• Bulletin of the Korean Chemical Society
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• 제30권4호
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• pp.787-790
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• 2009

The predictions of the radical reaction sites for phenol, 2-, 3- and 4-chlorophenols (CPs) and 4-chloronitrobenzene (CNB) were studied by atomic charge distribution calculations. The atomic charge distributions on each atom of these molecules were obtained using the CHelpG and MK (Merz-Kollman/Singh) methods with the optimized structural parameters determined by DFT calculation at the level of BLYP/6-311++G(d,p). By comparing the experimentally obtained hydroxyl addition site(s) and the calculated atomic charges on carbon atoms of phenol and CPs, we found that hydroxyl substitution by oxidation reaction mainly occurred to the carbon(s) with high atomic charges. With these results, we were easily able to predict the position(s) of the ·OH reaction site(s) of phenol, CPs and CNB through atomic charge distribution calculations.

• Applied Science and Convergence Technology
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• 제23권5호
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• pp.273-278
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• 2014

Using ab initio calculations, the effects of uniaxial, biaxial, and hydrostatic strains on the magnetocrystalline anisotropy of $L1_0$-orderd FePt and CoPt alloys were systematically investigated. Interestingly, the rates and the signs of magnetocrystalline anisotropy changes of FePt and CoPt were determined by the directions and dimensions of strains. The calculation results are consistent with the previous experimental observations and are expected to provide directions to tailor magnetic properties of various types of $L1_0$-ordered FePt and CoPt systems.

• Transactions on Electrical and Electronic Materials
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• 제17권1호
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• pp.29-32
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• 2016

Using the various exchange-correlation functionals, such as LDA, GGA-PBE, GGA-PBEsol and GGA-AM05 functionals, first principle studies were conducted to determine the structures of paraelectric and ferroelectric PbTiO₃. Based on the structures determined by the various functionals, the piezoelectric properties of PbTiO₃ are predicted under the density-functional perturbation theory (DFPT). The present prediction with the various GGA functionals are closer to the experimental findings compared to the LDA values. The present DFT calculations using the GGA-PBEsol functional estimate the experimental data more reasonably than the conventional LDA and GGA fucntionals. The GGA-AM05 functional also predicts the experimental data as well as the GGA-PBEsol. The piezoelectric tensor calculated with PBEsol is relatively insensitive to pressure.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제38회 동계학술대회 초록집
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• pp.360-360
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• 2010

Very initial stage of oxidation process of Si (001) surface was investigated using large scale molecular dynamics simulation. Reactive force field potential was used for the simulation owing to its ability to handle charge variation associated with the oxidation reaction. To know the detail mechanism of both adsorption and desorption of water molecule (for simulating wet oxidation), oxygen molecule (for dry oxidation) and their atom constituents, interaction of one molecule with Si surface was carefully observed. The simulation is then continued with many water and oxygen molecules to understand the kinetics of oxide growth. The results show that possibilities of desorption and adsorption depend strongly on initial atomic configuration as well as temperature. We observed a tendency that H atoms come relatively into deeper surface or otherwise quickly desorbed away from the silicon surface. On the other hand, most oxygen atoms are bonded with first layer of silicon surface. We also noticed that charge transfer is only occur in nearest neighbor regime which has been pointed out by DFT calculation. Atomic structure of the interface between the oxide and Si substrate was characterized in atomic scale.

• 한국재료학회지
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• 제18권12호
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• pp.673-677
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• 2008

Calcia (CaO) stabilized cubic-$HfO_2$ is studied by density functional theory (DFT) with generalized gradient approximation (GGA). When a Ca atom is substituted for a Hf atom, an oxygen vacancy is produced to satisfy the charge neutrality. The lattice parameter of a $2{\times}2{\times}2$ cubic $HfO_2$ supercell then increases by $0.02\;{\AA}$. The oxygen atoms closest to the oxygen vacancy are attracted to the vacancy as the vacancy is positive compared to the oxygen ion. When the oxygen vacancy is located at the site closest to the Ca atom, the total energy of $HfO_2$ reaches its minimum. The energy barriers for the migration of the oxygen vacancy were calculated. The energy barriers between the first and the second nearest sites, the second and the third nearest sites, and the third and fourth nearest sites are 0.2, 0.5, and 0.24 eV, respectively. The oxygen vacancies at the third and fourth nearest sites relative to the Ca atom represent the oxygen vacancies in undoped $HfO_2$. Therefore, the energy barrier for oxygen migration in the $HfO_2$ gate dielectric is 0.24 eV, which can explain the origin of gate dielectric leakage.

• Bulletin of the Korean Chemical Society
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• 제24권6호
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• pp.792-796
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• 2003

Polychlorinated dibenzo-p-dioxins (PCDDs) are extremely toxic and persistent environmental pollutants. Their chemical reactivities and other physicochemical/biological properties show a strong dependence on the chlorination pattern. With increasing the number of chlorines, dioxin congeners become more electronegative and gain higher electron affinities. The vertical electron affinities (VEA) are related with the LUMO energies of neutral molecules. LUMO energies of all PCDD congeners were calculated at the B3LYP/6-31G** level and those of some selected congeners at the level of B3LYP/6-311G**//B3LYP/6-31G** and B3LYP/cc-pvtz/ /B3LYP/6-31G**. The total energies of neutral and anionic species for dibenzo-p-dioxins (DD), 1469-TCDD, 2378-TCDD, and OCDD were calculated at the level of B3LYP/6-31G**, B3LYP/aug-cc-pvdz, and B3LYP/ aug-cc-pvtz//B3LYP/6-31G**. By using the four congeners with D2h symmetry as reference molecules, we could estimate VEA (B3LYP/aug-cc-pvdz) of 75 PCDD congeners based on the linear correlations between LUMO energy and VEA (B3LYP/6-31G**) and between VEA (B3LYP/6-31G**) and VEA (B3LYP/aug-ccpvtz// B3LYP/6-31G**). Results show that all PCDDs with the number of Cl ≥ 3 have positive electron affinities. The PCDD electron affinity values provided in this work can be a useful data set in understanding the congener-specific reactivities of dioxins in various environmental media.

• Bulletin of the Korean Chemical Society
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• 제30권1호
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• pp.205-213
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• 2009

This study reports on the geometry optimizations and electronic structure calculations for methyl pyropheophorbide (MPPa), tropolonyl methyl pyropheophorbides (TMPPa, ITMPPa), and cationic tropolonyl methyl pyropheophorbides ($TMPPa^+{{\cdot}BF_4}^-,\;ITMPPa^+{{\cdot}BF_4}^-,\;TMPPa^+,\;and\;ITMPPa^+$) using Local Spin Density Approximation (LSDA/ 6-31G*) and the Restricted Hatree-Fock (RHF/6-31G*) level theory. From the calculated results, we found that substituted cationic tropolonyl groups have larger structural effects than those of substituted neutral tropolonyl groups. The order of structural change effects is $ITMPPa^+ > ITMPPa^+{{\cdot}BF_4}^-$ > ITMPPa, as a result of the isopropyl group. Because it is an electron-releasing group, the substituted isopropyl group electronic effect on a 3-position tropolone increases the Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital (HOMO-LUMO) energy gap. It was constituted that the larger the cationic characters of these photosensitizers, the smaller the HOMOLUMO band gaps are. The orbital energies of the cationic systems and the ions are stronger than those of a neutral system because of a strong electrostatic interaction. However, this stabilization of orbital energies are counteracted by the distortion of chlorin macrocycle, which results in a large destabilization of chlorin-based compound HOMOs and smaller destabilization of LUMOs as shown in TMPPa (ITMPPa), $TMPPa^+{{\cdot}BF_4}^- (ITMPPa^+{{\cdot}BF_4}^-),\;and\;TMPPa^+\;(ITMPPa^+)$ of Figure 6 and Table 6-7. These results are in reasonable agreement with normal-coordinate structural decomposition (NSD) results. The HOMO-LUMO gap is an important factor to consider in the development of photodynamic therapy (PDT).

• 대한화학회지
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• 제53권3호
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• pp.244-256
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• 2009

소랄렌과 피리디민 염기와의 $C_4$-고리화 부가반응을 통한 가닥내 교차 결합(interstrand crosslinking) 에 의해 만들어진 소랄렌 복합체를 ab initio 방법인 HF와 DFT 6-31G 방법을 이용하여 연구하였다. 소랄렌과 피리디민 염기에 의해 만들어진 광생성물인 8-MOP< >Thy, Ps< >Cyt, Ps< >Thy, Ps< >Ps, Thy< >(3, 4)Ps(12, 13)< >Thy의 최적화된(optimized) 구조를 알 수 있으며, 8-MOP< >Thy은 (trans-syn) 구조, Ps(3, 4)< >Cyt은 (trans-anti) 구조, Ps(12, 13)< >Cyt은 (trans-anti) 구조, Ps(3, 4)< >Thy은 (trans-syn) 구조, Ps(12, 13)< >Thy은 (trans-syn) 구조가 가장 유리하다. Ps(3, 4)< >Thy과 Ps(12, 13)< >Thy의 Gibbs 자유 에너지 변화(${\Delta}{G^{\circ}}$)를 비교하면 Ps(12, 13)< >Thy의 광생성물(단일부가 생성물)이 이루어진 뒤에 Ps(3, 4)< >Thy의 광고리화 부가반응 생성물(이중부가 생성물)을 형성한다는 사실을 알 수 있다. Bispsoralen(psoralen dimer)에서는 Ps(12, 13)< >Ps(12, 13)(trans-anti) 구조가 가장 유리하며, Thy< >(3, 4)Ps(12, 13)< >Thy에서는 (cis syn)(cis anti) 형태가 가장 유리하다.

• 전기화학회지
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• 제16권3호
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• pp.177-183
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• 2013

고전압에서도 사용 가능한 바인더 개발에 대한 요구가 증대됨에 따라 이에 적합한 내산화성이 우수한 바인더를 양자화학적 모델링에 기반하여 제안하고자 하였다. 각 고분자 poly(acryl amide)(PAM), poly(methyl acrylate)(PMA), poly(vinylidene fluoride)(PVDF), poly(hexafluropropylene)(PHFP)에 대하여 반경험적 방법(Semi-empirical method) 및 밀도범함수 이론(Density Functional Theory, DFT) 방법을 이용하여 단량체부터 사량체까지의 고분자 바인더에 대한 최고 점유 분자 궤도함수(Highest occupied molecular orbital, HOMO) 에너지와 이온화 에너지(Ionization Potential, IP) 값을 구하여 실험 값과 비교하였다. 밀도범함수 방법으로 해석한 결과, PHFP, PVDF, PMA, PAM 순으로 고분자의 내산화성이 좋은 것으로 시뮬레이션을 통해 예측되었고, 이러한 결과는 선형 훑음 전압-전류법(Linear Sweep Voltametry, LSV)으로부터 얻은 실험값과 일치하였다. 또한 이 결과는 HOMO 오비탈의 구조를 분석하여 내산화성이 좋은 원인을 규명하였다.

• Korean Chemical Engineering Research
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• 제55권5호
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• pp.704-710
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• 2017

가스 하이드레이트 형성원리를 이용한 해수담수화는 이미 상용화된 역삼투 방식에 비하여 아직 실증화 단계이지만 그 공정이 비교적 단순하고 특히 냉매를 객체가스로 사용할 경우 아주 낮은 공정 온도가 필요하지 않아 에너지 소비량(thermal budget)이 향상될 가능성이 있기 때문에 여전히 많은 관심을 받고 있다. 따라서 본 연구에서는 수소불화탄소(HFC, hydrofluorocarbon) 및 수소염화불화탄소(HCFC, hydrochlorofluorocarbon) 계열의 냉매들을 객체가스로 한 가스 하이드레이트 형성 거동을 에너지적인 관점에서 해석하고자 하였고 이를 위해 밀도 범함수(DFT, density functional method) 이론을 기반으로 한 분자모델링을 도입하였다. 객체가스(guest gas)로 R-134a, R-227ea, R-236fa, R-141b를 선정하였으며 계산을 위하여 물 분자로 이루어진 $5^{12}$, $5^{12}6^2$, $5^{12}6^4$의 세가지 구조의 동공들(cavities)을 구성하였다. 동공, 객체가스, 그리고 객체가스가 삽입된 동공의 구조를 분자모델링을 이용하여 각각 최적화하였고 계산된 각 구조의 에너지로부터 동공과 객체가스의 결합에너지(binding energy)를 계산하였다. 마지막으로 결합에너지를 비교함으로써 어느 냉매가 가장 유리한 조건에서 가스하이드레이트를 형성할 지를 판단하였다. 결과적으로 R-236fa가 가장 자발적(spontaneous)으로 가스 하이드레이트를 형성할 것으로 예상되었고 사람에 대한 낮은 독성과 물에 대한 작은 용해도 측면에서도 가장 적절한 선택으로 평가되었다.