Browsing by Author "Witek, Henryk"

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Automatized Parametrization of SCC-DFTB Repulsive Potentials: Application to Hydrocarbons
(2009-10-29)
Gaus, Michael
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Chou, Chien-Pin
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Witek, Henryk
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Elstner, Marcus
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應用化學系
In this work, we derive and test a new automatized strategy to construct repulsive potentials for the self-consistent charge density functional tight-binding (SCC-DFTB) method. This approach allows one to explore the parameter space in a systematic fashion in order to find optimal solutions. We find that due to the limited flexibility of the SCC-DFTB electronic part, not all properties can be optimized simultaneously. For example, the optimization of heats of formation is in conflict with the optimization of vibrational frequencies. Therefore, a special parametrization, for vibrational frequencies is derived. It is shown that the performance of SCC-DFTB can be significantly improved using a more elaborate fitting strategy. A new fit for C and H is presented, which results in an average error of 2.6 kcal/mol for heats of formations for a large set of hydrocarbons, indicating that the performance of SCC-DFTB can be systematically improved also for other elements.
Communication: Three-dimensional model for phonon confinement in small particles: Quantitative bandshape analysis of size-dependent Raman spectra of nanodiamonds
(2014-01-28)
Korepanov, Vitaly I.
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Witek, Henryk
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Okajima, Hajime
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Osawa, Eiji
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Hamaguchi, Hiro-O
Raman spectroscopy of nano-scale materials is facing a challenge of developing a physically sound quantitative approach for the phonon confinement effect, which profoundly affects the phonon Raman band shapes of small particles. We have developed a new approach based on 3-dimensional phonon dispersion functions. It analyzes the Raman band shapes quantitatively in terms of the particle size distributions. To test the model, we have successfully obtained good fits of the observed phonon Raman spectra of diamond nanoparticles in the size range from 1 to 100 nm. (C) 2014 AIP Publishing LLC.
Infrared identification of the sigma-complex of Cl-C6H6 in the reaction of chlorine atom and benzene in solid para-hydrogen
(2013-02-21)
Bahou, Mohammed
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Witek, Henryk
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Lee, Yuan-Pern
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應用化學系
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應用化學系分子科學碩博班
The reaction of a chlorine atom with benzene (C6H6) is important in organic chemistry, especially in site-selective chlorination reactions, but its product has been a subject of debate for five decades. Previous experimental and theoretical studies provide no concrete conclusion on whether the product is a pi- or sigma-form of the Cl-C6H6 complex. We took advantage of the diminished cage effect of para-hydrogen (p-H-2) to produce Cl in situ to react with C6H6 (or C6D6) upon photolysis of a Cl-2/C6H6 (or C6D6)/p-H-2 matrix at 3.2 K. The infrared spectrum, showing intense lines at 1430.5, 833.6, 719.8, 617.0, and 577.4 cm(-1), and several weaker ones for Cl-C6H6, and the deuterium shifts of observed new lines unambiguously indicate that the product is a 6-chlorocyclohexadienyl radical, i.e., the sigma-complex of Cl-C6H6. Observation of the s-complex rather than the pi-complex indicates that the s-complex is more stable in solid p-H-2 at 3.2 K. The spectral information is crucial for further investigations of the Cl + C6H6 reaction either in the gaseous or solution phase. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790860]
PHYS 689-Rovibronic bands of the A(2)B(2) <- (XB1)-B-2 transition of C6H5O and C6D5O detected with cavity ringdown absorption near 1.15-1.32 mu m
(2008-04-06)
Cheng, Chi-Wen
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Witek, Henryk
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Lee, Yuan-Pern
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應用化學系
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應用化學系分子科學碩博班
PHYS 689-Rovibronic bands of the A(2)B(2) <- X(2)B(1) transition of C(6)H(5)O and C(6)D(5)O detected with cavity ringdown absorption near 1.15-1.32 mu m
(2008-04-06)
Cheng, Chi-Wen
;
Witek, Henryk
;
Lee, Yuan-Pern
;
應用化學系
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應用化學系分子科學碩博班
Rovibronic bands of the (A)over-tilde (2)B(2)<-(X)over-tilde (2)B(1) transition of C(6)H(5)O and C(6)D(5)O detected with cavity ringdown absorption near 1.2 mu m
(2008-10-21)
Cheng, Chi-Wen
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Witek, Henryk
;
Lee, Yuan-Pern
;
應用化學系
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應用化學系分子科學碩博班
We recorded several rovibronic bands of C(6)H(5)O and C(6)D(5)O in their (A) over tilde (2)B(2)<-(X) over tilde (2)B(1) transitions in the range 1.14-1.31 mu m with the cavity ringdown technique. While the electronic transition is forbidden, several vibronic bands are observed. By comparison of rovibronic contours of observed and simulated bands to determine their types of transition, and by consideration of vibrational wavenumbers of the upper state based on quantum-chemical calculations, we were able to provide vibronic assignments of observed bands and derive several experimental vibrational wavenumbers (given as nu in unit of cm(-1) in this paper) for the (A) over tilde (2)B(2) state, namely, nu(12)=947, nu(13)=793, nu(14)=417, nu(15)=964, nu(16)=866, nu(17)=723, nu(18)=680, and nu(19)=499 for C(6)H(5)O, and nu(12)=772, nu(13)=626, nu(14)=365, nu(15)=812, nu(17)=599, nu(18)=532, and nu(19)=436 for C(6)D(5)O. Transitions involving vibrationally excited levels of nu(20) were also observed; nu(20) of the (A) over tilde state is greater by 50 cm(-1) than the (X) over tilde state of C(6)H(5)O. A weak origin at 7681 cm(-1) for the (A) over tilde <-(X) over tilde transition of C(6)H(5)O (7661 cm(-1) for C(6)D(5)O) with a c-type contour was observed. Observed isotopic ratios of vibrational wavenumbers for the (A) over tilde state of C(6)H(5)O to those of C(6)D(5)O are in good agreement with the predictions from quantum-chemical calculations at the B3LYP/aug-cc-pVTZ level. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2992077]
Sub-stoichiometric reductive etherification of carbohydrate substrates and one-pot protecting group manipulation
(2020-04-28)
Chen, Chiao Wen
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Wang, Ching Chi
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Li, Xin Ru
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Witek, Henryk
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Mong, Kwok-Kong Tony
In this study, we report a new reductive etherification procedure for protection of carbohydrate substrates and its application for one-pot preparation of glycosyl building blocks. The reported procedure features the use of polymethylhydrosiloxane (PMHS) as a sub-stoichiometric reducing agent, which prevents the transilylation side reaction and improves the efficiency of the reductive etherification method. Application of the PMHS reductive etherification procedure for one-pot protecting group manipulation are described.
When finite becomes infinite: convergence properties of vibrational spectra of oligomer chains
(2018-10-01)
Chou, Chien-Pin
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Witek, Henryk
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Irle, Stephan
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交大名義發表
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應用化學系
We present a computational study of convergence properties of vibrational IR and Raman spectra for a series of increasingly long units of polyethylene, cis- and trans-polyacetylenes, and polyynes. Convergent behavior to the spectra of infinitely long polymers was observed in all cases when chains reached lengths of approximately 60 carbon atoms, both with respect to the positions of the bands and to their intensities. The vibrational spectra of longer chains are practically indistinguishable. The convergence rate depends on the degree of the conjugation in a studied system: Vibrational spectra for oligoethylenes converge noticeably faster than the spectra for the conjugated systems. The slowest convergence is observed for skeletal motions of the oligomer chains, which may require more than a hundred carbon atoms in the chain to show deviations smaller than 1cm(-1) to the corresponding solid-state calculations. The results suggest that the boundary between the properties of finite and infinite molecular systems fades away for a surprisingly small number of atoms.