Showing papers by "Kwang S. Kim published in 1998"

Structures, binding energies, and spectra of isoenergetic water hexamer clusters: Extensive ab initio studies

Abstract: We investigated five lowest energy structures of the water hexamer (ring, book, bag, cage, and prism) using extensive ab initio calculations. High levels of theory using various basis sets were employed. On the basis of Mo/ller–Plesset second order perturbation (MP2) calculations using a large basis set [9s6p4d2 f1g/6s4p2d]+diffuse(2sp/s), the lowest energy structure with zero point energy (ZPE) correction is the cage conformer, followed by the book (within 0.1 kcal/mol) and the prism (within 0.2 kcal/mol). The spectra of the five conformers have been investigated. The predicted rotational constants and dipole moments of the cage conformer are in good agreement with the experiment [Liu et al., Nature 381, 501 (1996)] as compared to other structures. This proves that the experiment surely found the cage structure, which was first reported by one of the authors [Kim et al., Chem. Phys. Lett. 131, 451 (1986)]. However, the five structures would still be nearly isoenergetic within 0.7 kcal/mol at 0 K. Above ∼...

Ionophores and receptors using cation-pi interactions: collarenes.

Abstract: Cation-π interactions are important forces in molecular recognition by biological receptors, enzyme catalysis, and crystal engineering. We have harnessed these interactions in designing molecular systems with circular arrangement of benzene units that are capable of acting as ionophores and models for biological receptors. [n]Collarenes are promising candidates with high selectivity for a specific cation, depending on n, because of their structural rigidity and well-defined cavity size. The interaction energies of [n]collarenes with cations have been evaluated by using ab initio calculations. The selectivity of these [n]collarenes in aqueous solution was revealed by using statistical perturbation theory in conjunction with Monte Carlo and molecular dynamics simulations. It has been observed that in [n]collarenes the ratio of the interaction energies of a cation with it and the cation with the basic building unit (benzene) can be correlated to its ion selectivity. We find that collarenes are excellent and efficient ionophores that bind cations through cation-π interactions. [6]Collarene is found to be a selective host for Li+ and Mg2+, [8]collarene for K+ and Sr2+, and [10]collarene for Cs+ and Ba2+. This finding indicates that [10]collarene and [8]collarene could be used for effective separation of highly radioactive isotopes, 137Cs and 90Sr, which are major constituents of nuclear wastes. More interestingly, collarenes of larger cavity size can be useful in capturing organic cations. [12]Collarene exhibits a pronounced affinity for tetramethylammonium cation and acetylcholine, which implies that it could serve as a model for acetylcholinestrase. Thus, collarenes can prove to be novel and effective ionophores/model-receptors capable of heralding a new direction in molecular recognition and host-guest chemistry.

Role of Lewis Acid(AlCl3)−Aromatic Ring Interactions in Friedel−Craft's Reaction: An ab Initio Study

Abstract: Electrophilic aromatic substitution reactions, which are highly exemplified by the widely used Friedel-Craft's reaction, have been extensively studied using theoretical and experimental techniques. A number of elegant mechanisms have been proposed for the Friedel-Craft's reaction till date. In all the proposed mechanisms, the role of the Lewis acid has been limited to the generation of the electrophile, which subsequently attacks the aromatic system to form either a π or a σ complex. A recent experimental report on the reaction of CO with benzene in zeolite catalysts intrigued us because the presence or absence of AlCl3 was found to govern the reaction product. These clearly indicated that AlCl3 has an additional role in the reaction. We probed this role theoretically by performing high-level ab initio calculations on two model systems C6H6−BCl3 and C6H6−AlCl3. Our results clearly indicate that one of the benzene carbon tends to become highly nucleophilic, thereby facilitating an attack by an incipient el...

Benzene-hydrogen halide interactions: Theoretical studies of binding energies, vibrational frequencies, and equilibrium structures

Abstract: High level ab initio calculations have been performed on the benzene-HCl and benzene-HF systems using the second-order Mo/ller-Plesset perturbation theory. In contrast to existing theoretical studies, the calculated binding energies indicate that HCl binds more strongly to benzene than HF. This is in accordance with the limited experimental data available on these systems. An explanation has been forwarded for the above observation by performing a molecular orbital analysis of both C6H6⋯HF and C6H6⋯HCl. In the global minimum of C6H6⋯HF, HF lies inclined to the benzene ring with the hydrogen atom pointing either towards a benzene carbon or the center of carbon-carbon bond. In the C6H6⋯HCl complex, HCl is found to lie along the C6 axis of the benzene ring for smaller basis sets, but it also tends to lie inclined to the benzene ring for a very large basis set. The quantum mechanical probabilistic characterization of the structure of the C6H6⋯HCl complex provides a more realistic description of the experiment...

Novel Amphi-Ionophores

Abstract: The interactions of cyclic peptides containing glycines with cations (Li+, Na+, Be2+, Mg2+) and anions (F- and Cl-) have been investigated using ab initio calculations. The cyclic peptides are foun...

Cation Affinities of [16]Starand Model. Comparison with 12-Crown-4: Crucial Role of Dipolar Moiety Orientations

Abstract: [16]Starand appears to be a promising ionophore because of its rigid structure with the spherical cavity into which Li+ can fit perfectly. Using ab initio calculations, we investigated if the stara...