• Journal of the Korean Chemical Society
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• v.43 no.6
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• pp.621-627
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• 1999

A new predictive scheme to calculate hydrocarbon enthalpies of formation from ab initio total energy calculations is described. The method improves a previous computation procedure (based on the total number of atoms) through the inclusion of bond parameters. Present results are good enough and the average absolute errors in the computed values of enthalpies of formation are lower than the experimental uncertainties. Some possible extensions are pointed out in order to reach definitive conclusions about the proposed methodology.

• Bulletin of the Korean Chemical Society
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• v.34 no.1
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• pp.179-187
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• 2013

We describe newly developed software named KPACK for relativistic electronic structure computation of molecules containing heavy elements that enables the two-component ab initio calculations in Kramers restricted and unrestricted formalisms in the framework of the relativistic effective core potential (RECP). The spin-orbit coupling as relativistic effect enters into the calculation at the Hartree-Fock (HF) stage and hence, is treated in a variational manner to generate two-component molecular spinors as one-electron wavefunctions for use in the correlated methods. As correlated methods, KPACK currently provides the two-component second-order M${\o}$ller-Plesset perturbation theory (MP2), configuration interaction (CI) and complete-active-space self-consistent field (CASSCF) methods. Test calculations were performed for the ground states of group-14 elements, for which the spin-orbit coupling greatly influences the determination of term symbols. A categorization of three procedures is suggested for the two-component methods on the basis of spin-orbit coupling manifested in the HF level.

• Bulletin of the Korean Chemical Society
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• v.25 no.11
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• pp.1657-1660
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• 2004

Several critical geometries associated with the rearrangement of $CH_3SNO_2\;to\;CH_3SONO$ are calculated with the density functional theory (DFT) method and compared with those of the ab initio molecular orbital methods. There are two probable pathways for this rearrangement, one involving the transition state of an oxygen migration and the other through the homolytic decomposition to radicals. The reaction barrier via the transition state is about 60 kcal/mol and the decomposition energy into radicals about 35 kcal/mol, suggesting that the reaction pathway via the homolytic cleavage to radical species is energetically favorable. Since even the homolytic cleavage requires large energies, the rearrangement reaction is unlikely without the aid of catalysts.

• Bulletin of the Korean Chemical Society
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• v.9 no.2
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• pp.101-105
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• 1988

The ab initio effective valence shell Hamiltonian method, based on quasidegenerate many-body perturbation theory, is generalized to calculate molecular properties as well as the valence state energies which have previously been determined for atoms and small molecules. The procedure requires the evaluation of effective operator for each molecular property. Effective operators are perturbatively expanded in powers of correlation and contain contributions from excitations outside of the multireference valence space. To demonstrate the validity of this method, calculations for dipole moments of several low lying valence states of OH, $OH^+$, and $OH^-$ to first order in the correlations have been performed and compared with configuration interaction calculations.

• Bulletin of the Korean Chemical Society
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• v.22 no.7
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• pp.693-698
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• 2001

Ab initio density functional calculations on the structural isomers, the hydration energies, and the hydrogen bond many-body interactions for gauche-, trans-protonated ethylenediamine-(water)3 complexes (g-enH+(H2O)3, t-enH+(H2O)3) have been performed. The structures and relative stabilities of three representative isomers (cyclic, tripod, open) between g-enH+(H2O)3 and t-enH+(H2O)3 are predicted to be quite different due to the strong interference between intramolecular hydrogen bonding and water hydrogen bond networks in g-enH+(H2O)3. Many-body analyses revealed that the combined repulsive relaxation energy and repulsive nonadditive interactions for the mono-cyclic tripod isomer, not the hydrogen bond cooperativity, are mainly responsible for the greater stability of the bi-cyclic isomer.

• Proceedings of the Korean Nuclear Society Conference
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• 2005.10a
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• pp.292-293
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• 2005

• Bulletin of the Korean Chemical Society
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• v.35 no.3
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• pp.839-844
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• 2014

The gas-phase radical-radical reaction $O(^3P)$ + $C_2H_5$ (ethyl) ${\rightarrow}$ $H(^2S)$ + $CH_3CHO$(acetaldehyde) was investigated by applying a combination of vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed beam configuration and ab initio calculations. The two radical reactants $O(^3P)$ and $C_2H_5$ were respectively produced by photolysis of $NO_2$ and supersonic flash pyrolysis of the synthesized precursor azoethane. Doppler profile analysis of the nascent H-atom products in the Lyman-${\alpha}$ region revealed that the average translational energy of the products and the average fraction of the total available energy released as translational energy were $5.01{\pm}0.72kcalmol^{-1}$ and 6.1%, respectively. The empirical data combined with CBS-QB3 level ab initio theory and statistical calculations demonstrated that the title exchange reaction is a major channel and proceeds via an addition-elimination mechanism through the formation of a short-lived, dynamical addition complex on the doublet potential energy surface. On the basis of systematic comparison with several exchange reactions of hydrocarbon radicals, the observed small kinetic energy release can be explained in terms of the loose transition state with a product-like geometry and a small reverse activation barrier along the reaction coordinate.

• Bulletin of the Korean Chemical Society
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• v.7 no.3
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• pp.218-224
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• 1986

The electronic structures of silane and chlorosilanes are studied by the SCF calculations using effective core potentials (ECP's). The results obtained with ECP's are in good agreement with corresponding all electron calculations demonstrating the reliability of ECP employed. The importance of polarization functions for the second row atoms is also evident in this study. The SCF calculations of silane and chlorosilanes are useful in qualitative understanding of many chemical properties since many trends are correctly obtained with the polarization functions included in basis sets of reasonable size.

• Proceedings of the Korean Society of Near Infrared Spectroscopy Conference
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• 2001.06a
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• pp.1274-1274
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• 2001

An anharmonicity is a fundamental quantity shaping the potential for stretching OH vibration in phenol and its derivatives. The phenomenon is examined both by experimental and theoretical methods. FT-IR and NIR spectra of series of phenols derivatives were measured in the range of fundamental and first two Overtones of $_{s}(OH)$ Vibrations in $CCl_4$ solutions. The electronic influence of substituents on the analyzed frequencies is discussed and correlated with $pK_{a}$ parameters. Ab initio MP2/6-31G(d,p) and B3LYP/6-31G(g,p) calculations of the potential for proton movement in OH group were performed. Equilibrium structures were also determined. The frequencies of fundamental and overtones were calculated by Numerov-type procedure. The results of calculations are compared with the experimental data. The best linear correlations were obtained for the results of MP2/6-31G(d,p) calculations. It was shown that some structural parameters are especially sensitive on substitution. The linear correlations were found between those parameters and spectroscopic data. The results of calculation are compared with available crystallographic data.

• Bulletin of the Korean Chemical Society
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• v.15 no.3
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• pp.221-227
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• 1994

The barrier heights of the internal rotations for ethyl halides calculated by ab initio methods differ from those of experiments by more than 0.2 kcal/mol. The use of basis sets larger than the $6-31G^{\ast}$ set and the inclusion of correlation do not improve the agreement between the calculated and experimental values. The zero-point vibration corrections are substantial in the HF calculations with $6-31G^{\ast}$ basis sets, but become negligible in the MP2 calculations with $6-311G^{{\ast}{\ast}}$ basis sets for $C_2H_5F\;and\;C_2H_5Cl$. It is shown that the rigid rotor approximation and the assumed shape of the potential curve as a cos2${\theta}$ curve could also be the sources of discrepancies between calculated and experimental values. Higher order perturbation corrections narrow the gap between experimental and theoretical values, but there still remains about 10% overestimate of 0.3 kcal/mol. Optimized geometries from the HF and MP2 calculations are in good agreement with those from experiments. Dipole moments calculated from the MP2 densities show slightly better agreement with experiments than those from the HF densities.