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Effect of Fluorination on Thiol Reactivity: Reaction of 4,4,4-Trifluorobutanethiol on Mo(110) M. E. Napier and C. M. Friend* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 Received May 9, 1995X The reaction of 4,4,4-trifluorobutanethiol on Mo(110) was studied using temperature-programmed reaction, Auger electron, and infrared spectroscopies. The chemistry of trifluoro-1-butanethiol on clean Mo(110) at saturation coverage closely resembles the chemistry observed for 1-butanethiol and trifluoroethanethiol, but there are important differences arising from the presence of fluorine and the length of the alkyl chain. The dominant decomposition pathway for CH3(CH2)3S-, CF3(CH2)3S-, and CF3CH2S- is C-S bond hydrogenolysis at approximately 300 K to form butane, trifluorobutane, and trifluoroethane, respectively. A secondary pathway is alkene elimination from CH3(CH2)3S-, CF3(CH2)3S-, and CF3CH2S- to form butene, trifluorobutene, and difluoroethylene, respectively, over a wide temperature range, 200-550 K. The primary difference in the chemistry comparing the two C4 thiolates, the fluorinated and nonfluorinated, is the observation of difluoroethylene from CF3(CH2)3S- at 540 K. No ethylene or other hydrocarbon products were formed at such high temperatures in the reactions of CH3(CH2)3S-. The observation of difluoroethylene from the reaction of CF3(CH2)3S- is most likely due to a difference in the competing rates of complete decomposition and selective elimination. The increased strength of C-F relative to C-H bonds, and the increased strength of the C-C and C-H bonds adjacent to the trifluoromethyl group, may help to preserve a fluorocarbon intermediate, possibly trifluoroethyl, up to the more elevated temperatures such that difluoroethylene is formed. In addition, calculations suggest that the reaction of trifluorobutanethiol yielding difluoroethylene is thermodynamically more favorable. The chemistry of C2and C4-fluorinated thiolates is also compared and interpreted in terms of both geometric and energetic factors.
Introduction The bonding and reactions of organosulfur molecules have been studied on a number of metal surfaces due to their relevance to catalytic processes.1-10 We undertook the investigation of fluorinated thiols, trifluoroethanethiol11 and trifluorobutanethiol, on Mo(110), in order to determine how fluorination affects the thermal stability of the alkanethiol layers, and to fully understand the effect of electron exposure on methyl- and trifluoromethylterminated organic monolayers. A complete understanding of the electron-induced chemistry is important to technologies, such as lithography and electron beam writing. In addition, comparison of the reactivity of fluorocarbon and hydrocarbon thiols could provide insight into the surface chemistry of thiols in general. As expected, the addition of a trifluoromethyl head group alters the reactivity of alkanethiols on Mo(110). Alkanethiols react on molybdenum surfaces with S-H bond scission at low temperatures, forming the adsorbed thiolate and surface hydrogen. The rate of C-S bond scission controls hydrocarbon formation. Ethane is formed at 300 K and ethylene at 340 K from CH3CH2S-, with the relative amounts of ethane and ethylene determined, in part, by the amount of hydrogen on the surface. Ethylene formation predominates at higher temperatures, as the X
Abstract published in Advance ACS Abstracts, March 15, 1996.
(1) Parker, B.; Gellman, A. J. Surf. Sci. 1993, 292, 223. (2) Jaffey, D. M.; Madix, R. M. Surf. Sci. 1994, 311, 159. (3) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (4) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (5) Albert, M. R.; Lu, J. P.; Bernasek, S. L.; Cameron, S. D.; Gland, J. L. Surf. Sci. 1988, 206, 348. (6) Roberts, J. T.; Friend, C. M. J. Am. Chem. Soc. 1986, 108, 7204. (7) Roberts, J. T.; Friend, C. M. J. Phys. Chem. 1988, 92, 5205. (8) Huntley, D. R. J. Phys. Chem. 1989, 93, 6156. (9) Friend, C. M.; Roberts, J. T. Acct. Chem. Res. 1988, 21, 394. (10) Castro, M. E.; Ahkter, S.; Golchet, A.; White, J. M.; Sahin, T. Langmuir 1991, 7, 126. (11) Napier, M. E.; Friend, C. M. J. Phys. Chem. 1995, 99, 8750.
surface hydrogen supply is depleted through recombination. 1-Butanethiolate reacts via C-S bond hydrogenolysis to form butane at 300 K and dehydrogenation to form butene at 265 and 340 K.6,12 Again, the relative amounts of butane and 1-butene are determined, in part, by the amount of hydrogen on the surface.12 Increasing the hydrogen coverage through adsorption of hydrogen or deuterium, prior to the adsorption of the alkanethiol, increases the amount of the hydrogenolysis product, alkane, relative to the dehydrogenation product, alkene. This study was specifically undertaken to determine the effect of the addition of the trifluoromethyl group on thiol chemistry. Trifluoroethanethiol is proposed to form the thiolate and surface hydrogen after low-temperature S-H bond scission, analogous to the low-temperature reaction of alkanethiols on Mo(110).11 However, the chemistry of trifluoroethanethiolate is quite complex and significantly altered from that observed for ethane thiolate.7,11 Most significant is the evolution of 1,1,1trifluoroethyl radical (CF3CH2•) at 265 K. Ethyl radical is not detected in the reaction of ethane thiolate on Mo(110).7,14 C-S bond hydrogenolysis, yielding trifluoroethane, and defluorination, yielding difluoroethylene, are of nearly equal importance, whereas C-S bond hydrogenolysis of ethanethiolate to form ethane predominates. Although much of the chemistry of trifluoro-1-butanethiol on clean Mo(110) at saturation coverage closely resembles the chemistry observed for 1-butanethiol and (12) In an initial report on butanethiol reaction on Mo(110),6 no lowtemperature (265 K) butene was observed. The absence of the lowtemperature butene state was due to a high hydrogen background in the chamber used for the initial experiments and consequently a high surface hydrogen concentration. The initial results could be replicated in the current system by preadsorbing hydrogen or deuterium prior to the adsorption of butanethiol. As discussed in the text, preadsorption of hydrogen alters the product distribution, such that the hydrogenolysis product, butane, is favored. (13) Weldon, M. K.; Friend, C. M. Rev. Sci. Instrum. 1995, 66 (11), 5192. (14) Napier, M. E.; Friend, C. M. Unpublished results.
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trifluoroethanethiol, there are significant differences arising from the presence of fluorine and the alkyl chain length. Again, the predominant trifluoro-1-butane thiolate pathway is C-S bond hydrogenolysis to form 1,1,1trifluorobutane at 305 K. A secondary pathway is dehydrogenation to form 4,4,4-trifluoro-1-butene over a wide temperature range, 200-550 K. Analogous to 1-butanethiol, the relative amounts of trifluorobutane and trifluorobutene are determined, in part, by the amount of hydrogen on the surface. The primary difference between 1-butanethiolate and trifluorobutanethiolate reactivity is the formation and evolution of a high-temperature product, difluoroethylene at 540 K, in conjunction with the hydrogen fluoride and a trace amount of ethylene. The primary difference in the reactivity of the C2 and C4 fluorinated thiolates is the observation of a low-temperature fluorine elimination pathway and trifluoroethyl radical evolution from the CF3CH2S- reaction. Radicals are not observed for the ethane, butane, or trifluorobutanethiolates. Geometric and energetic factors play a significant role in determining the effect of fluorination on the thiolate chemistry. Experimental Section Experiments were performed in an ultrahigh vacuum (UHV) chamber, with a base pressure below 1 × 10-10 Torr. The chamber was equipped with a UTI quadrupole mass spectrometer, lowenergy electron diffraction (LEED) optics, and an Auger spectrometer with cylindrical mirror analyzer (Perkin Elmer, Model 15-155), all described previously.15 The Mo(110) crystal (Metal Crystals, Ltd.) could be cooled to 100 K or heated to 900 K radiatively or to 2300 K by electron bombardment. The Mo(110) surface was cleaned before each experiment by oxidation at 1200 K in 1 × 10-9 Torr of O2 for 5 min. The crystal temperature was allowed to return to ∼200 K before flashing to 2300 K for 30 s to remove the residual oxygen. No surface carbon or oxygen were detected in the Auger electron spectra of the surface recorded prior to fluorinated thiol adsorption. A sharp (1×1) low-energy electron diffraction pattern was also observed. The trifluorobutanethiol sample was prepared using established methodology.16 The samples were degassed by several freeze-pump-thaw cycles before each use. Dioxygen (Matheson, 99.8%), dihydrogen, and dideuterium were used as received. The crystal was positioned approximately 2 mm from the aperture of the mass spectrometer shield during collection of temperature-programmed reaction data. The sample was biased at -60 V in order to reflect electrons generated in the mass spectrometer and, therefore, preclude electron-induced decomposition. The mass spectrometer was interfaced to a PC. The data reported here were collected with a program which allowed collection of up to 10 separate ion intensity-temperature profiles during a single experiment. The heating rate was constant with dT/dt ) 4 K/s between 110 and 650 K. All infrared reflection absorption spectra were collected using a single-beam, clean-air-purged, Fourier transform infrared spectrometer (Nicolet, Series 800) and averaged over 300 scans using an MCT detector at a 4 cm-1 resolution; the scan time was approximately 5 min.13 Sample spectra were ratioed against a background taken immediately after the sample scan by flashing the crystal to 900 K. The background scan was initiated after the crystal temperature had returned to ∼100 K.
Figure 1. Temperature-programmed reaction of condensed trifluorobutanethiol on initially clean Mo(110). The spectra for the ions representative of the three fluorocarbon products are shown: (a) trifluorobutane, m/e ) 29; (b) trifluorobutene, m/e ) 110; (c) difluoroethylene, m/e ) 64. Difluoroethylene evolves at 540 K only. The mass 64 peak at 300 K is due to the cracking of both trifluorobutane and trifluorobutene. Also observed: (d) trifluorobutanethiol, m/e ) 144; (e) dihydrogen, m/e ) 2; and (f) hydrogen fluoride, m/e ) 20.
Temperature Programmed Reaction Spectroscopy. Three fluorocarbon products, 1,1,1-trifluorobutane, CF3(CH2)2CH3, 4,4,4-trifluoro-1-butene, CF3CH2CHdCH2, and 1,1-difluoroethylene, CF2dCH2, are produced during temperature-programmed reaction of a saturation coverage of trifluorobutanethiol (Figure 1). The only other
gaseous products detected in a comprehensive search for products in the range of 2-290 amu were a trace amount of ethylene, CH2dCH2, (data not shown) gaseous hydrogen fluoride (HF), and gaseous dihydrogen (H2). All products are observed at saturation coverage and identified by quantitative analysis of mass spectrometer data. The identification of 1,1,1-trifluorobutane and 4,4,4-trifluoro1-butene was complicated due to the fact that neither reference mass spectra nor authentic samples were available for comparison. The specific procedure used for the mass spectral identification of trifluorobutane and trifluorobutene is described below. 1,1,1-Trifluorobutane evolves at 305 K (Figure 1a, m/e ) 29, and Table 1).17 Mass fragment ratios for trifluorobutane formed on the hydrogenated surface match within experimental error the fragment ratios for the 305 K trifluorobutane from the clean surface, with one exception. The mass ratios involving m/e ) 110, the trifluorobutene parent, from the hydrogen-saturated surface are smaller by approximately half, when compared
(15) Weldon, M. K.; Friend, C. M. Surf. Sci. 1994, 310, 95. (16) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
(17) Identification of 1,1,1-trifluorobutane was made by comparison to the mass fragmentation pattern obtained for trifluorobutane evolving from a hydrogen-presaturated surface (Table 1).
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Table 1. Mass Ratios from Hydrogen Precovered and Clean Mo(110) Surfaces temp, K 300 270 305 340
27
29
41
43
69
91
Hydrogen Pre-covered Surface 50 100 20 9 8 1 50 45 45
Clean Mo(110) Surface 100 32 7 7 100 18 7 6 100 23 6 6
3 1.5 1.5
93
110
0.5
0.2
0.6 0.6 0.5
1 0.5 0.7
to the same ratios for the clean surface. This suggests that a trace amount of trifluorobutene may be formed and desorbing at this temperature. Because the amount of trifluorobutene formed at 305 K is relatively small, there is little change in the fragments ratioed to mass 29. Trifluorobutene evolves in significant quantities at 265 and 340 K. Presaturation of a molybdenum surface with hydrogen or deuterium alters the product distribution of both alkanethiols and fluoroalkanethiols, such that the predominant product is the saturated hydrocarbon or fluorocarbon, in this case trifluorobutane.11,14 The fragmentation pattern from the hydrogen-presaturated surface was used to identify trifluorobutane, because of the lack of available reference spectra and lack of an authentic sample for mass spectral comparison. The mass fragment pattern measured for trifluorobutane evolving from the hydrogen presaturated surface at 305 K, and for the fluorocarbon reaction products from the clean surface at three temperatures, 270, 305, and 340 K, are given in Table 1. The intensities of the representative trifluorobutane parent masses, m/e ) 111, CF3(CH2)2CH2, and 112, CF3(CH2)2CH3, from the clean surface are small, but are in agreement with those measured for the surface presaturated with hydrogen. A detailed line shape analysis of the m/e ) 110 spectrum at saturation coverage reveals that 4,4,4-trifluoro-1-butene evolves over a wide temperature range, 200-550 K (Figure 1b, m/e ) 110). Again, neither a reference spectrum nor an authentic trifluorobutene sample was available for comparison. All trifluorobutene fragments are also fragments of trifluorobutane; however, the mass fragment ratios for the 270 and 340 K peaks do not perfectly match those for the trifluorobutane states on either the clean (305 K peak) or the hydrogen presaturated surface (Table 1), indicating that there is a second fluorocarbon product. The peak shapes and temperatures for m/e ) 110 and 112, the trifluorobutene and trifluorobutane parent masses, respectively, are distinctly different, clearly indicating that both are products. Identification was made by comparing the fragment ratios of ions associated with the two fluorocarbon products, to the parent ions. In all cases, the fragments which have undergone a loss of two hydrogens with respect to fully hydrogenated fragments (i.e., m/e ) 91, CF2CH2CHCH2, compared to m/e ) 93, CF2CH2CH2CH3), match the peak position and peak shape of the trifluorobutene parent, m/e ) 110 (Figure 2). For example, m/e ) 110, CF3CH2CHdCH2, the trifluorobutene parent ion; m/e ) 91, CF2CH2CHdCH2; and m/e ) 41, CH2CHdCH2, are all characteristic of trifluorobutene (Figure 2a) and have the same peak positions and shapes. Trifluorobutane masses, m/e ) 93, CF2(CH2)2CH3, and m/e ) 43, CH2CH2CH3 (Figure 2b), have a distinctly different peak shapes. Trifluorobutene evolves over a wide temperature range up to 550 K. The third fluorocarbon product, difluoroethylene (Figure 1c, m/e ) 64) evolves only at 540 K. Difluoroethylene was identified by comparison to the cracking of an authentic difluoroethylene sample in our vacuum system. The large mass 64 peak at 305 K can be attributed to the combined cracking of both trifluorobutane and trifluo-
Figure 2. Comparison of the peak shapes and positions of masses characteristic of trifluorobutene and trifluorobutane, demonstrating the formation of the the two products, during temperature-programmed reaction of condensed trifluorobutanethiol on initially clean Mo(110). The spectra for representative ions: (a) trifluorobutene parent, m/e ) 110, compared to trifluorobutane, m/e ) 93 and 43; and (b) trifluorobutene, m/e ) 110, 91, and 41. The trifluorobutene, m/e ) 110, evolution is observed from 200 to 550 K.
robutene and is not due to significant evolution of difluoroethylene at 300 K. Fluorination stabilizes the high-temperature intermediate leading to difluoroethylene formation. No hydrocarbon products, specifically ethylene, were observed at high temperature from the reaction of butanethiol. Condensed layers of trifluorobutanethiol sublime from Mo(110) in a sharp peak at 140 K (Figure 1d, m/e ) 144). As expected, this peak increases indefinitely with continued trifluorobutanethiol exposure. Dihydrogen (Figure 1e, m/e ) 2) evolves at 345 and 550 K, whereas hydrogen fluoride (Figure 1f, m/e ) 20) and a trace amount of ethylene, m/e ) 28 (data not shown) evolve concurrent with difluoroethylene at 540 K. Hydrogen adsorbed to saturation coverage on initially clean Mo(110) evolves at approximately 350 K.18 The lowtemperature dihydrogen is predominantly due to the recombination of the thiol hydrogens. In contrast, the high-temperature dihydrogen is reaction-limited. The high-temperature dihydrogen and hydrogen fluoride are attributed to a combination of nonselective decomposition of a surface intermediate containing both intact C-H and C-F bonds and defluorination to difluoroethylene. Both intact C-H and C-F bonds are observed in the infrared spectra taken at 480 K, prior to high-temperature dihydrogen and hydrogen fluoride evolution. At saturation coverage, the amount of sulfur deposited was found to be 0.26 ( 0.02 monolayers of thiolate based on the S(LMM) Auger electron intensity after heating condensed trifluoroethanethiol, compared to the intensity measured for a 0.35 monolayer sulfur overlayer.19 The saturation coverage was confirmed to be 0.26 ( 0.02 monolayers based on the sulfur yield at high temperatures, 1450-2080 K. The integrated area under the sulfur peak following trifluorobutanethiol reaction was compared to the integrated area under the sulfur peak for a known sulfur coverage of 0.35 monolayers, to derive the saturation coverage. Residual carbon and fluorine were also detected (18) Roberts, J. T.; Friend, C. M. Surf. Sci. 1987, 186, 201. (19) Weldon, M. K.; Napier, M. E.; Wiegand, B. C.; Friend, C. M.; Uvdal, P. J. Am. Chem. Soc. 1994, 116, 8328.
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Figure 3. The relative product yields, as a function of thiolate coverage (S deposited) for (a) trifluorobutane, (b) trifluorobutene, (c) difluoroethylene, and (d) hydrogen fluoride. The relative yields were calculated by dividing the peak height at each sulfur coverage by the maximum peak height observed for each species. The error bars are indicative of one standard deviation based on peak height measurements of five or more temperature-programmed reaction spectra.
by Auger electron spectroscopy, following temperatureprogrammed reaction to 750 K. The distribution of products evolved during temperature-programmed reaction of trifluorobutanethiol on initially clean Mo(110) depends on the initial trifluorobutanethiol coverage (Figure 3).20 At the lowest coverages studied, θ e 0.1 monolayers of thiolate, dihydrogen and minor amounts of the most intense hydrocarbon and fluorocarbon trifluorobutane fragments were observed in a broad, low-intensity state at approximately 350 K, suggesting formation of trifluorobutane. The low intensity of these masses preclude unequivocal identification, but it is clear that nonselective decomposition of trifluorobutanethiol is the predominant reaction pathway at low coverages. Dihydrogen is evolved in one state at 420 K, with a high-temperature shoulder at 500 K (data not shown). Hydrogen fluoride was not detected in this low coverage range. At intermediate coverages, 0.1 e θ e (20) Shown in Figure 3 are the relative yields of the three fluorocarbon products and hydrogen fluoride versus sulfur or thiolate coverage. The relative yields were calculated by dividing the peak height at each coverage by the maximum peak height observed for each species, giving a maximum relative yield of 1. The masses used to calculate the yields are as follows: mass 29 for trifluorobutane, mass 110 for trifluorobutene, mass 64 for difluoroethylene, and mass 20 for hydrogen fluoride. The error for each measurement is based on the calculation of one standard deviation for a minimum of five experiments.
0.20 monolayers of thiolate, trifluorobutane is the major fluorocarbon product desorbing from the surface at 305 K. Trifluorobutene evolves in one state at 320 K, with trace amounts continuing to evolve up to 550 K. Dihydrogen evolves in two states at 345 and 550 K, and hydrogen fluoride evolves in one state at 540 K. As discussed above, the 265 K trifluorobutene state was not observed until the thiolate coverage approached saturation. At exposures corresponding to approximately 0.15 monolayers of thiolate and above, difluoroethylene evolves at 540 K. Nonselective decomposition to form atomic carbon, fluorine, sulfur, gaseous dihydrogen, and gaseous hydrogen fluoride is a competing pathway. Nonselective decomposition predominates at low coverages. Carbon, fluorine, and sulfur remain on the surface at all coverages, as determined by Auger electron spectroscopy. The fluorine signal is at a maximum, when the high-temperature difluoroethylene state saturates at 0.26 monolayers. The carbon signal is also at a maximum at saturation coverage. The presence of deuterium or hydrogen significantly alters the product distribution for temperature-programmed reaction of a saturation coverage of trifluorobutanethiol and decreases the saturation coverage of thiolate. When a saturation coverage of hydrogen or
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Table 2. Infrared and Electron Energy Loss Assignments assignment
trifluorobutanethiolate
trifluoropropyl iodide Cu(111)
trifluoro-3-chloropropane
C-H stretch CH2 scissor CH2 wag CF stretch νa(C-F) CF stretch νs(C-F) CH2 twist C-C stretch CH2 rock
2982, 2950 1440 1392, 1314 1257 1160 1205 1086 1024
2985 1350
3010, 2980, 2965, 2920 1436 1395, 1313 1250 1150 1215 1042 980
Figure 4. Reflection infrared spectra of CF3(CH2)3S at (a) submonolayer, θ ) 0.1 monolayers, and (b) saturation, θ ) 0.25 monolayers.
deuterium are adsorbed on Mo(110), prior to trifluoroethanethiol exposure, the maximum amount of thiolate formed was reduced from 0.26 to 0.18 monolayers. Trifluorobutane is the major fluorocarbon product evolving from the surface in one state at 305 K. As surface hydrogen recombines and is depleted, trifluorobutene formation at 320 K accounts for a minor product pathway. Hightemperature evolution of trifluorobutene fragments is still observed, but significantly reduced in intensity from the clean surface. Only one deuterium is incorporated into the trifluorobutane product and no deuterium into the trifluorobutene, indicating that there is no H-D exchange at this temperature. The low-temperature trifluorobutene and the high-temperature difluoroethylene at 540 K are no longer observed. HF evolution at 540 K is significantly reduced by adsorbed H or D. When deuterium is preadsorbed, a minor amount of DF is observed, at 545 K, 5 K higher than HF evolution, suggesting that a small amount of H-D exchange may be occurring at higher temperatures. The reduction in the integrated yield of all dihydrogen isotopes and in the integrated yield of hydrogen fluoride indicates that preadsorption of deuterium significantly abates both irreversible decomposition and trifluorobutene formation. Infrared Spectroscopy. Infrared spectra were obtained for trifluorobutanethiolate coverages ranging from 0.1 to 0.25 monolayers (Figure 4a,b and Table 2). The spectra are quite complex with multiple modes below 1500 cm-1. Assignment of the modes at saturation coverage was made by comparison to infrared spectra of liquid phase 1,1,1-trifluoro-3-chloropropane21 and electron energy loss spectra of a monolayer of 1,1,1-trifluoro-3-propyl iodide on Cu(111) (Table 2).22 The S-H stretch at 2550 cm-1 was only observed for trifluorobutanethiol coverages above saturation. The absence of the S-H stretch for saturation coverage and below is consistent with, but does not prove, the theory that the thiolate is formed upon adsorption.23 The relative intensities of the asymmetric and symmetric C-F stretches at 1257 and 1160 cm-1, respectively, vary dramatically with the trifluorobutanethiol exposure (21) Kuramshina, G. M.; Pentin, Y. A. J. Chem. Thermodyn. 1979, 11, 1115. (22) Forbes, J. G.; Gellman, A. J. J. Am. Chem. Soc. 1993, 115, 6277.
1265 1153 1073 927
(Figure 4a,b), suggesting that the orientation of the trifluoromethyl group depends on the thiolate coverage. At the lowest coverages investigated (θ < 0.1 monolayers of thiolate), the C-F asymmetric stretch at 1257 cm-1 is significantly more intense than the C-F symmetric stretch at 1157 cm-1 (Figure 4a). As the coverage is increased to 0.25 monolayers of thiolate, there is a change in the relative intensity of the C-F modes with the symmetric stretch becoming much more intense than the asymmetric (Figure 4b). The change in the relative intensities suggests a coverage-dependent change in the orientation of the trifluoromethyl group. The almost total absence of intensity in the symmetric stretch mode suggests that the trifluoromethyl group is aligned nearly parallel to the surface at low coverages, ∼0.1 monolayers. Assuming that there is no change in the C-C backbone due to the formation of the thiolate, the absence of the C-F symmetric stretch also suggests that at low coverages the molecule is aligned nearly parallel to the surface. In the range 0.15-0.25 monolayers (saturation) of thiolate, a reorientation of the thiolate occurs with the symmetric C-F stretch becoming more intense than the asymmetric C-F stretch. The change in relative intensities suggests that the trifluoromethyl group is now aligned near to the surface normal. Again, assuming that there is no change in the C-C backbone due to formation of the thiolate, the dramatic increase in the relative intensity of the C-F symmetric stretch suggests that the thiolate is now aligned more upright. The relative intensities of both the CH2 wag and symmetric C-C stretch also change as a function of coverage, which is generally consistent with molecular reorientation. Discussion The trifluorobutanethiol is proposed to form the thiolate, CF3(CH2)3S-, on Mo(110) after low-temperature S-H bond scission (Figure 5), in analogy to the low-temperature reaction of butanethiol and other thiols on Mo(110).6 The absence of the S-H stretch in the infrared for coverages below saturation,23 the absence of molecular desorption, and the observation of a S-Mo bond in the X-ray photoelectron spectra7 for analogous methyl-terminated thiols are all consistent with thiolate formation. The trifluoro-1-butanethiolate reacts at saturation coverage via four competing pathways: (1) C-S bond hydrogenolysis at 300 K to yield 1,1,1-trifluorobutane; (2) dehydrogenation at the 2-carbon, in the range of 200-520 K, to yield 4,4,4-trifluoro-1-butene; (3) γ-fluorine elimination with accompanying C-C bond scission at 540 K to yield 1,1-difluoroethylene and a trace amount of ethylene; and (4) irreversible decomposition ultimately resulting in gaseous dihydrogen and hydrogen fluoride, as well as adsorbed sulfur, carbon, and fluorine. The proposed reaction scheme is consistent with all of the observed results (Figure 5). (23) Orientation effects could, in principle, make the ν(S-H) mode dipole forbidden and thus not observable in the IR. No other S-H modes were observed in the IR and the ν(S-H) was not observed for any coverage up to saturation, even though the molecule reorients in this range. This suggests that the lack of a ν(S-H) is not due to orientation.
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Langmuir, Vol. 12, No. 7, 1996 1805 Table 3. ∆Hf Values Used for ∆Hrxn Calculations compound
∆Hf (kcal/mol)
CF3(CH2)3SH(g) CH3(CH2)3SH(g) CF3CH2SH(g) CF2CH2 CH2CH2(g) CF3CH2•(g) HF(g) MoS2(s) MoF2(s)
-14529 -12129 -16029 -8129 +1224 -12524 -6524 -5624 -3930
Table 4. Estimates of the Standard Enthalpy Changes at 298.15 K
reaction
Figure 5. Proposed reaction scheme for trifluorobutanethiol on initially clean Mo(110).
Although much of the chemistry of trifluoro-1-butanethiol on clean Mo(110) at saturation coverage closely resembles the chemistry observed for 1-butanethiol and trifluoroethanethiol, there are important differences arising from the presence of fluorine and the alkyl chain length. The predominant decomposition pathway for CH3(CH2)3S-, CF3(CH2)3S-, and CF3CH2S- is C-S bond hydrogenolysis at ∼300 K to form butane, trifluorobutane, and trifluoroethane, respectively. A secondary pathway is alkene elimination to form butene, trifluorobutene, and trifluoroethane, respectively. Alkene formation occurs over a wide temperature range, approximately 200-550 K for all three thiols. The primary difference in the chemistry of the trifluorobutanethiolate vs 1-butanethiolate is the observation of difluoroethylene, a trace amount of ethylene and HF from CF3(CH2)3S- at 540 K. No high-temperature ethylene or any other hydrocarbon products were observed for CH3(CH2)3S-. Several kinetic effects may contribute to the absence of high temperature hydrocarbon products from the reaction of CH3(CH2)3S-. The trifluoromethyl group influences the homolytic bond strength of the adjacent C-H and C-C bonds of the 3-carbon, causing the weaker C-H and C-C bonds at the C-1 and C-2 positions to break more readily. For example, the C-H and C-C bond strengths in CF3CH3 are 6.5 and 13 kcal/ mol, respectively, greater than in CH3CH3.24 In addition, the higher C-F (115 kcal/mol) relative to C-H (90 kcal/mol) bond energy could result in more facile C-H bond scission, favoring nonselective decomposition of CH3(CH2)3S-.24 The higher C-F, C-H, and C-C bond strengths at the trifluoromethyl group and adjacent methylene, may also alter the nature of the intermediate present at high temperature, which leads to difluoroethylene formation. It is possible that trifluoromethyl is the intermediate leading to CF2CH2, but the complexity of the infrared spectra at elevated temperature precludes (24) CRC Handbook of Chemistry and Physics, 70th ed.; CRC Press, Inc.: Boca Raton, FL, 1990.
3Mo(s) + 4CF3(CH2)3SH(g) f 4CF2CH2(g) + 4CH2CH2(g) + H2(g) + 2HF(g) + 2MoS2(s) + MoF2(s) Mo(s) + 2CF3(CH2)3SH(g) f 2CF3CH2CHCH2(g) + 3/ H (g) + MoS (s) 2 2 2 Mo(s) + 2CF3(CH2)3SH(g) f 2CF3(CH2)2CH3(g) + H2(g) + MoS2(s) Mo(s) + 2CH3(CH2)3SH(g) f 4CH2CH2(g) + H2(g) + MoS2(s) Mo(s) + 2CF3CH2SH(g) f 2CF3CH3(g) + MoS2(s) 3Mo(s) + 4CF3CH2SH(g) f 4CF2CH2(g) + H2(g) + 2HF(g) + 2MoS2 + MoF2 Mo(s) + 2CF3CH2SH(g) f 2CF3CH2•(g) + H2(g) + MoS2 Mo(s) + 2CF3(CH2)3SH(g) f 2CF3(CH2)2CH2•(g) + H2(g) + MoS2(s)
∆H (kcal/ mol) +23 -106 -66 +34 -86 +35 +14 +32
identification of the intermediate. The structures of surface intermediates from CF3(CH2)3S- and CH3(CH2)3S- may also be different due to the substantially larger dipole moment of C-F vs C-H bonds. Dipoledipole interactions will be most pronounced at the high coverages and could contribute to the reorientation of the CF3 and may be important in evolution of high-temperature difluoroethylene. Again, the complexity of the vibrational spectra at elevated temperatures precludes resolution of this point. Thermodynamic factors also render formation of CF2CH2 from CF3(CH2)3SH more favorable than CH2CH2 from CH3(CH2)SH based on estimates of the standard enthalpy changes for these reactions (Tables 3 and 4). For example, the primary difference in the chemistry of the C2 and C4 fluorinated thiolates is the low-temperature fluorine elimination pathway and trifluoroethyl radical evolution from CF3CH2S- reaction. Both geometric and energetic factors probably play a significant role in dictating the reaction mechanism, as is evident from comparison of CF3(CH2)3S- and CF3CH2Sreactivity. At saturation coverage, trifluoroethanethiolate decomposition proceeds through three fluorocarbon formation pathways. Two of the pathways, C-S bond hydrogenolysis to form trifluoroethane and defluorination to form difluoroethylene, are of nearly equal importance and resemble the trifluorobutanethiolate decomposition mechanism. While the trifluorobutanethiolate reacts similarly, the trifluorobutene β-dehydrogenation pathway is of lesser importance, when compared to the C-S bond hydrogenolysis pathway. The more effective competition between C-S bond hydrogenolysis and defluorination, in the C2 case, can be rationalized in terms of the facility for β-fluorine elimination. The positioning of the fluorines β to the sulfur so that the fluorines are accessible to the surface, and the equivalent strengths of the Mo-F and C-F bonds contribute to making the defluorination pathway for CF3CH2S- energetically feasible.24 In fact, β-fluorine elimination from trifluoroethyl on Si(100)25 and
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Langmuir, Vol. 12, No. 7, 1996
Ag(111)26,27 has also been found to be very facile. In both cases, difluoroethylene is observed in the gas phase. Dehydrogenation of CF3(CH2)3S- will not be as energetically feasible, when compared to the C2 defluorination reaction, possibly due to the lower strength of the Mo-H relative to Mo-F bond. Increasing the distance of the trifluoromethyl group from the surface apparently disfavors the low-temperature fluorine elimination pathway observed for the trifluoroethanethiol reaction. This pathway does not become important until high surface temperatures, when the intermediate is less sterically constrained. Hence, there is little C-F bond scission in trifluorobutanethiolate below 500 K. The infrared results indicate that the trifluoromethyl group is nearly perpendicular to the surface at saturation and support the contention that the CF3 group is not accessible to the surface. A second important difference in the CF3(CH2)3Schemistry compared to that of CF3CH2S- is that no gaseous radicals were evolved from trifluorobutanethiolate even at saturation coverage, whereas trifluoroethyl radical at 265 K is a significant path for CF3CH2S- reaction.11 Trifluoroethyl radical was formed over a very narrow coverage range, 0.24-0.26 monolayers (saturation) of trifluoroethanethiolate. Radical formation was specific to CF3CH2S- on Mo(110), even though the saturation coverages for ethane, butane, trifluoroethane, and trifluorobutanethiolates are similar. Energetic factors most certainly contribute to the absence of ethyl, butyl and trifluorobutyl radical formation, however. Firstly, the increase in the adjacent C-H and C-C bond strengths, with the addition of the trifluoromethyl group, and the increased C-F bond strength relative to C-H will retard C-H and C-C bond scission at these positions. As a result, the amount of CF3CH2S- which irreversibly decomposes relative to ethane, butane, and trifluorobutanethiolates is lower. For example, 50% less CF3CH2S- irreversibly decomposes compared to CF3(CH2)3S- for similar coverages.28 An increase in the amount of irreversible decomposition in CF3(CH2)3S(25) Lin, J. L.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1995, 13, 178. (26) Paul, A.; Gellman, A. J. Submitted for publication in Langmuir. (27) Street, S. C.; Gellman, A. J. Submitted for publication in J. Chem. Phys. (28) The difference in the irreversible decomposition pathways is based on a comparison of carbon Auger signals for ethane, butane, trifluoroethane, and trifluorobutanethiolate after reaction at equivalent coverages.
Napier and Friend
relative to CF3CH2S- reduces the coverage of intact thiolate on the surface and consequently less steric inhibition to radical adsorption. Hence reactions, such as β-elimination, are expected to be more important for the C4 case. Furthermore, the formation of trifluoroethyl radical from CF3CH2SH is estimated to be more thermodynamically favorable than CF3(CH2)3• from CF3(CH2)3SH by ∼18 kcal/mol (Tables 3 and 4). This difference mainly reflects the estimated differences in the heats of formation of the two radicals (Table 4). Conclusion The reactions of 4,4,4-trifluorobutanethiol on Mo(110) are different than both CF3CH2SH and CH3(CH2)3SH. While the major reaction pathway is C-S bond hydrogenolysis for all thiols studied on Mo(110), including CF3(CH2)3SH, both thermodynamic and kinetic effects result in a high-temperature pathway exclusive to the CF3(CH2)3SH. Defluoroethylene and HF are produced at ∼540 K from reaction of the trifluorobutanethiolate at high coverage. Both steric and bond strength effects are proposed to result in the high-temperature pathway. The polar CF3 group reorients as a function of coverage such that the CF bonds are inaccessible to the surface; thus, fluorine elimination is inhibited. The stronger C-H and C-C bonds at the position adjacent to the fluorine also probably retard nonselective decomposition relative to CH3(CH2)SH, preserving the molecular intermediates that lead to difluoroethylene production. These studies demonstrate how fluorine substitution alters both product distributions and the kinetic lability of surface intermediates. Acknowledgment. We gratefully acknowledge the support of this work by the Department of Energy FG02-84-ER-13289 and Harvard University Materials Laboratory under DMR-89-20490. M. E. Napier acknowledges the support of an NSF postdoctoral fellowship, CHE-9302439. We would also like to thank Hans Biebuyck and Suk-Wah Tam-Chang for the synthesis of 4,4,4trifluorobutanethiol. LA950363Q (29) The heats of formation for the fluorinated thiol compounds are not available in the literature, so the bond additivity method was used to estimate these values. (30) JANAF Thermochemical Tables, 2nd ed.; Nat. Bur. Stand.: Gaithersburg, 1971.
