Browsing by Author "Nishimura, Yoshifumi"

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Automatized Parameterization of DFTB Using Particle Swarm Optimization
(2016-01)
Chou, Chien-Pin
;
Nishimura, Yoshifumi
;
Fan, Chin-Chai
;
Mazur, Grzegorz
;
Irle, Stephan
We present a novel density-functional tight-binding (DFTB) parametrization toolkit developed to optimize the parameters of various DFTB models in a fully automatized fashion. The main features of the algorithm, based on the particle swarm optimization technique, are discussed, and a number of initial pilot applications of the developed methodology to molecular and solid systems are presented.
Automatized Parameterization of DFTB Using Particle Swarm Optimization
(2016-01-01)
Chou, Chien-Pin
;
Nishimura, Yoshifumi
;
Fan, Chin-Chai
;
Mazur, Grzegorz
;
Irle, Stephan
We present a novel density-functional tight-binding (DFTB) parametrization toolkit developed to optimize the parameters of various DFTB models in a fully automatized fashion. The main features of the algorithm, based on the particle swarm optimization technique, are discussed, and a number of initial pilot applications of the developed methodology to molecular and solid systems are presented.
Automatized Parameterization of the Density-functional Tight-binding Method. II. Two-center Integrals
(2016-01-01)
Witek, Henryk A.
;
Chou, Chien-Pin
;
Mazur, Grzegorz
;
Nishimura, Yoshifumi
;
Irle, Stephan
We present an efficient numerical integration scheme (TWOCENT) to be used in the context of automatized parameterization of the density-functional tight-binding (DFTB) method. The accuracy of the integration process is assessed and its range of applicability is discussed. The functionality of the developed code is tested by reproducing the electronic portion of the existing mio parameter sets and by reproducing a series of reference DFT band structures of elemental solids.
Automatized Parameterization of the Density-functional Tight-binding Method. II. Two-center Integrals
(2016-01)
Witek, Henryk A.
;
Chou, Chien-Pin
;
Mazur, Grzegorz
;
Nishimura, Yoshifumi
;
Irle, Stephan
We present an efficient numerical integration scheme (TWOCENT) to be used in the context of automatized parameterization of the density-functional tight-binding (DFTB) method. The accuracy of the integration process is assessed and its range of applicability is discussed. The functionality of the developed code is tested by reproducing the electronic portion of the existing mio parameter sets and by reproducing a series of reference DFT band structures of elemental solids.
Critical interpretation of CH- and OH- stretching regions for infrared spectra of methanol clusters (CH3OH)(n) (n=2-5) using self-consistent-charge density functional tight-binding molecular dynamics simulations
(2014-09-07)
Nishimura, Yoshifumi
;
Lee, Yuan-Pern
;
Irle, Stephan
;
Witek, Henryk A.
;
交大名義發表
Vibrational infrared (IR) spectra of gas-phase O-H center dot center dot center dot O methanol clusters up to pentamer are simulated using self-consistent-charge density functional tight-binding method using two distinct methodologies: standard normal mode analysis and Fourier transform of the dipole time-correlation function. The twofold simulations aim at the direct critical assignment of the C-H stretching region of the recently recorded experimental spectra [H.-L. Han, C. Camacho, H. A. Witek, and Y.-P. Lee, J. Chem. Phys. 134, 144309 (2011)]. Both approaches confirm the previous assignment (ibid.) of the C-H stretching bands based on the B3LYP/ANO1 harmonic frequencies, showing that nu(3), nu(9), and nu(2) C-H stretching modes of the proton-accepting (PA) and proton-donating (PD) methanol monomers experience only small splittings upon the cluster formation. This finding is in sharp discord with the assignment based on anharmonic B3LYP/VPT2/ANO1 vibrational frequencies (ibid.), suggesting that some procedural faults, likely related to the breakdown of the perturbational vibrational treatment, led the anharmonic calculations astray. The IR spectra based on the Fourier transform of the dipole time-correlation function include new, previously unaccounted for physical factors such as non-zero temperature of the system and large amplitude motions of the clusters. The elevation of temperature results in a considerable non-homogeneous broadening of the observed IR signals, while the presence of large-amplitude motions (methyl group rotations and PA-PD flipping), somewhat surprisingly, does not introduce any new features in the spectrum. (C) 2014 AIP Publishing LLC.
Dramatic Reduction of IR Vibrational Cross Sections of Molecules Encapsulated in Carbon Nanotubes
(2011-06-01)
Kazachkin, Dmitry V.
;
Nishimura, Yoshifumi
;
Witek, Henryk A.
;
Irle, Stephan
;
Borguet, Eric
Combined temperature-programmed desorption and IR studies suggest that absorption cross sections of IR-active vibrations of molecules "strongly" bound to single-wall sidewalls as the origin of such "screening". The observed and identify dielectric screening by highly polarizable SWCNT endohedrally encapsulated molecules are dramatically reduced, carbon nanotubes (SWCNTs) are reduced at least by a factor of 10. Quantum chemical simulations show that IR intensities of intensity reduction originates from a sizable cancellation of adsorbate dipole moments by mirror charges dynamically induced on the nanotube sidewalls. For exohedrally adsorbed molecules, the dielectric screening is found to be orientation-dependent with a smaller magnitude for adsorption in groove and interstitial sites. The presented results clearly demonstrate and quantify the screening effect of SWCNTs and unequivocally show that IR spectroscopy cannot be applied in a straightforward manner to the study of peapod systems.
Infrared absorption spectrum of the simplest deuterated Criegee intermediate CD2OO
(2016-07-28)
Huang, Yu-Hsuan
;
Nishimura, Yoshifumi
;
Witek, Henryk A.
;
Lee, Yuan-Pern
;
應用化學系
We report a transient infrared (IR) absorption spectrum of the simplest deuterated Criegee intermediate CD2OO recorded using a step-scan Fourier-transform spectrometer coupled with a multipass absorption cell. CD2OO was produced from photolysis of flowing mixtures of CD2I2, N-2, and O-2 (13 or 87 Torr) with laser light at 308 nm. The recorded spectrum shows close structural similarity with the spectrum of CH2OO reported previously [Y.-T. Su et al., Science 340, 174 (2013)]. The four bands observed at 852, 1017, 1054, and 1318 cm(-1) are assigned to the OO stretching mode, two distinct in-plane OCD bending modes, and the CO stretching mode of CD2OO, respectively, according to vibrational wavenumbers, IR intensities, rotational contours, and deuterium-isotopic shifts predicted with extensive quantum-chemical calculations. The CO-stretching mode of CD2OO at 1318 cm(-1) is blue shifted from the corresponding band of CH2OO at 1286 cm(-1); this can be explained by a mechanism based on mode mixing and isotope substitution. A band near 936 cm(-1), observed only at higher pressure (87 Torr), is tentatively assigned to the CD2 wagging mode of CD2IOO. Published by AIP Publishing.
Infrared identification of the Criegee intermediates syn- and anti-CH3CHOO, and their distinct conformation-dependent reactivity
(2015-05-01)
Lin, Hui-Yu
;
Huang, Yu-Hsuan
;
Wang, Xiaohong
;
Bowman, Joel M.
;
Nishimura, Yoshifumi
The Criegee intermediates are carbonyl oxides that play critical roles in ozonolysis of alkenes in the atmosphere. So far, the mid-infrared spectrum of only the simplest Criegee intermediate CH2OO has been reported. Methyl substitution of CH2OO produces two conformers of CH3CHOO and consequently complicates the infrared spectrum. Here we report the transient infrared spectrum of syn- and anti-CH3CHOO, produced from CH3CHI + O-2 in a flow reactor, using a step-scan Fourier-transform spectrometer. Guided and supported by high-level full-dimensional quantum calculations, rotational contours of the four observed bands are simulated successfully and provide definitive identification of both conformers. Furthermore, anti-CH3CHOO shows a reactivity greater than syn-CH3CHOO towards NO/NO2; at the later period of reaction, the spectrum can be simulated with only syn-CH3CHOO. Without NO/NO2, anti-CH3CHOO also decays much faster than syn-CH3CHOO. The direct infrared detection of syn-and anti-CH3CHOO should prove useful for field measurements and laboratory investigations of the Criegee mechanism.
Mechanism of Back Electron Transfer in an Intermolecular Photoinduced Electron Transfer Reaction: Solvent as a Charge Mediator
(2014-10-06)
Narra, Sudhakar
;
Nishimura, Yoshifumi
;
Witek, Henryk A.
;
Shigeto, Shinsuke
;
應用化學系
Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron transfer reactions. Unfortunately, a detailed mechanism of BET reactions remains largely unknown in spite of their importance for the development of molecular photovoltaic structures. Here, we examine the BET reaction of pyrene (Py) and 1,4-dicyanobenzene (DCB) in acetonitrile (ACN) by using time-resolved near-and mid-IR spectroscopy. The Py dimer radical cation (Py-2(center dot+)) and DCB radical anion (DCB center dot-) generated after photoexcitation of Py show asynchronous decay kinetics. To account for this observation, we propose a reaction mechanism that involves electron transfer from DCB center dot- to the solvent and charge recombination between the resulting ACN dimer anion and Py-2(center dot+). The unique role of ACN as a charge mediator revealed herein could have implications for strategies that retard charge recombination in dye-sensitized solar cells.
Theoretical study of cellobiose hydrolysis to glucose in ionic liquids
(2014-05-30)
Nishimura, Yoshifumi
;
Yokogawa, Daisuke
;
Irle, Stephan
;
應用化學系
;
應用化學系分子科學碩博班
The S(N)1-type hydrolysis reaction of cellobiose in ionic liquids (ILs) was theoretically investigated. First principles and ab initio quantum chemical methods were used in conjunction with the \'reference interaction site model self-consistent field with spatial electron density distribution\' (RISM-SCF-SEDD) method. Reaction mechanism pathways are discussed and compared to calculations in gas phase and in aqueous solution. Analysis of solvation effects indicates strong interaction between hydrogen atoms of glucose hydroxyl groups and the anions in ILs, contributing to large stabilization of the reaction product. The calculated activation energy in ILs (24.5 kcal/mol) agrees quantitatively with the experimental value (26.5 kcal/mol). (C) 2014 Elsevier B. V. All rights reserved.