Browsing by Author "Chen, Jien-Lian"

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Core Excitation, Specific Dissociation, and the Effect of the Size of Aromatic Molecules Connected to Oxygen: Phenyl Ether and 1,3-Diphenoxybenzene
(2014-09-11)
Lin, Yi-Shiue
;
Lin, Shu-Yu
;
Lee, Yuan T.
;
Tseng, Chien-Ming
;
Ni, Chi-Kung
Near-edge X-ray absorption fine structure (NEXAFS) spectra of phenyl ether at the carbon K-edge and 1,3-diphenoxybenzene at both the carbon and oxygen K-edges were measured in the total ion yield mode using X-rays from a synchrotron and a reflectron time-of-flight mass spectrometer. Time-dependent density functional theory was adopted to calculate the carbon and oxygen K-edge NEXAFS spectra of phenol, phenyl ether, and 1,3-diphenoxybenzene. The assignments and a comparison of the experimental and calculated spectra are presented. The mass spectra of ionic products formed after X-ray absorption at various excitation energies are also reported. Specific dissociations were observed for the 1s --> pi* transition of phenyl ether. In comparison with phenol and phenyl ether, the dependence of the fragmentation on the excitation site and destination state was weak in 1,3-diphenoxybenzene, likely as a result of delocalization of the valence electrons and rapid randomization of energy.
Highly Selective Dissociation of a Peptide Bond Following Excitation of Core Electrons
(2015-06-18)
Lin, Yi-Shiue
;
Tsai, Cheng-Cheng
;
Lin, Huei-Ru
;
Hsieh, Tsung-Lin
;
Chen, Jien-Lian
The controlled breaking of a specific chemical bond with photons in complex molecules remains a major challenge in chemistry. In principle, using the K-edge absorption of a particular atomic element, one might excite selectively a specific atomic entity in a molecule. We report here highly selective dissociation of the peptide bonds in N-methylformamide and N-methylacetamide on tuning the X-ray wavelength to the K-edge absorption of the atoms connected to (or near) the peptide bond. The high selectivity (56-71%) of this cleavage arises from the large energy shift of X-ray absorption, a large overlap of the Is orbital and the valence pi* orbital that is highly localized on a peptide bond with antibonding character, and the relatively low bond energy of the peptide bonds. These characteristics indicate that the high selectivity on bond dissociation following core excitation could be a general feature for molecules containing peptide bonds.
Measurement and prediction of the NEXAFS spectra of pyrimidine and purine and the dissociation following the core excitation
(2015-09-01)
Lin, Yi-Shiue
;
Lin, Huei-Ru
;
Liu, Wei-Lun
;
Lee, Yuan T.
;
Tseng, Chien-Ming
Near-edge X-ray absorption fine structure spectra of pyrimidine and purine at the carbon and nitrogen K-edges were measured using synchrotron radiation and a reflectron time-of-flight mass spectrometer. Spectrum assignments based on the time-dependent density functional theory calculation were presented. From the comparison of the experimental and theoretical results, the nature of various core-excitation transitions and the chemical shifts in the absorption energies were elucidated. The dissociation fragments following the core excitation were analyzed, and the enhancements on the fragment branching ratios were observed. The possible identities and dissociation mechanisms of the major fragments were discussed. (C) 2015 Elsevier By. All rights reserved .
Selectivity of peptide bond dissociation on excitation of a core electron: Effects of a phenyl group
(2016-09-01)
Tsai, Cheng-Cheng
;
Chen, Jien-Lian
;
Hu, Wei-Ping
;
Lin, Yi-Shiue
;
Lin, Huei-Ru
The selective dissociation of a peptide bond upon excitation of a core electron in acetanilide and N-benzylacetamide was investigated. The total-ion-yield near-edge X-ray absorption fine structure spectra were recorded and compared with the predictions from time-dependent density functional theory. The branching ratios for the dissociation of a peptide bond are observed as 16-34% which is quite significant. This study explores the core-excitation, the X-ray photodissociation pathways, and the theoretical explanation of the NEXAFS spectra of organic molecules containing both a peptide bond and a phenyl group. (C) 2016 Elsevier B.V. All rights reserved.