• 대한화학회지
• /
• 제58권6호
• /
• pp.513-516
• /
• 2014

We measure the conductance of a 4,4'-diaminobiphenyl formed with Ni electrodes using a scanning tunneling microscope-based break-junction technique. For comparison, we use Au or Ag electrodes to form a metal-molecular junction. For molecules that conduct through the highest occupied molecular orbital, junctions formed with Ni show similar conductance as Au and are more conductive than those formed with Ag, consistent with the higher work function for Ni or Au. Furthermore, we observe that the measured molecular junction length that is formed with the Ni or Au electrodes was shorter than that formed with the Ag electrodes. These observations are attributed to a larger gap distance of the Ni or Au electrodes compared to that of the Ag electrodes after the metal contact ruptures. Since our work allows us to measure the conductance of a molecule formed with various electrodes, it should be relevant to molecular electronics with versatile materials.

• Bulletin of the Korean Chemical Society
• /
• 제26권8호
• /
• pp.1177-1184
• /
• 2005

Ab initio quantum chemical molecular orbital calculations have been performed for the unimolecular decomposition of methacrylonitrile ($CH_3C(CN)=CH_2$), especially for HCN and $H_2$ molecular elimination channels. Structures and energies of the reactants, products, and relevant species along the individual reaction pathways were determined by MP2 gradient optimization and MP4 single point energy calculations. Direct four-center elimination of HCN and three-center elimination of H2 channels were identified. In addition, H or CN migration followed by HCN or H2 elimination channels via the methylcyanoethylidene intermediate was also identified. Unlike the case of crotonitrile previously studied, in which the dominant decomposition process was the direct three-center elimination of HCN, the most important reaction pathway should be the direct threecenter elimination of $H_2$ in the case of methacrylonitrile.

• Bulletin of the Korean Chemical Society
• /
• 제26권11호
• /
• pp.1706-1710
• /
• 2005

The excited-state intramolecular proton transfer (ESIPT) fluorescence in the 2-(2'-hydroxyphenyl)benzoxazole (HBO) derivatives with different electron donor and acceptor substituents was studied by spectroscopic and theoretical methods. Changes in the electronic transition, energy levels, and orbital diagrams of HBO analogues were investigated by the semi-empirical molecular orbital calculation and were correlated with the experimental spectral position of ESIPT keto emission. It was found that the presence of substituents, regardless of their nature, resulted in the red-shifted absorption relative to HBO. However, the spectral change of the ESIPT fluorescence was differently affected by the nature of substituent: hypsochromic shift with electron donor and bathochromic shift with electron acceptor.

• Bulletin of the Korean Chemical Society
• /
• 제28권12호
• /
• pp.2369-2376
• /
• 2007

N2O has been found from experimental and theoretical considerations to bind on-top to the Pd(110) surface in a tilted end-on fashion via its terminal N atom. We use a frontier orbital description of the bonding interactions in the Pd-N2O system to obtain molecular insight into the catalytic mechanism of the activation of N2O by the Pd(110) surface giving rise to the formation of N2 and O on the surface. For the tilted end-on N2O binding mode, the LUMO 3π of N2O has good overlap with the Pd dσ and dπ orbitals which can serve as the electron donors. The donor-acceptor orbital overlap is favorable for electron transfer from Pd to N2O and is expected to dominate the surface reaction pathway of N2O decomposition.

• 대한화학회지
• /
• 제16권3호
• /
• pp.142-148
• /
• 1972

IMO 이론을 naphthalene, phenanthrene 및 1,2-benzanthracene 과 maleic anhydride 와의 Diels-Alder 반응에 적용하였다. Alternant hydrocarbon 에서 열적 부과반응이 일어 나리라 예상되는 위치가 실험결과와 잘 일치하였다. 계산된 안정화 에너지로부터 부가생성률 중의 이성체들의 상대적양이 설명되었으며 또 Brown 의 paralocalization 에너지와 안정화에너지 사이에 좋은 직전성이 있음을 알았다.

• 대한화학회지
• /
• 제45권3호
• /
• pp.207-212
• /
• 2001

1.4-Diaza-1,3-butadiene(1,4-DAB)과 acrolein 과의 Diels-Alder 반응의 반응성을 조사하기 위해 1,4-DAB의 말단 두 질소에 전자를 끌거나 주는 기들을 치환시킨 여러 치환 1,4-DAB들에 대해 DFT계산을 수행하였다. 최적화된 구조들에 대해 계산된 FMO(Frontier Mole cular Orbital) 에너지로서 상대적인 반응성을 설명할 수 있었다. 즉, 전자를 주는 기가 치환될 경우에는 normal electron demand 반응이 유리하며, 전자를 끄는 기가 치환될 경우에는 inverse electron demand 반응 메카니즘으로 진행됨을 알 수 있었다.

• 대한화학회지
• /
• 제29권1호
• /
• pp.3-8
• /
• 1985

Density functional theory에서의 전기음성도의 정의에 의해 전기음성도를 가전자(valence electron)만을 고려하여 구하였다. 전기음성도는 Hartree-Fock의 궤도 에너지(orbital energy)로 표시되는데 이로부터 구한 분자의 전기음성도 값은 다른 값들과 좋은 상관관계(correlation)를 보여준다. 또한 분자를 형성할 때 Sanderson의 법칙(electronegativity equalization principle)이 만족됨을 보였다.

• Bulletin of the Korean Chemical Society
• /
• 제34권2호
• /
• pp.415-420
• /
• 2013

Effect of chemical substitution at the para-position of the thiophenoxyl radical has been theoretically investigated in terms of energetics, structures, charge densities and orbital shapes for the ground and first electronically excited states. It is found that the adiabatic energy gap increases when $CH_3$ or F is substituted at the para-position. This change is attributed to the stabilization of the ground state of thiophenoxyl radical through the electron-donating effect of F or $CH_3$ group as the charge or spin of the singly-occupied molecular orbital is delocalized over the entire molecule especially in the ground state whereas in the excited state it is rather localized on sulfur and little affected by chemical substitutions. Quantitative comparison of predictions based on four different quantum-mechanical calculation methods is presented.

• 대한화학회지
• /
• 제20권2호
• /
• pp.136-140
• /
• 1976

오르토-터페닐계 화합물들의 광화학적 탈수소 고리화 반응을 섭동분자궤도론적으로 다루었으며 일차 들뜬 상태의 가동결합차수가 반응성의 좋은 지표가 될 수 있음을 밝혔다. 이 해석은 Woodward-Hoffman의 궤도함수 대칭성 보존법칙에도 부합된다.

• Bulletin of the Korean Chemical Society
• /
• 제5권1호
• /
• pp.37-41
• /
• 1984

The pseudolattice calculations in the CNDO/2 level of approximation are carried out for polymeric beryllium hydride, polyethylene and polymeric boron hydride. Since there is no evidence on the geometry for polymeric boron hydride, the two possible geometries are assumed. One is a polyethylene-type geometry and the other is a polymeric beryllium hydride-type geometry. In order to compare their relative stability, we calculate polyethylene and polymeric beryllium hydride and then compare with polymeric boron hydride having the assumed structures. The total energy calculation indicates that a polymeric beryllium bydride-type geometry is more stable than a polyethylene-type geometry. Our results obtained for polyethylene are in good agreement with those given by CNDO/2 crystal orbital. From the convergence problem with respect to the number of unit cells (M), the calculation with value of 4 for M can be considered to give the convergence limit results.