• Bulletin of the Korean Chemical Society
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• v.19 no.10
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• pp.1073-1079
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• 1998

The potential energy surface (PES) of the non-identical SN2 reactions, F- + CH3Cl → FCH3 + Cl and (H2O)F + CH3Cl → FCH3 + Cl-(H2O), were investigated with ab initio MO calculations. The ab initio minimum energy reaction path (MERP) of the F- + CH3Cl → FCH3 + Cl- was obtained and it was expressed with an intermediate variable t. The ab initio PES was obtained near around t. Analytical potential energy function (PEF) was determined as a function of the t in order to reproduce the ab initio PES. Based on Morse-type potential energy function, a Varying Repulsive Cores Model (VRCM) was proposed for the description of the bond forming and the bond breaking which occur simultaneously during the SN2 reaction. The MERP calculated with the PEF is well agreed with the ab initio MERP and PEF could reproduce the ab initio PES well. The potential parameters for the interactions between the gas phase molecules in the reactions and water were also obtained. ST2 type model was used for the water.

• Proceeding of EDISON Challenge
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• 2013.04a
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• pp.214-216
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• 2013

The atomic and electronic properties of molybdenum disurfide ($MoS_2$) nanostructures are investigated through density functional theory (DFT) calculations. We find that the band gap is indirect (about 1.79 eV) and direct (about 1.84 eV) in GGA for 2-dimensional $MoS_2$ in our calculations. On the other hand, 1-dimensional armchair nanoribbons have semiconductor properties (band gap is about 0.11~0.28 eV), while 1-dimensional zigzag nanoribbons are metallic.

• Bulletin of the Korean Chemical Society
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• v.25 no.11
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• pp.1657-1660
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• 2004

Several critical geometries associated with the rearrangement of $CH_3SNO_2\;to\;CH_3SONO$ are calculated with the density functional theory (DFT) method and compared with those of the ab initio molecular orbital methods. There are two probable pathways for this rearrangement, one involving the transition state of an oxygen migration and the other through the homolytic decomposition to radicals. The reaction barrier via the transition state is about 60 kcal/mol and the decomposition energy into radicals about 35 kcal/mol, suggesting that the reaction pathway via the homolytic cleavage to radical species is energetically favorable. Since even the homolytic cleavage requires large energies, the rearrangement reaction is unlikely without the aid of catalysts.

• Journal of the Korean Chemical Society
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• v.50 no.3
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• pp.203-207
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• 2006

Ab initio calculations of the structure and the binding energies of K+(C2H5OH)n, (n=1~5) complexes were carried out with MP2/ full gen 6d and MP2/ 6-311G** methods. The stable structures of the complexes with n=2 to 5 were linear, trigonal, tetrahedral and trigonal bipyramid respectively. The binding energies of complexes were increased with the number of ligands, but the incremental binding energies were decreased. These results agreed well with the results of K+ complexes with other solvents.

• Bulletin of the Korean Chemical Society
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• v.14 no.4
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• pp.476-480
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• 1993

The ab initio calculations were performed on ClO and $ClO^+$ using the configuration interaction and M${\phi}$ller-Plesset methods of several different levels of approximation. Three different basis sets, 66 contracted Gaussian-type orbitals,6-31$G^*$ and 6-311$G^*$, were employed in this calculation. The results of calculation were compared with the experimental values of ClO. The values from the calculation with 66cGTO basis set gave excellent agreement with the experimental values. The spectroscopic constants of $ClO^+$ were also predicted.

• Bulletin of the Korean Chemical Society
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• v.11 no.6
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• pp.511-515
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• 1990

Ab initio calculations with MP3 and CISD method were performed for the FCO and $FCO^+$. The equilibrium geometry of FCO shows the reasonable agreement with experimental values. $FCO^+$ has a linear geometry with $R_{CF}=1.213\;and\;R_{CO}=1.118{\AA}$. The quadratic force constants of $FCO^+$ are 23.21 md ${\AA}^{-1}$ for CO stretch and 12.38 md ${\AA}^{-1}$ for CF stretch. The cubic force constants and the other molecular constants are also calculated.

• Bulletin of the Korean Chemical Society
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• v.24 no.1
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• pp.55-59
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• 2003

This work is focused on analyzing ion-pair interactions and showing the effect of solvent induced inter-atomic attractions in various dielectric environments. To estimate the stability of ion-pairs, SCI-PCM ab initio MO calculations were carried out. We show that the solvent-induced attraction or ‘cavitation' energy of the ion-pair interactions in solution that arises mainly from the stabilization of the water molecules by the generation of an electrostatic field. In fact, even the strong electrostatic interaction characteristic of ion-pair interactions in the gas phase cannot overcome the destabilization or reorganization of the water molecules around solute cavities that arise from cancellation of the electrostatic field. The solvent environment, possibly supplemented by some specific solvent molecules, may help place the solute molecule in a cavity whose surroundings are characterized by an infinite polarizable dielectric medium. This behavior suggests that hydrophobic residues at a protein surface could easily contact the side chains of other nearby residues through the solvent environment, instead of by direct intra-molecular interactions.

• Bulletin of the Korean Chemical Society
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• v.2 no.4
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• pp.132-138
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• 1981

Ab initio molecular orbital calculations have been performed in an effort to determine which types of chemical interactions play essential roles for the system, , $H_2O+CH_2SH^+$, and $H_2O+ CH_2S$. The most important contribution to the interaction energy in controlling reaction path is the exchange repulsion energy, EX, which is largely responsible for the shape of the total interaction energy curve. In the ion-molecule reaction, prior protonation of thioformaldehyde or prior deprotonation of water leads to formation of the corresponding ionic adducts ($H_2O+CH_2SH$ and $HOCH_2S^-$), with no barrier to reaction, simulating specific acid and base catalysis, respectively, as in the case of formaldehyde. Otherwise, approach of water to thioformaldehyde gives rise to a completely repulsive interaction.

• Bulletin of the Korean Chemical Society
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• v.16 no.9
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• pp.827-831
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• 1995

Kinetic studies on the reactions of five secondary acylic alkyl arenesulfonates with anilines are carried out in acetonitrile at 65.0 ℃. The magnitude of ρXZ determined (ρXZ=0.12-0.13) is slightly greater than that for the alicyclic series (ρXZ=0.11) under the same experimental condition. Ab initio MO results are found to support the slightly tighter transition state expected from the greater magnitude of ρXZ for the acyclic series. Despite the small variations, the magnitude of ρXZ and the theoretical transition state tightness remain relatively constant for the secondary carbon centers. Secondary kinetic isotope effects involving deuterated aniline nucleophiles show a successively smaller kH/kD(<1.0) value for a more sterically crowded reaction center carbon. This is in accord with the later transition state for bond-making predicted by the Bell-Evans-Polanyi principle for the more endothermic nucleophilic substitution reaction. Further support is provided by the results of the AM1 MO calculations on the reactions of secondary alkyl benzenesulfonates with chloride nucleophile.

• Journal of the Korean Chemical Society
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• v.28 no.4
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• pp.217-230
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• 1984

The MINDO/3 method was used in determination of transition states and activation barriers for various 1,2-, 1,3- and 1,5-sigmatropic hydrogen rearrangements involving systems with central carbon atom. It was found that, besides the consideration of orbital symmetry, steric effect, aromaticity, and orbital interactions were found to dictate the stability of the transition state. For systems with hetero atoms, lone pair orbitals tend to ease orbital distortion required at the transition state by participating in hydrogen transfer process and were found to lower the activation barrier accordingly. Comparison of the relative barrier heights with those obtained by using more sophisticated ab initio MO calculations showed that the MINDO/3 results give qualitatively the same tendency of the relative order of the activation barriers.