Cycloaddition of Cyclopentadiene and Dicyclopentadiene on Si(100

The adsorption of cyclopentadiene and its dimer form, dicyclopentadiene, on the Si(100)-2×1 surface has been investigated using polarized multiple in...
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J. Phys. Chem. B 1999, 103, 6803-6808

6803

Cycloaddition of Cyclopentadiene and Dicyclopentadiene on Si(100)-2×1: Comparison of Monomer and Dimer Adsorption George T. Wang, Collin Mui, Charles B. Musgrave, and Stacey F. Bent* Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305 ReceiVed: May 7, 1999; In Final Form: June 23, 1999

The adsorption of cyclopentadiene and its dimer form, dicyclopentadiene, on the Si(100)-2×1 surface has been investigated using polarized multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy together with ab initio quantum chemistry calculations. The results show that cyclopentadiene adsorbs at room temperature to form a [4 + 2] cycloaddition product and at saturation covers less than twothirds of the surface. Dicyclopentadiene adsorbs intact, most likely by a [2 + 2] cycloaddition reaction, and is estimated to adsorb onto 30-40% more SidSi dimer sites than cyclopentadiene despite its larger size. Dicyclopentadiene is not found to react via a surface-catalyzed retro-Diels-Alder pathway whereby it splits into cyclopentadiene while chemisorbing, even though this pathway is theoretically predicted to be much more thermodynamically favored.

Introduction The growing importance of organic films in semiconductor technology has led to increased research efforts directed toward understanding and controlling hydrocarbon reactions with the silicon surface. One area in particular that has recently attracted attention is the cycloaddition reactions of alkenes with the Si(100)-2×1 surface, owing to the potential of such reactions for selective and controlled growth of ordered organic layers on semiconductor surfaces. The ability to covalently bond to semiconductor surface organic layers with desired functionality and known orientation may have potential applications in a number of areas, including lithography, optoelectronics, sensors, and low-dielectric materials. It is known that the Si(100)-2×1 surface consists of rows of silicon dimers and provides a uniform, ordered, and reactive template for surface chemistry. Recent studies have shown that the silicon dimer, which contains a weak π bond,1,2 can react with hydrocarbons containing one or more double bonds via a [2 + 2] cycloaddition reaction in which the π bonds of the SidSi dimer and CdC group are broken to form a fourmembered ring with two new Si-C σ bonds at the Si-C interface.3-7 Moreover, it has been predicted theoretically8 and shown experimentally9-11 that if the hydrocarbon contains a conjugated diene, it can also react alternatively via an analogue of the familiar Diels-Alder [4 + 2] reaction to produce a sixmembered ring at the Si-C interface. Bifunctional or multifunctional unsaturated organic molecules are of particular interest owing to their potential to undergo further cycloaddition or other reactions once bonded to the surface. Cyclopentadiene is well-known for its high reactivity in Diels-Alder reactions. In fact, at room temperature two cyclopentadiene molecules slowly undergo an intermolecular Diels-Alder reaction, dimerizing to form dicyclopentadiene. Although dicyclopentadiene is stable at room temperature, heating it to its boiling point of 443 K will cause the reverse or retro-Diels-Alder reaction whereby the dimer dissociates to give two cyclopentadiene molecules. * To whom correspondence should be addressed. E-mail: stacey.bent@ stanford.edu. Fax: 650-723-9780.

In this study we investigate the interaction of cyclopentadiene and its dimer, dicyclopentadiene, with the Si(100)-2×1 surface using polarized multiple internal reflection (MIR) infrared spectroscopy. As a conjugated diene, cyclopentadiene can react with the SidSi dimer by either a [2 + 2] or [4 + 2] reaction. Dicyclopentadiene, on the other hand, can react intact only via a [2 + 2] reaction using one of its CdC bonds. However, an alternative reaction pathway is proposed here in which the surface catalyzes a retro-Diels-Alder reaction whereby dicyclopentadiene dissociates into two cyclopentadiene molecules during adsorption. It will be shown theoretically that this alternative pathway results in a thermodynamically favored product over the [2 + 2] intact addition by a large margin. Our experimental results show that at room temperature cyclopentadiene chemisorbs onto the Si(100)-2×1 surface. Comparison with theoretically calculated frequencies and geometries for both the [2 + 2] and [4 + 2] cycloaddition products indicates that cyclopentadiene bonds with the SidSi dimer via a [4 + 2] reaction. Dicyclopentadiene also chemisorbs onto the surface, and comparison with cyclopentadiene indicates that the dimer does not undergo a surface-catalyzed retro-Diels-Alder reaction into the monomer but instead adsorbs intact at room temperature. Experimental Section All experiments were performed in an ultrahigh vacuum (UHV) chamber (base pressure of e7 × 10-10 Torr) that has previously been described in detail.12 The chamber is equipped with an unshielded quadrupole mass spectrometer and an ion gun for surface sputtering. Infrared data were collected in multiple internal reflection (MIR) mode using a Mattson Galaxy 4020 Fourier transform infrared (FTIR) spectrometer with a liquid nitrogen cooled, narrow-band HgCdTe detector. For each experiment, a background IR emissivity spectrum for the clean sample was recorded, and subsequent scans after adsorption were ratioed to this background spectrum and transformed to absorption spectra. The temperature-programmed desorption (TPD) studies were performed with a Eurotherm temperature controller to maintain a temperature ramp of 1 K/s.

10.1021/jp991528x CCC: $18.00 © 1999 American Chemical Society Published on Web 07/28/1999

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Wang et al.

Figure 1. Energy minimized adsorption configurations on Si(100): (a) [4 + 2] cycloaddition of cyclopentadiene; (b) [2 + 2] cycloaddition of cyclopentadiene; (c, d) R-bonded dicyclopentadiene; (e, f) β-bonded dicyclopentadiene.

The sample, a Si(100) crystal of trapezoidal geometry (1 mm × 20 mm × 50 mm, 45° beveled edges, Harrick), is mounted on a holder that is heated by a resistive tungsten heater and cooled by heat exchange with a liquid nitrogen coldfinger. The sample surface was cleaned by sputtering with Ar+ ions at room temperature followed by annealing to 1020 K for 15 min. Disilane (Si2H6) was dosed while the sample was cooling from 930 to 620 K in order to prepare a smooth Si(100)-2×1 surface.13,14 The quality of the surface was verified by the presence of sharp monohydride features at 2097 and 2086 cm-1 15,16 as determined via infrared spectra taken before flashing the hydrogen off at 860 K. Dicyclopentadiene of >98% purity was obtained from Aldrich, with 98.6% of the dicyclopentadiene in endo form and 1.4% in exo form. Cyclopentadiene was prepared by thermal cracking and distillation of the dicyclopentadiene.17 Both compounds were purified by repeated freeze-pump-thaw cycles before introduction into the chamber, and their purities were verified in situ by mass spectrometry. Disilane (Si2H6, gas, 99.998%, Voltaix) was used without further purification. Exposures are reported in units of Langmuir (L), where 1 L ) 10-6 Torr s. All exposures were performed by filling the chamber with the compound of interest for a given pressure and time. The pressures have not been corrected for ion gauge sensitivity. All ab initio calculations were done using the Gaussian 98 computational chemistry software.18 The structures, energies, and vibrational frequencies of the reaction products were predicted at the B3LYP/6-31G* level of theory.19,20 This method

has been shown to predict accurate geometries and vibrational frequencies for similar cycloaddition reactions on the Si(100)2×1 surface.21 The surface was modeled using a dimer cluster consisting of nine Si atoms, where the two atoms in the topmost layer of the cluster represent the surface dimer. The seven subsurface Si atoms are terminated with 12 hydrogen atoms to maintain the sp3 hybridization of the bulk Si atoms. All minimized structures obtained were verified by frequency analysis, whereby no imaginary frequencies were found in optimized product structures. Reaction energies reported are all zero-point-corrected. Results and Discussion The ab initio energy minimized structures of the [4 + 2] and [2 + 2] surface adducts of cyclopentadiene on Si(100)-2×1 are shown in parts a and b of Figure 1, respectively, along with the calculated binding energies. Our calculations predict the [4 + 2] product has a binding energy of -49.1 kcal/mol, versus a binding energy of -36.8 kcal/mol for the [2 + 2] product. This result is consistent with previous theoretical studies21 of other cycloaddition reactions on Si(100)-2×1, which predict the [4 + 2] product is thermodynamically more stable than the [2 + 2] product due to the extra strain associated with the fourmembered ring in the [2 + 2] product compared to the sixmembered ring in the [4 + 2] product. The calculations also predict that the [2 + 2] product is asymmetric with respect to the SidSi dimer, with the cyclopentadiene retaining a planar configuration. The [4 + 2] product, on the other hand, is

Adsorption on Si(100)-2×1

Figure 2. IR spectra of cyclopentadiene (500 L) adsorbed on Si(100)2×1 at 300 K for s- and p-polarized light. Theoretically predicted frequencies for the [4 + 2] and [2 + 2] reactions are shown and are scaled by a factor of 0.938. Short lines indicate frequencies predicted to have very weak intensities.

symmetric with respect to the SidSi dimer with the cyclopentadiene in a bent configuration, as shown in Figure 1a. Endodicyclopentadiene has the potential to react intact to form any of the four products shown in parts c-f of Figure 1, neglecting stereochemistry. This is due to the fact that dicyclopentadiene has two CdC double bonds, labeled here R and β, each of which can react via a [2 + 2] cycloaddition in two possible configurations, which we refer to as “up” (parts c and e of Figure 1) and “down” (parts d and f of Figure 1) for convenience. An inspection of the products suggests that the “down” configurations are unfavorable because of steric interactions between the nonbonding end of the dicyclopentadiene and the surface. Ab initio energy minimized structure calculations therefore were done for only the “up” configurations. These calculations predict a binding energy of -35.5 kcal/mol for the R-bonded adduct (Figure 1c) and -43.7 kcal/mol for the β-bonded adduct (Figure 1e). Another bonding possibility is for both double bonds of dicyclopentadiene to react with two adjacent surface dimers in the same row. Although we have not performed the calculations on a two-dimer cluster model, we suspect such a configuration would be highly strained for the following reasons. First, the calculated minimum-energy conformation of free dicyclopentadiene predicts the R and β double bonds to be nonparallel and also significantly twisted from a coplanar configuration. Also, the separation between the double bonds in dicyclopentadiene is as much as 0.89 Å shorter than the separation between adjacent surface dimers. Hence, the expected transition state and product for the adjacent dimer bonding configuration is likely to be unfavorable because of strain. Figure 2 shows the infrared spectra of a saturation dose (500 L) of cyclopentadiene on the Si(100)-2×1 surface at 300 K taken with s- and p-polarized light. Sharp, distinct peaks can be seen in the C-H stretching region associated with sp3 (2800-3000 cm-1) and sp2 (3000-3100 cm-1) hybridized carbon, indicating

J. Phys. Chem. B, Vol. 103, No. 32, 1999 6805 that cyclopentadiene has chemisorbed onto the surface. Theoretically calculated frequencies for both the [2 + 2] and [4 + 2] products are also shown, scaled by a factor of 0.938. This value is close to the scaling factor of 0.94 previously used by Bent and co-workers9 when comparing the experimental and theoretically calculated8 spectra for 2,3-dimethyl-1,3-butadiene on Si(100)-2×1. Comparison of the spectra in Figure 2 with the theoretically calculated frequencies shows closest agreement of the experimental spectra with the predicted [4 + 2] DielsAlder product, particularly for the peaks at 2841 and 2895 cm-1 as well as with the shoulder around 2915 cm-1. Agreement of the experimental spectra with the predicted [2 + 2] product is poorer, although based on the proximity of some of the predicted [4 + 2] and [2 + 2] modes, the presence of a small [2 + 2] side product cannot be entirely ruled out. Curiously, the peak seen at 2945 cm-1 is not predicted for either product. Although we are not certain of the origin of this feature, its position suggests that it may be related to some dicyclopentadiene impurity, as this is at the second strongest peak in the chemisorbed dicyclopentadiene spectrum. Further evidence supporting the assignment of the chemisorbed adduct to the [4 + 2] product comes from polarization studies. p-Polarized light has electric field components both parallel and perpendicular to the plane of the surface, the amplitudes of which are nearly equal in the MIR setup utilized here.16 s-Polarized light has its electric field entirely parallel to the surface plane, the amplitude of which is roughly equal to the electric field component parallel to the surface with p-polarized light. p-Polarized light, therefore, can probe vibrational modes both parallel and normal to the surface, whereas s-polarized light can only probe vibrational modes parallel to the surface. Figure 2 shows that the spectra for both s- and p-polarizations are nearly identical except that the peak at 2895 cm-1 is noticeably larger in the p-polarized spectrum than in the s-polarized spectrum. This suggests that the dipole moment for this mode has a significant component perpendicular to the surface, since spolarized light cannot probe modes perpendicular to the surface. The calculations for the [4 + 2] product indicate that this mode is the asymmetric stretch of the CH2 group. The transition dipole moment for this vibrational mode is calculated to be oriented at 67° from the surface plane, which agrees with the polarization studies showing that this mode has a significant component normal to the surface. Additionally, it can be seen in Figure 1a that this is the only mode that has a strong component of its dipole moment normal to the surface, consistent with the observation that the other peaks in the spectra exhibit virtually no differences between the s- and p-polarizations. Infrared spectra for a saturation dose (1500 L) of dicyclopentadiene on Si(100)-2×1 at 300 K and a multilayer exposure at 128 K are shown in Figure 3. Chemisorbed and multilayer (100 K) spectra of cyclopentadiene are also shown for comparison. It can be seen that the chemisorbed spectra of cyclopentadiene and dicyclopentadiene are distinctly different from their respective multilayer spectra, confirming that the molecules are chemisorbed to the surface rather than physisorbed at room temperature. As with cyclopentadiene, the chemisorbed spectrum for dicyclopentadiene shows several sharp, distinct peaks in the C-H stretching region from 2800 to 3100 cm-1, indicating adsorption onto the surface. Upon inspection, however, it is evident that the spectra of chemisorbed monomer and dimer differ in both the peak frequencies and intensities. If the surface does indeed catalyze a retro-Diels Alder reaction of dicyclopentadiene to cyclopentadiene during adsorption (the alternative pathway previously proposed), we expect the spectra

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Figure 3. Multilayer (unpolarized) and chemisorbed (p-polarized) IR spectra for dicyclopentadiene and cyclopentadiene on Si(100)-2×1. Some dicyclopentadiene impurity has been subtracted from the multilayer cyclopentadiene spectrum, although some small, incompletely subtracted peaks from 2840 to 2990 cm-1 can still be seen. A different cyclopentadiene sample was used for the chemisorbed spectrum and contained no detectable dicyclopentadiene as confirmed by mass spectrometry.

for dicyclopentadiene and cyclopentadiene would be quite similar, if not identical. Thus, it is immediately apparent that the major pathway for the adsorption of dicyclopentadiene at room temperature is not through a surface-catalyzed retroDiels-Alder reaction but rather most likely through a [2 + 2] cycloaddition leading to one or more of the products in parts c-f of Figure 1. The width of the alkene-like C-H stretch at 3050 cm-1 in the chemisorbed dicyclopentadiene spectrum provides further insight into the bonding of dicyclopentadiene at the surface. It should be noted that this peak is much sharper than a similar peak in the dicyclopentadiene multilayer also at 3050 cm-1 (fwhm ) 11 vs 28 cm-1, respectively). The broadness of the multilayer peak appears to be the result of two overlapping absorbances of the R and β alkene C-H stretches. Since a significant concentration of more than one adsorbed product (e.g., both R-up and β-up forms) would most likely result in a similar broadening of this peak in the chemisorbed spectrum, the data do not support the presence of multiple products. Instead, the data suggest that there may be one preferred adsorption product out of the possibilities shown in Figure 1 and that only one chemisorbed product is formed. Both monomer- and dimer-chemisorbed spectra show some absorbance in the Si-H stretching region between 2000 and 2150 cm-1, indicating potentially some dissociative adsorption. Interestingly, the Si-H feature for both cyclopentadiene and dicyclopentadiene exhibits a pronounced derivative-like shape, with a negative peak at 2087 cm-1, as shown in Figure 2 for cyclopentadiene. Although the origin of this feature is not known, it could signify the presence of a small amount of adsorbed hydrogen originating from trace contaminants in the

Wang et al. chamber or from residual hydrogen incompletely removed from the surface during flashing. Upon adsorption of the monomer or dimer, it may be possible that the adsorbed hydrogen is perturbed, causing the Si-H peak to shift down in frequency. Such a shift would result in the derivative shape seen in the spectra after adsorption of cyclopentadiene and dicyclopentadiene. However, we note that the infrared transition dipole is much stronger (∼25×) for Si-H than for C-H.12 Hence, the amount of adsorbed hydrogen is a small fraction at most of adsorbed hydrocarbon, and we conclude that the adsorption of cyclopentadiene and dicyclopentadiene is almost entirely molecular. This result is consistent with previous cycloaddition studies on Si(100)-2×1.5,10,22 To estimate an upper limit for the cyclopentadiene coverage, disilane (Si2H6) was used to gauge the extent of bare surface remaining after adsorption. In these experiments disilane was dosed at room temperature onto both the cyclopentadiene-saturated surface and the clean Si(100)-2×1 surface. Disilane dosed at room temperature onto the Si(100)-2×1 surface has been shown to dissociatively adsorb on the bare surface, forming mainly dihydride species with some monohydride and trihydride groups, and is not expected to react at occupied sites.13,23-25 By comparing the integrated intensities of the Si-H region for disilane dosed onto the clean and cyclopentadiene-covered surfaces, we estimate that at least one-third of the cyclopentadienecovered surface is bare. One-third is given as a lower limit because in addition to directly blocking sites, adsorbed cyclopentadiene may also hinder disilane adsorption at neighboring sites. Hence, cyclopentadiene adsorbed at 300 K covers less than twothirds of the surface. It should be noted that dosing disilane at room temperature onto the cyclopentadiene-covered surface had no effect on peaks in the C-H stretching region, indicating that (1) cyclopentadiene is not displaced upon disilane adsorption, (2) disilane does not add onto the remaining cyclopentadiene double bond, and (3) the cyclopentadiene double bond is not hydrogenated by the resulting Si-H surface species at 300 K. An estimate of dicyclopentadiene surface coverage relative to cyclopentadiene surface coverage was obtained by comparing the integrated intensities of the chemisorbed spectra at saturation doses and by assuming that an adsorbed dicyclopentadiene molecule results in roughly twice the integrated ν(C-H) intensity of a cyclopentadiene molecule. Using this method, we estimate that dicyclopentadiene adsorbs onto approximately 3040% more dimer sites than cyclopentadiene. This is a somewhat surprising result considering the larger size of the dimer over the monomer and suggests that the dimer packs onto the surface more efficiently than the monomer. Thus, we tentatively assign the dicyclopentadiene surface adduct to the R-up configuration in Figure 1c, which appears to be the least sterically hindered configuration with respect to adjacent SidSi dimers out of the four possible products. Moreover, the expected transition state structure of dicyclopentadiene on the surface leading to the β-up product in Figure 1e is expected to be sterically hindered by the CH2 bridge group, and hence, this reaction pathway may be unlikely. The presence of the alkene stretch in the chemisorbed spectrum also indicates that the configuration where both double bonds have reacted across adjacent dimers is not the major product, although the possibility of a small side product cannot be ruled out. Additionally, the large integrated intensity of chemisorbed dicyclopentadiene relative to cyclopentadiene (∼2.7:1 ratio) provides evidence that dicyclopentadiene adsorbs intact. If dicyclopentadiene were to dissociatively adsorb (i.e., as cyclopentadiene), the expected integrated intensity would be closer to that of cyclopentadiene.

Adsorption on Si(100)-2×1

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TPD studies were performed for both cyclopentadiene and dicyclopentadiene chemisorbed onto the Si(100)-2×1 surface. For cyclopentadiene, some molecular desorption is observed at a peak temperature of 570 K, along with a large H2 desorption peak at 780 K. This indicates that decomposition of the DielsAlder adduct to form surface carbon and hydrogen occurs upon heating. Thermal desorption studies of the dicyclopentadiene also yielded a similar but broader peak at 570 K corresponding to molecular desorption of dicyclopentadiene and possibly cyclopentadiene as well. Since the mass cracking patterns of the monomer and dimer are very similar, it is difficult to determine from the dicyclopentadiene TPD results whether some of the adsorbed dicyclopentadiene first converts to cyclopentadiene during desorption or if the dicyclopentadiene cracks to cyclopentadiene in the mass spectrometer. Observation of massto-charge ratios (m/e) of 91 and 132 in the dicyclopentadiene TPD spectrum, which are not present for cyclopentadiene mass spectra, confirm however that some of the dicyclopentadiene desorbs intact. As with cyclopentadiene, a large H2 desorption peak at 780 K indicates mainly decomposition of the dicyclopentadiene adduct upon heating. Discussion of Energetics As previously discussed, the retro-Diels-Alder pathway, in which dicyclopentadiene dissociates into cyclopentadiene during adsorption, provides a large thermodynamic incentive over an intact [2 + 2] pathway. That dicyclopentadiene does not appear to undergo a surface-catalyzed retro-Diels-Alder reaction during adsorption provides insightful information into the driving forces of the reaction. Parts a and b of Figure 4 show the potential energy diagrams for the retro-Diels-Alder pathway and the [2 + 2] pathway (leading to the product in Figure 1c) of dicyclopentadiene adsorption, respectively. For the retro-DielsAlder pathway, Figure 4a shows that the energy barrier for the retro-Diels-Alder reaction of dicyclopentadiene into 2 M equiv of cyclopentadiene in the gas phase is 33.7 kcal/mol.26 The energy barrier for the surface-catalyzed reaction is most likely smaller; thus, 33.7 kcal/mol is an upper limit for the reaction barrier. The cyclopentadiene then overcomes an energy barrier, which is likely to be minimal,21 undergoing the [4 + 2] reaction. This results in a energy change of -98.2 kcal/mol, since 2 M equiv of the monomer are formed from the dimer and the energy change of the monomer [4 + 2] reaction is -49.1 kcal/mol. Thus, the overall reaction energy of the retro-Diels-Alder pathway is approximately -81.2 kcal/mol. In contrast, Figure 4b shows that the [2 + 2] intact addition of dicyclopentadiene leads to a reaction energy change of only -35.5 kcal/mol, a difference of 45.7 kcal/mol. Despite the fact that the retro-Diels-Alder pathway leads to a product that is thermodynamically favored by a significant margin, our experimental results indicate that the [2 + 2] intact addition of the dimer is the dominant reaction pathway. This implies that thermodynamic factors do not play the dominant role in determining the reaction products of dicyclopentadiene at room temperature. We can infer then that kinetic factors control the reaction outcome in the form of either the activation barrier or the preexponential factor. We first discuss the activation barrier. If we were to assume that the preexponential factors for both reactions are nearly equivalent, we would conclude that even with the catalytic effect of the surface, the reaction barrier for dicyclopentadiene to split into cyclopentadiene is higher than the intact [2 + 2] reaction barrier. This argument would assign to the [2 + 2] reaction pathway an upper limit of 33.7 kcal/mol (or lower, depending on the extent to

Figure 4. Potential energy diagram for (a) the surface-catalyzed retroDiels-Alder reaction pathway and (b) the intact [2 + 2] reaction pathway leading to the product in Figure 1c for dicyclopentadiene on Si(100)-2×1.

which the barrier for the retroreaction is reduced at the surface). However, [2 + 2] reactions are known to have a high reaction probability on Si(100)-2×1 at room temperature,3-7 suggesting an activation barrier far below this upper limit. Another important possibility is that the preexponential factors for the two reactions differ significantly. There are likely to be severe geometric constraints on a transition state in which the dicyclopentadiene molecule has sufficient overlap with two silicon surface dimers to form two adsorbed cyclopentadiene species. Such a tight transition state would lead to a low preexponential factor relative to the [2 + 2] reaction. In this case, the prefactors must be considered when drawing conclusions about the relative reaction barriers of the two reactions. An interesting question is whether higher adsorption temperatures can facilitate the retro-Diels-Alder pathway. This possibility will be examined in future studies. Conclusions The cycloaddition of cyclopentadiene and dicyclopentadiene on the Si(100)-2×1 surface has been explored both theoretically and experimentally. Results indicate that cyclopentadiene chemisorbs at room temperature via a [4 + 2] Diels-Alder cycloaddition, while dicyclopentadiene adsorbs intact via a [2 + 2] reaction. Dicyclopentadiene does not appear to undergo a surface-catalzyed retro-Diels-Alder reaction during chemisorption despite the product being more thermodynamically favored by a large margin. Instead, it appears this reaction pathway has a slower rate than the intact [2 + 2] reaction. Future studies will examine whether higher temperatures can alter the reaction pathway of dicyclopentadiene on Si(100)-2×1. Acknowledgment. G.T.W. acknowledges financial support from a National Science Foundation Graduate fellowship. S.F.B.

6808 J. Phys. Chem. B, Vol. 103, No. 32, 1999 acknowledges financial support from a National Science Foundation CAREER award (DMR-9501774) and from the Beckman Foundation. C.B.M. acknowledges the Charles Powell Foundation. We thank Hailan Duan for her technical assistance. References and Notes (1) Appelbaum, J. A.; Baraff, G. A.; Hamann, D. R. Phys. ReV. B 1976, 14, 588. (2) Hamers, R. J.; Tromp, R. M.; Demuth, J. E. Surf. Sci. 1987, 181, 346. (3) Nishijima, M.; Yoshinobu, J.; Tsuda, H.; Onchi, M. Surf. Sci. 1987, 192, 383. (4) Hovis, J. S.; Lee, S.; Liu, H. B.; Hamers, R. J. J. Vac. Sci. Technol. B 1997, 15, 1153. (5) Hovis, J. S.; Hamers, R. J. J. Phys. Chem. B 1997, 101, 9581. (6) Hovis, J. S.; Hamers, R. J. J. Phys. Chem. B. 1998, 102, 687. (7) Kong, M. J.; Teplyakov, A. V.; Jagmohan, J.; Lyubovitsky, J. G.; Mui, C.; Bent, S. F. Manuscript in preparation. (8) Konecny, R.; Doren, D. J. J. Am. Chem. Soc. 1997, 119, 11098. (9) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Am. Chem. Soc. 1997, 119, 11100. (10) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Chem. Phys. 1998, 108, 4599. (11) Kong, M. J.; Teplyakov, A. V.; Lyubovitsky, J. G.; Bent, S. F. Surf. Sci. 1998, 411, 286.

Wang et al. (12) Kong, M. J.; Lee, K. S.; Lyubovitsky, J.; Bent, S. F. Chem. Phys. Lett. 1996, 263, 1. (13) Boland, J. J. Phys. ReV. B 1991, 44, 1383. (14) Lin, D. S.; Hirschorn, E. S.; Chiang, T. C.; Tsu, R.; Lubben, D.; Greene, J. E. Phys. ReV. B 1992, 45, 3494. (15) Chabal, Y. J. Surf. Sci. 1986, 168, 594. (16) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211. (17) Roberts, R. M. Modern Experimental Organic Chemistry, 3rd ed.; Saunders College: Philadelphia, 1979. (18) Frisch, M. J.; Trucks, G.W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J. Gaussian98, revision A.5;; Gaussian, Inc.: Pittsburgh, 1998. (19) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B. 1988, 37, 785. (20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (21) Konecny, R.; Doren, D. J. Surf. Sci. 1998, 417, 169. (22) Hovis, J. S.; Liu, H.; Hamers, R. J. Surf. Sci. 1998, 404, 1. (23) Wu, Y. M.; Baker, J.; Hamilton, P.; Nix, R. M. Surf. Sci. 1993, 295, 133. (24) Wang, Y. J.; Bronikowski, M. J.; Hamers, R. J. Surf. Sci. 1994, 311, 64. (25) Lubben, D.; Tsu, R.; Bramblett, T. R.; Greene, J. E. J. Vac. Sci. Technol. A 1991, 9, 3003. (26) Wassermann, A. Diels-Alder Reactions; Elsevier Publishing Co.: Amsterdam, 1965.