8254
Langmuir 2001, 17, 8254-8259
Low-Energy Collisions of Pyrazine and d6-Benzene Molecular Ions with Self-Assembled Monolayer Surfaces: The Odd-Even Chain Length Effect Kurt V. Wolf,† David A. Cole,‡ and Steven L. Bernasek*,† Department of Chemistry, Princeton University, Princeton, New Jersey 08544, and Evans East, East Windsor, New Jersey 08520 Received July 26, 2001. In Final Form: September 28, 2001 The low-energy reactive collisions of two independent probe ions, pyrazine and d6-benzene, illustrate an odd-even hydrocarbon chain length effect for a wide range of hydrocarbon self-assembled monolayer (SAM) surfaces. SAM surfaces prepared from alkanethiols ranging from CH3(CH2)10SH to CH3(CH2)17SH chemisorbed to polycrystalline gold are shown to exhibit an odd-even effect where the orientation of the terminal methyl group determines the reaction behavior of the thin film. X-ray photoelectron spectroscopy shows the surfaces to be homogeneously covered and provides evidence for the presence of a thiolate (Au-SR) on the SAM surface. When 30 eV incident ions were used, the extent of hydrogen addition to the incident probe ion was larger for odd carbon chain length SAM surfaces when compared to the even chain length films. For an odd chain length SAM surface, the terminal methyl group exposes a C-H bond perpendicular to the surface, increasing hydrogen addition reactivity for the probe ions. The low-energy probe ions also reacted with the surfaces to form alkyl addition products. The alkyl addition products also showed an odd-even effect. In this case, the even chain length SAM surfaces were more reactive than the odd chain length surfaces. For an even SAM surface, the terminal C-C bond is oriented quasi-perpendicular to the Au surface, allowing more direct access to the carbon atom on the terminal methyl group and increasing its reactivity.
Introduction Self-assembled monolayer (SAM) surfaces have been studied extensively in the last two decades.1 The impetus for such an explosion of research in monolayer assemblies such as SAM surfaces has to do with their relevance to biological interfaces and membranes, corrosion inhibition, wetting, adhesion, and microelectronic circuit fabrication.2 SAM surfaces also provide a very good adsorbate system for studying ion/surface interactions.3 SAM surfaces of alkanethiols on gold have been used extensively as target surfaces in ion/surface scattering for several reasons including (1) they are easily prepared, (2) they are relatively stable in a vacuum and in air, (3) their overall thickness can be controlled based on the choice of thiol chain length used, and (4) surface chemistry can be manipulated by changing functional group(s) attached to the alkanethiol. Alkanethiols self-assemble on Au(111) surfaces to form a well-ordered monolayer which is covalently bound to the surface through a sulfur-gold bond.4-7 The backbone of the SAM surface, which is composed of the methylene groups of the alkyl chains, is oriented in an all-trans conformation with an overall chain tilt angle of ∼30° from the surface normal, as determined by ellipsometry and IR spectroscopy.8 These studies have shown that the orien† ‡
Princeton University. Evans East.
(1) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Evans, C.; Pradeep, T.; Shen, J.; Cooks, R. G. Rapid Commun. Mass Spectrom. 1999, 13, 172. (4) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (5) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (6) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (7) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
tation of the terminal methyl group of alkanethiol SAM surfaces differs depending on whether the backbone of the SAM surface contains an even or an odd number of carbon atoms in the chain.9 In Figure 1, the orientation of n-pentanethiol (an odd chain) adsorbed on a gold surface is compared with the orientation of n-butanethiol (an even chain) adsorbed on gold. This schematic is not meant to imply anything about the two-dimensional unit cell of thiols adsorbed on gold or the detailed orientation of the hydrocarbon backbone in the ordered overlayer. Rather, the figure illustrates differences in the relative orientation of the terminal methyl group for well-ordered odd and even chain length overlayers. For an even chain SAM surface, the hydrogen atoms on the terminal methyl group are oriented such that all three hydrogen atoms are in the same plane approximately parallel to the gold substrate. However, for the odd alkanethiol chemisorbed to gold, the terminal methyl group contains two hydrogen atoms in the plane of the gold substrate and one hydrogen atom nearly perpendicular to the substrate. This physical difference is referred to as the odd-even effect and has been observed using many analytical techniques including surface-induced dissociation tandem mass spectrometry and contact angle measurements.7,10 Selective chemical modification of surfaces in order to manipulate their physical and chemical properties is a topic of growing interest for its many potential applications.11 Ion/surface collisions have been used to probe (8) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (9) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (10) Angelico, V. J.; Mitchell, S. A.; Wysocki, V. H. Anal. Chem. 2000, 72, 2603. (11) Miller, S. A.; Cooks, R. G. Fundamentals and Applications of Gas Phase Ion Chemistry, Proceedings of the NATO Advanced Study Institute, Grainau, Germany, Vol. 521, 1995; Jennings, K. R., Ed.; Kluwer: Boston.
10.1021/la011183l CCC: $20.00 © 2001 American Chemical Society Published on Web 11/22/2001
Collisions of Molecular Ions with SAMs
Langmuir, Vol. 17, No. 26, 2001 8255
both known to form product ions with hydrocarbon-covered surfaces. Pyrazine and benzene are shown to readily participate in hydrogen and alkyl addition reactions with surfaces through a sputtered ion mechanism:12
M+ + SX f M + S + X+ f MX+ + S
(1)
where M+ is the incident ion, S is the substrate, and X is an attached surface species. In the case of hydrogen addition, X is equal to hydrogen. In alkyl addition reactions, X is equal to CxHy. For benzene alkyl addition, the most likely mechanism is as follows:
C6H6+• + H-SAM surface f C6H6 + C3H5+ f C7H7+ + C2H4 (2) Figure 1. Models of the orientation of (a) 1-pentanethiol (an odd thiol) and (b) 1-butanethiol (an even thiol) on a Au surface. The orientation of the end methyl group depends on whether the alkanethiol is odd or even in chain length.
surfaces to understand the physical and chemical composition of modified surfaces as well as to modify the surfaces directly. Ion/surface collisions may be elastic, inelastic, or reactive, where reactive collisions include charge exchange and atom or molecular transfer processes. Inelastic collisions that lead to the dissociation of the incident ion are used to characterize biological ions and smaller organic ions in the technique known as surfaceinduced dissociation tandem mass spectrometry (SID MS/ MS). This method has received considerable attention due to the ion structural information that is obtained from the product ion spectra. In a typical SID MS/MS experiment, an energetic ( 15).14 This short-long chain trend has been observed in the structure of SAM surfaces in the standing-up phase and macroscopically in the phase diagrams of n-alkanethiols on Au(111).15 Pyrazine and d6-benzene molecular ions were used independently as probe ions to investigate this odd-even effect. These two compounds were chosen since they are (12) Phelan, L. M.; Hayward, M. J.; Flynn, J. C.; Bernasek, S. L. J. Phys. Chem. B 1998, 102, 5667.
The sputtered ion, C3H5+, was chosen over CH3+ based on the overall thermodynamics of the process; however, both ions were experimentally observed to form C7H7+, the tropylium ion.16 Experimental Section Materials. Seven n-alkanethiols (CH3(CH2)nSH, where n ) 10, 11, 12, 13, 14, 15, and 17) were used in these studies. The seven thiols were purchased commercially from Aldrich, TCI, and Pfaltz & Bauer, Inc. and used without further purification. Gold Substrate. All SAM surfaces were formed on vapordeposited gold substrates purchased from Evaporated Metal Films of Ithaca, NY. The gold substrates consist of 1000 Å of vapor-deposited Au on 50 Å of Ti deposited on silica glass where the surface dimensions are 11/16 in. × 17/32 in. The Au surfaces were precleaned in a 3:2 solution of concentrated H2SO4 and 30% H2O2 and rinsed with copious amounts of distilled water and absolute ethanol. Monolayer Preparation. Self-assembled monolayer surfaces were prepared by immersing a freshly cleaned Au substrate into a 1 mM alkanethiol solution in absolute ethanol. The surfaces were permitted to grow in the thiol solution for at least 2 weeks in order for the packing of the thin film to fully equilibrate. The SAM surfaces were rinsed five times with absolute ethanol. The surfaces were dried under a stream of dry nitrogen, attached to the sample holder, and pumped down to ultrahigh vacuum (UHV) conditions. X-ray Photoelectron Spectroscopy (XPS) Measurements. A Physical Electronics 5700LSci XPS with a monochromatic aluminum source with a power of 350 W was used. The analysis region was 2 mm × 0.8 mm, and the exit angle was held at 20° (defined as the angle between the surface plane and the axis of the electron analyzer lens). The surfaces of the specimens were examined initially by low-resolution survey scans to determine which elements were present. High-resolution spectra were acquired to determine the binding energy and concentration of the elements observed in the survey spectra. The quantification of the elements was accomplished by using the atomic sensitivity factors for the Physical Electronics model 5700L Sci ESCA spectrometer. The high-resolution spectra were charge corrected in all cases so that the binding energy for the C-(C, H) in the carbon 1s spectra was equal to 284.8 eV, the binding energy for adventitious carbon. Mass Spectrometry Measurements. The surface-induced dissociation tandem mass spectrometer consists of a custom compact design, which has been described earlier.17 The SID instrument is constructed from two Ametek Dycor M200M (13) Shen, J.; Grill, V.; Evans, C.; Cooks, R. G. J. Mass Spectrom. 1999, 34, 354. (14) Leung, T. Y. B.; Gerstenberg, M. C.; Lavrich, D. J.; Schreiber, F.; Scoles, G.; Poirier, G. Langmuir 2000, 16, 549. (15) Fenter, P. Self-assembled monolayers of thiols; Ulman, A., Ed.; Thin Films, Vol. 24; Academic Press: San Diego, CA, 1998. (16) Hayward, M. J.; Park, F. D. S.; Phelan, L. M.; Bernasek, S. L.; Somogyi, A.; Wysocki, V. H. J. Am. Chem. Soc. 1996, 118, 8375. (17) Bernasek, S. L.; Park, F. D. S.; Phelan, L. M.; Hayward, M. J. Isr. J. Chem. 1998, 38, 375.
8256
Langmuir, Vol. 17, No. 26, 2001
Figure 2. XPS survey spectrum for the corner 1 CH3(CH2)11SAu SAM surface. quadrupole mass spectrometers. The collision energy is determined by the difference between the potential on the electron impact ion source and that of the surface. By changing the potential of the ion source and setting the potential of the target surface to common ground, the collision energy of the incident ions can be varied. The SID instrument was fitted with a multiple surface holder. This allows for up to eight samples to be placed into the UHV chamber at one time. The sample holder can be rotated freely 360° and moved linearly in the x, y, and/or z directions. During each experiment, a selected surface is positioned in the path of the incident ion beam. The base pressure of the main chamber is held at 1 × 10-9 Torr; however, during the ion/surface reactions, the introduction of pyrazine or d6-benzene to produce the desired probe ions raises the pressure to 2 × 10-7 Torr, which is held throughout the duration of each experiment. Each mass spectrum is obtained by averaging 10 individual scans at random sites on the sample surface to increase the signalto-noise ratio. All data points presented represent the average of at least three independent measurements consisting of 10 scans each.
Results and Discussion XPS was used to determine if the surfaces were homogeneously covered and uncontaminated. Figures 2 and 3 illustrate survey and high-resolution XPS spectra, respectively, for the CH3(CH2)11SAu SAM surface. The major components observed in the survey spectrum were carbon, gold, sulfur, and oxygen. The XPS spectra were charge corrected by assigning the C-(C, H) in the carbon 1s spectra to 284.8 eV. Close examination of Figure 3b shows a high binding energy shoulder at 285.5 eV which may correspond to the carbon atoms bound to the sulfur in the alkanethiol chains. This asymmetry may also be due to differences in location of the probed carbon atom in the chain of the thiol overlayer. Part c in Figure 3 shows the sulfur 2p XPS spectrum. The S 2p3/2 binding energy was found to be 161.4 eV. This shift (from a binding energy of 164 eV) corresponds to partially reduced sulfur indicating that the Au-S bond is thiolate-like, which agrees with the current literature.5,19-22 (18) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Homes, J. L.; Jevin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1. (19) Bourg, M. C.; Badia, A.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 6562.
Wolf et al.
Figure 3. High-resolution XPS spectra for (a) oxygen 1s, (b) carbon 1s, (c) sulfur 2p, and (d) gold 4f for the corner 1 CH3(CH2)11S-Au SAM surface. Table 1. Concentration of Elements Detected by XPS (in Atomic Percenta) oxygen
carbon
sulfur
gold
CH3(CH2)10SAu SAM Surface corner 1 1.5 74.4 corner 2 1.1 74.1 corner 3 1.8 73.5 center 0.9 75.0 average 1.3 74.3 standard deviation 0.4 0.6 carbon/gold ratio 3.46 ( 0.07
3.0 2.9 2.9 2.8 2.9 0.1
21.2 21.9 21.8 21.2 21.5 0.4
CH3(CH2)11SAu SAM Surface corner 1 0.8 75.6 corner 2 0.7 76.0 corner 3 0.4 77.1 center 0.0 76.4 average 0.5 76.3 standard deviation 0.4 0.6 carbon/gold ratio 3.74 ( 0.08
2.7 3.1 2.6 2.9 2.8 0.2
20.9 20.2 19.9 20.6 20.4 0.4
a Concentrations are normalized to 100%. Note: XPS does not detect hydrogen or helium.
Table 1 reports the atomic concentrations of the highresolution XPS spectra at varying sampling points for both a CH3(CH2)10SAu and a CH3(CH2)11SAu SAM surface. It is clear from the carbon, sulfur, and gold atomic percent concentrations and their standard deviation from the varying sampling points that the surfaces do appear to be quite homogeneously covered, and at no time were bare gold regions found. The presence of oxygen in the films caused some concern. However, it is important to note that the sample points were measured in chronological order as they appear in Table 1 (reading from top to bottom). It is obvious that the amount of oxygen detected during the XPS measurements decreased over time to 0.0% oxygen recorded for the final measurement. It is (20) Okawa, H.; Wada, T.; Sasabe, H.; Kajikawa, K.; Seki, K.; Ouchi, Y. Jpn. J. Appl. Phys. 2000, 39, 252. (21) Khudorozhko, G. F.; Asanov, I. P.; Mazalov, L. N.; Kravtsova, E. A.; Parygina, G. K.; Mironov, V. E. J. Electron Spectrosc. Relat. Phenom. 1994, 68, 199. (22) Rodriguez, J. A.; Li, S. Y.; Hrbek, J.; Huang, H. H.; Xu, G. Q. Surf. Sci. 1997, 370, 85.
Collisions of Molecular Ions with SAMs
Langmuir, Vol. 17, No. 26, 2001 8257
Figure 5. The ratio of (MpH+ + (MpH-HCN)+)/(Mp+• + (MpHCN)+) for 30 eV pyrazine molecular ions versus the carbon atom chain length for the n-alkanethiols chemisorbed to Au.
Figure 4. The 30 eV collision of (a) pyrazine molecular ions Mp+• (m/z 80) and (b) d6-benzene molecular ions Mb+• (m/z 84) with a CH3(CH2)17S-Au surface.
likely that the majority, if not all, of the oxygen detected is in the form of ethanol since the SAM surfaces are grown in a dehydrated ethanol solution and rinsed thoroughly with ethanol before being pumped down to UHV conditions. The solvent could readily adhere or be trapped to the matrix of the thin film, and over time the ethanol would be pumped away as is seen in Table 1. XPS also proved to be sensitive enough to detect an increase in carbon going from a CH3(CH2)10SAu to a CH3(CH2)11SAu SAM surface. If the oxygen is excluded from the data and assumed to be ethanol, an increase of 1.8% in atomic carbon is observed which is in agreement with the increasing alkyl chain of the SAM. However, a simple carbon/gold ratio, as shown in Table 1, will illustrate the relative amount of carbon per surface. When the alkyl chain length was increased, the C/Au ratio went from 3.46 ( 0.07 to 3.74 ( 0.08. This trend corresponds to a direct increase of carbon and a decrease or attenuation of gold which is in agreement with what would be expected for an increase in length on a SAM surface in the standingup phase. The SID tandem mass spectra, illustrated in Figure 4, were obtained from a 30 eV collision of pyrazine molecular ions (m/z 80) and d6-benzene molecular ions (m/z 84) incident on a CH3(CH2)17S-Au surface. For the pyrazine mass spectrum, two significant reaction products are observed for the hydrocarbon SAM surface, as has been reported previously.10 The peak labeled MpH+ in Figure 4a (where Mp+ is the molecular ion for pyrazine) represents a reaction resulting in the addition of a hydrogen atom to the incident pyrazine molecular ion. The peak corresponding to m/z 95 is due to a direct methyl addition to the incident pyrazine molecular ion. The loss of HCN is a common pathway for the fragmentation of pyrazine molecular ions. At 30 eV collision energy, both the Mp+• and the MpH+ ions contain sufficient internal energy to further fragment producing (Mp-HCN)+ (m/z 53) and (MpH-HCN)+ (m/z 54) daughter ions, respectively. It is more difficult to accurately assign all of the daughter ions produced for the benzene probe ion since benzene has a
Figure 6. The ratio of (H+ + (Mb+H-C2D2)+ + (Mb+H-D2)+ + (Mb+H)+)/Mb+• for 30 eV d6-benzene molecular ions versus the carbon atom chain length for the n-alkanethiols chemisorbed to Au.
higher order of symmetry than pyrazine resulting in multiple equivalent fragmentation channels. This phenomenon is seen clearly when parts a and b of Figure 4 are compared, where Figure 4b contains a greater number of daughter ions. To facilitate the analysis of the spectra, isotopically labeled benzene was used. The direct hydrogen addition product along with the accompanying fragment ions can then be clearly assigned since any fragment ion with an odd mass would have undergone a hydrogen addition reaction. Therefore, the peaks at m/z 85, 81, and 57 correspond to MbH+, (MbH-D2)+, and (MbH-C2D2)+, respectively (where Mb+ is the molecular ion for d6benzene). The peak at m/z 96 is due to a unique alkyl addition reaction between the d6-benzene molecular ion and the SAM surface forming a tropylium cation (C7D5H2+) as the net product. For both pyrazine and d6-benzene, taking the ratios between the reacted and unreacted hydrogen addition reactions, the extent of H-addition can be measured and compared for the different hydrocarbon SAM films. Likewise, comparing the normalized peak areas for the alkyl addition products for both incident ions with the varying SAM surfaces, the extent of alkyl reactivity of the two incident ions can be compared. A plot of (MpH+ + [MpH-HCN]+)/(Mp+• + [Mp-HCN]+) versus the number of carbon atoms in the self-assembled monolayer is shown in Figure 5. Since the loss of HCN is clearly observed, the ratio of the hydrogen addition fragment ions versus the corresponding surface-induced dissociation daughter ions serves as a direct measure of the extent of the hydrogen addition reactions. A clear oddeven effect is observed for this system. The odd chain length films correspond to greater reactivity for hydrogen addition. The reactivity of d6-benzene molecular ions incident on the SAM films was measured in a similar manner. Figure 6 plots the ratio (H+ + (Mb+H-C2D2)+ + (Mb+H-D2)+ + (Mb+H)+)/Mb+• versus the number of carbon atoms in the n-alkanethiol SAM. In this case, the extent of hydrogen addition of the deuterated probe ion was measured by taking the ratio of the sum of the odd
8258
Langmuir, Vol. 17, No. 26, 2001
Figure 7. The normalized methyl addition (m/z 95) of pyrazine molecular ions versus the carbon atom chain length for the n-alkanethiols chemisorbed to Au.
Figure 8. The normalized alkyl addition (m/z 96) of d6-benzene molecular ions versus the carbon atom chain length for the n-alkanethiols chemisorbed to Au.
mass fragments versus the conserved even molecular ion. Again, an obvious odd-even pattern is observed for this probe ion. Since the odd-even effect is observed for two independent probe ions, it is concluded that the orientation of the alkanethiols on the gold surface is responsible for this effect. Therefore, the structure of an odd SAM film causes one or more hydrogen atoms on the terminal methyl group to be more reactive than the hydrogen atoms on an even SAM surface. The error bars in Figures 5 and 6 are most likely due to minor fluctuations in the SID MS/MS instrument, the reproducibility of the monolayer growth, and local morphology variations between the multiple sampled points monitored for each individual surface. Normalized alkyl addition plots for pyrazine and d6benzene are shown in Figures 7 and 8, respectively. The net result for pyrazine molecular ions incident on alkanethiol SAM surfaces is a direct methyl addition to the aromatic compound yielding a C5H7N2+ ion (m/z 95). Figure 7 shows an odd-even behavior that is opposite to the effect that was reported in Figure 5 for hydrogen addition. Here, the even SAM surfaces appear more reactive for alkyl addition reactions. Similarly, Figure 8 shows that even SAM targets are more reactive in alkyl addition reactions with the d6-benzene probe ion. In this case, d6-benzene forms the stable tropylium ion (m/z 96) as a net product ion instead of forming a direct methylated product ion. The obvious variation in reactivity for odd versus even alkanethiol self-assembled monolayer surfaces is in agreement with the chain length dependence of the terminal methyl group orientation of the film as illustrated in Figure 1. When the SAM surface contains an odd number of alkyl carbons, one of the hydrogen atoms on the terminal methyl group is aligned approximately perpendicular to the gold surface. This adopted orientation causes the odd chain length film to be more reactive in hydrogen addition reactions with an impinging ion beam. For odd SAM surfaces, it is also possible for the secondary methylene group (the methylene group composed of the carbon bound to the terminal methyl group) to donate
Wolf et al.
hydrogen atoms. The adopted trans conformation partially exposes these hydrogen atoms to the surface of the monolayer making them accessible to incoming incident ions. The exposed hydrogen atoms protect the underlying carbon atoms and restrict the amount of direct alkyl addition products that can be formed during the ion/ surface collisions. Conversely, for an even SAM surface the preferred orientation of the terminal methyl group aligns all three hydrogen atoms quasi-parallel to the gold substrate. This geometry along with the trans conformation of the secondary methylene group, which positions the hydrogen atoms pointing toward the Au substrate, reduces the amount of available hydrogen atoms which can undergo addition reactions. This arrangement also exposes the carbon atom on the terminal methyl group and orients the terminal C-C bond perpendicular to the surface, so that alkyl addition ion/surface reactions occur more readily for even versus odd SAM films. Previous studies of the formation of tropylium ions via a sputtered ion mechanism proposed that the reactive ion sputtered from the SAM surface was C3H5+ as illustrated in eq 2.16 This work also confirmed that tropylium cations could be formed from the addition of benzene and sputtered methyl cations. The alkyl addition of CH3+ to benzene to form the seven-membered aromatic ring is thermodynamically favorable and was experimentally observed for gas-phase ion/molecule reactions using a triple quadrupole mass spectrometer.16 Since a clear odd-even surface effect is seen in the formation of tropylium ions, which is a direct result of the orientation of the terminal methyl group of the SAM surface, the following sputtered ion pathway is strengthened:
C6D6+• + H-SAM surface f C6D6 + CH3+ f C7D5H2+ + H2 (3) This mechanism is supported by the fact that direct alkyl addition of C3H5 was not seen for d6-benzene or pyrazine. Since the extent of formation of the alkyl addition products is greater when an even SAM target is employed, the direct availability of the terminal carbon during the ion/surface collisions permits the terminal carbon-carbon bond to fragment. This heterolytic fragmentation of the surface could result in the direct chemical sputtering of CH3+ ions in addition to C3H5+ ions. Throughout these studies, it is important to note that there were no overall long-short chain effects observed in the reactivity. Using two independent probe ions at 30 eV incident energy, the odd-even effect is observed from a CH3(CH2)10S-Au surface to a CH3(CH2)17S-Au film. On the basis of the reproducibility of the data and the consistency of the odd-even effect with respect to both hydrogen and alkyl addition products for both probe ions, it is concluded that long-short effects do not drastically distort the orientation of the terminal methyl group of hydrocarbon SAM surfaces. The orientation of the terminal methyl group of an odd versus even SAM surface illustrates different chemical behavior as demonstrated in Figures 7 and 8. However, when the y-axes of the two plots are compared, it is clear that at 30 eV incident energy, the formation of tropylium cations (m/z 96) is favored over the formation of the direct methyl addition cation of pyrazine (m/z 95) by about a factor of 10. Table 2 illustrates the possible molecular structures of the products and the relative heats of formation for the C7D5H2+ and C5H7N2+ ions through the reaction of neutral d6-benzene and pyrazine with sputtered methyl cations from the SAM surface, respectively.18 It is
Collisions of Molecular Ions with SAMs
Langmuir, Vol. 17, No. 26, 2001 8259
Table 2. Molecular Structure and Relative Energies for the Formation of C7D5H2+ and C5H7N2+ via the Reaction of Neutral d6-Benzene and Pyrazine with Sputtered Methyl Cations, Respectively product number
molecular formula
molecular structure
1a
C7D5H2+
-327.2
2b
C5H7N2+
-418.3
∆Hreaction (kJ/mol)c
3b
C5H7N2+
-468.7
4b
C5H7N2+
-461.2
5b
C5H7N2+
-261.9
6b
C5H7N2+
-307.5
a Illustrates the physical properties of the tropylium product associated with the reaction C6D6 + CH3+ f C7D5H2+ + DH. b Illustrates the physical properties of the direct methylated pyrazine products associated with the reaction C4H4N2 + CH3+ f C5H7N2+. c Represents the appropriate heats of formation for the two independent reactions above. In these calculations, both gasphase experimental and theoretical heats of formation found using a standard AM1 method were used (ref 18).
unclear which of the five isomers is formed for the methylated pyrazine product. Comparing the thermodynamic data presented in Table 2 with the experimental observation that the alkyl addition for d6-benzene is favored over the methyl addition for pyrazine, products 5 or 6 are most likely the structure of the C5H7N2+ ion. The enthalpies of formation for structures 5 or 6 are less than for the formation of tropylium, due to the associated loss of aromaticity on formation of products 5 or 6. The recent literature demonstrates that d4-pyrazine molecular ions abstract hydrogen atoms from SAM surfaces much more readily than d6-benzene molecular ions at the same translational energy.12 Figure 4 demonstrates that when incident ions collide with an even alkanethiol SAM surface, hydrogen addition is still observed. If pyrazine molecular ions have a very large
affinity to bond with hydrogen atoms, it is possible that this strong affinity could simply limit the pathway for direct methyl addition. This bias characteristic of pyrazine molecular ions could reduce the alkyl addition of the incident ions resulting in fewer product ions formed which would coincide with the results of Figures 7 and 8. Due to the complex nature of ion/surface collisions, it is not possible to determine which of the structural models in Table 2 accurately describes the ionic structure of the C5H7N2+ product.18 Conclusions Low-energy ion-surface scattering is very sensitive to the surface structure of hydrocarbon self-assembled monolayer surfaces. In reactions between 30 eV pyrazine and d6-benzene molecular ions and the SAM surface, an odd-even effect was observed. In agreement with the earlier work of Angelico et al.10 for pyrazine ions interacting with thiol and alkoxyphenyl-benzenethiol monolayers, the difference in reactivity between the two independent incident ions and the sputtered hydrogen and alkyl addition reactions is ascribed to variations in the orientation of the terminal methyl group of the SAM surfaces in the standing-up phase. For the hydrogen addition reactions, the orientation of the terminal methyl group for an even carbon chain SAM film limits the extent of the reaction when compared to an odd SAM surface. For the odd chain length alkanethiol adsorbed on the gold surface, the preferred orientation of the overlayer exposes the hydrogen atom on the terminal methyl group where it can react more easily, yielding a hydrogenated product. During the alkyl addition reactions of the incident ions, an odd-even effect was also observed. In this case, the even carbon chain length SAM surfaces were more reactive when compared to the odd chain length films. Again, the orientation of the terminal methyl group governs this trend. For an even SAM surface, the terminal carbon atom is exposed to a greater extent causing a more efficient C-C bond excitation resulting in a larger efficiency for alkyl sputtering. These results are even more remarkable since at no time were the gold substrates or the SAM surface annealed. Therefore, under these condition and at room temperature, the preferred packing of the surfaces provides well-ordered thin films. Results presented here also demonstrate that the SAM surfaces are not only ordered at the molecular level, but XPS results confirmed that the surfaces are very homogeneous in nature indicating that the order is conserved throughout the two-dimensional matrix. Acknowledgment. We are grateful for the support of this work by the National Science Foundation, Division of Chemistry. We acknowledge Dr. Lynn Phelan for early works on this system. LA011183L
