Publications in the Year: 2011

Journal Article

Chen, S-Y, Tsai P-Y, Lin H-C, Wu C-C, Lin K-C, Sun BJ, Chang AHH.  2011.  I2 molecular elimination in single-photon dissociation of CH2I2 at 248 nm by using cavity ring-down absorption spectroscopy. Journal of Chemical Physics. 134, Number 3 AbstractWebsite

Following single-photon dissociation of CH2I2 at 248 nm, I2 molecular elimination is detected by using cavity ring-down absorption spectroscopy. The technique comprises two laser beams propagating in a perpendicular configuration, in which a tunable laser beam along the axis of the ring-down cell probes the I2 fragment in the B 3 ou + - X 1 g + transition. The nascent vibrational populations for v 0, 1, and 2 levels are obtained with a population ratio of 1:(0.65 0.10):(0.30 0.05), corresponding to a Boltzmann-like vibrational temperature of 544 73 K. The quantum yield of the ground state I2 elimination reaction is determined to be 0.0040 0.0025. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state CH2I2 via internal conversion, followed by asynchronous three-center dissociation. A positive temperature effect supports the proposed mechanism. © 2011 American Institute of Physics.
Kao, M-J, Chen C-H, Tsai P-Y, Lim T-S, Lin K-C, Luh T-Y.  2011.  Hydrogen-bonding-induced one-handed helical polynorbornenes appended with chiral alaninegland. Macromolecular Chemistry and Physics. 212:2328-2338., Number 21 AbstractWebsite

Polynorbornenes appended with anthracene and chiral alanine linkers are synthesized. Hydrogen bonding between the adjacent bisamidic linkers brings adjacent anthracene chromophores in a more suitable orientation for exciton coupling and renders one-handed helical structures for these polymers. Excimer formation is observed from their emission spectra. Monoamidic linkers provide only one hydrogen bond, which would be less robust and result in much lower circular dichroic response. Hydrogen bonding between the adjacent chiral alanine linkers brings appended anthracene in a more suitable orientation for exciton coupling and excimer formation, rendering one-handed helical structures in polynorbornenes. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Tsai, P-Y, Che D-C, Nakamura M, Lin K-C, Kasai T.  2011.  Orientation dependence for Br formation in the reaction of oriented OH radical with HBr molecule. Physical Chemistry Chemical Physics. 13:1419-1423., Number 4 AbstractWebsite

The orientation dependence of Br-atom formation in the reaction of the oriented OH radical with the HBr molecule using the hexapole electrostatic field was studied. Experimental results for the orientation dependence in the reaction were analyzed using a Legendre polynomial fit. The results show two reactive sites. It was found that O-end attack is most favored for this reaction, and that H-end attack also shows a pronounced reactivity. The reactivity of the side-ways attack was found to be small. By comparing the results of the orientation dependence in the reaction with studies of inelastic collisions and theoretical calculations, two reaction pathways are proposed. Reaction by O-end attack is followed by a direct abstraction of the H-atom from the HBr molecule. The mechanism for H-end attack may have H-atom migration from HBr to form the water molecule. © 2011 the Owner Societies.
Hsiao, M-K, Lin K-C, Hung Y-M.  2011.  Quasiclassical trajectory calculations for Li(22PJ) H2 → LiH(X1) H: Influence by vibrational excitation and translational energy. Journal of Chemical Physics. 134, Number 3 AbstractWebsite

Ab initio potential energy surfaces and the corresponding analytical energy functions of the ground 1A′ and excited 2A′ states for the Li(22P) plus H2 reaction are constructed. Quasiclassical trajectory calculations on the fitted energy functions are performed to characterize the reactions of Li(22P) with H2(v 0, j 1) and H2(v 1, j 1) as well as the reaction when the vibrational energy is replaced by collision energy. For simplicity, the transition probability is assumed to be unity when the trajectories go through the crossing seam region and change to the lower surface. The calculated rotational distributions of LiH(v 0) for both H2(v 0, j 1) and H2(v 1, j 1) reactions are single-peaked with the maximum population at j′ 7, consistent with the previous observation. The vibrational excitation of H2(v 1) may enhance the reaction cross section of LiH(v′ 0) by about 200 times, as compared to a result of 93-107 reported in the experimental measurements. In contrast, the enhancement is 3.1, if the same amount of energy is deposited in the translational states. This endothermic reaction can be considered as an analog of late barrier. According to the trajectory analysis, the vibrational excitation enlarges the H-H distance in the entrance channel to facilitate the reaction, but the excess energy may not open up additional reaction configuration. © 2011 American Institute of Physics.
Tsai, P-Y, Lin K-C.  2011.  Doublet rotational energy transfer of the SH (X 2Π, v′′ = 0) state by collisions with Ar. Physical Chemistry Chemical Physics. 13:8857-8868., Number 19 AbstractWebsite

The rotational energy transfer (RET) by Ar collisions within the SH X 2Π (v′′ = 0, J′′ = 0.5-10.5) state is characterized. The integral cross sections as a function of collision energy for each rotational transition are calculated using a quantum scattering method in which the constructed potential energy functions are based on a ground state potential energy surface (PES) reported previously. On the other hand, a laser-induced excitation fluorescence technique is employed to monitor the relaxation of the rotational population as a function of photolysis-probe delay time following the photodissociation of H2S at 248 nm. The rotational population evolution is comparable to its theoretical counterpart based on calculated Λ-resolved RET rate constants. The propensity in Λ-resolved RET transitions is found to approximately resemble the case of OH(X 2Π, v′′ = 0) + Ar. The Λ-averaged RET collisions are also analyzed and result in several propensity rules in the transitions. Most propensity rules are similar to those observed in the collisions of SH(A 2Σ+) by Ar. However, the behavior of the conserving ratio, defined as rate constants for spin-orbit conserving transition divided by those for spin-orbit changing transition, shows distinct difference from those described by Hund’s case (b). © the Owner Societies.
Chao, M-H, Tsai P-Y, Lin K-C.  2011.  Molecular elimination of methyl formate in photolysis at 234 nm: Roaming vs. transition state-type mechanism. Physical Chemistry Chemical Physics. 13:7154-7161., Number 15 AbstractWebsite

Ion imaging coupled with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) technique is employed to probe CO(v″ = 0) fragments at different rotational levels following photodissociation of methyl formate (HCOOCH 3) at 234 nm. When the rotational level, J″CO, is larger than 24, only a broad translational energy distribution extending beyond 70 kcal mol-1 with an average energy of about 23 kcal mol -1 appears. The dissociation process is initiated on the energetic ground state HCOOCH3 that surpasses a tight transition state along the reaction coordinate prior to breaking into CO + CH3OH. This molecular dissociation pathway accounts for the CO fragment with larger rotational energy and large translational energy. As J″CO decreases, a bimodal distribution arises with one broad component and the other sharp component carrying the average energy of only 1-2 kcal mol-1. The branching ratio of the sharp component increases with a decrease of J″CO; (7.3 ± 0.6)% is reached as the image is probed at J″CO = 10. The production of a sharp component is ascribed to a roaming mechanism that has the following features: a small total translational energy, a low rotational energy partitioning in CO, but a large internal energy in the CH3OH co-product. The internal energy deposition in the fragments shows distinct difference from those via the conventional transition state. © the Owner Societies 2011.
Liu, C-Y, Tsai M-T, Tsai P-Y, Liu Y-T, Chen SY, Chang AHH, Lin K-C.  2011.  Gas-phase photodissociation of CH3CHBrCOCl at 248 nm: Detection of molecular fragments by time-resolved FT-IR spectroscopy. ChemPhysChem. 12:206-216., Number 1 AbstractWebsite

By employing time-resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v=1-3), HBr (v=1), and CO (v=1-3) are detected in one-photon dissociation of 2-bromopropionyl chloride (CH3CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time-resolved high-resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0A ±0.2 and 7.4A ±0.6 kcal mol-1, respectively, while the rotational energy in HBr is evaluated to be 0.9A ±0.1 kcal mol-1. The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br2, in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH3CHBrCOCl. A two-body dissociation channel is favored leading to either HCl+CH3CBrCO or HBr+CH2CHCOCl, in which the CH 3CBrCO moiety may further undergo secondary dissociation to release CO. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Wu, C-C, Lin H-C, Chang Y-B, Tsai P-Y, Yeh Y-Y, Fan H, Lin K-C, Francisco JS.  2011.  Br 2 molecular elimination in photolysis of (COBr) 2 at 248 nm by using cavity ring-down absorption spectroscopy: A photodissociation channel being ignored. Journal of Chemical Physics. 135, Number 23 AbstractWebsite

A primary dissociation channel of Br 2 elimination is detected following a single-photon absorption of (COBr) 2 at 248 nm by using cavity ring-down absorption spectroscopy. The technique contains two laser beams propagating in a perpendicular configuration. The tunable laser beam along the axis of the ring-down cell probes the Br 2 fragment in the B 3Π + ou-X 1Σ g + transition. The measurements of laser energy- and pressure-dependence and addition of a Br scavenger are further carried out to rule out the probability of Br 2 contribution from a secondary reaction. By means of spectral simulation, the ratio of nascent vibrational population for v = 0, 1, and 2 levels is evaluated to be 1:(0.65 ± 0.09):(0.34 ± 0.07), corresponding to a Boltzmann vibrational temperature of 893 ± 31 K. The quantum yield of the ground state Br 2 elimination reaction is determined to be 0.11 ± 0.06. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state (COBr) 2 via internal conversion. A four-center dissociation mechanism is followed synchronously or sequentially yielding three fragments of Br 2 + 2CO. The resulting Br 2 is anticipated to be vibrationally hot. The measurement of a positive temperature effect supports the proposed mechanism. © 2011 American Institute of Physics.