Showing papers by "Kwang S. Kim published in 2001"

Ultrathin Single-Crystalline Silver Nanowire Arrays Formed in an Ambient Solution Phase

Abstract: We report the synthesis of single-crystalline silver nanowires of atomic dimensions. The ultrathin silver wires with 0.4 nanometer width grow up to micrometer-scale length inside the pores of self-assembled calix[4]hydroquinone nanotubes by electro-/photochemical redox reaction in an ambient aqueous phase. The present subnanowires are very stable under ambient air and aqueous environments, unlike previously reported metal wires of ∼1 nanometer diameter, which existed only transiently in ultrahigh vacuum. The wires exist as coherently oriented three-dimensional arrays of ultrahigh density and thus could be used as model systems for investigating one-dimensional phenomena and as nanoconnectors for designing nanoelectronic devices.

Olefinic vs. aromatic pi-H interaction: a theoretical investigation of the nature of interaction of first-row hydrides with ethene and benzene.

Abstract: The nature and origin of the π−H interaction in both the ethene (olefinic) and benzene (aromatic) complexes of the first-row hydrides (BH3, CH4, NH3, H2O, and HF) has been investigated by carrying out high level ab initio calculations. The results indicate that the strength of the π−H interaction is enhanced as one progresses from CH4 to HF. Unlike conventional H-bonds, this enhancement cannot be simply explained by the increase in electrostatic interactions or the electronegativity of the atom bound to the π H-bonded proton. The contributions of each of the attractive (electrostatic, inductive, dispersive) and repulsive exchange components of the total binding energy are important. Thus, the inductive energy is highly correlated to the olefinic π−H interaction as we progress from CH3 to HF. On the other hand, both electrostatic and inductive energies are important in the description of the aromatic π−H interaction. In either case, the contribution of dispersion energies is vital to obtain an accurate est...

Structures and spectra of iodide-water clusters I-(H2O)(n=1-6): An ab initio study

Abstract: To investigate the structures of I−(H2O)n=1–6, extensive ab initio calculations have been carried out. Owing to very flexible potential surfaces of the system (in particular for n=5 and 6), the lowest energy structures are characterized from various possible low-lying energy conformers. In contrast to some previously reported structures, we find a new lowest energy structure (followed by a few low-lying energy conformers) for n=5 and four nearly isoenergetic conformers for n=6. These conformers have surface and near-surface structures with the coordination number of 4. The present results provide the information of possible structures in recent profuse experiments of infrared spectra of I−(H2O)n=1–6 and charge transfer from the excited iodide ion to water molecules. Our predicted ionization potentials and OH stretching frequencies are in good agreement with the experimental data available, while only the cases of the OH frequencies for n=4 and the ionization potential for n=5 need consideration of conform...

Structures, energies, and vibrational spectra of water undecamer and dodecamer: An ab initio study

Abstract: Ab initio calculations have been carried out to study the water clusters of undecamer [(H2O)11] and dodecamer [(H2O)12]. At the level of Moller–Plesset second-order perturbation theory using TZ2P++ basis set, the lowest-energy conformer of the undecamer has the skeletal structure of Prism56 that a cyclic pentamer and a cyclic hexamer are fused into a prism shape with 16 hydrogen bonds (HBs). In this case, there are quite a number of nearly isoenergetic conformers with different hydrogen orientations. Among these, more stable conformers tend to have dangling H atoms separated (i.e., less clustered). The lowest energy conformer of the undecamer is different in hydrogen orientation from any previously suggested structure (including the ones obtained from various minimization algorithms). A second lowest energy skeletal structure is of Prism56B that a cyclic pentamer and an open-book hexamer are fused into a prism shape with 17 HBs. The most stable dodecamer is a fused cubic or tetragonal prism skeletal struc...

Weakly correlated one-dimensional indium chains on Si(111)

Abstract: Quasi-one-dimensional (1D) metals often exhibit a broken symmetry state. Here our first-principles density-functional theory calculations show that quasi-1D indium chains on the Si(111)-$(4\ifmmode\times\else\texttimes\fi{}1)$ surface are stabilized with $(4\ifmmode\times\else\texttimes\fi{}2)$ or $(8\ifmmode\times\else\texttimes\fi{}2)$ symmetry by lattice distortions of the two zigzag indium rows composing the chain. The ground state is almost degenerate, consistent with recent experiments which indicate that several phases coexist at low temperature.

Intramolecular charge transfer of π-conjugated push–pull systems in terms of polarizability and electronegativity

Abstract: We have derived a simple expression to evaluate the amount of intramolecular charge transfer (ICT) of π-conjugate push–pull systems from the properties of electronegativity (χ) and polarizability (α) of the corresponding push and pull systems. This simple model is verified from ab initio calculations of disubstituted benzenes, stilbenes, and butadienes (push–pull systems) and their monosubstituted (push or pull) systems with various donors and acceptors. The bond length alternation (BLA) is often used as a good structural parameter to describe the amount of ICT; however, it is not a complete parameter because the amounts of ICT for the same sets of donor/acceptor pairs are different for different bridge systems. Here, we report a parameter composed of polarizability and electronegativity to give a consistent amount of ICT for different bridge systems. In particular, when a highly electropositive donor is used, the polarizability of an acceptor is the most determining factor for ICT. On the basis of this m...

Structure and stability of fluorine-substituted benzene-argon complexes: The decisive role of exchange-repulsion and dispersion interactions

Abstract: The van der Waals complexes benzene-argon (BAr), fluorobenzene-argon (FAr), p-difluorobenzene-argon (DAr) are investigated at the second-order Moller–Plesset (MP2) level of theory using the 6-31+G(d), cc-pVDZ, aug-cc-pVTZ, and [7s4p2d1f/4s3p1d/3s1p] basis sets. Geometries, binding energies, harmonic vibrational frequencies, and density distribution are calculated where basis set superposition errors are corrected with the counterpoise method. Binding energies turn out to be almost identical (MP2/[7s4p2d1f/4s3p1d/3s1p]: 408, 409, 408 cm−1) for BAr, FAr, and DAr. Vibrationally corrected binding energies (357, 351, 364 cm−1) agree well with experimental values (340, 344, and 339 cm−1). Symmetry adapted perturbation theory (SAPT) is used to decompose binding energies and to examine the influence of attractive and repulsive components. Fluorine substituents lead to a contraction of the π density of the benzene ring, thus reducing the destabilizing exchange-repulsion and exchange-induction effects. At the same ...

Structural stabilities and self-assembly of Cucurbit[n]uril (n=4-7) and decamethylcucurbit[n]uril (n=4-6): A theoretical study

Abstract: Relative stabilities of cucurbit[n]uril homologues (CB[n]; n = 4−7) and decamethylcucurbit[n]uril homologues (DCB[n]; n = 4−6) have been inspected with ab initio and density functional theory (DFT) calculations. Results show that CB[6] and CB[7] are more stable among CB[n] homologues and DCB[5] among DCB[n] homologues, which confirms the previous experiments. This is also supported in consideration of the puckering angles of the respective dimeric building units. Our investigation on the template effect in the formation of CB and DCB macrocyclic structure suggests that H3O+ is important in tethering the building units being added to the macrocyclic structure in the course of the reaction. The presence of acid tends to favor the structures of smaller-size homologues. Thus, in consideration of the acid contents in the experiments, our calculations explain why CB[6] and DCB[5] are the major products. Moreover, it explains why considerable amount of CB[7] could be obtained in the recent report in which the am...

σ to π conformational transition: Interactions of the water trimer with π systems

Abstract: We investigate the interaction of the water trimer with the aromatic π systems (benzene, toluene, fluorobenzene, and p-difluorobenzene) at the second-order Moller–Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets. The minimum-energy structures, binding energies, and the harmonic vibrational frequencies were calculated for all these systems. The minimum-energy structures reveal that the water trimer has only a π type of interaction with benzene and toluene, but has both a π and σ type (H-bond with the fluorine atom) of interaction with fluorobenzene and p-difluorobenzene. The binding energies reveal that, contrary to what has been observed in previous theoretical studies, the strength of the interaction of these π systems with the water trimer is greater than with the water dimer. A good agreement is obtained between the calculated and experimentally measured vibrational frequencies of these complexes. An analysis of the calculated OH stretching modes reveals some unusual facets of ...

Ab initio studies of π-water tetramer complexes: Evolution of optimal structures, binding energies, and vibrational spectra of π-(H2O)n (n=1–4) complexes

Abstract: The optimal structures, binding energies, and harmonic vibrational frequencies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as π-(water tetramer) clusters (π: p-difluorobenzene, fluorobenzene, benzene, toluene) have been evaluated at the second order perturbation level of theory (MP2) using both the 6-31+G* and aug-cc-pVDZ basis sets. In sharp contrast to the complexes of smaller water clusters with these π systems, wherein the water subcluster is most strongly bound to toluene, the water tetramer is most strongly bound to fluorobenzene. This exceptionally high binding energy results from both a π⋅⋅⋅OH H-bond and a competing σ F⋅⋅⋅OH bond between the water tetramer moiety and the aromatic molecule. The magnitudes of the many-body energy terms and their contribution to the binding energies of these π-(water tetramer) systems indicates that the contributions of three- and higher-order terms are much smaller when compared to the neutral water clusters. The two-body terms associated with the π- and σ-type of interaction indicates that in both the fluorobenzene and p-difluorobenzene complexes, the increase in the size of the water cluster enhances the π-H-bonding interaction and weakens the σ F⋅⋅⋅H interaction. This observation is in consonance with the calculated and experimentally observed redshifts of the OH vibrational frequencies. Thus, with an increase in the size of a water cluster bound to the fluorinated π system, there is a lowering of the redshift induced by the σ F⋅⋅⋅H interaction and an increase in the redshift due to the π-H interaction. The calculated redshift of the π H-bonded OH mode is very much dependent on the basis set, with larger basis sets yielding shifts which are in better agreement with the experimentally determined shifts.

Oscillatory energetics of flat Ag films on MgO(001)

Abstract: The energetics and electronic structures of flat Ag films on the MgO(001) substrate are studied by first-principles density-functional calculations. An oscillatory variation of the film energetics showing the existence of multiple magic thicknesses for smooth growth is found. This oscillatory behavior correlates well with the quantum-well states, which themselves vary with the film thickness. The results demonstrate the importance of the confined motion of the conduction electrons in stabilizing epitaxial metal films, as emphasized in a recent ``electronic growth'' model.

Mechanistic insight into the inactivation of carboxypeptidase A by alpha-benzyl-2-oxo-1,3-oxazolidine-4-acetic acid, a novel type of irreversible inhibitor for carboxypeptidase A with no stereospecificity.

Abstract: On the basis of the active site topology and enzymic catalytic mechanism of carboxypeptidase A (CPA), a prototypical zinc-containing proteolytic enzyme, α-benzyl-2-oxo-1,3-oxazolidine-4-acetic acid...

On the microscopic interaction of para-chlorofluorobenzene with water

Abstract: The isolated (1:1) complex of para-chlorofluorobenzene with water has been investigated by IR/R2PI depletion spectroscopy and ab initio calculations on the geometry and IR spectra. Whereas three different isomers (σF, σCl, π) were located by theory, in the molecular beam experiments only the two σ isomers have been observed.

Model calculation of the band energy gap for poly(p-phenylenevinylene)

Abstract: The direct band gap of the conjugated polymer poly(p-phenylenevinylene) (PPV) is calculated in an effective single-band model by means of a renormalization scheme. This model provides a simple computational method for obtaining the band gap of a many-band conjugated π-electron system of long polymers. The calculation shows that, for the existing data for PPV, the present renormalization scheme works well and its results as regards the band energy gap are comparable to those obtained by other approaches including ab initio calculation.