J. Phys. Chem. 1982, 86, 3277-3279
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Surface-Enhanced Raman Study of Organic Sulfides Adsorbed on Silver: Facile Cleavage of S-S and C-S Bonds C. J. Sandroff' and D. R. Her~chbach'~~ Exxon Research and Engineering Company, Linden, New Jersey 07036 (Received: June 1, 1982)
Vibrational spectra observed by means of surface-enhanced Raman scattering (SERS) are reported for organic sulfides absorbed on silver island films. The compounds, RSSR and RSR, with R = C6H5- or C6H5CH2-, decompose on silver at room temperature to the corresponding mercaptide, implying facile cleavage of S-S and C-S bonds. Mercaptide formation from disulfides probably proceeds via a monosulfide intermediate rather than by homolytic cleavage. The ring-stretching frequencies in the mercaptides indicate that the phenyl species is "lying flat", whereas the benzyl species is "sticking up" from the surface. The decomposition reactions offer probes for SERS studies of the mechanism by which extreme pressure additives function in lubricating hydrocarbons. Monitoring of S-S bonds by SERS may also provide a means to examine catalytic activity of silver or silver sulfide surfaces. The observation of vibrational spectra of molecules adsorbed on metal surfaces at monolayer or submomolayer coverages has become almost commonplace by virtue of surfaceenhanced Raman scattering' (SERS). For suitably prepared silver surfaces, SERS amplifies the Raman cross sections more than a millionfold. In experiments applying SERS as a new approach to problems in boundary lubrication and catalysis, we have obtained vibrational spectra for many organic sulfides adsorbed on various surfaces.2 Here we report that diphenyl disulfide (DPhDS) and diphenyl sulfide (DPhS) adsorbed on silver undergo surface reactions involving facile cleavage of S-S and C-S bonds. We fiid analogous results for dibenzyl disulfide (DBDS) and dibenzyl sulfide (DBS). This suggests that S-S scission in disulfides and C-S scission in monosulfides occur generally in organic sulfides adsorbed on silver. Figure l a compares the conventional Raman spectra of bulk samples of DPhDS and DPhS. Both the vibrational frequencies and relative band intensities evince marked similarities for these compounds and the band at 542 cm-'-present in DPhDS but absent in DPhS-most clearly distinguishes them. Thus the 542-cm-l band (reported at 523 cm-' in liquid DPhDS3) can be safely assigned to the S-S stretch. Displayed in the top two plots of Figure l b are the SERS spectra of DPhDS and DPhS adsorbed on silver island films. The means of generating these films and the characterization of their optical properties have been described previously.4 The films employed here had a mass thickness of -50 A and were formed by evaporating silver at a rate of -1 s-' onto silica substrates held at 150 "C. The sulfides were applied to the island films from 0.01 M methanol solutions and the substrates were spun to ensure a thin, even adsorbate coating. The SERS spectra were obtained from a back-scattering sample geometry by using a Spex 1401 double monochromator to disperse the scattered radiation. Adsorbates were excited with 15 mW of 5145-A radiation and spectral resolution was set at -9 cm-'. Confirmation that the Raman spectra in Figure l b were from adsorbed molecules rather than a thick, bulk overlayer came from measurements of the excitation profile of the 1000-cm-' band. The profile shape was inconsistent with the u4 intensity dependence expected for scatterers from the bulk but closely parallels the silver island plasmon absorption: suggesting that the Raman scattering originated from species at or near the surface. The remarkable coincidence of the DPhDS and DPhS 'Exxon Faculty Fellow from Harvard University. 0022-3654/82/2086-3277$0l.25/0
SERS spectra in Figure l b demonstrates that they originate from the same species. Accordingly, either both DPhDS and DPhS react on the silver surface to give a common product or one of these species is converted to the other. Moreover, absence of the S-S stretch indicates that the disulfide linkage has been cleaved. The features observed in the SERS spectrum of DPhDS/DPhS at 416, 692,1000, 1020,1070, and 1572 cm-' are characteristic of the benzenethiogroup, Ca5S-, and analogous features can be found not only in bulk spectra of DPhDS and DPhS (Figure l a ) but in phenyl mercaptan as welL5 This, together with the absence of the S-S stretch, suggests that the species in Figure l b is either adsorbed DPhS or the common reaction product, a phenyl mercaptide. The SERS spectrum of phenyl mercaptan adsorbed on silver is shown below DPhDS and DPhS in Figure lb. The fidelity among these spectra clearly shows that both DPhDS and DPhS decompose on silver surfaces to the adsorbed phenyl mercaptide. These reactions imply, of course, the ready cleavage of S-S and C-S bonds. Also noteworthy in the SERS spectra are the vibrational frequency shifts exhibited by phenyl mercaptan upon adsorption as the mercaptide: the X-sensitive 1092-cm-' band3l5and the ring mode at 1581 cm-' in the mercaptan5 shift to 1072 and 1572 cm-', respectively. To explore the generality of the S-S and C-S cleavage reactions on silver surfaces we performed analogous SERS studies for the alkyl sulfides, DBDS, DBS, and benzyl mercaptan. The results are displayed in Figure 2 and can be interpreted exactly as before: DBDS and DBS decompose on silver surfaces to form the adsorbed benzyl mercaptide. Thus it appears that S-S and C-S linkages in organic disulfides and monosulfides are easily cleaved on silver under quite mild conditions. The reaction of DPhS or DBS to give the corresponding adsorbed mercaptide is presumably straightforward, involving only the cleavage of C-S bonds. From bond dissociation energies and the 10 kcal/mol bond weakening effect of adjacent phenyl groups on S-X bonds: one can
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~~
(1) For a recent review see "SurfaceEnhanced Raman Scattering",R.
K. Chang and T. E. Furtak, Ed., Plenum Press, New York, 1982. (2)C. J. Sandroff and D. R. Herschbach, Bull. Am. Phys. SOC.,27,No. 3 (1982). (3) J. H.S. Green, Spectrochim. Acta, Part A , 24, 1627 (1968). (4)D.A.Weitz, S.Garoff, and T. J. Gramila, Opt. Lett., 7,168(1982). ( 5 ) P.W. Scott, J. P. McCullough, W. N. Hubbard, J. F. Messerly, I. A. Hossenlopp, F. R. Frow, and C. Waddington, J. Am. Chem. SOC.,78, 5463 (1956). (6) S. W. Benson, Chem. Reo., 78,23 (1978). Thermochemical data on DBDS and DBS are not included in this review. Their behavior, however, should be similar to their n-alkyl analogues.
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The Journal of Physical Chemistry, Vol. 86, No. 17, 1982
Letters
estimate that C-S bonds in DPhS and DBS should be of nearly equal strength. Accordingly, the decomposition of diary1 and dialkyl monosulfides on silver, where a C-S bond is replaced by a strong Ag-S bond, should occur as readily with DPhS as with DBS. The mechanism for reaction of DPhDS and DBDS for form the corresponding mercaptide must be somewhat more complicated. Here, the apposite linkage-the S-S bond-is -20 kcal/mol weaker in DPhDS than in DBDS6 This is in marked contrast with the related monosulfides, where the cleaving C-S bonds are equally strong. Also, based on the monosulfide surface chemistry, one can postulate two mechanisms leading to conversion of the disulfides to the mercaptide. One involves homolytic cleavage of the S-S bonds and the other conversion of the disulfides to the corresponding monosulfides which subsequently yield the observed products. Thermochemical data for the organic sulfides6and silver sulfide7 imply that the reaction
RSSR
+ 2Ag
-+
RSR
+ Ag,S
Raman Shifl (cm')
with R E C6H5- or C6H,CH2- should proceed exothermically. In studies of catalytic decomposition of DBDS on iron powder DBS has been observed as the major product.8 This suggests that monosulfides are intermediates in mercaptide fromation from disulfides on silver surfaces. The apparently facile and complete reaction of DPhDS and DBDS to their mercaptides, despite the 20 kcal/mol difference in their S-S bond dissociation energies, is also consistent with a mechanism requiring the formation of a monosulfide intermediate. Finally, we mention the weak band at 1600 cm-', present in the SERS spectrum of DPhDS and DPhS but not in the phenyl mercaptide spectrum. Both the frequency of this band and its absence in the phenyl mercaptide spectrum suggests that it derives from an aromatic decomposition product of DPhDS and DPhS. Formation of such a by-product is inconsistent with homolytic cleavage of the S-S bond but would be expected if the disulfide yielded mercaptide through a monosulfide intermediate. Besides offering evidence that organic mono- and disulfides cleave on silver surfaces to give the corresponding mercaptides, Figures 1 and 2 contain information suggesting that phenyl and benzyl mercaptide assume different geometries on these surfaces. That both phenyl and benzyl mercaptide are attached to the silver surface via their sulfur atoms can be inferred from the vibrational frequency shifts in modes involving sulfur: in phenyl mercaptan, the X-sensitive 1092-cm-l bond shifts downward by 22 cm-' upon adsorption as the mercaptide, while the C-S stretch in benzyl mercaptan at 680 cm-' becomes a strong band at 652 cm-' in the mercaptide. Further evidence for a silver-sulfur interaction can be gleaned from the weak, broad signals found in the low-frequency region of the SERS spectra of the mercaptides. The band near 246 cm-' in phenyl mercaptide and 302 cm-I in benzyl mercaptide, having no obvious analogue in the Raman spectra of the neat mercaptans, could be attributable to a Ag-S stretch. That the Ag-S stretch has been reported to lie within the 150-250-cm-' range in several alkyl and aryl silver thiolatesQcould indicate different binding of the mercaptides on surfaces than in the bulk. One would not expect the rather large frequency shifts of sulfur-sensitive bands to be mirrored by the C=C ring
Figure 2. Raman spectra of compounds containing the C,H,CH,Sgroup: (a) normal Raman spectra of bulk samples of dibenzyl disulfide (DBDS) and dibenzyl sulfide (DBS); (b) SERS spectra of DBDS, DBS, and benzyl mercaptide on 50-A silver island films. Conclusions are analogous to those in Figure 1.
(7) "Handbook of Chemistry and Physics", 54th ed, Chemical Rubber Co., Cleveland, OH, p D-69. (8) C. H. Bovington and B. Dacre, ASLE Trans., 25, 44 (1982). (9) G. A. Bowmaker and L. C. Tan, Aust. J . Chem., 32, 1443 (1979).
Raman Shin (cm')
Figure 1. Raman spectra of compounds containing the C,H,S-group: (a) normal Raman spectra of bulk samples of diphenyl disulfide (DPhDS) and diphenyl sulfide (DPhS); (b) SERS spectra of DPhDS, DPhS, and pnenyi mercapran on 3 ~ - A siiver ismno riims. rtaeilry mween spectra shows that DPhDS and DPhS decompose to the mercaptide. No sign of a S-S stretch can be seen in any of the SERS spectra. Unassigned bands are from the bare island films or are extraneous.
J. Phys. Chem. 1082, 86, 3279-3281
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ducing the severity of wear for moving metal parts under high loads. The superior antiwear action of DBDS over DBS on steels is generally rationalized by assuming the disulfide more readily forms a mercaptide intermediate which later undergoes C-S bond cleavage to produce a sulfide film.12 Our results on silver, however, show that the di- and monosulfidesform adsorbed mercaptides with comparableease. Furthermore, starting from the disulfide, silver sulfide is probably formed before the mercaptide rather than after. We hope that SERS studies on silver in conjunction with classic wear tests will reveal the relationship between surface adsorbates and effective antiwear action. Such tests are feasible under realistic engineering conditions since SERS permits the observation of adsorbates even in the presence of a bulk liquid overlayer of lubricating hydrocarbons.2 The ready observation of SERS signals from decomposed DPhDS and DBDS even in the likely presence of Aga is also pertinent to studies of catalysis. Silver sulfide is believed to catalyze the formation of S2N2from S4N4on Ag gauze a t quite modest temperature^.'^ Formation of S4N4is the key step in producing the conducting polymer (SN),, a “nonmetallic metal”. Our results suggest SERS observations of this process should be feasible.
stretches since the latter are relatively insensitive to the sulfur environment. Indeed, in the family of compounds, DBDS (1602 cm-’), DBS (1598 cm-l), benzyl mercaptan (1602 cm-l), and benzyl mercaptide adsorbed on silver (1600 cm-’), scarcely any variation in ring-stretching frequency occurs. A distinctly different frequency behavior prevails for compounds containing the benzene thio group: DPhDS (1578 cm-’), DPhS (1582 cm-’), phenyl mercaptan (1581 cm-’), and phenyl mercaptide adsorbed on silver (1572 cm-l). Clearly, the ring-stretching frequency in the phenyl mercaptide is significantly affected by adsorption onto the silver surface, in sharp contrast to the adsorbed benzyl mercaptide. These data are consistent with the phenyl mercaptide “lying flat”on the silver surface and the benzyl mercaptide “sticking up” from the surface, with both compounds strongly bound to the silver surface through their sulfur atoms. It seems that the molecular geometry of the phenyl mercaptide (most likely planar, in contrast to the benzyl mercaptide) strongly influences adsorbate configuration. The observation of adsorbed organosulfur monolayers by SERS offers a powerful means to elucidate the mechanism of action of antiwear additives and other boundary lubricants.’0 DBDS, for example, is a prototype “extreme pressure” additive.lOJ1 Most theories of antiwear action presume DBDS decomposes on metal surfaces to form a protective, easily sheared metal sulfide layer thereby re-
Acknowledgment. We thank David A. Weitz for very helpful conversations and advice concerning SERS from island films.
(10)H.Czichos, ‘Tribology, A System Approach to the Science and Technology of Friction Lubrication and Wear”, Elsevier, New York,1978. (11)T. Sakurai, J.Lubr. Technol., 103,473(1981).
(12)B. Dacre and C. H. Bovington, ASLE Trans., 25, 272 (1982). (13)M.M.Labeg, P. Love, and L. F. Nichols, Chem. Rev., 79,1(1978).
Direct Detection of Spin-Polarized ESR Spectra of Biacetyi n7r* Triplet States in Organic Matrices at 77 K Hlsao Mural, Takashl Imamura, and Klnlchl Obl’ Department of Chemistry, Tokyo Institute of Technobgy, Ohokayam, Msguro, Tokyo, Japan (Received: June 9, 1982)
Transient spin-polarized ESR signals of the phosphorescent state of biacetyl have been studied in organic matrices. From analysis of the spectra, it is concluded that the D and E values are positive and negative, respectively, and the initial population to the 2 sublevel whose principal axis lies along the C=O direction is predominant. The increase of the Y sublevel populating rate in n-hexane suggests that the population of the spin sublevels through intersystem crossing is sensitive to minute distortions of the molecular frame.
Introduction The photophysical and photochemical properties of the lowest triplet states of biacetyl have been a target of recent spectroscopic research in conjunction with the electronic structure. Chan and his co-workers investigated small a-dimbonyh extensively using the ODMR technique and gained a lot of valuable spectroscopic and kinetic inforThey concluded mation concerning the triplet that the biacetyl triplet sublevel having the fastest populating rate by S1-T1 intersystem crossing was 2 whose principal axis lies approximately along the C=O direction and the most emissive level was also 2. The signs of the D and E values they suggested were negative and positive, (1)I. Y.Chan and R. H. Clarke, Chem. Phys. Lett., 19, 53 (1973). (2)I. Y.Chan and K. R. Walron, Mol. Phys., 34,65 (1977). (3)I. Y.Chan and S. Hsi, Mol. Phys., 34, 85 (1977). 0022-3654/82/2086-3279$01.25/0
re~pectively.~Recently we started time-resolved ESR studies of phosphorescent states and have already reported spin-polarized spectra of the 3 n ~ state * of benzophenone in glasses: In t h i s Lef% the application ofthis technique to biacetyl in o r g ~ matrices c and an important conclusion about the signs of D and E which are different from those be shown* reported by Chan and Hsi3
Experimental Section An X-band ESR spectrometer, Varian E-112, was modified for the time-resolved experiment. The experimental technique was the same as the one reported previously,4 which was introduced by the pioneering work of A MR-50E transient memory and a Weissman et (4) H. Murai, T. Imamura, and K. Obi, Chem. Phys. Lett., 87, 295 (1982).
0 1982 American Chemical Society
