Browsing by Author "Lin, Jim J."

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Barrierless reactions between two closed-shell molecules. I. Dynamics of F(2)+CH(3)SCH(3) reaction
(2008-03-14)
Lu, Yu-Ju
;
Lee, Lance
;
Pan, Jun-Wei
;
Xie, Tingxian
;
Witek, Henryk A.
A detailed experimental and theoretical investigation of the first-reported barrierless reaction between two closed-shell molecules [J. Chem. Phys. 127, 101101 (2007)] is presented. The translational energy and angular distributions of two product channels, HF+CH(2)SFCH(3) and F+CH(3)SFCH(3), determined at several collision energies, have been analyzed to reveal the dynamics of the studied reaction. Detailed analysis of the experimental and computational results supports the proposed reaction mechanism involving a short-lived F-F-S(CH(3))(2) intermediate, which can be formed without any activation energy. Other possible reaction mechanisms have been discriminated. The decay of the intermediate and competition between the two product channels have been discussed. (c) 2008 American Institute of Physics.
Barrierless reactions between two closed-shell molecules. II. Dynamics of F(2)+CH(3)SSCH(3) reaction
(2009-01-07)
Shao, Hua-Chieh
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Xie, Tingxian
;
Lu, Yu-Ju
;
Chang, Chih-Hsuan
;
Pan, Jun-Wei
A second example of a barrierless reaction between two closed-shell molecules is reported. The reaction F(2)+CH(3)SSCH(3) has been investigated with crossed molecular beam experiments and ab initio calculations. Compared with previous results of the F(2)+CH(3)SCH(3) reaction [J. Chem. Phys. 127, 101101 (2007); J. Chem. Phys. 128, 104317 (2008)], a new product channel leading to CH(3)SF+CH(3)SF is observed to be predominant in the title reaction, whereas the anticipated HF+C(2)H(5)S(2)F channel is not found. In addition, the F+C(2)H(6)S(2)F product channel, the analog to the F+C(2)H(6)SF channel in the F(2)+CH(3)SCH(3) reaction, opens up at collision energies higher than 4.3 kcal/mol. Angular and translational energy distributions of the products are reported and collision energy dependences of the reaction cross section and product branching ratio are shown. The reaction barrier is found to be negligible (< 1 kcal/mol). Multireference ab initio calculations suggest a reaction mechanism involving a short-lived intermediate which can be formed without activation energy.
Barrierless reactions between two closed-shell molecules. II. Dynamics of F-2+CH3SSCH3 reaction
(2009-01-07)
Shao, Hua-Chieh
;
Xie, Tingxian
;
Lu, Yu-Ju
;
Chang, Chih-Hsuan
;
Pan, Jun-Wei
A second example of a barrierless reaction between two closed-shell molecules is reported. The reaction F-2+CH3SSCH3 has been investigated with crossed molecular beam experiments and ab initio calculations. Compared with previous results of the F-2+CH3SCH3 reaction [J. Chem. Phys. 127, 101101 (2007); J. Chem. Phys. 128, 104317 (2008)], a new product channel leading to CH3SF+CH3SF is observed to be predominant in the title reaction, whereas the anticipated HF+C2H5S2F channel is not found. In addition, the F+C2H6S2F product channel, the analog to the F+C2H6SF channel in the F-2+CH3SCH3 reaction, opens up at collision energies higher than 4.3 kcal/mol. Angular and translational energy distributions of the products are reported and collision energy dependences of the reaction cross section and product branching ratio are shown. The reaction barrier is found to be negligible (< 1 kcal/mol). Multireference ab initio calculations suggest a reaction mechanism involving a short-lived intermediate which can be formed without activation energy.
Crossed molecular beam studies on the reaction dynamics of O(D-1)+N2O
(2006-10-07)
Lu, Yu-Ju
;
Liang, Chi-Wei
;
Lin, Jim J.
;
應用化學系
;
Department of Applied Chemistry
The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N-2+O-2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(D-1) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N-2+O-2 channel (f(T)=0.57) is much higher than that of the NO+NO channel (f(T)=0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The (N2O)-N-46 product of the isotope exchange channel, O-18+(N2O)-N-44 -> O-16+(N2O)-N-46, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%. (c) 2006 American Institute of Physics.
Dynamics of reactions between two closed-shell molecules
(2011)
Lin, Jim J.
;
應用化學系
;
Department of Applied Chemistry
The crossed molecular beam technique has been utilized to investigate a large number of elementary reactions. However, most of the studied reactions involve atoms or radicals; reactions between two stable molecular reactants are, in fact, seldom studied with the crossed molecular beam method. In this perspective, reactions between two stable molecules are reviewed and discussed. With crossed molecular beams and vacuum UV photoionization, the nascent products have been unambiguously identified. Consistent pictures of the reaction paths have been constructed based on the experimental data and ab initio calculations. Furthermore, there are intriguing features about the reaction barriers. The F(2) + organosulfur reactions are barrierless, demonstrating the first examples of such interactions between two closed-shell reactants. The barrier of F(2) + alkene reaction decreases with more methyl substitution groups at the C=C double bond, yet the absolute barrier heights from experiment and theory disagree with each other by similar to 2 kcal mol(-1), leaving an issue to be resolved in the future.
Dynamics of reactions O((1)D)+C(6)H(6) and C(6)D(6)
(2008-11-07)
Chen, Hui-Fen
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Liang, Chi-Wei
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Lin, Jim J.
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Lee, Yuan-Pern
;
Ogilvie, J. F.
The reaction between O((1)D) and C(6)H(6) (or C(6)D(6)) was investigated with crossed-molecular-beam reactive scattering and time-resolved Fourier-transform infrared spectroscopy. From the crossed-molecular-beam experiments, four product channels were identified. The major channel is the formation of three fragments CO+C(5)H(5)+H; the channels for formation of C(5)H(6)+CO and C(6)H(5)O+H from O((1)D)+C(6)H(6) and OD+C(6)D(5) from O((1)D)+C(6)D(6) are minor. The angular distributions for the formation of CO and H indicate a mechanism involving a long-lived collision complex. Rotationally resolved infrared emission spectra of CO (1 <=upsilon <= 6) and OH (1 <=upsilon <= 3) were recorded with a step-scan Fourier-transform spectrometer. At the earliest applicable period (0-5 mu s), CO shows a rotational distribution corresponding to a temperature of similar to 1480 K for upsilon=1 and 920-700 K for upsilon=2-6, indicating possible involvement of two reaction channels; the vibrational distribution of CO corresponds to a temperature of similar to 5800 K. OH shows a rotational distribution corresponding to a temperature of similar to 650 K for upsilon=1-3 and a vibrational temperature of similar to 4830 K. The branching ratio of [CO]/[OH]=2.1 +/- 0.4 for O((1)D)+C(6)H(6) and [CO]/[OD]>2.9 for O((1)D)+C(6)D(6) is consistent with the expectation for an abstraction reaction. The mechanism of the reaction may be understood from considering the energetics of the intermediate species and transition states calculated at the G2M(CC5) level of theory for the O((1)D)+C(6)H(6) reaction. The experimentally observed branching ratios and deuterium isotope effect are consistent with those predicted from calculations.
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
;
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 the simplest pi-bonding molecule
(2008-05-14)
Lu, Yu-Ju
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Xie, Tingxian
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Fang, Jing-Wen
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Shao, Hua-Chieh
;
Lin, Jim J.
The reaction of F(2)+C(2)H(4) has been investigated with crossed molecular beam experiments and high level ab initio calculations. For a wide range of collision energies up to 11 kcal/mol, only one reaction channel could be observed in the gas phase. The primary products of this channel were identified as F+CH(2)CH(2)F. The experimental reaction threshold of collision energy was determined to be 5.5 +/- 0.5 kcal/mol. The product angular distribution was found to be strongly backward, indicating that the reaction time scale is substantially shorter than rotation. The calculated transition state structure suggests an early barrier; such dynamics is consistent with the small product kinetic energy release measured in the experiment. All experimental results consistently support a rebound reaction mechanism, which is suggested by the calculation of the intrinsic reaction coordinate. This work provides a clear and unambiguous description of the reaction dynamics, which may help to answer the question why the same reaction produces totally different products in the condensed phase. (C) 2008 American Institute of Physics.
Dynamics of the F-2 Reaction with Propene: The Effect of Methyl Substitution
(2009-04-23)
Fang, Jing-Wen
;
Xie, Tingxian
;
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 F-2+CH3SCH3 reaction: A molecule-molecule reaction without entrance barrier
(2007-09-14)
Lu, Yu-Ju
;
Lee, Lance
;
Pan, Jun-Wei
;
Witek, Henryk A.
;
Lin, Jim J.
The F-2+CH3SCH3 reaction was studied with crossed molecular beam techniques and high level ab initio calculations. Significant reactivity was observed even at low collision energies, consistent with the negligible barrier height obtained from the ab initio calculations. All experimental findings are consistent with a weakly bound reaction intermediate of F-F-S(CH3)(2) structure, which possesses a special type of three-center four-electron bonding. Analogous intermediates can also explain the reactions of F-2 with CH3SH and CH3SSCH3.
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
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Garofalo, Lauren
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Smith, Mica
Photodissociation cross sections of ClOOCl at 248.4 and 266 nm
(2009-11-07)
Lien, Chien-Yu
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Lin, Wei-Yen
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Chen, Hsueh-Ying
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Huang, Wen-Tsung
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Jin, Bing
This study utilized a mass-resolved detection of ClOOCl to determine its photodissociation cross section, which is the product of the absorption cross section and dissociation quantum yield. An effusive molecular beam of ClOOCl was generated and its photodissociation probability was determined through measuring the decrease in the ClOOCl beam intensity upon laser irradiation. By comparing with a reference molecule, the absolute cross sections of ClOOCl were obtained without knowing its absolute concentration. The determined cross section of ClOOCl at 248.4 nm is (8.85 +/- 0.42)x10(-18) cm(2) at 200 K, significantly larger than previously reported values. The temperature dependence of the cross section was investigated at 248.4 nm in the range of 160-260 K; only a very small and negative temperature effect was observed. Because 248.4 nm is very close to the peak of the UV absorption band of ClOOCl, this work provides a new calibration point for normalizing relative absorption spectra of ClOOCl. In this work, the photodissociation cross section at 266 nm and 200 K was also reported to be (4.13 +/- 0.21)x10(-18) cm(2).
Photolysis Cross-Section of Ozone Dimer
(2011-11-04)
Chen, I-Cheng
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Chen, Andrew F.
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Huang, Wen-Tsung
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Takahashi, Kaito
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Lin, Jim J.
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.
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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.
;
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.