• Bulletin of the Korean Chemical Society
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• 제12권6호
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• pp.674-678
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• 1991

Methanolysis of a ${\beta}$-lactam ring of a cephalosporin was simulated with AM1 semiempirical quantum mechanical calculation. The tetrahedral intermediate TD1 from an O-protonated cephalosporin and a methanol transfers the proton intramolecularly to the C-4 carboxylate to generate an oxyanion, i.e., second tetrahedral intermediate TD2, which undergoes the amide bond cleavage without further protonation on the N-5. For this cleavage a low-energy barrier TS2 was located. According to the energy diagram, tetrahedral intermediates easily undergo ring cleavage even without the protonation on the amide nitrogen.

• EDISON SW 활용 경진대회 논문집
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• 제4회(2015년)
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• pp.91-98
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• 2015

Formamide의 중성가수분해 mechanism은 QM/MM (quantum mecahnics/molecular mechanics) molecular dynamics simulations 및 CPMD과 같은 방법으로 연구되어왔다. 본 연구에서는. Umbrella sampling을 이용한 QM/MM-MD simulation을 사용하여 4가지 반응의 free energy surface를 도출해냈다. 전체적으로, 가장 선호되는 메커니즘은 two step으로 구성된 water assisted stepwise mechanism이었으며 모든 mechanism은 ab-initio calculation과 QM/MM-MD simulation이 수행되었다. water assisted stepwise mechanism을 살펴보면, 첫 번째 step에서 formamide의 carbonyl group이 hydrate되면서 gem-diol intermediate를 형성한다. 다음 step에서, intermediate의 hydroxyl group으로부터 amino group으로 water-assisted proton transfer이 일어난다. 두 반응 모두에서 물이 proton transfer를 직접적으로 도와주는 것을 관찰할 수 있었다. 특히, ab-initio calculation과는 다르게 QM/MM-MD에서는 gem-diol intermediate가 안정화되는 것으로 solvent effect를 잘 보여준다.

• Bulletin of the Korean Chemical Society
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• 제33권12호
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• pp.4098-4102
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• 2012

The potential energy surface (PES) for the loss of HCN from the pyrimidine molecular ion has been explored using quantum chemical calculations. Possible reaction pathways to form five $C_3H_3N^{+{\bullet}}$ isomers have been obtained with Gaussian 4 model calculations. The rate constant for the HCN loss and the product branching ratio have been calculated using the Rice-Ramsperger-Kassel-Marcus theory on the basis of the obtained PES. The resultant rate constant agrees with the previous experimental result. By a kinetic analysis, it is proposed that the formation of $CH=CHC{\equiv}NH^{+{\bullet}}$ is favored near the dissociation threshold, while the formation of $CH=CHN{\equiv}CH^{+{\bullet}}$ is favored at high energies.

• 대한화학회지
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• 제55권5호
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• pp.733-740
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• 2011

Multiple regression analysis was used for the calculation of pKa values of 15 substituted benzaldoximes by using various types of descriptors as parameters. These descriptors are based on quantum mechanical treatments. They were derived by employing semi-empirical calculation represented by the PM3 model and an Abinitio method expressed by Hartree-Fock(HF) model performed at the 6-311 G(d, p) level of theory. The parameters tested for their ability to represent the variations observed in the experimental pKa(s) are atomic and structural properties including Muliken charges on the atoms of hydroxyl group and C=N bond, the angle $C_6-C_1-C_7$, and length of O-H bond. Molecular properties are also used like energies of HOMO and LUMO, hardness(${\eta}$), chemical potential(${\mu}$), total energy(TE), dipole of molecule(DM), and electrophilicity index(W). The relation between pKa values and each of these parameters of the studied compounds is investigated. Depending on these relations, two sets of parameters were constructed for comparison between the PM3 and HF methods. The results obtained favor the Abinitio method for such applications although both models proved to have high predictive power and have sufficient reliability to describe the effect of substituents on pKa values of benzaldoxime compounds under consideration which is clear from the values of correlation coefficient $R^2$ obtained and the consistency between the experimental and the calculated values.

• 대한전자공학회논문지SD
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• 제42권3호
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• pp.9-16
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• 2005

[ $Si_{1-x}Ge_x$ ] 위의 Si 반전층에서의 이동도를 반전층에서의 양자현상(버금띠 에너지와 파동함수)과 완화시간어림셈을 고려하여 계산하였다. 반전층에서의 양자현상은 슈뢰딩거 방정식과 포아슨 방정식을 자체 모순없이 계산하여 얻었다 완화시간은 밸리내 산란과 밸리사이 산란을 고려하여 계산하였다. 그 결과 Ge 함량이 증가됨에 따라 이동도가 증가되는 이유는 4-폴드 밸리에 존재하는 전자의 이동도보다 2-폴드 밸리에 존재하는 전자의 이동도가 약 3배 정도 크며 대부분의 전자가 밸리의 분리에 의해 2-폴드 밸리에 존재하기 때문이라는 것을 알 수 있었다. 한편, 포논 산란만을 고려한 이동도를 실험치와 일치시키기 위하여 전체 이동도에는 반전층 계면에서의 산란과 쿨롱 산란을 포함시켰다. 계산된 전계, 온도, 그리고 Ge 함량에 의존하는 이동도는 실험치와 근접하도록 변형포텐셜을 설정하였으며 정확한 결과를 위해서는 Si 에너지띠의 비포물성을 고려해야함을 확인하였다.

• Bulletin of the Korean Chemical Society
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• 제23권2호
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• pp.229-240
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• 2002

Quantum mechanical calculation is performed for the $O(^1D)$ + HCl ${\rightarrow}$OH + Cl reaction using Reactive Infinite Order Sudden Approximation. Shifting approximation is also employed for the l ${\neq}$ 0 partial wave contributions. Various dynamical quantities are calculated and compared with available experimental results and quasiclassical trajectory results. Vibrational distributions agree well with experimental results i.e. product states mostly populated at $v_f$ = 3, 4. Our results also show small peak at $v_f$ = 0, which indicates bimodal vibrational distribution. The results show two significant broad peaks in ${\gamma}_i$ dependence of the cross section, one is at ${\gamma}_i$ = $15^{\circ}-35^{\circ}$ and the another is at ${\gamma}_i$= $55^{\circ}-75^{\circ}$ which can be explained as steric effects. At smaller gi, the distribution is peaked only at higher state ($v_f$ = 3, 4) while at the larger gi, both lower state ($v_f$ = 0) and higher state ($v_f$ = 3, 4) are significantly populated. Such two competing contributions (smaller and larger ${\gamma}_i$) result in the bimodal distribution. From these points we suggest two mechanisms underlying in current reaction system: one is that reaction occurs in a direct way, while the another is that reaction occurs in a indirect way.

• JSTS:Journal of Semiconductor Technology and Science
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• 제4권1호
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• pp.63-73
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• 2004

A simple analytical model has been presented for the study of the optical bistability using a $GaAs-Al_{0.32}Ga_{0.68}As$ multiple quantum well (MQW) p-i-n diode structure. The calculation of the optical absorption is based on a semi-emperical model which is accurately valid for a range of wells between 5 and 20 nm and the electric field F< 200kV/cm . The electric field dependent analytical expression for the responsivity is presented. An attempt has been made to derive the analytical relationship between the incident optical power ( $(P_{in})$ ) and the voltage V across the device when the diode is reverse biased by a power supply in series with a load resistor. The relationship between $P_{in}$ and $P_{out}$ (i.e. transmitted optical power) is also presented. Numerical results are presented for a typical case of well size $L_Z=10.5nm,\;barrier\;size\;L_B=9.5nm$ optical wave length l = 851.7nm and electric field F? 100kV/cm. It has been shown that for the values of $P_{in}$ within certain range, the device changes its state in such a way that corresponding to every value of $P_{in}$ , two stable states and one unstable state of V as well as of $P_{out}$ are obtained which shows the optically controlled bistable nature of the device.

• Bulletin of the Korean Chemical Society
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• 제30권1호
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• pp.67-76
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• 2009

The development of a variety of methods like AM1, PM3, PM5 and DFT now allows the calculation of atomic and molecular properties with high precision as well as the treatment of large molecules with predictive power. In this paper, these methods have been used to calculate a number of quantum chemical descriptors (like Klopman atomic softness in terms of $E_n^{\ddag}\;and\;E_m^{\ddag}$, chemical hardness, global softness, electronegativity, chemical potential, electrophilicity index, heat of formation, total energy etc.) for 75 alkanes to predict their boiling point values. The 3D modeling, geometry optimization and semiempirical & DFT calculations of all the alkanes have been made with the help of CAChe software. The calculated quantum chemical descriptors have been correlated with observed boiling point by using multiple linear regression (MLR) analysis. The predicted values of boiling point are very close to the observed values. The values of correlation coefficient ($r^2$) and cross validation coefficient ($r_{cv}^2$) also indicates the generated QSPR models are valuable and the comparison of all the methods indicate that the DFT method is most reliable while the addition of Klopman atomic softness $E_n^{\ddag}$ in DFT method improves the result and provides best correlation.

• 한국광학회:학술대회논문집
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• 한국광학회 1995년도 광학 및 양자전자학 워크샵 논문집
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• pp.127-137
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• 1995

Anomalously large real space charge transfer through thick barries in GaAs asymmetric double quantum wells is studied by photoluminesence exitation. This inter-well excitonic transfer is very large when the barrier is the Al0.3Ga0.7As alloy, but disappears when the barrier is GaAs/AlAs digital alloy with an equivalent Al concentration of 0.28. These resilts combined with observed x and barrier thickness depence suggest that the spatial fluctuation of the atomic arrangment of Ga and Al in the alloy may be responsible for this transfer. This picture is supported by the quantum mechanical calculation in three dimensions which takes into account the side fluctuation effects.

• 한국산학기술학회논문지
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• 제19권7호
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• pp.95-104
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• 2018

이성질체의 형태는 수용액 상태에서 종종 안정성과 반응성 등의 기본상태 뿐만 아니라 사슬성장 및 접힘 과정으로 인하여 형태형성에 영향을 주기 때문에 올리고펩티드의 형태를 이해하는 것이 중요하다. 본 논문에서는 L-알라닌(LA), 글리신(G) 5량체 모델의 무수 및 수화물(수화율; h/1) 상태의 구조와 에너지를 4가지 형태이성질체 (베타-확장형;= t-/t+, $PP_{II}$형; g-/t+, $PP_{II}$-유사형; g-/g+ 및 알파-나선형; g-/g-)에 대하여 B3LYP/6-31G(d,p)를 이용하여 양자화학계산(QCC) 방법으로 분석하였다. 구조최적화는 밀도함수 이론(DFT)으로써 B3LYP를 사용하였으며, 기본설정(Basic set)으로는 6-31G(d,p)를 이용하였다. 이미노 양성자(NH)를 갖는 LA와 G에서 베타-확장형, $PP_{II}$-유사형, 알파-나선형의 3가지 형태가 얻어졌으며, 대부분 물 분자가 $PP_{II}$-유사형과 알파-나선형에서는 CO-HN 분자 내 수소결합 사이에 주로 삽입되었고, 베타-확장형은 CO기에 부착되었다. 또한, LA와 G에서 $PP_{II}$-유사형 형태이성질체가 무수 및 수화물 상태에서 가장 안정적이었으며, $PP_{II}$ 형태이성질체는 얻어지지 않았다. LA에 대한 결과는 알라닌 올리고펩티드의 안정적인 형태가 주로 $PP_{II}$라고 보고한 다른 연구의 실험적 및 이론적인 결과와는 상이했다. 올리고펩티드 형태이성질체의 생성패턴과 안정성이 CO-HN의 분자 내 수소결합의 존재 여부 또는 출발 아미노산 내 $NH_2$기의 존재 여부에 강한 영향을 받는 것을 알 수 있었다.