• Bulletin of the Korean Chemical Society
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• 제27권12호
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• pp.1997-2001
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• 2006

The photodissociation dynamics of formic acid (HCOOH) at 206 nm have been investigated from rotationally resolved laser induced fluorescence spectra of OH ($^2\Pi$) fragments produced exclusively in the ground state. From the spectra, the rotational energy of the fragments was measured to be $820\;{\pm}\;50\;cm^{-1}$. The translational energy released in the products, which is 87% of the total available energy of the system, was also measured from analyses of the Doppler profiles. Joining these data with quantum chemical molecular orbital calculations, we have concluded that the dissociation should take place along the S1 surface with an exit channel barrier and also that the energy partitioning is determined at the exit channel.

• 대한화학회지
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• 제21권1호
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• pp.16-22
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• 1977

$CH_3COCl$의 카르보닐 탄소에서의 $Cl^-$의 2분자 치환반응을 여러 거리에서 전방 및 후방 공격할 때의 에너지 변화(EHT)와 전자분포(CNDO/2)를 계산함으로서 분자궤도론적으로 연구하였다. 다른 실험 및 MO결과들과 함께 검토하여 본 결과 이 치환반응은 $S_N2-$보존형 메카니즘을 알았다.

• Bulletin of the Korean Chemical Society
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• 제6권2호
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• pp.103-107
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• 1985

Quantum chemical calculations were carried out to explain how the electronic states of some series compounds vary with metabolic activation. Compounds studied included aromatic amines and amides, polycyclic hydrocarbons, and a few alkylating agents that do not require metabolic activation. The 1, 2 and 4 positions forming the trans-butadiene frame of a molecule, henceforth referred to as "the trans 1, 2, 4 region", have seen to be important positions for the prediction of carcinogenic activity of these compounds. It is also evident that their electrophilic properties increase with metabolic activation. That is the sum of ${\pi}$-electron densities of the trans 1, 2, 4 region in the lowest unoccupied molecular orbital (LUMO) has been found to increase in the order of precarcinogens < proximate-ones < the carbocation ultimate-ones. This is consistent with the fact that chemical carcinogens become more strongly electrophilic with activating. This region not only provides a unified view of structurally diverse carcinogens, but also predicts uniformity in their reactive sites. Accordingly, we suggest that an understanding of the trans 1, 2, 4 region would be helpful in elucidating the mechanism of carcinogenesis.

• Bulletin of the Korean Chemical Society
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• 제19권1호
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• pp.110-114
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• 1998

The hydrogen-bonding interactions between thioacetamide (TA) and urea derivatives such as tetramethylurea (TMU) and dimethyldiphenylurea (DMDPU) have been studied using near-infrared absorption spectroscopy. Thermodynamic parameters for the interactions between TA and urea derivatives were determined by analyzing the $v^{as}_{N-H}$+Amide Ⅱ combination band of TA at 1970 nm. The ΔH° values, indicating the intrinsic strength of hydrogen bonding, are - 23.0 kJ/mole and - 19.8 kJ/mol for TMU and DMDPU, respectively. This is well explained by the inductive effects of substituents. Ab initio molecular orbital calculations for the proton affinity of TMU, N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA) in gas phase have been carried out at HF/3-21G ad HF/6-31G(d) levels, showing that the proton affinity of TMU is larger than that of DMA, which agrees well the experimental results.

• Bulletin of the Korean Chemical Society
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• 제2권4호
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• pp.132-138
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• 1981

Ab initio molecular orbital calculations have been performed in an effort to determine which types of chemical interactions play essential roles for the system, , $H_2O+CH_2SH^+$, and $H_2O+ CH_2S$. The most important contribution to the interaction energy in controlling reaction path is the exchange repulsion energy, EX, which is largely responsible for the shape of the total interaction energy curve. In the ion-molecule reaction, prior protonation of thioformaldehyde or prior deprotonation of water leads to formation of the corresponding ionic adducts ($H_2O+CH_2SH$ and $HOCH_2S^-$), with no barrier to reaction, simulating specific acid and base catalysis, respectively, as in the case of formaldehyde. Otherwise, approach of water to thioformaldehyde gives rise to a completely repulsive interaction.

• Bulletin of the Korean Chemical Society
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• 제15권11호
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• pp.933-939
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• 1994

Using an atom superposition and electron delocalization molecular orbital (ASED-MO) method, we have investigated the vibrational and chemisorptive properties of adsorbates on a Pt(100) surface during CO oxidation. The calculated vibrational stretching frequency for a predicted structure of $[CO{\cdot}{\cdot}{\cdot}O]^*$ complex is 1642 $cm^{-1}$. The CO bond stretches by 0.05 ${\AA}$ when adsorbed on one-fold site, and is tilted by 30 ${\AA}$ from the surface normal. We find the decrease in CO vibrational frequency on going from the one-fold to the high coordination sites. Binding at the two-fold site is predicted to be favored for $Pt_{18}(100)$ and at the 1-fold site for $Pt_{23}(100)$. From the calculations of the steric interactions, we have found that pre-adsorbed oxygen modifies the surface so that CO is adsorbed on the one-fold site ordered in a $(\sqrt{2}{\times}{\sqrt}{2})R45^{\circ}$. Our results are in good agreement with recent experimental findings of Hong et al. [J.Phys. Chem. 1993, 97, 1258].

• Bulletin of the Korean Chemical Society
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• 제18권10호
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• pp.1094-1099
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• 1997

The rotational barriers of thioacetamide (TA) and acetamide (AA) were studied using the ab-initio molecular orbital theory and NMR spectroscopy. The calculated rotational barriers using MP2/6-31G**//MP2/6-31G** for TA was 72.26 kJ/mol and 58.19 kJ/mol for AA, respectively. These results are good agreement with the experimental data. The tendency for the change of structural parameters is consistent with the result of formamide. In both amides, the rotational barrier arises from the pyramidalization of nitrogen. The chemical shifts of both amides are shifted upfield when temperature is raised, which confirms pyramidalization of nitrogen. The lineshape of 1H-NMR spectra of TA shows quintet which is contributed from two triplet spectra. This means that the distribution of electrons around the nitrogen is rather symmetric. Ab-initio calculations of electric field gradient for both amides confirm the above results. The above experimental results are well understood by Keith's view on thioamides, which excludes the contribution of resonance structure and considers the origin of rotational barrier to be the same in both thioamides and in corresponding amides.

• Bulletin of the Korean Chemical Society
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• 제19권12호
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• pp.1314-1319
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• 1998

Electronic structures and properties of the organometallic precursors [Me2AlNHR]2 (R =Me, iPr, and tBu) have been calculated by the semiempirical (ASED-MO, MNDO, AM1 and PM3) methods. Optimized structures obtained from the MNDO, AM1, and PM3 calculations indicate that the N-C bond lengths are considerably affected by the change of the R groups bonded to nitrogen, but the bond lengths of the Al-N and Al-C bonds are little affected. This result is useful in explaining the experimental results for the elimination of the R groups bonded to nitrogen, and could serve as a guide in designing an optimum precursor for the AlN thin film formation.

• Bulletin of the Korean Chemical Society
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• 제17권7호
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• pp.612-616
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• 1996

The new 1,2-dithiolene, 1,4-butanediyldithioethylene-1,2-dithiolate (BDDT2-), has been isolated. In addition, new monoanionic bis-complexes with nickel and copper have been prepared and characterized. In order to investigate the detailed electronic structure of the metal complexes of the new ligand, BDDT2-, in terms of the oxidation state of the central metal ions, we have carried out molecular orbital (MO) calculations of Ni(BDDT)2-and Cu(BDDT)2- utilizing an Extended Huckel method. Cyclic voltammetry data for both complexes were obtained with a potentiostat. We have also compared these results to the previously synthesized Ni(PDDT)2-, Ni(DDDT)2-,Cu(PDDT)2-, and Cu(DDDT)2-.

• Bulletin of the Korean Chemical Society
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• 제14권2호
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• pp.191-195
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• 1993

Molecular orbital calculations at the extended Huckel level have been carried out for an electron deficient cluster, $Tl_3(FeL_3)_2{(FeL_4)_3}^{-3}$, where L=CO or $H^-$. The LUMO, $2a_2$", is destabilized by the secondary interaction of the LUMO with $1a_2$" on $(FeL_3)_2$ fragment. This is one of six skeletal bonding orbitals which are associated with $Tl-FeL_3$ bonds. Overlap population analysis has been applied to account for two kinds of Tl-Fe bonds. Replacement of the terminal $C_{3v}$, $FeL_4$, by the $C_{2v}$, $FeL_4$ units in cluster results in slight energy stabilization of the cluster.