Browsing by Author "Shen, Zhitao"

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First-principles study on sum-frequency generation spectroscopy of methanol adsorbed on TiO2(110) surface: Effects of substrate and molecular coverages
(2019-05-14)
Shen, Zhitao
;
Lin, Chih-Kai
;
Zhu, Chaoyuan
;
Lin, Sheng Hsien
;
交大名義發表
In this work, starting from the general theory of sum-frequency generation (SFG), we proposed a computational strategy utilizing density functional theory with periodic boundary conditions to simulate the vibrational SFG of molecules/solid surface adsorption system. The method has been applied to the CH3OH/TiO2(110) system successfully. Compared with the isolated molecule model, our theoretical calculations showed that the TiO2 substrate can significantly alter the second-order susceptibilities of a methanol molecule which is directly related to the SFG intensity. In addition, the SFG spectra have obvious changes while the methanol coverage increases, especially for the OH vibration peaks. Our theoretical spectra agree reasonably well with experimental measurements at 1 ML coverage, and an interesting peak which is absent in the theoretical spectra is tentatively assigned to some CH3 stretch vibration of methanol adsorbed on the oxygen vacancy of TiO2.
Spectral Characterization of Three-Electron Two-Center (3e-2c) Bonds of Gaseous CH3-S therefore S(H)CH3 and (CH3SH)(2)(+) and Enhancement of the 3e-2c Bond upon Protonation
(2018-07-05)
Xie, Min
;
Shen, Zhitao
;
Wang, Dandan
;
Fujii, Asuka
;
Lee, Yuan-Pern
The three-electron two-center (3e-2c) bond plays an important role in structures and electron communication in biological systems involving cationic sulfur compounds. Although the nature of 3e-2c bonds and their theoretical formalism have attracted great interest, direct spectral identifications of 3e-2c-bound molecules are scarce. We observed the infrared spectra of the weakly 3e-2c-bound CH3S:.S(H)CH3 and the strongly 3e-2c-bound (CH3SH)(2)(+) in a supersonic jet using infrared (IR) dissociation with vacuum-ultraviolet photoionization and time-of-flight detection. Protonation of CH3S:.S(H)CH3 to form [CH3(H)S therefore S(H)CH3](+) significantly enhances the 3e-2c bond, characterized by a large red shift of the SH-stretching band with enhanced IR intensity, shortening of the calculated S-S distance from 3.00 to 2.86 angstrom, and a dissociation energy increased from similar to 23 to 162 kJ mol(-1).
Vibrational autoionization of state-selective jet-cooled methanethiol (CH3SH) investigated with infrared plus vacuum-ultraviolet photoionization
(2017-11-21)
Xie, Min
;
Shen, Zhitao
;
Pratt, S. T.
;
Lee, Yuan-Pern
;
應用化學系
Vibrational autoionization of Rydberg states provides key information about nonadiabatic processes above an ionization threshold. We employed time-of-flight mass detection of CH3SH+ to record vibrational-state selective photo-ionization efficiency (PIE) spectra of jet-cooled methanethiol (CH3SH) on exciting CH3SH to a specific vibrationally excited state with an infrared (IR) laser, followed by excitation with a tunable laser in the vacuum-ultraviolet (VUV) region for ionization. Autoionizing Rydberg states assigned to the ns, np, nd and nf series are identified. When IR light at 2601 (nu(3), SH stretching mode) and 2948 cm(-1) (nu(2), CH3 symmetric stretching mode) was employed, the Rydberg series converged to the respective vibrationally excited (nu(3) and nu(2)) states of CH3SH+. When IR light at 3014 cm(-1) (overlapped nu(1)/nu(9), CH3 antisymmetric stretching and CH2 antisymmetric stretching modes) was employed, Rydberg series converging to two vibrationally excited states (nu(1) and nu(9)) of CH3SH+ were observed. When IR light at 2867 cm(-1) (2 nu(10), overtone of CH3 deformation mode) and 2892 cm(-1) (2 nu(4), overtone of CH2 scissoring mode) was employed, both Delta nu = 01 and Delta nu = -2 ionization transitions were observed; there is evidence for direct ionization from the initial state into the CH3SH+ (nu(+)(4) = 1) continuum. In all observed IR-VUV-PIE spectra, the ns and nd series show intensity greater than the other Rydberg series, which is consistent with the fact that the highest-occupied molecular orbital of CH3SH is a p-like lone pair orbital on the S atom. The quantum yields for autoionization of various vibrational excited states are discussed. Values of nu(1) = 3035, nu(2) = 2884, nu(3) = 2514, and nu(9) = 2936 cm(-1) for CH3SH+ derived from the converged limits agree satisfactorily with values observed for Ar-tagged CH3SH+ at 3026, 2879, 2502, and 2933 cm(-1).