Browsing by Author "Chiang, SY"

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Dissociation of energy-selected c-C2H4S+ in a region 10.6-11.8 eV: Threshold photoelectron-photoion coincidence experiments and quantum-chemical calculations
(2005-08-01)
Fang, YS
;
Lin, IF
;
Lee, YC
;
Chiang, SY
;
應用化學系
Dissociation of energy-selected c-C2H4S+ was investigated in a region of 10.6-11.8 eV with a threshold photoelectron-photoion coincidence technique and a synchrotron as a source of vacuum ultraviolet radiation. Branching ratios and average releases of kinetic energy in channels of formation of c-C2H4S+, CH3CS+, and HCS+ were obtained from well-resolved time-of-flight peaks in coincidence mass spectra. Measured average releases of kinetic energy for channel CH3CS++H of least energy are substantial and much greater than calculated with quasiequilibrium theory; in contrast, small releases of kinetic energy near the appearance onset for channel HCS++CH3 agree satisfactorily with statistical calculations. Calculations of molecular electronic structures and energetics of c-C2H4S+ and C2H3S+ isomers and various fragments and transition states were also performed with Gaussian 3 method to establish dissociation mechanisms. A predicted dissociation energy of 11.05 eV for c-C2H4S -> HCS++CH3 agrees with a linearly extrapolated threshold at 10.99 +/- 0.04 eV and a predicted dissociation mechanism that c-C2H4S+ isomerizes to CH3CHS+ before dissociating to HCS++CH3 supports the experimental results. The large releases of kinetic energy for channel CH3CS++H might result from a dissociation mechanism according to which c-C2H4S+ isomerizes to a local minimum CH3CSH+ and then dissociates through a transition state to form CH3CS++H. (C) 2005 American Institute of Physics.
Dissociation of energy-selected c-C3H6S+ studied with threshold photoelectron-photolon coincidence experiments and calculations
(2006-05-10)
Chiang, SY
;
Fang, YS
;
Lin, CN
;
應用化學系
;
Department of Applied Chemistry
The dissociation of energy-selected c-C3H6S+ was investigated in a region 10.2-10.9 eV with a threshold photoelectron-photoion coincidence technique. Branching ratios and average releases of kinetic energy for channel c-C3H6S+ -> H2CS+ + C2H4 were derived from coincidence mass spectra. The measured small releases of kinetic energy near the appearance onset agree with statistical calculations; a linearly extrapolated threshold at 10.39 +/- 0.01 eV agrees with a predicted energy 10.35 eV with the GAUSSIAN-3 method. We discuss plausible mechanisms for c-C3H6S+ dissociating to CH3CS+ + CH3 based on G3B3 calculations to rationalize the absence of CH3CS+ signal in these experiments. (c) 2006 Elsevier B.V. All rights reserved.
Experimental and quantum-chemical studies on dissociative photoionization of c-C2H4S
(2005-06-01)
Chiang, SY
;
Fang, YS
;
應用化學系
;
Department of Applied Chemistry
The dissociative photoionization of thiirane (c-C2H4S) in a region similar to 9-20 eV was investigated with photoionization mass spectroscopy and calculations of molecular energies with the Gaussian-3 method. An adiabatic ionization energy of 9.051 eV for c-C2H4S agrees with a prediction of 9.07 eV for formation of c-C2H4S+; predictions of energies of formation of ring-opened isomers CH3CHS+, CH2CHSH+ and CH2SCH2+ are 8.85, 8.91 and 9.37 eV, respectively. Major fragment ions C2H3S+, C2H2S+, HCS+, H2S+ and C2H3+ were observed with their respective appearance energies at 10.71, 13.07, 11.13, 11.96 and 12.58 eV. Based on comparison of determined appearance energies and predicted reaction energies, we established six dissociation channels c-C2H4S+ -> CH3CS+ + H, HCS+ + CH3, H2S+ + C2H2, C2H3+ + HS, CH2CS+ + H-2 and CHCSH+ + H-2. Ring-opening and H migration are involved in these processes. (c) 2005 Elsevier B.V. All rights reserved.
Experimental and quantum-chemical studies on photoionization and dissociative photoionization of CH2Br2
(2004-02-15)
Chiang, SY
;
Fang, YS
;
Sankaran, K
;
Lee, YP
;
應用化學系
The dissociative photoionization of CH2Br2 in a region similar to10-24 eV was investigated with photoionization mass spectroscopy using a synchrotron radiation source. An adiabatic ionization energy of 10.25 eV determined for CH2Br2 agrees satisfactorily with predictions of 10.26 and 10.25 eV with G2 and G3 methods, respectively. Observed major fragment ions CH2Br+, CHBr+, and CBr+ show appearance energies at 11.22, 12.59, and 15.42 eV, respectively; minor fragment ions CHBr2+, Br+, and CH2+ appear at 12.64, 15.31, and 16.80 eV, respectively. Energies for formation of observed fragment ions and their neutral counterparts upon ionization of CH2Br2 are computed with G2 and G3 methods. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. An upper limit of DeltaH(f,298)(0)(CHBr+)less than or equal to300.7+/-1.5 kcal mol(-1) is derived experimentally; the adiabatic ionization energy of CHBr is thus derived to be less than or equal to9.17+/-0.23 eV. Literature values for DeltaH(f,298)(0)(CBr+)=362.5 kcal mol(-1) and ionization energy of 10.43 eV for CBr are revised to be less than 332 kcal mol(-1) and 9.11 eV, respectively. Also based on a new experimental ionization energy, DeltaH(f,298)(0)(CH2Br2+) is revised to be 236.4+/-1.5 kcal mol(-1). (C) 2004 American Institute of Physics.
Experiments and calculations on photoionization and dissociative photoionization of CH2CO
(2004-08-01)
Chiang, SY
;
Fang, YS
;
Bahou, M
;
Sankaran, K
;
應用化學系
The dissociative photoionization of molecular-beam cooled CH2CO in a region of similar to10-20 eV was investigated with photoionization mass spectrometry using a synchrotron radiation as the light source. Photoionization efficiency curves of CH2CO+ and of observed fragment ions CH2+, CHCO+, HCO+ C2O+ CO+, and C2H2+ were measured to determine their appearance energies. Relative branching ratios as a function of photon energy were determined. Energies for formation of these observed fragment ions and their neutral counterparts upon ionization of CH2CO are computed with the Gaussian-3 method. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. The principal dissociative processes are direct breaking of C=C and C-H bonds to form CH2+ + CO and CHCO+ + H, respectively; at greater energies, dissociation involving H migration takes place.