Cycloaddition Reactions of Phenylazide and Benzylazide on a Si(100

Feb 26, 2008 - The reaction of phenylazide (C6H5−N3) with a Si(100)-2 × 1 surface is analyzed with multiple internal reflection Fourier-transform i...
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J. Phys. Chem. C 2008, 112, 4297-4303

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Cycloaddition Reactions of Phenylazide and Benzylazide on a Si(100)-2 × 1 Surface Timothy R. Leftwich and Andrew V. Teplyakov* Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: December 1, 2007; In Final Form: January 5, 2008

The reaction of phenylazide (C6H5-N3) with a Si(100)-2 × 1 surface is analyzed with multiple internal reflection Fourier-transform infrared spectroscopy (MIR-FTIR), temperature-programmed desorption (TPD), and density functional theory (DFT). After examining multiple possible structures, it was found that phenylazide undergoes 1,3-cycloaddition through the -N3 functionality. The 1,2-cycloaddition observed previously for other azides does not contribute substantially to the reaction of phenylazide with this substrate. A kinetic argument is established for why 1,2-addition is observed with the reaction of benzylazide (C6H5-CH2-N3) but not with the reaction of phenylazide with this surface. The 1,2-adduct of benzylazide converts to the 1,3-adduct below room temperature. In addition, the last step in the reactions of both azides, that of nitrogen elimination, is found to occur above room temperature where it is predicted and observed that nitrogen is released at a higher temperature for benzylazide than for phenylazide.

I. Introduction

II. Computational Methods

The attachment of organic layers on silicon surfaces has been an area of active research that has accrued significant interest.1-10 Microelectronics, catalysis, and biological sensing are a few fields where this research may be applied. Aromatic compounds packed on the surface in a prearranged pattern may hold the key to lateral conductance of electric signals because of the possibility of overlapping among π-orbitals of neighboring molecules.11-13 Si(100)-2 × 1 is an ideal substrate for model studies of aromatic compounds because its 2 × 1 reconstruction14-16 can allow for the attachment of organic compounds along the silicon dimer rows, effectively using this reconstruction as a template to produce lines of molecules. In addition, these dimers are buckled, with the “up” silicon atom being slightly negative and the “down” silicon atom being slightly positive, and this buckling exhibits dynamic flipping at room temperature.17 Unfortunately, there are few known methods for attaching aromatic compounds to the surface. Some of these attachment techniques have problems such as oxygen migration18,19 and multiple configurations or major products. Thus, new chemical schemes for attaching aromatic groups to semiconductor substrates are in constant demand. An interesting method of attaching an aromatic compound to a semiconductor surface is based on the cycloaddition reaction of an azido group. This has been previously demonstrated with benzylazide on a Si(100)-2 × 1 surface.20 That study, along with studies of other azides on Si(100)-2 × 1 in ultrahigh vacuum have shown that nitrogen elimination follows azide attachment without surface oxidation.20-23 In this study, we examine the reaction of phenylazide with a Si(100)-2 × 1 surface, comparing the possible 1,2- and 1,3-addition pathways, using temperature programmed desorption (TPD), multiple internal reflection Fourier-transform infrared spectroscopy (MIRFTIR), and density functional theory (DFT). Additional parallels will also be drawn between the reactions of benzylazide and phenylazide with this surface.

Electronic structure calculations, transition state investigations, and vibrational frequency predictions were performed using the B3LYP hybrid density functional24,25 and a split valence polarized basis set with diffuse functions, 6-311+G(d,p),26-34 as implemented in the Gaussian 03 suite of programs.35 The Si(100)-2 × 1 surface was modeled by a Si9H12 cluster, representing one bare dimer, and a Si15H16 cluster, representing two neighboring bare dimers in the same row, where in both clusters the subsurface silicon atoms were terminated with hydrogen to preserve their hybridization. In the case of the Si15H16 cluster, the atoms in the third and higher layers were fixed at the positions of the optimized bare twodimer cluster. All of the frequencies in the calculated vibrational spectra were scaled by the factor 0.95 to minimize systematic errors. Frequency calculations were also preformed with deuterium atoms terminating the subsurface silicon atoms to allow for a better examination of the azide stretch region, around 2100 cm-1, avoiding interference from characteristic Si-H absorption features. GridChem’s computational resources were utilized for some of the results presented in this publication.36,37

* Corresponding author. Tel.: (302) 831-1969. Fax: (302) 831-6335. E-mail: [email protected].

III. Experimental Details The data presented here were colleted at the University of Delaware using two ultrahigh vacuum (UHV) chambers. Both chambers have a base pressure of approximately 5 × 10-10 Torr and are equipped with an Auger electron spectrometer (AES), a setup for low-energy electron diffraction (LEED), and an ion gun for surface sputtering. Auger electron spectra are used to confirm a clean Si(100)-2 × 1 surface after sputtering the silicon crystals with 1.0 keV Ar+ ions at room temperature for 45 min and subsequent annealing at 1150 K for 15 min. Argon (Matheson, 99.999%) was used without additional purification. One UHV chamber was utilized for infrared studies and is coupled to an infrared spectrometer (Nicolet, Magna 560) set up in a multiple internal reflection geometry with an external liquid nitrogen-cooled MCT detector. Spectra were collected with a resolution of 4 cm-1 and 2048 scans. Background spectra

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were collected before dosing of the appropriate amount of compounds into the chamber through a variable leak valve, and collecting the foreground. The cleanliness of the dosing compound was determined in situ using an unshielded mass spectrometer (SRS 200). All the infrared spectra were collected at the same temperature as the background: the silicon crystal (Harrick Scientific Corporation, 25 mm × 20 mm × 1 mm, 45° beveled edges) was allowed to cool after any brief annealing. This sample was mounted on a manipulator capable of heating the sample to ∼1200 K, using an e-beam heater (McAllister Technical Services), and cooling the sample to ∼100 K. The other chamber is used for temperature-programmed desorption (TPD) studies utilizing a shielded differentially pumped mass spectrometer (Hiden Analytical), with the 2 mm aperture of the shield positioned 2 mm away from the crystal during data collection. This mass spectrometer is also used to verify the cleanliness of the dosing compound in situ. The 1 cm × 1 cm silicon sample used in these experiments was cut from a Si(100) wafer (Wafer World, Inc.) and mounted on a manipulator that allowed the sample to be cooled to 150 K and heated to 1150 K. Phenylazide was synthesized and donated by Dr. Travis Shay and Professor Klaus Theopold of the Department of Chemistry and Biochemistry, University of Delaware. Phenylazide is explosive under distillation conditions. Phenylazide was purified by using several freeze pump-thaw cycles before introduction into a UHV chamber and its purity was confirmed in situ by mass spectrometry, with the spectrum obtained compared to the data available in the NIST database.38 All exposures in these studies are reported in Langmuirs (1 L ) 10-6 Torr s) and are uncorrected for ion gauge sensitivity. IV. Results and Discussion IV. 1. Computational Predictions. The reaction of phenylazide with a Si(100)-2 × 1 surface was examined computationally and spectroscopically. Several possible products of this reaction were considered and are summarized in Figure 1. All of these structures involve the reaction of the azide group and not the phenyl group with the surface. The retention of aromaticity has been shown both experimentally and computationally to be a strong driving force for directing reactions on Si(100)-2 × 1.13, 18-21, 39-69 The reactions of a phenyl group in a multifunctional reactive hydrocarbon are rare. In fact, even a substituent as simple as a methyl group has been reported to compete with an aromatic ring in toluene in a reaction with a Si(100)-2 × 1 substrate.48,50,51 More reactive substituents generally dominate the initial attachment chemistry in aromatic compounds.10 In our computational analysis of the attachment chemistry characteristic of the azide group in phenylazide, two stable structures, I and II, corresponding to relatively weakly bonded phenylazide molecules through the formation of a dative bond between one of the nitrogen atoms of an azide group and the “down” atom of the silicon surface dimer on the Si(100)-2 × 1 substrate were found. Of these two, structure I is found to be more stable. Therefore, it will be used as the starting point for further reactions with the surface; however, it should be again pointed out that both these local energetic minima represent relatively weakly bound structures. Structures III, IV, V, and VI represent possible cycloaddition products of the azide group, with structures III and V being substantially more stable (and therefore more likely to exist) compared to structures IV and VI. It should be noted that structures III and V represent 1,3- and 1,2-cycloadditions, respectively, similar to those that have been observed previously with other

Figure 1. Optimized structures and energies (relative to a phenylazide molecule, a Si9H12 model cluster and also, N2 for structure VIII) for adsorption through N1 (I); adsorption through N3 (II); 1,3-addition (III); 2,3-addition (IV); 1,2-addition (V); chemisorption through N3 (VI); C-N dissociation (VII); and chemisorption through N1 after N2 desorption (VIII) of phenylazide with a Si(100)-2 × 1 surface. Energies without zero point correction are given in parentheses. Color coding: green, silicon; blue, nitrogen; gray, carbon; white, hydrogen. The same color coding is used in all other figures. Hydrogen atoms terminating the silicon cluster are omitted for clarity. All energies are reported in kJ/mol.

compounds on silicon. 1,3-Addition was initially predicted computationally for diazomethane,21 methyl thiosulfinylamine,21 nitrileoxide,21 nitrilesulfide,21 methylazide,21 and nitromethane.21,70 It was later observed experimentally for nitromethane,71 nitroethane,72 and nitrobenzene.18,19 1,2-Addition has been reported for a number of substituted aromatic compounds;13,20,54-66 of the two studied cases where there is a potential for 1,3addition,20,61 only the reaction of phenyl isothiocyanate with silicon showed a strong preference for 1,2-addition.61 Despite the fact that structure VII corresponding to the C-N bond dissociation is thermodynamically stable, the kinetic barrier for this transformation starting with structure I is 43.2 kJ/mol, which is 19.8 kJ/mol higher than the desorption barrier. This is expected based on the known reactions involving C-N bond dissociation in alkylamines.39-42,46,47,73-90 As will be shown below, the spectroscopic signature of this structure is not observed in our experimental studies. Finally, structure VIII is expected to be the product of nitrogen elimination from either 1,2- or 1,3-cycloaddition processes involving the azide group,

Reactions of Phenylazide and Benzylazide its stability presented in Figure 1 includes a N2 molecule released into the gas phase. This is exactly the type of structure that is a target of successful chemical schemes delivering aromatic entities to the semiconductor surfaces without contamination and in a highly ordered fashion. Since phenylnitrene (C6H5N) is a stable molecule, the stability of structure VIII compared to that of phenylnitrene in the gas phase and the clean Si(100)-2 × 1 surface mimicked by the Si9H12 cluster. This yields a desorption barrier of 474.4 kJ/mol, if it is assumed that the adsorption process for phenylnitrene is barrierless. All of the structures described above involve the reaction of phenylazide with a single silicon surface dimer. The possibility of phenylazide reacting with more than one dimer was also examined by using a double dimer cluster. It was found to be substantially less favorable than reactions involving a single dimer. For example, the interdimer analog to structure VIII, an interdimer nitrogen bridge, is only 123.0 kJ/mol more stable than the reactants. This is not only less stable than structure VIII but also less stable than structures III and V, the 1,3- and 1,2-cycloadducts, respectively; therefore, no further examinations involving reactions with more than one dimer were explored. After analyzing the possible surface processes that can lead to the formation of stable adducts, two reactions, that of 1,3addition followed by subsequent nitrogen elimination and 1,2addition with subsequent nitrogen elimination, were investigated in detail and are shown in Figure 2. The reaction barriers for both 1,3- and 1,2-addition are below the desorption energy, as are the barriers for N2 elimination. This not only means that these reaction pathways are possible but that they likely compete with each other. To determine the dominant addition pathway, the reaction barriers for the first reaction, the transformation of the weakly bonded intermediate, structure I, to either the 1,3or the 1,2-adduct, must be examined. The predicted barrier for the reaction to the 1,3-adduct is only 0.7 kJ/mol while the barrier for the reaction to the 1,2-adduct is 11.6 kJ/mol. This would correspond to an approximate probability ratio of 80:1 at 300 K and 6250:1 at 150 K for 1,3- to 1,2-addition. In fact, the predicted barrier for 1,3-addition is so low that the phenylazide molecule may have sufficient energy to overcome the reaction barrier even at cryogenic temperatures. It should be noted that these probability ratios along with the others referred to later in the text would parallel the ratio of the rates if the preexponential factors for the two paths were equal. Although these pre-exponential factors may not be the same they are likely similar and allow for the establishment of an approximate kinetic estimate from the probability ratios. IV.2. Infrared Analysis. According to the computational investigation, 1,3-addition is so much more favored than 1,2addition that no experimental observation of 1,2-addition is expected. This is indeed the case. The left panel of Figure 3 shows the azide stretch region in the infrared spectra of a submonolayer coverage of phenylazide before and after its reaction with the Si(100)-2 × 1 surface as well as the predicted spectrum for structure V, the 1,2-adduct. It should be noted that the spectral region presented in Figure 3 is the most informative, since any azide stretches are expected to fall in this spectral window and since structures V and VII have very different spectroscopic signatures within this frequency range, while the characteristic absorption bands for structures III, IV, VI, and VIII all lie well outside of it. As shown in the left panel of Figure 3, only the signature of the physisorbed phenylazide, identical to the spectroscopic signature of this compound in the condensed multilayer (as presented below) is observed at

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Figure 2. 1,3-Addition with subsequent N2 elimination surface reaction pathway, top, and 1,2-addition with subsequent N2 elimination surface reaction pathway, bottom, for the reaction of phenylazide with a Si(100)-2 × 1 surface represented by a Si9H12 cluster. Computations were performed at the B3LYP/6-311+G(d,p) level of theory. Hydrogen atoms representing silicon cluster termination are omitted for clarity. All energies are reported in kJ/mol.

cryogenic temperatures. No qualitative changes in this infrared spectral region are observed upon annealing, as only the intensity of the azide absorption bands decreases with temperature. The lack of an azide stretch after chemisorption not only eliminates

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Figure 3. Infrared studies of phenylazide, left, and benzylazide, right, on Si(100)-2 × 1: (a) 5L submonolayer coverage of phenylazide condensed on a Si(100)-2 × 1 surface at 113 K; (b) Same as (a) but briefly annealed to 153 K; (c) same as (a) but briefly annealed to 182 K; (d) predicted infrared spectrum for structure V (a 1,2-adduct of phenylazide on a Si(100)-2 × 1 surface); (e) predicted infrared spectrum for a dimer of phenylazide; (f) 10 L submonolayer coverage of benzylazide condensed on a Si(100)-2 × 1 surface at 100 K; (g) same as (f) but briefly annealed to 185 K; (h) same as (f) but briefly annealed to 254 K; (i) predicted infrared spectrum for a 1,2-adduct of benzylazide on a Si(100)-2 × 1 surface; (j) predicted infrared spectrum for a dimer of benzylazide. The frequencies of all predicted spectra are scaled by 0.95. The spectra corresponding to the adsorbed benzylazide and phenylazide are reported with a deuterium-substituted silicon cluster to avoid interference from Si-H vibrations.

the presence of structure V but also structure VII. The predicted azide vibrational mode for structure V presented in Figure 3, spectrum (d), seems to agree quite well with one of the azide stretches in spectra (a) and (b). However, Figure 4 shows that the intensity of this absorption band increases with exposure of phenylazide far above the formation of a monolayer. Although the possibility that there may be a small component contributed by 1,2-adducts cannot be completely ruled out, it is clear that this absorption band corresponds to the molecular phenylazide. The splitting in the azide stretch seen at cryogenic temperature can be explained by the formation of multimers of phenylazide. To illustrate this, a spectrum of a dimer of phenylazide is shown in Figure 3e. This spectrum has two closely spaced vibrational modes, 2127 and 2129 cm-1, corresponding to azide stretches. In addition, the formation of dimers and perhaps by extension multimers is shown to have a thermodynamic driving force, since the dimer structure is 3.3 kJ/mol more stable than two individual phenylazide molecules. The splitting of the azide stretch has been seen previously with a similar compound benzylazide, where an analogous prediction for a dimer of benzylazide not only showed splitting of the azide stretch, as can be seen in the right panel of Figure 3, spectrum (j), but was also found to be 5.5 kJ/mol more stable than two individual benzylazide molecules.20

Leftwich and Teplyakov

Figure 4. Infrared studies of phenylazide on a Si(100)-2 × 1 surface at cryogenic temperatures (114 K) at various exposures of phenylazide with the predicted spectrum of a phenylazide molecule (bottom).

A comparison of phenylazide and benzylazide can prove to be insightful to the reactivity of both molecules with a Si(100)-2 × 1 surface. Although no infrared evidence for 1,2addition has been observed with phenylazide, evidence for 1,2addition of benzylazide has been reported,20 and the comparison is shown in Figure 3. The agreement of the predicted azide stretch for spectrum (i), the 1,2-adduct, with an azide stretch seen in spectrum (g), a spectrum of benzylazide exposed to a Si(100)-2 × 1 surface and briefly annealed to 185 K, confirms the presence of this product on the surface. The reason that 1,2addition is found to occur with benzylazide is that like phenylazide all the reaction barriers in the 1,3- and 1,2-addition pathways, the analog to the pathways presented in Figure 2 for phenylazide, are below the activation energy required for desorption; however, in the case of benzylazide the reaction barriers for the conversion of a weakly adsorbed state to either the 1,3- or the 1,2-adduct are more comparable. The predicted barriers are 9.8 and 15.5 kJ/mol, respectively. This would correspond to an approximate probability ratio of 10:1 at 300 K and 100:1 at 150 K. This means that although 1,3-addition is still predicted to be more favorable there is a substantial chance for the 1,2-addition to be observed with infrared spectroscopy. Although the 1,2-adduct of benzylazide can be confirmed spectroscopically at cryogenic temperatures, by room-temperature it is no longer present. This is illustrated in Figure 3, spectrum (h), by the lack of any observable azide stretch after a brief annealing to 254 K. The lack of an azide stretch at room temperature can be informative about what may be present on the surface. Since the benzylazide analogues to the structures in Figure 1 have been previously examined by our group and found not only to follow the same thermodynamic trends but also similar predicted infrared spectra, this means that not only is the 1,2-adduct absent at room temperature but the analog to

Reactions of Phenylazide and Benzylazide

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Figure 6. Temperature-programmed reaction/desorption studies of (a) 15L of phenylazide and (b) 40L of benzylazide adsorbed on a Si(100)-2 × 1 surface at cryogenic temperatures. m/e+ ) 28 (molecular nitrogen) was followed.

Figure 5. Infrared studies of phenylazide: (a) monolayer of phenylazide on a Si(100)-2 × 1 at room temperature; (b) same as (a) but briefly annealed to 503 K; (c) predicted infrared spectrum for structure III (1,3-adduct of phenylazide on a Si(100)-2 × 1 surface); (d) predicted infrared spectrum for structure VIII; (e) predicted infrared spectrum for structure IV (a 2,3-adduct of phenylazide on a Si(100)-2 × 1 surface); (f) predicted infrared spectrum for structure VI. The predicted vibrations around 2100 cm-1 are due to the Si-H absorption bands corresponding to cluster terminations only.

structure VII and an additional structure, created by the dissociation of the C-C bond between the phenyl and methylazido groups across a dimer, are also not present because they have a pronounced predicted azide stretch. Figure 5 shows two experimental infrared spectra of phenylazide reacted with the Si(100)-2 × 1 surface, as well as the predicted infrared spectra for structures III, the 1,3-adduct, and VIII, the product of nitrogen elimination. As shown in the next section, the first of these experimental spectra are collected prior to nitrogen elimination, while the second is collected after nitrogen elimination. Although both predicted spectra are similar to each other, they agree well with the experimental spectra. In addition, the presence of C-H stretches from only the aromatic ring and the lack of any Si-H stretches after chemisorption indicate that the phenyl group does not react with the surface. Based on the thermodynamic and kinetic arguments obtained in the computational investigation, production of structure III, the 1,3-adduct, should dominate adsorption of phenylazide on

Si(100)-2 × 1 surface, as the possibility of C-N dissociation is kinetically unlikely, while the possible result of 2,3-cycloaddition (structure IV) and N-bonded structure VI are substantially less stable than any other structures considered. Also, contrary to the experimental observation, the predicted spectra for structures IV and VI only exhibit CdC stretches of exceptionally small intensity relative to the rest of the spectrum, as can be seen in Figure 5. In addition, even if the alternative 1,2cycloaddition observed for benzylazide does take place for phenylazide, the result of this addition is easily converted into structure III. The similarities in predicted spectroscopic signatures for structures III and VIII, the 1,3-adduct and the product of nitrogen elimination, respectively, do not allow us to unequivocally distinguish them as dominant upon room-temperature adsorption; however, based on the comparison with the experimentally recorded room-temperature spectra, it is likely that either structure III or structure VIII are mostly populating the silicon surface at room temperature. Since structure VIII is produced by a release of N2 into the gas phase from structure III, it should be possible to mark the conversion temperature of structure III into structure VIII by temperatureprogrammed desorption analysis described in the next section. IV.3. Temperature-Programmed Desorption Analysis. As noted above, it is difficult with infrared analysis alone to determine the temperature at which nitrogen elimination occurs, marking the production of structure VIII; however, it can be readily determined by following molecular nitrogen evolution in a TPD experiment. Figure 6 shows the TPD spectra of both phenylazide and benzylazide exposed to Si(100)-2 × 1 at cryogenic temperatures where the evolution of molecular

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Leftwich and Teplyakov respectively. Using the Redhead analysis for a first-order process with a pre-exponential factor of 1013 s-1 and a heating rate of 2 K/s, this would correspond to a reaction barrier of approximately 82 kJ/mol and 105 kJ/mol, respectively. The computational results also predict that the barrier for nitrogen elimination from the 1,3-adduct for phenylazide is smaller than that for benzylazide. These results are 61.4 kJ/mol for phenylazide and 89.8 kJ/mol for benzylazide. The overall major surface reaction is illustrated for phenylazide in the top panel of Figure 2 (and for the major pathway of benzylazide in the Supporting Information). One final question that should be examined is “what happens to the 1,2-adduct of benzylazide”? There are two possibilities: one, that it undergoes nitrogen elimination similar to the last step in the bottom reaction diagram in Figure 2 and two, that it rearranges to another species. Although the first possibility cannot be completely discarded because the temperature at which the 1,2-adduct disappears is close to the multilayer desorption temperature, a small reaction barrier of 2.2 kJ/mol for the conversion of the 1,2-adduct to the 1,3-adduct was predicted computationally for benzylazide. This rearrangement would result in the two end-nitrogens in the azide group, furthest from the phenyl ring, to switch positions. When this barrier is compared to the barrier for nitrogen elimination, 19.3 kJ/mol,20 by examining the probability ratio around the temperature that the 1,2-adduct disappears, 250 K, it is found that this would correspond to a ratio of approximately 3740:1 rearrangement to elimination. This would heavily favor rearrangement to the 1,3-adduct over nitrogen elimination. Conclusions

Figure 7. Temperature-programmed reaction/desorption studies of phenylazide adsorbed on a Si(100)-2 × 1 surface at cryogenic temperatures. m/e+ ) 119 (molecular ion of phenylazide) was followed as a function of phenylazide exposure. The inset shows the integrated peak areas as a function of the initial exposure.

nitrogen (m/e+ ) 28) was followed. Two peaks can be observed in each spectrum. The first peak corresponds to the multilayer desorption. Conformation of this assessment for phenylazide is shown in Figure 7. Here the molecular ion of phenylazide (m/e+ ) 119) was followed after variable exposures of Si(100)-2 × 1 to phenylazide at cryogenic temperatures. The exposure required for monolayer formation at cryogenic temperatures was found to be approximately 15 L, as confirmed by the peak area plot presented in the inset of Figure 7. In addition, the multilayer desorption peak, found to occur at 225 K, correlates well with the lower temperature peak in Figure 6, spectrum (a). A similar analysis was conducted by the our group for benzylazide where the exposure required for multilayer formation in this experimental setup using the same dosing geometry at cryogenic temperatures was found to be 20 L and the multilayer desorption peak was found to occur around 215 K. This also coincides with the lower temperature peak in Figure 6, spectrum (b). The second peak in each spectrum in Figure 6 originates from nitrogen elimination. Since by room temperature there is no 1,2adduct present for either phenylazide or benzylazide and since molecular nitrogen is produced above room temperature in both cases, the peaks at 315 K in spectrum (a) and at 398 K in spectrum (b) in Figure 6 should correspond to nitrogen elimination from the 1,3-adduct for phenylazide and benzylazide,

The reaction of phenylazide with a Si(100)-2 × 1 surface was examined by TPD, MIR-FTIR, and DFT with a detailed side-by-side comparison made to the reaction of benzylazide with the same surface. It was established that phenylazide undergoes 1,3-addition followed by nitrogen elimination. Nitrogen elimination was found to occur above room temperature, as confirmed by TPD analysis. It was determined that the reason that 1,2-addition was observed spectroscopically for benzylazide and not for phenylazide is that although 1,3-addition is still favored over 1,2-addition the reaction barriers for 1,2and 1,3-addition are more comparable for benzylazide than for phenylazide. Benzylazide is predicted to be much more likely to undergo 1,2-addition over 1,3-addition than phenylazide. The 1,2-adduct of benzylazide is shown to convert to the 1,3-adduct by room temperature and this conversion is predicted to be heavily favored over nitrogen elimination. Nitrogen elimination in a benzylazide reaction with Si(100)-2 × 1, presumably from the 1,3-adduct is found to occur at higher temperatures, around 400 K, compared to phenylazide, which agrees with theoretical predictions. Both phenylazide and benzylazide are found to cleanly deliver an aromatic ring to a Si(100)-2 × 1 surface. Acknowledgment. This work was supported by the National Science Foundation (CHE-0415979). The authors are grateful to Dr. Travis Shay and Professor Klaus Theopold of the University of Delaware for the synthesis and donation of phenylazide. Dr. Semyon Bocharov, Dr. Olga Dmitrenko and Dr. Lucila P. Me´ndez De Leo are acknowledged for their earlier work with benzylazide. GridChem is acknowledged for computational resources and services for selected results used in this publication. Supporting Information Available: Cartesian coordinates and infrared frequencies for the structures considered, the

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