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Cheol Ho Choi

Publications and source records attributed to Cheol Ho Choi.

14 recordsLinked to original sources
PubMed2006

Adsorption reactions of dimethylaluminum isopropoxide and water on the H/Si(100)-2 x 1 surface: initial reactions for atomic layer deposition of Al2O3.

The surface reaction pathways of dimethylaluminum isopropoxide (DMAI) and water with the H/Si(100)-2 x 1 surface were theoretically investigated with SIMOMM:MP2/6-31G(d). The oxygen atom in DMAI stabilizes an initial complex, facilitating the approach of DMAI to the surface. The methane loss reaction, propane loss reaction, methylation, hydrogen loss reaction, and ring closing reaction channels of the DMAI-surface reactions were identified. Among these, the methane loss reaction depositing -Al(CH3)OCH(CH3)2 was found to be the major channel due to low barrier height and large exothermicity. The ring closing reaction is kinetically the second most accessible channel, even though it is not thermodynamically favorable. On the basis of these theoretical results, recent experimental data were reinterpreted such that the experimentally observed peaks of CH4 and CH(CH3)2OH are in fact the products of these two channels. The propane loss reaction is kinetically the third most probable channel. It produces the surface Si-O bond, which is a reaction unique to DMAI as compared to trimethylaluminum. In summary, the oxygen substitution not only affects the basic nature of the existing potential energy surfaces but also opens new possibilities.

Journal Article↗
PubMed2006

A theoretical study of thermal [1,3]-sigmatropic rearrangements of 3-trimethylsilyl-1-pyrazoline: concerted vs. stepwise mechanisms.

Possible reaction mechanisms of 1,3-silyl and 1,3-hydrogen thermal rearrangements of trimethylsilyl-1-pyrazoline and its model systems were theoretically explored using B3LYP, MP2, CR-CCSD(T), CASSCF(6,5), and MRMP2(6,5) theories. Nitrogen substitution at the center position of allylic moiety turned out to have a special stabilizing effect on diradical intermediates, allowing a stepwise pathway. This substitutional effect was attributed to the nitrogen lone pair electrons, which form strong pi-conjugations with diradicals. The second nitrogen substitution at the terminal allylic position selectively reduces the reaction barrier of antarafacial retention pathway, creating a competition between concerted and stepwise channels. The introduction of a five-membered ring imposes ring strain on the allylic moiety and increases steric hindrance, allowing no antarafacial channels. The combined effect of the nitrogen substitution and the five-membered ring further removes the possibility of concerted reaction pathways. Therefore 1,3-silyl migrations of 3-trimethylsilyl-1-pyrazoline were found to occur only through stepwise mechanisms, implying that the Woodward-Hoffmann rule is not operative. The 1,3-hydrogen migration also occur via a stepwise mechanism; however, it would not occur easily because its reaction barrier is much higher than that of 1,3-silyl migrations. Current study shows that a stepwise mechanism can be the dominant reaction pathway of some particular [1,3]-sigmatropic rearrangements.

Models, Chemical↗
PubMed2005

Surface-stabilized amorphous germanium nanoparticles for lithium-storage material.

Amorphous Ge nanoparticles with the particle size of approximately 10 nm were prepared by capping butyl groups and were characterized using XAS, TEM, FT-IR reflectance, and electrochemical cycling. The XAS results for the first-cycle Ge nanoparticles exhibited either a little particle aggregation after reformation of the Ge-Ge metallic bond or reformation of Ge-Ge metallic bond followed by a little particle aggregation. More interestingly, butyl groups, being electrochemically stable, remained after cycling, and the quantum mechanical calculation of the thermodynamic energy of the reaction using the GAMESS (General Atomic and Molecular Electronic Structure System) program suggested the formation of a very stable surface Ge-C bond that cannot be easily subjected to the subsequent chemical reactions. Initial charge capacity is 1470 mAh/g with an irreversible capacity ratio of 12%; no capacity fading was observed out to 30 cycles. Even at 5 C rate discharging, capacity retention was 98%, compared to that at 0.2 C rate discharging. In addition, the capacity was fully recovered at 0.2 C rate cycling.

Journal Article↗
PubMed2005

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.

Journal Article↗
PubMed2005

Surface S(N)2 reaction by H2O on chlorinated Si(100)-2 x 1 surface.

The potential energy surfaces of one, two, and three water molecule sequential adsorptions on the symmetrically chlorinated Si(100)-2 x 1 surface were theoretically explored with SIMOMM:MP2/6-31G(d). The first water molecule adsorption to the surface dimer requires a higher reaction barrier than the subsequent second water molecule adsorption. The lone pair electrons of the incoming water molecule nucleophilically attack the surface Si atom to which the leaving Cl group is bonded, yielding an S(N)2 type transition state. At the same time, the Cl abstracts the H atom of the incoming water molecule, forming a unique four-membered ring conformation. The second water molecule adsorption to the same surface dimer requires a much lower reaction barrier, which is attributed to the surface cooperative effect by the surface hydroxyl group that can form a hydrogen bond with the incoming second water molecule. The third water molecule adsorption exhibits a higher reaction barrier than the first and the second water molecule adsorption channels but yields a thermodynamically more stable product. In general, it is expected that the surface Si-Cl bonds can be subjected to the substitution reactions by water molecules, yielding surface Si-OH bonds, which can be a good initial template for subsequent surface chemical modifications. However, oversaturations can be a competing side reaction under severe conditions, suggesting that the precise control of surface kinetic environments is necessary to tailor the final surface characteristics.

Journal Article↗
PubMed2005

Comparative study of surface cycloadditions of ethylene and 2-butene on the Si(100)-2 x 1 surface.

Multireference wave functions were used to study the ethylene and 2-butene surface reactions on Si(100) in their lowest energy singlet states. In addition to the diradical pathway, a pi-complex pathway on the ethylene surface was found. The net barrier for the latter process is 4.5 kcal/mol higher than that for the former, making the pi-complex pathway kinetically less accessible. Therefore, although there is a competition between the two initial channels, the diradical path is slightly favored, and rotational isomerization is possible. However, since the initial potential energy surfaces of the two channels are different, depending on experimental conditions, the branching ratio between the two channels may change. Consequently, the combined effects that would favor one channel over the other may not derive directly from the initial reaction barrier. This provides an explanation of the experimental controversy. As a result, the final distributions of surface products may depend on the experimental kinetic environment, especially when the population change due to the rotational isomerization is expected to be very small. A significantly different reaction channel is found in the 2-butene surface reaction on Si(100), in which a methyl hydrogen easily transfers to the surface yielding a new type of surface product other than the expected [2 + 2] cycloaddition product, with a comparatively small activation barrier. Consequently, the overall surface reactions of ethylene and 2-butene may be quite different. Therefore, direct comparisons between ethylene and 2-butene experimental results would be very useful.

Journal Article↗
PubMed2004

Synthesis of Ru(II) complexes of N-heterocyclic carbenes and their promising photoluminescence properties in water.

Novel complexes 1 and 2 based on N-heterocyclic carbenes, which are analogous to Ru(bpy)(3)(2+) and Ru(terpy)(2)(2+), respectively, were synthesized. The complex, which is analogous to Ru(terpy)(2)(2+), exhibited promising photoluminescence properties with a long lifetime of 820 ns in acetonitrile and 3100 ns in water at room temperature, respectively. In addition, ab initio calculations were carried out.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2004

Direct determination of multipole moments of Cartesian Gaussian functions in spherical polar coordinates.

A new way of generating the multipole moments of Cartesian Gaussian functions in spherical polar coordinates has been established, bypassing the intermediary of Cartesian moment tensors. A new set of recurrence relations have also been derived for the resulting analytic integral values. The new method furnishes a conceptually simple and numerically efficient evaluation procedure for the multipole moments. The advantages over existing methods are documented. The results are relevant for the linear scaling quantum theories based on the fast multipole method.

Journal Article↗
PubMed2004

Surface reaction mechanisms of hydrazine on Si(100)-2 x 1 surface: NH3 desorption pathways.

Multireference as well as single-reference wave functions were adopted to study the surface reaction mechanisms of hydrazine. The initial surface mechanisms resemble those of ammonia and its methyl derivatives. MRMP2 values indicate that the lifetime of initial molecularly adsorbed species should be longer than previously suggested. High energy path as well as low energy path of subsequent surface reactions were found. The theoretical initial surface product of low energy path is consistent with the experimentally suggested structure. Both paths eventually lead to very stable surface products, which are also consistent with the experimentally suggested structures. The reaction channels of the experimentally observed NH3 desorptions were also revealed. It was shown that the high reactivity of hydrazine as compared to ammonia and its methyl derivatives is due to the high nucleophilic ability of the additional nitrogen atom of hydrazine.

Adsorption↗
PubMed2002

Passive and active oxidation of Si(100) by atomic oxygen: a theoretical study of possible reaction mechanisms.

Reaction mechanisms for oxidation of the Si(100) surface by atomic oxygen were studied with high-level quantum mechanical methods in combination with a hybrid QM/MM (Quantum mechanics/Molecular Mechanics) method. Consistent with previous experimental and theoretical results, three structures, "back-bond", "on-dimer", and "dimer-bridge", are found to be the most stable initial surface products for O adsorption (and in the formation of SiO(2) films, i.e., passive oxidation). All of these structures have significant diradical character. In particular, the "dimer-bridge" is a singlet diradical. Although the ground state of the separated reactants, O+Si(100), is a triplet, once the O atom makes a chemical bond with the surface, the singlet potential energy surface is the ground state. With mild activation energy, these three surface products can be interconverted, illustrating the possibility of the thermal redistribution among the initial surface products. Two channels for SiO desorption (leading to etching, i.e., active oxidation) have been found, both of which start from the back-bond structure. These are referred to as the silicon-first (SF) and oxygen-first (OF) mechanisms. Both mechanisms require an 89.8 kcal/mol desorption barrier, in good agreement with the experimental estimates of 80-90 kcal/mol. "Secondary etching" channels occurring after initial etching may account for other lower experimental desorption barriers. The calculated 52.2 kcal/mol desorption barrier for one such secondary etching channel suggests that the great variation in reported experimental barriers for active oxidation may be due to these different active oxidation channels.

Journal Article↗
PubMed2002

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.

Acrylonitrile↗