Research Center for Computational Science

Okazaki, Japan

Research Center for Computational Science

Okazaki, Japan
SEARCH FILTERS
Time filter
Source Type

Dhital R.N.,Graduate University for Advanced Studies | Kamonsatikul C.,Mahidol University | Somsook E.,Mahidol University | Bobuatong K.,Research Center for Computational Science | And 4 more authors.
Journal of the American Chemical Society | Year: 2012

This paper describes the unique catalytic activity of bimetallic Au/Pd alloy nanoclusters (NCs) for Ullmann coupling of chloroarenes in aqueous media at low temperature. The corresponding reaction cannot be achieved by monometallic Au and Pd NCs as well as their physical mixtures. On the basis of quantum chemical calculation, it was found that the crucial step to govern the unusual catalytic activity of Au/Pd is the dissociative chemisorption of ArCl, which is unlikely in the monometallic Au and Pd NCs. © 2012 American Chemical Society.


Takahashi O.,Hiroshima University | Tashiro M.,Japan Institute for Molecular Science | Tashiro M.,Research Center for Computational Science | Ehara M.,Japan Institute for Molecular Science | And 4 more authors.
Chemical Physics | Year: 2011

We have investigated the K-2, K-1L-1, and L-2 double core hole (DCH) states of the SiX4 (X = H, F, Cl, and CH3) molecules using the CASSCF and DFT methods aiming at the DCH electron spectroscopy. The Si 1s IPs and DIPs of the present molecules and the generalized relaxation energies are compared and analyzed. The values extracted from the excess relaxation energy agree well with the generalized relaxation energy. The effect of the substituents (H, F, Cl, and CH3) surrounding the central Si atom is examined. The present results illustrate that the DCH electron spectroscopy for K-2, K-1L -1, and L-2 DCH states is useful for the chemical analysis. © 2011 Elsevier B.V. All rights reserved.


Tashiro M.,Japan Institute for Molecular Science | Tashiro M.,Research Center for Computational Science | Ehara M.,Japan Institute for Molecular Science | Ehara M.,Research Center for Computational Science | Ueda K.,Tohoku University
Chemical Physics Letters | Year: 2010

We have theoretically investigated the double core-hole (DCH) states of the open-shell molecules and examined the possibility of DCH spectroscopy by means of X-ray two-photon photoelectron spectroscopy (XTPPS). Energies of many DCH states were obtained by the CASSCF calculations and the generalized intra-and inter-atomic relaxation energies were evaluated. We show that XTPPS can extract these quantities by the measurement of single and double core-hole ionization potentials. We discuss the influence of chemical environment on the DCH states with two holes at the same atomic site and at two different atomic sites. © 2010 Elsevier B.V. All rights reserved.


Takahashi O.,Hiroshima University | Tashiro M.,Japan Institute for Molecular Science | Tashiro M.,Research Center for Computational Science | Ehara M.,Japan Institute for Molecular Science | And 4 more authors.
Journal of Physical Chemistry A | Year: 2011

Double-core-hole (DCH) spectra have been investigated for pyrimidine, purine, the RNA/DNA nucleobases, and formamide, using the density functional theory (DFT) method. DCH spectra of formamide were also examined by the complete-active-space self-consistent-field (CASSCF) method. All possible single- and two-site DCH (ssDCH and tsDCH) states of the nucleobases were calculated. The generalized relaxation energy and interatomic generalized relaxation energy were evaluated from the energy differences between ssDCH and single-core-hole (SCH) states and between tsDCH and SCH states, respectively. The generalized relaxation energy is correlated to natural bond orbital charge, whereas the interatomic generalized relaxation energy is correlated to the interatomic distance between the core holes at two sites. The present analysis using DCH spectroscopy demonstrates that the method is useful for the chemical analysis of large molecular systems. © 2011 American Chemical Society.


Bobuatong K.,Research Center for Computational Science | Karanjit S.,Japan Institute for Molecular Science | Fukuda R.,Research Center for Computational Science | Fukuda R.,Japan Institute for Molecular Science | And 3 more authors.
Physical Chemistry Chemical Physics | Year: 2012

The aerobic oxidation of methanol to formic acid catalyzed by Au 20 - has been investigated quantum chemically using density functional theory with the M06 functional. Possible reaction pathways are examined taking account of full structure relaxation of the Au 20 - cluster. The proposed reaction mechanism consists of three elementary steps: (1) formation of formaldehyde from methoxy species activated by a superoxo-like anion on the gold cluster; (2) nucleophilic addition by the hydroxyl group of a hydroperoxyl-like complex to formaldehyde resulting in a hemiacetal intermediate; and (3) formation of formic acid by hydrogen transfer from the hemiacetal intermediate to atomic oxygen attached to the gold cluster. A comparison of the computed energetics of various elementary steps indicates that C-H bond dissociation of the methoxy species leading to formation of formaldehyde is the rate-determining step. A possible reaction pathway involving single-step hydrogen abstraction, a concerted mechanism, is also discussed. The stabilities of reactants, intermediates and transition state structures are governed by the coordination number of the gold atoms, charge distribution, cooperative effect and structural distortion, which are the key parameters for understanding the relationship between the structure of the gold cluster and catalytic activity in the aerobic oxidation of alcohols. © 2012 The Owner Societies.


Fukuda R.,Japan Institute for Molecular Science | Fukuda R.,Research Center for Computational Science | Fukuda R.,Japan Science and Technology Agency | Ehara M.,Japan Institute for Molecular Science | And 4 more authors.
Procedia Computer Science | Year: 2011

We developed the direct SAC-CI (symmetry-adapted cluster-configuration interaction) method for accurate calculations of electronic excited states of large molecules. The direct SAC-CI method uses an efficient MO (molecular orbital) direct scheme that is compatible with the perturbation-selection technique with using sparse linear algebra techniques. The direct SAC-CI can calculate large molecules with moderate computational cost. Using the direct SAC-CI method, excited states of free-base tetraazaporphyrin, phthalocyanine, naphthalocyanine, and anthracocyanine were studied. An interesting feature of the observed spectra is that the splitting of the Q-bands decreases with increasing molecular size. The SAC-CI calculation well reproduced the observed trend of excitation energies and showed the importance of electron correlation effects in the excited states. © 2011 Published by Elsevier Ltd.


Karanjit S.,Japan Institute for Molecular Science | Bobuatong K.,Research Center for Computational Science | Fukuda R.,Japan Institute for Molecular Science | Fukuda R.,Research Center for Computational Science | And 3 more authors.
International Journal of Quantum Chemistry | Year: 2013

The mechanism of the aerobic oxidation of methanol to formic acid catalyzed by Au8 - has been systematically investigated using density functional theory with the M06 functional. The reaction pathways were examined by taking into account the full structural relaxation of the Au 8 -. Stepwise and concerted reaction mechanisms are proposed. The stepwise mechanism is initiated by the hydrogen abstraction of a methoxy species by a superoxo-like anion on the gold cluster, resulting in the formation of formaldehyde. Subsequently, the formaldehyde is activated by the hydroxyl group of a hydroperoxyl-like complex, leading to the formation of a hemiacetal intermediate. The formation of formic acid in the final step is achieved by hydrogen abstraction of the hemiacetal intermediate by atomic oxygen attached to the gold cluster. Our calculations indicate that the first step of the stepwise mechanism, that is, hydrogen abstraction of the methoxy species, is the rate-determining step. Another possible reaction pathway involving a single-step hydrogen abstraction, a concerted mechanism, is also discussed. This mechanism may also be responsible for the reasonable catalytic activity of aerobic oxidation of methanol on Au8 - because of the low activation energy barrier. © 2012 Wiley Periodicals, Inc.


Fukuda R.,Japan Institute for Molecular Science | Fukuda R.,Research Center for Computational Science | Fukuda R.,Venture Institute | Fukuda R.,Japan Science and Technology Agency | And 5 more authors.
Journal of Chemical Physics | Year: 2010

Electronic excited states, electronic absorption, and magnetic circular dichroism (MCD) spectra of free-base tetraazaporphyrin (TAP), phthalocyanine (Pc), naphthalocyanine (Nc), and anthracocyanine (Ac) were studied by quantum chemical calculations using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. Not only optically allowed states including the Q- and B-bands but also optically forbidden states were calculated for transitions whose excitation energies were lower than 4.5 eV. The present SAC-CI calculations consistently assigned the absorption and MCD peaks as optically allowed π → π excitations, although these calculations using double-zeta basis limit quantitative agreement and discussion. For Nc and Ac, excited states beyond the four-orbital model appeared in the low-energy region. The low-energy shifts of the Q-bands with the extension of molecular size were explained by the orbital energies. The splitting of the Q-bands decreases with extension of the molecular size. This feature was reproduced by the SAC-CI calculations but the configuration interaction with single excitations and time-dependent density functional theory calculations failed to reproduce this trend. Electron correlation in the excited states is important in reproducing this splitting of the Q-bands and in describing the energy difference between the B2u and B3u states of free-base porphyrins. © 2010 American Institute of Physics.


Fukuda R.,Japan Institute for Molecular Science | Fukuda R.,Research Center for Computational Science | Fukuda R.,Kyoto University | Ehara M.,Japan Institute for Molecular Science | And 2 more authors.
Bulletin of the Chemical Society of Japan | Year: 2013

The excited electronic states and optical absorption spectra of coronene (C24H12), hexa-peri-hexabenzocoronene (HBC) (C42H18), and circumcoronene (C 54H18) were studied using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. For coronene and HBC, the SAC-CI calculations reproduced the experimental spectra well and predicted optically forbidden excited states. For HBC, the symmetry lowering enhanced the intensity of the S2 state that corresponds to the p-band, and the SAC-CI calculation predicted the existence of the second and third optically allowed states around the β-band region near 4.0 eV. For circumcoronene, the SAC-CI calculation predicted a strong absorption of the β-band in the visible light region. The mechanisms of energy splitting for the HOMOLUMO transition were investigated. Electron correlation was the most important factor for the energy splitting between the lowest and the nextlowest states. Configuration interaction with single excitations (CIS) calculations could not correctly predict the relative energies of these states in coronene and circumcoronene. For HBC, on the other hand, the CIS calculation provided the same energy order as the SAC-CI calculation. © 2013 The Chemical Society of Japan.


Dhital R.N.,Osaka University | Ehara M.,Research Center for Computational Science | Sakurai H.,Osaka University
Yuki Gosei Kagaku Kyokaishi/Journal of Synthetic Organic Chemistry | Year: 2015

The Palladium nanoclusters (NCs)- catalyzed carbon- carbon bond- forming reaction has emerged as a powerful tool in organic synthesis and has been intensively studied over the past few decades. Recently, a series of gold NC- catalyzed carbon- carbon bond formation has also been introduced even though contamination by PPB levels of palladium impurities has caused debate. Besides the development of gold and palladium NCs, new strategies for the design of gold- based bimetallic catalysts have subsequently been introduced. These burgeoning strategies are remarkable for achieving reaction under ambient conditions, with unique selectivity, and high activity. Among the various bimetallic NCs, Au/Pd has attracted a great deal of attention because of its superior catalytic activities in various types of reaction. In attempts to rationalize the synergistic effects observed with these catalysts, two major factors have been suggested, the ensemble effect and the ligand effect and these effects have been well documented in recently published reviews. This account provides an overview of recent developments in the use of gold- palladium bimetallic NCs for C- X bond activation. A specific objective is to demonstrate the effect of inclusion of Au as nearest heteroatom in the Au/Pd alloy in C- X bond activation from the viewpoint of both experimental observations and quantum chemical calculations.

Loading Research Center for Computational Science collaborators
Loading Research Center for Computational Science collaborators