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Showing papers by "Kwang S. Kim published in 2007"


Journal ArticleDOI
TL;DR: The presence of substituents on the aromatic ring, irrespective of their electron withdrawing or donating nature, leads to an increase in the binding energy, and the displaced-stacked conformations are more stabilized than the T-shaped conformers, which explains the wide prevalence of displaced stacked structures in organic crystals.
Abstract: Interactions involving aromatic rings are important in molecular/biomolecular assembly and engineering. As a consequence, there have been a number of investigations on dimers involving benzene or other substituted π systems. In this Feature Article, we examine the relevance of the magnitudes of their attractive and repulsive interaction energy components in governing the geometries of several π−π systems. The geometries and the associated binding energies were evaluated at the complete basis set (CBS) limit of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using a least biased scheme for the given data set. The results for the benzene dimer indicate that the floppy T-shaped structure (center-to-center distance: 4.96 A, with an axial benzene off-centered above the facial benzene) is isoenergetic in zero-point-energy (ZPE) corrected binding energy (D0) to the displaced-stacked structure (vertical interplanar distance: 3.54 A). However, the T-shaped structure i...

602 citations


Journal ArticleDOI
TL;DR: The Hg2+-promoted ring opening of spirolactam of the rhodamine moiety induced cyclic guanylation of the thiourea moiety, which resulted in the dual chromo- and fluorogenic observation (OFF-ON).

420 citations


Journal ArticleDOI
TL;DR: The effect of hydration, messenger atom Ar, and temperature are crucial to elucidating the nature of vibrational spectra of Eigen and Zundel forms and to assigning the vibrational modes of small protonated water clusters.
Abstract: The spectral properties of protonated water clusters, especially the difference between Eigen (H3O+) and Zundel (H5O2+) conformers and the difference between their unhydrated and dominant hydrated forms are investigated with the first principles molecular dynamics simulations as well as with the high level ab initio calculations. The vibrational modes of the excess proton in H3O+ are sensitive to the hydration, while those in H5O2+ are sensitive to the messenger atom such as Ar (which was assumed to be weakly bound to the water cluster during acquisitions of experimental spectra). The spectral feature around ∼2700 cm-1 (experimental value: 2665 cm-1) for the Eigen moiety appears when H3O+ is hydrated. This feature corresponds to the hydrating water interacting with H3O+, so it cannot appear in the Eigen core. Thus, H3O+ alone would be somewhat different from the Eigen forms in water. For the Zundel form (in particular, H5O2+), there have been some differences in spectral features among different experime...

139 citations


Journal ArticleDOI
TL;DR: Quinoxaline derivatives (1-4) bearing two imidazolium moieties are found to strongly bind anions and show unique charge-transfer fluorescent responses to pyrophosphate and acetate, whereas they show excimer formation with other anions.

102 citations


Journal ArticleDOI
TL;DR: Detailed ab initio calculations on some model cation-pi-anion ternary complexes are carried out and the nonpair potential terms, three-body contributions, and attractive and repulsive energy components of the interaction energy are evaluated.
Abstract: Cation-pi and the corresponding anion-pi interactions have in general been investigated as binary complexes despite their association with counterions. However, a recent study of the ammonia channel highlights the important but overlooked role of anions in cation-pi interactions. In an effort to examine the structural and energetic consequences of the presence of counterions, we have carried out detailed ab initio calculations on some model cation-pi-anion ternary complexes and evaluated the nonpair potential terms, three-body contributions, and attractive and repulsive energy components of the interaction energy. The presence of the anion in the vicinity of the pi system leads to a large redistribution of electron density and hence leads to an inductive stabilization. The resulting electronic and geometrical changes have important consequences in both chemical and biological systems. Compared to cation-pi-anion ternary complexes, the magnitude of the cation-pi interaction in pi-cation-anion ternary complexes is markedly lower because of charge transfer from the anion to the cation.

99 citations


Journal ArticleDOI
TL;DR: Based on density functional theory, a program code is developed to investigate the electron transport characteristics for a variety of nanometer scaled devices in the presence of an external bias voltage and k‐point sampling for the realistic modeling of the bulk electrode.
Abstract: Based on density functional theory (DFT), we have developed algorithms and a program code to investigate the electron transport characteristics for a variety of nanometer scaled devices in the presence of an external bias voltage. We employed basis sets comprised of linear combinations of numerical type atomic orbitals and k-point sampling for the realistic modeling of the bulk electrode. The scheme coupled with the matrix version of the non-equilibrium Green function method enables determination of the transmission coefficients at a given energy and voltage in a self-consistent manner, as well as the corresponding current-voltage (I-V) characteristics. This scheme has advantages because it is applicable to large systems, easily transportable to different types of quantum chemistry packages, and extendable to describe time-dependent phenomena or inelastic scatterings. It has been applied to diverse types of practical electronic devices such as carbon nanotubes, graphene nano-ribbons, metallic nanowires, and molecular electronic devices. The quantum conductance phenomena for systems involving quantum point contacts and I-V curves are described for the dithiol-benzene molecule in contact with two Au electrodes using the k-point sampling method.

86 citations


Journal ArticleDOI
TL;DR: By linking the urea moiety at the 1,8 positions of the carbazole fragment, the host systems 1, 2, and 3 are synthesized having both chromogenic and fluorogenic signaling subunits, which enables us to differentiate the given set of anions.
Abstract: By linking the urea moiety at the 1,8 positions of the carbazole fragment, we synthesized host systems 1, 2, and 3 having both chromogenic and fluorogenic signaling subunits. The spectral changes in both the signaling subunits could be easily analyzed via a simple 2-dimensional (2D) analytic approach described here, which enables us to differentiate the given set of anions. Structural studies are also reported.

72 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that metallic carbon nanotubes (CNTs) can form asymmetric couplings even if symmetric structures are employed, and that the NDR phenomenon can be obtained by tuning the threshold voltage for the asymmetric coupling by a proper choice of a molecule.
Abstract: An intricate problem in molecular electronics is to control the molecule-electrodes contacts. Asymmetric couplings between both contacts are important in driving novel nonlinear transport characteristics like negative differential resistance (NDR). We find that in the presence of an applied field, metallic carbon nanotubes (CNTs) can form asymmetric couplings even if symmetric structures are employed. This origin is due to the CNT itself, while the NDR phenomenon can be obtained by tuning the threshold voltage for the asymmetric couplings by a proper choice of a molecule.

70 citations


Journal ArticleDOI
TL;DR: In this paper, a mini-review of intermolecular interactions ranging from hydrogen bonding to ionic interactions to aromatic interactions is presented, and the conformational changes between stacked and edge-to-face conformers in benzoquinone-benzene complexes are discussed.
Abstract: In this mini-review we briefly describe intermolecular interactions ranging from hydrogen bonding to ionic interactions to aromatic interactions. Manifestation of these interaction forces is in the design and realization of various ionophores with chemo-sensing capability for biologically important cations and anions. We also explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled organic nanotubes. We further discuss the conformational changes between stacked and edge-to-face conformers in benzoquinone-benzene complexes, which are controlled by alternating electrochemical potential. The resulting flapping motion illustrates a promising pathway toward the design of nanomechanical devices.

69 citations


Journal ArticleDOI
TL;DR: In this article, two new imidazolium systems bearing two pyrene groups have been synthesized and the binding properties of these hosts were examined via fluorescent changes, and the fluorescent changes of host 2 upon the addition of anions were also utilized as a NOR logic gate and an INH logic gate.

58 citations


Journal ArticleDOI
TL;DR: The vibrational frequency shifts were investigated as a function of the number of water molecules and the frequency characteristics with and without the presence of outer-shell water molecules to identify specific hydrogen-bonding structures present in the experimental spectra.
Abstract: The design of cesium-selective ionophores must include the nature of cesium-water interactions. The authors have carried out extensive ab initio and density functional theory calculations of hydrated cesium cations to obtain reasonably accurate energetics, thermodynamic quantities, and IR spectra. An extensive search was made to find the most stable structures. Since water⋯water interactions are important in the aqua-Cs+ clusters, the authors investigated the vibrational frequency shifts as a function of the number of water molecules and the frequency characteristics with and without the presence of outer-shell water molecules. The predicted vibrational frequencies were then compared with the infrared photodissociation spectra of argon-tagged hydrated cesium cluster ions. This comparison allowed the identification of specific hydrogen-bonding structures present in the experimental spectra.

Journal ArticleDOI
TL;DR: A conformational inhomogeneity model is proposed, in which a broad distribution of conformations of the open isomers in the ground state is projected into two minima in the excited electronic potential surface to lead to the slow and the fast reaction pathways.
Abstract: Photochromic ring closure reaction dynamics of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluoro cyclopentene and its derivatives in solution has been studied by femtosecond time-resolved fluorescence. Time-resolved spontaneous fluorescence of the open isomer reveals a fast component of around 1 ps and a slow component on the order of 100 ps. Fluorescence time profiles, reaction quantum yields, and relative populations of the parallel (C(s) symmetry) and antiparallel (C(2) symmetry) conformations indicate that both time components are attributable mostly to the C(2) conformer that undergoes the ring closure reaction. The fast component is assigned to the direct ring closure reaction, and the slow component is assigned to the reaction through conformation change. Time constants of the slow component for the derivatives are inversely proportional to the reaction quantum yields, suggesting that the rate of the conformational dynamics is comparable to the rate of other population relaxation processes. The relative amplitude and exact time constant of the fast component depend on the detection wavelength displaying a higher relative amplitude with shorter time constant at longer wavelengths. The results allow us to propose a conformational inhomogeneity model, in which a broad distribution of conformations of the open isomers in the ground state is projected into two minima in the excited electronic potential surface to lead to the slow and the fast reaction pathways.

Journal ArticleDOI
TL;DR: In this paper, the authors applied the extended Drude model (EDM) to noble metals of Cu, Ag, and Au and obtained self-consistently the Drude parameters without using the sum rule or any approximation.
Abstract: We have applied the extended Drude model (EDM) to noble metals of Cu, Ag, and Au. Drude parameters are obtained self-consistently without using the sum rule or any approximation. The core polarization contribution e∞ is shown to be large due to shallow d-orbitals. The frequency dependent mass enhancement factor λ (ω) and the scattering rate 1/τ (ω) are obtained. The Drude region can be identified when 1/τ (ω) deviates from the Fermi liquid behavior due to interband transitions. The onset frequency for the interband transition is found to be lower than the frequency value at the minimum of the imaginary part of the dielectric constant. The validity of the approximation of ωτ ≫ 1, which is commonly employed in analyzing optical data, has been checked based on the EDM for noble metals. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal ArticleDOI
TL;DR: The ionic dissociation of salts was examined with a theoretical study of KX hydrated by up to six water molecules KX(H2O)n and single point energy calculations using the coupled cluster theory with single, double, and perturbative triple excitations were performed on the MP2 optimized geometries.
Abstract: The ionic dissociation of salts was examined with a theoretical study of KX (X=F,Cl,Br,I) hydrated by up to six water molecules KX(H2O)n (n=1-6). Calculations were done using the density functional theory and second order Moller-Plesset (MP2) perturbational theory. To provide more conclusive results, single point energy calculations using the coupled cluster theory with single, double, and perturbative triple excitations were performed on the MP2 optimized geometries. The dissociation feature of the salts was examined in terms of K-X bond lengths and K-X stretch frequencies. In general, the successive incorporation of water molecules to the cluster lengthens the K-X distance, and consequently the corresponding frequency decreases. Near 0 K, the KX salt ion pairs can be partly separated by more than five water molecules. The pentahydrated KX salt is partly dissociated, though these partly dissociated structures are almost isoenergetic to the undissociated ones for KFKCl. For the hexahydrated complexes, KF is undissociated, KClKBr is partly dissociated, and KI is dissociated (though this dissociated structure is nearly isoenergetic to a partly dissociated one). On the other hand, at room temperature, the penta- and hexahydrated undissociated structures which have less hydrogen bonds are likely to be more stable than the partly dissociated ones because of the entropy effect. Therefore, the dissociation at room temperature could take place for higher clusters than the hexahydrated ones.


Journal ArticleDOI
TL;DR: In this paper, the authors discuss ionophores with chemo-sensing capability for biologically important cations and an- ions and explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled nanotubes from calix(4)hydroquinone (CHQ).
Abstract: For the design of functional molecules and nanodevices, it is very useful to utilize nanorecognition (which is governed mainly by interaction forces such as hydrogen bonding, ionic interaction, π-H/π-π interactions, and metallic interactions) and nanodynamics (in- volving capture, transport, and release of electrons, photons, or protons). The manifestation of these interaction forces has led us to the design and realization of diverse ionophores/re- ceptors, organic nanotubes, nanowires, molecular mechanical devices, molecular switches, enzyme mimetics, protein folding/unfolding, etc. In this review, we begin with a brief dis- cussion of the interaction forces, followed by some of our representative applications. We discuss ionophores with chemo-sensing capability for biologically important cations and an- ions and explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled nanotubes from calix(4)hydroquinone (CHQ). The binding study of neutral and cationic transition metals with the redox system of hydroquinone (HQ) and quinone (Q) predicts what kind of nanostructures would form. Finally, we look into the conformational changes between stacked and edge-to-face conformers in π-benzoquinone- benzene complexes controlled by alternating electrochemical potential. The resulting flap- ping motion illustrates a promising pathway toward the design of mobile nanomechanical de- vices.

Journal ArticleDOI
TL;DR: Alkali-noble metal alloy wires show better linearity and stability than either pure alkali metal or noble metal wires on the basis of first-principles calculations of clusters and one dimensional infinitely long subnanowires of the binary systems.
Abstract: On the basis of first-principles calculations of clusters and one dimensional infinitely long subnanowires of the binary systems, we find that alkali-noble metal alloy wires show better linearity and stability than either pure alkali metal or noble metal wires. The enhanced alternating charge buildup on atoms by charge transfer helps the atoms line up straight. The cesium doped gold wires showing significant charge transfer from cesium to gold can be stabilized as linear or circular monoatomic chains.

Journal ArticleDOI
TL;DR: In this paper, it was shown that as the diameter and length of Bi nanorods decrease below ∼10 and ∼35 nm, respectively, the quantum confinement effect drastically increases.
Abstract: From the electron energy loss spectroscopy (EELS) study of plasmon excitation of Bi nanorods, we find that as the diameter and length of Bi nanorods decrease below ∼10 and ∼35 nm, respectively, the quantum confinement effect drastically increases. Thus, such Bi nanorods would be extremely useful for thermoelectric devices and applications.

Journal ArticleDOI
TL;DR: The structures, binding energies, thermodynamic quantities, IR spectra, and electronic properties of the hydride anion hydrated by up to six water molecules, and the dynamics of the detachment process of a hydrogen radical upon the excitation is discussed.
Abstract: On the basis of density functional theory (DFT) and high level ab initio theory, we report the structures, binding energies, thermodynamic quantities, IR spectra, and electronic properties of the hydride anion hydrated by up to six water molecules. Ground state DFT molecular dynamics simulations (based on the Born-Oppenheimer potential surface) show that as the temperature increases, the surface-bound hydride anion changes to the internally bound structure. Car-Parrinello molecular dynamics simulations are also carried out for the spectral analysis of the monohydrated hydride. Excited-state ab initio molecular dynamics simulations show that the photoinduced charge-transfer-to-solvent phenomena are accompanied by the formation of the excess electron-water clusters and the detachment of the H radical from the clusters. The dynamics of the detachment process of a hydrogen radical upon the excitation is discussed.

Journal ArticleDOI
TL;DR: This simple and straightforward qualitative analysis, using the synthesized novel compounds herein, can be considered a new innovative tool for discriminating 1 degrees -3 degrees amines as an alternative to the historical Hinsberg test.
Abstract: The Hinsberg test to recognize the type of amine (1 degrees -3 degrees amines), which has been established for more than 100 years and well documented in textbooks, is not possible without conducting complicated organic reactions. We report for the first time unique chemosensors that are capable of showing selective color changes toward 1 degrees -3 degrees amines as a color version of the Hinsberg test. This simple and straightforward qualitative analysis, using the synthesized novel compounds herein, can be considered a new innovative tool for discriminating 1 degrees -3 degrees amines as an alternative to the historical Hinsberg test.

Journal ArticleDOI
TL;DR: In this paper, the theoretical investigations of various cluster systems comprising of water clusters, π-containing clusters, and metallic clusters are highlighted, and the authors illustrate how these investiga...
Abstract: In this account, we highlight the theoretical investigations of various cluster systems comprising of water clusters, π-containing clusters, and metallic clusters. We illustrate how these investiga...

Journal ArticleDOI
TL;DR: In this article, the effect of an excess electron on the hexahydated hydroiodic acid was investigated, where the dissociated structure [H3O+(H2O)5I−] was shown to be much more stable than the undissociated one.
Abstract: A hydroiodic acid molecule is dissociated by more than four water molecules. Here, the effect of an excess electron on the hexahydated hydroiodic acid where the dissociated structure [H3O+(H2O)5I−] is much more stable than the undissociated one [(H2O)6HI], is investigated. Upon binding an excess electron (e−), the cluster releases a hydrogen radical and forms the stable hexahydrated iodide [(H2O)6I−] when the initial kinetic energy is above ∼200 K, due to the small transition barrier (∼0.5 eV). The system with the hydrogen radical released, [(H2O)6I− + H·], is much more stable than the systems with an excess electron, [(H3O)+{e−(H2O)5}I−] or [e−(H2O)2(H3O)+(H2O)3I−].

Journal Article
TL;DR: Theoretical investigations of gas phase clusters enable the evaluation of intrinsic molecular properties and intermolecular interactions based on the microscopic determination of the properties of individual atoms, molecules, or clusters, one can predict the macroscopic properties of bulk matter as mentioned in this paper.
Abstract: Theoretical investigations of gas phase clusters enable the evaluation of intrinsic molecular properties and intermolecular interactions Based on the microscopic determination of the properties of individual atoms, molecules, or clusters, one can predict the macroscopic properties of bulk matter With the aid of theoretical insights into the interaction forces holding these clusters and subsequent properties of a large number of cluster systems (ranging from simple water clusters to large π-systems), we have investigated the properties of various novel molecular systems including endo/exohedral fullerenes, nanotori, nonlinear optical materials, ionophores/receptors, polypeptides, enzymes, organic nanotubes, nanowires, and electronic and nano-mechanical molecular devices This mini-review highlights some of the interesting results obtained in the course of our extensive theoretical investigations of clusters toward the molecular design approach of nanomaterials