Adsorption of Structural and Stereoisomers of Cyclohexanediamine at

Article pubs.acs.org/JPCC

Adsorption of Structural and Stereoisomers of Cyclohexanediamine at the Ge(100)‑2 × 1 Surface: Geometric Effects in Adsorption on a Semiconductor Surface Keith T. Wong and Stacey F. Bent* Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: The effects of structural isomerism and stereoisomerism in molecular adsorption at surfaces are studied through the reaction of four cyclohexanediamine isomerscis-1,2-, trans-1,2-, cis-1,4-, and trans-1,4-cyclohexanediamineat the Ge(100)-2 × 1 semiconductor surface. Using a combination of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, we find that all isomers primarily adsorb by reacting with both functional groups, either forming an N−Ge dative bond to the surface or undergoing N−H dissociation to form Ge−N and Ge−H ordinary covalent bonds. Among the four isomers, differences are observed in the relative amounts of dative bonded versus dissociated amines. Density functional theory calculations explain these results by showing that strain of the surface− adsorbate bonds and in the cyclohexane backbone of the adsorbate give rise to differences in the reaction energetics. In particular, trans-1,4-cyclohexanediamine is the only isomer that must adopt the twist-boat conformation to interact with the surface via both functional groups, and the energetic penalty of this requirement leads to its distinct product distribution in which N dative bonding is more prevalent than for the other three isomers studied.

1. INTRODUCTION The nonplanar conformations of cyclohexane were first proposed in the 1890s1−3 and started to gain widespread acceptance following the work of Ernst Mohr in 1918.4,5 Disubstituted cyclohexane compounds have long been studied in the context of chelation with metal ions, and both the relative positions of substituents on the cyclohexane ring (structural isomerism and stereoisomerism) and the conformation of the ring (conformational isomerism) are known to affect chelation.6−12 More recently, regioselectivity and stereoselectivity of reactions of disubstituted cycloalkanes with other organic molecules have been studied for identification or separation of biomolecules.13,14 Such work has led to the ability to identify stereoisomers by means such as chemical ionization mass spectrometry.15−18 Identification of specific stereoisomers has particular importance for biologically active compounds, for which activity sometimes differs by many orders of magnitude between stereoisomers. The present work extends the past studies of regio- and stereoselectivity of disubstituted cyclohexanes, which focused on liquid- or gas-phase reactivity, to their reaction at a solid substrate, namely, the 2 × 1 reconstructed Ge(100) surface. Reaction of organic molecules at semiconductor surfaces is important because it offers the ability to tune the properties of semiconductor surfaces and of inorganic−organic interfaces. By combining established knowledge of inorganic semiconductor processing techniques with the tailorability of organic molecules, novel device structures may be created. Applications © 2013 American Chemical Society

of organic functionalization are varied and include organic electronics,19,20 surface passivation,21−23 chemical and biological sensors,24,25 organic thin film growth,26,27 nanoscale patterning,27,28 and molecular electronics.29−31 The 2 × 1 reconstruction of the germanium surface creates ordered rows of dimers, and two key properties of these surface dimers enable analogies to be drawn with organic chemistry.32,33 First, the 2 × 1 reconstruction reduces the number of surface dangling bonds by forming a σ-bond and partial π-bond between the dimer atoms. The presence of a partial π-bond imparts olefinic character that enables cycloaddition-like reactions.32−34 Second, uneven charge distribution between the dimer atoms is associated with tilting of the dimers out of the surface plane. The resulting up-tilted dimer atoms are electron rich, and the down-tilted dimer atoms are electron deficient. This creates electrophilic/nucleophilic and Lewis acid/base characteristics that enable surface reactions involving dative bonding or proton transfer.32,34 These reactions are the most relevant for the cyclohexanediamines studied in this work, and Figure 1 shows the expected reaction pathway for an amine functional group with a Ge(100)-2 × 1 surface dimer. Reaction of other amines, including several methylamines, with Ge(100)-2 × 1 and the related Si(100)-2 × 1 have been previously studied.35,36 It was found that methylamine, Received: June 28, 2013 Revised: August 14, 2013 Published: September 5, 2013 19063

dx.doi.org/10.1021/jp406423n | J. Phys. Chem. C 2013, 117, 19063−19073

The Journal of Physical Chemistry C

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phenylenediamine isomers is eliminated. In addition, the current study employs density functional theory (DFT) calculations directly aimed at elucidating how adsorbate geometry affects strain and, thus, adsorption energetics. Using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), we find that bifunctionality enables proton transfer reaction not observed for monofunctional alkyl amines on the Ge(100)-2 × 1 surface at room temperature,35 and the results show that most cyclohexanediamine molecules react through both amine functional groups. For convenience, such adducts will be called “bidentate”, although in this context the term is not meant to imply bonding to a single metal center. While cis- and trans-1,2-cyclohexanediamine exhibit similar reactivity, significant differences are observed between the cis- and trans-1,4 diastereomers. DFT calculations suggest that this disparity is driven by strain imposed by the chair−boat conformational change required for trans-1,4-cyclohexanediamine to form a bidentate linkage to the surface. Similarly, small differences in the amounts of N dative bonded versus N−H dissociated amines between cis-1,4cyclohexanediamine and the two 1,2-diastereomers result from different strain of the Ge−N bonds between the surface and adsorbate. The use of the crystalline germanium surface in this work provides an ordered array of reactive sites with fixed positions from which the geometric differences among structural isomers and stereoisomers of cyclohexane can be probed and compared with homogeneous chemistry. As described above, this study shows clear differences in reactivity with Ge(100)-2 × 1 among the four cyclohexanediamine isomers. The results demonstrate the importance of geometric effects and strain in reaction of disubstituted cyclohexane isomers with a crystalline surface. Importantly, these results parallel past studies showing the effects of stereoisomerism or structural isomerismeven as minimal as chair−boat conformational changeson chelated metal complex stability and reaction selectivity and indicate that these fundamental concepts also apply to reactions of organic molecules at solid semiconductor surfaces.7,9−12,42

Figure 1. Schematics of the four cyclohexanediamine isomers studied in their lowest energy conformation (chair conformation with, where applicable, diequatorial amines) and the expected reaction pathway for an amine functional group with a Ge(100)-2 × 1 surface dimer. Only α-hydrogens on the cyclohexanediamine isomers are shown for simplicity.

dimethylamine, and trimethylamine adsorb molecularly on Ge(100)-2 × 1 at room temperature by donation of a lone pair of electrons from nitrogen to the a down-tilted Ge dimer atom, forming an N−Ge dative bond. On Si(100)-2 × 1, the dative bonded state serves as a precursor for methylamine and dimethylamine to undergo N−H dissociation to form ordinary covalent Si−N and Si−H bonds (i.e., proton transfer from the amine to the surface). These reactions are shown schematically in Figure 1 for the general case of an amine reacting with a Ge(100)-2 × 1 surface dimer; the same reactions are possible for the cyclohexanediamines studied in this work. Previous studies have also investigated reaction of other bifunctional molecules at the semiconductor surface.22,37−41 Of particular relevance, the reactivity of phenylenediamine isomers was found to vary due to a combination of geometric and electronic effects.39 The m-phenylenediamine isomer was found to bond to Ge(100)-2 × 1 through both amines more so than did the ortho- or para-isomers. It was concluded that electronic interactions between the two amine groups (e.g., by resonance or inductive effects) on m-phenylenediamine contribute to its higher degree of attachment through both functional groups. It was also inferred from differences in reactivity between o-phenylenediamine and p-phenylenediamine, which are expected to be electronically similar, and from calculated adsorption energies, that geometric effects also play a role in determining the amount of adsorbates bound through one versus both amines. Therefore, in the phenylenediamine system, both geometric and electronic effects are coupled. The current study compares reaction at the Ge(100)-2 × 1 surface of the four cyclohexanediamine structural and stereoisomerscis- and trans-1,2-cyclohexanediamine and cis- and trans-1,4-cyclohexanediamineshown schematically in Figure 1. The cyclohexane moiety is expected to be inert with respect to the surface and provide for negligible electronic interaction between the functional groups. Hence, differences in reactivity among cyclohexane isomers must result from geometric effects or from surface-mediated electronic interactions, and the complication of electronic interaction between amines of

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. FTIR Spectroscopy. FTIR spectroscopy studies were performed in an ultrahigh vacuum (UHV) chamber described previously35 with a base pressure less than 2 × 10−10 Torr. Two cycles of argon ion sputtering (1 kV accelerating voltage, 20 mA emission current, 9−10 μA sample current) for 20 min followed by annealing to 880 K for 5 min were used to prepare the clean, reconstructed Ge(100)-2 × 1 surface. Temperature was monitored by a type K thermocouple attached directly to the surface of the crystal, and low-energy electron diffraction (LEED; Physical Electronics) verified the 2 × 1 reconstruction. A Nicolet 6700 spectrometer in a multiple internal reflection (MIR) geometry was used in conjunction with a liquid nitrogen cooled mercury−cadmium−telluride (MCT) detector to collect FTIR spectra. The unpolarized IR beam was coupled into and out of the UHV chamber through differentially pumped CaF2 viewports. The beam path outside of the UHV chamber was sealed and purged with air treated by a purge gas generator (Parker Filtration) to eliminate spectral features from H2O and CO2. Absorption by the CaF2 windows below ∼1050 cm−1 limits the spectral range of our system. Trapezoidal Ge crystals (19 mm × 14 mm × 1 mm, 45° bevels, Harrick Scientific) were used as the substrate. IR spectra were corrected for baseline fluctuation by manually subtracting spline functions 19064



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fit to points devoid of spectral features. Physisorbed multilayer spectra were collected by holding the substrate at low temperature to enable condensation of dosed precursor onto the surface. 2.2. XPS. XPS studies were performed in a separate UHV chamber also described previously43 with base pressure less than 2 × 10−10 Torr. The Ge substrate (approximately 8 mm × 8 mm × 1 mm, MTI Corp.) was cleaned by a single cycle of argon ion sputtering (1 kV accelerating voltage, 20 mA emission current, 18−20 μA sample current) for 30 min followed by annealing to approximately 800 K. Temperature was monitored by a type K thermocouple attached to the sample holder but not in direct contact with the crystal due to the chamber design. LEED (Princeton Research Instruments) was used to verify the 2 × 1 reconstruction, and XPS showed that levels of carbon, sulfur, and oxygen were below the detection limit of our system following the cleaning procedure. C(1s) photoelectron spectra were collected using Al Kα radiation, while N(1s) spectra were collected using Mg Kα radiation (both nonmonochromated and operating at 300 W, 20 mA emission current, 15 kV anode voltage; SPECS Surface Nano Analysis GmbH) due to interference from the Ge Auger series. All spectra were collected with a 25 eV pass energy using a five-channel hemispherical analyzer. Before fitting photoelectron spectra, background curvature was removed by subtracting a spectrum of the clean Ge(100)-2 × 1 surface. Spectra were then fit using Unifit 201244 with a Shirley baseline and a chemically realistic number of pure Gaussian components constrained to the same full width at half-maximum (fwhm) within a given spectrum. Due to the resolution limitation of our nonmonochromated X-ray sources, fitting of N(1s) spectra required additional parameters to be fixed or limited. N(1s) spectra were fit with three peaks with 1.9 eV fwhm (typical for our system) in the following energy ranges: 397.0−397.9, 399.1−399.2, and 400.1−401.0 eV. These energy ranges are what is expected for dissociated amines, unreacted amines, and dative bonded amines, respectively, on the basis of previous experiments,37,45 multilayer spectra (see Supporting Information, section A), and experiments with cyclohexylamine (see Supporting Information, section B). Within these constraints, the fit with the lowest χ2 was found. This fitting procedure correctly fit the cyclohexylamine N(1s) spectrum with a peak of zero area at 399.1−399.2 eV (as a monofunctional molecule, chemisorbed cyclohexylamine cannot leave unreacted amines at the surface). The Ge(3d5/2) photoelectron peak was used as an internal standard for calibrating the energy scale and peak intensity. For multilayer spectra in which charging of the surface may occur, the energy scale was calibrated using the position of a photoelectron peak that is not expected to shift upon chemisorption [i.e., the C(1s) photoelectron peak]. Relative sensitivity factors determined for our system for the different photoelectron lines were used in quantitative comparisons. Intensities in all photoelectron spectra shown have been scaled by the transmission function of our analyzer. 2.3. Precursors and Dosing Methods. trans-1,4-Cyclohexanediamine (≥99%, Sigma-Aldrich) is a light brown, crystalline solid at room temperature. cis-1,4-Cyclohexanediamine (≥98%, TCI America), (±)-cis-1,2-cyclohexanediamine (≥97%, Acros Organics), and (±)-trans-1,2-cyclohexanediamine (≥98%, Acros Organics) are all clear to light yellow liquids at room temperature. Liquid precursors underwent several freeze−pump−thaw cycles before being dosed by backfilling the UHV chamber through a variable leak valve. trans-

1,4-Cyclohexandiamine was pumped out via a turbomolecular pump before being dosed through a variable leak valve connected to a directed doser. All precursors were stored and transferred under nitrogen. Identities of the dosed compounds were verified by in situ quadrupole mass spectrometry. Exposures are reported in units of langmuir (1 L = 10−6 Torr·s) and are not corrected for ion gauge sensitivity. Exposures were typically much larger than necessary to saturate the surface for experimental simplicity, and the reported exposures are not necessarily representative of different sticking coefficients. 2.4. Computational Methods. Quantum chemical calculations were carried out using the Gaussian 03 software suite46 to determine both energetics of reaction pathways and theoretical IR spectra of potential surface species. We used the Becke3 Lee−Yang−Parr (B3LYP) three-parameter density functional theory (DFT) method, which has been previously shown to provide predictive results for similar systems.32,35,47−50 To reduce the number of calculations, it was necessary to use only the most stable conformation of each precursor (e.g., cyclohexane in chair conformation with, where applicable, diequatorial amines) as a starting point for optimizations, except in cases where a different conformation was deemed likely. The Ge(100)-2 × 1 surface was modeled by a Ge23H24 two-dimer cluster representing two dimers across a trench. Ge dimer atoms and all adsorbate atoms were modeled using the triple-ζ 6-311++G(d,p) basis set. Subsurface Ge atoms were modeled using the LANL2DZ pseudopotential in order to reduce the computational cost of calculations and were terminated with hydrogens, modeled using the 6-31G(d) basis set, in order to fill their valence and approximate neighboring Ge atoms. For optimization calculations, all terminating hydrogens and all cluster Ge atoms except for the top two planes of atoms (including the dimer atoms) were frozen to prevent nonphysical distortion. Local minima and transition states were verified by frequency calculations at the optimized geometries using the same basis sets. All reported energies are zero-point energy corrected; non-zero-point corrected adsorption energies are reported in the Supporting Information. Terminating hydrogens were assigned a mass of 74 amu in frequency calculations to eliminate artificial Ge−H vibrations in the calculated IR spectra, and all calculated frequencies have been scaled by a factor of 0.96;51,52 4 cm−1 fwhm Lorentzian line shapes with the calculated intensities have been used to represent IR bands in calculated spectra.

3. RESULTS 3.1. XPS Results. Cis-1,2-, trans-1,2-, cis-1,4-, and trans-1,4cyclohexanediamine at saturation coverages on Ge(100)-2 × 1 were studied by XPS, FTIR spectroscopy, and DFT calculations. Note that 1,3-cyclohexanediamine was only readily available as a mixture of cis and trans isomers, and therefore, it is not included in this study. C(1s) XP spectra (included in the Supporting Information, section C) show no evidence of C−H dissociation to form a Ge−C covalent bond, and the total C(1s) to N(1s) peak areas are close to the expected 6:2 ratio. Figure 2 shows N(1s) XP spectra for saturation doses of each of the cyclohexanediamine isomers. Each N(1s) spectrum is fit by three Gaussian components; the details of the fit procedure are described in section 2.2. The middle peak (shown in green in Figure 2) is centered at a binding energy of 399.2 eV. This is the binding energy expected for N(1s) core electrons in an unreacted amine functional group based on multilayer data (see 19065



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versus two-peak fitting of the N(1s) spectra. Results of fitting with two peaks are included parenthetically in the following discussion. Nonetheless, we can conclude that monodentate adducts, which leave unreacted amines, account for only a small portion of adsorbed cyclohexanediamines for all of the isomers studied. Differences among isomers are observed in the ratio of dative bonded to dissociated amines. In particular, trans-1,4-cyclohexanediamine behaves most differently from the other three isomers. Cis-1,4-, trans-1,2-, and cis-1,2-cyclohexanediamine all form significantly more of the N−H dissociated product than N dative bonded product. The amount of dissociated amines for these isomers are 3.9, 1.8, and 2.3 times as much as the amount of dative bonded amines, respectively [3.2, 1.7, and 1.6 using two-peak N(1s) fits]. In contrast, this ratio is 1.0 for trans-1,4cyclohexanediamine [0.8 using two-peak N(1s) fit], indicating that it adsorbs to form roughly equal amounts of dissociated and dative bonded amines or even slightly more of the latter. Also shown in Figure 2 are the absolute surface coverages in monolayers (ML; 1 ML = 1 adsorbate molecule per surface atom) at saturation calculated by comparing the total N(1s) peak area to that of a saturation dose of pyridine, which is known to saturate on Ge(100)-2 × 1 at 0.25 ML coverage.55 All four isomers achieve saturation coverages slightly below the 0.25 ML limit expected in the case when all adsorbates form bidentate products; these saturation coverages agree with our conclusion that only a small percentage of free, unreacted amines from monodentate adducts are present at the surface. 3.2. FTIR Spectroscopy Results. FTIR spectra of Ge(100)-2 × 1 exposed to saturation doses of the four cyclohexanediamine precursors are shown in Figure 3 (black spectra) along with the corresponding spectra of physisorbed multilayers at low temperature (blue spectra). Calculated spectra for probable adsorption products of trans-1,4-cyclohexanediamine on a Ge23H24 cluster are also shown in Figure 3 (purple spectra). Calculated spectra for the other isomers comprised similar features and, for simplicity, are not shown. For all precursors, both chemisorbed and multilayer spectra contain similar features corresponding to N−H stretching (3120−3350 cm−1), aliphatic C−H stretching (2850−2860 cm−1 symmetric, 2920−2925 cm−1 asymmetric), NH2 scissoring (1550−1605 cm−1), and C−H bending modes (1225−1450 cm−1). The only distinct feature of the chemisorbed spectra compared to the multilayer spectra is the presence of a mode at ∼1930 cm−1 attributed to Ge−H stretching. C−H dissociation of aliphatic hydrocarbons is rarely observed on the Ge(100)2 × 1 surface at room temperature; therefore, the Ge−H stretching peak likely results from formation of Ge−H species at the surface upon N−H dissociation of an amine functional group. This is in agreement with the XPS data, indicating that N−H dissociated adducts comprise a significant portion of the product distribution for all four precursors. The presence of NH2 scissor modes in all of the chemisorbed spectra indicates that some amine functional groups remain unreacted or form an N dative bond to the surface without undergoing N−H dissociation. The calculated spectra indicate that N dative bonding causes a red-shift of the NH2 scissor mode by 20−30 cm−1 from its location in an unreacted amine. However, DFT calculations for cyclohexylamine (the monofunctional analogue of cyclohexanediamines) show that the frequency of the NH2 scissor mode is also quite sensitive to the adsorbate orientation. For example, red shifts of 13 and 23 cm−1 (relative to a free molecule) were calculated for

Figure 2. Fitted XP spectra of N(1s) photoelectron region for 500 L of trans-1,4-cyclohexanediamine, 20 L of cis-1,4-cyclohexanediamine, 10 L of trans-1,2-cyclohexanediamine, and 50 L of cis-1,2-cyclohexanediamine on Ge(100)-2 × 1 at room temperature. The exposures used were more than is necessary to saturate the surface. Surface coverage for each adsorbate is also shown (1 ML = 1 adsorbate molecule per surface atom).

the Supporting Information, section A), which may result from diamines bound to the surface through only one functional group (hereafter referred to as monodentate products for convenience). Donation of a lone pair of electrons from an amine nitrogen to the germanium surface leads to a decrease in electron density on nitrogen and results in a higher N(1s) binding energy. Thus, the higher binding energy peaks in Figure 2 centered at 400.1 eV (shown in blue) are attributed to dative bonded amine functional groups. Finally, an amine functional group may undergo N−H dissociation upon adsorption at the Ge(100)-2 × 1 surface. On the basis of the Pauling electronegativities of nitrogen (3.04), germanium (2.01), and hydrogen (2.20), dissociation of an N−H bond and formation of a Ge−N ordinary covalent bond is expected to increase electron density on nitrogen resulting in lower N(1s) binding energy. Thus, the lower binding energy peaks in Figure 2 (397.5−397.8 eV; shown in orange) are attributed to N−H dissociated amines. These assignments are in agreement with those for other amines adsorbed on Ge(100)-2 × 1 and Si(100)-2 × 1.37,38,53,54 It is apparent from the fits of the XP spectra in Figure 2 that the majority of functional groups react by either forming a Ge−N dative bond or undergoing N−H dissociation to form Ge−N and Ge−H covalent bonds for all four cyclohexanediamine isomers. Free, unreacted amines account for only 3−17% of the nitrogen signal. Since our nonmonochromated spectrometer cannot completely resolve three N(1s) peaks, it was necessary to restrict some of the fit parameters to achieve the fits shown, as described in section 2.2. This fitting procedure yields roughly the maximum possible amount of unreacted amines; the N(1s) spectra can also be well fit by two peaks (excluding the central peak corresponding to unreacted amines), which corresponds to the minimum possible amount of unreacted amines. Most of our analysis focuses on the relative amounts of dative bonded and N−H dissociated amines, which is only slightly affected by the choice of three19066



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with what was observed by XPS: cis- and trans-1,2-cyclohexanediamine have similar ratios, whereas cis-1,4-cyclohexanediamine has relatively more Ge−H stretching from N−H dissociation. 3.3. Theoretical Results. 3.3.1. Absence of Intramolecular Electronic Interactions. It is possible for bifunctional molecules to have an electronic interaction between the two functional groups through the molecular backbone linking the two functional groups. In organic chemistry, this is commonly described in terms of inductive and resonance effects, and such intramolecular electronic interactions have been found to be important for phenylenediamines.39 For cyclohexanediamines, however, we expect that the nonconjugated cyclohexane backbone does not enable resonance interactions between functional groups as an aromatic phenylene ring does. To demonstrate this, we have performed DFT calculations to determine adsorption energies of the monodentate dative bonded and N−H dissociated adsorption products for cyclohexylamine and the four cyclohexanediamines studied. These adsorption energies are shown in Table 1. For cyclohexylamine, dative bonding is 23.4 kcal/mol Table 1. DFT Calculated Adsorption Energies on a Ge23H24 Clustera

Figure 3. FTIR spectra of 50 L of cis-1,4-cyclohexanediamine, 10 L of trans-1,4-cyclohexanediamine, 10 L of cis-1,2-cyclohexanediamine, and 10 L of trans-1,2-cyclohexanediamine chemisorbed at saturation at room temperature (black spectra) and physisorbed at low temperature (blue spectra; scaled) on Ge(100)-2 × 1 and calculated IR spectra of probable surface species for trans-1,4-cyclohexanediamine (purple spectra). Calculated spectra for monodentate N dative bonded (N dat.), monodentate N−H dissociated (N−H diss.), and bidentate combinations are included. For simplicity, only calculated spectra of trans-1,4-cyclohexanediamine are shown.

calculated adsorption energy (kcal/mol) cyclohexylamine cis-1,2-cyclohexanediamine trans-1,2-cyclohexanediamine cis-1,4-cyclohexanediamine trans-1,4-cyclohexanediamine

N dative bonded

N−H dissociated

−23.4 −28.1 −27.6 −21.3 −23.0

−30.6 −29.6 −31.2 −28.4 −31.4

a

For the diamines, only one functional group is reacted with the cluster.

cyclohexylamine with the cyclohexane ring lying closer to the surface versus standing up from the surface, respectively, despite the two orientations having negligibly different energies (see details in the Supporting Information, section D). Thus, even for the case of cis-1,4-cyclohexanediamine, where there are two clearly resolved NH2 scissor peaks in the chemisorbed spectrum, it is unclear whether the two peaks result from different surface species (i.e., dative bonded versus free amine functional groups), orientational changes, or other factors that may shift the NH2 scissor peak such as coupling between adsorbates or hydrogen bonding. For the other isomers, broad NH2 scissor peaks likely comprised of two or more modes are observed. Thus, for all isomers the NH2 scissor peaks are consistent with the presence of dative bonded and free amines, but the two states cannot be conclusively differentiated on the basis of FTIR spectra. The integrated area of the Ge−H stretching peak may be compared to the integrated area of the NH2 scissor peaks. Fitting a Gaussian function (or two Gaussians in the case of cis1,4-cyclohexanediamine NH2 scissoring for which two peaks can be clearly resolved) by a least-squares method yields Ge−H stretching/NH2 scissoring peak area ratios of 1.30, 0.81, and 0.92 for cis-1,4-, trans-1,2-, and cis-1,2-cyclohexanediamine, respectively. The smaller and broader peaks (likely due to its lower surface coverage) prevent accurate calculation of this ratio for trans-1,4-cyclohexanediamine. Note that these ratios are not necessarily the ratios of dissociated amines to dative bonded or unreacted amines, as the absorption cross sections may differ for these vibrational modes, but comparisons among the isomers may be made. The ratios are in good agreement

exoenergetic, and N−H dissociation is 30.6 kcal/mol exoenergetic. The dative bonded and N−H dissociated adsorption energies for the four cyclohexanediamines differ from these values by no more than 2.2 kcal/mol except in the case of cis- and trans-1,2-cyclohexanediamine dative bonding. Dative bonding of cis- and trans-1,2-cyclohexanediamine is 4.7 and 4.2 kcal/mol more favorable than for cyclohexylamine, respectively, but these differences can be attributed to stabilization from intramolecular hydrogen bonding between amine functional groups of the adsorbed products. In these cases, a hydrogen of one amine was situated near the nitrogen of the other amine at a distance of ∼2.15 Å. In fact, for cis-1,2cyclohexanediamine, dative bonded states with and without this intramolecular hydrogen bonding could be calculated, and their adsorption energies were 28.1 and 22.9 kcal/mol, respectively, indicating that 4−5 kcal/mol stabilization is of the correct magnitude to be attributed to hydrogen bonding. Since the differences in dative bonded and dissociated adsorption energies among cyclohexylamine and all four cyclohexanediamine isomers are small, we conclude that the presence of a second amine functional group on the cyclohexane ring does not significantly affect the reaction energetics of the first amine functional group via resonance or inductive effects. Calculated monodentate adsorption energies for cis- and trans-1,3cyclohexanediamine were also similar (see section E of the Supporting Information for an expanded version of Table 1 including these adsorption energies) showing that the similarity 19067



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Figure 4. DFT-calculated reaction pathways for cis- and trans- isomers of 1,2- and 1,4-cyclohexanediamine on a Ge23H24 cluster representing two dimers across a trench. Energies of key points are shown in kcal/mol and are connected by lines to guide the eye. A dotted line is used to connect points that are likely separated by an energetic barrier that could not be found in calculations.

Table 2. Calculated Surface-Induced Adsorbate Strain Energies for Cyclohexanediamines Adsorbed on a Ge23H24 Cluster via Both Functional Groups calculated surface-induced adsorbate strain energy (kcal/mol) cis-1,2-cyclohexanediamine trans-1,2-cyclohexanediamine cis-1,4-cyclohexanediamine trans-1,4-cyclohexanediamine

N dative/N dative

N−H diss./N dative

N−H diss./N−H diss.

2.9 2.5 4.5 12.6

4.4 2.4 10.8 14.7

3.6 5.0 11.1 16.0

bonding of the second amine. As will be discussed later, this is a key difference that enables dissociation of diamines at room temperature, unlike monofunctional alkyl amines. Figure 4 shows that the pathways for cis- and trans-1,4cyclohexanediamine differ significantly from each other and from the 1,2-cyclohexanediamine isomers. The energetic differences of the monodentate N dative bonded state are attributed to intramolecular hydrogen bonding in both of the 1,2-isomers, which provides approximately 5 kcal/mol of additional stabilization, as previously discussed. Without this hydrogen bonding, all four isomers would have nearly equal adsorption energies in the monodentate dative bonded state. However, for the subsequent bidentate states, the distance between surface dimers prevents intramolecular hydrogen bonding between amines, and the energetic differences among isomers arise from a different source: surface-induced distortion or strain of the adsorbate. To show that strain of the adsorbate introduced by bidentate bonding to the surface is largely responsible for the energetic differences observed in the pathways in Figure 4, the strain energy of each molecule in its adsorbed geometry was calculated. To do this, the energy of each bidentate adsorbate fixed in its adsorbed geometry (but with the cluster removed) was compared to the energy of the free, fully optimized molecule. For N−H dissociated amines, the N−Ge bond was replaced by N−H, and the length of the resulting N−H bond was optimized. This method restored the amine to its nondissociated state for comparing calculated energies while preserving the geometry of the adsorbed molecule. The resulting surface-induced adsorbate strain energies (hereafter referred to as adsorbate strain energies for brevity) are shown in

in adsorption energies is not simply a result of the particular isomers studied in this work. 3.3.2. Adsorption Pathways. To better understand the different adsorption behavior of trans-1,4-cyclohexanediamine compared to the three other cyclohexanediamine isomers studied, DFT calculations were performed to determine energies of key points in the reaction pathways. Figure 4 shows the reaction pathway on a Ge23H24 cluster for all four isomers in which both amines dative bond to the surface before undergoing N−H dissociation. For simplicity, only pathways for “cross-trench” reaction on a Ge23H24 cluster representing two dimers across a trench are shown. Calculations were also performed for “end-bridge” reaction on a Ge15H16 cluster representing two neighboring dimers in the same row; however, reaction of both functional groups in this configuration was in all cases less favorable. The calculated reaction pathways in Figure 4 highlight a few key differences among the cyclohexanediamine isomers. The cis- and trans-1,2-cyclohexanediamine pathways (purple and magenta pathways in Figure 4, respectively) are essentially identical with, at most, 1.5 kcal/mol difference between equivalent points in these pathways. Thus, the small change in geometry from cis- to trans-1,2-cyclohexanediamine does not change the reaction energetics, in agreement with experimental results showing essentially identical reactivity. Although the activation barriers are quite large (∼24 and ∼35 kcal/mol for the first and second N−H dissociation, respectively), the barriers lie below the reactant energy, unlike for cyclohexylamine (calculated N−H dissociation transition state energy 0.9 kcal/mol above the reactants) or monofunctional alkyl amines,35 owing to the added stabilization provided by dative 19068



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Table 2. For cis- and trans-1,2-cyclohexanediamine, the adsorbate strain energies are small (2−5 kcal/mol). cis-1,4Cyclohexanediamine has a relatively small (4.5 kcal/mol) adsorbate strain energy for the dual dative bonded state and larger (∼11 kcal/mol) adsorbate strain energies for the other two bidentate products. trans-1,4-Cyclohexanediamine has still larger adsorbate strain energies of 12−16 kcal/mol. The differences in adsorbate strain energies account for much of the energetic difference between pathways in Figure 4, and the remainder is likely attributable to strained Ge−N bonds (either dative or ordinary covalent) between the surface and adsorbate. The end result is that the reaction pathway for cis-1,4cyclohexanediamine is shifted up in energy from that of the 1,2 isomers, and the reaction pathway for trans-1,4-cyclohexanediamine is shifted up in energy yet more due to even greater adsorbate strain necessary to bond to the surface via both functional groups. The large adsorbate strain energy for trans-1,4-cyclohexanediamine arises from the necessity for the cyclohexane ring to adopt a twist-boat conformation in order for both functional groups to interact with the surface. The twist-boat conformation of cyclohexane is a local minimum in energy, but due to steric strain from flagpole hydrogens and nearly eclipsed C− H bonds, it is higher in energy than the most stable chair conformation. Figure 5 shows the DFT-optimized geometries

the free molecule indicate that the barrier height is approximately 11 kcal/mol. A dotted connecting line is used in Figure 4 to indicate that an energetic barrier likely separates the single and dual N dative bonded states for trans-1,4cyclohexanediamine. One important result of the energetic penalty of adopting a twist-boat conformation is that bidentate dual N dative bonding is 1.3 kcal/mol less energetically favorable than monodentate single N dative bonding according to our calculations for trans-1,4-cyclohexanediamine. As a result, adsorbates that do not pass over the transition state to dissociation are more likely by a factor of almost 10 (∼400 including estimated entropic corrections from DFT calculations) to be bound to the surface by dative bonding of only one functional group than by dative bonding of both functional groups. As discussed further below, this may have implications for subsequent N−H dissociation of trans-1,4-cyclohexanediamine.

4. DISCUSSION Experimental results show that cis- and trans-isomers of 1,2- and 1,4-cyclohexanediamine adsorb on the Ge(100)-2 × 1 primarily by dative bonding and N−H dissociation. The fact that both dative bonded and N−H dissociated products are present indicates that kinetics rather than thermodynamics must control the product distribution for the following reason. The dual N−H dissociated product is more energetically stable than any other product, as shown in Figure 4, and even for cis-1,2cyclohexanediamine, which has the smallest difference in adsorption energies between the dual N−H dissociated and the second most favorable product, the second product would account for less than 1% of the product distribution under thermodynamic equilibrium. Hence, the observed product distribution is inconsistent with a thermodynamically controlled reaction and instead must be kinetically controlled. Fits of the N(1s) XP spectra in Figure 2 suggest that free amines account for small percentages of the nitrogen signal, and this is further supported by the saturation coverages, which are all below 0.25 ML, the limit for the case when all adducts form bidentate products. Cis- and trans-1,2-cyclohexanediamine and cis-1,4-cyclohexanediamine preferentially undergo N−H dissociation with the dissociated product being 1.8−3.9 times as prevalent [1.6− 3.2 based on two-peak N(1s) fits]. The higher quantity of N−H dissociated amines indicates that at least some adsorbates form the dual N−H dissociated product. The saturation XP spectra of these three cyclohexanediamines are similar to that of ethylenediamine on Ge(100)-2 × 153 and indicate a notable difference from the behavior of methylamine and dimethylamine, which only form dative bonded products on Ge(100)2 × 1 at room temperature.35 As described for ethylenediamine,53 bifunctionality may facilitate dissociation of aliphatic amines: the two functional groups of a diamine may interact with two surface dimers, thereby affording additional stabilization that moves transition states for N−H dissociation below the reactant energy. Although the barrier heights for dissociation are quite large from the preceding intermediates, as shown in Figure 4, the barriers all lie below the reactant energy. It has previously been found experimentally that, in an effect attributed to incomplete thermal accommodation in a precursor state, reactions may occur despite large activation barriers when the transition state lies near or below the energy of the reactants.36,50,53 Thus, the larger percentage of N−H dissociated than dative bonded functional groups may be

Figure 5. DFT-optimized geometries of each cyclohexanediamine isomer dual N−H dissociated on a Ge23H24 cluster. Subsurface atoms and all hydrogens aside from amine hydrogens are hidden to facilitate viewing the cyclohexane conformation. cis-1,2-, trans-1,2-, And cis-1,4cyclohexanediamine adopt a chair conformation, while trans-1,4cyclohexanediamine adopts a twist-boat conformation. Schematics of the cyclohexane conformation of each isomer are shown in orange beside the DFT structures to further clarify the conformations.

of each isomer dual N−H dissociated on a Ge23H24 cluster to illustrate the necessity for the conformational change of trans1,4-cyclohexanediamine. The geometry of the cyclohexane ring is similar for dual N dative bonded or N dative bonded/N−H dissociated products. For a free trans-1,4-cyclohexanediamine molecule, the change from a chair to twist-boat conformation results in a 7.3 kcal/mol energetic penalty according to DFT calculations. To achieve this conformational change, there is also an energetic barrier. Calculations were unable to locate this transition state for trans-1,4-cyclohexanediamine adsorbed on a Ge23H24 cluster by single N dative bonding, but calculations for 19069



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Table 3. Ge−Ge−N Bond Angles from DFT-Optimized Structuresa calculated Ge−Ge−N bond angle (deg) cis-1,2-cyclohexanediamine trans-1,2-cyclohexanediamine cis-1,4-cyclohexanediamine


N−H diss./N−H diss. TS


116.2 118.6 94.1

132.3 128.1 95.8

114.9 117.4 93.1

a

Bond angles are measured between the germanium dimer atoms and the nitrogen that has undergone N−H dissociation and are shown for the N− H dissociated/N dative bonded product, the dual N−H dissociated product, and the transition state between these two states.

[0.8 using two-peak N(1s) fit] times the amount of N dative bonded amines according to XPS results. As previously described, trans-1,4-cyclohexanediamine must adopt a twistboat conformation to form bidentate products, and the twistboat conformation is energetically less stable than the chair conformation due to steric strain. The energetic difference between the chair and twist-boat conformation (7.3 kcal/mol according to DFT calculations for trans-1,4-cyclohexanediamine) would lead to the chair conformation being over 105 times more abundant than the twist-boat conformation at equilibrium at room temperature in the gas phase. When comparing the monodentate N dative bonded (chair) versus the bidentate dual N dative bonded (twist-boat) products, the additional stabilization of the second dative bond partially offsets the energetic penalty of adopting the twist-boat conformation, but the bidentate state is still 1.3 kcal/mol less stable than the monodentate state. On the basis of this energetic difference, the monodentate product will be nearly 10 times as abundant, as mentioned previously (∼400 times as abundant if estimated entropic corrections are included). Thus, for trans-1,4-cyclohexanediamine, the concentration of bidentate dual N dative bonded surface specieseffectively the reactant for producing dissociated productsis quite small, leading to a relatively slow N−H dissociation reaction. Interestingly, the fit of the N(1s) XP spectrum shows that unreacted amines from monodentate products account for, at most, a minority of amines, indicating that most trans-1,4cyclohexanediamine molecules do, in fact, form bidentate products. Given the nearly equal amounts of N dative bonded and N−H dissociated amines, most trans-1,4-cyclohexanediamine molecules must remain in the N−H dissociated/N dative bonded state without undergoing N−H dissociation of the second amine. Incomplete thermal accommodation is likely necessary for N−H dissociation of trans-1,4-cyclohexanediamine, similar to the other isomers. We speculate that the slow first N−H dissociation of trans-1,4-cyclohexanediamine due to the requirement of partial ring inversion (an activated and thermodynamically unfavorable process) may allow it to lose more thermal energy, compared to the other isomers, while the N−H dissociated/N dative bonded adduct is formed. This extra energetic accommodation may combine with the higher transition state energy for N−H dissociation of the second amine on trans-1,4-cyclohexanediamine (only 7.2 kcal/mol below the reactants) to limit the amount of dual N−H dissociated products it is able to form. It is possible for monodentate N dative bonded species to undergo N−H dissociation to form the monodentate N−H dissociated product (as observed for cyclohexylamine; see the Supporting Information, section B), but the transition state for this reaction lies 1.2 kcal/mol above the energy of the reactants, and consequently, this reaction is also expected to occur slowly as well.

attributed to dissociation occurring before complete thermal accommodation. Looking in more detail at the ratio of N−H dissociated to N dative bonded functional groups, we find that cis- and trans-1,2cyclohexanediamine have very similar ratios of 2.3 and 1.8 [1.6 and 1.7 based on two-peak N(1s) fits], respectively, as expected on the basis of their negligibly different reaction energetics in Figure 4. On the other hand, cis-1,4-cyclohexanediamine has a slightly stronger preference to undergo dissociation (ratio of 3.9 or 3.2 from three- and two-peak N(1s) fits, respectively) despite the dual N−H dissociated product being ∼10 kcal/mol less thermodynamically favorable than for the 1,2 isomers. This provides further evidence that the reactions of cyclohexanediamines on Ge(100)-2 × 1 are under kinetic control, since the energetic barrier to dual N−H dissociation (second transition state in Figure 4) is only 22.0 kcal/mol for cis-1,4-cyclohexanediamine compared to 36.6 and 34.2 kcal/mol for cis- and trans-1,2-cyclohexanediamine, respectively. Interestingly, adsorbate strain energy causes all points on the cis-1,4-cyclohexanediamine pathway to be significantly higher in energy than the pathways of the 1,2 isomers, except for the second N−H dissociation transition state. The greater distance between amine nitrogens of the cis-1,4 isomer allows the second N−H dissociation to occur with less strain of the Ge−N bond of the first amine that has already undergone N−H dissociation. This is demonstrated by the Ge−Ge−N bond angles for DFT-optimized structures shown in Table 3. This angle, formed by the germanium dimer atoms and the nitrogen of the amine that has already undergone dissociation, is ideally near 109.5° (e.g., 109.9° for the optimized structure of N−H dissociated cyclohexylamine). The Ge−Ge−N bond is strained for all adsorbates in the N−H dissociated/N dative bonded and dual N−H dissociated states, as demonstrated by the bond angle differing significantly from 109.5°. Most important is that the Ge−Ge−N angle increases 9.5°−16.1° from the N−H dissociated/N dative bonded state to the subsequent N−H dissociation transition state for the 1,2-cyclohexanediamine isomers and becomes more strained at the transition state. On the other hand, for cis-1,4-cyclohexanediamine the Ge−Ge−N angle increases only 1.7° and actually becomes closer to the ideal angle of 109.5°; thus, the Ge−Ge−N bond of cis-1,4cyclohexanediamine is less strained at the transition state. Incomplete thermal accommodation is necessary for N−H dissociation, as discussed earlier, but even for the case of partial thermal accommodation, the size of the subsequent energetic barrier may impact the product distribution. Thus, the greater percentage of N−H dissociated amines for cis-1,4-cyclohexanediamine may be attributed to its smaller second N−H dissociation activation energy due to less strain of the surface− adsorbate bond. trans-1,4-Cyclohexanediamine shows markedly different adsorption behavior than the other three cyclohexanediamine isomers studied. The amount of N−H dissociated amines is 1.0 19070



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Table 4. Predicted Adsorption Energies of Bidentate Productsa predicted adsorption energy (kcal/mol) cis-1,2-cyclohexanediamine trans-1,2-cyclohexanediamine cis-1,4-cyclohexanediamine trans-1,4-cyclohexanediamine a

N dative/N dative



−53.3 −52.7 −38.1 −33.4

−53.3 −56.4 −38.9 −39.7

−55.6 −57.4 −45.7 −46.8

Predictions are based on combining monodentate adsorption energies (Table 1) and adsorbate strain energies (Table 2).

In summary, we find that the differences in amounts of dative bonded versus N−H dissociated amine functional groups are controlled by kinetics, and the dynamics of thermal accommodation are important in determining the product distributions, since N−H dissociation would otherwise be inaccessible owing to the large activation energies. Most important, strain of surface−adsorbate bonds and the adsorbed molecule differ significantly among the four structural and stereoisomersparticularly for trans-1,4-cyclohexanediamine, which must adopt a more sterically strained twist-boat conformation in order for both functional groups to interact with the surfaceand play large roles in determining reaction energetics. The adsorption energies of bidentate states may be predicted by summing adsorption energies of monodentate states from Table 1 and the appropriate calculated surface-induced adsorbate strain energy from Table 2. The results of this simple prediction are shown in Table 4, and these predicted adsorption energies can be compared with the calculated adsorption energies shown in Figure 4. The predicted energies in Table 4 are similar (within 6 kcal/mol) to the energies in Figure 4 for N−H dissociated/N dative bonded and dual N−H dissociated products. This similarity suggests that different amounts of adsorbate strain required for bidentate adsorption of the four structural and stereoisomers are primarily responsible for the differences in reaction energetics. Some error is expected since the estimated adsorbate strain energies in Table 2 do not account for strain of the surface−adsorbate bonds. However, the predicted adsorption energies of dual N dative bonded products in Table 4 are much larger (≥10 kcal/ mol larger) than the adsorption energies in Figure 4, suggesting that the model of an adsorbate dual N dative bonded on a cluster captures an effect not present in this simplistic prediction of adsorption energies. A possible cause of this discrepancy is nonlocal charge transfer through the cluster upon dative bonding. Nonlocal charge redistribution has been the topic of several studies primarily focused on adsorption of ammonia on Si(100)-2 × 1.56−59 The key result of these nonlocal charge transfer effects is that dative bonding on adjacent dimers is hindered. Nonlocal effects and resulting coverage-dependent adsorption behavior of alkyl amines require further investigation and will be the topic of a future publication.

species. Both 1,2-cyclohexanediamine isomers and cis-1,4cyclohexanediamine form more N−H dissociated amines than N dative bonded amines at saturation, whereas trans-1,4cyclohexanediamine forms roughly equal quantities of the two products. Comparison of the saturation product distributions with the calculated reaction pathways shows that reactions of cyclohexanediamines are under kinetic control at room temperature, and that the dynamics of thermal accommodation must play a role in determining the product distribution. DFT calculations show that the presence of a second amine functional group on the cyclohexane backbone has negligible influence by inductive or resonance effects on the reactivity of the first amine. Instead, geometric differences among the cyclohexanediamine structural and stereoisomers account for the different reactivities. The distinct behavior of trans-1,4cyclohexanediamine appears to be driven by the necessity for its cyclohexane ring to adopt a more strained twist-boat conformation in order to form bidentate adducts. Additionally, DFT calculations show that different amounts of strain of the surface−adsorbate bonds also impact the reaction energetics when comparing among the three cyclohexanediamine isomers that do not undergo a ring inversion. These results show that the cyclohexane spacer offers enough flexibility to allow bidentate adsorption of nearly all adducts regardless of where the two amine functional groups are located on the cyclohexane ring. However, even the relatively small energetic penalties associated with strained surface−adsorbate bonds or a conformational change from the lowest energy chair conformation to a twist-boat conformation impact the reaction energetics sufficiently to change the product distribution. These conclusions mirror past results from other fields of chemistry, showing that stereo, structural, or conformational isomerism can affect the stability of chelated organometallic complexes6−12 or reaction selectivity13−18,42 in solution, and they extend these basic principles to reactions of organic molecules at a crystalline surface. By changing one or both of the functional groups or modifying the molecular backbone to impose additional geometric constraints, one may be able to leverage the different reactivities of the various isomers to achieve high regioselectivity or stereoselectivity of reaction with the Ge(100)-2 × 1 surface.

5. CONCLUSIONS In this work, we have investigated adsorption of cis-1,2-, trans1,2-, cis-1,4-, and trans-1,4-cyclohexanediamine at the Ge(100)2 × 1 surface by a combination of XPS, FTIR spectroscopy, and DFT calculations. At saturation, each cyclohexanediamine isomer forms a mixture of bidentate adducts involving combinations of N−H dissociation and N dative bonding, and monodentate adducts comprise a minority of surface

* Supporting Information

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ASSOCIATED CONTENT

S

Cyclohexanediamine multilayer XP spectra, cyclohexylamine XP spectra, cyclohexanediamine C(1s) XP spectra, calculated NH2 scissor-mode frequencies for cyclohexylamine in different orientations, additional calculated monodentate adsorption energies, non-zero-point corrected adsorption energies, and the complete listing of ref 46. This material is available free of charge via the Internet at http://pubs.acs.org. 19071



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(22) Kachian, J. S.; Tannaci, J.; Wright, R. J.; Tilley, T. D.; Bent, S. F. Disulfide Passivation of the Ge(100)-2 × 1 Surface. Langmuir 2010, 27, 179−186. (23) Loscutoff, P. W.; Bent, S. F. Reactivity of the Germanium Surface: Chemical Passivation and Functionalization. Annu. Rev. Phys. Chem. 2006, 57, 467-495. (24) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species. Science 2001, 293, 1289−1292. (25) Lin, Z.; Strother, T.; Cai, W.; Cao, X. P.; Smith, L. M.; Hamers, R. J. DNA Attachment and Hybridization at the Silicon(100) Surface. Langmuir 2002, 18, 788. (26) George, S. M.; Yoon, B.; Dameron, A. A. Surface Chemistry for Molecular Layer Deposition of Organic and Hybrid Organic− Inorganic Polymers. Acc. Chem. Res. 2009, 42, 498−508. (27) Zhou, H.; Bent, S. F. Molecular Layer Deposition of Functional Thin Films for Advanced Lithographic Patterning. ACS Appl. Mater. Interfaces 2011, 3, 505−511. (28) Hurley, P. T.; Ribbe, A. E.; Buriak, J. M. Nanopatterning of Alkynes on Hydrogen-Terminated Silicon Surfaces by Scanning ProbeInduced Cathodic Electrografting. J. Am. Chem. Soc. 2003, 125, 11334. (29) Guisinger, N. P.; Basu, R.; Baluch, A. S.; Hersam, M. C. Molecular Electronics on SiliconAn Ultrahigh Vacuum Scanning Tunneling Microscopy Study. Ann. N.Y. Acad. Sci. 2003, 1006, 227. (30) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Self-Directed Growth of Molecular Nanostructures on Silicon. Nature 2000, 406, 48. (31) Wolkow, R. A. Controlled Molecular Adsorption on Silicon: Laying a Foundation for Molecular Devices. Annu. Rev. Phys. Chem. 1999, 50, 413−441. (32) Filler, M. A.; Bent, S. F. The Surface as Molecular Reagent: Organic Chemistry at the Semiconductor Interface. Prog. Surf. Sci. 2003, 73, 1−56. (33) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Cycloaddition Chemistry of Organic Molecules with Semiconductor Surfaces. Acc. Chem. Res. 2000, 33, 617−624. (34) Tao, F.; Bernasek, S. Functionalization of Semiconductor Surfaces; Wiley-VCH: New York, 2012. (35) Mui, C.; Han, J. H.; Wang, G. T.; Musgrave, C. B.; Bent, S. F. Proton Transfer Reactions on Semiconductor Surfaces. J. Am. Chem. Soc. 2002, 124, 4027−4038. (36) Mui, C.; Wang, G. T.; Bent, S. F.; Musgrave, C. B. Reactions of Methylamines at the Si(100)-2 × 1 Surface. J. Chem. Phys. 2001, 114, 10170−10180. (37) Kachian, J. S.; Bent, S. F. Adsorption Behavior of Bifunctional Molecules on Ge(100)-2 × 1: Comparison of Mercaptoethanol and Mercaptamine. J. Phys. Chem. C 2010, 114, 22230−22236. (38) Kachian, J. S.; Jung, S. J.; Kim, S.; Bent, S. F. Coverage Dependence of Glycine Adsorption on the Ge(100)-2 × 1 Surface. Surf. Sci. 2011, 605, 760−769. (39) Kachian, J. S.; Squires, K. H.; Bent, S. F. Competing Geometric and Electronic Effects in Adsorption of Phenylenediamine Structural Isomers on the Ge(100)-2 × 1 Surface. Surf. Sci. 2013, 615, 72−79. (40) Shong, B.; Wong, K. T.; Bent, S. F. Reaction of Hydroquinone and p-Benzoquinone with the Ge(100)-2 × 1 Surface. J. Phys. Chem. C 2012, 116, 4705−4713. (41) Wong, K. T.; Chopra, S. N.; Bent, S. F. Single Versus Dual Attachment in the Adsorption of Diisocyanates at the Ge(100)-2 × 1 Surface. J. Phys. Chem. C 2012, 116, 12670−12679. (42) Ogawa, H.; Chihara, T.; Taya, K. Selective Monomethyl Esterification of Dicarboxylic Acids by Use of Monocarboxylate Chemisorption on Alumina. J. Am. Chem. Soc. 1985, 107, 1365−1369. (43) Filler, M. A.; Van Deventer, J. A.; Keung, A. J.; Bent, S. F. Carboxylic Acid Chemistry at the Ge(100)-2 × 1 Interface: Bidentate Bridging Structure Formation on a Semiconductor Surface. J. Am. Chem. Soc. 2006, 128, 770−779. (44) Hesse, R. Unifit, 2012 ed.; Unifit Scientific Software GmbH: Leipzig, Germany, 2012.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: (650) 723-0385. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE 1213879).

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REFERENCES

(1) Sachse, H. Chem. Ber. 1890, 23, 1363−1370. (2) Sachse, H. Z. Phys. Chem. 1892, 10, 201−241. (3) Sachse, H. Z. Phys. Chem. 1893, 11, 185−219. (4) Mohr, E. J. Prakt. Chem. 1918, 98, 315. (5) James, L. Nobel Laureates in Chemistry 1901−1992; Wiley: New York, 1993. (6) Gillard, R. D.; Irving, H. M. Conformational Aspects of Chelate Rings. Chem. Rev. 1965, 65, 603−616. (7) Martell, A. B. The Effect of Ligand Isomerism on the Stability of Metal Chelate Compounds. J. Phys. Chem. 1955, 59, 308−312. (8) Dwyer, F. Chelating Agents and Metal Chelates; Elsevier Science: New York, 1964. (9) Choppin, G. R.; Dadgar, A.; Rizkalla, E. N. Thermodynamics of Complexation of Lanthanides by Dicarboxylate Ligands. Inorg. Chem. 1986, 25, 3581−3584. (10) Hancock, R. D. Molecular Mechanics Calculations and Metal Ion Recognition. Acc. Chem. Res. 1990, 23, 253−257. (11) Irving, H.; Pettit, L. D. 292. The Stabilities of Metal Complexes of Some C-Substituted Derivatives of Glycine. J. Chem. Soc. 1963, 0, 1546−1553. (12) Martell, A. E.; Hancock, R. D.; Motekaitis, R. J. Factors Affecting Stabilities of Chelate, Macrocyclic and Macrobicyclic Complexes in Solution. Coordin. Chem. Rev. 1994, 133, 39−65. (13) Kikuchi, Y.; Kato, Y.; Tanaka, Y.; Toi, H.; Aoyama, Y. Molecular Recognition. 15. Molecular Recognition and Stereoselectivity: Geometrical Requirements for the Multiple Hydrogen-Bonding Interaction of Diols with a Multidentate Polyhydroxy Macrocycle. J. Am. Chem. Soc. 1991, 113, 1349−1354. (14) Roy, C. D.; Brown, H. C. Stereoisomer-Differentiating Esterification of Diols with Methylboronic Acid. A Simple Method for the Separation of cis- and trans-1,2-Diols. Tetrahedron Lett. 2007, 48, 1959−1961. (15) Mancel, V.; Sellier, N. Stereochemical Differentiation of Cyclic Glycols by Ion−Molecule Reactions in a Quadrupole Mass Spectrometer Using Dimethyl Ether Acetonitrile and 2-S-Pyrrolidinemethanol. Rapid Commun. Mass. Spectrom. 2000, 14, 80−85. (16) Thompson, R. S.; Guler, L. P.; Nelson, E. D.; Yu, Y.-Q.; Kenttämaa, H. I. Mechanistic Study of Stereoselective Gas-Phase Reactions of Phosphenium Ions with cis- and trans-1,2-Diaminocyclohexanes. J. Org. Chem. 2002, 67, 5076−5084. (17) Splitter, J. S.; Turec̆ek, F. Applications of Mass Spectrometry to Organic Stereochemistry; VCH: New York, 1994. (18) Moraes, L.; Eberlin, M. Ketalization of Gaseous Acylium Ions. J. Am. Soc. Mass. Spectrom. 2001, 12, 150−162. (19) Guisinger, N. P.; Greene, M. E.; Basu, R.; Baluch, A. S.; Hersam, M. C. Room Temperature Negative Differential Resistance through Individual Organic Molecules on Silicon Surfaces. Nano Lett. 2004, 4, 55. (20) Hanson, E. L.; Guo, J.; Koch, N.; Schwartz, J.; Bernasek, S. L. Advanced Surface Modification of Indium Tin Oxide for Improved Charge Injection in Organic Devices. J. Am. Chem. Soc. 2005, 127, 10058−10062. (21) Avasthi, S.; Qi, Y.; Vertelov, G. K.; Schwartz, J.; Kahn, A.; Sturm, J. C. Silicon Surface Passivation by an Organic Overlayer of 9,10Phenanthrenequinone. Appl. Phys. Lett. 2010, 96, 222109. 19072



Article

(45) Bent, S. F.; Kachian, J. S.; Rodríguez-Reyes, J. C. F.; Teplyakov, A. V. Tuning the Reactivity of Semiconductor Surfaces by Functionalization with Amines of Different Basicity. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 956−960. (46) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, Rev. D.01 ed.; Gaussian, Inc.: Wallingford, CT, 2004. (47) Konecny, R.; Doren, D. J. Cycloaddition Reactions of Unsaturated Hydrocarbons on the Si(100)-(2 × 1) Surface: Theoretical Predictions. Surf. Sci. 1998, 417, 169−188. (48) Lu, X.; Lin, M. C. Reactions of Some [C,N,O]-Containing Molecules with Si Surfaces: Experimental and Theoretical Studies. Int. Rev. Phys. Chem. 2002, 21, 137−184. (49) Wang, G. T.; Mui, C.; Tannaci, J. F.; Filler, M. A.; Musgrave, C. B.; Bent, S. F. Reactions of Cyclic Aliphatic and Aromatic Amines on Ge(100)-2 × 1 and Si(100)-2 × 1. J. Phys. Chem. B 2003, 107, 4982− 4996. (50) Wong, K. T.; Chopra, S. N.; Bent, S. F. Dissociative Adsorption of Dimethyl Sulfoxide at the Ge(100)-2 × 1 Surface. J. Phys. Chem. C 2012, 116, 26422−26430. (51) Andersson, M. P.; Uvdal, P. New Scale Factors for Harmonic Vibrational Frequencies Using the B3LYP Density Functional Method with the Triple-ζ Basis Set 6-311+G(d,p). J. Phys. Chem. A 2005, 109, 2937−2941. (52) Scott, A. P.; Radom, L. Harmonic Vibrational Frequencies: An Evaluation of Hartree-Fock, Møller-Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors. J. Phys. Chem. 1996, 100, 16502−16513. (53) Kim, A.; Filler, M. A.; Kim, S.; Bent, S. F. Ethylenediamine on Ge(100)-2 × 1: The Role of Interdimer Interactions. J. Phys. Chem. B 2005, 109, 19817−19822. (54) Kugler, T.; Thibaut, U.; Abraham, M.; Folkers, G.; Göpel, W. Chemically Modified Semiconductor Surfaces: 1,4-Phenylenediamine on Si(100). Surf. Sci. 1992, 260, 64−74. (55) Cho, Y. E.; Maeng, J. Y.; Kim, S.; Hong, S. Formation of Highly Ordered Organic Monolayers by Dative Bonding: Pyridine on Ge(100). J. Am. Chem. Soc. 2003, 125, 7514−7515. (56) Owen, J. H. G. Competing Interactions in Molecular Adsorption: NH3 on Si(001). J. Phys.-Condens. Mat. 2009, 21, 443001. (57) Wang, Y.; Hwang, G. S. Origin of Nonlocal Interactions in Adsorption of Polar Molecules on Si(001)-2 × 1. J. Chem. Phys. 2005, 122, 164706−164709. (58) Widjaja, Y.; Musgrave, C. B. A Density Functional Theory Study of the Nonlocal Effects of NH3 Adsorption and Dissociation on Si(100)-(2 × 1). Surf. Sci. 2000, 469, 9−20. (59) Widjaja, Y.; Musgrave, C. B. Indirect Adsorbate−Adsorbate Interactions Mediated through the Surface Electronic Structure of the Si(100) Surface. J. Chem. Phys. 2004, 120, 1555−1559.

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