• Bulletin of the Korean Chemical Society
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• v.10 no.1
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• pp.60-66
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• 1989

Semiempirical computations using the AM1 and MNDO methods were carried out in order to elucidate allowed mechanisms for 1,3-group(X) rearrangement processes with X = $BH_2$, $CH_3$, CN, F, $NH_2$, OH, Cl and SH. The reactivity of the group migration was largely controlled by the steric effect in the 4-membered ring transition state, an antarafacial process having a greater energy barrier due to a greater steric repulsion. For the groups with lone pair electrons, the participation of the lone pair orbital is found to ease the steric effect by enabling the FMO interation with highly polarizable, high lying, lone pair electrons at relatively distant range; the involvement of lone pairs in the transition state causes an alteration of the symmetry selection rule to that of a 6-electron system with an allowed 1,3-suprafacial migration in contrast to an allowed 1,3-antarafacial migration for a 4-electron system. Various stereoelectronic aspects were analysed in some detail.

• Bulletin of the Korean Chemical Society
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• v.16 no.5
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• pp.410-416
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• 1995

The proton transfer energies of gas phase glycine and alanine and those of hydrated glycine and alanine were calculated both with Hartree-Fock and $M{\Phi}ller-Plesset$ ab initio molecular orbital (MO) calculations with 6-31G** basis set. The transition states of the proton transfer of gas phase glycine was also investigated. For zwitterions, both for glycine and alanine, the water bound to -NH3+ site stabilize the complex more compared with the water bound to -CO2-. The proton transfer energy, ΔEpt, of glycine, alanine, mono-hydrated glycine, mono-hydrated alanine, di-hydrated glycine and di-hydrated alanine were obtained as 30.78 (MP2: 22.57), 31.43, 23.99 (MP2: 17.00), 24.98, 22.87, and 25.63 kcal/mol, respectively. The activation energy for proton transfer from neutral (Nt) glycine to zwitterion (Zw) glycine, Ea, was obtained as 16.13 kcal/mol and that for reverse process, Ear, was obtained as 0.85 kcal/mol. Since the transition state of the proton transfer of gas phase glycine locate near the glycine zwitterion on the potential energy surface and the shape of the potential well of the zwitterion is shallow, the zwitterion easily changed to neutral glycine through the proton transfer.

• Bulletin of the Korean Chemical Society
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• v.20 no.12
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• pp.1493-1500
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• 1999

The electronic transitions from the ground state to low-lying excited states of CF₃OH have been investigated using high level ab initio quantum mechanical techniques. Also the possible photodissociation procedures of CF₃OH have been considered. The highest level employed in this study is TZP CCSD(T) level of theory. The possible four low-lying excited states can result by the excitation of the lone pair electron (n) in oxygen to σ$^*$ molecular orbital in C-O or O-H bond. The vertical transition (n → σ$^*$) energy is predicted to be 220.5 kcal/mol (130 nm) at TZ2P CISD level to theory. The bond dissociation energies of CF₃OH to CF₃O +H and CF₃+OH have been predicted to be 119.5 kcal/mol and 114.1 kcal/mol, respectively, at TZP CCSD level of theory. In addition, the transition state for the unimolecular decomposition of CF₃OH into CF₂O + HF has been examined. The activation energy and energy separation for this decomposition have been computed to be 43.6kcal/mol and 5.0 kcal/mol including zero-point vibrational energy corrections at TZP CCSD(T) level of theory.ed phenols were also estimated.

• Bulletin of the Korean Chemical Society
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• v.14 no.6
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• pp.669-673
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• 1993

The CO substitution reactions of the complex, $Cp(S-2,4-Me_2C_5H_5)CrCo$ with $PR_3(PR_3=PMePh_2,\;P(OCH_3)_3,\;PMe_2Ph)$ were investigated spectrophotometrically at various temperatures. From the reaction rates, it was suggested that the CO substitution reaction took place by first-order (dissociative) pathway. Activation parameters in decaline were ${\Delta}H^{\neq}\;=\;22.0\;kcal{\cdot}mol^{-1}$, ${\Delta}S^{\neq}=\;-3.8cal{\cdot}mol^{-1}{\cdot}K^{-1}$. Unusually low value of ${\Delta}S{\neq}$ suggests an ${\eta}^5-S{\to}{\eta}^5-U$ conversion of the pentadienyl ligand. This suggestion was confirmed by the Extended-Huckel molecular orbital (EHMO) calculations, which revealed that the total energy of $Cp(S-2,4-Me_2C_5H_5$)CrCO is about 0.42 kcal/mol more lower than that of $Cp(U-2,4-Me_2C_5H_5)CrCO$ and the energy of $[Cp(U-2,4-Me_2C_5H_5)Cr{\cdots}CO]^{\neq} $ transition state is about 2.43 kcal/mol lower than that of $[Cp(S-2,4-Me_2C_5H_5)Cr{\cdots}CO]^{\neq}$ transition state.

• Bulletin of the Korean Chemical Society
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• v.14 no.2
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• pp.207-211
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• 1993

The CO substitution reactions in the complex, $Cp(S-C_5H_7)CrCO$ with $PR_3(PR_3=PMePh_2,\;P(OCH_3)_3,\;PMe_2Ph)$ were investigated spectrophotometrically at various temperatures. From the reaction rates, it was suggested that the CO substitution reaction takes place by first-order (dissociative) pathway. Activation parameters in decaline are ${\Delta}H^{\neq}\;=\;24.58\;kcal\;{\cdot}\;mol^{-1},\;{\Delta}S^{\neq}\;=\;3.05 cal\;{\cdot}\;mol^{-1}{\cdot}K^{-1}$. Unusually low value of ${\Delta}S^{\neq}$ suggests an ${\eta}^5-S\;{\to}\;{\eta}^5-U$ conversion of the pentadienyl ligand. This was confirmed by the extended-Huckel molecular orbital (EHMO) calculations, which revealed that the total energy Of $Cp(S-C_5H_7)CrCO$ is about 6.84 kcal/mol more stable than that of $Cp(U-C_5H_7)CrCO$ and the energy of $[Cp(U-C_5H_7)CrCO^{\neq}$ transition state is about 4.25 kcal/mol lower than that of $[Cp(S-C_5H_7)Cr]^{\neq}$ transition state.

• Bulletin of the Korean Chemical Society
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• v.6 no.6
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• pp.362-366
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• 1985

The hydrogen atom transfer reaction between substituted methane, $CH_3X,$ and its radical, $CH_2X(X=H,F,CH_3,CN,OH\;and\;NH_2$ was studied by MINDO/3 method. The transition state(TS) structure and energy barriers were determined and variation of the transition state and of the reactivity due to the change of X were analyzed based on the potential energy surface characteristics. It was found that the greater the radical stabilization energy. the looser the TS becomes; the TS occurs at about 15% stretch of the C-H bond, which becomes longer as the radical stabilization energy of $CH_2X$ increasers. The intrinsic barrier, ${\Delta}E*_{x.x},$ of the reaction with X was found to increase in the order $H The degree of bond stretch of the C-H bond stretch of the C-H bond at the TS also had the same order indicating that the homolytic bond cleavage of the C-H bond is rate-determining. Orbital interactions at the TS between LUMO of the fragment $C{\ldots}H{\ldots}C$ and the symmetry adapted pair of nonbonding, $n{\pm}(=n_1{\pm}n_2),$ or pi orbitals of the two X atoms were shown to be the dominant contribution in determining tightness or looseness of the TS. The Marcus equation was shown to apply to the MINDO/3 barriers and energy changes of the reaction.

• Journal of the Korean Chemical Society
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• v.30 no.1
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• pp.9-18
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• 1986

MNDO and STO-3G calculations were performed to determine relative stabilities of rotamers for ${\alpha}$-substituted acetones, $CH_2XCOCH_3$, X = F, Cl, OH, SH, and $NH_2$. It was found that rotamers corresponding to gauche forms are preferred for all the ${\alpha}$-substituents except for X = F and NH$_2$, for which the cis forms were the preferred ones. The stability of gauche form was dictated by the stabilizing two-orbital-two-electron interaction ${\sigma}_{cx}$-${\pi}_{co}^*$, operating uniquely in the gauche form due to the substantial vicinal overlap and energy gap narrowing between ${\sigma}_{cx}$ and ${\pi}_{co}^*$ orbitals. The energy gap narrowing was caused by the lowering of ${\pi}_{co}^*$ level due to the hyperconjugative ${\sigma}_{cx}^*$-${\pi}_{co}^*$ interactions; the red shift in the n-${\pi}^*$ transition was another effect of the relatively large ${\sigma}_{cx}^*$-${\pi}_{co}^*$ splitting. Various ${\sigma}-{\pi}$ interactions in the gauche form were found to be stronger in the third-row hetero atom system, X = Cl and SH. Interactions between nonbonding orbital on N, $n_N$ and vicinal C-C ${\sigma}$ bond were shown to be stronger in the trans than in the cis orientation.

• Journal of Korean Vacuum Science & Technology
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• v.6 no.4
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• pp.153-157
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• 2002

The electronic structures of Gd$_{5}$Si$_2$Ge$_2$ compound, which has a giant magnetocaloric effect, in the monoclinic and orthorhombic phases were calculated using the tight-binding linear-muffin-tin-orbital method within the atomic-sphere approximation. The calculated total energies of the monoclinic and orthorhombic structures in the paramagnetic phase confirm that the orthorhombic structure is more stable than monoclinic structure. The density of states (DOS) at the Fermi level of the orthorhombic phase is higher than that of the monoclinic phase in the paramagnetic phase, fulfilling the Stoner criterion. The calculated charge density verified the breaking of Ge(Si)-Ge(Si) bonding in the basal plane upon the orthorhombic-monoclinic phase transition. The DOS curve fairly well reproduces the photoemission spectrum.m.

• Journal of the Korean Chemical Society
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• v.13 no.4
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• pp.281-287
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• 1969

Rate constans for the chloride exchange of some arylmethylchloride in dry acetone have been determined, and activation parameters have been evaluated. The reactivities of substates are explained with perturbational molecular orbital (PMO) method and HSAB principle. It was found that carbon-chlorine resonance integral at the transition state is a out 67% of ${\beta}$, the carbon-carbon resonance integral.

• Journal of Magnetics
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• v.14 no.3
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• pp.114-116
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• 2009

The electronic properties of Cu or Pd-doped GaN at several concentrations are examined using the full-potential linear muffin-tin orbital method. For ($Cu_{0.055}Ga_{0.945}$)N, the model reveals a magnetic moment of $1.47{\mu}B$ per supercell. The range of concentrations that are spin-polarized should be restricted within narrow limits. A paramagnetic to ferromagnetic phase transition is found to occur at a Cu concentration of 5.55%.