Showing papers by "Kwang S. Kim published in 1993"

Water-cluster distribution with respect to pressure and temperature in the gas phase

Abstract: In spite of decades of extensive studies of water, the experimental information of water clusters larger than the trimer is hardly available yet. To aid the better analysis of certain cluster properties in an experiment, we studied the small-water-cluster distribution (particularly for the minimum-energy structures) in the gas phase. Utilizing the thermodynamic information in the range from the water monomer to the octamer (except for the heptamer) by ab initio calculations, we investigated the mole fractions of the water clusters along the vapor pressure of the condensed phase. These mole fractions increase with increasing temperature or pressure, while the higher clusters increase still more. The entropy increment of the cyclic pentamer relative to the cyclic tetramer is particularly small; thus, the cyclic pentamer shows thermodynamically unusual characteristics. For the trimer, the cyclic structure is more stable than the linear structure at temperatures lower than \ensuremath{\sim}400 K, while above this temperature, the latter is more stable due to the entropy effect. Similar phenomena are also expected for the higher clusters. The mole fractions of the higher cyclic clusters are found to be very small in a vapor unless they are uncondensed with insufficient water molecules.

Origin of diastereoselectivity in the nitrile oxide cycloadditions with Oppolzer's chiral sultams: coulombic interaction as the key role in diastereofacial differentiation

Abstract: Diastereofacial selectivity in the nitrile oxide cycloadditions with Oppolzer's chiral sultams cannot be explained with the conventional face shielding by sterically bulky groups. To find the origin of the diastereoselectivity, the transition states for the cycloadditions have been studied with semiempirical quantum mechanical calculations using the PM3 method. The calculated product ratios are in good agreement with experiment. We find that the activation barriers for the favored and disfavored transition states are strongly correlated with the Coulombic repulsions between the dipolar oxygen and the sultam oxygens. Such correlation is also found from ab initio calculations using the PM3 optimized geometries