Cycloaddition reactions of acrylonitrile on the Si(100)-2 x 1 surface

Multi-reference as well as single-reference quantum mechanical methods were adopted to study the potential energy surface along three possible surface reaction mechanisms of acrylonitrile on the Si(100)-2 x 1 surface. All three reactions occur via stepwise radical mechanisms. According to the computed potential energy surfaces, both [4+2] and [2+2](CN) cycloaddition products resulting from the reactions of surface dimers with the C[triple bond]N of acrylonitrile are expected, due to the negligible activation barriers at the surface. Another possible surface product, [2+2](CC), requires a 16.7 kcal/mol activation energy barrier. The large barrier makes this route much less favorable kinetically, even though this route produces the thermodynamically most stable products. Isomerization reactions among the surface products are very unlikely due to the predicted large activation barriers preventing thermal redistributions of the surface products. As a result, the distribution of the final surface products is kinetically controlled leading to a reinterpretation of recent experiments. An intermediate Lewis acid-base type complex appears in both the [4+2] and [2+2](CN) cycloadditions entrance channels, indicating that the surface may act as an electrophile/Lewis acid toward a strong Lewis base substrate.     

Cycloaddition of benzene on Si(100) and its surface conversions

A comprehensive ab initio study of the adsorption of benzene on the silicon(100) surface is presented. Five potential candidates ([2+2] adduct, [4+2] adduct, two tetra-sigma-bonded structures, and one radical-like structure) for the reaction product are examined to determine the lowest energy adsorption configuration. A [4+2] butterfly structure is determined to be the global minimum (-29.0 kcal/mol), although one of the two tetra-sigma-bonded structures (-26.7 kcal/mol) is similar in energy to it. Multireference perturbation theory suggests that the [4+2] addition mechanism of benzene on Si(100) is very similar to the usual Diels-Alder reaction (i.e., small or zero activation barrier), even though benzene adsorption entails the loss of benzene aromaticity during the reaction. On the other hand, the [2+2] cycloaddition mechanism is shown to require a relatively high activation barrier (17.8 kcal/mol), in which the initial step is to form a (relatively strongly bound) van der Waals complex (-8.9 kcal/mol). However, the net activation barrier relative to reactants is only 8.9 kcal/mol. Careful examination of the interconversion reactions among the reaction products indicates that the two tetra-sigma-bonded structures (that are energetically comparable to the [4+2] product) can be derived from the [2+2] adduct with activation barriers of 15.5 and 21.4 kcal/mol. However, unlike the previous theoretical predictions, it is found that the conversion of the [4+2] product to the tetra-sigma-bonded structures entails huge barriers (>37.0 kcal/mol) and is unlikely to occur. This suggests that the [4+2] product is not only thermodynamically the most stable configuration (lowest energy product) but also kinetically very stable (large barriers with respect to the isomerization to other products).     

Cycloaddition reactions of cyanogen (C2N2) on the Si(100)-2x1 surface

Multireference as well as density functional theories in combination with the surface integrated molecular orbital molecular mechanics were adopted to study the surface reactions of cyanogens on Si(100)-2x1 surface. Three different products were identified as minima in the initial surface reaction. Among these, the [2+2] product is both kinetically easily accessible and thermodynamically the most stable. Therefore, it can be considered as the experimentally found strongly bound surface species. Unlike other conjugated systems, the [4+2] product is less stable than the [2+2] product. Subsequent surface isomerization studies revealed that kinetically favorable channels exist between the initially formed low-temperature species and the high-temperature species, indicating that surface morphology changes gradually as a function of surface temperature. Theses two channels eventually lead to the same final surface products, which is consistent with experiment. Current study shows that the subsequent surface isomerizations are the key reactions to better understand the complex surface structures and their properties.     

Density-functional study of the cycloaddition of acrylonitrile on the Si(100) surface

Using a density functional approach, we have explored the cycloaddition of acrylonitrile on the Si(100) surface. The buckling of the surface dimers characteristic for the (2x1) reconstructed surface is shown to favor structures with a dipolar moment such as the resonant form of acrylonitrile with cumulative double bonds. The bond of acrylonitrile via a single C atom is a possible intermediate leading to the nitrile structure of the adsorbed molecule.     

Precursor mediated cycloaddition reaction of ethylene to the Si(100)c(4 x 2) surface

We report the direct observation of a precursor state for the cycloaddition reaction (the di-sigma bond formation) of ethylene on Si(100)c(4 x 2) using high-resolution electron energy loss spectroscopy at low temperature, and the meta-stable precursor state is identified as a weakly bonded pi-complex type. The activation energy from the pi-complex precursor to the di-sigma bonded species is experimentally estimated to be 0.2 eV. First-principles calculations support the pi-complex precursor mediated cycloaddition reaction of ethylene on Si(100)c(4 x 2).     

Diradical mechanism for the [2 + 2] cycloaddition of ethylene on Si(100) surface

Density functional cluster model calculations have been performed to explore the reaction mechanism for the adsorption of ethylene on Si(100). It is shown that the [2 + 2] cycloaddition of ethylene on a Si=Si dimer of Si (100) surface follows a diradical mechanism, via a pi-complex precursor and a singlet diradical intermediate, and the rate-determining step for the overall reaction is the formation of the diradical intermediate.     

2+2] Cycloaddition reactions of ethylene derivatives with the Si(100)-2 x 1 surface: a theoretical study

Possible mechanisms of the [2+2] cycloaddition reactions of ethylene (1), propylene (2), vinyl chloride (3), and styrene (4) with the Si(100)-2 x 1 surface have been investigated by theoretical calculations with the unrestricted density functional theory (DFT) and the second-order M?ller-Plesset perturbation theory (MP2). Facile occurrence of the studied reactions is supported by the low activation energies (2.45-5.76 kcal/mol) in the rate-determining steps. The buckled Si(100) surface facilitates the reactions via the low-symmetric pathways. The reactions follow the diradical mechanism of thermal [2+2] cycloaddition reactions between pi-electron donors (the ethylene derivatives) and acceptors (the Si surface) through a pi-complex precursor and a singlet diradical intermediate. The influence of substituents on the relative reactivity takes a qualitative sequence of 1 < 2 < 3 < 4. The natural bond orbital (NBO) analysis and the released heat of some model reactions suggest that the relative reactivity might be partially understood by the pi-electron-donating abilities of the substituent to stabilize the radical centers at the transition states of the rate-determining steps.     

Cycloaddition isomerizations of adsorbed 1,3-cyclohexadiene on Si(100)-2x1 surface: first neighbor interactions

The initial and subsequent surface reaction mechanisms of 1,3-cyclohexadiene on the Si(100)-2x1 surface were theoretically explored, focusing on the possible first-neighbor interactions. Five different initial reaction channels leading to nine different surface products were identified, confirming previous experimental reports of inter-dimer structures. Among the nine identified products, five of these surface products are new species that have not previously been reported. Potential energy surface studies reveal that the net reaction barriers within a given channel are very small, indicating that the final product distributions within that channel are determined by thermodynamics. On the other hand, thermal isomerizations between different channels are not expected to occur easily. Therefore, the surface product distributions among the five different channels are more likely to be determined by kinetics. As a result, understanding the relationships among the available reaction channels both kinetically and thermodynamically is essential for properly interpreting the experimental results. The current study shows that the subsequent surface chemical reactions of unsaturated initial surface products are strongly coupled with the first-neighbor interactions.     

Hydrazine-1-carbonitriles: new synthesis approach to and reactions with carbonyl compounds

Abstract  

A series of 1-(pyrimidin-2-yl)hydrazine-1-carbonitriles was synthesized by base-catalyzed condensation of 3,4-diamino-1,2,4-triazole hydrobromide with several 1,3-dinucleophilic compounds. These final products were formed by ring opening of the 1,2,4-triazolium ring via intermediate 3-amino[1,2,4]triazolo[1,5-a]pyrimidinium bromides.     

UPS of multilayer nitrogen‐bearing compounds on the Si(100) surface

Ultraviolet photoemission spectra (UPS) are presented for condensed layers of three ethylated amines; mono‐, di‐, and triethylamine (TEA) on the Si(100) surface at 100 K. The photoemission peaks associated with the nitrogen lone pair electrons are identified in the amines and compared with the corresponding spectra for condensed ammonia. Shifts in the lone pair binding energy for the ethyl‐substituted amines are shown to be consistent with conventional chemical paradigms. Also, for comparison purposes spectra for two nonethylated amines, trimethylamine (TMA), and its silicon analog, trisilylamine (TSA), are presented and discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Azide reactions for controlling clean silicon surface chemistry: benzylazide on Si(100)-2 x 1

A combination of experimental and computational studies presents direct proof of a novel reaction pathway that delivers aromatic compounds onto a Si(100)-2 x 1 substrate. Benzylazide chemisorbs on a Si(100)-2 x 1 surface, and this chemisorption is followed by nitrogen elimination, leading to a stable surface adduct based on a Si-Si-N cyclic entity. This reaction occurs via a stable surface intermediate with the surface-bound nitrogen molecule stabilized by the presence of a neighboring aromatic group, which eventually releases nitrogen into the gas phase and forms the final product.     

Grafting cavitands on the Si(100) surface

Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.     

Molecular design by cycloaddition reactions—36: Cycloaddition reactions of 7-azabenzonorbornadiene with troponoid compounds and photochemistry of the adducts

Reactions of 7-azabenzonorbornadiene 1 with tropone, tropolone and 2-aminotropone gave exclusively endo-exo adducts 3a–c in good yields. Similar reaction of other troponoid compounds like 2-acetylamino-, 2-acetoxy- and 2-methoxytropone afforded two isomeric cycloadducts in each case. Photolyses of tropone-adduct 3a in various solvents gave the corresponding cyclopropylcarboxylic acid derivatives 7–11 in high yields. Photochemical behaviors of these adducts 3c, e, f, 5e–f and 4d were also examined.     

Adsorption of trisilylamine on the Si(100) surface

Adsorption of trisilylamine (TSA) on the Si(100) surface has been studied using temperature programmed desorption (TPD) and time‐of‐flight electron stimulated desorption (TOFESD). TPD spectra exhibit the presence of three desorption states denoted by β₁, β₂, and β₃ associated with the presence of a mono‐, di‐, and tri‐hydride state respectively. This behavior is identical with previously observed desorption studies resulting from atomic hydrogen adsorption, indicating that the nitrogen species in the adsorbate has minimal impact on the surface structure of the hydride. Preliminary electron irradiation studies are reported and indicate that the formation of a thin silicon nitride layer is induced as a result of the irradiation. Copyright © 2008 John Wiley & Sons, Ltd.     

Competition and selectivity of organic reactions on semiconductor surfaces: reaction of unsaturated ketones on Si(100)-2 x 1 and Ge(100)-2 x 1

A combined experimental and theoretical study of a model system of multifunctional unsaturated ketones, including ethyl vinyl ketone (EVK), 2-cyclohexen-1-one, and 5-hexen-2-one, on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces was performed in order to probe the factors controlling the competition and selectivity of organic reactions on clean semiconductor surfaces. Multiple internal reflection infrared spectroscopy data and density functional theory calculations indicate that EVK and 2-cyclohexen-1-one undergo selective [4 + 2] hetero-Diels-Alder and [4 + 2] trans cycloaddition reactions on the Ge(100)-2 x 1 surface at room temperature. In contrast, on the Si(100)-2 x 1 surface, evidence is seen for significant ene and possibly [2 + 2] C=O cycloaddition side products. The greater selectivity of these compounds on Ge(100) versus Si(100) is explained by differences between the two surfaces in both thermodynamic factors and kinetic factors. With 5-hexen-2-one, for which [4 + 2] cycloaddition is not possible, a small [2 + 2] C=C cycloaddition product is observed on Ge(100) and possibly Si(100), even though the [2 + 2] C=C transition state is calculated to be the highest barrier reaction by several kilocalories per mole. The results suggest that, due to the high reactivity of clean semiconductor surfaces, thermodynamic selectivity and control will play important roles in their selective functionalization, favoring the use of Ge for selective attachment of multifunctional organics.     

Comparative study on reactions and self-directed growth mechanisms of styrene molecules on H-terminated Si(111) and Si(100): combining quantum chemistry and molecular mechanics simulations

A comparative study on mechanisms of radical initiated self-directed growth of styrene molecules on the H-terminated Si(111) and Si(100) has been carried out by using quantum chemical and molecular mechanics methods. Several possible H-abstraction pathways through formations of transition states containing five-, six-, and even eight-membered ring structures are investigated with the aid of surface cluster models and density functional theory calculations. It has been demonstrated by employing periodic surface models and molecular mechanics simulations that the surface pattern and intermolecular interactions between phenyl groups play important roles in the self-directed growth processes. The formation of cluster-shaped aggregation of styrene molecules on H-Si(111) results from the undirectional chain reactions, due to the isotropic hexagonal arrangement of surface sites. On the contrary, the anisotropic style of H-Si(100) induces a strong directional preference for H-abstractions, following an order of the inter Si-Si dimer > the intra Si-Si dimer > the inter Si-Si dimer row. The one-dimensionally ordered structures of single and double lines along the Si-Si dimer row are thus formed on H-Si(100). The self-directed growths of styrene molecules on both H-Si(111) and H-Si(100) are revealed to be stage-dependent.     

Diradical mechanisms for the cycloaddition reactions of 1,3-butadiene,benzene, thiophene,ethylene, and acetylene on a Si(111)-7x7 surface

The cycloaddition chemistry of several representative unsaturated hydrocarbons (1,3-butadiene, benzene, ethylene, and acetylene) and a heterocyclic aromatic (thiophene) on a Si(111)-7x7 surface has been explored by means of density functional cluster model calculations. It is shown that (i) 1,3-butadiene, benzene, and thiophene can undergo both [4+2]-like and [2+2]-like cycloadditions onto a rest atom-adatom pair, with the former process being favored over the latter both thermodynamically and kinetically; (ii) ethylene and acetylene undergo [2+2] cycloaddition-like chemisorptions onto a rest atom-adatom pair; and (iii) all of these reactions adopt diradical mechanisms. This is in contrast to the [4+2] cycloaddition-like chemisorptions of conjugated dienes on a Si(100) surface and to the prototype [4+2] cycloadditions in organic chemistry, which were believed to adopt concerted reaction pathways. Of particular interest is the [4+2]-like cycloaddition of s-trans-1,3-butadiene, whose stereochemistry is retained during its chemisorption on the Si(111) surface.     

Theoretical study on reactions of nitroethylene with the Si(100)-2 x 1 surface

Possible reaction pathways of nitroethylene with the Si(100)-2 x 1 surface have been investigated by unrestricted density functional theory. The facile occurrence of the studied reactions was demonstrated by the low activation energies of the rate-determining steps (1.07-5.23 kcal/mol). It was found that the [4 + 2] cycloaddition reaction of nitroethylene is most kinetically favorable. The isomerization reactions of the addition products were also investigated. The [3 + 2] cycloaddition product may further undergo a rearrangement by overcoming a 12.37 kcal/mol activation energy barrier into an isomer, with an oxygen atom of the nitryl group inserted between two silicon atoms of the Si(100) surface.