Although the FBMT and ClBMT mono-

Szafranski et al.

layers did not show any definitive signs of reduction at -1000 mV, the adsorbed bromo compound may be slightly more electroactive. This suggestion would be consistent with the trend observed for the halo-BTs. In any case, reduction of the adsorbed BrBMT to BMT is much less extensive than reduction of ClBT or BrBT to BT. This difference arises because electron transfer is facilitated for the halo-BTs by the sulfur atom being bonded directly to the ring. This explanation is supported by the work of Frost and co-workers, who have shown that there are considerable interactions between the π-electrons of the ring and the lone pair electrons on the sulfur atom in BT but only weak interactions in BMT.18 The BrBMT monolayer completely desorbs at +800 mV and ca. -1200 mV. The potential range over which the BrBMT monolayer remains on the electrode surface is similar to those of ClBMT and BMT and is wider than that of BrBT. We conclude that monolayers of the haloBMTs are more stable on the gold surface than monolayers of the halo-BTs. Conclusions Para-substituted halobenzenethiols (halo-BTs) and halobenzenemethanethiols (halo-BMTs) form stable monolayers on roughened gold electrode surfaces. All of the compounds studied here adsorb as thiolates via the sulfur atom, as was observed for aromatic thiols on gold in previous studies.6,8 The bond between the sulfur atom and the gold surface provides an anchor for the molecules in the monolayer. Varying the halogen atom in the para position of the halo-BTs and -BMTs produces spectroscopically distinguishable monolayers. All of the thiols in this study orient with the benzene ring tilted with respect to the local surface normal. The orientations of the thiols vary little upon changing the applied potential; however, orientations more parallel to the surface can occur when the surface coverage is decreased. Desorption occurs only at the positive and negative potential extremes: at potentials greater than +800 mV and less than -1000 mV vs SCE. The chloro- and bromo- aromatic thiols have related Raman and SER spectra and behave similarly on the electrode. The spectra of the fluoroaromatic thiols are rather different. This behavior reflects the similarity in the electron-withdrawing and -donating abilities of chlorine and bromine, in contrast to fluorine. BrBT is reduced to BT more easily at -1000 mV than is ClBT, while FBT is not reduced at all. BrBT also desorbs more extensively from the electrode at +800 mV than do ClBT and FBT. The electrochemical stability of aromatic thiolate monolayers is strongly affected by the electronic properties of substituents on the benzene ring, particularly their electronegativity and electron-donating or -accepting character. If a more physically and/or electrochemically stable monolayer is desired, a fluorine-substituted BT would be the best choice. On the other hand, the ability to reduce the ClBT and BrBT monolayers provides a means of altering the solution/monolayer interface in situ and creating mixed monolayers electrochemically. The monolayers derived from the three para-halosubstituted BMTs have similar properties. The halo-BMT monolayers remain on the surface over a slightly wider potential range than do the halobenzenethiols. They are also less electroactive, as evidenced by the fact that very little reduction to BMT is observed. The halo-BMT monolayers are less electrochemically reactive than the (18) Frost, D. C.; Herring, F. G.; Katrib, A.; McDowell, C. A.; McLean, R. A. N. J. Phys. Chem. 1972, 76, 1030-1034.

Halogenated Aromatic Thiols on Gold Electrodes

halo-BT monolayers because the methylene spacer group inhibits electron transfer from the metal to the aromatic ring. This property may be useful in fabricating monolayers with greater chemical stability. Acknowledgment. The authors thank Prof. George M. Whitesides (Harvard) and Dr. Ellen J. Zeman for helpful discussions in the early phases of this work. Support from the National Science Foundation (Grant DMR 8451962), the Office of Naval Research (Grant

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N00014-91-J-1363), BP America, the Eastman Kodak Co., and the PPG Foundation is gratefully acknowledged. Supporting Information Available: Tabulated positions, relative intensities, and vibrational assignments for frequencies between 150 and 3200 cm-1 for the Raman spectra of XBT and XBMT (X ) F, Cl, and Br) and the SER spectra of adsorbed monolayers of these compounds on gold at +800 or +500 mV, -600 mV, and -1000 mV vs SCE (7 pages). Ordering information is given on any current masthead page. LA970251U