Browsing by Author "Lee, Yuan T."

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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
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Lin, Shu-Yu
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Lee, Yuan T.
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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.
Dynamics of the F(2) Reaction with Propene: The Effect of Methyl Substitution
(2009-04-23)
Fang, Jing-Wen
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Xie, Tingxian
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Chen, Hsueh-Ying
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Lu, Yu-Ju
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Lee, Yuan T.
The reaction of F(2) + C(3)H(6) has been investigated with the crossed molecular beam technique. The only observed primary product channel is F + C(3)H(6)F while the HF + C(3)H(5)F channel cannot be found. The reaction cross section was measured as a function of collision energy and the reaction threshold was determined to be 2.4 +/- 0.3 kcal/mol. Compared to the reaction threshold of the F(2) + C(2)H(4) reaction, the methyl substitution effectively reduces the reaction threshold by about 3 kcal/mol. The product time-of-flight spectra and angular distributions were measured and analyzed. The angular distribution displays strongly backward, indicating that the reaction is much faster than rotation. All experimental results support a rebound reaction mechanism, which agrees with the structure of the calculated transition state. The transition state geometry also suggests an early barrier; such dynamics is consistent with the observed small kinetic energy release of the products. Except for the different values of the reaction thresholds, the dynamics of the F(2) + C(2)H(4) and F(2) + C(3)H(6) reactions are remarkably similar.
Dynamics of the F-2 Reaction with Propene: The Effect of Methyl Substitution
(2009-04-23)
Fang, Jing-Wen
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Xie, Tingxian
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Chen, Hsueh-Ying
;
Lu, Yu-Ju
;
Lee, Yuan T.
The reaction of F-2 + C3H6 has been investigated with the crossed molecular beam technique. The only observed primary product channel is F + C3H6F while the HF + C3H5F channel cannot be found. The reaction cross section was measured as a function of collision energy and the reaction threshold was determined to be 2.4 +/- 0.3 kcal/mol. Compared to the reaction threshold of the F-2 + C2H4 reaction, the methyl substitution effectively reduces the reaction threshold by about 3 kcal/mol. The product time-of-flight spectra and angular distributions were measured and analyzed. The angular distribution displays strongly backward, indicating that the reaction is much faster than rotation. All experimental results support a rebound reaction mechanism, which agrees with the structure of the calculated transition state. The transition state geometry also suggests an early barrier; such dynamics is consistent with the observed small kinetic energy release of the products. Except for the different values of the reaction thresholds, the dynamics of the F-2 + C2H4 and F-2 + C3H6 reactions are remarkably similar.
Dynamics of the O-18(D-1)+(CO)-O-16 reaction by crossed beam methods and implications for non-mass-dependent oxygen isotope effects in bulk photochemistry experiments
(2014-08-10)
Estillore, Armando D.
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Jin, Bing
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Huang, Chia-Pin
;
Garofalo, Lauren
;
Smith, Mica
Excited-state dissociation dynamics of phenol studied by a new time-resolved technique
(2018-02-21)
Lin, Yen-Cheng
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Lee, Chin
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Lee, Shih-Huang
;
Lee, Yin-Yu
;
Lee, Yuan T.
Phenol is an important model molecule for the theoretical and experimental investigation of dissociation in the multistate potential energy surfaces. Recent theoretical calculations [X. Xu et al., J. Am. Chem. Soc. 136, 16378 (2014)] suggest that the phenoxyl radical produced in both the X and A states from the O-H bond fission in phenol can contribute substantially to the slow component of photofragment translational energy distribution. However, current experimental techniques struggle to separate the contributions from different dissociation pathways. A new type of time-resolved pump-probe experiment is described that enables the selection of the products generated from a specific time window after molecules are excited by a pump laser pulse and can quantitatively characterize the translational energy distribution and branching ratio of each dissociation pathway. This method modifies conventional photofragment translational spectroscopy by reducing the acceptance angles of the detection region and changing the interaction region of the pump laser beam and the molecular beam along the molecular beam axis. The translational energy distributions and branching ratios of the phenoxyl radicals produced in the X, A, and B states from the photodissociation of phenol at 213 and 193 nm are reported. Unlike other techniques, this method has no interference from the undissociated hot molecules. It can ultimately become a standard pump-probe technique for the study of large molecule photodissociation in multistates. Published by AIP Publishing.
Measurement and prediction of the NEXAFS spectra of pyrimidine and purine and the dissociation following the core excitation
(2015-09-01)
Lin, Yi-Shiue
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Lin, Huei-Ru
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Liu, Wei-Lun
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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 .
Near-Edge X-ray Absorption Fine Structure Spectra and Site-Selective Dissociation of Phenol
(2014-03-06)
Lin, Yi-Shiue
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Lu, Kun-Ta
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Lee, Yuan T.
;
Tseng, Chien-Ming
;
Ni, Chi-Kung
A time-of-flight mass spectrometer with orthogonal acceleration and soft X-rays from synchrotron radiation were utilized to measure near-edge X-ray absorption fine structure (NEXAFS) spectra of carbon and oxygen in phenol and the corresponding ionic fragments following core excitation. The photon energies were in the range of 284-298 eV for the carbon K-edge and 529.5-554.5 eV for the oxygen K-edge. The total ion yield, ion intensity for each ionic fragment, and ion intensity ratio, defined as ion intensity divided by total ion yield, were measured as a function of photon energy. Possible mechanisms of dissociation are proposed and enhancements of specific products of dissociation are reported. In general, the enhancement of these specific products is small in the carbon K-edge region but is clear for some products at the oxygen K-edge. In particular, elimination of the H atom from the hydroxyl group was observed only at the oxygen K-edge. One remarkable result is that an excitation of a core-level electron of oxygen greatly enhanced the cleavage of specific C-C bonds.
Photodissociation Dynamics of Benzaldehyde (C(6)H(5)CHO) at 266, 248, and 193 nm
(2011-11-04)
Bagchi, Arnab
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Huang, Yu-Hsuan
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Xu, Z. F.
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Raghunath, P.
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Lee, Yuan T.
The photodissociation of gaseous benzaldehyde (C(6)H(5)CHO) at 193, 248, and 266 nm using multimass ion imaging and step-scan time-resolved Fourier-transform infrared emission techniques is investigated. We also characterize the potential energies with the CCSD(T)/6-311+G(3df,2p) method and predict the branching ratios for various channels of dissociation. Upon photolysis at 248 and 266 nm, two major channels for formation of HCO and CO, with relative branching of 0.37:0.63 and 0.20:0.80, respectively, are observed. The C(6)H(5)+HCO channel has two components with large and small recoil velocities; the rapid component with average translational energy of approximately 25 kJ mol(-1) dominates. The C(6)H(6)+CO channel has a similar distribution of translational energy for these two components. IR emission from internally excited C(6)H(5)CHO, nu(3) (nu = 1) of HCO, and levels nu <= 2, J <= 43 of CO are observed; the latter has an average rotational energy of approximately 13 kJ mol(-1) and vibrational energy of approximately 6 kJ mol(-1). Upon photolysis at 193 nm, similar distributions of energy are observed, except that the C(6)H(5)+HCO channel becomes the only major channel with a branching ratio of 0.82 +/- 0.10 and an increased proportion of the slow component; IR emission from levels nu(1) (nu = 1) and nu(3) (nu = 1 and 2) of HCO and nu <= 2, J <= 43 of CO are observed; the latter has an average energy similar to that observed in photolysis at 248 nm. The observed product yields at different dissociation energies are compared to statistical-theory predicted results based on the computed singlet and triplet potential-energy surfaces.
Photodissociation Dynamics of Benzaldehyde (C6H5CHO) at 266, 248, and 193 nm
(2011-11-04)
Bagchi, Arnab
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Huang, Yu-Hsuan
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Xu, Z. F.
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Raghunath, P.
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Lee, Yuan T.
The photodissociation of gaseous benzaldehyde (C6H5CHO) at 193, 248, and 266 nm using multimass ion imaging and step-scan time-resolved Fourier-transform infrared emission techniques is investigated. We also characterize the potential energies with the CCSD(T)/6-311+G(3df,2p) method and predict the branching ratios for various channels of dissociation. Upon photolysis at 248 and 266 nm, two major channels for formation of HCO and CO, with relative branching of 0.37:0.63 and 0.20:0.80, respectively, are observed. The C6H5+HCO channel has two components with large and small recoil velocities; the rapid component with average translational energy of approximately 25 kJ mol(-1) dominates. The C6H6+CO channel has a similar distribution of translational energy for these two components. IR emission from internally excited C6H5CHO, nu(3) (nu = 1) of HCO, and levels nu <= 2, J <= 43 of CO are observed; the latter has an average rotational energy of approximately 13 kJ mol(-1) and vibrational energy of approximately 6 kJ mol(-1). Upon photolysis at 193 nm, similar distributions of energy are observed, except that the C6H5+HCO channel becomes the only major channel with a branching ratio of 0.82 +/- 0.10 and an increased proportion of the slow component; IR emission from levels nu(1) (nu = 1) and nu(3) (nu = 1 and 2) of HCO and nu <= 2, J <= 43 of CO are observed; the latter has an average energy similar to that observed in photolysis at 248 nm. The observed product yields at different dissociation energies are compared to statistical-theory predicted results based on the computed singlet and triplet potential-energy surfaces.
Photodissociation dynamics of nitrobenzene and o-nitrotoluene
(2007-02-14)
Lin, Ming-Fu
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Lee, Yuan T.
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Ni, Chi-Kung
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Xu, Shucheng
;
Lin, M. C.
Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO2 elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO2 elimination channels were determined to be NO/NO2=0.32 +/- 0.12 (193 nm), 0.26 +/- 0.12 (248 nm), and 0.4 +/- 0.12(266 nm) for nitrobenzene and 0.42 +/- 0.12(193 nm) and 0.3 +/- 0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results. (c) 2007 American Institute of Physics.
Photodissociation dynamics of nitrobenzene and o-nitrotoluene
(2007-02-14)
Lin, Ming-Fu
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Lee, Yuan T.
;
Ni, Chi-Kung
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Xu, Shucheng
;
Lin, M. C.
Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO2 elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO2 elimination channels were determined to be NO/NO2=0.32 +/- 0.12 (193 nm), 0.26 +/- 0.12 (248 nm), and 0.4 +/- 0.12(266 nm) for nitrobenzene and 0.42 +/- 0.12(193 nm) and 0.3 +/- 0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results. (c) 2007 American Institute of Physics.
Photodissociation dynamics of phenol
(2007-09-27)
Tseng, Chien-Ming
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Lee, Yuan T.
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Lin, Ming-Fu
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Ni, Chi-Kung
;
Liu, Suet-Yi
The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H2O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or HO occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 <= v <= 4) in the spectral region of 1860-2330 cm(-1) were detected upon photolysis at 193 nm. After a correction for rotational quenching, CO (v 4) shows a nascent rotational temperature of similar to 4600 K. The observed vibrational distribution of (v = 1)1(v 2)/(v = 3)/(v = 4) = 64.3/22.2/9.1/4.4 corresponds to a vibrational temperature of 3350 +/- 20 K. An average rotational energy of 6.9 +/- 0.7 kcal mol(-1) and vibrational energy of 3.8 +/- 0.7 kcal mol(-1) are observed for the CO product. The dissociation channels, translational energy distributions of the photofragment, and vibrational and rotational energies of product CO are consistent with potential energy surfaces from quantum chemical calculations and the branching ratios from an RRKM calculation.
Selectivity of peptide bond dissociation on excitation of a core electron: Effects of a phenyl group
(2016-09-01)
Tsai, Cheng-Cheng
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Chen, Jien-Lian
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Hu, Wei-Ping
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Lin, Yi-Shiue
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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.
UV Absorption Cross Sections of ClOOCl Are Consistent with Ozone Degradation Models
(2009-05-08)
Chen, Hsueh-Ying
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Lien, Chien-Yu
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Lin, Wei-Yen
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Lee, Yuan T.
;
Lin, Jim J.
Recently, discrepancies in laboratory measurements of chlorine peroxide (ClOOCl) absorption cross sections have cast doubt on the validity of current photochemical models for stratospheric ozone degradation. Whereas previous ClOOCl absorption measurements all suffered from uncertainties due to absorption by impurities, we demonstrate here a method that uses mass-selected detection to circumvent such interference. The cross sections of ClOOCl were determined at two critical wavelengths (351 and 308 nanometers). Our results are sufficient to resolve the controversial issue originating from the ClOOCl laboratory cross sections and suggest that the highest laboratory estimates for atmospheric photolysis rates of ClOOCl, which best explain the field measurements via current chemical models, are reasonable.
UV Photolysis of ClOOCl and the Ozone Hole
(2011-07-04)
Lin, Jim J.
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Chen, Andrew F.
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Lee, Yuan T.
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應用化學系
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Department of Applied Chemistry
The photochemistry of the ClO dimer (ClOOCl) plays a central role in the catalytic destruction of polar stratospheric ozone. In spite of decades of intense investigations, some of its laboratory photochemical data had not reached the desired accuracy to allow a reliable simulation of the stratospheric ozone loss until recently. Inevitable impurities in ClOOCl samples have obstructed conventional measurements. In particular, an absorption measurement of ClOOCl in 2007, which gave much lower cross sections than previous studies, implied that the formation of the ozone hole cannot be explained with cur-rent chemical models. Scientists have wondered whether the model is insufficient or the data is erroneous. Efforts aiming to resolve this controversy are reviewed in this paper, which emphasizes newly developed experiments to determine two critical photochemical properties of ClOOCl-its absorption cross section and product branching ratio-including the first reported product branching ratio at 351.8 nm photolysis.