• Bulletin of the Korean Chemical Society
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• v.14 no.1
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• pp.55-58
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• 1993

In order to determine the cobalt substituting site in $AlPO_4-5$ framework, ASED-MO theory has been used. The substitution of cobalt for aluminum is energetically more favorable than that for phasphorous. The stabilized energy of the former is 51 eV lower than that of the latter. The calculated net charge was +1.27 for Al, +0.85 for P, and +1.56 for Co, respectively. The valence electron population (VEP), reduced overlap population (ROP) and net charge for the charged cluster models were compared for $AlPO_4-5$ and $CoAlPO_4-5$ systems. Then, twe find that the covalency of P-O bond was greater than that of Al-O bond.

• Bulletin of the Korean Chemical Society
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• v.15 no.11
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• pp.996-1000
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• 1994

Ab initio calculations with various basis sets have been carried out to investigate the geometries and ring inversion barrier of $R_2TiC_3H_6\;and\;R_2TiS_3$, R=Cp and Cl. Optimized geometries of $R_2TiC_3H_6$ showed the four membered ring was planar on Cs symmetry. However, $R_2TiS_3$ complexes were optimized to be stable in the puckered form. The smallest Basis III with STO-3G on Cp ligands gave reasonable results for the calculations of metallocene. The energy barrier for the ring inversion of metallacyclosulfanes, $Cp_2TiS_3$ was computed to be 8.72 kcal/mol at MP2 level. For the Cl system, we reproduced the molecular structure and ring inversion energy with Basis V.

• Bulletin of the Korean Chemical Society
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• v.8 no.6
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• pp.444-449
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• 1987

Solvent effects on gas phase reactions of methoxide ion with substituted ethylenes, $CH_2$ = CHR where R = CN, CHO and $NO_2$, are investigated theoretically using the AM1 method. Results show that the methoxide approaches in-plane in all reactions, but subsequently rotate out-of-plane to form tetrahedral complexes in additon reactions. All reactions of a bare methoxide are found to be exothermic, the exothermicity being the greatest in the ${\beta}$-addition, in which the excess energy is forced to be contained within the ${\beta}$-adduct rendering extreme instability. However a part of the excess energy can be removed by a solvate molecule giving a stable complex prior to the product formation. The hydride transfer processes were found to be unfavorable due to the high activation barriers. The ${\alpha}-H^+$ abstraction process from acrylonitrile becomes endothermic as a result of monosolvation of the methoxide, in agreement with experimental results.

• Bulletin of the Korean Chemical Society
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• v.12 no.5
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• pp.468-473
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• 1991

Viscoelastic properties of PET filament fibers on stress relaxation were investigated in the solvents of $H_2$O, 0.05% NaOH and 50% DMF using an Instron (UTM4-100 Tensilon) with solvent chamber. The theoretical stress relaxation equation derived by applying the Ree-Eyring's hyperbolic sine law to dashpot of three element non-Newtonian model was applied to the experimental stress relaxation curves, and the model parameters $G_1,G_2$, ${\alpha}$ and ${\beta}$ were obtained. By analyzing temperature dependency of the relaxation time, the values of activation entropy, activation enthalpy and activation free energy for flow in PET filament fiber were evaluated, the activation free energy being about 25.7 kcal/mol. The self diffusion coefficient and hole distance were obtained from parameters ${\alpha}$, ${\beta}$ and crystallite size in order to study the self diffusion and the orientation of crystallites in amorphous region and the effect of solvent.

• Bulletin of the Korean Chemical Society
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• v.12 no.5
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• pp.537-540
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• 1991

Electrophilic methylation reactions of five-membered heteroaromatic compounds, furan, pyrrole and thiophene, with the dimethylfluoronium ion, ${CH_3}{FCH_3}(+), have been investigated theoretically by the MNDO method. The site selectivity of ${\alpha}, {\beta}$ and heteroatom (X) is related to charge density of the site, indicating that the site selectivity is dictated by electrosatic interaction between two reaction centers. The reactivity order between the three heteroaromatics can not be determined decisively since the order differs depending on which site is compared, with relatively low activation enthalpies, ${\Delta}{H^\neq}$= 20-30 kcal/mol, in all cases. These site and substrate selectivity behaviors are consistent with the gas-phase experimental results.

• Bulletin of the Korean Chemical Society
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• v.12 no.6
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• pp.658-661
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• 1991

The gas-phase nucleophilic substitution reaction of pentafluoroanisole with $OH^-$ and ${NH_2}^-$ nucleophiles have been studied theoretically using the AM1 method. Three reaction channels, $S_N2$, IPSO and $S_NAr$ (scheme 1), are all very exothermic so that all are accessible despite the varying central energy barriers which are much lower than the reactants level. In the IPSO and $S_NAr$ channels, the reactants form directly a stable ,${\sigma}$-anion complex which proceeds to form a proton transfer complex via a transition barrier corresponding to a loose ${\pi}$-type complex with the F-(or ${OCH_3}^-$) leaving group. Due to a greater number of probable reaction sites available for $S_NAr$ compared to the other two processes, the $S_NAr$ channel is favored as experimentally observed.

• Korean Journal of Optics and Photonics
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• v.2 no.4
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• pp.227-232
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• 1991

The possibility for the high intensity and energy possessing a short pulse in the HF chemical laser system which contained fluoride molecules (RF) was demonstrated theoretically through the numerical model simulation. The calculation was accomplished by assuming that the thermal branched chain mechanism of RF was occurred in the initiation step of $H_2+F_2$ chain reaction. Variations of the major chemicals and the temperature in the system were calculated as a function of time. An analysis was also performed to evaluate output pulse profile through parametric studies.

• Bulletin of the Korean Chemical Society
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• v.7 no.3
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• pp.183-185
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• 1986

Self-atom polarizability were calculated for 17 polycyclic aromatic hydrocarbons and their metabolites by LCAO-MO method and examined the relation with the carcinogenicity. It has been found that ${\pi}_{1.2.4}$, the sum of self-atom polarizability of 1, 2 and 4 positions forming trans-butadiene frame in a compound, agree quite well with the observed carcinogenic activity, and also, ${\pi}_{1.2.4}$ value increase with the metabolic activiting in agreement with the experimental facts that parent carcinogens activated with metabolism. Accordingly, we suggest that the 1, 2 and 4 positions in the carcinogenic compounds play the most important role in the process of chemical carcinogenesis, and also self-atom polarizability, as one of theoretical reactivity indices, is to be used as a measure of carcinogenic activity.

• Bulletin of the Korean Chemical Society
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• v.11 no.3
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• pp.194-200
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• 1990

Various mechanistic aspects of the A2 hydrolysis of methyl acetimidate were explored using the MNDO method. As in thecorresponding reactions of acetamide and methyl carbamate, a proton transfer pre-equilibrium exists between the N-protonated and the O-protonated tautomers, and the subsequent hydrolysis proceeds from the more stable N-protonated form. Of the two reaction pathways, the $A_{AL}2$ path is favored in the gas phase and in concentrated acid solutions, whereas the $A_{AC}2$ path is favored in less acidic solutions with a stable cationic tetrahedral intermediate formed in the rate determining step. Negative charge development on the alkoxy oxygen in the transition state suggested a rate increase with the increase in the electron withdrawing power of the alkoxy group. Calculations on the reaction processes with AM1 indicated that MNDO is more reliable in this type of work, although AM1 is better than MNDO in reproducing hydrogen bonds.

• Bulletin of the Korean Chemical Society
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• v.7 no.3
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• pp.231-235
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• 1986

MNDO and ab initio calculations for gas phase reactions of alkoxide-exchange at silicon and carbon centers have been performed. Results show that MNDO values of ${\Delta}$E's of these reactions closely parallel those of the STO-3G method. The alkoxide-exchange at silicon is shown to be facile due to the formation of stable five-coordinate intermediate while the reaction at carbon is predicted to proceed with high barrier; the difference in this substitution behavior between carbon and silicon is shown to be due to an easy valence shell expansion of silicon in accommodating an extra bond in the formation of stable five-coordinate intermediates.