• Journal of the Korean Chemical Society
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• v.54 no.4
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• pp.363-373
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• 2010

Computational studies were carried out on the opiates morphine, heroin, codeine, pentazocine, and buprenorphine, under the density functional theory. The geometric parameters of the pharmacophore and substituents were evaluated at the B3LYP/6-31+G(d) level of theory. The electronic structure calculations were performed using the same hybrid functional at the B3LYP/6-311++G (d,p) level of theory. The atomic charges were obtained by Mulliken population analysis. Given the reported biological activity, calculated partition coefficients, and electronic and geometric analysis, pentazocine and buprenorphine were chosen as models for proposed analogues. These analogues were then studied and compared with the model molecules. The study reveals that the geometry and electronic structure of the pharmacophore remains consistent in the presence of different substituents. Because the proposed analogues preserve the studied properties of the model molecules, it is likely that these analogues display biological activity.

• Journal of the Korean Chemical Society
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• v.55 no.1
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• pp.7-13
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• 2011

As an aid towards improving the treatment of exchange and correlation effects in electronic structure calculations, it is desirable to have a clear picture of concepts of exchange-correlation functionals in computational calculations. For achieving this aim, it is necessary to perform different theoretical methods for many groups of materials. We have performed hybrid density functional theory (DFT) methods to investigate the density charges of atoms in rings and cages of carbon nanotube. DFT methods are engaged and compared their results. We have also been inclined to see the impression of exchange and correlation on nuclearnuclear energy and electron-nuclear energy and kinetic energy. With due attention to existence methods, B3P86, B3PW91, B1B96, BLYP and B3LYP have used in this work.

• Journal of the Korean Chemical Society
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• v.61 no.1
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• pp.16-24
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• 2017

The adsorption of various atmospheric harmful gases ($CO_x$, $NO_x$, $SO_x$) on graphene-like boron nitride(BN) and aluminum nitride(AlN) sheets was theoretically investigated using density functional theory (DFT) and MP2 methods. The structures were fully optimized at the $B3LYP/6-31G^{**}$ and $CAM-B3LYP/6-31G^{**}$ levels of theory and confirmed to be a local minimum by the calculation of the harmonic vibrational frequencies. The MP2 single-point binding energies were computed at the $CAM-B3LYP/6-31G^{**}$ optimized geometries. Also the zero-point vibrational energy (ZPVE) and 50%-basis set superposition error (BSSE) corrections were included. The adsorptions of gases on the BN sheet were predicted to be a physisorption process and the adsorptions of gases on the AlN sheet were predicted to be a physisorption process for $CO_x$ and $NO_x$ but to be a chemisorption process for $SO_x$.

• Journal of the Korean Chemical Society
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• v.60 no.6
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• pp.410-414
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• 2016

The anharmonic frequency of a local OH stretching mode of a water monomer and dimer was calculated using various levels of density functional theory. The quantum chemical potential energy curves as a function of the OH bond distance were calculated, and they were fitted with the Morse potential function to analytically obtain the fundamental transition frequency. By comparing those values with the frequencies similarly calculated using an ab initio quantum chemical method, the coupled cluster theory including both single and double excitations with the perturbative inclusion of triple excitation in the complete basis limit, the accuracy of various density functional methods in the calculation of anharmonic vibration frequency of water molecules was assessed. For a water monomer, X3LYP and B3LYP methods give the best accuracy, whereas for a water dimer, B972, LCBLYP, ${\omega}B97X$, ${\omega}B97$ methods show the best performance.

• Bulletin of the Korean Chemical Society
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• v.24 no.6
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• pp.853-858
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• 2003

The reaction mechanism of the pyrolysis of sulphonyl oximes ($CH_3-C_6H_4-S(O)_2O-N=C(H)-C_6H_4Y$), in the gas phase is studied theoretically at HF/3-21G, ONIOM (B3LYP/6-31G**:HF/3-21G) and ONIOM (MP2/6- 31G**:HF/3-21G) levels. All the calculations show that the thermal decomposition of sulphonyl oximes is a concerted asynchronous process via a six-membered cyclic transition state. The activation energies (Ea) predicted by ONIOM (B3LYP/6-31G**: HF/3-21G) method are in good agreement with the experimental results for a series of tosyl arenecarboxaldoximes. Five para substituents, Y = $OCH_3$, $CH_3$, H, Cl, and $NO_2$, are employed to investigate the substituent effect on the elimination reaction. Linear Hammett correlations are obtained in all calculations in contrast to the experimental finding.

• Bulletin of the Korean Chemical Society
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• v.32 no.7
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• pp.2301-2305
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• 2011

The potential energy surface (PES) for the loss of HCN or HNC from the pyrazine molecular ion was determined based on quantum chemical calculations using the G3//B3LYP method. Four possible dissociation pathways to form four $C_3H_3N^{+{{\bullet}}$ isomers were examined. A Rice-Ramsperger-Kassel-Marcus quasi-equilibrium theory model calculation was performed to predict the dissociation rate constant and the product branching ratio on the basis of the obtained PES. The resultant rate constant for the HCN loss agreed with the previous experimental result. The kinetic analysis predicted that the formation of $CH=CHN{\equiv}CH^{+{\bullet}}+HCN$ was predominant, which occurred by three consecutive steps, a C-C bond cleavage to form a linear intermediate, a rearrangement to form an H-bridged intermediate, and elimination of HCN.

• Bulletin of the Korean Chemical Society
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• v.25 no.2
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• pp.260-262
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• 2004

In agreement with the results of previous MP2 calculations by Sauers, B3LYP, CASSCF, and CASPT2 calculations on the parent 2-carbena-1,3-dioxolane show that it fragments to ethylene plus $CO_2$ by a concerted pathway with only a small energy barrier. Not only is fragmentation via stepwise C-O bond cleavage computed to be a much higher energy pathway, but the singlet diradical that would be an intermediate along such a reaction path is not even computed to be a local minimum on the potential energy surface.

• Bulletin of the Korean Chemical Society
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• v.28 no.12
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• pp.2310-2314
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• 2007

Stable molecular conformations were calculated for the p-tert-butylcalix[4]arene crown ether bridged at the lower rim with pyridyl unit (1) in the various conformers and their potassium-ion complexes. The structures of three distinct conformations have been optimized using DFT B3LYP/6-31G(d,p) method. Relative stability of free host 1 is in following order: cone (most stable) > partial-cone > 1,3-alternate conformer. For two different kinds of complexation mode, the potassium cation in the crown-ether moiety (cr) has much better complexation efficiency than in the benzene-rings (bz) pocket for all three kinds of conformation of host molecule 1. The relative stability of complex (1+K+) in the cr-binding mode is in following order: partial-cone (most stable) ~ cone > 1,3-alternate conformer.

• Bulletin of the Korean Chemical Society
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• v.29 no.10
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• pp.1893-1897
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• 2008

We have performed DFT calculations to investigate the conformational characteristics and hydrogen bonds of the p-tert-butylcalix[5]arene (1) and p-tert-butylcalix[4]arene (2). The structures of different conformers of 1 were optimized by using B3LYP/6-31+G(d,p) method. The relative stability of the various conformers of 1 is in the following order: cone (most stable) > 1,2-alternate > partial-cone > 1,3-alternate. The relative stability of four conformers of 2 is in the following order: cone (most stable) > partial-cone > 1,2-alternate > 1,3-alternate. The primary factor affecting the relative stabilities of the various conformers of the 1 and 2 are the number and strength of the intramolecular hydrogen bonds. The hydrogen-bond distances are discussed based on different calculation methods.

• Bulletin of the Korean Chemical Society
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• v.29 no.3
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• pp.541-545
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• 2008

Stable molecular isomers were calculated for the azobenzene crown ether p-tert-butylcalix[4]arene (1) in the host and their alkali-metal-ion complexes. The structures of two distinct isomers (cis and trans) have been optimized using DFT B3LYP/6-31G(d,p) method. Trans isomer of 1 is found to be 11.69 kcal/mol more stable than cis analogue. For two different kinds of complexation mode, the alkali-metal-cation in the crown-ether moiety (exo) has much better complexation efficiency than in the benzene-rings (endo) pocket for both isomers of 1. Sodium ion has much better complexation efficiency than potassium ion in all kinds of complexation mode with host 1. The Na+ complexation efficiency of the trans-complex (1) in the exo-binding mode is 8.24 kcal/mol better than cis-exo analogue.